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The influence of cavity deformation on the shrinkage and warpageof an injection-molded part Abstract As soon as the gate solidifies or the nozzle isclosed, the amount of melt inside a cavity remains constant.At this moment, the cavity deformation affects the finalproduct shape and size. A set of simulation procedures hasbeen developed in this study to estimate the cavitydeformation which arises during an injection-moldingprocess. A mold-filling program was applied to calculatethe molding variables. The estimated cavity pressure,temperature distribution, and clamping force were em-ployed as the boundary conditions in the mold-deformationanalysis. 42560
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A structural analysis program was developed topredict the cavity deformation. Molding experiments werecarried out for polymethyl methacrylate (PMMA) wedge-shaped parts. To verify the structural analysis, a straingauge was installed on the sidewall of the mold. Themeasured strains agreed with the simulated results. Acommercial simulation software package was also used topredict the shrinkage and warpage of injection-moldedparts. Numerical results showed the improvement inpredicting the shape and size of a final product by takingthe cavity deformation into account. The influences of thepacking pressure, mold temperature, or melt temperatureon shrinkage and warpage were also investigated.Keywords Injection molding . Shrinkage . Warpage .Deformation1 IntroductionThe injection-molding process involves the injection of apolymer melt into a mold, where the melt cools andsolidifies to form a plastic product. The process comprisesfilling, packing, and cooling phases. During these processes, the residual stress is produced due to highpressure, temperature change, and the relaxation of poly-mer chains, resulting in shrinkage and warpage of the part.In order to yield a product with high precision, theoptimum mold geometry and processing parameters mustbe found. To reduce the cost and time required at the designstage, it is important to simulate shrinkage and warpage ofthe injection-molded part considering residual stress [1].Shrinkage and warpage are two important factorsdetermining the quality of injection-molded parts. Thepacking pressure, melt temperature, mold temperature, andpacking time affect the shrinkage and warpage behavior[2]. Injection-molding processes can be simulated withsome commercial software. The shrinkage and warpage ofinjection-molded parts can also be predicted. However, asignificant discrepancy was found between the simulatedand experimental results [3]. One reason for this is that theshrinkage of injection-molded parts could be anisotropic.The complicated dimensional change is caused by unevencooling, non-uniform planar volumetric shrinkage, anddifferential thermal strain due to geometric effects. Thiseffect increases the difficulty in molding simulation.Another possible reason is that commercial software doesnot consider the mold deformation. If the cavity isdistorted, the shape or size of the molding is affected.The industry continuously requires more accurate andprecise results from simulation software. Simulations haveto consider more detailed aspects of the process, thematerial, and the mold. Therefore, the obtained results canbe closer to reality and contain all of the informationrequired to describe product properties accurately andcompletely [4].As the injected melt solidifies because of the cold moldwalls, the screw presses additional melt into the mold undera holding pressure to compensate for the shrinkage of thepolymer. The result is that air bubbles and sink marks in themolding may be eliminated, and shrinkage and warpagecan be minimized.Pressure gradients during the packing phase depend onthe process and design parameters, and are also affected bythe mold elasticity. Mold elastic deformation can play a significant role in the cavity pressure–time history. Toassess the importance of mold deformability, Leo andCuvelliez [5] added a strain gauge to the setup andmonitored the flexural strain of the mold backplate.
Thedeflection and cavity pressure observations have beenfound to agree with a simple model of the holding phase,including an elastic volumetric expansion of the cavityunder pressure.Tests were performed by Pantani et al. [4] with differentholding programs up to pressures high enough to giveevidence of the effect of mold deformation. Molding testswere simulated by means of a software code with the aim ofunderstanding the role and relevance of mold deformation,and of some options for thermal boundary conditions to themodeling and simulation of the post-filling steps. Com-parison between the simulation results and the experi-mental data shows that considering a rigid mold can lead topredicted values of post-filling pressure profiles muchdifferent from experimental profiles.The pressure in the melt can reach very large valuesduring the packing process. Because of such internalpressures, molds are exposed to a high mechanical loadingthat induces a deformation. The main cavity deformation isconcentrated along the direction perpendicular to thelargest surface, i.e., along the thickness direction.eTable 2 Values of the double-domain Tait model for the PMMAused in the numerical simulation To prevent the mold from being opened, an appropriateclamping force is applied by the clamping unit. The mold iscompressed along the thickness direction. The clampingforce compresses the mold and which also results in adeformation.The original mold size is measured at 25°C. The mold isconditioned by cooling channels to a specified moldtemperature. The cavity wall is also heated when it comesin contact with the injected melt. Thermal loading leads tothe thermal expansion of the mold.The purpose of this paper is to assess the influence ofmold deformation on the product accuracy. A numericalapproach was developed to estimate mold deformation.First, mold-filling analysis was conducted to obtain themolding information. Then, the predicted cavity pressureand temperature, along with the applied clamping force,were applied as the boundary conditions in the molddeformation analysis. The predicted strains were comparedwith the strains measured by installing a strain gauge on thesidewall of the mold. After verifying the strain data, themold deformation analysis can be applied. The shrinkageand warpage of injection-molded parts were calculatedbased on the mold-filling program by taking cavitydeformation into account. To check the accuracy of thenumerical approach, the predicted product shape and sizewere compared with the measured results. The influence ofthe packing pressure, mold temperature, or melt temper-ature on shrinkage and warpage were also studied. 2.1 MaterialThe material used in this study is a commercially availableinjection-molding grade of polymethyl methacrylate(PMMA). The material was pre-conditioned at 120°C for4 h using a dehumidifying drier before molding. Therelevant properties of the material are summarized below.Viscosity curves of the polymer are well described in thewhole pressure, temperature, and shear rate ranges ofinterest by a 7-constant Cross-WLF (Williams-Landell-Ferry) model [6–8]:ηη01η0γ:τ 1 n(1)η0 D1 expA1 T TA2 T T (2)T D2 D3p (3)A2e A2 D3p (4)The values of the constants are listed in Table 1.A double-domain Tait model [1, 8] was applied torepresent the PMMA used in this study:vT; p v0 T 1 C ln 1pBT vt T; p(5)where C=0.0894. The double-domain representation isthen given as:v0 T b1;m b2;mTb1;s b2;sTifT > TgT < Tg(6)BTb3;m exp b4;mT
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