Technical Information○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○

Processing Guide

IntroductionThis report provides guidelines for the efficientmixing and processing of Viton® fluoroelastomers.Suggested machine settings are given for variousprocessing techniques. Since mixing and processingequipment can vary with age, maintenance, manufac-turer and design, this report only purports to offertypical conditions, and not absolutes.

Polymer FormViton® typically is supplied in slab form, either asgum polymer, or as a precompound (gum polymer,plus curatives). Slabs of Viton® are nominally1.25 cm (0.5 in) thick and 36 cm (14 in) wide. Thelength of the slab may vary but there are typically10 slabs per box. Some Viton® gums are supplied asfree-flowing pellets.

PackagingViton® comes packaged in rectangular 61 x 41 x22 cm (24 x 16 x 8.75 in) Kraft boxes. The boxes aredesigned to fit a standard 1.0 x 1.2 m (40 x 48 in)pallet, using a 5 box per layer pattern. The standardpackage weight is 25 kg (55.1 lb) per box.

Handling and StorageBefore handling or processing Viton® polymers orprecompounds the user should read and understandthe DuPont Dow Elastomers technical bulletin“Handling Precautions for Viton® and RelatedChemicals.” It is generally advisable to avoid havingphysical contact with the polymer, all compoundingingredients and/or any of the fumes or dust associatedwith the processing of the polymer/compound.

Like most rubbers, Viton® and the ingredients used incompounding should be stored in cool dry locationsand kept free of contamination. Although the Viton®

polymers tend to have excellent storage stability,storage in hot and/or damp areas should be avoided.To avoid contamination, such items as atmosphericmoisture, oils and greases from machinery or othercompounds, other polymers and/or polymeric com-pounds should be prevented from coming in contactwith Viton® polymers.

2

Table of Contents

Processing .............................................................................................................................................. 3Mill Mixing ....................................................................................................................................... 3

Mill Mixing Recommended Practices ......................................................................................... 3Curative Addition ........................................................................................................................ 3Acid Acceptors ............................................................................................................................ 4Processing Aids ............................................................................................................................ 4Mill Mixing Trouble Shooting Guide .......................................................................................... 4

Excessive roll sticking ............................................................................................................ 4Excessive bagging ................................................................................................................... 4Poor Dispersion ....................................................................................................................... 5

Cooling and Storage ..................................................................................................................... 5Internal Mixing ................................................................................................................................. 5

Definitions and Practices ............................................................................................................. 5General Recommendations .......................................................................................................... 8Internal Mixing Trouble Shooting Guide .................................................................................... 8

Poor Dispersion ....................................................................................................................... 8Poor Scorch Characteristics .................................................................................................... 8

Extrusion ........................................................................................................................................... 8Set up Conditions ......................................................................................................................... 9

Calendering ....................................................................................................................................... 9General Recomendations/Set up .................................................................................................. 9Stock Warm up and Calender Feeding ........................................................................................ 9Curing Calender Sheeting ............................................................................................................ 9Calendering Trouble Shooting Guide ........................................................................................ 10

Molding ........................................................................................................................................... 10General Practices ....................................................................................................................... 10Compression Molding ................................................................................................................ 11Transfer Molding ....................................................................................................................... 11Injection Molding ...................................................................................................................... 11Trouble Shooting Guide ............................................................................................................. 14

Backrinding ........................................................................................................................... 14Blisters .................................................................................................................................. 14Sponged Areas ...................................................................................................................... 16Poor Knit ............................................................................................................................... 17Non-fills ................................................................................................................................ 17Tearing .................................................................................................................................. 17Mold Shrinkage ..................................................................................................................... 18

Adhesion ......................................................................................................................................... 19Adhesives ................................................................................................................................... 19General Practices ....................................................................................................................... 19Types of Primers and Adhesives ............................................................................................... 20

Oven Postcuring .................................................................................................................................. 21General Practices ....................................................................................................................... 21Postcure Oven Fires ................................................................................................................... 22

Suggested Mold Release Systems ....................................................................................................... 22

3

ProcessingViton® polymers can be mixed using conventionalrubber processing equipment. Demands in the marketplace for high quality goods that meet increasinglystringent performance criteria place special emphasison product quality. Economic constraints, coupledwith increased competition have made it essential thatproducts be made properly from the beginning of theprocess, rather than relying on end-of the line inspec-tions, where rejects invoke higher added costs. Forrubber, getting it right from the start means getting itright during the mixing cycle.

Mill MixingMill mixing is one of the oldest and most basicmethods of mixing rubber compounds. It consists oftwo, counter-rotating steel rolls turning at differentspeeds. The different speeds of the rolls create ashearing action at the point where the two rolls areclosest. By passing the polymer through the nipbetween the two rolls, the polymer is masticated andsqueezed to form a band on one of the rolls. Afterforming a band, the dry and liquid ingredients areincorporated, by the grinding and shearing action ofthe two rolls. To further enhance the mixing anddispersion of the ingredients, the mill operator mustcut, fold and refine the compound.

It is generally recommended that Viton® be mixed onas cool a mill as possible (23°C or 75°F). The use ofchilled water minimizes scorch and promotes highershear, thus improving dispersion. Once the polymerhas been banded, the addition of the other ingredientscan start immediately. If two polymers of dissimilarviscosities are to be blended, the higher viscositypolymer should be banded first, followed by theaddition of the lower viscosity polymer. Once a bandof the polymers has been formed, the band should becut several times to enhance the blending and then theother ingredients may be added. It is important that thebatch being mixed on the mill is properly sized for themill being used. Too large or too small a batch tends toreduce dispersion and/or significantly increase themilling time required to obtain good dispersion. Thefollowing are suggested batch sizes for various millsizes:

Mill Roll Length Batch Weights

0.91 m (36 in) 13–15 kg (28–33 lb)1.02 m (40 in) 16–18 kg (35–40 lb)1.22 m (48 in) 21–25 kg (47–55 lb)1.52 m (60 in) 34– 41 kg (75–90 lb)

Recommended Mill MixingPractices• Always use a clean mill, free of contamination from

other elastomers, oils, greases and sulfur-bearingchemicals.

• Viton® is typically a “back roll” polymer. Thismeans that the polymer will go to the back roll(fastest roll) when banded. Forcing the polymer orcompound to the slower roll can sometimes be anoption, but the compound may exhibit extensivebagging and splitting to both rolls if this isattempted.

• Use chilled water (10°C [50°F]) whenever available.The best dispersion occurs at maximum shear. Asthe polymer heats up during the mix cycle, theviscosity will decrease along with shear stress.Keeping the stock temperature cool is especiallyimportant for polymers that have Mooney viscositiesthat are 30 and lower, in order to prevent mill rollsticking and to maximize dispersion.

• Typically, ingredients that melt are easier to disperseif added after the filler has been incorporated. Thesematerials can be added late in the mix, when thebatch temperature has increased sufficiently to meltthem.

• Generally, it is best to allow the mixed stock tocondition for at least 12 hr prior to molding parts.This conditioning allows the interaction between thefiller system and polymer to take place, resulting innoticeable improvements in mold flow characteris-tics and physical properties.

Curative Addition• Diak™ #1 should be added to the batch last, just prior

to the cross cutting and refining steps. Diak™ #3 and#4 are safer curatives and can be incorporated at thesame time the fillers are added.

• VC-20 and VC-30 should be added to the bandedpolymer as early in the mixing operation as possible.Neither of the chemicals will melt under normalmixing conditions, and depend on good shear fordispersion. Adding these chemical masterbatcheslate in the mix will result in dispersion problems.

• VC-50 is supplied in the form of free flowingpastilles that will readily break up into smallerparticles during incorporation. However, the VC-50must melt in order to be properly dispersed. VC-50melts at 80°C (176°F). Therefore the batch shouldreach a temperature of 100°C (212°F) to ensuremelting.

4

Acid Acceptors• Always preblend the acid acceptors (MgO, Ca(OH)2,

ZnO, etc.) with the filler(s) for the best dispersion.The addition of metal oxides alone (especially MgO)can cause caking and sticking to the rolls.

• Moist magnesium oxide will badly cake on the millrolls and will be more difficult to disperse. Further,moist acid acceptors can cause scorch and cureproblems.

• These materials are hygroscopic and should be keptdry.

Processing Aids• Do not use stearate types of process aids, such as

zinc stearate; these materials reduce the processingsafety of bisphenol cure systems.

• Add process aids late in the mix, or last. Processingaids, in general, are easily dispersed, but, if addedtoo early in the mix, they can reduce the mixingshear that is needed for good dispersion of ingredi-ents that do not melt.

• Recommended process aids for bisphenol-curedtypes of Viton® include Struktol WS280, CarnaubaWax, VPA 1, VPA 2, and VPA 3. The VPA 3 shouldnot be used at levels higher than 1.25 phr. Levels ofVPA 3 at or above 1.50 phr will result in poorscorch safety.

• For peroxide-cured types of Viton®, Struktol®

WS280 and Armeen® 18D (<1.25 phr) are recom-mended for improved mill handling and moldrelease.

Mill Mixing Trouble-Shooting GuideExcessive sticking of stock to mill rolls• Do not add MgO separately to the batch, at any stage

of the mix.• Increase flow, and/or reduce the temperature of

water passing through rolls.• Do the rolls have excessive scale deposits, thus

reducing the heat transfer efficiency of the rolls?• Add 0.5–1.0 phr of carnauba wax to bisphenol-cured

Viton® formulations. If peroxide cured formulationsare being used, add 0.5–1.0 phr of Struktol® WS280or Armeen® 18D.

• If possible, use a higher viscosity grade of Viton®,or blend current polymer with a higher viscosityversion.

Excessive bagging of the stock• Try warming the mill rolls, above the current

temperature, and/or using a tighter mill nip opening.• The formulation should not contain excessive

amounts of process aids: e.g., no more than a total of1.5 phr. Armeen® 18D should not be used at levelshigher than 0. 5 phr.

• If bagging occurs on the front roll, move the batch tothe back roll.

Suggested Mill Mixing Procedure

Time, min Operation

–10 Sweep the mill pan, clean the rolls and guides to eliminate potentialcontamination. Adjust mill roll temperature to 32 ± 5°C (90 ± 10°F).Water on full.

–5 Premix filler(s), magnesium oxide and calcium hydroxide, or other metaloxides in a clean rigid container to obtain a homogeneous blend.

0 Band the polymer and adjust the width of the nip to obtain a rolling bank.

2–3 Add all the premixed ingredients into the nip at a rapid, but uniform rate across the length of therolls. Allow loose ingredients to fall on the pan. Sweep the pan of any ingredients and reintroducethem into the nip. Sweep and re-add the loose ingredients as often as necessary until noneremain.

9–12 When all the ingredients have been fully incorporated, close the mill nip. Cut and blend the stockthree times from each side of the mill roll. ”Cigar” the stock and pass it through a tight nip settingat least 4 times.

13–15 After refining the stock, open the rolls to the desired width for final sheet thickness and cut thestock across the width of the roll to remove the stock in a sheet. Allow the stock to air cool.Note: Air-cooling is preferred. However, the stock may be water cooled if precautions are taken tolimit the amount of time the stock is dipped and if forced air is used to completely rid the stock ofany residual water before storing.

5

Poor dispersion of curatives• VC-20, VC-30: Add these curatives to the batch

immediately after banding the polymer. Make surethat the batch does not get too hot, too quickly.

• VC-50: Make sure that the batch reaches a minimumtemperature of 100°C (212°F), in order to ensurethat the curative will melt.

Poor dispersion of metal oxides/acidacceptors• Premix acid acceptors with filler(s), prior to adding

to the banded polymer.• Keep the mill rolls as cool as possible.• Make certain the metal oxides have been stored in a

cool, dry place and kept in tightly closed containers.• Check the metal oxides for lumps, which could be an

indication of moisture contamination.• Use masterbatch metal oxides.• Allow the stock to condition for 12–24 hr after

mixing and then refine the stock.

Cooling and StorageCompounds based on Viton® should be cooled asquickly as possible after mixing. Cooling can beaccomplished by immersing the sheeted stock into adip tank, by exposure to fans, or by water spray. Thebest cooling results from using water spray and fans.If water is used, it is critical that ALL the water beremoved from the surface of the stock before placinginto storage.Under no circumstances should the stock be stackedfor storage if the internal temperature is above 32°C(90°F). The stock should be cool to the touch. Apartitioning agent is required to prevent mixedcompound from sticking to itself, especially forcompounds made with lower viscosity polymers.Talc is preferred. Do not use a dusting agent thatcontains stearates.Mixed stock should be kept in a cool (18°C [65°F]),dry location and protected from airborne contami-nants and moisture. When taken out of storage, keepthe stock covered while allowing it to come to roomtemperature. Any moisture that condenses on thestock should be quickly removed, to prevent blister-ing and scorch problems during the curing process.

Internal MixingAlthough there are several variations of internalmixers, the basic actions they perform are the same.Internal rubber mixers consist of two intermeshing,counter-rotating bladed rotors turning at the samespeed. The rotors are set at a specific distance fromeach other and the walls of the chamber, in which they

are housed. A ram is positioned in the throat of theunit, leading to the chamber. During the loadingprocess, the ram is retracted to allow the variousingredients to be charged into the chamber. However,during the mixing cycle, the ram is lowered to helpfeed the materials to the rotors and to force the materi-als against the rotors while mixing. The polymer andcompounding ingredients are charged into the chambervia chute or throat. The mixing is accomplished byshearing action of the rotors against the walls of thechamber and by the squeezing and shearing actionbetween rotor blades. When the polymer is in contactwith the rotors, the speed of the rubber is equal to thespeed of the rotor, whereas the speed of a particlelocated on the fixed walls of the chamber is zero. Dueto the peripheral speed of the rotor tip and the staticnature of the mixers internal chamber, the shear ratevalues are very high.

Mixers of this type can range in batch sizes fromapproximately 2.2–363 kg (5–800 lb). Mix times willgenerally run from 3–5 min, depending on the condi-tion of the mixer and the formulation to be mixed. It ispreferred that mixers of this type have chilled orrefrigerated water to provide ample cooling of thebatch. As with mill mixing, the best dispersion occurswhen the polymer viscosity is the highest. As the mixtemperature increases due to shear, the viscosity willdecrease along with the dispersion efficiency.

It is generally recommended that for processinguniformity and consistency, an internal mixer shouldbe outfitted with the following:• A timer for measuring the cycle time• A method for measuring the internal temperature of

the batch• Variable rotor speed• Variable ram pressure• A method for determining power consumption

during the mixing cycle• A method to extract heat from the batch

Definitions and RecommendedPracticesTotal Mix Cycle Time—This is the time recordedfrom the start of feeding the materials into thebanbury, to the moment the batch is discharged.Because the rate of temperature increase of the workinput to the batch can be affected by the conditioningof the raw materials, feedstock and mixer temperature,‘total mix cycle time’ is not normally a primary controlparameter. However, it can be used as an effect alert orindicator that the desired processing or work input isdifferent.

6

Rotor Speed—The rotor speed will determine the rateof temperature increase. Typically, the ability tomonitor, adjust and record rotor speeds during themixing cycle will allow for significant improvementsin mixing uniformity and performance.

Ram Down Cycle Time—The mixing in the banburyis initiated with the pressurization of the batch, causedby the ram being lowered under pressure.

Power/Work Input—The work input to the materialis a measure of the energy consumed by the mixer,while combining the ingredients. In theory, the conver-sion of mechanical energy into heat through theshearing action of the materials passing between therotor tips and the mixer’s chamber wall should beuniform for a given set of operating conditions anduniform feedstock conditions. Variations in thefeedstock temperature, either from seasonal changes orprocess variations, and changes in the mixer tempera-ture will have a significant affect on energy consump-tion. Other ways to measure energy consumption orwork input:

– Peak Power or the maximum instantaneous powerutilized by the mixer. Usually the maximumdispersion occurs at the peak power plateau.

– Motor Torque is a calculated value and can beused at various points during the mixing cycle asan indication of the composition’s viscosity.

– Batch Gauge Temperature—A continuousindication of the batch temperature is essential forinternal mixers. Batch temperature defines anddetermines the point in the cycle that the variousingredients can be added to the batch, whenmaterials will be better dispersed, when themelting of various ingredients will occur and whenthe compound can be discharged from the mixer. Itis very important that this instrument be properlymaintained and checked for accuracy. Generally,the internal temperature of a compound will be10–25°C (20–50°F) higher than that indicated bythe mixer’s gauge. Batches should be dischargedwhen the mixer’s internal temperature gauge is nohigher than 110-115°C (230–240°F) range.

With few exceptions, most compounds based onViton® can be ‘one pass’ mixed in an internal mixer.Mixing times generally range from 3–5 min. If theproper mixing procedures are followed, mixing in aninternal mixer can be as safe as mixing on a large mill.As with mill mixing, availability and use of chilled orrefrigerated water is very important. The use of chilledwater allows better control of the stock temperatureduring mixing, including keeping the viscosity of thestock higher for increased shear and dispersion.

The recommended load factor for most compounds ofViton® is approximately 70–72%. Too high a loadfactor can result in scorching the batch and poordispersion. Likewise, if the load factor is too low, themix time will increase and poor dispersion can result.The load factor is the percentage of the mixer’schamber to be filled by the stock. The actual volume ofthe chamber is the chamber volume minus the volumeoccupied by the rotors or ‘net chamber volume.’ Sincerubbers are incompressible, a certain volume of thechamber must be left unfilled, to facilitate movementof the stock and mixing.

Batch Size (weight) = (Net Chamber Volume) x(Load Factor) x (Compound Specific Gravity)

The load factor defines that portion of the net chambervolume actually occupied by the mixed batch. It doesnot take into account such variables as:• The condition of the mixer. Wear on the mixing

chamber and rotors increases the net chamber sizeand thus may require a slightly higher load factor.

• The rheological properties of the polymer. Polymersthat tend to quickly thermally soften may require ahigher load factor.

• The stock viscosity or stiffness. High viscositycompounds generate heat quickly and make controlof the stock temperature more difficult and mayrequire a slightly reduced load factor in order tohave better temperature control of the stock.

• The cooling efficiency and available mixing power.The load factor may have to be reduced to improvecontrol of the temperature and to remain within thepower capabilities of the mixer. It should be notedhowever, that less than optimum load factors tend toincrease the mixing cycle and increase the risk ofpoor dispersion.

Internal Mixing GeneralRecommendationsWhen mixing any compound based on Viton®, aproperly cleaned mixer is required in order to avoidcontamination. A thorough cleaning of the mixer isparticularly important if sulfur or sulfur-bearingcompounds were previously used in that mixer. Sulfuris extremely detrimental to the bisphenol cure system.The presence of as little as 0.15 phr of elemental sulfurcan cause the complete loss of cure activity in abisphenol-cure compound. Likewise, aromatic oils cancause decreased cure activity in peroxide curedpolymers. Therefore, care should be used in selectingthe oils used for lubricating the rotor seals.

7

Chilled water (10°C [50°F]) is typically recommendedfor obtaining the best dispersion of ingredients that donot melt below 100°C (212°F). Maximum shear isobtained by keeping the polymer viscosity as high aspossible.

For highly filled compounds of Viton® it is recom-mended that an “upside down” mix or a “sandwichmix” be used. The “upside down” mix involvesloading the fillers and acid acceptors first, followed bythe polymer(s). A ‘sandwich’ mix involves charginghalf the polymer into the mixer first, followed by thefillers and acid acceptors, and then the remainingpolymer.

If variable rotor speed control is available, a slow rotorspeed is recommended over that of a fast rotor speed.Typically, rotor speeds of 25–45 rpm are preferred formost mixing. Rotor speeds that are too high willgenerate heat too quickly, reducing the viscosity of thepolymer and the ability of the mixer to disperse theingredients. Further, the excessive heat that is gener-ated may cause scorching of the compound. Too slowa rotor speed may cause excessively long mix cyclesand failure to reach a temperature high enough to meltspecific ingredients.

It is a good practice to determine the actual batchtemperature with a probe, after the batch has beendischarged from the mixer. All internal mixers tend toexhibit some discrepancy between the actual batchtemperature and the batch gauge temperature in themixing chamber. The difference between the twotemperatures may be caused by the location of themixer’s thermocouple, the batch size and/or the batchviscosity. Knowing the differences between the twotypes of temperature measurements will influence

when the batch should be dropped, to avoid scorchproblems.

Use the drop-mill only for sheeting and taking heatout of the batch. Mixed compound dropped from aninternal mixer will typically be relatively low inviscosity, and will not provide sufficient shear foreffective, additional dispersion of the ingredients. Ifadditional mixing is required, use the drop mill tolower the batch temperature, allow the batch to cometo room temperature and then refine it on the mill later,in a second pass.

Many mixers are designed to pump a lubricant into therotor dust seals, to prevent various ingredients in thecompound from entering the bearings. Make certainthat the internal mixer is set up to pump a minimumamount of the lubricant, to avoid contamination of thebatch. Dioctylphthalate is recommended for use as abearing lubricant, but excessive amounts of thismaterial in a compound of Viton® can cause moldingdefects, shrinkage variations, and can potentiallydegrade the physical properties or lower the heatresistance of the final vulcanizates.

If a blend of polymers is to be used, the higher viscos-ity polymer should be added first, allowed to warm-up,and then the lower viscosity polymer added. Thistechnique is only required if the difference in polymerviscosity is greater than 10 points. Polymers that have10 points or less differences in their viscosities maycharged into the mixer at the same time.

Viton® curatives, VC-20 and VC-30 should be addedas early as possible (after the polymer) to the batch, inorder to get good dispersion. Viton® curative VC-50can be premixed with the acid acceptor/filler blend.

Suggested Internal Mixing Procedure

Time, min Operation

–5 Clean the mixing chamber.Run Full cooling circulating water through chamber shell and rotorsRotor Speed: 25–35 rpmRam Pressure: 420–500 kpa (60–80 psi)

0 Add all fillers and acid acceptors followed by the polymer(s)0.5 Allow all fillers, acid acceptors to dry blend. Sweep the throat of the mixer and lower the ram.1.0 Mixing begins1.5 Raise the ram, add internal processing aids if required2.0 Lower the ram2.5 Continue the mixing process

3.0–4.0 Dump or drop the batch (110 ± 5°C [230 ± 10°F])

8

In the case of peroxides, and Diak™ #7, the twomaterials can be preblended with the fillers, or theDiak™ #7 may be preblended with the fillers and theperoxide added after the fillers.

The best dispersion of Acid Acceptors is obtained byblending them with the fillers before adding them tothe mixer.

Do not use stearate (i.e., zinc stearate) type processingaids. These types of materials will cause prematurecuring with bisphenol cure systems. Process aid(s)should be added late in the mixing cycle, or during thelast sweep. If added too early in the mix, they tend toreduce the shear that is required for good mixing of theother ingredients.

Internal Mixing Trouble ShootingGuidePoor dispersion• If the batch fails to go together, the load factor may

be too small for the mixer and/or the wrong chambersize is being used in the calculation of the batchsize. Check to make certain that the batch isproperly sized for that particular mixer. Also makecertain that the batch is not too large for the mixer.A load factor between 70 and 72% is generallyrecommended.

• Make certain the processing aids are added late inthe mixing cycle.

• Check the dust seal lubrication system to makecertain it is not over lubricating the bearings, andthus leaking into the mixing chamber and contami-nating the stock.

• Check the ram pressure. It should be pressurizedhigh enough to force the material into the mixingchamber of the mixer, but low enough to allowmovement during mixing. Some slight movement ofthe ram allows the batch to turnover during themixing process, thus preventing dead spots in thechamber.

Poor and/or changed scorchcharacteristics• Check the rate at which the batch is heating up. If

heat is generated too fast, the polymer’s viscositymay become too low to provide effective shear foradequate dispersion. Also, the stock may precurebefore adequate dispersion is obtained. To preventthis, one or all of the following may be required:

• Increase the amount of cooling water flow.• Clean the water channels of buildup to improve the

cooling efficiency.• Reduce the rotor speed.• Check the differences between the mixer’s gauge

temperature and the actual (probe) temperature.

• Make certain the mixing chamber is clean. Contami-nation from other compounds may effect the curerate and state.

• Acid acceptors and many mineral fillers are hygro-scopic. Introducing moisture into a bisphenol cureFKM compound will decrease the scorch safety.All moisture sensitive materials should be stored intightly closed containers, in a cool dry location,especially during the months of high humidity.Further, it is a good practice to minimize the numberof times a container is opened and closed, and usepre-packaged/weighed units of the chemicals.

• Care must be taken to prevent moisture from con-densing on the stock during storage or after beingremoved from storage.

ExtrusionThe extrusion characteristics of compounds made withViton® are a function of the compound viscosity, typeand amount of processing aid, extruder conditions, aswell as the size and shape of the extrudate. In orderto obtain smooth extrudates, the incorporation of1.25–1.50 phr of an extrusion aid, such as VPA No. 2or carnauba wax, is recommended. It is very importantto recognize that compounds of Viton® extrude bestover a comparatively narrow temperature range, andgood temperature control is essential for consistentresults. Simple screw designs may be effective undersome conditions but the best performance will comefrom mixing/plasticizing configurations.

Compounds based on low viscosity Viton® types maygive easy extrusion, but may also exhibit collapse afterextrusion or during the early stages of curing. Theimportance of this feature will depend on the shape ofthe extruded profile. The best results are obtained fromenhanced rheology copolymer precompounds ofmedium viscosity, e.g., blend of A-201C/A-401C orViton® A-331C.

When extruding preforms of compounds made withViton®, a relatively cool barrel and screw will increasethe stiffness of the stock, thus minimizing the tendencyto entrap air. The stock should be free of moisture andat ambient temperature when being fed to the extruder.If the compound is taken from refrigerated storage,water or moisture will sometimes condense on thestock. This moisture must be removed before the stockis fed to the extruder. Moisture will manifest itself asblisters on the surface of the extrudate.

One of the most critical factors affecting the smooth-ness of the Viton® extrudate is the extrusion speed.Extruding at too high a speed can cause melt fracture.The critical conditions for the onset of melt fracturedepend on the die entry geometry, die dimensions,temperature and the polymer. Typically, the onset of

9

melt fracture depends on the nature of the polymer andits macromolecular characteristics.

Another, equally important factor in obtaining smoothextrudates with Viton® is the die temperature:bisphenol cure compounds, in particular, require a hotdie—between 115–125°C (239–257°F).

Screw Extruder Setup

Stock Temperature 15–27ºC [60–80ºF]

Screw Temperature 27–90ºC [80–195ºF]Typically the screw is cool atthe feed section and graduallyincreases in temperature asit nears the head of theextruder.

Screw Speed 10–25 rpm

Barrel Temperature 27–71ºC [80–160ºF]

Head Temperature 65–85ºC [150–185ºF]

Die Temperature 115–125ºC [239–257ºF]

CalenderingIMPORTANT: Do not use silicone-treated releasepaper as liner material when calendering peroxide-cure types of Viton®. Peroxide-cure types of FKMhave been known to exhibit a significant degree ofadhesion to these types of paper.

The degree of quality obtained in calendering sheetstock compounds of Viton® depends upon manyfactors, but largely on the viscosity of the compoundat the calender. For best results, every effort should bemade to ensure that the compound to be calendered isuniform in dispersion, viscosity, temperature andvolume of flow.

Typically, compounds of Viton® tend to calendersmoothly and without roll sticking, using the followingsuggested conditions:

The temperatures outlined below are only suggestedconditions. The actual temperatures employed willdepend on the compound formulation (i.e., with orwithout process aids), the desired sheet thickness,compound viscosity, and the speed of operation.

Stock Warm-up and Calender Feed:The warm-up mill temperature should be within –5 or+10°C (–10 or +20°F) of the top roll temperature ofthe calender. A sufficient amount of stock should beloaded on the mill to complete one full pass. Particularcare should be taken not to over load the mill with too

much stock. Once the stock is banded, it should be cutfrom alternate sides of the mill to ensure a uniformtemperature (viscosity). If more stock is required tofeed the calender, the warmed stock should be movedto one side of the mill and the fresh stock added to theother side. Once the fresh stock has been warmed tothe proper temperature, it can then be moved to thefeed side of the mill and blended with the othermaterial. Only uniformly warmed stock should be fedto the calender.

Uniform feed into the calender nip is important forproducing air-free calendered sheets, with goodsurface integrity. The amount of feedstock in the nipshould be kept as small as possible. Large banks ofstock will have large temperature variations, due tosurface cooling. This results in changes in the stockviscosity and variations in the smoothness and thick-ness of the sheet.

The calender should be continuously and evenly fedacross the width of the rolls, to maintain a uniformbank. Feeding the compound into the calender nip withlarge amounts of compound should be avoided.

After calendering, it is a good practice to allow thewrapped stock to stress relax in the liner for approxi-mately 24 hr before further handling.

Curing Calendered SheetCalendered sheet, under proper tension (pressure) maybe cured in steam or hot air. The curing time requiredgenerally depends upon the steam or hot air tempera-ture, number of wraps on the curing drum and theinsulating effect of the liner material. It is importantthat the core of the bundle reach cure temperature foran adequate amount time. When steam curing, it isbest to slowly increase and lower the pressure, in orderto prevent blistering. Also, it is best to protect thestock from direct contact with the steam by coveringthe outer layer of compound with an impermeablemembrane (PTFE, or FEP sheet, for example).

To facilitate release of the cured sheet stock fromthe liner with a minimum of distortion, the liner shouldbe stripped from the stock as quickly as possible aftercuring. To postcure the sheet, it is best done byfestooning in a forced hot air oven. A distance of10 cm (4 in) between the festoons is suggested, toallow proper air circulation and even heat transfer.Sheets greater than 6.4 mm (0.25 in) should be steppostcured to prevent the formation of blisters in thestock.

10

Calendering Trouble Shooting GuideRecommended PracticesIt is extremely important to supply only the minimumamount of warm compound to the calendar nip that isnecessary to maintain a continuous sheet.

Mixed compound must be pre-warmed, prior to beingplaced in the calender nip. The stock to be calenderedshould be warmed on a calender mill (minimum shear)to at least 45°C [115°F].

Internal release agents should be kept to a minimum,to prevent slipping on the calendar rolls. Carnaubawax and Struktol® WS-280 are effective internalrelease aids for use in calendering bisphenol-curetypes of Viton®. The level of these process aids shouldnot exceed 1.25 phr in the compound.

Rough SurfaceIf the surface of the calendered sheet is rough, checkto make sure that:• The compound is warm enough before placing in the

calender nip• A minimum amount of material is added to the nip

at any given time, and that excess material is notallowed to ‘hang’ at the nip, thereby losingtemperature

• The calendar rolls are hot enough

Lacey SheetA ‘lacey’ appearance (e.g., has numerous holes andtears, randomly occurring throughout the sheet) can bethe result of excessively high roll temperatures and/orusing a polymer that is too low in molecular weight toprovide the necessary degree of green strength. If thisproblem occurs, it is recommended that lower calenderroll temperatures be evaluated, and/or that a higherviscosity polymer be used in the compound.

Suggested 3-Roll Calendar Setup*

Temperatures, °C [°F]Cure System Top Roll Middle Roll Bottom Roll

Diak™ #3 45–50 [113–122] 45–50 [113–122] Cool or Ambient(Diamine)

VC20/VC30 or VC-50 60–75 [140–167] 50–65 (122–149] Cool or Ambient(Bisphenol)

Diak™ #7, #8 60–75 [140–167] 55–70 [131–158] Cool or Ambient(Peroxide)

* Different temperature ranges are recommended for compounds based on Viton® that employ different crosslinking systems:diamine, bisphenol, and peroxide

Uneven Thickness Across Sheet WidthThe higher the viscosity of the polymer, the greaterwill be the difficulty in maintaining consistent thick-ness across the width of the calendar rolls. The use oflower viscosity polymer, or increased roll temperature(if practical) should be evaluated, to alleviate this typeof problem.

Slipping/Bagging On the Calendar RollsIf the stock fails to grab, and feed consistently throughthe nip, it is probably the result of too high a level ofinternal process aid, and the levels of such materialsmay have to be reduced.

MoldingGeneral PracticesTypically, compounds of Viton® exhibit mold shrink-age of 2.5–3.5% after post-cure. All molds shouldclose tightly and cleanly at the flash line. Also, themold should be designed to give good venting oftrapped air. Press platens should be free fromdistortion.

Typical molding conditions employ temperatures of160–190°C (320–374°F), pressures in the15–20 MPa(2175–2900 psi) range and the use of delayed pressbumping, to allow air to escape. This procedureensures good compound flow, forces out air and helpsto minimize backrinding. The curing cycle time will bedependent on formulation, temperature, part size andconfiguration.

Mold temperature, as opposed to platen temperature orsteam line pressure, should be checked with a cali-brated pyrometer or by wax sticks prior to molding.This elementary precaution minimizes the risk ofproducing defective parts due to undercure. Further, tohelp minimize temperature lost and/or temperatureextremes, the platen heaters should be periodicallychecked for proper operation and the press/moldsshould not be subjected to cold air-streams during themolding operation.

11

Mold staining and dirtying can be a problem withcertain types of compound formulations, particularlythose that are amine cured (i.e., Diak™ 1, 3 and 4cures). Careful attention should be given to thecondition of molds and a routine inspection/cleaningshould be made before the molds are used. Hardchrome plating of mold surfaces is recommended, tominimize mold fouling.

Mold release agents, both internal and external,facilitate the removal of parts and reduce mold fouling.Some suggested systems are detailed in the followingTable. In the case of the external mold release agents,pre-conditioning the mold prior to actually makingparts is often beneficial.

Compression MoldingCompression molding is the most common process formolding compounds based on Viton® polymers. It isrecommended that Viton® polymers having a viscosityof at least 30 (ML 1+10 at 121°C) be used for com-pounds that are to be compression molded.

The primary factors effecting molding quality in thisprocess are:• Preform weight — preforms must be of a weight

adequate to provide for complete cavity filling, flow(typically 6–10% higher weight than finished part)

• Preform density (must be dense, and free of trappedair)

• Consistency of pressure, across all cavities in themold (to assure consistent shrinkage across allcavities)

Transfer MoldingTransfer molding requires the use of relatively lowviscosity polymers and compounds. It is recommendedthat Viton® polymers having a viscosity lower than50 (ML 1+10 at 121°C) be used for compounds thatare to be transfer molded.

Transfer molding involves significant shear-heatgeneration, in transferring the compound form the pot,through the mold sprues, and into the mold cavities.Thus, in addition to providing for low compoundviscosity, it is critical that the compound be formulatedfor adequate scorch safety, such that the compoundwill exhibit adequate flow (mold filling) prior to theonset of cure.

Four major factors effect mold filling in transfermolding processes, and can be altered singly or incombination, to optimize mold filling.

Compound viscosity: Low polymer viscosity must bebalanced against the slightly better physical propertiesthat are typical of compounds based on high molecularweight polymers (high compound viscosity).

Compound scorch safety (ts1, ts2 – onset of cure):The onset of cure must be balanced against the timerequired to attain a state of cure adequate fordemolding of the cured parts. Too ‘safe’ a compoundmay require impractically long molding cycles.

Sprue size: It is desirable to use the smallest sizesprue hole practical, to minimize damage to the partsupon demolding, and the tearing of the sprue from themolded part. The sprue size must be large enough,however, to provide for sufficient flow of the com-pound, given the molding pressure that is available,and given the scorch safety of the compound.

Mold temperature (165–175°C [329–347°F] —recommended): For the same basic mold cycle time,temperatures for transfer molding operations cantypically be lower than those used for compressionmolding, because of the significant amount of shearheat that is generated in the compound, in the act oftransferring material from the transfer pot into themold cavities.

Injection MoldingThere are two basic types of injection molding ma-chines used in the rubber industry. One is called the‘ram’ or ‘piston’ type and the other is called the‘reciprocating screw’ type.

The ram or piston injection-molding machine is anoutgrowth of the transfer molding process. In opera-tion, the rubber compound, in strip or extruded form isfed into a heated cylinder, where it is warmed to apredetermined temperature. The softened elastomericcompound is then forced by a hydraulic ram through anozzle into mold runners and restrictive gates, into aheated mold cavity. In the mold, the material isshaped, cured and then removed.

In the case of the reciprocating screw injection-molding machine, the rubber compound can be in theform of pellets or strips, however, strips are mostcommonly used. The compound is fed into a heatedbarrel, where the material is heated and homogenizedby a rotating screw. The rotation and reciprocation ofthe screw meters a predetermined amount of thecompound into the forward portion of the barrel. Thiswarmed material is then injected by the screw, actingas a ram, into the nozzle runners and gates, and intothe heated mold. During the early stages of the cure,the screw is maintained in the injection position, at a

12

predetermined pressure to consolidate the molding.Then, at a preset time, the screw starts to turn andmoves back to a feeding position, where it preparesmore material for the next injection, or shot.

Listed in the following table are some of the advan-tages of the ‘ram’ type injection-molding machineversus the ‘reciprocating screw’ type.

In general, if the compound is high in viscosity,scorchy, or requires good mixing, then a screw type ismore desirable. If the mold design is complex (runnersystem), then a vertical ram or screw type machinemay be more desirable. Typically if a floor space is ata premium, then a vertical machine is dictated. If it isexpected that future work will involve high viscositycompounds, then a screw type of machine would beadvisable. However, if initial cost is critical, then aram type is dictated. At the same clamping tonnage,ram type injection-molding machines are generallylower in price. If a lower price per part is critical, thenthe screw type will generally give you shorter moldingcycles. Regardless of the type of injection moldingmachine purchased, it should be designed for rubberprocessing.

The projected area of the parts to be made generallydictates the tonnage requirement of a machine. Asgeneral rule, the clamping tonnage can be estimated asfollows:

Required Clamp kg = [(Pa x NC) + Ra] x (281.1)

Where:Pa = the area of the parts, cm2

NC = the number of cavities in the moldRa = the runner area in cm2

The shot capacity requirements can be estimated asfollows:

Shot Capacity = ([wt per part] x no. of cavities) +runner wt + 15% material wtretention in the barrel)

DefinitionsInjection-molding process—A procedure wherebyan accurately metered amount of plasticized rubbercompound is moved under controlled temperature,pressure and time into a closed heated mold, where itis to be vulcanized.

Ram (plunger) type injection-molding machine—A machine that utilizes a ram within a heated cylinderto force heat plasticized rubber compound into aclosed, heated mold for the purpose of vulcanization.

Reciprocating screw injection molding machine—An injection molding machine that utilizes a screwwithin a heated barrel to plasticize and convey stock toa certain position, and then is used as a ram to forcethe stock through runners to a closed heated mold forthe purpose of vulcanization.

Injection time—The time required by the screw orram to force the stock into the mold.

Hold time—The time immediately following thecompletion of the injection cycle when a reducedpressure is maintained on the stock in the mold toconsolidate the molding during the initial curing stageof the compound.

Cure time—The time the compound is held in themold, before the mold is opened.

Clamp time—The sum of the injection time and thecure time.

Cycle time—The time required to inject the com-pound, cure the part, remove the cured part(s) from themold and close the mold again. It is the overall timesbetween successive closing of the mold, and mayinclude cleaning of the mold surface, if necessary.

Injection pressure—The pressure exerted on theplasticized stock, in the barrel or cylinder, to force itinto the mold cavities.

Hold pressure—This is the reduced pressure on theinjected stock, used to consolidate the molding duringthe initial curing stage.

Clamping pressure—This pressure is usually ex-pressed in tonnage. It is the amount pressure exertedon the material in the mold, by the mold, during thecuring or vulcanization cycle.

Back pressure—The pressure exerted on the com-pound in the barrel, by the screw, to compact of thestock before injection shot.

13

Recommended Practices forInjection MoldingThe following are typical operating conditions forinjection molding compounds of Viton®:

Ram-type Screw-type

Temperature, °CBarrel

Feed zone 80–90 25–40Middle zone 80–90 70–80Front zone 80–90 80–100

Nozzle 90–100 100–110Nozzle extrudate 115–120 115–120

Mold extrudate 165–170 165–170Mold 205–220 205–220

Pressures (MPa)Injection (Sufficient to give an injection time

of 3–5 sec.Typically this pressure may rangefrom 14–115 MPa)

Hold pressure — 1/2 injection press.Back pressure — 0.3–1.0 MPaClamping pressure max. max.

Times (seconds) for thin pieces [< 5mm thick]Injection 3–5 3–5Hold — 10–15Cure 45–60 30–45Clamp (injection and cure) 48–65 33–50Cycle 58–75 43–60

Screw Speed — 40–60 rpm

Compounding for InjectionMoldingAn injection-moldable compound of Viton® shouldhave good scorch safety characteristics, to withstandthe preinjection plasticization temperature in the barrelwithout scorching. It should flow easily through thenarrow nozzle, runners and gates, to allow fast fillingof the mold cavities. Its shear/viscosity relationshipshould be such that the heat build-up that occursduring the passage from the barrel to the mold iscontrollable. On reaching the hot mold, the compoundshould then cure quickly. In general, many of thecompound characteristics that are favorable fortransfer molding are applicable to injection molding.

Although no elaborate stock preparation proceduresare required for injection molding compounds, thefollowing recommendations are advisable:• Care should be taken to prevent the incursion of

moisture into the stock. Moisture in compoundsmade of Viton® can decrease the scorch safety andcause blisters and internal porosity. It is advisable tomake certain that all the ingredients used in thecompound are as moisture-free as possible and that

the stock is protected during storage to prevent theabsorption of atmospheric moisture.

• The actual viscosity, scorch characteristics and curerate required will depend on the part size, molddesign, machine specifications and machine operat-ing conditions.

• The preform that is fed into the barrel should containa minimum amount of trapped air. This is especiallyimportant when using ram type injection-moldingmachines. With the screw type of injection moldingmachines, much of the air is able to escape backdown the flights of the screw. (See the “HandlingPrecautions for Viton® and Related Chemicals”technical information bulletin on the “diesel” effectand other related precautions.)

• The compounds being prepared for injection mold-ing should be exposed to as little heat history aspossible prior to actual usage. This would includeheat generated during mixing, milling, extruding,storage, etc.

• Each batch should be handled in a consistent fashionto reduce batch-to-batch variations, thus insuringuniform moldings and less rejects or poor qualityparts.

Trouble Shooting InjectionMolding ProblemsThere are no set rules for trouble shooting injectionmolding problems, since they may be due to a combi-nation of things. Each problem should be handled onan individual basis and analyzed with regard to:• The compound being used, and the preparation of

the stock• The part being made• The injection molding machine and it’s operation;

and• The mold

General Guidelines• Use a systematic approach when investigating or

analyzing a problem• Examine the compounding parameters carefully for

all preparation variations. Also compare the controltest results to previous records

• Check the equipment for mechanical failure and/orwear

• Check the machine operation. Go through the cycles;check the temperatures and pressures at each portionof the molding cycle. Also, compare the results toprevious records

• Record the cause and corrective action for futurereferences

14

Trouble Shooting Guide—Problemsand Corrective ActionsThe following is a list of some of the more commonproblems encountered when injection molding rubber.It is by no means an all-inclusive list. One must alsokeep in mind that many factors are interrelated andthat changing one parameter in the operating condi-tions may often have an affect on other moldingconditions or molding quality.

Problem—Air Entrapment in the MoldAny air trapped in the mold cavity will prevent themold cavity from filling properly. To avoid trappingair:• Make certain the feed stock, especially for ram type

machines, is free of entrapped air• Provide sufficient backpressure at the nozzle to

adequately compress the stock in the barrel orcylinder

• Increase the injection time• Lower the injection pressure, and/or• Make certain the mold is properly vented

Problem—BackrindingThis type of problem may involve the compoundacceleration system (speed of curing), too long a ‘holdtime,’ too high a hold pressure, and/or too high a moldtemperature. In addition to backrinding, ‘sink backs’may occur at the gates.

• Decrease the level of the accelerator system orformulate a slower cure rate

• Reduce the mold temperature• Decease the hold time• Decrease the hold pressure

Problem—BlistersBlisters can originate from a variety of causes, such asentrapped air in the feed stock, entrapped moisture inthe compound, incorrect stock viscosity, too large anozzle, too short an injection time, inadequate moldventing and/or the volatilization of one of thecompound’s ingredients.

• Make certain the stock has not been contaminatedwith moisture, either from the compounding ingredi-ents or improper storage

• Check the feedstock for trapped air. Remill orextrude at a lower temperature if necessary

• Adjust the viscosity of the stock to insure goodcompaction in the barrel or in the cylinder. Usuallya higher viscosity will help

• Decease the barrel or cylinder temperature toincrease the backpressure, thereby obtaining bettercompaction of the stock

• Eliminate all compounding ingredients that mayvolatilize (decompose) or produce a gaseous productat the processing temperature

• Check the nozzle extrudate for air bubbles. Decreasethe nozzle size to increase the backpressure ifnecessary

• Decrease the injection pressure• Increase the injection time• Increase the hold time and/or pressure• Increase the cure state of the article• Make certain the mold is properly vented

Problem—Distortion or Rough SurfaceScorched stock, too long an injection time, too hot amold or an undersized runner and/or gates may causedistortion of the molded article after removal from themold.

• Use fresh stock, the old stock may have precured• Recompound for better stock flow or lower

viscosity• Decrease the temperature of the stock entering the

mold by:– Decreasing the compound viscosity by filler or

polymer modification– Decrease the cylinder temperature– Decrease the injection pressure– Decrease the injection time– Increase the nozzle size– Increase the gate size

• Reduce the mold temperature

Problem—Excessive Mold FlashThis problem is usually associated with too low astock viscosity, too high an injection pressure, too longan injection time, too large a shot size, or a poor fittingmold. To eliminate this problem:• Increase the stock viscosity• Decrease the injection pressure• Decrease the injection time• Make certain the shot size is not too large• Check the alignment of the mold halves• Increase clamping pressure

Problem—Excessive Nozzle FlashNozzle flash is usually due to a worn nozzle or nozzlebushing surface, too large a nozzle, too high aninjection pressure, or too low a compound viscosity.

• Check the nozzle and/or nozzle bushing surfaces forwear, and refinish them if necessary

• Decrease the injection pressure• Increase the stock viscosity• Decrease the nozzle size• Decrease the back pressure

15

Problem—Long Cure CyclesToo low a barrel or mold temperature will cause longcure cycles. Also, an inadequately formulated com-pound will result in a long cure cycle.

• Adjust the compound cure system to increase therate of cure

• Preheat the reserve stock (ram machines only).• Increase the temperature of the stock entering the

mold by:– Increasing the compound viscosity– Increase the barrel or cylinder temperature– Increase the injection pressure– Reduce the nozzle and/or gate size

Problem—Poor KnittingPoor knitting may be due to excessive mold release(lubricant), too high a mold temperature, too fast acure rate, or inadequate stock flow.

• Reduce the amount of internal process aid(s) and/orexternally applied mold lubricant

• Lower the compound viscosity, reduce the rate ofcure, or increase the scorch safety

• Lower the mold temperature

Trouble Shooting General MoldingProblemsBackrindingBackrinding appears as very rough edges on a moldedpart, caused by the expansion of the part upondemolding, at the nearest area of relief, usually at theparting line of the mold cavity.

Causes and Corrective Measures—Stock viscosity istoo high. Reduce the compound viscosity by using alower viscosity polymer.Inadequate mold flow. Use a bump cycle on themold, or change existing bump cycle by lengtheningthe time when the mold is bumped.Compound is too scorchy. Reduce mold temperatureand/or increase scorch safety of the stock. If thescorch safety has changed, determine the reason forthe change and/or use fresh stock.

BlistersThere are a number of different types and causes ofblisters. The appearance, relative size, and number ofblisters in molded parts can be quite different depend-ing on the actual cause of the problem.

Blisters caused by undispersed particlesAppearance—Usually a relatively small numberof blisters per part, small in size, and usually visibleonly near the surface of the part. This type of

blister is readily identified by the presence of anundispersed particle inside the blister. Such particlesusually are readily seen, by cutting mixed stock,preforms, or molded parts with a sharp knife.Undispersed particles large enough to cause blistersin molded parts will be visible to the naked eye.Causes and Corrective Measures—Blisters of thistype result from poor dispersion. Make certain thestock is refined prior to making the preforms.Review mix procedures to maximize the dispersionof all powdered ingredients. Check compoundingingredients for the presence of hard, undispersableparticles/grit, or contamination.

Blisters caused by contamination of thecompound with a different compound,based on a nonfluorinated rubber

Appearance—These types of blisters are less obvi-ous than those made with undispersed lumps of lightcolored acid acceptors or mineral fillers. Typicallythese types of blisters may appear as a crack, or avoid inside of a molded part. If the contaminant isnear the surface of the molded part, it may pop outupon demolding, thus leaving an irregular-shapedvoid in the part.Causes and Corrective Measures—Before mixing orprocessing (by any method) a compound based onViton®, make sure to clean the equipment to removeall non-fluoroelastomer based compound. This alsoincludes such contamination as oils, from all equip-ment involved in the processing of the stock, includ-ing mixers, preformers, and molding operations.

Blisters caused by trapped airAppearance—Trapped air usually appears as arelatively small number of blisters, randomly locatedthroughout the part. Depending on their size, indi-vidual blisters may have a spongy appearance, buthave no evident particle or contaminant within theblister itself.Causes and Corrective Measures—In low viscositystocks, and particularly in the case of using a ramextruder to make preforms, it is possible to trap airin preforms. If the preform is not subjected toadequate flow/squeeze in the molding operation, theair will not be forced out, and will create blisters.Make sure all molding preforms are dense, and donot contain pockets of trapped air. A breaker plateand screen pack at the extruder head may be used toincrease pressure to a degree that is sufficient toforce air out of the preform at the die entrance.Low preform weight can also result in trapped air.Make certain the preform weight is sufficient toprovide adequate stock flow and pressure in themold cavities.

16

Blisters caused by inadequately dis-persed process aid

Appearance—Blisters of this type tend to have a‘wet’ look on their interior surface.Causes and Corrective Measures—Pockets ofundispersed process aid, such as waxes, may vapor-ize at molding temperatures, thereby formingblisters. Make certain that adequate mixing time isprovided for the complete incorporation and disper-sion of all process aids used in the stock.

Blisters caused by entrained waterAppearance—This type of blister usually appears asa number of small-sized blisters per part, with noevident particles or contaminants within the blisteritself.Causes and Corrective Measures—Stocks that arestored in a cold room, and which are then broughtonto the shop floor in humid weather can collectsignificant amounts of moisture on the surface.Subsequent processing may cause this surfacemoisture to be trapped within the compound.When a compound is stored under cold conditions itmust be covered with plastic, and when retrieved foradditional processing, the plastic must be removedonly after the compound has come to room tempera-ture. There have also been cases in which internalmixer rotors and extruder screws developed cracks,thus allowing cooling water to be introduced into thecompound.If water is thought to be the cause of blistering, andcondensation from cold storage of compound isproven not to be the cause, pressure-test internalmixer rotors and extruder screws for possible cracks/leakage of cooling water or steam.

Blisters caused by poor dispersion ofaccelerator or curative

Appearance—This type of blister typically shows upas small areas of very small blisters. The blisteredarea will typically have a sponge-like appearanceand, when probed, will exhibit obvious signs ofundercure.Causes and Corrective Measures—Poor dispersionof either accelerator or curative will result in aninadequate state of cure in the immediate area ofpoor dispersion. During molding, these areas will‘blow,’ resulting in blistered, or sponged areas.Good dispersion of the accelerator and crosslinkingagents is critical in avoiding this type of blister.

Blisters caused by undercureAppearance—Parts that are undercured typicallyexhibit extensive blistering, throughout the entirepart. The interiors of these blisters are spongy inappearance, and if probed with a metal pick, exhibitobvious signs of undercure.Causes and Corrective Measures—Too low a moldtemperature, too short a time in the mold, or grosslyunder-weight preforms will result in extensiveblistering. The corrective action may require one ormore of the following:• Increase the mold temperature• Increase the molding time• Increase level of accelerator in the compound formulation• Make certain the preform weight is adequate

Sponged areas, splits, fissures afterpostcuring

Appearance—This type of molding defect occursmost frequently in parts that have a cross-sectionalthickness greater than 5 mm (0.20 in). Such partsmay appear to be without flaws when initiallydemolded, but will exhibit internal voids, or fissures,after postcuring.Causes and Corrective Measures—Areas having asponge-like appearance in the interior of parts areusually the result of an inadequate state of cure.Provide sufficient time in the mold to allow com-plete transfer of heat through the bulk of the part toits interior. For example, parts that are 6–9 mm(0.25–0.35 in) thick may require 15–20 min at 175°C(347°F) to fully cure, while a part madeusing the same compound that is less than 2.5 mm(0.10 in) thick, may need only 2.5 min to be fullycured.Internal fissures or splits are significantly differentin appearance from blisters or sponge. Instead ofbeing rounded, discrete voids, fissures and splits are,as the names suggest, distinctly planar and depend-ing on the cause, their interior surfaces can be veryrough in appearance. Fissures typically are caused byeither one or two different problems.Fissures are most commonly seen in post-cured partsthat are thicker than 5 mm (0.20 in). Fissuring, or‘blowing’ is typically the result of the violent escapeof volatiles trapped within a part during postcure.Parts thicker than 5 mm should always be oven‘step-postcured,’ and the cure cycle should start at90°C (194°F) for 2–4 hr. This will allow anyvolatiles that are trapped inside the part to escapegradually without ‘blowing’ the part.

17

If the preforms are built up from multiple plies ofstock, and if too much internal process aid is used inthe formulation and/or there is insufficient preformweight or mold pressure to provide for adequatecompaction (knit) of the layers, the layers mayseparate, either upon de-molding, or after thepostcure cycle. Multiple plies or layers of stock canbe successfully used to make preforms for thickmolded parts, but care must be taken to insure that:• The surfaces of all layers are clean, and free ofprocess aid, which may ‘bleed’ out of the stockwhile sitting at room temperature• An adequate amount of preform weight, and moldclamping pressure is used, to insure sufficientcompaction and knit of the individual layers.

Knit Lines/Flow linesAppearance—Knit, or flow lines, appear as small,groove-like lines in the surface of the molded parts.These flaws are caused by inadequate flow andknitting of the compound.Causes and Corrective Measures—Knit linesproblems are likely to occur in molding applicationshaving the following conditions:• Where the compound has to flow a relatively longdistance to meet with another portion of the partflowing from the opposite direction• Where the compound only has to flow a shortdistance, but under very low shear and pressureOne of the two principal causes of knit lines, underlong flow conditions, is the onset of cure beforecompletely filling of the mold cavity. Premature cure(scorch) will create a surface that is high in viscos-ity, thus decreasing the compound’s ability tocompletely fill the mold cavity. Further, this highviscosity interface will also interfere with thecompound’s ability to knit.A common cause of knit lines is the use of excessivemold lubricant (release) or internal process aid. Inmolds where an excessive amount of mold lubricantis used and where a long flow path is required, thecompound can wipe the excess lubricant from themold. In doing so, the lubricant can buildup in frontof the compound and prevent bonding and/or knit.Internal processing aids are effective because theyare incompatible with the fluoroelastomer com-pound. Because of their incompatibility they tend tobleed to the surface of the stock. If an excessiveamount of internal mold lubricant is added to thecompound, it may bleed to the surface of the partduring the molding, and be pushed along in front offlowing stock or coat the compound surface. Thesurface coat and/or accumulation of the matingsurfaces may prevent their knitting.

These knit lines which are the result of excessiveexternal or internal mold lubricant will exhibitdecidedly ‘wet’ looking surfaces, and, in severecases, the presence of excessive mold lubricant canbe determined by a wet slippery feel in the area ofthe knit line. Care should be taken to avoid over-spraying molds. The level of internal mold releaseagents will be restricted by the type and amount offiller in the compound, the molding process, and thesize and design of the mold (especially as regardsthe stock’s flow path). A compound designed forcompression molding a small, simple-shaped part,with little flow involved will accommodate a greateramount of internal lubricant than, for instance, acompound which is to be injection molded into acavity that involves a long flow path, throughrunners and gates.

Non-FillsAppearance—Non-fills usually appear as small,depressed areas on a molded partCauses and Corrective Measures—Like knit lines,the following may cause this type of flaw:• Premature curing (scorch) of the compound• Insufficient preform weight• Inadequate cavity pressure, and/or• Uneven mold closure (resulting in varied pressuresbetween mold cavities)

If non-fills occur, check for premature scorch bymolding the part at a lower temperature and byrunning a Mooney scorch test.Mold temperature or ratio of accelerator tocrosslinker (VC-20–VC-30 ratio) can be reduced, toprovide for increased scorch safety.Verify that the preform weight is adequate for thefinished part weight and for sufficient flashMake sure that all mold cavities are of the sameheight and receive the same clamping force.

TearingAppearance—Tears can occur in molded parts invarying degrees. They can range from small cracksin the surface, or partially separated corners or edgesof molded parts, to total separation or breakage of amolded part into two or more separate pieces.Causes and Corrective Measures—Tears experi-enced during demolding are usually caused byinadequate amount of hot elongation necessary toextract the part from the mold. It should be notedthat elongation values measured at room temperaturedo not adequately reflect how well a compound willperform at demolding temperatures. To gain a better

18

idea of how the compound will perform underdemolding conditions, tensile properties should bemeasured at or above the demolding temperature,before postcuring. For example, a compound with a120–130% elongation, measured at 177°C (350°F),for instance, is usually adequate for demolding valvestem seals, or shaft seals. Higher elongation valuesmay be required for more complex shapes.Poor release of the part from the mold surface cancreate tears that, given good mold release perfor-mance, would not otherwise occur. Good moldrelease is critical in terms of preventing demoldingtears from occurring.Poor mold flow/filling and/or knit lines can alsocause tears. See the suggestions for knit lines andflow problems for corrective actions.The tear resistance of fluoroelastomer vulcanizatesis very dependent on the tensile strength andelongation properties of the compound. Tensile andelongation values decrease, with increasing tempera-ture. Thus lower mold temperatures may signifi-cantly reduce part tearing, providing the moldrelease is adequate. However, increasing the accel-erator level of a compound or a faster-curing versionof the polymer type may be desirable to avoid anypenalty in molding cycle time due to lower moldingtemperatures.The elongation at break of a vulcanizate is highlydependent on the amount of cross-linking agent thatis added to the compound. Reducing the level ofcurative (crosslinking agent) will have a significantimpact on improving hot elongation at break and,consequently, demolding tear.Tearing can also be caused by a foreign inclusion,and/or poor dispersion, which can create stressrisers. These stress-risers are weak points in the partwhereby tear can be easily initiated due to theconcentration of force at that location, when extract-ing the part form the mold. Check for foreigninclusions and poor dispersion. Further, mold cavitydesign can also have an effect on producing stress-risers. Sharp corners or angles can also create stress-risers and make demolding without tearing verydifficult. Where possible, all inserts and moldcavities should have the largest radiuses allowable,to distribute the demolding forces.

Mold shrinkageVariable shrinkageWhen variations in shrinkage of finished parts occur,and the cause cannot be traced to any specific, consis-tent difference, such as batch to batch, or (consistent)variation between mold cavities, then the postcureoven should be looked at carefully as a possible sourceof the problem. If the postcure oven is not set up toprovide for proper air flow and internal air circulation,and if it is near its rated limit for high temperaturecapability, the temperature within the oven can vary asmuch as 50–75°C (122–167°F), from the center of theoven to the various corners. Such variations in oventemperature will cause variations in shrinkage.

Variable Shrinkage Within Molding HeatsVariable shrinkage in multiple-cavity molds can resultfrom number of causes. When investigating variableshrinkage, it is extremely valuable to identify andrecord the shrinkage for the individual cavities. Often,a pattern or trend can be identified if the followingevents or factors are at fault in creating the shrinkagevariation:

If specific cavities consistently provide differentshrinkage compared to other cavities, this is likely theresult of either uneven heights of flash groove cut-off,a cocked platen, or the result of a mold being placedoff center of the molding platen or ram. In the case ofuneven platens, or off-center mold placement, shrink-age between cavities typically will differ by a consis-tent, increasing amount, in going from the area ofhighest clamp pressure to the area of lowest pressure.

Variable Shrinkage Between MoldingHeatsWhen variations in shrinkage occur between consecu-tive heats, and:• Occur within the same batch of mixed compound• Occur using the same batch of mold preps, and• Are noted between heats which have been postcured

in the same oven, using the same postcure cycle

The cause of these variations is most likely due tovariations in mold temperature between heats.Changes in mold temperature, sufficient to effectdifferences in shrinkage, can result from having themold out of the press for different periods of timebetween heats. Or, significant variations in the mold‘loading and demolding cycle’ can also affect thetemperature. How long the mold is out of the pressbetween heats, or significant ‘breaks’ in the normalcycle of loading, curing, and demolding, as in the caseof the first heat being run after a lunch break duringwhich the mold sat unused in the press.

19

Changes in molding pressure can also cause differ-ences in shrinkage between batches, but it is unusualfor presses to be at fault in this manner. This type ofshrinkage variation can occur, however, if multipledeck presses are not loaded in a consistent manner. Forexample, if a press is typically run with two molds, itshould always be run this way, and both molds shouldalways be loaded with preforms.

Variable Shrinkage—Batch to BatchIf differences in shrinkage occur between differentmixes of compound (and the postcure ovens have beenruled out as a source of the problem) this will mostlikely be the result of differences in scorch and/orviscosity between batches. Such differences canthemselves result from differences in mixing historyand/or the age of the mixed compound.

Process aids can be another possible source of differ-ences in shrinkage between batches, particularly forprocess aids that are relatively fugitive. In these cases,it is important that heat histories of the batches be asnearly equivalent as possible. That should include themixing cycles and the sheet-off cycle, and any subse-quent preparation of the compound. Variations in mixtemperature can result in significant differences in theresidual levels of the process aid, which will directlyimpact the final shrinkage values of the cured parts.

AdhesionGood adhesion can be obtained between compoundsof Viton® and to other substrates if the surfaces areproperly prepared, and if a suitable primer adhesiveis used. For detailed information on adhering Viton®

to metal substrates, see DuPont Dow Elastomerstechnical bulletin “Adhering Viton® to Metal DuringVulcanization.”

Cured compounds of Viton® can readily be bonded toitself, but, in general, the best bonds are achievedbetween vulcanizates that have not been oven post-cured.

In order to obtain a bond between two pieces ofvulcanized Viton®, the surfaces that are to be matedshould be roughened slightly with sandpaper andwiped with a solvent-soaked cloth. Isopropanol,ketones (acetone, methylethyl ketone), or low molecu-lar weight acetates (ethyl acetate) may be used toremove any traces of grease or oil, and to ‘pre-swell’the surface, allowing better penetration of theadhesive.

‘Skive’ or angled cuts are strongly recommended,versus ‘butt’ splices, particularly if the bonded splicewill be subjected to compressive stresses while inservice. Angled cuts on both ends of the pieces, to bebonded together, will provide a considerably larger

surface area for adhesion than a ‘butt’ splice. The‘skive’ cuts should be made such that the plane of theresulting splice is parallel to any compressive force towhich the finished part will be subjected.

AdhesivesThere are a number of adhesives available that havebeen expressly designed for bonding vulcanizedfluoroelastomer to vulcanized fluoroelastomer. Theseadhesives are based on fluoroelastomer compoundsdissolved in solvent(s) and are commercially availablefrom the following companies:

Eagle Elastomers, Stow, OH—Telephone No.: (330)923-7070

Pelmor Laboratories, Newtown, PA—Telephone No.: (800)772-6969

Thermodyn Corporation, Toledo, OH—Telephone No.: (800)654-6518

In addition, cyanoacrylate adhesives and two-partepoxy adhesives may be used for this purpose. In thecase of two-part epoxies, thinning the adhesive withxylene or toluene provides bonds that are more flexiblethan those obtained using the undiluted epoxy adhe-sives.

General PracticesBonding Viton® to Metal (See “Adhering Viton® toMetal During Vulcanization.”)

High mold cavity pressure is extremely important inbonding, especially for good and consistent bonds tometal inserts. Depending on the pressure capability ofthe press, the number of cavities in the mold may haveto be restricted or reduced, in order to increase theeffectiveness of the available platen ram pressure.

Silane-based primers/adhesives are very sensitive tomoisture, and metal inserts treated with silane adhe-sives should be used within several days of beingcoated. In seasons of high humidity, prepared insertsshould be kept covered and/or used within a day ortwo of being coated.

The thickness of the primer/adhesive layer is criticalto obtaining good bonds. Too thick a layer will notprovide strong bonds, and therefore most silane-typeadhesives must be thinned with a compatible solvent.The percent dilution of silane adhesives will vary,depending on a number of factors. At least two differ-ent concentrations should be tried, before settling on,or discounting any particular primer. For example,there have been cases where a 100% Chemlok® 607treatment provided no bond, but a 25% Chemlok®/75%ethanol mixture resulted in 100% stock tear bonds. Indiluting silane-based adhesives, it is critical that drysolvents be used.

20

Silane types of adhesives/primers are particularlysuited for the dipping processes. Dipping allows thetreated inserts to dry quickly and tack-free. Sprayapplication is less preferable, as the spraying allowsfor ambient humidity to have a greater effect on theapplication. Applying the primers by brush is the leastpreferred method, as it is difficult to obtain 100%coverage of a consistent thickness.

Wiping of the adhesive by the Viton® compound as itflows around or on to the insert can occur during themold filling process. Prebaking the treated metalinserts for approximately 5–15 min at 125–150°C(257–302°F) can reduce this problem. Lower viscositycompounds will exhibit less tendency to wipe theadhesive from the insert than higher viscosity stocks.

Compounds with very short scorch times will typicallybe more difficult to bond to metal than identicalcompounds having a longer scorch (ts2) time.

Oven postcuring of molded parts must take intoaccount the thermal stability of the adhesive. Typi-cally, postcure temperatures should not exceed 200°C(392°F). Some primers have higher thermal stabilitythan others, check with the manufacturer to be certainof the upper temperature capabilities.

The compound formulation should not contain exces-sive amounts of waxes or other internal process aidsthat can migrate out of the polymer under heat andshear. Similarly, care must be taken to assure thatexternal mold spray does not ‘land’ on primer-treatedinserts.

Bisphenol-cured Viton® compounds that are to bebonded to metal should be formulated with low(2–3 phr) levels of calcium hydroxide. Typically,Viton® recipes call for combinations of 3 phr magne-sium oxide and 6 phr calcium hydroxide. Using levelsof 6–8 phr magnesium oxide and 2–3 phr calciumhydroxide will provide significantly better bonding tometal. In addition, this combination of metal oxideswill provide essentially the same cure rates, withslightly better tear resistance, and no sacrifice intensile properties, or compression set. Also, the use of15–17 phr of a low activity magnesium oxide, incombination with 2 phr of calcium hydroxide has beenfound to be particularly effective in promoting metalbonding with bisphenol-cured compounds of Viton®.

Compounds filled with mineral fillers typically pro-vided better adhesion to metal than carbon black-filledstocks. Furnace blacks are noticeably poorer thanThermal grades in this regard.

Various metals will also differ in the ease with whichcompounds of Viton® to metal bonds can be obtained.

This is due in large part to how well the surface can beroughened. A relative rating of metals and the relativeease of bonding is as follows:

Steel > Stainless Steel > Brass > Aluminum

Steel can be sand or grit-blasted to create a roughenedsurface, or it can be treated with a phosphatizingagent. Both techniques will create roughened surfaces,thereby increasing the surface area for better bonding.

Stainless steel must be ‘pickled’—etched withhydrochloric acid. The end result, like phosphatecoating, is to increase the surface area.

It is more difficult to obtain a roughened surface onbrass or aluminum, since they are softer metals,however, brass may be etched with ammoniumpersulfate.

Types of Primers and AdhesivesFor multicavity, small part production, silane-typeprimers are preferred. The silane-type primers arepreferred for these types of productions due to theinherent capabilities of the primer to coating largenumbers of metal inserts without the tendency for theprimed parts sticking together. The following arerepresentative silane-based primers known to beeffective in bonding compounds of Viton® to metal:

Silane TypesLord Corporation, Erie, PA (U.S.)

Chemlok® 5150 Good for bisphenol,diamine cures

Chemlok® 607 Good for bisphenol,diamine cures

Chemlok® 5151

Rohm & HaasThixon® XAV-273/66* Good for bisphenol,

diamine curesThixon® 310Thixon® 3010Thixon® 300/311

*Note: Thixon® XAV-273/66 is a two part system with a limitedshelf life when mixed together.

Henkel GMBH, Dusseldorf, GE

Chemosil® 511 Good for bisphenol,diamine cures

Chemosil® 512 General purpose:including peroxide-cures

Rohm & HaasMegum 3290-1 General purpose:

including peroxide-cures

21

Yokohama Kobunshi Kenkyujo Co. Ltd.(JA)Monicas D-602 General purpose:

including peroxide-curesMonicas CF-5M Good for bisphenol,

diamine cures

Epoxy TypesEpoxy adhesives are effective for very difficultbonding applications. However, two-part epoxy-typesof adhesives are not well suited for treating largenumbers of metal inserts, since the adhesive coatingdoes not dry tack-free in ambient air. Semi automatedadhesive application lines can be set up to take advan-tage of the excellent bonding capability. These semiau-tomatic lines must either provide for maintainingseparation between the coated inserts or include a passof the coated inserts through an oven, sufficient tobake dry the epoxy coating. Typically a baking time of5–10 min at 150°C is required.

It should be noted that epoxy adhesives are particularlyuseful in the manufacture of relatively large parts,particularly where limited mold pressure is available.An example of this would be in the case of moldingbutterfly valve liners.

The following are representative epoxy-based adhe-sives known to be effective in bonding compounds ofViton® to metal:

Chemical Ingredients, Ltd. (UK)Cilbond 30/31

Yokohama Kobunshi Kenkyujo Co. Ltd.(JA)Monicas V16A/B

Epoxy types of adhesives can be thinned with xyleneor toluene, as needed, to provide thinner coatings andto obtain better coverage, with reduced slumping oruneven buildup.

Cements/Tie-Coats Based on Viton®For particularly difficult bonding applications, whereepoxy type adhesives may not be adequate, cementsbased on compounds of Viton® can be prepared. Thesecements have been proven to provide excellent bond-ing to metals. Such cements, like epoxies, are bestsuited for large parts, particularly when limitedmolding pressure is available. An example of a ‘tie-coat’ formulation is shown below.

Tie-Coat base compound phrViton® A, or Viton® B 100.0Low Activity MgO 15.0MT Carbon Black 20.0Diak™ #3 3.0

After mixing the above formulation, the compound isdissolved at 20–30 wt%, in methylethylketone (MEK),

methylisobutylketone (MIBK), or a mixture of the twosolvents. MIBK evaporates more slowly than MEK,and can serve to prevent a ‘skin’ from forming on thecoating. By using a mixture of MEK and MIBK thedrying time is faster than using MEK by itself.

When the compound is fully dissolved, add 5 parts ofChemlok® 607 or Chemosil® 512 to 100 parts of thesolution. The fully mixed cement will typically bestable for several days before showing signs of gelling.The cement should be applied to a grease-free, grit-blasted metal surface, and be allowed to fully air dry,before attempting to mold. The formulation asdescribed is also an effective cement for bondingcured compounds of Viton® to cured compounds ofViton®. See technical information bulletin, “AdheringViton® to Metal During Vulcanization,” for moreinformation.

Oven PostcuringGeneral PracticesParts to be postcured should be deflashed prior topostcuring, and care should be taken to assure that allloose particles of flash are removed from the parts.Small pieces of flash, falling onto the heating elementsin the oven can be a source of fires. (See technicalinformation bulletin “Effect of Oven Post-Cure Cycleson Vulcanizate Properties.”)

Tensile strength and compression set are the twophysical properties most affected by oven postcuring.Fluid resistance, in terms of permeation rates orvolume increase, is affected only very slightly.

Typically, 80–90% of the improvements that can beobtained in tensile strength and compression setresistance can be achieved within 1–4 hr at 232°C(450°F) or 250°C (482°F). In many cases, postcureovens are run for 24 hr simply for convenience infactory shift scheduling. Refer to the bulletin refer-enced above for more detailed information on theeffects of oven postcure cycles on vulcanizateproperties.

In order to obtain the best possible resistance tocompression set, compounds of Viton® with bisphenolcure systems must be oven postcured for at least 16 hrat 230–250°C (450–482°F).

Substantial improvements in tensile strength andresistance to compression set (compared to values forthose properties with no postcure), can be obtained inoven postcures at temperatures as low as 150°C(302°F), for periods of time as short as 1–2 hr.Depending on the required tensile strength and/orcompression set values that are required, postcurecycles can be considerably shorter than the ‘standard’24 hr that is normally used.

22

Fluoroelastomer parts should not be placed in apostcure oven with other parts made from differentelastomers. In particular, parts made with siliconerubber should not be postcured with a fluoroelastomer.Postcuring VMQ or FVMQ and FKM parts togethercan result in the total loss/destruction of the entirebatch of parts. This is due to the chemical interactionsbetween silicone rubber and the small amounts of HFgenerated during the postcuring of the fluoroelastomercompound.

Postcure ovens must be equipped to provide adequate(fresh) airflow. Generally a minimum of 0.14–0.20 m3

(5.0–7.0 ft3) of air per minute should be introduced ona continuous basis into an oven having interior dimen-sions of approximately 0.23 m3 (8 ft3).

Parts to be postcured must be placed evenly through-out the oven. The parts must not be piled too deeply,thus preventing adequate air circulation around theparts. Adequate airflow around the individual parts iscritical, not only to obtain consistent physical proper-ties within a given batch of parts, but also to preventoven fires.

Postcure oven temperatures should be monitored on aregular (at least quarterly) basis. It is critical that aneven temperature be maintained throughout the entireoven, and that temperature differences between thecenter of the oven and any corner area not exceed 5°C(10°F).

Molded parts having cross-sectional thickness inexcess of 5 mm (0.20 in) should be ‘step’ postcured.A recommended ‘step’ postcure cycle would start at90°C (195°F) for 2–4 hr. The oven temperature canthen be increased in increments of 25–40°C each hour,until the desired final temperature is reached. Theremaining time can be run at the final temperature. The‘step’ postcure cycle is used to allow moisture that isgenerated in the press cure, to escape gradually.

Bonded parts (Viton®/metal inserts) should bepostcured at temperatures no higher than 200°C(392°F), in order to prevent degradation of the primer,and a subsequent loss of the bond.

Postcure Oven FiresRefer to Viton® bulletin “Handling Precautions forViton® and Related Chemicals.”

There are several possible causes of postcure ovenfires, which have already been mentioned above,under General Practices. Some of the most obvioustechniques to prevent oven fires are by:

– Preventing small pieces of molding flash fromfalling onto the oven heater elements;

– Maintaining adequate rate/amount of airflowthrough the oven;

– Never loading too many parts or by piling them toodeeply into an oven, thus preventing the requiredairflow from reaching each individual part.

High levels of hydrocarbon process aids, such ascarnauba wax, or low molecular weight polyethylenein fluoroelastomer compounds can, over time, poten-tially cause oven fires. Such materials may be boiledout of the compounds during the oven postcure cycle,and can condense or collect on the inside of the ovenventing pipe. The resulting accumulated residue canthen be ignited by a small flame source, such as aglowing piece of flash being blown up into the oven,and subsequently into the vent pipe. These residuesmay also cause fires by being heated to their ignitionpoint.

Suggested Mold Release SystemsFor Viton®Amine Cures

Internal 0.5 parts low molecular wtpolyethylene1 or 1.0 partvegetable wax2

External Polyethylene or siliconeemulsions

Bisphenol Cures

Internal 0.5–1.0 parts vegetable wax2

0.5–1.0 sulphone compound3

External Silicone or PTFE telomeremulsions

Peroxide Cures

Internal 3 component systems,consisting of:0.2–0.3 stearic acid0.2–0.5 Armeen® 18D0.2–0.5 vegetable wax2

External PTFE telomer emulsions

1 e.g., AC Polyethylene 17022 DuPont Dow Elastomers VPA No. 2 or carnauba wax flakes3 DuPont Dow Elastomers VPA No. 1 and VPA No. 3

23

NOTES

(07/03) Printed in U.S.A.Reorder No.: VTE-H90171-00-A0703

The information set forth herein is furnished free of charge and is based on technical data that DuPont Dow Elastomers believes to be reliable. It is intended for use by persons having technical skill, at their owndiscretion and risk. Handling precaution information is given with the understanding that those using it will satisfy themselves that their particular conditions of use present no health or safety hazards. Becauseconditions of product use and disposal are outside our control, we make no warranties, express or implied, and assume no liability in connection with any use of this information. As with any material, evaluationof any compound under end-use conditions prior to specification is essential. Nothing herein is to be taken as a license to operate under or a recommendation to infringe on any patents.

CAUTION: Do not use in medical applications involving permanent implantation in the human body. For other medical applications, discuss with your DuPont Dow Elastomers Customer Service Representativeand read Medical Caution Statement, H-69237-1.

Viton® ia a registered trademark of DuPont Dow Elastomers.

Copyright © 2003 DuPont Dow Elastomers. All Rights Reserved.

For further information on Viton® or other elastomers please contact one of the addresses below, or visitus at our website at www.dupont-dow.com

USA – GlobalHeadquartersDuPont Dow Elastomers L.L.C.300 Bellevue Parkway, Suite 300Wilmington, DE 19809Tel. +1-800-853-5515

+1-302-792-4000Fax +1-302-792-4450

AustriaDolder GmbHBrucknerstrasse 6/2/5AA-1040 WienTel. +43-1-504 21 80Fax +43-1-504 21 93

BeneluxN.V. Sepulchre S.A.19, av. des NénupharsB.P. No. 6B-1160 BruxellesBelgiumTel. +32-2-672 23 35Fax +32-2-673 67 82

DenmarkNordica A/SPilestraede 43Postbooks 2241DK-1019 Kobenhaven KTel. +45-3315 2855Fax +45-3315 2161

Geneva – EuropeanHeadquartersDuPont Dow Elastomers S.A.2, chemin du PavillonCH-1218 Le Grand-Saconnex,Geneva, SwitzerlandTel. +41-22-717 4000Fax +41-22-717 4001

FinlandOy Algol ABNuutisarankatu 15FI-33900 TempereTel. +358-3-266 1948Fax +358-3-266 0212

FranceSafic-AlcanDépartementDuPont Dow Elastomers3, rue BelliniF-92806 Puteaux CedexTel. +33-1-46 92 64 32Fax +33-1-46 67 04 42

GermanyDuPont Dow Elastomers GmbHDuPont Strasse 1D-61343 Bad HomburgTel. +49-6172-87 13 55Fax +49-6172-87 13 51

ItalyDolder-MassaraVia Caduti Bollatesi 38/b20021 Bollate (MI) ItalyTel. +39-02-350081Fax +39-02-38300725

South & CentralAmerica HeadquartersDuPont Dow Elastomers Ltda.Alameda Itapecuru, 506 - Sala 12Alphaville - Barueri - SPCEP 06454-080BrasilTel. +55-11-4166-8978Fax +55-11-4166-8989

PortugalSafic-Alcan PortugalRua D. Marcos da Cruz 1351P-4455-482 PerafitaTel. +351-22-999 82 90Fax +351-22-996 39 27

SpainSafic-Alcan Espana, S.A.Division IsisaRocafort, 241-243E-08029 BarcelonaTel. +34-93-322 04 53Fax +34-93-410 69 78

SwedenNordica Elastomers ABHamntorget 1P.O. Box 10104S-43422 KungsbackaTel. +46-300-73 250Fax +46-300-73 251

SwitzerlandDolder AGImmengasse 9CH-4004 BaselTel. +41-61-326 66 00Fax +41-61-326 47 81

Singapore – Asia PacificHeadquartersDuPont Dow Elastomers Pte Ltd.1 Maritime Square #10-54HarbourFront CenterSingapore 099253Tel. +65-6275 93 83Fax +65-6275 93 95

JapanDuPont Dow Elastomers Ltd.Dempa Bldg. 3F, 11–15Higashi-gotanda, 1-chomeShinagawa-kuTokyo, Japan 141-0022Tel. +81-3-3444-5166Fax +81-3-3444-6095

Korea OfficeDuPont Dow Elastomers Ltd.Suite 401, City Air Terminal Bldg.159-6, Samsung-dong, Kangnam-kuSeoul, KoreaTel. 011-82-2-551-7443Fax 011-82-2-551-7455