Property modification of bulk molding compounds for use in injection molding

Property Modification of Bulk Molding Compounds for Use in Injection Molding

M. PRITCHARD* and A. G. GIBSON

Department of Materials Science and Engineering University of Liverpool

Liverpool L69 3BX, United Kingdom

Although impact and flexural strength of injection- molded bulk molding compounds increase initially with glass fiber content, these properties level out at a glass volume fraction between 0.1 and 0.2, limiting the achiev- able properties. Use of “special purpose” polyester resins gives no significant improvement in impact. The impact strength limitation is not worsened, however, by using the maximum processable level of filler, this being true for all fillers commonly used in polyester compounds. Replace- ment of a fraction of the glass by poly(ethy1ene terephthalate) fibers results in a substantial improvement in impact strength.

INTRODUCTION ulk molding compound (BMC) offers the de- B signer a very useful combination of me-

chanical, electrical, and thermal properties. Al- though BMC, until recently, has been processed mainly by compression molding, there is now increasing emphasis on injection molding because of the many advantages the process offers.

Early experiences in BMC injection molding highlighted two points: First, the properties of components tend to be inferior to those produced by compression molding and, second, compound development and evaluation for the injection field must be performed under realistic conditions using an injection-molding machine. Compounds developed and optimized for compression molding do not necessarily perform well on injection.

The first aim of the work described here was to investigate systematically the effects of vary- ing reinforcement and filler contents on the two key properties of injection-molded BMC components: impact and flexural strength. Although it is possible to injection mold BMC with low levels of fiber degradation by careful design of processing equipment (1) . properties still tend to be inferior to those of compression-molded products because of the higher level of flow- induced fiber orientation that occurs. The most important property, impact strength, is the one most affected. Despite this problem, injection molding is finding increasing use for BMC be-

* Present address: Fibreforce Composites, Falroak Lane, Whitehouse. Runcorn WA7 3DL. U.K.

cause of the benefits it offers: shorter cycle times, a higher level of automation, and improved surface finish.

Unfortunately it is not possible to improve properties simply by increasing the reinforcement content. Distinct bounds to the range of processable compositions are imposed by the packing of fibers (2) and fillers (3) . Within the region of processability, impact strength does increase initially with increasing reinforcement content but, as will be shown, it reaches a plateau value at a relatively low glass content. Although this plateau value of impact strength is not greatly affected by changes in filler content, it still represents a fundamental restric- tion to the wider application of injection-molded BMC.

The second objective of the work was therefore to examine ways in which the impact strength limitation might be overcome. Two ap- proaches were investigated: use of modified resin systems and use of organic reinforcements in combination with glass.

Using a standard compound formulation, a range of resin systems was studied, including toughened, flexibilized, vinyl ester, and epoxy- modified resins. The degree of property improvement attainable by making changes in the resin system appeared to be minimal. This result contrasts with the results attainable by using special resins in, for instance, sheet molding compounds (SMC).

Experiments were performed using a variety of organic reinforcements in hybrid compounds with glass. The most promising of these, poly- ethylene terephthalate (PET) fibers, gave a very

POLYMER COMPOSITES, APRIL 1988, Vol. 9, No. 2 131

M . Pritchard and A. G. Gibson

useful increase in impact performance at the expense of a small reduction in flexural strength and modulus. The PET/glass hybrid route appears to be the most promising one for overcoming impact limitations, particularly in molding situations where high levels of fiber degradation are unavoidable with glass.

EXPERIMENTAL Materials

For the trials with BMCs containing conventional constituents, compounds were produced, unthickened, from the materials shown in Ta- ble 1 using a 2-blade mixer. For the experiments involving alternative resins, the description and sources of the resins are given in Table 2. The descriptions are based on information given by the resin suppliers. All the resins investigated were materials that showed potential

Table 1. Material Used in Bulk Molding Compounds. ~ ~~ ~

Supplier and Product Material Description Code

1. RESIN PHASE Unsaturated isophthalic

polyester resin. (For alternative resins, see Table 2.)

Low profile additive (33.3 p. h.r.)

Initiator (1.5 p.h.r.)

2. FILLER Stearate-coated calcite.

Mold release agent

3. REINFORCEMENT 6-mm chopped glass strands

(hard sized) (For details of PET

reinforcements, see text.)

Scott Bader (Crystic D4029)

Freeman (40-1 080)

Akzo-Chemie (t-butyl peroctoate)

Croxton and Garry (Omya BLR3)

Croxton and Garry (zinc stearate)

Fibreglass Reinforcement (FGCS 1401)

for property enhancement with BMC. They were all capable of being processed using conventional polyester BMC technology. Finally, for the measurements involving glass-PET hybrid compounds, 6-mm chopped PET fibers were used (Cornpet@ IW7 1, supplied by Allied Fibers).

Processing The mechanical property results reported

here were obtained on injection-molded plaques measuring 150 mm by 100 mm by 4 mm and on samples cut from 250-mm diameter, 4-mm thick compression-molded discs. To obtain con- sistent and reproducible results on injection- molded samples, it is necessary to make allowance for both fiber degradation effects and mold flow orientation.

Fiber degradation was kept to a minimum by using a screw injection-molding machine with a screw of sufficiently large flight depth, with a 6-mm diameter nozzle and generous sprue dimensions. Quantitative aspects of the design of screws and nozzles for minimum fiber degradation have been discussed elsewhere (1). The fiber orientation problem is illustrated, for the plaque molding, by the X-ray radiograph shown in Fig. 1. The divergent flow that takes place in the gate region leads to fiber orientation in the direction perpendicular to mold flow. Initial orientation of this type is common in the majority of injection-molded components and leads to loss of impact strength, particularly in the gate region. The transverse fiber orientation can be seen to persist throughout the molding resulting in a strong anisotropy of properties, which must be taken into account in component design. The bounds within which properties such as impact and flexural strength can be expected to lie were therefore determined by cutting samples from near the center of the plaque, in directions par-

Table 2. Resin Systems Used in Experimental BMCs.

Mechanical Properties of Unfilled Cured Resin

Elongation Modulus Strength to break

(GPa) W a ) (”/.)

Freemans Stypol 40-6022 Orthophthalic hot press molding 3.5 32 1.1

Derakane 790 Vinyl ester resin with high strength 3.5 75 4.5

Manufacturer and Product Code General Description of Resin

polyester.

polyester. Rigid, highly reactive.

and temperature performance.

chemical resistance.

resistance and high H.D.T.

Scott Bader Crystic D4029 lsophthalic hot press molding 3.0 54 1.7

Scott Bader Crystic 600E Epoxy-modified polyester with 3.1 70 4.0

Freemans Stypol 40-8200 Bisphenol A resin with chemical 3.2 43 1.5

Freemans Stypol 40-821 0 Rigid isophthalic wet lay-up resin. 3.0 35 2.3 Freemans Stypol 40-391 0 Tough, resilient isophthalic SMC 2.8 40 2.1

Freemans Stypol 40-391 9 As above, with greater resilience - Scott Bader 390 lsophthalic neopentyl glycol. - Scott Bader D4029 with 25% lsophthalic resin with urethane -

resin. - - - -

Flexible gel-coat resin. - -

NV1080 acrylate toughening additive.

132 POLYMER COMPOSITES, APRIL 1988, Vol. 9, No. 2


Fig. 1 . X - r a y radiograph of injection-molded BMC plaque. Thickness: 4 mrn. Glass volume fraction: 0.14.

allel and perpendicular to the major mold flow direction.

In contrast to the behavior seen in reinforced thermoplastic moldings, fiber orientation does not vary significantly through the thickness of moldings. Orientation is not greatly influenced in the vicinity of the mold surface since there is no solidifying layer. Since the tool surface is considerably hotter than the compound, the resin in this region undergoes a rapid initial fall in viscosity. This tends to promote a flat plug flow velocity profile across the thickness of the molding.

Use of Ternary Diagrams to Describe Composition

In reinforced thermoset systems, where a great many different components can be pres-

POLYMER COMPOSITES, APRIL 1988, Vol. 9, No. 2

ent, it is useful to have a general means of describing and comparing compositions of different molding materials. Any component of a thermoset molding system can be placed under one of the following three headings:

1 . resin 2. filler 3. reinforcement.

The use of a ternary diagram is therefore ap- propriate. Since processing behavior and most mechanical properties are largely determined by volume rather than mass fractions, it is more relevant to describe composition on a volume fraction basis. This enables the effects of changing composition with components of different densities to be determined more readily. The volume fraction, Vi, of each individual component can be readily converted to its weight fraction, W,, and vice versa by equations of the form

and

where p l , p2 . . . pi are densities of components. Expressing BMC compositions in volumetric

form also enables a region of permissible compositions to be defined and a region of processable compositions to be determined experimen- tally. Figure 2 shows the limitations imposed by packing of filler particles and reinforcement particles.

The maximum volume fraction of filler that can be incorporated in resin in the absence of fibers is generally in the region of 0.6 to 0.7, although higher packing fractions can be obtained (3) by using fillers of mixed particle size distributions. The filler packing limitation gives rise to the line, AB.

The volume fraction of reinforcements that

REINFORCEMENT

A

Fig. 2. Ternary diagram for BMCs, based on volume fractions, showing area of processable compositions.

133

M. Pritchard and A. G. Gibson

can be incorporated depends on the bundle aspect ratio, and for random dispersion is given by (2)

V3max = kD/L (3) where LID is the aspect ratio and k is a constant with a value in the region 4 to 5.5.

The limitation corresponding to the measured aspect ratio of 6-mm chopped glass strands is shown as line CD in Fig. 2. Attempts to incor- porate higher volume fractions than those corresponding to this limit usually result in compounds that will not flow from the feed cylinder into the barrel of the machine. When flow can be induced, it is frequently accompanied by a high level of fiber degradation.

The region of processable compositions is bounded, therefore, by lines BE and DE. The majority of commercial dough molding compounds have compositions near the upper left corner of the region of processability. In draw- ing the lines AB and CD, no allowance has been made for the effects of packing and particle size interactions between the fibers and filler. Such effects have been investigated in detail by Mi- lewski (3) . If these secondary effects were incorporated into the diagram, the two intersecting lines would be replaced by a curved boundary, the exact location of which would depend on the ratio of reinforcement diameter to filler diameter. The simpler type of boundary with the intersecting straight lines is sufficiently ac- curate for present purposes however.

With a hard-sized glass, some breakup of bundles and glass strands is inevitable, even in low work processing, so a n array of effective reinforcement lengths and bundle diameters will be present in the final product.

It should be noted that, although the ternary diagram using volume fractions gives a useful means of comparing compositions, it does not imply that compounds of the same volume composition would have identical properties or processing characteristics. Obviously these properties will be influenced by factors such as filler surface area and particle shape and size distri- bution.

The region of processable compositions is bounded, a t low fiber or filler volume fractions, by practical limits connected with processable viscosity. Below a certain viscosity level, compounds become sticky and difficult to handle. Reinforcement tends to separate out rather than flow with the compound.

The overall objectives of compound formulation are twofold: (i) to achieve the maximum effect ive reinforcement volume fraction, and (ii) to maximize filler content without detriment to mechanical performance.

RESULTS AND DISCUSSION Effect of Glass Fiber Volume Fraction

Figure 3 shows the changes that take place in two key properties: flexural strength and

1 1 I 0.2

GLASS V, 0

b

I I

GLASS V, 0 0,z

Fig. 3. Influence of volumetric glass content, V,, at a constant filler-to-resin ratio, V,/V, = 1 . (a) Unnotched Charpy impact strength: (b)flexural strength. Key: Injec- tion-molded plaques, transverse, T , to moldflow H: par - allel, P. to moldflow 0; compression-molded plaques, C , + average values.

impact strength as the glass volume fraction, V3, is varied at a constant filler-to-resin ratio. Results for injection-molded plaques (parallel and transverse to mold flow) and compression molded plaques are shown. Each experimental point represents a mean of ten test results. The scatter of the results, as is often the case with polyester molding compounds, was high-es- pecially in the case of the impact tests. An indication of the scatter is given by the error bars shown in each figure, which were calcu- lated for a 90% confidence level. For clarity, the error bars are shown for only one point in each figure.

Both flexural and impact strength increase initially with increasing glass content, but be- gin to level out at V3 values above about 0.1. The most cost-effective use of glass reinforcement would therefore appear to be at levels below this. The effect of molding anisotropy is shown by the difference between the parallel and perpendicular values.

The observation that properties show plateau behavior with increasing V3 is in keeping with the effects of the fiber packing problems mentioned above. On approaching the packing level predicted by Eq 3 the fibers become prone to breakage in compounding and flow, with con-


Property Modvication of Bulk Molding Compounds for U s e in Injection Molding

sequent limitations on their reinforcing capa- bility.

Toughening in BMC is almost entirely due to energy absorption by fiber pullout from the cracked matrix. Matrix cracking occurs around discontinuities or stress concentrations. A “damage zone” in which debonding and pullout occur forms around such regions. The damage zone can be regarded in many respects as anal- ogous to the plastic zone that forms around cracks or discontinuities in a ductile material. The size of the damage zone and, hence, toughness are related to fiber length (5, 6).

As mentioned above, the moldings described in this paper were performed under “low work” conditions, where every effort was made to min- imize fiber breakage. In many real molding op- erations, precautions of this type are not taken. When excessive fiber breakage does occur, the main effect is to substantially reduce the transverse flow property value. In the worst case, transverse direction of impact strength can decrease to a value only 30% higher than the parallel value.

When designing injection-molded BMC components, is is necessary to take account of the high levels of anisotropy that can occur. I t is particularly important, when siting gates, to bear in mind the poor impact strength of the highly oriented material around the gate area. Divergent flow in the vicinity of the gate leads to strong tangential orientation, so that an impact in this region may tend to “punch out” a circular plug.

Weld line strength is another critical factor. Our own measurements with BMC components suggest that the only safe design procedure is to assume that the weld line strength of this material is zero. In some, but by no means all, components containing cored holes where weld lines would be unavoidable in a normal molding, the injection-compression technique may be used to advantage. Not all components are amenable to injection-compression, however, and the vertical flash tooling required can be significantly more expensive than conventional tooling.

An attractive alternative to injection-compression is the use of movable cores. This technique can be used where a molding is required to contain one or more molded holes in a direction parallel to the closing direction of the tool. The cores are withdrawn during mold-filling and pushed in hydraulically or pneumatically just as cure is beginning. Several core pins can be mounted on a core plate that is situated adjacent to the ejector plate and separately ac- tivated. In multicavity tools where runners are present, this same technique can be used to cut off mold cavities from the runner system. This enables gates of generous dimensions to be used, minimizing fiber degradation, while blem- ish-free moldings are produced that separate easily from the runners.

.

Effect of Filler-to-Resin Ratio Maintaining a constant glass volume fraction,

V3, of 0.14, we investigated compounds with different filler-to-resin ratios, so that the compositions all lay on a horizontal line in Fig. 2. Values of filler volume fraction, V,, in the range 0.23 to 0.51 were covered.

The mechanical property results for these compounds, shown in Fig. 4, provide ample justification for raising the filler content to the maximum possible level permitted by packing and processability. Although matrix embrittle- ment might have been expected at high filler content, filler level appears in practice to have very little effect on the strain value at which matrix cracking first appears in BMC. The fairly small variation in impact strength seen in Fig. 4a is in keeping with the assertion that toughness in this material is mainly derived from fiber pullout and, hence, is determined primar- ily by fiber length and orientation.

Property Limitations with Conventional BMC From the results of the work described so far,

it must be concluded that there is a fundamental limit on the mechanical properties, particularly impact strength, that can be achieved in injection-molded BMC. The plateau value of properties cannot be exceeded by increasing $ 1 a -- 4

I b

I I 1 1

0.4 0.6 Ot2 FILLER V,

0

Fig. 4. Influence of filler content, V,, at a constant glass content, V, = 0.14. (a) Unnotched Charpy impact strength; (b)Jexural strength. Symbols explained in caption for Fig. 3.

POLYMER COMPOSITES, APRIL 1988, Vol. 9, No. 2 135

M. Pritchard and A. G. Gibson

glass content because of packing and degradation problems. This is a disappointing result, bearing in mind that impact strength limitations are hindering injection-molded BMC, which has many other desirable characteristics, from entering some key application areas.

I t was decided to examine the possible ways of improving impact through changes of the constituents. Two general routes were possible:

1 . To increase the area under the stress-strain curve by increasing the strain to failure of the matrix.

2. To examine the use of alternative reinforcements less prone to fiber degradation than glass.

Both routes were examined as described below-.

Use of Resins with Improved Properties Several alternative types of polyester resin

were investigated in compounds having a standard formulation containing a glass volume fraction, V,, of 0.13 and a filler volume fraction, V z , of 0.48. These were injection molded and compression molded as described previously.

Most of the candidate resins employed in this study showed increased strength or elongation to break in the cured, unfilled state. The vinyl ester and epoxy-modified resins, for instance, offer greatly improved unfilled strength and elongation compared with conventional polyes- ters. Two of the other resins (Freeman 40-3910 and 40-391 9) were hot press molding resins, known to give tough, resilient behavior in SMC. Finally, a flexible resin (normally employed as a gel coat) and an isophthalic resin containing a urethane acrylate toughening additive were investigated.

The results obtained on the molded compounds are shown in Table 3. If impact strength of the injection-molded products is the most important criterion, it can be seen that none of the resins performed significantly better than the standard isophthalic polyester, although the gel-coat resin, the epoxy-modified resin, and the rubber-toughened isophthalic performed about as well as the standard. Despite having the

highest unfilled strength, the vinyl ester resin gave the poorest combination of molded properties.

The resins that gave tough, resilient behavior in SMC (Freemans 40-3910 and 40-3919) were marginally poorer in overall molded properties than the standard material. Clearly the criteria for tough behavior in SMC and in injection- molded BMC are different. In SMC a significant amount of fiber fracture takes place on failure. It appears that in BMC, where virtually no fiber fracture occurs and toughness is dominated by fiber pullout, the impact performance is almost independent of resin properties.

The conclusions of this section, although rather negative, are quite important. The first is that for injection-molded BMC there is little to be gained by the use of special resin matrices rather than standard isophthalic or orthophthalic polyester. Secondly, the mechanical properties of an unfilled resin, or even its performance in a material such as SMC, are no indication of behavior in injection-molded BMC.

Use of PET Reinforcement as an Impact Modifier

Following the premise that toughness arises mainly from fiber pullout, a number of organic reinforcements were investigated. In general, these reinforcements are tougher and more flexible than glass and offer the possibility of improved packing as well as passage through the molding machine without fiber degradation. Reinforcements examined included jute, poly- vinyl alcohol (PVA), and PET. Neither jute nor PVA fibers showed great improvement over glass. Moreover, both types of reinforcement presented a possible problem with water absorption. However, the PET reinforcement did show a very useful impact improvement.

The PET fibers investigated (Allied Fibres, Competa IW71) were in the form of 6-mm chopped strands. These were specially developed for use in BMC, being thermally stable and sized with a high integrity, but resin-soluble binder (7). . ---.-

Figure 5 shows the results of tests on the

Table 3. Mechanical Property Results Attained with Experimental BMC’s Using Nonstandard Resins (V, = 0.13, V, = 0.48).

Scott Bader D4029 Freeman 40-6022 Derakane 790 Scott Bader 600E Freeman 40-8200 Freeman 40-821 0 Freeman 40-391 0 Freeman 40-391 9 Scott Bader 390 Scott Bader D4029 +

Compression-molded

Impact Flex Flex Strength Strength Modulus

kJ/m2 MN/mZ GN/m2

17.1 85.9 10.5 13.3 65.2 11.0 10.0 76.3 9.3 16.8 81.1 11.5 10.6 75.5 11.4 11.2 62.1 10.1 13.1 75.3 10.3 13.1 65.5 8.1 17.9 85.2 9.2

Injection-molded Parallel


kJ/m2 MN/m2 GN/ma

5.4 5.4 4.6 4.9 6.4 4.8 5.1 4.2 6.0

48.7 51.4 45.6 54.8 65.3 40.9 44.7 40.3 36.7

7.9 9.3 6.1 8.7

10.4 8.5 7.6 5.8 6.5

Injection-molded Perpendicular


kJ/m2 MN/m2 GN/m2

12.8 88.3 10.6 11.1 90.8 10.4 9.0 79.0 8.3

12.6 98.7 10.3 10.6 69.1 10.4 14.7 90.0 10.3 12.5 86.4 9.3 13.3 81.3 7.9 12.9 82.3 9.0

25% NV1080 16.1 82.3 9.4 5.4 48.7 7.9 12.8 88.3 10.6



I I I I

1 I I I 0 50 100

PET Yo

1.7 lb

PET o/o

I c

hybrid compounds where some of the glass reinforcement was replaced by a n equivalent volume of PET fibers. It can be seen that flexural strength and stiffness reduce somewhat, though not catastrophically, with increasing PET content. This is probably because of the lower stiffness compared with glass. However, there is a considerable improvement in impact strength. This is probably due to the ability of the PET fibers to disperse well and to flow through the molding machine virtually intact. There is also some evidence that because of their greater flexibility, they can pack more

easily when there is restricted movement: a situation in which glass fibers are liable to fracture.

The comparison of fracture surfaces in Fig. 6 shows that, although glass reinforcement does give some fiber pullout, badly placed bundles or fiber agglomerations can also act as areas of weakness. The PET fibers show a more uniform dispersion with less preferential orientation. There has been considerable fiber pullout. The ability of the PET fibers to pack more uniformly because of their flexibility is also apparent. The results on hybrid PET compounds show similar trends to those reported by other workers (8).

To determine whether the benefits of hybrid compounds persist under conditions of severe shear, some "difficult" moldings were made on a tool known to give fiber degradation and impact strength problems. This was a multicavity tool for an electrical component. The mold had small diameter runners and gates of quite restricted cross-sectional area. The molded component took the form of a rectangular box. Fig- ure 7 shows the load-deflection traces that were obtained in a probe flexure test on the flat area of the base of the molding. The hybrid compounds show a very useful increase in fiber pullout, after the maximum load is reached. There is a considerable increase in toughness with increasing PET content, accompanied by

Fig, 6. Comparison of fracture surfaces of plaques with and without PET reinforcement. Scanning electron rnicro- graph [a) 100% glass; [bJ 50% glass, 50% PET.

POLYMER COMPOSITES, APRIL 1988, YO/. 9, NO. 2 137

M . Pritchard and A . G . Gibson

1 LOOC n

~ODEFLECTION mm ’ O 0 ~ODEFLECTION mm ’ O U

Fig. 7. Effect of PET content on load-deflection traces for probe penetration tests performed on the base of an injection-molded BMC box.

a small decrease in strength. It appears, therefore, that PET fibers are capable of changing the fracture mode from a quasi-brittle one to one involving fiber pullout, even when the compound is subjected to considerable shear during molding. The change in failure mode on addi- tion of PET is evident from the photographs of the box shown in Fig. 8.

A possible disadvantage is that at high PET contents, moldings are difficult to deflash and degate. Advanced deflashing techniques have been developed to deal with this problem (9). However, hybrid compounds with 10% to 25% of the glass replaced by PET do not show such severe deflashing problems. The fact that the flash has greater integrity when PET fibers are present may not always be disadvantageous. The possibility of flash adhering to tool surfaces after ejection of the component sometimes re- quires the presence of a n operator to inspect and clean tooling, thus preventing full auto- matic cycling. With PET, the flash almost in- variably leaves the tool along with the molding.

When used at moderate levels, PET reinforcement gives a worthwhile improvement in impact performance for a modest cost increase compared with conventional compounds. The heat deflection temperature of hybrid compounds only begins to fall significantly a t volume ratios in excess of 50/50.

CONCLUSIONS 1. Impact and flexural strength of injection-

molded BMC increase initially with increasing glass content, but level off a t a volume fraction between 0.1 and 0.2. This presents a fundamental limitation to the impact strength of compounds containing only glass reinforcement.

2. Increasing filler-to-resin ratio within the limits permitted by packing and processability has no significant effect on either impact or

Fig. 8. Failure modes resulting from penetration tests on molded boxes containing glass and PET reinforcements. (aa)aU glass; (b) equal volumetric mixture of glass and PET; (c) all PET.

flexural strength. It is therefore desirable to use high filler-to-resin ratios.

3. Use of modified, rubber-toughened or flexible resins is of little use in improving the impact behavior of injection-molded BMC. More- over, properties of unfilled resin or properties when used in other polyester compounds, such as SMC, give no reliable guidelines to behavior in injection-molded BMC.

4. The use of PET fibers as a hybrid reinforcement along with glass gives a very desirable improvement in impact strength, combined with a small decrease in flexural strength.

ACKNOWLEDGMENT This work is part of the program of the Com-

posites Group a t Liverpool University and is supported by the U.K. Science and Engineering Research Council in collaboration with Scott Bader Co. and GEC Henley. We are also grateful to the Allied Signal Corporation for support and for the supply of Compet fibers.

REFERENCES 1. A. G. Gibson and G. A. Williamson. Plast. Rubber

Process. Applications, 4, 203 (1984). 2. K. E. Evans and A. G. Gibson, Compos. Sci. Technol.,

25, 149 (1986). 3. J. V. Milewski, in “Handbook of Fillers and Reinforce-

ments for Plastics,” Van Nostrand Reinhold. New York (1 978).

4. A. G. Gibson, Plas t . Rubber Process. Applications, 5 , 95 (1985).

5. D. W. Lamb, F. W. Noble, and A. G. Gibson, Proc. Churchill Conference on Deformation, Yield and Frac- ture of Polymers, Cambridge, U.K. (1982).

6. D. W. Lamb, Ph.D. thesis, University of Liverpool. U.K. (1985).

7. D. S. Cordova, D. R. Coffin, J. A. Young, and H. H. Rowan. Proc RP/C 39th Annual Conference, SPI, Hous- ton, Texas (1984).

8 . D. S. Cordova, H. H. Rowan, and L. C . Lin, Proc. RP/C 41st Annual Conference, SPI, Atlanta, GA (1986).

9. D. S. Cordova, D. R. Coffin, and H. H. Rowan, “Practical Techniques for the Use of PET/Glass Fiber Reinforce- ments,” Allied Signal Corporation, New York (1985).

138 POLYMER COMPOSITES, APRIL 1988, Yo/. 9, No. 2

Documents

Property modification of bulk molding compounds for use in injection molding