ampco_mold_design_guidelines.pdf

7/27/2019 ampco_mold_design_guidelines.pdf

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CDA Sponsors Mold Design

GuidelinesLook for ideasthat will allow

faster processingof plastic andproduction ofhigher-quality

parts

in MM&TKeep an eye out for comingissues of Modern Mold andTooling, which will containinjection mold design guide-lines developed to make youmore productive. These infor-mative and collectable fact-filled design guidelines arebeing developed for the injec-tion molder, mold designer andmold builder. The informationcontained in the guide-lineswill maximize the mold's cycletime and improve part qualitywith the use of copper alloys inthe mold. The articles willbegin in the May issue.These information packedInjection Mold DesignGuidelines are being developedand generated by The Copper-Alloy Molds Marketing TaskGroup. The group is a networkof copper-alloy suppliers, dis-tributors, and fabricators whohave joined together to assist

the moldmakers, molders andmanufacturers to improve theprocessing of plastic materials.The task group, supported bythe trade associations of thecopper industry, is dedicatedto research and disseminatingthe information you need totake advantage of the superiorperformance of moldscontaining copper-alloys. Also,the association is dedicated todeveloping an infrastructure ofcopper producers, fabricators,suppliers and mold makerswho are in the plastics chain.Research work, performed atWestern Michigan University, isconducted to address technicalissues and remove barriers tothe use of copper alloys forplastic processing. Thedevelopment of these injectionMold Design Guidelines is aresult of this research and inaddition to empirical dataderived from industryapplications.


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Several molds, funded by thetask group, were built and testedto conduct research under actualproduction conditions. Onestudied the cycle time advantagethe copper alloys offered overtraditional mold steels.Additionally, due to the superiorthermal conductivity of thecopper alloys, part qualityimprovements including lesswarpage, better dimensionalstability and more uniform moldtemperatures resulted. Otherresearch and testing con-centrated on eliminating moldsweating under humid operatingconditions. This is accomplishedby running higher mold operatingtemperatures with copper alloymold cores. The test resultsprove that better partdimensional stability can beobtained at shorter mold coolingtimes without mold sweatingwhen compared with moldsteels.Exhaustive wearstudy is under wayAn exhaustive wear study isunder way testing the effects ofelectroless nickel, hard chrome,titanium nitriting, thin densechrome and thin dense chromewith diamond particulate inextending the mold life of thecopper alloys.As a service to the plasticsindustry, the Task Group is fund-ing the publication of theseguidelines in Modern Mold andTooling. The greatest benefit tothe people who deal with moldsand molding will be to collecteach issue to use as a referencein both the applications of the

copper alloys and the molddesign principles.Subjects for the Injection MoldDesign Guidelines willinclude:1. Sprue Bushings and RunnerBars2. Mold Cores, Core Pins andChill Plates3. Mold Cavities and "A" Sideinserts4. Slides, Lifters and RaisingMold Members

5. Ejector Pins, Ejector Sleevesand Ejection6. Mold Temperature ControlSystems, Bubblers, Baffles,Diverters and Plugs7. Wear plates, Slide Gibs,

Interlock Plates, Leader pins andGuided Ejector Bushings8. Plating and Coating of CopperAlloys9. Application of Copper Alloys ininjection and Blow Molds

These guidelines will include

properties of the various copperalloys most commonly utilized fortheir thermal and bearingproperties, compared withtraditional mold steels. Charts,graphs, formulas and descrip-tions will provide the user withpertinent data not available fromother sources.


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Injection Mold Design

GuidelinesMaximizingPerformance UsingCopper Alloys

Copper Alloys for ConveyingPlastic in Injection Molds

The high thermal conductivity ofcopper alloys makes them idealmaterials for the injection moldsprue bushing and runner bars.Three alloys typically are utilizedfor the mold components, whichwill have contact with plastic. Thecopper alloys are:

Ampcoloy 83, high hardnessberyllium-copper alloy Ampcoloy 95, high conductivityberyllium-copper alloy Ampcoloy 940, NiSiCrhardenedhigh conductivity copper alloy

These Copper alloys have six tonine times greater heat transferrates than conventional moldsteels as indicated by the thermalconductivit .

and runner faster, allowing moreefficient ejection or removal bysprue pickers or robots.

Sprue Bushing Radiusin North America two injectionmold nozzle and sprue bushingradii are used, 1/2 and 3/4 inch.To insure proper fit up, the nozzleradius is nominal -.015 inch, whilethe sprue radius is nominal +. 015inch, required tolerances to use.

Swing points and tolerances usedin establishing the radius on asprue bushing are shown inillustration I.

Sprue Bushing OrificeMachine nozzle orifices come innominal 1/16" fractional inchsizes. To insure that the slug inthe nozzle will pull through thesprue, the orifice must be .031(1/32 inch) larger in diameter. This

dimension is referred to as the "0"dimension. The relationship isshown in this chart.

Illustration I,Sprue Radius

Mold Material Thermal

Conductivity(BtU/Hr/Ft2/F)

Ampcoloy 83 60

Ampcoloy 95 135

Ampcoloy 940 125

H-13 17

P-20 20

420 SS 14Nozzle O" Sprue O"

1/16" 3/32"

1/8" 5/32"

3/16" 7/32"

3/8" 9/32"

5/16" 11/32"

Nominal +1/32'

The sprue or runner system mustnever control the cooling phase

and/or overall molding cycle.Plastic in contact with copperalloys will set the sprue


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Also, a sprue bushing that is notkeyed will rotate creatingmisalignment with the runnermachined into the face of thesprue bushing and the runnersystem. To prevent theseproblems, retain and key thesprue into position with the use ofa cap screw as illustrated infigure IV.

Sprue FitHeat must be transferred fromthe sprue through the copperalloy sprue bushing to the moldplates, interference fit is rec-ommended for optimum cooling.The bore through the "A" plateshould be nominal size to plus

.0005 inches with a surface finishof at least 16 RMS. The shank ofthe sprue bushing should be thenominal size, plus .0005 to plus.001 inches.

Standard Sprue BushingAvailability

Copper alloy sprue bushings withpatented stainless steel nozzleseats are commercially available.An insulator between the nozzleand sprue is beneficial incontrolling the flow of heat fromthe nozzle to the sprue. Specialsprue bushings may beconstructed to suit using standard1/2 inch per foot sprue bushingtapered drills and reams. Spruebushings with tapers of up to 3/4inch per foot have been used fordifficult to remove plastics. Caremust be taken to insure that thetaper is draw polished and freefrom undercuts or rough surfacesthat could hinder sprue removal.

Conventional Injection MoldRunner Systems

The shape of the runner, fullround or trapezoidal, or otherconfiguration, is dictated by molddesign. The most efficient runnercross section is full round. Theefficiency of the runner crosssection can be calculated with aformula, figure V, the larger theratio the better.

Sprue Bushing TaperTo aid in the removal of the spruefrom the bushing, a taper of one-half inch per foot is normally usedin injection molding. Calculate thesprue orifice at the parting lineface, multiply the tangent of thetaper angle times the length, plusthe "02". Knowing this dimension,informed decisions can be madeon primary runner sizing.

The sprue frequently controls themolding cycle when larger orificeconventional steel sprue bushingsare used. The application of acopper alloy sprue bushing coolsthe sprue more quickly andefficiently, allowing the moldingcycle to be controlled by the piece

part.

Pressure loss is high in the sprue.This is the only place in the feedsystem where the channelprogresses from a smaller area tolarger. Frequently, smaller orificesare used on long sprue bushingsin an effort to reduce the mass.This results in extremely highinjection pressure losses, makingthe part hard to fill.

The chart in illustration lll is aguide for determining the effectthat the specific "0" dimensionhas on the pressure required todeliver plastic through the lengthof a sprue. Note that the differ-ence between a 3/32 and 9/32-inch sprue is about 1,000 PSIover a short sprue and almost1,500 pounds on a long sprue.

Using a copper alloy spruebushing allows for an increasedsize orifice, thus reducingpressure loss while maintainingreasonable cooling times.

Sprue Retention andAnti-RotationPressure acting on the partingline face of the sprue, due toprojected area of the runnersystem or part detail, exertspressure on the sprue bushing.

Illustration II, Sprue taper

Illustration III, Pressure,Sprue Length

Illustration IV, Anti rotationscrew

Illustration V, Formula runnersystem


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Injection Mold Runner BarsRunner systems for high cavita-tion molds normally have largerdiameters due to runner balancing.The runner system extends themolding cycles as heat is slowly

transferred from the thick plastic tosteel mold plates. Inserting copperalloy runner bars in the mold "A"and "B" plates, cooling the runnerfaster, is beneficial, in reducing theoverall molding cycle.

Runner SizingRunner sizing is dependent onmany things, including: plasticmaterial; part size, weight and wallthickness; molding machinecapabilities and processingparameters and, the number andplacement of the cavities.

Each mold is unique and thedesigner must consider allparameters and options availableon an individual case by case basis.Several mold design softwarepackages are available, includingMold Flow and C-Mold, whichaddress sizing of the runner system.

One method of runner sizing andbalancing used by mold designersstarts at the sprue and then workstoward the gate. Other designersstart with the part wall thickness andwork back to the sprue outlet orifice.The normally recommendedprocedure is that, in the direction ofplastic flow, the runner area alwaysgoes from larger to smaller. Neverfrom a smaller area to a larger area.

When the primary runner diameter is

known, the sum of the areas of themultiple connecting runners must beequal or smaller in area than thepreceding runner.When working backfrom the part,some designers size thefinal runner channel size (that runnerwhich feeds the gate) to equal thethickest wall section in the part. Eachrunner intersection then is a functionof the area of that runner times thenumber of connecting runners,usuallytwo. Therefore, the area of theupstream runner is always at least

equal to or larger in area than thesum of the branches. Note that a

runner with one-half the area is notthe same as a runner of one-halfthe diameter.

Formulas for calculating the area ofthe runner:

Runner Bar Mating

Best results are obtained by

Full Round Runner

A = 0.7854 d2

Trapezoidal Runner

A = (w1 + w2) h/2

A = area, d = diameter, I = length,

w = width, h = height

Illustration VI, Runner Barsdesigning and building the runnerbars to have zero to negativecontact with each other when the

mold is closed. This will prevent anydeformation on the parting linesurfaces that could result from highclamping pressures exceeding thecompressive strength of the alloy.To accomplish this, the "A" and "B"runner bars should be flush tominus .001 inch on each side of themold. This allows the mold baseand/ or cavity and core inserts toreceive machine clamp force, notthe runner bars.

Care must be taken to understandthe characteristics of the plasticbeing molded and clearanceshould be short of allowing therunner system to flash.Additionally, it is important to insurethat the mating halves of the runnersystem are in perfect alignment,with no mismatch at the parting line,to maximize plastic flow efficiency.

Runner Bar CoolingThe runner system must nevercontrol the molding cycle. To insureproper temperature control of therunner bars, cooling channelsshould be placed directly into theboth the "A" and "B" side inserts.The cross-drilled holes should beblocked with a plug containing an"0" ring and a straight thread plug.Due to the high thermal conductivityof the copper alloys and thetendency to thermal cycle rapidly,tapered thread systems must beavoided in the copper alloys toprevent cracking.

Illustration VII

With the increased cooling rate ofthe copper alloys and propercooling arrangements, largerdiameter runners can be used In amold equipped with copper alloyrunner bars. Almost withoutexception, runner diameters one ortwo sizes larger can be set upquicker with the copper alloys, overtraditional mold steels.

Sprue PullerA reverse taper sprue puller, 3 forstiffer materials and 5 for flexiblematerials, is recommended to insuresprue removal. To rapidly cool theundercut machine, the puller directlyinto the runner bars or a copper

alloy insert. Illustration Vll givesmore details.


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Guidelines

Maximizing Performance UsingCopper Alloys

The Injection Mold Core

A mold core is any member thatforms the interior of a plastic part,usually on the "B" side of the moldparting line. Mold cores can bemachined from a solid piece ofcopper alloy or inserted to aid inconstruction or allow for easierreplacement if a component wouldever be damaged in molding.

This picture shows a large copperalloy core, about 24 inches long

and seven inches high, used tomold a PVC bezel for a KitchenAid dishwasher manufactured byWhirlpool Corporation, FindlayOhio. The copper alloy wasspecified primarily to eliminatewarpage on the part, which is bothfunctional and esthetic in nature.The cycle time advantage of about20% by using the high thermalconductive copper alloy was anadded bonus to the improved partquality which was the main

objective.

Properly designed molds withcopper alloys used in strategiclocations, usually the core, haveproven to reduce injection moldingcooling cycles by 20 to 50 percent. The mold core is responsiblefor removing from 65 to 75 percent of the heat from the

The copper alloys normally selected formold cores, core pins, inserts, slides andraising mold members are; AA83 a highhardness beryllium-copper alloy;

plastic molding due to the material

shrinking around the standingfeatures of the mold.

Copper alloys have adequatehardness levels to hold up againstnormal injection pressures found inconventional injection moldingmachines. The normal press (pos-itive interference, or crush) is notused due to the higher ductility ofcopper alloys and to avoid anypeening or hobbing at shut offs andat the parting line. Rather zero tonegative press is recommended.

Negative press or clearance of themating components must obviouslybe less than that where plastic willflash. Hardness levels of thecopper alloys and mold steels arelisted in the following chart:

Hardness Levels

CopperAlloy

Hardness

S

teel

A

lloy

Hardness

AA83 41 RC H-13 38/52 RC

AA95 96 Rb P-20 28-48 Re

AA940 94 Rb 420SS

27-50 Re

Picture of Whirlpool mold: Acore built from a copper alloyfor a large dishwasher part.

AA95 a high conductivity beryllium-copper; and, AA940 a NiSiCrhardened high conductivity copperalloy. These alloys, with six to ninetimes greater heat trans-


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fer rates than steels (see InjectionMold Design Guidelines, number 1for details) have proven over timeto be the best choices for plasticforming mold components. Othercopper alloys, including the alu-

minum bronzes, have attributesconsistent with specificapplications in the mold notassociated with plastic forming.These include frictional wear andguiding surfaces where theirexcellent frictional properties canbest be utilized.

Types of Core Construction

Mold cores can be machined froma solid mold "B" plate but are morecommonly inserted into the "B"plate for ease of manufacture.When inserting the core, it isnormally retained with a heel orcap screws. The heel on a core,see Illustration A, typicallyextending .125 for small cores and.250 for larger cores should have alength ratio of one to two times theheel for maximum strength. The

corresponding counter bore ismachined into the plate withclearance around the perimeter,allowing the main core body toalign the insert. The depth of theheel pocket should match theinsert to plus .0002 to insure thatthe core does not move in themolding cycle.

Other means of holding the coreinclude blind pocketing, illustrationB, or self-sealing, Illustration C.The self-sealing insert is a popularchoice for deep pocket inserting inapplications where most of thepart, outer molding surfaces, areformed on the "B" side of the mold.As the portion of the pocketedinsert aligns the insert, the depthmust be great enough to withstandany side pressures imposed inmolding.

Tensile StrengthTensile strength is extremelyimportant attribute when selectinga suitable mold material, if therewere a scale that measuredtoughness we would want to use

that for a mold material. The cop-per alloys exhibit a good combi-nation of tensile strength andductility, making them tough andideal candidates for mold compo-nents, not withstanding the high

thermal conductivity properties.Tensile strength of the three cop-per alloys and three common moldsteels are compared in thefollowing tensile strength table.

Tensile Strength (ksi)

CopperAlloy

ksi Steel Alloy ksi

AA83 190 H-13 206

AA95 110 P-20 146

AA940 100 420 SS 125-250

Illustration A: Heeled core with

bubbler

Mold CoolingThe injection molding cycle ismade up of a number of elements.They include the filling portion,sometimes referred to as fill, packand hold, the cooling portion andthe mold open portion. The coolingportion is always the longest and

frequently represents greater than65 per cent of the overall cycle.Therefore, the longest element inthe overall cycle is where thegreatest benefit can be obtained inimproving the injection moldingcycle and where copper alloyswork to your best advantage.

Illustration B: Inserted core

with water channels

The principles of heat flow in aninjection mold are: 1. Heat flowsfrom the body with the highertemperature to a body of lowertemperature (from the plastic to themold component the plastic is incontact with). 2. The temperaturedifference, not the amount of heatcontained, determines flow of heat.3. The greater the difference intemperatures between the plasticand the mold component, thegreater the flow of heat. 4.Radiation, conduction and/orconvection transfer heat.Conduction is the main method ofheat transfer in an injection mold.

Illustration C: Self sealingcore insert

The amount of heat that must beconducted from a mold can becalculated. It must be rememberedthat 65 to 75 percent of this heatmust be removed through the coreof the mold. The formula is asfollows:

Illustration D: Heeled core

with water passages


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Where: H = Quantity of heat in Btuconducted

H=KAT(tp-tcL

K = Thermal conductivityfactor of mold material inBtu/hr/ft2/F/A = Surface area of mold incontact in square feetT = Time in hourstp = Temperature of plastic

tc = Temperature ofcoolantL = Distance from surface

of mold to coolant channel(Note: H-13, P-20 and 420 SS thermalconductivity ranges from 12 to 20, the threecommonly use copper alloys range from 61

to 135)The importance that the highthermal conductivity of properties ofcopper alloys has in removing heatfrom the mold can be determinedfrom the formula. Obviously, theother elements of the formula areimportant considerations that mustalso be taken into account whendesigning an efficiently cooledinjection mold. However, changingthe mold material and the resultantthermal conductivity factor is

normally the simplest and mosteffective means of achieving thebest cycle time.

Coolant OptionsThe injection mold core is one ofthe more difficult areas to place andinstall the proper coolant channelsdue to the limited space availableand ejection options necessary forpart removal, it is important that thecoolant system be one of the firstconsiderations made in mold

design, as the overall success ofthe mold project is dependant onhow efficient the mold cooling cyclecan be made. The use of copperalloys and their inherent superiorthermal conductivity is the bestguarantee the mold has at success.

Copper alloys will insure that thesurface temperate is as even aspossible and will extract heat awayfrom the plastic part. To maintainbest operating conditions and short

cooling cycles, it is imperative thatthe heat is efficiently removed formthe molds core. Coolant lines, in theform of through-drilled channels orwith the use of bubblers or baffles,should be installed in the mold

core similar to a steel core. This willprovide the best results and yieldthe most efficient cooling. Shouldthe same coolant diameters andplacement not be possible, thecopper alloys are more forgiving

than mold materials with lowerthermal conductivity. Typically thecopper alloys will allow greaterliberties in placement of coolantchannels, while cooling moreefficiently than their steelcounterparts. Caution should beused to insure that adequateprovisions have been made forremoval of the heat from any moldcomponent.

If drilled coolant channels aremachined directly into the moldcore, the edge of the coolantchannel should be about two timesthe diameter away from the moldingsurface. The distance between thecoolant channels should be fromthree to five times the diameter, seeIllustration D. Positioning thecoolant channel any closer to themolding surface does not neces-sarily result in better cooling and insome cases provides a gradientdifferential in surface temperature,

which could leave residual stressesin the plastic part. More details oncooling options will be presented inthe sixth article in this series.

Cycle Time ImprovementsExtensive testing was conducted atWestern Michigan Universitycomparing the use of the threemost commonly utilized copperalloys in injection molds AA83 (A),AA95 (B) and AA940 (C) againstH-13 and 420 SS. A single cavitytest mold for a 50-mm

polypropylene closure was obtainedand optimized to run at the lowestcycle time possible. Identical moldcomponents were fabricated fromthe three copper alloys and steelmaterials. Identical processingconditions were established andeach core material was tested withthe only variable being cooling time.Graph A illustrates the cycleadvantages and the reduction incooling times made by the copperalloys when compared directly to

the conventional mold steels.

Each test was conducted after acontrolled stabilization period. Forpurposes of evaluating the

Graph A: Actual comparisons of bestachievable cycle and cooling times in thesame mold, the only change was the corematerial

Graph B: Amount of part warpage Icompared

to cycle time for copper alloys vs tool steel

results of cycle time, cooling time was theonly variable. The only mold change wasthe core itself. The only processingchange allowed was to cooling time. Melttemperature, cooling temperatures,injection time, gate seal and otherprocessing conditions were monitored toinsure identical conditions. This test wasperhaps the first time ever that exactingcomparisons were made, under pro-duction type conditions, that physically

demonstrated the advantages of thesuperior thermal conductivity and effectson the injection molding cooling cycle andthe resultant overall molding cycle.

Graph B compares part warpage, Inmillimeters, between the three copperalloys and two steel materials at variouscycle times. The copper alloys removeheat so efficiently that part warpage isminimal, even at shorter cycle times. Thebenefit of improving part quality at faster

molding cycles over steel is obvious.However, the greatest advantage might bethe better consistency imparted into theplastic parts as a result of even moldsurface temperatures.


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This test was conclusive, confirm-ing anecdotal experience fromothers where cooling cycle timeimprovements of 20 to 50 per centare common on production moldswhen copper alloys are properly

utilized.

Chill Plate ApplicationsFrequently molds with smalldiameter cores, those too small forcooling lines, benefit by seatingcopper alloy core pin heads on acopper chill plate of the same alloy.When ever core size or designallows you should install coolantlines using baffles, bubblers ordrilled channels, to optimize mold

cooling and temperature control.When either the number of corepins or the diameter prevents theinstallation of the coolant channels,the chill plate concept should beconsidered. Testing has found thatthe best results are obtained whenthe core pins and chill plate havethe same high thermal conductivitynumbers. Obviously the higher thethermal conductivity number thebetter. Coolant channels areinstalled directly into the chill plate

to remove the heat and maintainthe proper mold core temperature.in small diameter cores, where sizelimitations prevent water channel isthe core, this concept has beenshown to be almost as effective ascores with small coolant channels.The chill plate concept is not aseffective as direct water coolingand should not be used for largerdiameter cores where directcoolant is possible, illustration Eshows a chill plate application.

Note, the chill plate can beinstalled under the main core orwhen using sleeve ejection,mounted to the back of the ejectorhousing.

Draft ConsiderationsThe molded plastic part must beejected from the core. To enhancepart ejection draft, tapering of thepart feature to assist in moldrelease is necessary. The draftangle specified should result afterconsultation

with the plastic material supplier,plastic part designer, molder andmold designer. Draft should be asgenerous as possible and normallymatches the draft on the cavityside to insure an even and

consistent wall thickness, seeIllustration F.

Ribs on the other hand present adifferent problem. Large draftangles results in thick wall sectionswhere the rib joins the main wallsection. Normally, the junction ofthe rib to the wall should be one-half to two thirds of the mating wallthickness. The use copper alloys isof great benefit in these situations.The rapid removal of heat from the

thicker ribs, due to the moreefficient cooling of the copperalloys, will normally reduce oreliminate the sink mark, which istypically caused by the delayedsolidification of plastic at thejunction of ribs at the wall of thepart. Without the benefit of thesuperior cooling of the copperalloy, injection pressure and holdtimes are often extended. This notonly results in longer cycle timesbut also increases the incidence of

flash, warpage and over packing ofthe molded part. a

DisclaimerThese guidelines are a result of research atWMU and Industry experience gained withthe use of copper alloys in injection molding.While the Information contained is deemedreliable, due to the wide variety of plasticsmaterials, mold designs and possiblemolding applications available, no war-ranties are expressed or Implied in theapplication of these guidelines.

Contact information

Information on copper alloys is availablefrom the Copper Development Association,1-800-232-3282.

Illustration E: Chill plateapplication

Illustration F: Self sealingcore with draft


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Guidelines

Maximizing Performanceusing Copper Alloys

The Injection Mold CavityA mold cavity forms the exterior ofthe plastic part and almost alwaysis located on the "A" side of themold. However, there are

situations where the cavity islocated on the opposite sideof the parting line oroccasionswhere the injection-moldedpart is symmetrical and thecavity is on both sides of theparting line. In these instancesit is called a "B" side cavity.

Cavity InsertsMolds containing more thatone cavity, multipleimpression molds, are usuallyconstructed individually, or ifsmall, ganged into cavityblocks, and inserted into the

mold "A" or cavity plate. Separateinserting allows for ease ofmanufacturing the cavity from avariety of mold materials andmakes replacement easier should

damage ever occur. Round insertsare a natural, as round insertpockets are easy to machine withgreat accuracy into mold plates.The round insert is an idealapplication for "surround" coolingof the insert. "0" rings are used toseal the water channels andprevent coolant leakage. Theyshould be designed to be placed incompression and not in shear, for

ease of insert installation and leakfree operation.

Mono Block Construction

Frequently in low run singleimpression and/or large molds thecavity is machined directly into the"A" plate. This type of moldconstruction is referred to as"Mono-Block" construction andeliminates the step of machining aninsert pocket into the plate.Additionally, it offers a very rigidtype mold construction withopportunities for excellent coolingchannels surrounding the cavity.

Illustration A: Edge gate showing gatewidth (W), depth (D) and gate land (L).

Cavity CoolingCooling channels of appropriatediameter or channel size should beincorporated into the mold cavityblock. Placement of thesechannels should follow similarrecommendations made for thecore of the mold. The coolantchannel should be placed abouttwo times the diameter away fromthe cavity with a pitch of three tofive times the diameter. While it is

common practice to run the cavitywarmer than the core for aestheticreasons, the best running moldsare those where even surfacetemperatures and goodtemperature control can bemaintained.

To insure turbulent water flow inmold cooling circuits a mold cool-ing analysis is conducted prior tomold building. This analysis checksand determines place-


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run molds. P-20 or Number3 steel is used for long runmolds and when part detail ismachined into the mold plate.When a heat-treated mold plateis required, some cavity detail

can be formed on the mold plateand the cavity built up withlaminations, H-13 is a logicalchoice for these high run molds.Another material used for highvolume and long running moldsor with corrosive plastics is type420F stainless steel.

Cavity MaterialsWhen high thermal conductivity isrequired, either to more rapidlycool the plastic and promotefaster cycles or to maintain amore even surface temperaturefor better dimensionalconsistency, the copper alloysdesignated Ampcoloy AA83,AA95 or AA940 are used.

Gate Placement,Types and SizeGate placement, normally in thecavity, is critical to the plasticcomponent ultimate aestheticand physical properties. Theideal location will, to a degree,

dictate the type and size of thegate used for the mold. The bestgate location, along with the typeand size of the gate, is one of theearly and difficult mold designdecisions that must be madecorrectly to insure a good runningmold.

In injection molding the morecommonly used gates are theedge, submarine (tunnel) andpinpoint gate, which constitutes

about 65% of molds built andwhich we will elaborate on.However, some parts are moreideally suited to different gateconfigurations includingrunnerless molding systemdrops, fan, tab, sprue, ring,diaphragm, flash and post gates.

Edge GatesThe edge gate is the mostcommon type of gate on

conventional molds. (IllustrationA) Typically, the gate depth (d) is50 to 80% of the wall that it isconnected to. The depth of thegate controls gate freeze off andis the most critical

ment, number and size of coolant

channels required.Afterthe mold isbuilt coolant flow can be measuredto determine if adequate flow ratesare being maintained. The followingtable lists pipe size and minimumamount of flow in gallons per minutewhich guarantees turbulent flow.

Nominal PipeSize

(NPT

ChannelSize(diameter)

Min.

FlOW

(Gal/Min.)

1/16 .250 0.34

1/8 .313 0.45

1/4 .438 0.56

3/8 .562 0.75

1/2 .688 1.30

Illustration B: Submarine or tunnelgate, diameter and angle of coneillustrated

Core Inserts in CavitiesHoles in the part and other male coredetail formed from the cavity side arenormally achieved with core insertsin the cavity. Consideration has to bemade for removal of heat from thesecomponents. Parts like television

backs with air circulation slots areidea candidates for inserting highthermal conductivity copper alloys,Ampcoloy AA83, AA95 or AA940 intothe cavity, as are other hard to installcoolant channels in other sections ofthe molds.

A pure mold cavity for an item like adrinking glass, where the plasticshrinks away from the cavity, onlyhas to remove about 25 to 33% ofthe heat from the plastic. Plastic

parts with contoured configurationssuch as toys sometimes are moldedwith 50% of the heat removedthrough the cavity. These partsbenefit from the high thermalconductivity of the copper alloys andthe more even mold surfacetemperatures offered.

Mold Base MaterialsAdditionally the mold plate can bebuild from the material most ideallysuited for the type and amount ofcycles the mold will run. The moldplates and/or mold base are normallybuilt from 1030 plain carbon steel,Number1 steel for prototypes orvery short runs. Number 2 steel,4130-4140, is most often used formedium

Illustration C: Pin point gate,used in three plate molds


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dimension in determining pressureloss. Width of the gate (w) isgenerally two to four times thedepth, depending upon the volumeof plastic required to fill the part.The gate land (I) should be short toavoid large pressure losses.

Additionally, a short gate land willassist in breaking or degating thepart from the runner.

Submarine GateThe submarine (frequently referredto as a tunnel or sub) gate is apopular choice on conventionalmolds when an automatic methodof separating the part from therunner system is desired. Thediameter of the gate (Illustration B)is generally 50 to 70% of the wall

section, with 60% a commonchoice. The angle of the cone isnormally 30. However, with stiffermaterials it frequently is less. Caremust be taken so as not to placethe cone too close to the cavitythereby avoiding thin cavity materialsections and premature failure.Flexible plastics allow the angle toincrease, as the plastic will still pullfrom the cone. Most submarinegates are placed to enter on thecavity side of the mold. In these

situations, ejector pins with pullersare incorporated in the runnersystem to hold the runner duringgate separating and then eject therunner.

Pin Point GatePin point gates (top gating) areused in modified and three platemolds. Typically, and almost with-out exception, round parts likeclosures and caps have the gatelocated in the center of the part.

The cone, connecting the runnersystem with the gate starts with afull radius at the end and typicallyhas a 2-3 angle to assist inextraction with the assistance of asucker pin. The gate portion tapersdown at about a 10 angle. Theintersection of the gate to the cavitymust be sharp providing for a goodbreak off. Generally, the amount ofgate projection from a pin pointgate is one-half the gate diameter.The gate diameter (Illustration 0 is

30 to 60% of the wall that it isconnected to. The ideal gatediameter is a function of the flowproperties of the plastic; the easierthe flow the smaller the gatediameter. The harder the flow, the

larger the diameter. Obviously, thesmaller the gatediameter the better the cosmeticson the part and the smaller the gatevestige.

The ideal location for the gate islisted below, unfortunately,sometimes trade offs must be madeand not every criteria can besatisfied. Therefore, a decision hasto be made as what is the bestcompromise in gate placement. 1.Plastic must have the ability to fillthe entire part without usingextreme processing conditions andmaximum injection pressures.(Multiple gates may be required onsome parts) 2.Material, flowing fromthe gate, will push gasses towardthe parting line or other areas wherethey can be effectively vented andwill not entrap gas. 3. Plastic willflow into the thickest section of thepart and will flow from thicker tothinner sections. 4. Flow to allpoints on the part will be of equaldistance and the part will notexperience areas of over packing.5. The gate must be located in anarea not subjected to high stressesdepth, information available fromthe plastic material supplier is avery reliable source for technicalinformation.

VentsVents (Illustration D) should beinstalled at the mold parting line.The depth should be slightly lessthen where flash will form. Ventsshould be 1/4 to 1/2 inch in width.After a short land length, typically.020 inch, the vent depth should bemachined .03 to .04 deep and mustlead to atmosphere outside themold. Conventional runner systems

should be vented also. Any gas thatcan be vented prior to arriving at thecavity is less volume that has to beallowed to escape. Typically runnersystem vents are twice the depth asthe part if runner flash is not anissue and installed at the end of therunner and at junctions. Stoppingshort of the outside of the mold withany vent is extremely inefficient inallowing gasses to escape andconsidered bad practice.

Round parts with top gates,(Illustration E) lend themselves torelief around the entire part. Acollector ring assists in collecting

(the gate area could well be the

weakest section on the part). 6.Plastic entering the part from the gatemust impinge on a wall or moldmember to create a small backpressure, avoid jetting. 7. The locationhas to be in an area that will minimizeweld, flow or knit lines especially onclass "A" surfaces. 8. The gate mark

should not be located on anappearance or functional surface. 9. Ifthe gate is of the type that must betrimmed, it must be located in anaccessible area.

Methods of ventingInjection molds must be vented toallow volatiles released from theplastic pellets during the plasitifi-cation process and the air trapped inthe closed mold to escape. Adequatevents must be installed on the partingline, runner systems and in any placein the mold where entrapment ofthese gasses occur. The depth of thevent must be less than that which theplastic will flash. Experience hasproven that the location of the vent inproximity to the gate will determinethe depth to which plastic will flash.The closer the vent is to the gate theeasier the material will flash. Thelonger the flow distance from the gate,the less likely plastic will flash at thesame


13/39

stone or blasting. These cate-gories then have three sub-groups defining the lastbenching or polishing operationthat will yield the desired finishor appearance on the plasticpart. in addition to the listing ofthe SPI finish standard numberand the correspondingoperation, we have included theRa and RMS measurementtaken from a finished

component. It is important toremember that the measurementis not part of the SPI standard,nor is it endorsed by SPI. It issimply included as a reference.

the gasses and bleed offs allowthem to escape to atmosphere.The volume of the bleed offs mustequal or be greater than thevolume of the collector rings to beefficient.

Areas in the cavity that can bevented other than the parting lineare at insert lines, core pins andareas where sintered vents canbe installed. Ejector pins are oftenoverlooked as excellent places to

install vents, as they are selfcleaning due to their movement atejection. An effective vent can beachieved by grinding relief on thetop diameter (Illustration F) toallow the gasses to escapearound the diameter to a collectorring and then spiral bleed offs tothe ejector housing. This methodkeeps the ejector pin centered inthe ejector pin hole, preventingshifting of the pin causing flash onone side of the pin and shuttingoff the vent on the other side andis much more effective than anunder-size pin in an oversizedejector pin hole.

Mold FinishThe Society of Plastics Industry(SPI) developed and publishes astandard for mold finish that isuniversally used to specify thedesired effect on the plastic part.The standard is based on fourdifferent methods, polishing with

diamond, paper,

SPIFinish

Definition of LastOperation

Ra RMS

A-13 Diamond Buff 0.6 0.8



B-1600 Grit Paper 2.1 2.6

B-2400 Grit Paper 3.8 4.9B-3320 Grit Paper 4.8 6.2

C-1600 Stone 4.4 5.7

C-2400 Stone 8.0 10.2

C-3320 Stone 8.9 11.4

D-1 Dry Blast #11 8.8 11.1

D-2 Dry Blast # 240Al Oxide

13.2

17.0

D-3 Dry Blast # 24 AlOxide

80.8

104.5


14/39


Guidelines

FOURTH IN A SERIES: NOMENCLATUREMaximizing Performance Using Copper Alloys

Nomenclature for the types of moldsis somewhat diverse but usuallyfollows an order describing the typeof runner system, mold action andejection method used. A mold isconsidered a standard mold when it

has a conventional runner system,the part is pulled without any actionand the mold only has an opening atthe parting line. Occasionally wehear the term two-plate applied tothis type of conventional mold. Thisis not necessarily a correctdescription and perhaps is only usedto differentiate it from a three-platemold.

A three-plate mold has the runner

system installed between a separateparting line and the parts are gatedwith a pin point gate. The advantageis that the cold runner is separatedfrom the parts on mold opening. Thistype of mold was popular for topgating parts and now frequently arunner-less molding system is usedin its place.

Runnerless molding systems (RMS)account for nearly 30% of the molds

built today. RMS can be internally orexternally heated. If internallyheated, the mold has distributortubes and/or probes with electricheaters placed in the distributionchannels to maintain the melttemperature of the plastic as it flowsaround the tubes toward the gate.Each cavity needs at least

one probe to feed plastic for themold to be a true runnerless moldingsystem. Often hybrid systems areused, especially on small partswhere one probe is used to feed aconventional runner system, which

then feeds multiple parts. Copperalloy probes have proven to holdmore even temperature profiles thansteel alloys, especially at the tip end.

Internally heated systems incorpo-rate a manifold with balanced andstreamlined passages installed forthe plastic to flow from the inlet tothe nozzles. Nozzles normallyequipped with coil heaters aroundthe periphery and a thermocouple tocontrol the temperature, feed plastic

from the manifold to the gate. A verypopular divergent flow styleincorporates a copper alloy tip,usually made from AA83 or AA940copper alloy, to aid in maintaining aneven temperature profile at the gateentrance to the cavity. The melt flowtypically diverges from the centerflow passage through orificesallowing the copper alloy tip toextend down to or into the gateorifice. The tip then maintains controlof the gate area, freezing the gate

during part removal and maintainingthe correct temperature to open thegate for the next cycle.

Molds can have many actions,depending on what has to beaccomplished, to free undercuts andremove the part from the mold.External threads for example, if hotstrippable, utilize slide action orexpandable cavities to free externalfeatures.

Illustration A: Stripper ring ejection

of an undercut


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Internal threads can be formed oncollapsible cores or in unscrewingmolds. Several methods are usedfor unscrewing molds, includinghydraulic motors, splines, variousgearing methods and for largemultiple cavity molds, racks andpinions are used.

Other mold actions include lifters,wedges orslifters (a new term),raising members and slides(sometimes referred to as splits,cams and side action). Moldnomenclature then typicallydescribes the type of ejectorsystem used, normally ejectorpins, sleeves, stripper rings orplates. Therefore, molds arenormally refereed to as "a threeplate, slide action sleeve ejectedmold", or"runnerless collapsiblecore, stripper plate mold".

Mold SlidesUndercuts, features on the plasticpart that are not in line withnormal mold opening, are fre-quently encountered. When theundercut is small, typicallydefined as a percentage of theoverall part dimension, the bestand least expensive option is todetermine if the part will flexenough to strip off the cavity orcore without the use of a mold

action. Freeing the plasticundercut is first dependent uponthe plastic material, its flexibilityand hardness. The greater theflexibility and more compressivethe plastic the greater theundercut can be. The stifferandmore rigid the plastic material, theless the undercut must be.Undercuts are defined as thepercentage difference between"d", the amount of the undercut,and "D" the diameter or

dimension that the undercut hasto snap off (see Illustration A).

Seals are molded from flexiblePVC with undercuts greater than.375 inch and a 1.500 diameter,resulting in undercuts of 25%.Modified Closure Manufacturer'sAssociation (CMA) threads arefrequently stripped on closuresizes above 24mm in polyethyl-ene and polypropylene, especiallyin co-polymers. Acme or buttress

threads typically will not strip dueto the sharp and flat thread profileperpendicular to the direction ofdraw.

External part features, thosenormally found on the cavity

side of the mold, require that thecore be removed prior toattempting to free even theslightest undercut, as the flexingplastic must have a place tocompress or expand into if thepart is manufactured withoutmoving mold members. Whenthe undercut is too great, the

mold cavity can be split or mov-ing cams installed to release theundercut. These plastic part fea-tures with details connected tothe main wall tend to have thethickest sections. Copper alloyswith their ability to cool fasterthan conventional mold steelshave proven to be the bestchoice of mold materials in theseareas, copper alloys will providethe most even surfacetemperatures necessary to take

the heat away from the moldingsurfaces. Frequently, the front ofthe slides are faced or inserted. Acopper alloy is inserted on a steelslide carrier and coolant channelsare machined through the carrierinto the copper alloy insert. Withthis design, the copper slide faceacts as a watered heat sink,drawing heat away from the part.

In all other designs the slideshould be designed with thesame concept and mold coolantchannel as the molds cores andcavities. The coolant channelscould include looping flow, bafflesor bubbles. A coolant-circulatingcascade is available from anumber of standard mold com-ponent supplies and is ideal forgetting coolant into hard to reachareas like those found on a slide.The best practice is to placethese coolant lines about twodiameters of the cooling channel

away from the molding surfaces.This standard works well with thecopper alloys, as well as moldsteels. However, ifyou cannotget that close to the moldsurface, the more efficient copperalloys, with their higher thermalconductivity, will perform wellwhen the coolant lines are notideally located.

Mold LiftersA lifter is a component in themold that is normally attached to

and actuated by the ejectorsystem and moves at an angle tofree internal molding detail (seeIllustrations B and C). They aretypically attached between theejector retainer and ejector plateswith some mechanism

Illustration B: A lifter shown with the moldclosed. Lifter angle is exaggerated, should notexceed 5.

Illustration C: lifter actuation shown duringejection.Lifter angle is exaggerated, should not exceed

5.


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allowing the fixed end to slide orpivot to compensate for the move-ment of the lifter position as itmoves at the desired angle. Liftersare frequently used when segment-ed plastic undercuts (raised moldcore detail) is necessary. The lifterhas to move out of the mold core atan angle, typically 5 or less, toclear the plastic from the mold lifter

detail. This angle is critical for tworeasons. First, if the angle were toogreat the forward motion of theejector system would put too muchpressure against the lifter body.This pressure would create bindingof the lifter and lead to excessivewear or premature failure. Shouldthe angle be too shallow, theejector plate travel would beexcessive. Therefore, carefulengineering and good judgementhas to be made.

Due to their function, lifters are nor-mally long and narrow. Coolantchannels are nearly impossible tomachine into them. The AA83,AA95 and AA940 copper alloys,normally used in the mold cavityand core, will remove the heatefficiently from the lifter. However,because this is a high wear areaand when the mold core is builtfrom one of the alloys already,aluminum bronzes make excellent

choices for lifter materials. Moreinformation on aluminum bronzeswill be included in article eight ofthis series. To avoid seizing thelifter, one copper alloy ridingagainst another copper alloy is notgood engineering practice. One ofthe components should be plated orcoated. The plating or coatingshould be carefully chosen, as itmust provide a low coefficient offriction between the two surfaces.Surface treatments should provide

dry lubrication and not be affectedby contact with the plastic materialand thermal cycling of the moldcomponent due to the moldingprocess.

As the lifters have to move inwardfrom the inside wall of the plasticpart to free the undercut, the partmust be devoid of any detail thatwould prohibit or impede liftermovement. Should the part designnot allow this required movementthe only choice to form this partdetail may be with the aid of inter-nal or hidden slides. The problemwith internal slides is the amount ofroom they take to position and butwhen moved forward free movethem in a core.

Wedges or Moving Members

Wedges are mold components thathave a shape that allows them to fittight in the molding position butwhen moved forward freethemselves from the pocket tomove away from the plastic wall(see illustrations D and E). Theyhave a guiding system allowingthem to move forward and awayfrom internal or external undercutson the plastic part. The wedges arenormally located on the "B" side ofthe mold and are either pulled witha mechanical attachment from the"A" side of the mold, or pushed bythe ejector system or cylinders.While less common, wedges can beinstalled on the "A" side of the mold.

The wedge must be guided as itmoves forward. The two guide sys-

tems most frequently encounteredare the "T" slot or dovetails. Moldswith wedges utilizing dovetails toguide and hold the wedge in posi-tion are being called "slifters" in thetooling community. Wedges orslifters have a commonality withlifters. The angle in which they raisemust be steep enough to free theundercut within the movementrange and yet shallow enough soas not to bind or be exposed toconditions where excessive wear

could occur.

By design, these mold membershave large areas in contact with theplastic part. Therefore it isnecessary to build them withcoolant channels and frommaterials with high thermalconductivity rates. While thesewedges and slifters are ideal candi-dates for the AA83, AA95 andAA940 copper alloys for the plasticforming contact areas, they are not

the best choices for the "T" or dove-tail guiding systems. Therefore,several options should beconsidered in their construction.One preferred method is tolaminate hardened tool steel to thecopper molding face and install theguiding system in the tool steel.Aluminum bronze materials can belaminated to the opposing memberof the mold to reduce friction andavoid common tool steels acting asbearing surfaces.

Raising Mold Members

Occasionally plastic parts will haveextreme contours. Automotive "A","B" and "C" pillars, for example,which have geometry where theonly way to free the part is to raise

Illustration D: Wedge/slifter with themold closed.

it out of the mold and physically ormechanically flex the plastic to removeit from the mold core. These molds arefrequently considered raising coremolds. This type of arrangementcomplicates the installation of coolant

channels due to their contour andshape. Placement of the coolantchannels can be far from ideal.Typically a tool steel core member inthese applications results in areaswhere cooling is compromised. Copperalloys have proven time and again thatthey will, due to their high cooling rates,run cooler and have more evenlydistributed surface temperate than asteel counterpart would have. Theplastic product almost always has lesswarp, twist, sink and is more

dimensional consistent, due toimproved temperature control of thisraising mold member.

Other Mold Movements.

The injection-molding machine providesone movement when the machineplates separate. The subsequent moldopening provides the mold designerwith motion that can be used tomechanically create movement inanother plane. Plate movement,commonly referred to as floating of the

plates, creates the conditions where thedesired mold actions can beincorporated.

One example is the movement ofconventional mechanical slides on the"B" side of the mold with an angle pin(see Illustration F). The angle pin(s) islocated on the "A" side of the mold andwhen the parting line separates the pin,due to the angle, moves the slide out. Ifthe same movement is required on the"A" side of the mold the problem of

clearing the undercut prior to the mainparting line must be overcome. Onesolution may be to pull the slides withhydraulic or pneumatic cylinders prior tothe mold opening. If the slide has plasticforming projected area against it thecavity pressure must be overcome bysome locking method.


17/39

Should the area and pressures besmall the cylinder may haveenough force to prevent move-ment. If the pressures are great,then a locking cylinder must beused. in any event, the timing of

the cylinder retraction andadvancement must be tied intothe molding machine and mea-sures taken to insure that theslide is in the proper position onmold opening and closing.

To move the "A" half slidesmechanically a mold movementhas to be established where the"A" plate floats (retaining theplastic part) creating forwardmovement so that angle pins

mounted in the top clamp platecan actuate the slides away fromthe part and clearing the under-cut. Once the part is clear fromthe slide the plate movement ispositively stopped, normally withshoulder or stripper bolts, and themain parting line is allowed toopen.

The movement of plates is typi-cally accomplished with a pullermechanism. Frequently externalmounted commercially availablelatch lock devices are mountedon the mold. These mechanismsare solidly attached to the moldmember that will be actuating theplate and the opposite end of thedevice will contact and lock theplate when moving it and releasewhen the desire travel has beenreached.

TimingThe expression used to describethe proper sequence of events in

mold action is timing. Theopening and closing of a stan-

dard mold is straight forward, thesequence of events is that themold closes, plastic is injected,the plastic is cooled, the moldopens, the parts are ejected andthe cycle continues. When mold

actions, items like slides, lifters,wedges, floating plates, etc. areincorporated, the sequence ofevents must be pre-determinedand the mold designed and builtto insure that the proper eventhappens and that the plate ormovement has traveled thecorrect amount prior to the nextsequence starting. Additionally, itis important that the mold actionsreturn in the proper order. Overthe years, almost any action or

movement has been installed inproduction molds. We are onlylimited by our imagination on howto positively insure that theproper mold action will take placeat the correct time and thenreverse the process to preparefor the next molding cycle.

There is no room for error insequencing of mold actions. Eachoperation must be preciselycarried out in the proper

sequence with the movementrequired exactly carried out. Ifany plate or action is left tochance, damage will occursometime during the molding run.The correct way to design themold is to positively achieve thedesired movement at the righttime, while providing a method ofdetermining that the sequencehas occurred prior to allowing themold process to continue to thenext step.

Illustration E: Slifter actuation duringejection, showind the wear plates and

dove tail guides

Illustration F: Copper faced slide showing theuse of an angle pin. The cooling circuit pathand O rings between copper and steel arenot visible in this view.


18/39

injection Mold DesignGuidelines

FIFTHINA SERIES

Maximizing ,

Performance UsingCopper Alloys

By Dr. Paul Engelmannand Bob Dealey

for the Mold MarketingTask Croup of the CopperDevelopment Association^

Illustration A: Core pin forming a holeIn the plastic part. The core pin

transfers heat to a chill plate for fastercooling cycles.

Copper Alloy Core PinsThe fastest, easiest and quickest method ofproving benefits from the high thermalconductivity properties of copper alloys is toreplace a core pin in a troublesomeapplication. Core pin, as the name implies,forms the interior of a plastic part feature.Problem areas in the mold that will benefitfrom the core pin replacement Include heavy

wall sections that control cycle time, interiorpart features that cannot be cooled efficiently,sections prone to sink marks and features thatrequire tighter and more consistentdimensional control. (illustration A)

The principles of heat flow should beunderstood and applied in theinjection mold design as the moldacts as a heat exchanger during themolding cycle. Those principles are:1. Radiation, conduction andconvection transfer heat.(Conduction is the main method ofheat transfer in a mold and is the

most efficient means of cooling) 2. Heatflows from the body with the highertemperature to a body of lower tem-perature. (You cannot transfer cold). 3. Thetemperature difference, not the amount ofheat contained, determines flow of heat. 4.The greater the difference in temperaturesbetween the bodies, core and plastic, thegreater the flow of heat. 5. The thermalconductivity of the mold materials will havea dominant affect on the amount of heatenergy transferred.

The following mold-cooling formula isnormally used for engineering molddesigns for efficient operation:


19/39

ference between them. The fitdimensions and tolerances of core pinsare critical to the success in theirfunction in the injection mold. Core fit atits mounting surface, is typically aninterference fit of -.0000 to -.0005depending on its size and frequency ofremoval from the mold. As a generalrule the length of the fit area should beat least twice the diameter.

As the core pin forms Its detail in thesurrounding plastic, the heat given offfrom the plastic must be absorbed and

transferred through the core pin to anarea of the mold where the heat can betransferred Into cooling lines. As withtool steel cores, the most efficient

Chart B: Chart illustratesadvantages of copper alloys inreducing part warpage and cycletimes.

had less warpage than the partmolded with a steel core with watercirculating, even at a 22% longermolding cycle.

From the Western MichiganUniversity test data one canconclude that "L" is ratherinsignificant when the combinationof the copper alloy core pins andchill plate of the same material andthermal conductivity is used in amold design. This is an important

discovery and technologyimprovement in the efficiency ofmold building and injectionmolding.

Coefficient of ThermalExpansionThe coefficient of thermalexpansion must be consideredwhen designing molds withmaterials that expand at differentrates. The degree of thermalexpansion is critical in both the fitof the components and the correctdimensions to design and build themold core, components and cavity.Copper alloys have larger expan-

sion coefficients than tool steelsand are listed in Illustration C.

Both the plastic material shrink rateand thermal expansion of the moldcavity and core must be taken intoconsideration in the design of closetolerance molds. Plastic shrinkrates, when using copper alloys inthe mold, may be reduced whencompared with steel components. Ifthe plastic material shrink rate isaffected by mold temperature, thencompensation must be made.Typically, the mold surfacetemperature will be moreconsistent and lower with the useof copper alloys, if the mold will berun at elevated temperatures, as isthe case with many of the hewengineering grades of materials,the thermal expansion of moldcavities and cores must beconsidered when specifying moldsizes. This same considerationshould be taken into account wheninserting a copper alloy into a steelretainer. The final fit should becalculated at operatingtemperatures.

Ejector Sleeves and Core Pins

Core pins fitting into ejector

sleeves requires specialconsiderations. Standard offthe shelf ejector sleeves arebuilt to accept pins with toler-ances applicable to ejector pinsand not core pins. Apparentlywhen ejector sleeves were firstintroduced the only closetolerance pins available wereejector pins and the precedentwas established. As ejectorsleeves are utilized to forceplastic off mold detail, it impliesthat a core pin should be used

in the application. The core pinmust rapidly transfer heatremoved from the plastic toanother part of the mold. Thehigh thermal conductivity ofcopper alloys performs thisfunction efficiently and

Mold Material andDescription

Coefficient of ThermalExpansion 10-6/F

Applications in Molds

420 SS Stainless Steel 6.1 High Gloss Cavities

H-13 Tool Steel 7.1 Hardened cavities and Cores

P-20 Tool Steel 7.1 Pre-Hard Cavities

Ampcoloy 83 9.7 Cores, Core Pins, Cavity inserts,Slides, Etc. Where Higher Hardness is

Ampcoloy 95 9.8 Cores, Core Pins, Cavity inserts,Slides, Etc. Where Higher Thermal

Ampcoloy 940 9.7 Cores, Core Pins, Cavity inserts,Slides, Etc. Where Higher Thermal

Ampco 18 9.0 Lifters, Bushings, Bearings, WearPlates, Gibs and High Wear Areas

Ampco 21 9.0 Ejector Sleeves, Bushings, Bearings,wear Plates, Gibs and Load Bearing

Areas

Illustration C: Chart listing various mold materials, coefficient

of thermal expansion and applications, in injection molds.removing the heat is with an internalcoolant passage with cooling mediumcirculating in the core itself.

Tests at Western Michigan universityhave proven the effectiveness oftransferring heat from the plasticthrough a copper alloy core pin andthen into a copper alloy chill plate.(Chart B). in this illustration, theresults of exhaustive testing in thesame mold are shown; steel and cop-per core pins with and without watercirculating in them were tested. The

tests prove the effectiveness of corepins made from copper alloys, withhigher thermal conductivity rates thantool steels, will cool a part faster withfar less warpage at shorter cycletimes than their steel counterparts.Note that the warpage of a partmolded with copper alloy core pins,resting on a copper alloy chill plate,


20/39

results in a very consistent and uniformshot-to-shot component temperature.

Care must be taken to provide theproper clearance between the ejectorsleeve and copper alloy core pin.

Always check your ejector sleevesupplier's dimensions and tolerances,the ejector sleeve has an internaltolerance of the nominal dimension +.0005 -.0000 inches. The copper alloy

core pin should have approximately.0010 to .0015 clearance, dependingupon the diameter and at whatclearance plastic will flash. Copperalloy core pins can not justautomatically be used with standardejector sleeves as the tolerances do notallow enough clearance and galling ofthe components will result. Properconsideration must be made inproviding the proper sliding fit.

The other design necessity is to insurethat the proper bearing length betweenthe ejector sleeve and core pin is used.

(Illustration D) Bearing length should bea function of the core pin diameter. Thegeneral rule of thumb is that thebearing length should be two times thediameter. We think when the bearinglength exceeds one-half to three-quarters of an inch, problems will occuras a result of too great of a bearinglength. Experience in the mode offailure between the sleeve and pinshow that 90% of the time the bearinglength is too long. Standard ejectorsleeves are provided with allowancesfor cutting to the desired length and thebearing length is purposely long toaccommodate all possible sleevelengths in that size range. Therefore,when the sleeve is cut for just a shortlength, the bearing length is long andthat is generally when problems occur.

Copper Alloy Ejector SleevesThin wall ejector sleeves built from

Ampco 21 aluminum bronze and thenplated are successfully utilized in highspeed and high cavitation unscrewingmolds. The sleeves offer advantagesover their H-13 counterparts as theyprovide an extremely low coefficient of

friction. More importantly, they holdtheir roundness better in thin wallsleeve applications. Diameters of 2.000inch with wall thickness of .040 inch inthe ejection area have been known torun 1,000,000 cycles. When the platingbegins to show evidence of wear orexposes the copper alloy the moldcomponents are stripped, refit andplated again. Due to similar materials, ittypically is not a good practice to runcopper against copper in ejector sleeveapplications. However, with the properplating on both components, successhas been achieved in high volume

molds.

Copper Alloy Ejector and SpruePuller PinsThe placement of ejector pins, sleevesrings or bars in the mold is crucial tothe efficient removal of the plastic part.

First and foremost, the ejectorcomponent must push the plastic off themold member. Placing an ejector pin ona surface that creates a pulling actionon a plastic wall results in a greaterresistance to the removal of the part.

Ejector and sprue puller pins built fromthe copper alloys, AA83, AA940, A18and A21 are successfully used in high

volume molds. The copper alloy ejectorpins require slightly greater clearancesbetween the pin and the ejector pinholeto compensate for their higher thermalexpansion. These pins work well whenadditional heat must be taken away andhave proven beneficial when used inthick wall applications for reducing oreliminating sink marks.

One of the most impressive successstories involves the use of a copperalloy sprue puller pin. (illustration E)Problems are frequently encountered incooling the sprue puller enough to

efficiently pull the sprue. The use of thecopper alloy sprue puller is highlyeffective and recommended whenflexible materials are molded andproblems pulling the sprue areencountered. Again, the bearing lengthon the sprue puller pin should be twotimes the diameter of the pin.

The most common mode of failure of anejector pin is galling created by overlylong bearing lengths. This failure modeis typically observed when a tensilefailure occurs. (Illustration F) Buckling ofejector pins is the second most common

mode of failure.

When small diameter pins are used,whether in steel or copper alloy,stepped ejector pins should be used formaximum resistance against bending.Euler's Formula for long and slendercolumns can be used to determine howlong an ejector pin can be in relation toits diameter. Most ejector pin failuresoccur, due to the column being slenderwhere bending or buckling actionpredominates over compressivestresses.

Ejector Rings, Bars and Air PoppetsWhen concerns regarding wear ofejector components are encountered,copper alloy stripper rings or bars canbe used. The ductility of the copperalloys, along with the low coefficient offriction between them and tool steelsmake them ideal candidates for thesemold components. Stripper ringsinserted into guided steel stripper platesdesigned with minimal clearance resultsin long maintenance-free operation, inthe event of the plate cocking or shifting

of the core, damage to the expensivemold component can be minimized withthe use of the copper alloys.

Additionally, the hard-to-cool area of thestripper plate

Illustration D: Bearing length ofejector sleeves should be no greaterthan two times the diameter.

contact surface is accommodated nicelyby the high thermal conductivity of thecopper alloys.

Ejector bars, similar to ejector rings butstraight, are being used to contact longwall sections and are replacing the

large number of small diameter ejectorpins commonly used. This concept,using the low friction properties of thealuminum bronze copper alloys,provides a robust means of ejecting onlarge surface areas. Fewer ejector pinimpression marks are encountered atshorter cooling and cycle times with theuse of ejector bars.

Head Clearances for Ejector Sleevesand PinsTo allow for any misalignment inmachining of the multiple plates in themold and to compensate for any

adverse thermal expansion, clearancesmust be provided for the ejectorcomponents in the mounting area. Thecounter bore depth in the ejectorretainer plate for the sleeve or pin headshould be .001 inch greater than theactual head dimension. This allows thefixed end of the column to seek itsproper alignment in relationship to thecorresponding hole in the mold core.

The head counter bore diameter shouldbe .015 inch larger than the sleeve orpin head diameter. Through clearancein the ejector retainer plate for the

sleeve or pin shank should be .005inch. These clearances will hold thehead end of the ejector componentsecure, yet allow enough movement tonot create binding when the device triesto seek its own location.

Clearances through the support and "B"plate should be .032 inch greater thanthe ejector component diameter. Eachplate leading edge should becountersunk with a 45 degree taper forease of assembly and to prevent

damage to the outside edge of theejector component. The mold should beassembled with the ejector plate sepa-rated from the ejector retainer plate.Each ejector sleeve or pin should beindi-


21/39

Illustration E: Copper alloy sprue pullerpin used to firm up puller and reducemolding cycles.

vidually positioned and loaded into itsproper location. The components,once committed to a location, shouldbe properly identified and alwaysreturned to that position.

Guided Ejector System

Every mold that utilizes small diam-eter ejector pins or is heavy enoughto cause the pins to flex should beequipped with a four post-guidedejector system. The most efficientsystems utilize hard surfacegrooveless leader pins and Ampco18 or Ampco 21 aluminum bronzebushings. The guides should belocated on the four corners of theejector system to provide the mostaccurate alignment of the compo-nents to the mold core. The objectiveis to remove any load from theejector components. Debates range ifthe leader pins should be installed inthe ejector housing or through thesupport plate. We prefer installing theguide pins in the support plate as thisallows the ejector retainer plate to beheld in position when the ejectorcomponents are loaded.In those instances when the supportplate will be subjected to greaterthermal expansion than the ejectorplate, additional clearances can beaccommodated in the fit of the bush-ings to the ejector and ejector retain-er plate. Placement of the guide pins

in the ejector housing removes theelement of thermal expansion fromthe equation, but makes it more diffi-cult to assemble the mold.

Ejector System ReturnEvery mold should have ejectorreturn pins to insure that the ejectorsystem has positively returned for thestart of the next cycle. The mostcommon method is using the stan-dard four return system found onevery standard mold base. This posi-tive method of ejector plate return,with the pin head resting on the

ejector plate and the tip located atthe parting line, only ensures fullreturn when the mold is closed.Frequently, it is desired to either takethe load off the return pins or assistin the return of the ejector system.Connecting the ejector system to themachine's hydraulic knock out plateswith an ejector rod is common. Whenthe machine ejector system returns,the ejector plate and ejector systemalso returns.

Many molds incorporatecompression springs to aid in thereturn of the ejector system. Foursprings, often located around thereturn pins, are used. Care must betaken not to over compress thesprings and cause premature failure.This method is not entirely fool proof.

The ejector system is not positivelyreturned after each cycle and shouldnever be used as the only means ofreturn if damage will result should anejector component not be returnedprior to mold closing. Ejector returnsprings should be replaced in sets,never individually to ensure that evenpressure is supplied against theejector plate.

When the ejector system must beabsolutely and positively returnedprior to the mold closing, early ejec-tor system mechanisms are used.

Small molds sometimes useinternally mounted early ejectorreturn systems. Medium and largemolds use externally mounted togglemechanisms to ensure that theejector plates have been positivelyreturned so they will not prevent themold from closing.We believe that having a limit switchor electrical signal to ensure thepositive return of the ejector systemis an important safety consideration.Using a switch alone, without theassistance of an early return system,can be dangerous and result in molddamage should a system electricalfailure or false signal occur.

Illustration F: Bearing length of ejectorpins is crucial to mold life.

AcknowledgementsThe Injection mold design guidelines were written by Dr. Paul Engelmann, AssociateProfessor, western Michigan university and Bob Dealey, Dealey's Mold Engineering,with the support of Dr. Dale Peters, for the Mold Marketing Task Group of theCopper Development Association. Kurt Hayden, graduate research assistant, WMU,generated the Illustrations. Research conducted by WMU plastic program students.

DisclaimerThese guidelines are a result of research at WMU and industry experience gainedwith the use of copper alloys In injection molding. While the information contained isdeemed reliable, due to the wide variety of plastics materials, mold designs andpossible molding applications available, no warranties are expressed or Implied in

the application of these guidelines.Contact informationInformation on copper alloys is available from the Copper Development Association,at 800-232-3282. Technical clarification of the guidelines can be made by contactingBob Dealey Dealey's Mold Engineering at 262-245-5800


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GuidelinesSIXTH IN A SERIES

Maximizing Performance using Copper Alloys

ejected correctly without harming thepart. Normally we think of theprocess as just cooling of the mold,but sometimes the mold is heated.Our ultimate objective is to controlmold temperature within a range thatyields a product within specificationsat acceptable cycle times.

Placement of Coolant Channels

Ideal placement of water channels incopper alloys will enhance analready good mold temperaturecontrol material. Good designpractice calls for the edge of thechannels to be placed two times thediameter of the channel away fromthe molds plastic forming surfaces,see Illustration A. This distance hasproven to be effective in providingenough support to preventdeformation of the molding surfaceand ideal for providing an even moldsurface temperature. Closerplacement to the plastic formingsurface could result in greatertemperature variation across themold surface by over-cooling areasin closer proximity.

The pitch, distance between coolantchannels, is also an importantdesign consideration. The recom-mended distance between thesechannels is two to five times thediameter of the coolant channel.

These recommendations haveproven effective in mold applicationsusing copper alloys. Frequently insimilar situations with molds builtfrom tool steels, therecommendations are to place thecoolant channels closer to the sur-face with reduced pitch distances.The superior thermal conductivity ofthe copper alloys allows greaterfreedom in channel placement.

ing plastic partdimensionalstability andrepeatability, crit-ical in three andsix sigma mold-ing, is to exposeeach and everycavity and mold-ing cycle toexactly the sameconditions. Themolding machineand/or processcontrols providethe ability tocontrol melttemperatures,screw recovery,injection ratesand pressures,cycle time andother parametersassociated

Cooling With Copper AlloysTypically the AA83, AA95 andAA940 copper alloys are used inplastic forming areas of moldsbecause of their high thermal con-ductivity and unique abilities toattain a more even molding surfacetemperature.

The key to obtaining and maintain-

Illustration A: Water channel placementshowing position between channel and edgeof cavity forming alloy.

with the process. Control of both the

mold surface temperature and thenthe range of these temperatures is aseparate and frequently overlookedprocess.

After cavity filling, mold temperaturecontrol is the single most importantfactor influencing dimensionalcontrol of the molded part. Allthermoplastics have to be cooledfrom their melt temperature to atemperature where they can be


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A fluid circulating pump withcapability of achieving turbulentflow rates is an important part ofthe equation. When using coldmold temperatures, typicallybelow 50 degrees F, closed sys-tems with mixtures of water andethylene glycol are typicallyused. These systems requirehigher horsepower motors toachieve the same flow rates aswater as the viscosity of the fluidchanges. Temperature rangesbetween 50 and 210 degrees Fusually use plain water.Processes over the boiling pointof water generally rely on oil andusually the mold is beingheated, even though the moldhas to cool the plastic to eject it.

Reynolds NumbersA method used in mold designdescribe the mold temperaturecontrol fluid flow in a mold, eitherlaminar or turbulent, is by adimensionless number. TheReynolds number takes intoaccount the pressure, volume

and viscosity of the coolant, theresistance to flow, length anddiameter of the channels and thepressure loss in the circuit.Laminar flow in a plastic mold,described by Reynolds numbersbelow 2,000, indicates conditionswhereby heat is not efficientlytransferred from the channel wallto the circulating media.Turbulent flow, Reynoldsnumbers above 5,000, describeconditions where efficient transfer

of heat is made from the coolantchannel wall to the circulatingmedia. Heat transfer duringturbulent conditions can be asmuch as three to five timesgreater than with laminar flow.Numbers falling between 2,000and 3,500 describe a transitionphase and typically is ineffectivein closely controlling mold surfacetemperatures. A simplifiedformula for determining theReynilds number for systems

using water appears in InjectionMolding Handbook,

edited by Dominick V. Rosatoand Donald V. Rosato. It takesinto account the fluid velocity infeet per second times thediameter of the coolant passagetimes a constant of 7740 dividedthe viscosity of water. Waterviscosity changes astemperatures increase. At 32Fthe viscosity of water is about 1.8centistokes, at 100F it haschanged to about 0.7 and at200F about 0.3. This explainswhy, on occasion, increasingcoolant temperature reduces partwarpage and cycle time. Lessonslearned in production moldinghave shown that with the use ofcopper alloys higher coolanttemperatures can be used,reducing sweating of the moldand supply lines, while producinga better part at lower cycle times.

Normally mold cool programs areused to analyze effectiveness ofheat transfer in the mold due tonumber of variables affecting thecalculation. While better coolingis achieved with higher Reynolds

numbers, a point of diminishingreturns will be reached. When thecirculating media has thecapability of removing heat fasterthan the plastic will give it up,which is typically the case withthe proper application of copperalloys, energy in cooling orheating and pumping thecirculating media is wasted.Correctly designed coolantsystems are important factors inobtaining fast and economical

cycle time. The higher thermalconductivity of the copper alloysallows more freedom in thisdesign over traditional tool steels.

An effective method of testingexisting mold temperature controlsystems is to remove an exit lineand measure the coolant flowthrough that circuit. The followingtable lists the flow nominal size(pipe), drilled whole diameter andthe minimum water flow required

insuring turbulent flow.

Illustration B: Baffle in series coolantcircuit, positioned to force flow up andover baffleand not around.

Ilustration C: Bubbler in parallelcoolant circuit. Area of center of tubeshould equal area of return.

PipeSize

DrilledChannelDiameter

Min. Flow (gal/min)

1/16-IMT 1/8-NPT1/4-NPT3/8-NPT1/2-NPT

.250

.3125

.4375 .562

.6875

.33 .44 .55 .75 1.3


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Chill PlatesEarlier injection mold design guide-lines describe the effective use of achill (temperature control) platemade from the same copper alloyto insure the same thermalconductivity. Testing at westernMichigan university has proven theeffectiveness of cooling multiplesmall cores that have smalldiameters preventing waterpassages. It is necessary that the

core pin heads be firmly seatedagainst a clean and oxidation freeplate surface to insure efficienttransfer of heat.

Temperature control channelswith BafflesChannels that divert temperaturecontrol fluids from one level toareas where heat is concentrated inthe mold can use baffles,Illustration B, to positively direct theflow through the channel. This type

of coolant direction is referred to asseries flow when multiple bafflesare used. Proper mold design startswith the diameter and area of theinlet channel. The hole for thebaffle, after taking the areaoccupied by the baffle into account,must be twice the area of the inletchannel, to prevent flow restrictionsand high-pressure losses.Remember when calculating flowchannels that twice the area is notthe same as twice the diameter.

Brass baffle and pressure plugs,which resists the build up of waterdeposits, work best in copperalloys. Most standard off the shelfbaffles use a dry seal design,where standard pipe taper is 3/4inch/foot, the dry seal designfeatures 7/8 inch/foot taper. Toprevent high hoop stresses on thecopper alloys straight threadpressure plugs must be usedinstead of either tapered or dry seal

pressure plugs.Another important consideration isthe clearance area between the tipof the baffle and the drilled hole.General design practice is to allowthe same gap as the diameter ofthe baffle hole. Make sure that thebaffle is installed at a 90 angle tothe flow of the coolant to positivelyforce the flow up and over thebaffle. Otherwise leakage aroundthe baffle will result in inefficientcooling. An effective method is to

braze the baffle blade to thepressure plug and mark the outsideof the plug with a line indicating theblade orientation. Check to insurethat the blade is properly positioned

when the pressure plug is tight.

As temperature control fluid flow ispositively directed through eachchannel, care must be taken toinsure that the outlet temperaturedoes not exceed the inlettemperatures by more than 3to5F. High temperature differentialsbetween individual cavities or theirmold sections results in undesirablepart consistency. Therefore, seriescircuits typically have a maximum ofsix to eight baffles.

Spiral baffles are useful in longslender cores as the coolant flowsaround the baffle, exposing thediameter of the coolant channel tomore even temperatures than whatcould result from having up oneside and down the other side of acore. Incorrect assumptions havebeen made that spiral baffles create

turbulent flow, the fact is thatspiraling water does not createturbulent flow or result in higherReynolds numbers, by the fact thatthe coolant is turning.

Temperature Control Channelswith Bubblers

Bubblers are also used to stepcoolant into areas of the mold thatrequire heat removal. The major dif-ference between the bubbler andthe baffle is that water flows up atube in the center of the coolantchannel and cascades down theoutside to the outlet, Illustration C.These cooling circuits, when morethan one bubbler is used are calledparallel circuits. The inlet has tohave greater volume than the sumof the bubbler internal diameters toinsure that each circuit will have thesame flow ratesDesign of thecoolant channel and the bubbler isimportant to successful moldtemperature control. The area ofthe internal diameter of the bubblertube, D2 must be exactly the sameas D3 to insure that high-pressurelosses are not encountered. Criticalto the calculation is determining thebubbler wall thickness, D1 and thearea it occupies. The coolant inletmust feed the bottom of the bubblertube. The outlet for the coolant isaround the outside diameter of thebubbler tube. Each mold coolantchannel inlet and outlet must beclearly marked to insure thatoutside connections are correctlymade, insuring the proper flow.Excessive looping can result inhigh-pressure losses with these cir

Illustration D: Recommended straighttreaded pressure plug and steal.

cults and must be avoided toachieve optimum mold cooling.

Drilling and PluggingCoolant ChannelsLong coolant channels are typicallygun drilled in mold plates, cavities

and cores. Typically, even withaccurate gun drilling, the hole canwander and the tolerance of holelocation is normally understood tobe .0.001 per inch of length.Smaller diameter drills tend towander more than larger diameters.Care must be taken when coolantlines pass close to holes in themold and adequate clearancesmust be allowed to prevent breakthough or leaving a weak section ofthe mold. With copper alloys the

minimum recommended distance isapproximately .100", dependingupon coolant diameter, distancefrom drill start and the size andlocation of the cross hole. Coolantchannels should not run parallel orin close proximity with sharp cavitycorners to guard against prematurefailure.

The coolant channels, illustration D,should be blocked with a fabricatedstraight threaded brass plug toavoid excessive hoop stresses onthe copper alloys. An effectivemethod in leak prevention is tocounter bore the plug hole and thenuse an o-ring installed incompression. The O-ring should bereplaced each time the plug isremoved or at major moldmaintenance cycles. Cross-drilled

ampco_mold_design_guidelines.pdf

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