Properties enhancement of short glass fiber-reinforced thermoplastics via sandwich injection molding

Properties Enhancement of Short Glass Fiber-ReinforcedThermoplastics via Sandwich Injection Molding

Somjate Patcharaphun, Gunter MennigInstitute of Mechanical and Plastics Engineering, Chemnitz University of Technology, D-09126 Chemnitz,Germany

This article demonstrates using sandwich injectionmolding in order to improve the mechanical propertiesof short glass fiber-reinforced thermoplastic parts byinvestigating the effect of fiber orientation, phase sepa-ration, and fiber attrition compared to conventional in-jection molding. In the present case, the effect of shortglass fiber content (varying from 0–40 wt%) within theskin and core materials were studied. The results showthat the mechanical properties strongly depend not onlyon the fiber concentration, but also on the fiber orienta-tion and the fiber length distribution inside the injection-molded part. Slight discrepancies in the findings can beassumed to be due to fiber breakage occurring duringthe mode of processing. POLYM. COMPOS., 26:823–831,2005. © 2005 Society of Plastics Engineers

INTRODUCTION

The use of reinforced thermoplastics in engineering ap-plications is increasing and diversifying. Short glass fibershave been extensively used in the automotive industry formany years because of their high strength-to-weight ratio,high stiffness, corrosion resistance, and ease of fabrication.They can be processed with the same techniques used forunfilled thermoplastics, e.g., injection molding, so process-ing cost is low. Moreover, their intrinsic recyclability iswidely recognized as a strong driving force for furtherapplications. The properties of short-fiber-reinforced ther-moplastics, however, pose a problem associated with fiberorientation, which in turn depends on the processing con-ditions and the geometrical shape of the mold such asgating, inserts, and section thickness [1–7]. During conven-tional/single-component injection molding, complex flow

fields in the cavity induce a fiber alignment pattern thatresults in different fiber orientations between the skin andcore layers, as schematically demonstrated in Fig. 1. This inturn affects the mechanical and thermal properties of themoldings and can lead to part distortion due to differentialshrinkage between sections with different fiber orientations[8].

The sandwich injection molding process consists of twoinjection units which sequentially inject different polymermelts into the mold. The first polymer entering the mold willbecome the skin material of the final part, and the corematerial is then embedded within this solidified layer. Theadvantage of this process over single-component injectionmolding is that special polymers can be used as skin mate-rial to provide good appearance, strength, chemical resis-tance, electromagnetic interference (EMI) shielding appli-cations, etc., while recycled or inexpensive materials canserve as the core material. Thus, part quality can be im-proved and the cost can be lowered. Many articles [9–11]have been published concerning the polymer melt flow andweldline strength of injection moldings made by using thecoinjection molding technique. Little attention, however,has been given to the effect of molding parameters on themechanical properties of coinjection moldings. Seldezn [12]studied the effect of molding conditions on material distri-bution and mechanical properties of sandwich moldedplates. The results suggested that three parameters, injectionvelocity, core temperature, and core content, were the mostsignificant affecting skin/core material distribution. More-over, the mechanical properties of sandwich molded partsshowed a high correlation with the skin/core material dis-tribution. The SCORIM method, developed by Allan andBevis [13], can control the fiber orientation of injection-molded parts using a two live-feed device located betweenthe injection unit and the mold cavity. It was shown thatSCORIM can enhance fiber alignment and tensile moduluswhen measured parallel to the flow direction.

In the case of short-fiber-reinforced thermoplastics, it ispossible to enhance the mechanical properties of moldedparts by using the sandwich injection molding technique.The melt flow front of the material is injected first, which

Results partly presented at the International Conference Polymeric Mate-rial 2004, September 2004, Halle/Saale, Germany.Somjate Patcharaphun is also at the Faculty of Engineering, KasetsartUniversity, Bangkok 10900, Thailand.Correspondence to: Prof. Dr.-Ing. Gunter Mennig; e-mail:[email protected] 10.1002/pc.20149Published online in Wiley InterScience (www.interscience.wiley.com).© 2005 Society of Plastics Engineers

POLYMER COMPOSITES—2005

will become the skin material, and develops a parabolicvelocity profile. Near the mold wall the fibers are generallyaligned in the flow direction due to the high velocity gra-dient. Prior to the skin material’s reaching the end of thecavity, the second material is injected to form the core. Thismaterial develops a second flow front, pushing the skinmaterial ahead of it. As shown in Fig. 2, the velocity at thecenter of the core material is higher than that at the skin flowfront because the material injected first solidifies as it comesinto contact with the cold wall of the mold. Thus, it acts asa second mold wall inside the mold cavity, narrowing theflow channel. The higher the velocity gradient of corematerial, the higher the fiber orientation in the core layer,which results in improved mechanical properties along theflow direction.

EXPERIMENTAL

The materials used in this study were unfilled polypro-pylene (PP-H 1100 L), supplied by Targor (Poland), andpolypropylene filled with 20 and 40 wt% short glass fiber(PP32G10-0 and PP34G10-9) marketed by Buna (Ger-many). Materials were provided in granular form. The testspecimens were molded using an Arburg Allrounder (Ger-many) two-component injection molding machine (Model320S 500-350), which can be employed for both conven-tional/single-component and sandwich injection molding.The core materials were colored prior to injection to facil-itate identification of the interface between skin and corematerials. Injection speed and skin/core volume ratio werechosen as follows: the speed of the first injection unit (skinmaterial) was kept higher in order to achieve a good surfacefinish and to prevent premature solidification of the melt,whereas a lower speed was used for the second injectionunit (core material). The latter was done in order to assessthe uniform core extension along the flow direction without

the breakthrough of the core material at the far end of thebar [11, 12]. Several settings were tried and those leading toan overall satisfying quality with regard to visual propertieswere finally chosen. All the specimens were molded afterthe machine had attained a steady state at the recommendedmelt and mold temperatures. The mold temperature was55°C and the five heating zones (from nozzle to feed zone)were set to 250°C, 240°C, 230°C, 220°C, and 210°C, re-spectively. The processing parameters and materials used inthis study are summarized in Tables 1 and 2.

Tensile and Charpy impact tests were preformed in ac-cordance with DIN EN ISO 527 and DIN EN ISO 179,respectively. The fiber orientation distribution between skinand core layers was assessed by optical microscopy (Olym-pus, Lake Success, NY, model PMG3) and computer-aidedimage analysis (a4i Analysis v. 5.1 and Image-Pro Plus,Media Cybernetics, Silver Springs, MD). For observingfiber orientation the specimens were prepared by cuttingthem at the middle into various layers parallel (see Fig. 3).The sections were then polished using a metallurgical tech-nique and mounted on a stage. In the present case, 500 fibersper sample were measured, establishing histograms andcalculating the fiber orientation variation across only halfthe thickness of the sections assuming the symmetry offlow. In order to determine planar fiber orientation in theskin and core layers, the second-order orientation tensor,a11, introduced by Advani and Tucker [14], was calculatedusing the following equation:

a11 �1

N�i

�n�1

N�i

cos2�i , (1)

where �i is the angle between the individual fibers and thelocal flow direction and N�i

is the number of fibers with acertain angle �i to the local flow direction. For perfect

FIG. 1. Schematic diagram of mold-ing indicating the fiber orientation inthe skin and core layers.

824 POLYMER COMPOSITES—2005

alignment along the flow axis, the orientation average (a11)would be equal to 1, whereas when fibers are randomlyoriented to the flow direction it would be 0.5.

For investigation of fiber lengths within the skin and corelayer the tensile specimens were cut into seven sections, as

shown in Fig. 3. For the separation of skin and core mate-rials a microtome technique was employed. Short glassfibers were isolated from the composite materials using an

FIG. 2. Schematic of polymer melt flow profile and resulting fiber layer structure in sandwich injection-moldedshort-fiber reinforced thermoplastics.

TABLE 1. Processing parameters.

Processing parametersSingle

molding

Sandwich molding

Firstinjection

unit

Secondinjection

unit

Injection pressure (bar) 1000 1000 1000Holding pressure (bar) 800 — 800Holding time (sec) 25 — 25Back pressure (bar) 20 20 20Cooling time (sec) 40 — 40Injection flow rate (ccm/s) 18.5 18.5 8.8Screw speed (m/min) 12 12 12Injection volume (ccm), (%) 37 (100%) 14.8 (40%) 22.2 (60%)

TABLE 2. Materials used in this study.

No. Single molding Sample code

1 PP PP2 PP�SGF 20 wt% SFRPP203 PP�SGF 40 wt% SFRPP40

Sandwich moldingSample code(skin/core)Skin material Core material

4 PP�SGF 20 wt% PP SFRPP20/PP5 PP PP�SGF 20 wt% PP/SFRPP206 PP�SGF 20 wt% PP�SGF 20 wt% SFRPP20/SFRPP207 PP�SGF 40 wt% PP SFRPP40/PP8 PP PP�SGF 40 wt% PP/SFRPP409 PP�SGF 40 wt% PP�SGF 20 wt% SFRPP40/SFRPP20

10 PP�SGF 40 wt% PP�SGF 40 wt% SFRPP40/SFRPP40

POLYMER COMPOSITES—2005 825

incineration method, according to DIN EN 60. Magnifiedfiber images were then digitized semiautomatically with thehelp of Image-Pro Plus software running on a personalcomputer. The fiber length distribution (FLD) was deter-mined by the average fiber length, which was calculatedfrom a minimum of 500 length measurements on fibersrecovered from the incineration of the specimen sections.The percent difference between the average fiber lengthinside the granules and the overall glass fiber length insidethe molded part (%�l) was used to describe the results. Forthis purpose the following equation was employed:

%�l � � lj � lG

lG� � 100, (2)

with lG being the average fiber length inside the granulesand lj the local fiber length inside the individual layers (skinand core layers) of the sections of the parts.

The phase separation analyses of sandwich and singleinjection moldings were also conducted using an incinera-tion method. The same specimen subdivisions as thoseshown in Fig. 3 were selected. The statistical calculationand the experimental procedure were employed in accor-dance with the work carried out by Hegler et al. [15]. Thepercent difference between the local filler concentration ofthe sectioned parts and the overall glass content inside themolded parts (%�Mj) was used to illustrate the results onthe basis of Eq. 3:

%�Mj � �Mj � Mtot

Mtot� � 100, (3)

where Mj is the local filler concentration and Mtot the

average total mass of specimen, which in turn is calculatedby Eq. 4:

Mtot �1

n �i

n ��

j

mG

�j

mP�i

, j � 1, . . . , 7, (4)

where mP is the weight of the specimen prior to incinera-tion, mG the weight of the remaining glass and n is thenumber of samples (n � 5).

RESULTS AND DISSCUSSION

Fiber Orientation Distribution

As Fig. 4a shows, it was found that there are a number ofdistinct regions within the moldings with different fiberalignments. This has also been identified by several otherstudies [1, 3–7, 13, 16–20]. The layer at the mold wall,referred to as the surface layer, tends to have fibers ran-domly oriented or slightly flow aligned. This randomlyoriented region is caused by fountain flow near the meltfront [21]. In particular, fibers from the core region near themelt front move outward to the wall passing through thefountain flow region. In the skin region the fiber orientationis predominately parallel to the flow direction. This is due toelongational forces arising during fountain flow at the frontand to shear flow after the front has passed. In contrast, arandom in-plane alignment of fibers is observed in the corelayer due to a slower cooling rate and lower shearing.Moreover, the micrographs clearly reveal that voids aremostly located within the core layer. Figure 5 shows the

FIG. 3. Location of areas for this investigation.


FIG. 4. a: Optical micrographs in the z-y plane of single injection moldings; SFRPP20 and SFRPP40. b:Optical micrographs in the z-y plane of sandwich injection moldings; SFRPP20/SFRPP20 and SFRPP40/SFRPP40.


measured values for the fiber orientation tensor (a11) vs. therelative thickness (zi/h). It can be observed that the a11 inthe core region of SFRPP20 is higher than in the core ofSFRPP40. These results are in agreement with the workcarried out by previous researchers [22, 23] in that anincrease of the thickness of the core layer of injection-molded short-fiber-reinforced thermoplastics appears to bemore pronounced as the glass fiber content increases.

Figure 4b shows the fiber orientation inside the skin andcore layers of sandwich molded specimens with differentglass fiber contents. For SFRPP20/SFRPP20 and SFRPP40/SFRPP40, it can also be seen in Fig. 5 that the values for a11

within the core layer are higher than those obtained forsingle injection moldings. This can be due to two possiblereasons. First, it can be the result of the previously men-tioned solidification of the skin material and consequentnarrowing of the flow channel with the core material stillexisting as a melt at the center. This behavior has also beenobserved in previous works [1, 24] dealing with the corelayer and the presence of voids rising with increasing thick-ness of the mold cavity. Second, the slower the injectionspeed of the second material during the filling stage, thehigher the thickness of the solidified skin layer restrictingthe cross-sectional area available for the flowing melt. Thisleads to a higher velocity gradient that tends to increase thefiber orientation of the adjacent melt layer. These observa-tions are also in accordance with previous work [7, 17–19,25–26] according to which the fibers at the mid-plane be-come more flow aligned and the thickness of the core regiondecreases with the thickness of the cold boundary layerincreasing.

Phase Separation Analysis

The effects of fiber content and different processingmethods on the phase distribution of short glass fibers havebeen analyzed in this work. It was found that %�Mj doesnot change with the varying short glass fiber concentrations.

These results strongly coincide with previous findings [15,27]. In addition, it was observed that there are no significantvariations of phase separation effects detectable in bothsandwich moldings with the same fiber content in skin andcore and single injection moldings. However, as shown inFig. 6, inhomogeneities become considerably pronouncedwhen a sandwich injection process with skin and corematerials of different fiber concentrations is used. In thecase of SFRPP20/PP and SFRPP40/PP, it can be seen that atthe gate region there is a shortage of glass fibers, whichfinally turn into excess at the far end of the bar. Contrary toSFRPP20/PP and SFRPP40/PP, the fiber contents of PP/SFRPP20 and PP/SFRPP40 are relatively high at the gateregion and fairly low at the end of the bar. This discrepancyin %�Mj can be explained by the fact that the skin/corevolume fraction is lower at the gate region than at the end ofthe bar (see Fig. 6). Therefore, when core material with ahigher fiber concentration is injected, a decrease in the%�Mj is observed at the end of the bar (position 7). As canbe seen in the case of SFRPP20/PP, SFRPP40/PP, andSFRPP40/SFRPP20, an opposite behavior can be noticedwhen the core material contains lower fiber concentration.

Fiber Length Distribution

Figure 7 shows the percent differences between the meanfiber length of the granules and the overall glass fiber lengthinside the molded part (%�l). In all cases the fiber length ismuch lower in the injection moldings than in the granules,which is due to the fact that fiber length is always reducedto a limiting value depending on melt viscosity, the intensityof the shear field, and the residence time [28–30]. Further-more, it is obvious that the mean fiber length for eachsubdivision of tensile specimens decreases with the increaseof the glass fiber concentration. This has been observed bymany authors [2, 4, 31–34] who mainly attribute a higherfiber concentration to a higher degree of fiber–fiber interac-tion and increased fiber–wall contacts. Moreover, it can beseen that the fiber attrition inside the skin layer of theinjection moldings is higher than that in the core layers. Ourexperiments with simple molded geometry showed that thereduction of fiber length for SFRPP20 is �10–15% in thecore and �20–25% in the skin layer. As pointed out byprevious researchers [22, 23, 35], the fiber length is obvi-ously higher within the core layer than the skin layer. Thisis due to the following mold filling characteristics. The corecomponent is filled with a higher velocity and little defor-mation occurs due to the shear forces being primarily re-tained within the peripheral zone, or skin region. While thecore is still being injected, the melt in the skin region beginsto solidify as soon as it comes into contact with the coldwall of the mold. Therefore, less deformation is applied tothe fibers at the center, which results in a higher averagefiber length in the core region. For the higher fiber concen-tration, a higher degree of fiber degradation inside the skinand core layers, which accounts for �30% in the core layerand 40–45% in the skin layer, can be observed. The occur-

FIG. 5. Variation of the a11 component of orientation tensor through thehalf thickness of single and sandwich molded specimens.


rence of more pronounced fiber length degradation for thehigher fiber concentration is believed to arise from an in-creased fiber–fiber interaction in the more viscous melt.

With respect to fiber attrition in the longitudinal directionof the bar, it can be noted that the effects of differentprocessing types and glass fiber concentrations do not leadto significant changes of fiber length. In all cases, onlyinsignificant differences between the subdivisions were ob-servable. Probably the effect of the simple mold geometryused for this investigation on fiber length destruction issmaller than that of complicated mold geometry [16, 36].Comparing the effect of sandwich and single injectionmolding processes on the fiber length inside the skin region,only minor differences were observed. As mentioned ear-lier, this is due to a higher shear rate near the mold surfaceand fiber interactions with the mold wall. The effect ofdifferent processing types on fiber length, however, is morepronounced in the core region. The fiber length distributionwithin the core layer of the sandwich molding (SFRPP20/SFRPP20) is slightly lower than the values obtained for thesingle injection molding (SFRPP20). For a higher fiberloading the fiber length inside the core region of SFRPP40and SFRPP40/SFRPP40 becomes higher. This cannot beexplained only by the narrower flow channel and the highershear rate occurring during the sandwich molding process,

but also by the higher fiber loading itself, which results inmore frequent fiber–fiber interactions and, thus, higher fiberdestruction in the core region of sandwich moldings [36].

Mechanical Properties

Figure 8 illustrates the tensile and impact properties ofsandwich molding specimens containing different glass fi-ber concentrations within the skin and core materials incomparison to those of single injection molding specimens.As is generally known, the addition of glass fibers results inan enhancement of the tensile and impact properties [1–3,31–36]. The mechanical properties of PP coinjected withglass fiber-reinforced PP are generally at an intermediatelevel between those of PP and glass fiber-reinforced PPalone [37]. It is interesting to note that, for the sandwichinjection moldings (SFRPP20/SFRPP20 and SFRPP40/SFRPP40), the maximum tensile stress and impact strengthare higher than for the single injection moldings (SFRPP20and SFRPP40). This improvement of the mechanical prop-erties is considered to be due to a higher degree of fiberorientation within the core layer (the influence of voids isneglected since the area fraction between the total area ofvoids and the cross-sectional area of the specimen is ap-proximately less than 0.25%). However, comparing the

FIG. 6. Comparison of phase distribution analysis of short glass fiber-reinforced PP for sandwich injectionmolding process at various positions of the tensile specimen.


FIG. 8. Effect of glass fiber contentson maximum tensile stress and impactstrength.

FIG. 7. Fiber attrition in the skin and the core layers at various positions of the tensile specimens. a: For singleand sandwich molding processes with 20 wt% short glass fibers. b: For single and sandwich injection moldingprocesses with 40 wt% short glass fibers.


mechanical properties of sandwich molding and single in-jection molding, it can be observed that the maximumtensile stress and the impact strength of sandwich specimensare not as high as one might have expected. This is probablydue to the higher fiber attrition that occurs during thesandwich molding process.

CONCLUSIONS

A sandwich injection molding technique was employedto enhance the mechanical properties of thermoplastic com-posites with respect to the orientation of short glass fiberswithin the skin and core region. The results showed a rise inthe maximum tensile stress and impact strength as theconcentration of the short glass fibers was increased. Thesemechanical properties of sandwich injection moldings wereobserved to be slightly higher than those of single injectionmoldings, which is attributed to the higher fiber orientationwithin the core layer. Investigating the influence of differentprocessing types on the phase distribution of short glassfibers, no significant phase separation effects were observedbetween sandwich and single injection moldings, exceptwhen a sandwich injection process with different fiber con-tents was used. The results obtained by analyzing the fiberattrition inside the skin and core regions in the longitudinaldirection of tensile specimens showed that the degree offiber degradation inside the skin layers was higher than inthe core layers. There were only minor differences in theskin region fiber length observed between sandwich andsingle injection molding processes; this effect was morepronounced in the core region and for the higher fiberconcentration.

ACKNOWLEDGMENTS

The authors thank TARGOR GmbH and BUNA GmbH,Germany, for materials, the Faculty of Engineering, Kaset-sart University, Thailand, for financial support, and techni-cal staffs from the Institute of Mechanical and PlasticsEngineering, especially Dipl.-Ing. Helmut Puschner for in-jection molding of the specimens.

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Properties enhancement of short glass fiber-reinforced thermoplastics via sandwich injection molding