Cooling With Copper AlloysTypically the C17200, C17510 andC18000 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-ing plastic partdimensional sta-bility 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 parame-ters associated

with the process. Control of boththe mold surface temperature andthen the range of these tempera-tures is a separate and frequentlyoverlooked process.

After cavity filling, mold tempera-ture control is the single mostimportant factor influencing dimen-sional control of the molded part.All thermoplastics have to be cooledfrom their melt temperature to atemperature where they can be

ejected correctly without harmingthe part. 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 rangethat yields a product within specifi-cations at acceptable cycle times.

Placement of Coolant ChannelsIdeal placement of water channelsin copper alloys will enhance analready good mold temperaturecontrol material. Good designpractice calls for the edge of thechannels to be placed two timesthe diameter of the channel awayfrom the molds plastic formingsurfaces, see Illustration A. Thisdistance has proven to be effectivein providing enough support toprevent deformation of the mold-ing surface and ideal for providingan even mold surface temperature.Closer placement to the plasticforming surface could result ingreater temperature variationacross the mold surface by over-cooling areas in 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 applica-tions using copper alloys.Frequently, in similar situations withmolds built from tool steels, therecommendations are to place thecoolant channels closer to the sur-face with reduced pitch distances.The superior thermal conductivityof the copper alloys allows greaterfreedom in channel placement.

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Injection Mold Design

GuidelinesMaximizingPerformance UsingCopper Alloys

SPECIAL ADVERTISING SECTION

By Dr. Paul Engelmannand Bob Dealey

for the Mold MarketingTask Group of the CopperDevelopment Association

Illustration A: Water channel placement showing positionbetween channel and edge of cavity forming alloy.

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A fluid circulatingpump with capabilityof achieving turbulentflow rates is an impor-tant part of the equa-tion. When using coldmold temperatures,typically below 50degrees F, closed sys-tems with mixtures ofwater and ethyleneglycol are typicallyused. These systemsrequire higher horse-power motors toachieve the same flowrates as water as theviscosity of the fluidchanges. Temperatureranges between 50 and210 degrees F usuallyuse plain water.Processes over theboiling point of watergenerally rely on oiland usually the mold isbeing heated, eventhough the mold hasto cool the plastic toeject it.

Reynolds NumbersA method used in molddesign to describe themold temperature con-trol fluid flow in amold, either laminar orturbulent, is by adimensionless number.The Reynolds numbertakes into account thepressure, volume andviscosity of the coolant,the resistance to flow,length and diameter ofthe channels and thepressure loss in the cir-cuit. Laminar flow in aplastic mold, describedby Reynolds numbersbelow 2,000, indicatesconditions whereby

heat is not efficiently transferredfrom the channel wall to the cir-culating media. Turbulent flow,Reynolds numbers above 5,000,describe conditions where effi-cient transfer of heat is madefrom the coolant channel wall tothe circulating media. Heat trans-fer during turbulent conditionscan be as much as three to fivetimes greater than with laminarflow. Numbers falling between2,000 and 3,500 describe a transi-tion phase and typically is inef-fective in closely controllingmold surface temperatures.

A simplified formula for deter-mining the Reynolds number forsystems using water appears in

Injection Molding Handbook,edited by Dominick V. Rosato andDonald V. Rosato. It takes intoaccount the fluid velocity in feetper second times the diameterof the coolant passage times aconstant of 7740 divided the vis-cosity of water. Water viscositychanges as temperaturesincrease. At 32°F the viscosity ofwater is about 1.8 centistokes, at100°F it has changed to about 0.7and at 200°F about 0.3. Thisexplains why, on occasion,increasing coolant temperaturereduces part warpage and cycletime. Lessons learned in produc-tion molding have shown thatwith the use of copper alloyshigher coolant temperatures canbe used, reducing sweating ofthe mold and supply lines, whileproducing a better part at lowercycle times.

Normally mold cool programsare used to analyze effectivenessof heat transfer in the mold dueto number of variables affectingthe calculation. While bettercooling is achieved with higherReynolds numbers, a point ofdiminishing returns will bereached. When the circulatingmedia has the capability ofremoving heat faster than theplastic will give it up, which istypically the case with the properapplication of copper alloys,energy in cooling or heating andpumping the circulating media iswasted. Correctly designedcoolant systems are importantfactors in obtaining fast and eco-nomical cycle time. The higherthermal conductivity of the cop-per alloys allows more freedomin this design over traditionaltool steels.

An effective method of testingexisting mold temperature con-trol systems is to remove an exitline and measure the coolantflow through that circuit. Thefollowing table lists the flownominal size (pipe), drilled wholediameter and the minimumwater flow required insuring tur-bulent flow.

Illustration B: Baffle in series coolant circuit, positioned to forceflow up and over baffle and not around.

Illustration C: Bubbler in parallel coolant circuit. Area of center oftube should equal area of return.

Pipe Size Drilled Min. FlowChannel (gal/min)Diameter

1/16-NT .250 .331/8-NPT .3125 .441/4-NPT .4375 .553/8-NPT .562 .751/2-NPT .6875 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 alloy toinsure the same thermal conductivi-ty. Testing at Western MichiganUniversity has proven the effective-ness of cooling multiple small coresthat have small diameters prevent-ing water passages. It is necessarythat the core pin heads be firmlyseated against a clean and oxidationfree plate surface to insure efficienttransfer of heat.

Temperature Control Channels withBafflesChannels that divert temperaturecontrol fluids from one level to areaswhere heat is concentrated in themold can use baffles, Illustration B,to positively direct the flow throughthe channel. This type of coolantdirection is referred to as series flowwhen multiple baffles are used.Proper mold design starts with thediameter and area of the inlet chan-nel. The hole for the baffle, after tak-ing the area occupied by the baffleinto account, must be twice the areaof the inlet channel, to prevent flowrestrictions and high-pressure loss-es. Remember when calculatingflow channels that twice the area isnot the same as twice the diameter.

Brass baffle and pressure plugs,which resists the build up of waterdeposits, work best in copper alloys.Most standard off the shelf bafflesuse a dry seal design, where stan-dard pipe taper is 3/4 inch/foot, thedry seal design features 7/8inch/foot taper. To prevent highhoop stresses on the copper alloysstraight thread pressure plugs mustbe used instead of either tapered ordry 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 of thebaffle hole. Make sure that the baffleis installed at a 90° angle to the flowof the coolant to positively force theflow up and over the baffle.Otherwise leakage around the bafflewill result in inefficient cooling. Aneffective method is to braze the baf-fle blade to the pressure plug andmark the outside of the plug with aline indicating the blade orientation.Check to insure that the blade isproperly positioned when the pres-sure plug is tight.

As temperature control fluid flow ispositively directed through eachchannel, care must be taken to

insure that the out-let temperaturedoes not exceed theinlet temperaturesby more than 3°to5°F. High tempera-ture differentialsbetween individualcavities or theirmold sectionsresults in undesir-able part consisten-cy. Therefore, seriescircuits typicallyhave a maximum ofsix to eight baffles.

Spiral baffles are use-ful in long slendercores as the coolantflows around thebaffle, exposing thediameter of the coolant channel tomore even temperatures than whatcould result from having up one sideand down the other side of a core.Incorrect assumptions have beenmade that spiral baffles create tur-bulent flow, the fact is that spiralingwater does not create turbulentflow or result in higher Reynoldsnumbers, by the fact that thecoolant is turning.

Temperature Control Channels withBubblersBubblers 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 the out-side 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 rates.

Design of the coolant channel andthe bubbler is important to success-ful mold temperature control. Thearea of the internal diameter of thebubbler tube, D2 must be exactly thesame as D3 to insure that high-pres-sure losses are not encountered.Critical to the calculation is deter-mining the bubbler wall thickness,D1 and the area it occupies. Thecoolant inlet must feed the bottomof the bubbler tube. The outlet forthe coolant is around the outsidediameter of the bubbler tube. Eachmold coolant channel inlet and out-let must be clearly marked to insurethat outside connections are cor-rectly made, insuring the properflow. Excessive looping can result inhigh-pressure losses with these cir-

cuits and must be avoided toachieve optimum mold cooling.

Drilling and Plugging CoolantChannelsLong coolant channels are typicallygun drilled in mold plates, cavitiesand cores. Typically, even with accu-rate gun drilling, the hole can wan-der and the tolerance of hole loca-tion is normally understood to be.0.001 per inch of length. Smallerdiameter drills tend to wander morethan larger diameters. Care must betaken when coolant lines pass closeto holes in the mold and adequateclearances must be allowed to pre-vent break though or leaving a weaksection of the mold. With copperalloys the minimum recommendeddistance is approximately .100",depending upon coolant diameter,distance from drill start and the sizeand location of the cross hole.Coolant channels should not runparallel or in close proximity withsharp cavity corners to guard againstpremature failure.

The coolant channels, Illustration D,should be blocked with a fabricatedstraight threaded brass plug to avoidexcessive hoop stresses on the cop-per alloys. An effective method inleak prevention is to counter borethe plug hole and then use an O-ringinstalled in compression. The O-ringshould be replaced each time theplug is removed or at major moldmaintenance cycles. Cross-drilledconnecting channels should havethe drill point run out in the con-necting channel, avoiding stress ris-ers.

Series and Parallel ChannelsCoolant channel placement has tobe considered and engineered intothe mold design from the onset.Efficient mold temperature control

Illustration D: Recommended straight threaded pressure plug and seal.

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has to have the same priority in amold design as gating and partejection; it cannot be an after-thought. Coolant circuits canloop inside the mold with con-necting channels or outside themold with external connections.

When the mold design calls forseries internal looping, and thecoolant could flow in more thanone direction, flow must be posi-tively routed in the desired chan-nel. A number of diverter plugsare commercially available toblock the unused channels anddirect flow though the designat-ed route. However, a simple andrecommended method is tomachine a brass plug .003/.005smaller than the coolant channelwith the plug length twice thediameter, Illustration E. The plugshould be inserted into the chan-nel with a light press fit to insureit remains firmly in the correctposition. The location should bemeasured by inserting a rod intothe coolant channel and theentire circuit water tested toinsure that proper routing with-out restriction has been achieved.

O-Rings for Sealing CoolantChannelsO-rings, when placed in com-pression, have proven to be themost effective method of provid-ing a seal between two joiningcomponents in a mold. They areplaced between mating compo-nents when baffles or bubblersare used. Additionally, O-ringsare placed between cavity andcore components when coolantis directed through the "A", "B"and support plates. The O-ringmaterial type must have a com-patible temperature rating with-in the range of the coolant and

mold operation temperature. Onoccasion a reaction can occurbetween copper alloys, in thepresence of water and/or otherfluids and certain mold steelswhere corrosion or pitting couldtake place. To prevent the possi-bility of this electrolytic actiontaking place, the copper alloy canbe chromium or nickel-plated inthe O-ring area. The objective isto prevent direct contactbetween the two materials byusing a third compatible materi-al. If the copper alloy compo-nent will have a coating or plat-ing applied to the molding sur-face anyway, covering the wholecomponent will normally suffice.A separate or different coatingfor the O-ring area should not benecessary.

All Water Lines are Not StraightThrough Mold ComponentsThe design and routing ofcoolant channels can be chal-lenging, especially in moldcores. Straight through drilledpassages are not always possi-ble due to mold configuration,mounting and ejector pin holesand other obstacles.Machining or drilling channelsthat intersect and directcoolant flow to the desiredlocation, Illustration F, shouldbe considered. The use of inno-vative design methods, includ-ing baffles and bubblers, toinsure proper mold tempera-ture control is achieved payshandsome rewards in obtainingan efficient running mold.Coupling these design princi-ples with the use of copperalloys and their superior ther-mal conductivity provides thebest opportunity in achievingoptimum molding conditions ■

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For more information about the use of copper alloys in tooling, please write in 674 on the reader service card.

Illustration E: Looped water circuit with diverter.

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 con-tained is deemed reliable, due to the wide variety of plastics materials, molddesigns and possible molding applications available, no warranties are expressed orimplied 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

Illustration F: Intersection drilled coolant channels in hardto reach locations.