Mold Side-Actions: How, Why and When They Work

1 As advances in technology have drivenmarket globalization and shortenedproduct life cycles, mold design andproduction methods have been pressured tokeep pace. The success of moldmakers inthe future will be defined by their ability todiscern the advantages and disadvantages ofthe ever-broadening options. With a fundamental understanding ofthese various side - action methods, mold -makers can choose and apply the optimalsolutions to today s applications. Solutionsmust benefit all of the players involved inthe process designers, moldmakers,molders and product manufacturers. Withdemands for improved quality, lower costs,shorter production times and increased partcomplexity, the need for a deeper under-standing by all participants is becomingparamount.

2 The need to think outside ofthe box and embrace advanced techniqueshas never been Fit Actions Cam Pin MethodThe most basic and familiar of all side -actions employs an angled pin to movethe core with heel block backup duringinjection. In the ideal mold , the com-ponents form a perfect fit when the moldis closed, with the desire that the com-ponents do not move or change shapeduring injection(see Figure 1). The campin method, however, has a fundamentallimitation in that steel is compressibleand flexible. Because the load forces in a tool duringinjection can be very large relative to themodulus of steel, assumptions of rigidityand incompressibility are not with a perfect fit, during injection,plastic pressure is applied to the core faceand the metal is flexed, side loaded orcompressed toward the heel block,resulting in slide face backup.

3 The degreeof the side loading, flex and compressionwill determine the resulting flash andother issues. The magnitude of these problems is afunction of the variation in injectionpressure and core/ mold geometry. Locationof stops, size of stops, guides, core length,shutoff area, etc. will all affect the amountof these errors. To understand how thecompressible nature of steel impacts thecam pin method, the engineering relation-ships of stress and strain on cores must bereviewed. For more detail on compressionof steel in general see Ruins the FitEngineering calculations for materialsoften focus on yield strength, with littleattention to change in shape.

4 Becauseinjection forces are relatively high andsmall deviations in materials leads to poorpart quality, attention to how steel changesshape under load is critical to good molddesign. While the mold is machined,assembled and spotted cold, it would bemore correct to manufacture the steelcomponents at operating temperature andunder the anticipated injection forces. In mold applications, the known factorsare often the injection pressure (P), core areaexposed to plastic (C), length of core (L), themajor diameter area of core (A), and materialproperty modulus (E). What is unknown isthe amount of deflection (D) of the core injection forces are relatively high and small deviations inmaterials leads to poor part quality, attention to how steelchanges shape under load is critical to good mold Side-Actions: How, Why and WhenThey WorkUnderstanding the effects of injection on the core, slide and associatedcomponents is critical to selecting the best side - action methods for a givenapplication.

5 This first of two articles will discuss the basic physics underlyingall side -actions as well as the fundamentals of side - action performance. Mark ScanlanMOLDDESIGNF igure 1 Cam pin action with courtesy of John M. Technology September, 2001 1To calculate the deflection for a given force,the FLEA formula can be = P x C D = (F x L)/(E x A)Example:Given a 10" long core pin with a "diameter major diameter (area of 1 ), acore face exposed to plastic of .5 aninjection pressure of 10,000 ,theparameters are:P = 10,000 = .5 = P x C = 5,000 = 10 inchesE = 30,000,000 = 1 = FL/EA = .0017 can be seen, a moderate size core areaof.

6 5" (half the pin area) can move " under a moderate injection pressure of10,000 psi. Note that this corresponds to amachine hydraulic line pressure of approxi-mately 1,000 psi1x 10 for the face area equal to the pin diameter (1"2),deflection would be approximately .004"due to pin compression alone. Changingmaterial to a glass-filled nylon could doubleinjection pressure to 20,000 psi, resulting " factors such as flex, thermalshrinkage and timing must be added tocompression to determine overall deviationfrom the desired shape. Of course there aremany other concerns for the componentsthemselves.

7 A short, yet informative, descrip-tion of slide design and other issues can befound in section of Menges andMohren s How to Make Injection of forces on the pin, most favoredangles, wear on heel blocks, recommendedslide tapers, concern for thermal effects anddelayed lifter designs are Simple ChartA simplification assumes constant corecross sectional area, core exposed areaequals core slide cross-section area and con-stant material modulus. Thus a plot can bemade of deflections for various pressuresand length (see Figure 2).Close inspection of the data below 20,000psi shows that all cores greater than 3" inlength have at least a.

8 001" compression(core deflection). For greater pressuresapproaching 20,000 psi, any core length willresult in some compression beyond .001". Thus it has been demonstrated that for thelimiting case where the core cross sectionalarea approaches exposed area, generally allcaptured cores with perfect size-on-sizetiming will demonstrate at least .001"deflection or backup of the core face duringinjection. In most cases, for common corelengths between 6" to 10" and injectionpressures of 10,000 to 15,000 psi (1,000 to1,500 hydraulic line pressure),thedeflection during injection will be " to.

9 005". Note that this movement isstrictly due to compression alone and mustbe added to the core movement due to flex,timing error, mismatches and thermalcontraction. Without getting involved in estimates ofcost or timesavings of various methods, thetraditional cam pin system can usually bedescribed as providing the following opera-tional and performance advantages in pro-duction: It is intrinsic to the mold itself requiring no external setup, hookups oradjustments by mold setup personnel oroperators. It has unlimited not appropriate for a variety ofcomplex applications which can be listedas disadvantages relative to other methodsto be discussed in Part II a few disadvan-tages are: It is complex to manufacture.

10 It has a one-off design. Flash or inconsistent parts occur due tocompression under note should be made here regarding theuse of hydraulic cylinders with heel blocks(see Figure 3). While the use of standardhydraulic cylinders can improve or helpsolve other problems with cam pin actions,they react in the same manner as a cam pinsystem during injection. Core compressionis a function of the length of the core to theheel block and is the same for any methodusing a heel block. The cylinder simply re-places the pin as another means to move thecore in and out when the mold is ,00015,00010,0005,000012345678 Core length in inchesInjection pressure (psi)9101112131415 Deflection of core in inchesD=(PxL) ".