Integration of residual stresses and deformations induced by the injection molding … · injection...

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Integration of residual stresses and deformations induced by the injection molding process in structural finite element simulation of polymeric components. Jorge Miguel Penedo Batista de Sousa [email protected] Instituto Superior T´ ecnico, Lisboa, Portugal May 2015 Abstract The mechanical properties of certain plastic components obtained by injection molding depend not only on the polymeric material that it is made of, but also on its geometry, the characteristics of the mold and the process settings. These aspects influence the material flow during molding, the molecular orientation, the material cristalinity, the degree of internal stresses, and so on. The material obtained under these conditions has different properties from those provided by the supplier. In a structural analysis this can lead to an innacurate results. The injection molding simulation tools and the finite element analysis softwares have been in use for several years by several companies from different industrial sectors. These two types of softwares are, however, used independently, i.e., there is not an integration of the injection molding simulation results in the finite element analysis softwares. The combination of these two types of analyses is very impportant to predict and evaluate the behaviour under service and the mechanical properties of the polymeric components. Keywords: Injection molding, Moldex3D, Finite element analysis, ANSYS, Deformations, Residual stresses 1. Introduction The mechanical behaviour of a particular compo- nent in service can be predicted/simulated using a structural analysis software. However, in a struc- tural analysis of a polymeric component obtained by injection molding it is only considered its nomi- nal shape and material characteristics with isotropic mechanical behaviour provided by the manufac- turer. A structural analysis of a polymeric compo- nent in these conditions can lead to a less accurate prediction of its mechanical behaviour. In fact, the mechanical characteristics of poly- meric components are highly dependent on the molding process. During the injection, depending on the component geometry, the characteristics of the mold and the processing conditions, deforma- tions are induced in the component, as well as inter- nal residual stresses, anisotropic mechanical prop- erties, especially in the presence of fiber reinforce- ments, among others. [1]. Currently there are injection molding simulation softwares that not only determine these variables induced by the process, but also contain interfaces that allow to export them to structural analysis softwares. Particularly Moldex3D features an inte- grated interface that exports results to a structural analysis. In this study it was conducted the integration of some of the injection molding simulation results, in particular the displacement and residual stresses, in a structural analysis with the purpose of providing a more accurate simulation of polymeric components mechanical behaviour. For the injection molding simulation was used the Moldex3D R12.0, as for the structural analyses was used the ANSYS Work- bench 14.0 software. 2. Background During the filling of the mold cavity several phe- nomena of high importance occur that determines the existence of residual stresses and deformations in the component when it is ready for service. These phenomena, as well as the creation of residual stresses mechanism will be described in this section. 2.1. Polymer flow during the filling phase Consider the filling step, when the polymer enters the mold cavity. As soon as the first layers of poly- mer contact the mold wall, which is at a lower tem- perature, they solidify instantly, while the centre of the flow remains molten. As more material is in- 1

Transcript of Integration of residual stresses and deformations induced by the injection molding … · injection...

Page 1: Integration of residual stresses and deformations induced by the injection molding … · injection molding process in structural nite element simulation of polymeric components.

Integration of residual stresses and deformations induced by the

injection molding process in structural finite element simulation

of polymeric components.

Jorge Miguel Penedo Batista de [email protected]

Instituto Superior Tecnico, Lisboa, Portugal

May 2015

Abstract

The mechanical properties of certain plastic components obtained by injection molding depend notonly on the polymeric material that it is made of, but also on its geometry, the characteristics of themold and the process settings. These aspects influence the material flow during molding, the molecularorientation, the material cristalinity, the degree of internal stresses, and so on. The material obtainedunder these conditions has different properties from those provided by the supplier. In a structuralanalysis this can lead to an innacurate results. The injection molding simulation tools and the finiteelement analysis softwares have been in use for several years by several companies from differentindustrial sectors. These two types of softwares are, however, used independently, i.e., there is not anintegration of the injection molding simulation results in the finite element analysis softwares. Thecombination of these two types of analyses is very impportant to predict and evaluate the behaviourunder service and the mechanical properties of the polymeric components.Keywords: Injection molding, Moldex3D, Finite element analysis, ANSYS, Deformations, Residualstresses

1. Introduction

The mechanical behaviour of a particular compo-nent in service can be predicted/simulated using astructural analysis software. However, in a struc-tural analysis of a polymeric component obtainedby injection molding it is only considered its nomi-nal shape and material characteristics with isotropicmechanical behaviour provided by the manufac-turer. A structural analysis of a polymeric compo-nent in these conditions can lead to a less accurateprediction of its mechanical behaviour.

In fact, the mechanical characteristics of poly-meric components are highly dependent on themolding process. During the injection, dependingon the component geometry, the characteristics ofthe mold and the processing conditions, deforma-tions are induced in the component, as well as inter-nal residual stresses, anisotropic mechanical prop-erties, especially in the presence of fiber reinforce-ments, among others. [1].

Currently there are injection molding simulationsoftwares that not only determine these variablesinduced by the process, but also contain interfacesthat allow to export them to structural analysissoftwares. Particularly Moldex3D features an inte-

grated interface that exports results to a structuralanalysis.

In this study it was conducted the integration ofsome of the injection molding simulation results, inparticular the displacement and residual stresses, ina structural analysis with the purpose of providing amore accurate simulation of polymeric componentsmechanical behaviour. For the injection moldingsimulation was used the Moldex3D R12.0, as forthe structural analyses was used the ANSYS Work-bench 14.0 software.

2. Background

During the filling of the mold cavity several phe-nomena of high importance occur that determinesthe existence of residual stresses and deformationsin the component when it is ready for service.These phenomena, as well as the creation of residualstresses mechanism will be described in this section.

2.1. Polymer flow during the filling phase

Consider the filling step, when the polymer entersthe mold cavity. As soon as the first layers of poly-mer contact the mold wall, which is at a lower tem-perature, they solidify instantly, while the centre ofthe flow remains molten. As more material is in-

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jected, it flows in the centre of the flow, displacingthe previously existing material towards the moldsurfaces, forming a new flow front, as shown in Fig-ure 1. This type of flow is often called ”fountainflow” or ”bubble flow”. The flow front resembles abubble being blown at the center.

Figure 1: Fountain flow and heat transfer. [2]

The melted plastic flows continuously along thecavity, adding new hot material which undergoes ahigh shear rate, generating a large amount of heatdue to friction. Both the heat added by the moltenplastic and the heat generated by friction are dis-sipated over the solidified layer towards the mold”cold” surface.

Initially, the solidified layer is very thin, and theheat is dissipated very quickly. As the polymer so-lidifies, the frozen layer thickness increases, and theheat flow to the mold decreases until a stage of equi-librium is reached, in which the heat dissipated byconduction is equal to the heat introduced by themolten plastic flow and the heat generated by fric-tion. This equilibrium is reached very quickly, usu-ally in tenths of a second. Therefore, the frozenlayer reaches the equilibrium state even at the be-ginning of the filling phase. [2]

During filling stage the polymer is subjected toshear stresses and shear rates, both responsible forthe orientation of the material in the flow direc-tion. The shear rate is zero in the component sur-face, where the plastic solidifies initially abruptlyincreases to a maximum in the vicinity of the solid-ified layer, decreasing towards the center, as shownin Figure 2.

Figure 2: Shear rate profile across the thickness dur-ing filling. [2]

In the interior of the component there is a lowcooling rate, allowing the relaxation of the poly-

mer molecular chains, resulting in a reduced molec-ular orientation. On the other hand, the surfaceof the component undergoes rapid cooling, freezingthe orientation of the molecular chains not allow-ing their relaxation, and trapping the shear stressesinduced by the flow, as shown in Figure 3. Thispattern will affect the level of residual stresses ofthe component. [2]

Figure 3: Molecular orientation over the componentthickness. [2]

2.2. Residual stressesThere are two types of residual stresses in a plasticcomponent: the flow induced residual stresses andthe thermally induced residual stresses. The firstare created due to the frozen molecular orientationduring the filling stage.

The thermal residual stresses on the other handare developed due to several reasons. One reason isthe non-uniform volumetric shrinkage of the mate-rial during the injection component, Figure 4.

Figure 4: Thermal induced residual stresses causedby uneven material shrinkage. [3]

Consider a polymer melt at a uniform tempera-ture that is suddenly constrained by the walls of amold at a lower temperature. In the early stages,the outer surfaces cool and contract to the solidstate, while the core at a higher temperature re-mains molten and free of contractions Fig. 4 (a).As the core cools and solidifies, its specific volumedecreases. Additionally, there is a constraint on thethermal contraction imposed by the previously so-lidified outer layers, Fig. 4 (b). This constraintbetween layers resulting thermally induced residualstress, which distribution along the thickness is rep-

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resented in Fig. 4 filler (c). On the external layersof the component there are compressive stresses andthe core is under tensile stresses. These generallyaffect the mechanical behaviour of the component.[3]

The unbalanced cooling is another source ofthermal residual stresses. When the cooling ratethrough the thickness is not equal on both sides,thermally induced residual stresses are derivedasymmetric. There is an asymmetric tensile-compressive pattern along the thickness resultingon a bending moment that causes warpage of thecomponent, as shown in Figure 5. Components withnon-uniform thicknesses or poor cooling systems arealso subject to thermal residual stresses induced byunbalanced cooling. [3]

Figure 5: Unbalanced cooling. [3]

Another reason for the existence of thermally in-duced residual stresses is the differential contrac-tion. The temperature profile along the thicknessof a generic component is illustrated in Figure 6(a). The section is divided into eight layers of equalthickness to illustrate the temperature at whicheach solidifies. The pressure profile along thicknessis represented in Fig. 6 (b). The pressure levels p1to p8 correspond to the temperature layers, T1 toT8. The PvT graph in Fig. 6 (c) can be determinedby the specific volume for each layer.

Figure 6: Development of different frozen-in specificvolumes through the part thickness. [3]

The component deforms due to the interactionof the several layers with different specific volumes.Figure 7 shows a schematic representation of dif-ferential contraction. If there was an unbundlingbetween the layers, these would contract freely andindependently according to their variation in vol-ume as shown in Fig. 7 (b). In reality, the layers

are connected between themselves, resulting in ashrinkage illustrated in Fig. 7 (c), therefore induc-ing also residual stresses. [3]

Figure 7: Differential shrinkage. [3]

2.3. DeformationsThe pressure and temperature variations inducechanges in the specific volume of the polymer ac-cording to the PvT relations. The shrinkage duringthe packing stage is highly related to the packingpressure. During this stage the pressure must becontinuously applied until the gate freezes. Thus,the volumetric shrinkage of a molded component isin part related to pressure and time of packing. Ifthese two factors are controlled correctly, the com-ponent will have less problems of this nature.

During the cooling stage the main factors affect-ing the volumetric shrinkage are the mold temper-ature and cooling rate. A high temperature gradi-ent between the mold and the component inducesa more rapid cooling of the material, whereby themolecular chains of the polymer will have less timeto relax and reorganize, resulting in a smaller con-traction of the component. Moreover, a smallertemperature gradient between the mold and thecomponent results in a higher degree of molecu-lar organization, and therefore a greater volumetriccontraction of the component.

Finally, from the moment in which the compo-nent is ejected from the mold until it reaches roomtemperature additional contractions and deforma-tions are developed due to the cooling outside themold. [4]

Note that for fiber filled polymeric components,these phenomena are exacerbated, i.e., the residualstress values significantly higher compared to thesame values for cases without fiber reinforcement.The reason for this phenomenon is that the fiberribs do not allow the contraction induced by thepolymer matrix, particularly in the main directionof orientation, leading to significantly higher resid-ual stresses and lower contractions components.

3. MethodsThis study was conducted using Moldex3D to per-form the injection molding simulation, and ANSYSto perform the structural analysis. In this sectionwill be introduced the different kinds of solid meshesused in Moldex3D. Next will be described the sev-

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eral calculation options that influence the warpageresults in Moldex3D. Finally it will be addressedexportation methodology of variables to ANSYS,as well as the relevant files to this study exportedby Moldex3D.

3.1. Solid mesh in Moldex3DThe Moldex3D supports almost all types of com-mon elements for solid models such as tetrahedral,hexahedral, pyramidal and prismatic elements, asshown in Figure 8.

Figure 8: Solid elements suported by Moldex3D. [4]

Generally, the easiest method of solid mesh mod-eling is generating tetrahedral elements. However,sometimes it may be inappropriate to use only onetype of elements in the mesh geometry. Thus,Moldex3D provides a technology that allows theuser to apply hybrid meshes on analyses of solidmodels. By this method the total number of el-ements is reduced, making the simulation compu-tationally lighter, maintaining an accuracy in theresults.

The boundary layer mesh (BLM) was introducedby Moldex3D. This is a hybrid mesh that containstwo kinds of elements, prismatic elements of re-duced thickness on the surface and tetrahedral el-ements inside. It is possible to set one, two, orany layer of these elements in the vicinity of eachsurface. For example, by setting any layer of theseelements it is obtained purely tetrahedral mesh. Onthe other hand, by defining two layers of prism el-ements on the surface there are obtained at leastfive layers of elements along the thickness of thecomponent. These five layers elements consist oftwo small thickness prismatic element layers, like a”shell” covering the entire surface, followed by oneor more inner layers of tetrahedral elements as il-lustrated in Figure 9. This way it is obtained arefinement in the near-surface areas and a coarsermesh inside, significantly reducing the mesh ele-ment count. Moldex3D BLM simulates with greataccuracy the phenomena occurring at the compo-nent surface during filling, providing results withgreater accuracy. [4]

Additionally, in order to have satisfactory resultsof the injection molding simulation it is required togenerate a mesh with good quality. Moldex3D hasfour quality criteria such as aspect ratio, skewness,orthogonality and smoothness that must be satis-fied in order to have a good quality mesh.

Figure 9: Boundary Layer Mesh. [4]

3.2. Warpage calculation options in Moldex3DThe warpage results of the injection molding simu-lation are dependent of several calculation optionsdefined prior to the simulation. Two options rele-vant for the studies conducted are the considerationof the in-mold constraint effect and the consider-ation of flow-induced residual stresses in warpageanalysis. By default Moldex3D does not considerthese effects in its warpage analyses. Therefore,to obtain more realistic warpage results, these ef-fects must be considered. Nevertheless it is possibleto consider just one of these effects in the warpageanalysis.

3.3. Exportation methodology from Moldex3D toANSYS

Moldex3D features an integrated interface (FEA In-terface) that allows the exportation and mappingof several injection molding simulation results to astructural analysis mesh. The integration of theinjection molding simulation results into the struc-tural analysis software involves a particular proce-dure that consists of several steps divided betweenMoldex3D and ANSYS Workbench, illustrated onthe flowchart of the Figure 10.

Figure 10: Moldex3D FEA Interface flowchart. [5]

As Fig. 10 shows, FEA Interface uses the struc-tural mesh as mapping target. Therefore, a previ-ous pre-processing of the structural analysis, mustbe made before the result exportation.

FEA Interface module has several exportationparameters and exports several files. In this studyonly the following files were used: Thermal stress

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output, Flow induced residual stress output,Initial strain output and Material propertiesoutput. These will be briefly described below.

Thermal stress output Corresponds to the ten-sor of thermally induced residual stresses in thecomponent according to the three directions (σxx,σyy, σzz, τxy, τyz, τxz). This file plays an importantrole in this work because it allows to incorporate theresidual stresses induced by the injection moldingprocess in structural analysis of in the component.However, associated with this file by itself, does notarise any deformation in the component from theinjection process.

Flow induced residual stress output Has thesame morphology of the Thermal stress output.However concerns only residual stresses induced bythe flow.

Initial strain output As a result of the injec-tion process the molded component will have de-formations and residual stresses. The transmissionof these variables to a structural model is made inMoldex3D by creating a temperature distributionthat reproduces them. This thermal problem hasto be solved in the structural program forcing theinitial temperature of the part to reach the ambienttemperature. Consequently, it will be obtained thedeformations and residual stresses resulting fromthe injection process in the structural analysis.

Material file output By default, the FEA Inter-face module creates a file with data of the compo-nent material to be input in the structural analysis.If it consists of an unfilled polymer, the softwareassumes the properties of the material as isotropic,with a linear elastic behavior, and assigning justone material to all elements of the mesh. There-fore the following properties are exported: Young’smodulus, Poisson’s ratio, the linear coefficient ofthermal expansion and density. With this databoth Moldex3D as ANSYS can solve Hooke’s lawfor isotropic materials. On the other hand, if thecomponent is constituted by a fiber filled polymer,various materials are created, with anisotropic char-acteristics. Each onde of these is defined by 21 rigid-ity constants, according to the three normal direc-tions xx, yy, zz, and three cutting directions xy, yzand xz. These constants make up the compliancematrix of stress-strain relation for anisotropic mate-rials. Additionally, there are also exported for eachmaterial three coefficients of thermal expansion forthe directions x, y and z, as well as their density.

Once these files are introduced in ANSYS Work-bench through the command line, the static struc-tural analysis can be performed.

4. Results and discussionThis study was divided in two main approaches.The first approach was considered a componentwith a simple geometry and an unfilled polymerin which there were conducted several mesh stud-ies within Moldex3D and ANSYS. The componentused in this approach is was the test specimen type1A according to ISO 527. In the second approach,it was considered a more complex component andperformed studies with two types of material, anunfilled polymer and a fiber filled polymer. Thecomponent of the second approach was a plastichandle, ilustrated in Figure 11.

Figure 11: Plastic handle.

Regarding the materials used, the unfilled poly-mer consisted of Stanyl TE300, a polyamide PA46produced by DSM. The fiber filled polymer wasStanyl 46HF5040, also a polyamide PA46 reinforcedwith 40% in volume of fiber glass, produced byDSM.

Moreover, all the Moldex3D analyses were con-ducted using either BLM meshes or pure tetrahe-dral meshes (BLM mesh with any layer of prismaticelements on the surface) with linear elements. Onthe ANSYS analyses were only used pure tetrehe-dral meshes, also with linear elements.

4.1. First approach: Mesh studies conducted withthe test specimen

The first mesh study was conducted withinMoldex3D and had the purpose to evaluate themost suitable mesh type for the injection moldingsimulation. There were generated two meshes, oneBLM and a pure tetrahedral mesh, with an elementcount in the same order of magnitude, as shown inTable 1.

Table 1: BLM and pure tetrahedral meshes elementcount.

Mesh Pure tetrahedral BLMElement count 86070 81206

Figure 12 illustrates a cross section of the speci-men in which the difference between the two meshesis shown.

There were conducted two complete injectionmolding simulations, both with the same process

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(a) Pure tetrahedralmesh.

(b) BLM.

Figure 12: Cross section of the specimen.

parameters and the same polymer (Stanyl TE300 ).Some of the results are shown below.

Figure 13 illustrates the resulting shear rate dis-tribution of both meshes.

Figure 13: Shear rate distribution in thickness.

BLM mesh presents the abrupt increase in theshear rate with close to the surface, as expectedaccording to Fig. 2 The result of the pure tetra-hedral mesh on the other hand, displays an irreg-ular increase in shear rate along the thickness, inany way resembling Fig. 2. Additional results suchas velocity distribution, end of cooling temperaturedistribution and volumetric shrinkage for instancepresented the same trend of the shear rate distribu-tion.

These differences between pure tetrahedral meshand BLM are reflected in warpage results, partic-ularly in the component displacement and residualstresses, Figures 14 and 15 respectively.

Figure 14: Total displacement.

According to these results it is concluded that theBLM is most suitable for the simulation of the injec-

Figure 15: Normal residual stresses in the x direc-tion.

tion molding process. However, as regards the map-ping of variables between meshes, the higher thesimilarity between the donor and receiver meshes,in terms of element type, number and quality, themore reliable will the transmission of variables be.In safer perspective, the most reliable mappingwould be between two tetrahedral meshes. Thus,for the following studies only meshes with lineartetrahedral elements were used, both in Moldex3Dand ANSYS.

In order to decrease the time and computing re-sources, it was performed a study that examinedthe influence of passing variables from Moldex3Dto structural meshes with different densities. Thisstudy is important in order select a computation-ally ”lighter” mesh without compromising the qual-ity and fidelity of the exported results. It will beconsidered as an indicator the number of layers ofelements along the thickness of the component. Theresults of Moldex3D pure tetrahedral mesh (MDXPTM) will be compared to the same results of threestructural meshes: a mesh with 4 layers of ele-ments along the thickness, with the element countin the same order of magnitude of the Moldex3Dpure tetrahedral mesh as shown in Table 2; andtwo coarser meshes, each with 3 and 2 layers of el-ements along the thickness. These three structuralmeshes were designated 4CAM, 3CAM and 2CAMrespectively.

It was exported the Initial strain output and theMaterial properties output from Moldex3D FEA In-terface to each onde of the three structural meshes.The results of total displacement and residualstresses induced on the component are shown inFigures 16 and 17 respectively.

It is noted that the 4CAM and 3CAM anal-ysis show very similar distributions and slightly

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Table 2: Element count of meshes in study.

Element RatioMesh count ANSYS/MDXMDX PTM 86070 -ANSYS 4CAM 86461 ∼ 1ANSYS 3CAM 36836 0.43ANSYS 2CAM 11502 0.13

Figure 16: Total displacement for the different anal-yses.

higher maximum displacement compared with theMoldex3D analysis. In contrast, the 2CAM to-tal displacement results show significant differencesfrom the Moldex3D result. It is concluded that asthe number of layers of elements along the thicknessof structural meshes decreases, the respective valuesof total displacement deviate from the Moldex3Dreference values.

Figure 17: Normal residual stress in the x directionfor the different analyses.

The same analysis is done considering the nor-mal residual stress of the component in the x di-rection. There is the same tendency of the total

displacement results. Both 4CAM 3CAM analy-ses present similar distributions to the Moldex3Dreference case. On the other hand, the 2CAM anal-ysis shows substantial differences compared to theMoldex3D reference case, mainly inside of the com-ponent section.

These differences occur mainly due to the dif-ferent densities between the donor and receivingmeshes. The mapping can not transmit accuratedata for a mesh if it does not have enough reso-lution to correctly capture the distributions of thevariables to be exported. One conclusion is thatthe 2CAM mesh is not be the most appropriateto this type of interface because of its lack reso-lution. Moreover, as mentioned above, despite thegood quality of the interface results in 4CAM mesh,it is also not appropriate because of its high density,being very ”heavy” for the structural calculation.

It therefore comes to the conclusion that the bestsuited mesh resolution for this procedure is the3CAM mesh. Containing approximately half of theinjection molding simulation mesh, it presents re-sults in terms of displacement and residual stressesvery close to the respective Moldex3D results, notcompromising computing performance.

This conclusion can be extrapolated for a generalcase of integration of injection molding simulationresults into a structural analysis. It is believed thatthe use of a structural mesh with a minimum ofthree layers of elements along the section with thesmaller thickness provides a more reliable mappingof variables.

4.2. Second approach: Plastic handle studiesThe studies of the plastic handle were conductedconsidering an unfilled polymer and fiber filled poly-mer. On both of them was applied the samemethodology. In this approach only pure tetrahe-dral meshes were used, both in Moldex3D and AN-SYS.

One goal of this interface is the reduction of thestructural mesh density relative to the injectionmolding simulation mesh, without the loss of vari-able quality in the mapping stage between meshes.It was previously concluded that to have an accu-rate variable mapping between Moldex3D and AN-SYS, the structural mesh must contain at least 3layers of elements along thickness. To this end, forthe plastic handle was generated a structural meshwith a density which allow the existence of these 3layers of elements in the section with less thicknessof the handle, as shown in Figure 18

Table 3 shows the element count of the Moldex3Dand ANSYS meshes. It should be noted that thereis a significant difference between their densities,having structural mesh approximately 60% less ele-ments compared to the injection molding simulationmesh.

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Figure 18: Section with less thickness of the handle.

Table 3: Moldex3D and ANSYS plastic handlemeshes element count.

Mesh Element countMoldex3D 261730

ANSYS 104925

Consider the study of the unfilled polymer. Itwas divided in three parts. Firstly, an analysisof displacements and residual stresses resulting inMoldex3D considering different warpage calculationoptions was done. Then it was conducted the Ther-mal stress output integration in the structural anal-ysis, in order to compare the residual stresses of thehandle between Moldex3D and ANSYS. Finally, itwas conducted the integration of the Initial strainoutput, with the displacement and residual stress in-formation, again in the structural analysis, also tomake a comparison between Moldex3D and ANSYSresults.

Warpage analysis in Moldex3D As mentionedabove, it is possible to define different conditionsfor calculating the warpage results in Moldex3D.Specifically the consideration of the in-mold con-straint effect and the flow-induced residualstresses effect in warpage analysis will be dis-cussed in this study. The most realistic warpagescenario is obtained considering both effects. Thereare therefore four hypotheses of warpage analysisin Moldex3D to be compared: (a) complete (con-sidering both effects mentioned above); (b) with-out the flow-induced residual stress effect (with thein-mold constraint effect); (c) without the in-moldconstraint effect (with the effect of flow-inducedresidual stress); (d) without any of the aforemen-tioned effects (analysis defined by default). Thecontribution of each effect was weighted studyingthe displacements and residual stresses of each ofthe four cases, as shown in Figures 19 and 20.

The total displacement values range from 0 (inblue) to 3.5 millimetres (in red). Cases (c) and (d)show higher total displacement values compared to

(a) Complete analy-sis.

(b) Analysis w/othe flow-inducedresidual stress effect.

(c) Analysis w/o thein-mold constrainteffect.

(d) Analysis w/oany effects.

Figure 19: Total displacement in Moldex3D.

the cases (a) and (b). It is believed the reason forthis difference is the presence or absence of the in-mold constraint effect.

(a) Complete analy-sis.

(b) Analysis w/othe flow-inducedresidual stress effect.

(c) Analysis w/o thein-mold constrainteffect.

(d) Analysis w/oany effects.

Figure 20: Moldex3D normal residual stresses inthe x direction.

The residual stress values range from -215 MPa(in blue) to 125 MPa (in red). For the residualstresses it is verified the same tendency of the to-tal displacement distributions, analyses (a) and (b)exhibit very similar results, as well as analyses (d)and (c).

It is concluded that the in-mold constraint effecthas a major contribution between both effects. Onthe other hand, the flow induced residual stress ef-fect has minimal contribution to the results results.Thus, among all cases, analyses (b) and (d) are con-sidered redundant.

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Thermal stress output integration in thestructural analysis Consider a completewarpage analysis in Moldex3D, in which areaccounted the in-mold constraint and flow-inducedresidual stress effects. The resulting residualstresses in the handle from the Moldex3D warpageanalysisare illustrated in Figure 21 (a). Theseresults are transmitted to an ANSYS static struc-tural analysis through the Thermal stress outputfile, resulting in the stress distributions shown onFigure 21 (b).

(a) Moldex3D. (b) ANSYS.

Figure 21: Normal residual stress in the x direction.

The stress values range from -135 MPa (in blue)to 125 MPa (in red). Analysing the Fig. 21, it ap-pears that the stress distribution between the twoanalyses is similar. It is concluded that the passageof data between Moldex3D and ANSYS using theThermal stress output is reliable for unfilled poly-mers.

Initial strain output integration in the struc-tural analysis The other warpage results inte-gration option is through the file Initial strain out-put. As stated above this transmits data related tothe component deformation induced by the injec-tion molding process from Moldex3D to ANSYS.This file also provides the residual stress induced inthe component by the injection molding process tothe structural analysis.

Consider the complete warpage analysis of thehandle in Moldex3D. Figure 22 (a) shows the to-tal displacement of Moldex3D analysis. Once inte-grated the Initial strain output file in the ANSYSstructural analysis, it is obtained the total displace-ment of the component shown in Figure 22 (b).

The total displacement values range from 0 (inblue) to 3.6 millimetres (in red). There is a sig-nificant difference between the total displacementdistributions of Moldex3D and ANSYS. The struc-tural analysis result, in Fig. 22 (b), shows largertotal displacement compared with the result of thereference Moldex3D, Fig. 22 (a).

The respective Moldex3D and ANSYS stress dis-tributions are shown in Figure 23.

Again, the stress values range from -135 MPa (inblue) to 125 MPa (in red). The Moldex3D and AN-

(a) Moldex3D. (b) ANSYS.

Figure 22: Handle total displacement distributions.

(a) Moldex3D. (b) ANSYS.

Figure 23: Handle normal residual stress in the xdirection distributions.

SYS stress distributions also show significant differ-ences between them.

These results of displacements and stresses leadto the conclusion that, for a complete warpage anal-ysis in Moldex3D, the results are being transferredto the ANSYS analyses with a deficit.

In order to discover the origin of this deficit anew integration considering the Initial strain outputfile was conducted. Consider the warpage analysisin which the Moldex3D not account the in-moldconstraint effect (only one considers the flow in-duced residual stress effect). In addition to the Ini-tial strain output, the Flow induced residual stressoutputwas also integrated in the structural analy-sis. The integration of these two files in ANSYSis expected to result in similar total displacementan residual stress distributions to the case in whichonly the flow induced residual stress effect is con-sidered (without the in-mold constraint effect) inMoldex3D warpage analysis. Figure 24 shows thetotal displacements of these analyses.

In this case there is a significant reduction of thetotal displacement differences between Moldex3Dand ANSYS.

With regard to this case stress distributions,these are shown in Fig. 25.

Although not exactly the same, the stress distri-bution of Moldex3D and ANSYS are quite similar,showing the same trend of the displacement results.

These results suggest that the Initial strain out-put file does not consider both the in-mold con-straint effect and the flow induced residual stress ef-fect. It is possible to add the Flow induced residual

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Page 10: Integration of residual stresses and deformations induced by the injection molding … · injection molding process in structural nite element simulation of polymeric components.

(a) Moldex3D. (b) ANSYS.

Figure 24: Handle total displacement distributionsw/o the in-mold constraint effect.

(a) Moldex3D. (b) ANSYS.

Figure 25: Handle normal residual stress in the xdirection distributions w/o the in-mold constrainteffect.

stress output file to this type of structural analysis.However, there is no way to consider the in-moldconstraint effect in these kind of structural analy-ses. This means the results of a complete warpageanalysis in Moldex3D (closest case of reality) can-not be integrated in a structural analysis.

The same methodology was applied to the studywith the fiber filled polymer. However, this pre-sented the same trend of results of the unfilled poly-mer study with higher values of residual stressesand smaller values of total displacements. There-fore, this study will not be showed.

5. Conclusions

In this study was studied a methodology to exportvariables from an injection molding simulation to astructural analysis, as well as the requirements forthis passage to have the maximum possible accu-racy.

Firstly, a mesh study in Moldex3D between thepure tetrehedral mesh and BLM was conducted.BLM proved more accuracy in the simulation ofthe injection process rather than pure tetrahedralmesh.

An objective of this study is the reduction el-ements between the injection molding simulationmesh and the structural mesh. The analyses con-ducted recommend that the structural mesh mustcontain at least three layers of elements along thethickness of the component, so as to ensure a reli-

able variable reception.With regard to Moldex3D warpage results, it

is possible to define several warpage options be-fore the software calculation. The considerationof the in-mold constraint effect in this calculationsignificantly affect the results particularly the de-formations and residual stresses of the component.The flow induced residual stress effect on the otherside does not have a noticeable variation in results.However, the inclusion of these two effects in thewarpage calculation provides a more accurate sim-ulation.

Regarding the integration of Moldex3D variablesin ANSYS, data transmission using the Thermalstress output proved to be reliable, resulting in sim-ilar residual stresses distributions in both softwares,regardless of the type of polymer used (filled or un-filled polymer). The in-mold constraint effect andflow induced residual effect discussed in the previ-ous paragraph are also considered in the Thermalstress output file. The only limitation of this file isthat it does not give results of the deformations in-duced by the injection molding process in the struc-tural analysis.

On the other hand, the Initial strain output hasnot revealed as reliable as the Thermal stress out-put. In particular, the in-mold constraint and flowinduced resiual stress effects are not considered inthis file. Therefore the results presented in AN-SYS have a deficit in relation to a complete warpageanalysis in Moldex3D, in which both of the effectsstated above are considered. This effect can bemitigated by introducing the Flow induced residualstress output in conjunction with the Initial strainoutput, in which the effect of flow stresses is con-sidered a structural analysis. However there is noway to include the most dominant effect (in-moldconstraint effect) in an ANSYS structural analysis.These phenomena occur in the absence and pres-ence of the polymer fiber reinforcements.

References[1] A. Peng, W. Yang, and D. Hsu. Enhanced struc-

ture cae solution with molding effect for auto-motive parts. CoreTech System Co., Ltd.

[2] J. Shoemaker. Moldflow Design Guide: A Re-source for Plastics Engineers. Hanser Gardner,2006.

[3] A. stergren. Prediction of residual stressesin injection moulded parts. Master’s thesis,Chalmers University of Technology, 2013.

[4] Moldex3d help.

[5] Coretech System.

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