Mold Design - Copper

Primary Runner Sizing Calculation02 0 1 L ( TAN 2.386 )Illustration II, Sprue taperPressure in Sprue *- - 3/32. 5/32. 7/32-9/322000 t--t----t---::;. ., r---;!I!1600II.600 ------b-" --- -- ------ - i ,- t-F -t ::l H- - 11132t--J:::§::F ;;:;;:: t-13 For Polystyrene4Length of SprueIllustration Ill, Pressure, Sprue Length-lllliiiiii;;::-, Illustration IV,Anti- rotation screwAER pER Efficiency RatioA Area of runnercross sectionP Periphery of runnercross sectionIllustration v,Formularunner systemSprue Bushing TaperTo aid in the removal of thesprue from the bushing, ataper of one-half inch per footis normally used in injectionmolding. Calculate the sprueorifice at the parting line face ,multiply the tangent of thetaper angle times the length,plus the "o 2 " . Knowing thisdimension, informed decisionscan be made on primary runner sizing.The sprue frequently controlsthe molding cycle when largerorifice conventional steelsprue bushings are used. Theapplication of a copper alloysprue bushing cools the spruemore quickly and efficiently,allowing the molding cycle tobe controlled by the piecepart.Pressure loss is high in thesprue. This is the only place inthe feed system where thechannel progresses from asmaller area to larger.Frequently, smaller orifices areused on long sprue bushingsin an effort to reduce themass. This results in extremely high injection pressure losses, making the part hard to fill .The chart in illustration 111 is aguide for determining theeffect that the specific "O"dimension has on the pressure required to deliver plastic through the length of asprue. Note that the difference between a 3/32 and9/32-inch sprue is about 1,000PSI over a short sprue andalmost 1,500 pounds on a longsprue.Using a copper alloy spruebushing allows for anincreased size orifice, thusreducing pressure loss whilemaintaining reasonable coolingtimes.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.Also, a sprue bushing that isnot keyed 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 theuse of a cap screw as illustrated in figure IV.Sprue FitHeat must be transferred fromthe sprue through the copperalloy sprue bushing to the moldplates. Interference fit is recommended for optimum cooling. The bore through the "A"plate should be nominal size toplus .0005 inches with a surfacefinish of at least 16 RMS. Theshank of the sprue bushingshould be the nominal size,plus .0005 to plus .001 inches.standard Sprue BushingAvailabilityCopper alloy sprue bushingswith patented stainless steelnozzle seats are commerciallyavailable. An insulatorbetween the nozzle and sprueis beneficial in controlling theflow of heat from the nozzleto the sprue. Special spruebushings may be constructedto suit using standard 112 inchper foot sprue bushingtapered drills and reams.Sprue bushings with tapers ofup to 3/4 inch per foot havebeen used for difficult toremove plastics. Care must betaken to insure that the taperis draw polished and free fromundercuts or rough surfacesthat could hinder sprueremoval.Conventional Injection MoldRunner SystemsThe shape of the runner, fullround or trapezoidal, or otherconfiguration , is dictated bymold design. The most efficient runner cross section isfull round. The efficiency of therunner cross section can becalculated with a formula,Figure V, the larger the ratiothe better.

Runner BarInjection Mold Runner BarsRunner systems for high cavitation molds normally have largerdiameters due to runner balancing. The runner system extendsthe molding cycles as heat isslowly transferred from the thickplastic to steel mold plates.Inserting copper alloy runner barsin the mold "A" and "B" plates.cooling the runner faster, is beneficial. in reducing the overallmolding cycle.Runner SizingRunner sizing is dependent onmany things, including: plasticmaterial; part size, weight andwall thickness; molding machinecapabilities and processing parameters and, the number andplacement of the cavities.Each mold is unique and thedesigner must consider all parameters and options available on anindividual case by case basis.several mold design software packages are available, including MoldFlow and C-Mold, which addresssizing of the runner system.one method of runner sizing andbalancing used by mold designers· starts at the sprue and thenworks toward the gate. Otherdesigners start with the part wallthickness and work back to thesprue outlet orifice. The normallyrecommended procedure is that,in the direction of plastic flow.the runner area always goes fromlarger to smaller. Never from asmaller area to a larger area .When the primary runner diameter is known. the sum of the areasof the multiple connecting runners must be equal or smaller inarea than the preceding runner.When working back from the part.some designers size the final runner channel size !that runnerwhich feeds the gatel to equal thethickest wall section in the part.Each runner intersection then is afunction of the area of that runner times the number of connecting runners, usually two.Therefore. the area of theupstream runner is always at leastequal to or larger in area than thesum of the branches. Note that arunner with one-half the area isnot the same as a runner of onehalf the diameter.StraightThreaded., Formulas for calculating the areaof the runner:Runner Bar MatingBest results are obtained byFull Round RunnerA 0.7854 d2Trapezoidal RunnerA (W1 W2l h/2A area, d diameter, I length,w width. h heightdesigning and building the runner bars to have zero to negativecontact with each other whenthe mold is closed. This will prevent any deformation on theparting line surfaces that couldresult from high clamping pressures exceeding the compressivestrength of the alloy. To accomplish this, the "A" and "B" runnerbars should be flush to minus .001inch on each side of the mold.This allows the mold base and/ orcavity and core inserts to receivemachine clamp force, not therunner bars.care must be taken to understand the characteristics of theplastic being molded and clearance should be short of allowingthe runner system to flash .Additionally, it is important toinsure that the mating halves ofthe runner system are in perfectalignment. with no mismatch atthe parting line, to maximize plastic flow efficiency.Runner Bar CoolingThe runner system must nevercontrol the molding cycle. Toinsure proper temperature controlof the runner bars. cooling channels should be placed directly intothe both the "A" and "B" sideinserts. The cross-drilled holesshould be blocked with a plug containing an "O" ring and a straightthread plug. Due to the high thermal conductivity of the copperalloys and the tendency to thermalcycle rapidly, tapered thread systems must be avoided in the copper alloys to prevent cracking.With the increased cooling rate ofthe copper alloys and properIllustration VI, Runner BarsIllustration VIIcooling arrangements. largerdiameter runners can be used in amold equipped with copper alloyrunner bars. Almost withoutexception, runner diameters oneor two sizes larger can be set upquicker with the copper alloys,over traditional mold steels.Sprue PullerA reverse taper sprue puller, 3 forstiffer materials and 5 for flexiblematerials, is recommended toinsure sprue removal. To rapidlycool the undercut machine. thepuller directly into the runnerbars or a copper alloy insert.Illustration VII gives more details.AcknowledgementsThe injection mold design guidelines werewritten by Dr. Paul Engelmann , AssociateProfessor, western Michigan University andBob Dealey, Dealey's Mold Engineering , withthe support of Dr. Dale Peters, for the MoldMarketing Task Group of the copperDevelopment Association . Kurt Hayden ,graduate research assistant, WMU , generated the Illustrations. Research conducted byWMU students in the plastics program .DisclaimerThese guidelines are a result of research atWMU and industry experience gained withthe use of copper alloys in injection mold ing. While the information contained isdeemed reliable , due to the wide variety ofplastics materials, mold designs and possi ble molding applications available, no warranties are expressed or implied in theapplication of these guidelines.Contact InformationInformation regarding copper alloys formolds and molding is available from theCopper Development Association,1-800-232-3282.

Injection Mold DesignGuidelinesMaximizingPerformance UsingCopper AlloysThe Injection Mold CoreA mold core is any member thatforms the interior of a plasticpart. usually on the "B" side ofthe mold parting line. Moldcores can be machined from asolid piece of copper alloy orinserted to aid in construction orallow for easierreplacement if a component would ever bedamaged in molding.Picture of Whirlpool mold: Acore built from acopper alloy for a large dishwasher partThis picture shows alarge copper alloycore,about24incheslong and seven incheshigh, used to mold aPVC bezel for aKitchen Aid dishwasher manufactured byWhirlpool corporation .Findlay Ohio. The cop per alloy was specifiedprimarily to eliminatewarpage on the part.which is both functional and esthetic innature. The cycle time advantageof about 20% by using the highthermal conductive copper alloywas an added bonus to theimproved part quality, which wasthe main objective.Properly designed molds withcopper alloys used in strategiclocations. usually the core, haveproven to reduce injection molding cooling cycles by 20 to 50 percent. The mold core is responsible for removing from 65 to 75per cent of the heat from theBy Dr. Paul Engelmannand Bob Dealeyfor the Mold MarketingTask croup of the CopperDevelopment Associationplastic molding due to the material shrinking around the standingfeatures of the mold.copper alloys have adequatehardness levels to hold up againstnormal injection pressures foundin conventional injection moldingmachines. The normal press !positive interference, or crush is notused due to the higher ductilityof copper alloys and to avoid anypeening or hobbing at shut offsand at the parting line. Ratherzero to negative press is recommended. Negative press or clearance of the mating componentsmust obviously be less than thatwhere plastic will flash . Hardnesslevels of the copper alloys andmold steels are listed in the following chart:Hardness LevelsCopperAlloyHardness SteelAlloyHardnessC1720041 Re96 RbH·13P-2038/ 52 ReC17510C1800094 Rb420 SS27 -50 Re28-48 ReThe copper alloys normallyselected for mold cores. corepins. inserts. slides and raisingmold members are; C17200 a highhardness beryllium-copper alloy;C17510 a high conductivity beryl l

tion molder, mold designer and mold builder. The informa tion contained in the guide lines will maximize the mold's cycle time and improve part quality with the use of copper alloys in the mold. The articles will begin in the May issue. These information packed Injection Mold Design Gu