Warpage of a large-sized orthogonal stiffened plate produced by injection molding and injection compression molding

Warpage of a Large-Sized Orthogonal StiffenedPlate Produced by Injection Molding and InjectionCompression Molding

Sung Hee Lee,1 Seong Yun Kim,2 Jae Ryoun Youn,2 Baek Jin Kim1

1Precision Molds and Dies Team, Korea Institute of Industrial Technology, Songdo-Dong, Yeonsu-Gu,Incheon 406-840, Korea2Department of Materials Science and Engineering, Research Institute of Advanced Materials (RIAM),Seoul National University, Sillim-Dong, Gwanak-Gu, Seoul 151-744, Korea

Received 9 July 2009; accepted 27 November 2009DOI 10.1002/app.31873Published online 22 February 2010 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Rectangular plates of the size of 1800 �600 � 12 mm3 and 1200 � 600 � 12 mm3 were selectedfor injection molding and injection compression molding,respectively, in order to investigate warpage characteris-tics of the large-sized polymer plates with orthogonal stiff-ener. To determine the mold system and to reducewarpage of the specimen, numerical analyses for injectionmolding and injection compression molding were per-formed by using a commercial simulation code. Experi-ments were performed to verify the suggested moldsystem and warpage of the specimen. Relatively largewarpage of the injection molded product was observed

and small warpage of the injection compression moldedproduct was generated. Compression force of the injectioncompression molding was only 6% of the clamp force ofthe injection molding. Warpage of the product wasreduced significantly by using the injection compressionmolding. The injection compression molding will be usedto substitute expensive and disused wood forms withinexpensive and recyclable polymer plates for concretecasting. VC 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116:3460–3467, 2010

Key words: molding; plastics; simulations; stress

INTRODUCTION

Wooden plates are generally used as concrete formsin public works. Recently, two types of the forms suchas euro-form and single body form are widely used.Euro-forms are inexpensive but all the used materialsare disused after 7–8 usages. On the contrary, singlebody forms can be used for 30–40 times and recycledbut its price is 10 times as expensive as that of theeuro-forms.1 Hence, inexpensive and recyclable mate-rials for the forms in public works have beenrequired. Recyclable and cheap polymeric materialsare paid attention to because the forms can be manu-factured by the relatively fast and cheap injectionmolding process. However, shrinkage of an injectionmolded polymeric product is inevitable because mol-ten polymers are filled in the mold cavity and cooled

during injection molding cycles. Shrinkage of poly-meric materials during injection molding depends onthe kind of polymer materials and is generally lowerthan 5% of the cavity volume.2

Although shrinkage of the polymeric material is con-sidered for design of the mold cavity, warpage can begenerated if nonuniform in-mold shrinkage generatedin thickness or in-plane direction because volumetricshrinkage of the material is simply considered for themold design. Warpage of injection molded products areinfluenced by many parameters such as material prop-erties, part geometry, tooling and injection molding con-ditions, etc.3 Warpage causes numerous problems suchas distortion of the finished part, difficulties in satisfyingthe target dimensions, higher internal stresses, etc. In es-pecial, warpage frequently occurs in the cases of large-sized injection molded products and flat-molded articlesbecause nonuniform in-mold shrinkage is easily gener-ated due to the geometric effect. Therefore, precisioncontrol of the part shrinkage should be required inorder to satisfy dimensional accuracy of the moldedproducts. Although many studies on warpage of injec-tion molded products have been reported,4–7 there arefew studies on warpage of large-sized orthogonal stiff-ened plastic plates.Injection compression molding, combination of

conventional injection molding and compression

Correspondence to: J. R. Youn ([email protected]).Contract grant sponsor: Korea Institute of Industrial

Technology (KITECH) and Korea Science and EngineeringFoundation (KOSEF) grant funded by the KoreanGovernment (MEST) through the Intelligent Textile SystemResearch Center (ITRC); contract grant number: R11-2005-065.

Journal ofAppliedPolymerScience,Vol. 116, 3460–3467 (2010)VC 2010 Wiley Periodicals, Inc.

molding, was developed to incorporate the advan-tages of both molding processes. The compressionstage can be introduced after partial filling of thecavity. It can also be activated to replace the packingand holding stages of the conventional injectionmolding. When compared with the typical injectionmolding, injection compression molding has suchadvantages as decreasing molding pressure, reduc-ing residual stress, decreasing uneven shrinkage andwarpage, and increasing dimensional accuracy.Because of these advantages, injection compressionmolding can be recommended to avoid the disad-vantages of injection molding.8–11

The purpose of this study is to reduce warpage ofthe large-sized orthogonal stiffened plastic platewith many ribs. Orthogonal stiffened plastic platespecimens with the size of 1800 mm � 600 mm � 12mm and 1200 mm � 600 mm � 12 mm in length,width, and total thickness were prepared, respec-tively, in order to investigate warpage of the injec-tion molded and injection compression moldedproducts. Ribs of 3 mm thickness were attached onthe plate in order to improve bending stiffness ofthe structure. According to the Ref. 12, bending stiff-ness of the honeycomb structure is the highest butmold manufacturing and injection molding process-ability of the structure are poor and expensive.Bending stiffness of the orthogonal rib structure isthe second highest and injection molding process-ability is excellent. In addition, the tooling cost ofthe latter is much less than that of the former. Con-sequently, the orthogonal rib structure is selected inthis study. Warpage and optimum processing condi-tions of the injection and injection compressionmolding were obtained by numerical flow analysis

using MoldflowTM (6.0 version, Moldflow Pty. Ltd,Victoria, Australia). Warpage for the product wasmeasured experimentally and the measurementresults were compared with the numericalpredictions.

EXPERIMENTAL

Materials and characterization

Glass fiber reinforced polypropylene (PP/GF, Ther-mocomp MEV-1006, LNP Engineering Plastic, USA)containing 30 wt % glass fibers was used for injectionand injection compression molding of large-sized or-thogonal stiffened plastic plates. Viscosity and PVTproperties of the PP/GF resin were obtained becausethose of the polymer resin were the most importantproperties for flow analysis. Rheological properties ofthe polymer resin were measured by Moldflow lab inthe shear rate range of 1–100,000 s�1 at 200�C,226.7�C, 253.3�C, and 280�C. PVT relation of the resinwas also measured by Moldflow lab at the tempera-ture range of 25–290�C at 0, 50, 100, 150, and 200 MPa.Shear viscosity and pressure–viscosity–temperature(PVT) properties of the resin were shown in Figures 1and 2 with respect to the shear rate and pressure,respectively. Typical shear thinning behavior wasobserved and nondifferentiable PVT properties rang-ing from 175�C to 200�C was those of typical semi-crystalline polymers.

Injection molding and injectioncompression molding

The PP/GF resin was dried at 80�C in vacuum for 4h before injection and injection compression molding

Figure 1 Viscosity variation of the ThermocompMEV-1006with respect to shear strain rate at 200�C, 226.7�C, 253.3�C,and 280�C. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

Figure 2 Specific volume of the Thermocomp MEV-1006with respect to temperature at 0, 50, 100, 150, and 200MPa. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

WARPAGE OF A LARGE-SIZED ORTHOGONAL STIFFENED PLATE 3461

Journal of Applied Polymer Science DOI 10.1002/app

to minimize the effect of moisture. Large-sizedorthogonal stiffened plastic plates of the size of 1800� 600 � 12 mm3 and 1200 � 600 � 12 mm3 weremolded by using an injection molding machine[DIMA1050 TON (screw diameter: 150 mm, maxi-mum injection pressure: 1316 kg/cm2, maximuminjection rate: 8.8 cm/s), Dongshin Hydraulics co.,Busan, Korea]. An injection compression moldingmachine [ID 3000 HM (screw diameter: 615 mm,maximum injection pressure: 1356 kg/cm2, maxi-mum injection rate: 30 cm/s), LS Mtron, Jeollabuk-Do, Korea] was also used to produce orthogonalstiffened plastic plates. Experimental setup of theinjection and injection compression molding forlarge-sized orthogonal stiffened plastic plates areshown in Figures 3 and 4 and molding conditions ofboth methods are listed in Table I. In the case of theinjection compression molding, the mold was in-stalled in a typical press and polymer resin was fedto the mold at the side as shown in Figure 4.

Numerical flow simulation

The generalized Hele-Shaw model has provided rel-atively exact results for flow of molten polymer resinin a thin three-dimensional cavity. Governing equa-tions for typical injection molding assuming inelas-tic, nonisothermal, and non-Newtonian fluid floware as follows13–15:

@

@zg@u

@z

� �� @q

@x¼ 0 (1)

@

@zg@v

@z

� �� @q

@y¼ 0 (2)

@q@t

þ @

@xquð Þ þ @

@yqvð Þ ¼ 0 (3)

qCp Tð Þ @T

@tþ u

@T

@xþ v

@T

@y

� �¼ @

@zk Tð Þ@T

@z

� �þg _c2 (4)

Figure 3 Experimental setup for injection molding of large-sized orthogonal stiffened plastic plate. [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 4 Experimental setup for injection compression molding of large-sized orthogonal stiffened plastic plate. [Colorfigure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

3462 LEE ET AL.


@

@xS@p

@x

� �þ @

@yS@p

@y

� �¼ G

@p

@tþ F (5)

where,

S¼Z b

0

qZ b

�z

z

gdzd�z (6)

G¼Z d

0

@ql@p

� �r

dzþZ b

d

@qs@p

� �r

dz (7)

F¼Z d

0

@ql@T

� �p

@T

@tdzþ

Z b

d

@qs@T

� �p

@T

@tdzþ ql�qsð Þz¼d

@d@t

(8)

where, P, T, u and v represent pressure, tempera-ture, x directional velocity of the molten polymerand y directional velocity of the molten polymer,respectively. _c, g, p, Cp, and k are shear rate, viscos-ity, density, specific heat under uniform pressure,and thermal conductivity, respectively. Non-Newto-nian viscosity can be represented by the modifiedCross model.13,16,17

g T; _c;pð Þ ¼ g0 T;pð Þ1þ g0 T;pð Þ _c=s�ð Þ1�n

(9)

where, g is viscosity, g0 is the zero shear rate viscos-ity, _c is shear rate, s* is the shear stress at the transi-tion between Newtonian and Power-law behavior,and n is Power-law index. g0 can be represented asa function of temperature by the Williams-Landel-Ferry (WLF) equation.

g0 T;pð Þ ¼D1 exp � A1 T�T� pð Þð Þ~A2þD3pþ T�T� pð Þð Þ

!(10)

where, T*(p) ¼ D2 þ D3p. There are seven constantsin eqs. (9) and (10), and the values of the Thermo-comp MEV-1006 are listed in Table II. The modified

Tait equation describes the PVT relationship of thepolymeric material as below.

v p;Tð Þa0sþals T�Tg

� �� 1�0:0894ln 1� P

Bs

� �� T�Tg

� �a0mþalm T�Tg

� �� 1�0:0894ln 1� P

Bm

� �� T > Tg

� �8<:

(11)

where,

Tg ¼ Tg0 þ B2p

Bm ¼ B0me�BlmT (12)

Bs ¼ B0se�BlsT

Cp Tð Þ ¼ C1 þ C2�T þ C3 tan h C4

�T� �

(13)

T ¼ T�Cs and C1–C5 are specific heat parameters.Convection heat transfer by cooling channels, heatconduction through the mold wall, and viscous heatgeneration during both filling and post-filling stagesare accounted for in the thermal analysis. Thecoupled thermal and flow fields are solved with thecontrol volume approach to handle automatic meltfront advancement by using a hybrid FEM/FDMscheme. An implicit numerical scheme is used tosolve the discretized energy equation.18 Properties ofthe PP/GF resin (Thermocomp MEV-1006, LNPEngineering Plastic, USA) was used as properties ofthe resin for injection and injection compressionmoldings. Selected material properties for the poly-mer material are summarized in Table III and mold-ing conditions for both numerical calculation andexperiments were the same.

RESULTS AND DISCUSSION

Preliminary prediction was performed in order toobtain optimum injection molding conditions byconsidering filling of the cavity.19–22 High injectionpressure was required due to large size and numer-ous ribs of the product and the high injection pres-sure can cause processing problems such as non-uniform pressure distribution and warpage. Gate,runner, and cooling channel system considered flu-idity of the product is shown in Figure 5. Variationof the injection pressure at the nozzle is also shownwith respect to injection time in Figure 5. The mini-mum injection pressure was obtained at the gate

TABLE IProcessing Conditions for Injection and Injection

Compression Moldings

Injectionmolding

Injectioncompressionmolding

Mold temperature (�C) 50 50Melt temperature (�C) 240 240Injection time (s) 8.5 5Cooling time (s) 100 150Compression delay time (s) – 6Compression time (s) – 20Compression force (ton) – 150–160

TABLE IIConstants of the Modified Cross Model with WLF

Equation for the Thermocomp MEV-1006

n s* (Pa) D1 (Pa s) D2 (K)D3

(K/Pa) A1~A2 (K)

0.2784 19,133 2.22e þ 018 263.15 0 39.013 51.6



when injection time of about 8.5 sec was applied.When the injection time was used as stand point,shorter injection time caused higher injection pres-sure due to higher viscous effect of the polymermelt and longer injection time also caused higherinjection pressure because of lower temperature ofthe polymer melt at the end of filling. Injection con-ditions that have been listed in Table I were deter-mined based on the preliminary prediction.

Warpage of the large-sized orthogonal stiffenedplastic plate observed by the experiment is shown inFigure 6 with numerically predicted results. Bothresults showed large warpage such that the surfacewithout any rib was bent outward. The large warp-age was attributed to thermal residual stressesdeveloped in injection molding and the residualstresses are caused by nonuniform cooling of thelarge-sized product. Residual stress distribution inthe thickness direction was predicted by the numeri-

cal simulation and stresses of about 15–43 MPa wereconfirmed as shown in Figure 7. Pressure distribu-tion lower than 100 MPa was also predicted at theend of filling but a large clamp force of about 2500ton was required to produce the large-sized product.Thereafter, a very large injection molding machinewas needed to carry out the production.

TABLE IIIMaterial Properties of the Polymer Resin for Numerical

Flow Simulation

PropertiesThermocompMEV-1006

Melt density (kg/m3) 971.4Solid density (kg/m3) 1132.4Elastic modulus (MPa) 5212Poisson’s ratio 0.437Thermal expansion coefficient (K�1) 3.024E-5Specific heat (J/kg K) 2407 (at 254.5�C,

temperature dependent)Thermal conductivity (W/m K) 0.18 (at 215.5�C,

temperature dependent)

Figure 5 Gate, runner, and cooling channel system oflarge-sized orthogonal stiffened plastic plate and injectionpressure variation with respect to injection time. [Colorfigure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]

Figure 6 Warpage of injection molded large-sized orthog-onal stiffened plastic plate predicted by numerical simula-tion. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

Figure 7 Residual stress distribution of injection moldedlarge-sized orthogonal stiffened plastic plate with respectto part thickness before ejection. [Color figure can beviewed in the online issue, which is available atwww.interscience.wiley.com.]

3464 LEE ET AL.


Because polymeric materials, an injection moldingmachine, and additional operations are selected byconsidering the property requirement of the endproduct at designing stage, changing the materialsand processing conditions is impossible in manycases. Residual stresses should be minimized duringinjection molding in order to minimize warpage ofthe product. However, minimizing residual stressesof the product is limited because of relatively longflow path of the injected polymer melt in the case oflarge-sized products. Injection compression moldingmethod was utilized in order to overcome the limita-tion of the injection molding. Schematic diagram ofthe injection compression molding is exhibited inFigure 8. The mold cavity is filled by the moltenpolymer resin and the final product is compressionmolded after closing the mold by applying theclamp force. Filling and packing pressure becomes

lower and the flow is shorter than those of the injec-tion molding in the case of the injection compressionmolding, where uniform mold pressure is applied.Preliminary numerical modeling was performed

in order to determine number of gates before pro-duction of the mold for the injection compressionmolding. Three cases were considered for the predic-tion, that is, one gate, two gate, and three gate sys-tem. Flow advancements for the three cases areshown in Figure 9. Flow advancements for the

Figure 8 Schematic diagram showing two stages of injection compression molding. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

Figure 9 Comparison of flow advancement betweeninjection molding and injection compression molding.[Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

Figure 10 Residual stress distribution of injection com-pression molded large-sized orthogonal stiffened plasticplate with respect to part thickness before ejection. [Colorfigure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]



injection molding are shown in Figure 9(a) and thosefor the injection compression molding are illustratedin Figure 9(b). Filling of the cavity by the injectioncompression molding was much faster than that bythe injection molding. Parallel flow path wasobserved at both ends of the cavity in the case ofone gate system. It was expected that less warpagewould occur because strength of the part wasincreased in the flow direction due to fiber orienta-tion. Therefore, one gate system was selected forproduction of the composite plate.

Residual stress distribution of about 28–41 MPawas expected by the numerical simulation as shownin Figure 10 and the results indicated that amplitudeof residual stress distribution of the injection com-pression molded part was smaller than that of theinjection molded part. Consequently, excellentdimensional stability of the product is expected dueto lower residual stresses of the injection compres-sion molded product. When comparing betweencompression force for the injection compressionmolding and the clamp force for the injection mold-ing, the compression force required for the large-sized orthogonal stiffened plastic plate was lowerthan one-tenth of the clamp force for the injectionmolding.

Warpage of the large-sized orthogonal stiffenedplastic plate produced the injection compression

molding was obtained by experiments and numeri-cal calculation, which is shown in Figure 11. Warp-age whose magnitude was similar to the thickness ofthe specimen was observed, and both experimentaland predicted results were in good agreement witheach other. Warpage of the injection compressionmolded plate was much less than that of the injec-tion molded product. Numerical analyses were car-ried out in this study to predict the warpage exactlyfor the injection molded and injection compressionmolded products. Warpage of the composite platewas reduced significantly by using the injectioncompression molding. The injection compressionmolding will be utilized to substitute expensive anddisused wooden plates for frame materials in con-struction with inexpensive and recyclable polymerbased composite plates.

CONCLUSIONS

Warpage of large-sized orthogonal stiffened plasticplates that were prepared by injection molding andinjection compression molding was investigated.Although warpage is inevitable due to thermalshrinkage of polymeric materials and large size ofthe product, the warpage was minimized by numeri-cal prediction and experimental studies. Relativelylarge warpage was observed in the case of the injec-tion molded part but small warpage of 1.5% wasgenerated in longitudinal direction by the injectioncompression molding. All the numerical predictionswere in good agreement with the experimentalresults. Compression force for the injection compres-sion molding was only 6% of the clamp force for theinjection molding. Therefore, Warpage of the injec-tion and injection compression molded products waspredicted by the numerical analysis and controlledefficiently by using the injection compression mold-ing method.

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Figure 11 Warpage of injection compression moldedlarge-sized orthogonal stiffened plastic plate predicted bynumerical analysis. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

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Warpage of a large-sized orthogonal stiffened plate produced by injection molding and injection compression molding