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. 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.

2 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. 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.

3 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. 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.

4 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 .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.

5 Of course there aremany other concerns for the componentsthemselves. 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 .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.

6 001"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 .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.

7 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) ".003".002".001".005"Figure 2 Constant core deflection vs. core length and courtesy of Mark 3 Cylinder with heel block with cylinder-only application is noteworthy in that the core is stillcompressed under load, but the compression is done by thecylinder toward the core face prior to Technology September, 2001 Compression Fit Actions Cylinder OnlyMethodUsing the hydraulic cylinder to positionthe core with the heel block to hold it and using it to both position and hold thecore are two completely different applica-tions.

8 In the cylinder only method, while thecore may have to be set prior to closing themold due to a design issue, most often it isset and pulled with the mold closed (seeFigure 4). During injection, full hydraulicpressure is maintained on the cores to pre-vent full hydraulic pressure duringinjection is not commonly available andusing standard hydraulic cylinders to move,hold and compress the cores duringinjection is not normally a viable option the physics of operation and the advantagesprovided by such a method are instructive toother methods discussed later. There may bespecial circumstances where use of thismethod, despite its drawbacks, provides thebest choice for the Is the FitA hydraulic cylinder is used to hold thecore during injection by ensuring that theforce from the cylinder rod exceeds theforce on the slide due to plastic pressure. Todetermine the expected force on the slide(F), the known factors of maximuminjection pressure (P) and core area exposedto plastic (C) are = P x CThe cylinder force must exceed the slideforce to prevent movement of the core force from a standard cylinder (M) is afunction of the piston area based on borediameter (B) in inches and suppliedhydraulic pressure (H) in = H x ( x B2)/4 Using the same data from the cam pinexample, force on the core is:P = 10,000 =.

9 5 = P x C = 5,000 an available hydraulic pressure of2,000 psi and a 2" bore cylinder, the forceavailable from the cylinder is:M = 2,000 x ( x 2 x 2)/4 = 6,280 that if the available hydraulicpressure was only half or 1,000 psi theforce available from the cylinder would be:M = 1,000 x ( x 2 x 2)/4 = 3,140 the first case, the force of the cylinderis greater than the force of the plastic andthe core face will not move. In the secondcase, the plastic force exceeds the cylinderforce and the core will move substantiallyuntil the shot volume is expended. Thisillustrates the problem of hydraulic pressuredropping out or decreasing during plasticinjection. Without hydraulic pressure, thecylinder force is zero. If the above formulae are set equal andsolved for bore size, the minimum cylinderbore diameter can be determined. At theminimum bore size, M = = SQRT[(4 x P x C)/(H x )] Figure 5shows the minimum bore sizeneeded for various exposed areas andinjection pressures at 1,000 psi hydraulicon the cylinder.

10 Before using this method,verify that full hydraulic pressure isavailable during injection and then verify itagain. Even if you are lucky enough tohave full injection pressure available, notethat it doesn t take much core area (aboutone square inch) to require a 4" borecylinder at a minimum. A three-square-inch area gets you quickly to a 6" bore. Fora detailed discussion of the relationshipbetween press line injection pressure andactual nozzle injection pressure, reviewMenges and example, with an injection linepressure of 1,500 psi, converted to injectionpressure of 15,000 psi, and a core area offour square inches, the point lies above , 1,000 psi line and below the , 1,000line. Thus at 1,000 psi cylinder pressure, acylinder larger than an 8" diameter borewould be needed. Since a 10" bore is abovethe point, it would meet the , use the formula and find theclosest larger bore size Core MovementThe cylinder-only application is note-worthy in that the core is still compressedunder load, but the compression is done bythe cylinder toward the core face prior toinjection.