Calculation of Shrinkage Ratefor Injection Molding Based on Moldflow

Fangcheng Xiao, Lei Chen and Xiao Wang

Abstract During the process of injection molding, shrinkage rate varies on plastictype, product structure, processing parameter, flow gate size and location. Tradi-tional shrinkage rate estimated from mold designer’s experience makes it quitedifficult to meet the accuracy requirements on products, especially in terms of bigsize items like automotive interior and exterior. This paper introduces the accuratecalculation of shrinkage rate for injection molding by computer simulation viaMOLDFLOW software, and researches the factors influencing shrinkage rate,which holds guiding significance on improving the pass rate of big size injectionmolded items.

Keywords MOLDFLOW � Shrinkage rate � Mold design � Injection molding �Computer simulation

1 Foreword

The shrinkage rate of the traditional injection molding products is based on theexperience of mold engineers who just select an empirical value, or on theintermediate value from the range of shrinkage rate provided by the materialmanufacturer. This means that as long as the plastic type is the same, the sameshrinkage rate is adopted for all the injection molded parts and all the locations of

F2012-E13-007

F. Xiao (&) � L. Chen � X. WangDepartment of Polymer Product Performance Development, Changan Automobile GlobalR&D Center, 401120 ChongQing, People’s Republic of Chinae-mail: sunqiemail@163.com

SAE-China and FISITA (eds.), Proceedings of the FISITA 2012 WorldAutomotive Congress, Lecture Notes in Electrical Engineering 196,DOI: 10.1007/978-3-642-33738-3_48, � Springer-Verlag Berlin Heidelberg 2013

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that part, in spite the fact that the product shape, mold structure, number, location,and size of gates may vary a lot. In fact, shrinkage rate of injection molded productvaries according to many influencing factors such as plastic types, productstructure, processing parameter, and size and location of gates, etc. [1]. Therefore,great errors exist between the actual shrinkage rate and the shrinkage rate adopted,and therefore it becomes difficult to control the size of large injection moldingparts. Especially in terms of injection molding for automobile interior and exterior,if the size is not properly controlled, gaps and rat-holes will appear, which willseriously affect the grade of the product.

For the purpose of better dimensional accuracy and surface quality of injectionmolding parts, a more accurate shrinkage rate is needed. Yet, neither the traditionalempirical estimation of the engineer nor the plastic supplier could provide ashrinkage rate against the part’s structure and specific material which is highlyclose to the actual shrinkage. However, this can be realized by simulating theinjection molding process with MOLDFLOW software. Using this simulation, ashrinkage rate highly close to the actual shrinkage can be calculated out withcomprehensive consideration on the influence of product structure, material type,gate number and location, and process parameter on the molding shrinkage. Theshrinkage mechanism of injection molding products, the calculation method andinfluencing factors of shrinkage rate will be introduced in the following text fromexamples of simulation calculation on automobile exterior and interior.

2 The Shrinking Mechanism of Injection Molding Parts

When the injection molding machine injects the plastic melt into the mold cavitythrough nozzle, runner, and gate, polymers orient along the melt-flow directionand the conformational entropy reduces. But this kind of orientation does nothappen in the whole polymer chain as the polymer is formed by a number of chainsegments and entanglement between those segments determines that orientationstretch could only happen in some segments.

After the mold cavity is fully filled and the pressure feeding stage is ended, theshear stress along the flow direction gradually disappears. According to the prin-ciple of entropy increase, the polymer chain tends to the largest natural curlyconformation entropy state, resulting in disorientation. At the same time, when thecavity pressure drops to the equal value of the atmospheric pressure, post-moldshrinkage process begins. As the volume of the polymer is composed by the volumeof the polymer chains and gaps between these polymer chains, volume shrinkage(including thermal shrinkage and crystal shrinkage) in plastic product is mainly theresult of the gap-narrowing between the molecules, which is realized by polymerchain’s constant exchanging places with the gaps and gradually transferring thegaps to the surface of the plastic piece [2, 3]. Therefore, the motion state ofpolymers on the surface layer of the plastic product controls its volume shrinkage.During the shrinking process of the plastic product, disorientation determines that

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polymers contract along the flow direction; while in the thickness direction, drivenby the intermolecular attractive force, polymers on the plastic surface layer movecloser to the core of the plastic product, which means that shrinkage in the thicknessdirection points from the plastic product’s surface to the core.

3 Processing Parameter Setting

After the structure design of the rear bumper I for a certain car was completed inCATIA, the pre-treatments of meshing, mesh repair, and runner establishment werecompleted in MOLDFLOW. The product size is 735 mm*1610 mm*540 mm. Theproduct is 3.0 mm thick. Figure 1 shows the product structure and the runner sys-tem. The material was PP filled with 10 % talc. Molding shrinkage rate in paralleldirection provided by the manufacturer was 0.4538 * 0.9043 %, and that of theperpendicular direction was 0.7559 * 1.367 %. The mold temperature was 35 �C.The melt temperature was 205 �C. The injection time was 5 s. Before the calculationof the cavity shrinkage dimension, a reference dimension was needed to be set, asshown in Fig. 2. The reference dimension was set according to the size of thetolerance. After the calculation was completed, the results were compared withthe tolerance range to judge whether the molding shrinkage rate selected wasqualified.

4 Calculation of Shrinkage Rate

In order to calculate the shrinkage rate of a specific material, process parameter andproduct structure, a simulated calculation of Fill + Pack + Shrink in MOLDFLOWwas needed. After the calculation was completed, MOLDFLOW would provide theentire product’s molding shrinkage rate, as shown in Fig. 3. The molding shrinkagerate of the rear bumper I was 1.1 ± 0.16 %. Molding shrinkage rate in all directionswas shown in the product molding shrinkage report, as shown in Fig. 4, togetherwith the rear bumper I’s recommended molding shrinkage rate range.

Fig. 1 Mesh of rear bumper I

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The mold design engineer could calculate the mold processing size in X, Y, Zdirection according to this table, and thus improve the machining accuracy of thecritical dimensions of the injection molding product in all directions.

For the recommended shrinkage rate of 1.1 ± 0.16 %, MOLDFLOW calcu-lated whether the size of different position of the simulated product met therequirement, as shown in Fig. 5. Green means the recommended product shrinkagerate could meet the requirement of the product’s size. Red means it could not meetthe requirement size of the product. Yellow means size of this part needs to betreated with caution. It is between ‘‘could meet the requirement’’ and ‘‘could notmeet the requirement’’. As shown in Fig. 5, by adopting the molding shrinkagerate of 1.1 ± 0.16 %, most part of the rear bumper was green, which meant thissize met the requirement. Two installation holes’ (point A and point B in thepicture) size was red. The two holes were kidney-shaped, and the size deviationwas of ±3 mm. This size was set as the reference size to perform the simulationanalysis. As shown in Fig. 6, the distance between A and B was 807.39 mm. Afterinjection molding, the predicted distance was 805.24 * 809.54 mm which waswithin the tolerance range and met the operation requirement.

Fig. 2 Reference dimensionof rear bumper I

Fig. 3 Molding shrinkagerate of rear bumper I

Fig. 4 Molding shrinkagerate of rear bumper I

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5 The Influencing Factors on Molding Shrinkage Rate

5.1 Gate’s Influence on Molding Shrinkage Rate

In terms of injection mold design, it is known that with fixed product structure andmaterial, the structure of the mold gates is a key influencing factor on the product’smolding shrinkage rate [4–8]. To research on the influence of gates on moldingshrinkage rate, the product structure, material, and processing parameter were keptthe same, and the gate’s number was reduced to 3. As shown in Fig. 7, the size andlocation of the 3 gates were set the same with the corresponding gates in Fig. 1.

The simulation calculation result was shown in Figs. 8 and 9. It could be seenthat after the adoption of 3 gates, the recommend molding shrinkage rate of therear bumper lowered 0.03 %, and shrinkage rate in X, Y, Z direction changed too.In Z direction, the decrease was the most obvious of 0.03 %. In X direction, themolding shrinkage rate increased 0.01 %. Therefore, when the mold was designed,key dimension in X and Z direction should be seriously considered to ensure theproduct size precision.

Fig. 5 Shrinkage rate of rearbumper I

Fig. 6 Reference dimensionreport of rear bumper I

Fig. 7 Running system ofrear bumper I

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5.2 Material’s Influence on Molding Shrinkage Rate

Materials have significant influence on the molding shrinkage rate. Crystallinematerial’s molding shrinkage rate is generally far larger than amorphous materi-als’. Different types of materials have different molding shrinkage rates. Even forthe same type of material, the molding shrinkage rate could be different as crys-tallizability of the material, filler type and quantity, orientation of the polymerchain segment will affect molding shrinkage rate.

After keeping the gate’s location and size, process parameter, and productstructure the same and changing the molding material into modified PP filled with13 % talc, the simulation calculation result of MOLDFLOW was shown inFigs. 10 and 11.

With a comparison of the calculation result of Figs. 10 and 11 with that ofFigs. 3 and 4, it is concluded that the molding shrinkage rate of the rear bumper Idecreased 0.46 % after using modified PP filled with 13 % talc, and the moldingshrinkage rate in X, Y, Z direction also greatly decreased. Therefore, it can beconcluded that different material’s molding shrinkage rate varies a lot. Therefore,when the mold is designed, as long as the material changes, the molding shrinkagerate recommended by simulation calculation in MOLDFLOW with the materialused in production should be adopted. Otherwise the accuracy requirement of theproduct can not be achieved.

Fig. 8 Shrinkage rate of rearbumper after using 3 gates

Fig. 9 Molding shrinkagerate in X, Y, Z direction ofrear bumper I after using 3gates

Fig. 10 Molding shrinkagerate of rear bumper afterchanging material

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5.3 Cooling System’s Influence on Molding Shrinkage Rate

After keeping the gate’s location and size, processing parameter, molding material,and product structure the same as shown in Fig. 12, the molding shrinkage rate ofrear bumper II was analyzed separately when core temperature was kept the samewith cavity temperature and adopting cooling system as shown in Fig. 13.

The MOLDFLOW calculation result was shown in Figs. 14 and 15. Comparingthe two Figures, we can see the molding shrinkage rate of rear bumper II increased0.15 %, and that in Y direction increased 18 %. The Y direction is the lengthdirection of rear bumper II, whose size is a key size of the product. If size in thisdirection can not meet the product accuracy requirement, a gap will appear after itis installed into the car, which will reduce the grade of the car.

5.4 Product Structure’s Influence on Molding Shrinkage Rate

Keeping the processing parameter, molding material, and other conditions thesame, the product structure’s influence on molding shrinkage rate was researchedwhen runner system of rear bumper II was kept same with that of rear bumper I.

MOLDFLOW’s simulation calculation result was shown in Fig. 16. ComparingFig. 16 with Figs. 3 and 4, it can be seen that the molding shrinkage rate of rearbumper II increased 0.06 % compared with that of rear bumper I. The shrinkagerate in X, Y, Z direction all increased. Especially in Y direction it increased 0.1 %.

Fig. 11 Molding shrinkagerate in X, Y, Z direction ofrear bumper I after changingmaterial

Fig. 12 Mesh and runnersystem of rear bumper II

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This proves that product structure will influence its molding shrinkage rate.Accordingly, when the mold is designed, the same molding shrinkage rate shouldnot be used just because the product material is the same.

Fig. 13 Cooling systemdiagram of rear bumper II

Fig. 14 Molding shrinkagerate of rear bumper II

Fig. 15 Molding shrinkagerate diagram of rear bumperII after using cooling system

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6 Conclusion

1. Using MOLDFLOW simulation analysis, molding material, gate location, gatenumber and size, product structure, and cooling system’s influence on moldingshrinkage rate can be thoroughly considered in order to calculate out theproduct’s proper molding shrinkage rate. On this basis, the molding result of thekey size can be predicted. Accordingly, the mold engineer can adjust the mold’sdimension to improve product accuracy.

2. Molding material is an important influencing factor on molding shrinkage rateof injection molding products. Different material’s molding shrinkage ratevaries a lot. Before designing the mold, simulation calculation on the moldingshrinkage of the selected material should be conducted.

3. Different gate location, quantity and size will affect the orientation and crys-tallization of the polymer chains and the feeding of the packing stage. Thus,location, quantity and size of the gates have a significant influence on moldingshrinkage rate.

4. Cooling system is another key influencing factor of molding shrinkage rate.Therefore, when the mold is designed, the molding shrinkage rate can beadjusted by optimizing the mold’s cooling system.

5. Product structure is one of the important influencing factors on the injectionmolding shrinkage rate. The same molding shrinkage rate should not be usedjust because the molding material is the same. Simulation calculation of Cool +Fill + Park + Shrink in MOLDFLOW should be carried out. The recommendedmolding shrinkage rate should be adopted with consideration on productstructure and other influencing factors.

Fig. 16 Molding shrinkagerate diagram of rear bumperII after using cooling system

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bending. China Plastics 20(11):8–105. Li H, Gu Y, Shen C (2002) Bending analysis and molding size predict of injection. China

Mach Eng 13(10):820–8226. Dong B, Shen C, Li Q (2005) Shrinking and bending reasons of thin injection molding

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Author Biography

XIAO Fangcheng Master, who graduated from Sichuan University in 2009, and majored in materialprocessing engineering, he has published 8 papers. Now he is working for the department of polymerproduct performance and development in Changan Automobile Global R&D Center. He is engaged inMOLDFLOW analysis and CAE work.

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