Injection molding of glass fiber reinforced phenolic composites. 2: Study of the injection molding process

Injection Molding of Glass Fiber Reinforced P he no I ic Corn posi tes .

2: Study of the Injection Molding Process

R. SINGH, F. CHEN, d F . R. JONES

Department of Engineering Materials The University of Sk@eld

Sk@eeld S1350, United Kingdom

This study of injection molding of glass fiber reinforced phenolic molding compounds examines fiber breakage and fiber orientation with key material and processing variables, such as injection speed, fiber volume !?action, and the extent of resin pre-cure. The fiber orientation, forming discrete skin-core arrangements, is related to the divergent gate to mold geometrical transition, the extent of pre-cure and injection speed functions of the melt viscosity. Transient modifications to the melt viscosity during mold filling produce variations in skin/core structure along the flow path, which are correlated to the mechanical properties of injection moldings. The melting characteristics of the phenolic resin during plasticization impose a severe environment of mechanical attrition on the glass fibers, which is sequen- tially monitored along the screw, and during subsequent flow through runners and gates of various sizes. Differences found between the processing characteristics of thermosets and thermoplastics raise questions concerning the applicability of thermoplastic injection molding concepts for thermosets.

1. INTRODUCTION

enolic matrix composites are used in many spe- p" cific applications, either as continuous or as short discontinuous fiber composites. Phenolic resin inher- ently gives fire resistance and toxic fume emission characteristics superior to those attainable with most other thermosets and thermoplastics (1. 2). However, even after their long existence in field use, they still failed to make an impact in the world of materials selection (3). This, paradoxically, suggests that tradi- tional methods of property characterization have made it difficult to portray its advantages.

A resurgence of interest, resulting from more strin- gent fire regulations, has given impetus to the development of a new range of high performance phenolics, taking advantage of the inherent properties of tradi- tional resins. In certain "high risk" sectors, such as public and government buildings, mass transport systems, and aerospace applications, phenolics are perceived as the obvious choice. For continuous fiber composites, this is due to developments in resin chemistry, processing, and carefully controlled cure schedules, producing void-free laminates (4). Other applications include new phenolic sheet molding compound (SMC) and dough molding compound (DMC), with comparable mechanical properties to those of re-

inforced polyester and epoxy (4, 5). The adoption of injection molding for short glass fiber reinforced phenolics offers many new engineering applications, and supersedes compression molding for many applications (6). However, specific literature on the injection molding of phenolic composites is still limited, in contrast to the detailed studies on thermoplastics, which range from fiber length degradation, fiber orientation. to mechanical performance (7-23).

The injection molding of thermosets differs from that of thermoplastics, and conclusions derived from the literature on injection molding of thermoplastics and our experimental observations on the injection molding of phenolics indicate several fundamental differences, which previously were assumed to be absent (24). The differences in the molding methods stem from technology used for thermosets and thermoplastics, which are as follows (6): First. for granular thermosetting compounds, limited amounts of conducted heat are required to aid the fluxing of the molding material in the injection barrel. Normally a fluid jacket to heat the barrel for thermosets is preferred to a system of temperature-controlled band heaters, which are used for thermoplastics. since there is little need for steep temperature gradients to be applied externally along the barrel. It is necessary to maintain good tempera-

POLYMER COMPOSITES, FEBRUARY lm, Vol. 19, No. 1 37

R. Singh, F. Chen, and F. R. Jones

ture stability against external or internal temperature fluctuations in the fluxing zone to reduce the risk of premature curing in the barrel. Further, owing to the differences in the solid to liquid transition from thermoplastics, the screw for thermosets may be designed with a smaller length-to-diameter ratio than a thermoplastics injection or extrusion machine. This is dis- cussed Section 3.1. Second, while thermoplastics, after injection into the mold, require cooling until the molding solidifies, thermosetting materials require further heating to achieve final cure and solidification. Further, a breathe facility is necessary to allow the escape of volatiles during final curing of phenolic compounds.

To understand how the process variables affect the mechanical properties, which show considerable dependence on fiber length distribution (FLD), and fiber orientation distribution (FOD), this paper concen- trates on those process parameters that most influence FLD and FOD.

1.1. Fiber Length D e m o n

The conventional compounding technique ensures good dispersion of fibers, but it also results in considerable reduction of fiber length, which downglades the mechanical properties (25). Some studies on fiber attrition during compounding of polypropylene have shown that glass fiber strands, usually around 3 mm length, are reduced to below 0.4 mm, following tumble- mixing and extrusion compounding (9). Gupta and Mittal(10) have conducted a study of the fiber length degradation during injection molding of polypropylene composites, and observed that considerable fiber attrition occurred during plasticization. On the contrary, Williamson and Gibson (26) studied the injection molding of thermoset DMC, and found only nominal fiber breakage during plasticization. This indicates that the polymer melting and rheological characteristics play important roles in fiber degradation during molding.

Two possible processing routes exist for injection molding of reinforced thermosets: liquid resin systems [DMC and bulk molding compound (BMC)] and dry or solid resin systems. As mentioned above, liquid resin systems containing fibers of moderate length can be processed with * * 1 fiber length degradation (26), which differs greatly from the solid resin systems, such as the phenolic molding compounds. The prob- lem is more complex with solid resin systems, since the liquid systems are simply conveyed by the screw; the solid systems must also be melted by the action of the screw rotation.

Our investigations indicate, and other workers agree (26, 27), that the fiber length is reduced at a number of key stages of injection molding. These are:

In the screw during plasticization; While passing through the converging region of the

In the gate region of the mold; During sudden changes of dimensions in the mold.

nozzle;

Our study examines the fiber breakage along the screw and barrel, and during flow through gates of differing size. Material variations, such as fiber volume fraction and the extent of resin pre-cure, are also considered.

The mechanical properties of short fiber reinforced composites show considerable dependence on fiber length and fiber length distribution, as well as the interaction between the glass fiber and phenolic resin, indicated by the critical fiber length, 4. A wide range of 4 values is reported for thermosets and thermoplastics. For example, for short fiber reinforced epoxy resin values of many millimeters are quoted, whereas for nylon and polypropylene the values lie below 1 mm, indicating an order of magnitude difference (22, 28, 29). It has been reported that the critical length of glass fiber reinforced phenolic composites is in the range of 5 to 8 mm (30). determined from the multiple matrix fracture of a unidirectional continuous fiber composite and the fitting of a modified composite the- ory to the tensile stress-strain response of injection molded discontinuous composites. Therefore, to reach a similar degree of reinforcement efficiency as thermoplastics, greater retention of fiber length is needed to enhance the mechanical properties.

1.2. Fiber Orientation

Many factors influence fiber orientation in injection molding. Numerous studies have examined the orientation of glass fibers in short fiber reinforced thermoplastic moldings (31, 32). In thermoplastics, the fibers are generally oriented through the thickness of the molding in three or five discrete layers of preferred orientation. This is commonly explained by the sequen- tial flow-freeze-flow mechanism (23). In the simple case, a skin-core structure, with fibers predominately aligned parallel to the flow direction in the skin regions and a random-in-plane to transverse fiber orientation in the core region, can be related to the process parameters and the mold geometry (33).

Few fiber orientation studies have been made on short fiber reinforced thermosets. Agassant et al. (34) recorded the fiber orientation during injection of BMC using various mold geometries and conditions, and took into account the effect of temperature, shear rate, and the degree of cure on viscosity. Gibson et al. (35, 36) have detailed fiber orientation during convergent flow of BMC and thermoplastics. Numerical techniques, based on constitutive equations, producing planar (laminar) predictions of fiber orientation are also a subject of attention (37, 38).

For thermoplastic composites, there is general ac- ceptance that the pseudoplastic response of the polymer melt has a significant influence on fiber orientation (23). For thermoset composites, the situation is further complicated by how the extent of cure affects the rheology of the resin. In injection molding of DMC and BMC, the cure reaction is delayed until the material enters the mold cavity, and does not affect fiber orientation (35). However, during injection molding of

38 POLYMER COMPOSITES, FEBRUARY 1998, Yo/. 19, No. 1

lnjection Motding of Glass Fiber Reinforced Phenolic

Table 1. Standard Injection Molding Conditions.

Process Parameters Settings

Mold temperature 165 - 170°C Barrel temperature 65 - 75°C Curing time 70 sec Screw rotation speed 75-80rpm Injection pressure (first stage) Injection pressure (second stage) Injection speed

116 MPa 58 MPa 2 x 10-5 m%ec

the solid form of phenolics, the cure reaction is insti- gated immediately when the material enters the barrel. The combined effect of injection speed and the cure reaction on fiber orientation is investigated.

2. EXPERUUENTAL

A thermoset injection molding machine, BOY 22s Dipronic. equipped with a 24 mm diameter thermoset screw, a thermoset nozzle, and an external temperature control unit for the barrel with water heating media was used. Narrow rectangular and one disc mold cavities were employed. Their dimensions are shown in FQ. 1 a, and the standard injection molding conditions are shown in Table 1. The molding compounds used are listed in Table 2.

The composites were ashed in a furnace at 600OC for 4 h. The fiber volume fraction was determined by the weight differences before and after ashing, and the fiber length in the remnant was measured with an image analyzer. Details of this procedure are given elsewhere (30). Fiber orientation was studied using conventional metallographical polishing and optical microscopy techniques.

The properties of the moldings were measured with the Mayes SM200, a servo-mechanical universal testing machine. A span of 40 mm was used for the flexural tests (3-point bending), and 11.75 mm for inter- laminar shear tests. A crosshead movement of 0.5 mm/min was employed, and the deflection of the specimens was measured with a transducer.

3. RESULTS AND DISCUSSION

3.1. Fiber Length Degradation

The function of a screw in an injection molding machine is to convey and plasticize the polymer, producing a reservoir of molten polymer for subsequent injection into the mold cavity (25). The term “melting of a material during plasticization” refers to a transition of solid granules to a viscous liquid. In the context of this paper, it refers to the transition of the phenolic granules in a glassy state to a viscous, putty-like material. It should not be confused with the melting of crystallites within thermoplastics.

The mechanism of plasticization in the screw and barrel is based on friction of the material with both the screw and the barrel. This leads to drag flow that produces a complex array of shearing, compressive forces and differential motion of liquid polymer to the solid counterpart during the solid to melt transition. The accumulative effect imposes severe damage on the glass fibers.

The design of the screw for thermoset injection molding is different from that for thermoplastics, as shown in FXg. 2. The flight depth for the thermoset screw is uniform over the whole screw length, while for the thermoplastic screw there are three distinct zones-compression, melting, and metering-and the depth for each zone is different. The screw length for thermoset composites is also considerably shorter than that for thermoplastics. The Merent designs of the two types of screw exist to accommodate the differences in melting characteristics of thermosets and thermoplastics (6).

The helix of material removed from the screw is shown in Fig. 3. It consisted largely of compacted, un- melted granules, suggesting that much of the screw length serves only to preheat the material. Melting actually commences along the h n t two screw flights, and may be assisted by the transmission of back pressure from the material in front of the screw travel- ing back along the screw. Fkrthermore, the helix angle

Table 2. Types of Injection Molding Compounds Used.

Molding Compounds SupplierlManufacturing Details Composition

Perstorp A2630 ~~~~

Perstorp Ferguson Ltd. Contains 30% by weight of short glass fiber & 19% filler

Perstorp A271 0 Perstorp Ferguson Ltd. Contains 38% bv weight of short alass fiber & 11 % filler.

GB 72 Glassbond (NW) Ltd. Contains 10% by weight mineral filler-no glass fibers,

GB 87 ~~ ~~~~~

Glassbond (NW) Ltd. Contains 40% by weight of short glass fiber and 10% mineral filler.

PMC (pultruded molding compound)

Laboratory synthesized: glass fiber tow pultruded through a phenolic resin solution tank, and chopped to produce (Silenka). pellets containing known and uniform fiber length (12.7 mm).

Contains Novolac phenolic powder CS366 (TBA composites) and 084- M-19 1200 tex E-glass fiber

PDMC (phenolic dough molding compound)

Laboratory synthesized: physical mixture of resin + chopped glass fiber of 6.4 mrn in length.

The constituents are the same as PMC. 30.7% by weight of glass fiber.

POLYMER COMPOSITES, FEBRUARY 1998, Vol. 19, No. 1 39


40

Fan Gate, No Gate (length \entrance \ eut 5

3mm

1 2 3

Gate Testpiece Axes +

e,

90 dep

__-

FQ. la Mold and gate geometries and dimenswns under study.

FYg. lb. Sectioning of disc moldings for mechanical and structural ad- nation

Thermoplastic Screw L/D = Large

Thermoset Screw L D = Small

m. 2. n~rnences between thermoplastic and thermoset irjection screws.

POLYMER COMPOSITES, FEBRUARY 1998, Vol. 19, No. 1

Injection Molding of Glass Fiber Reinforced Phenolic

Rg. 3. Fiber length degradation duringplasticization stage in the screw and barreL (4) (top) Helical ribbon takenfrom the screw during plasticization. (b) (bottom) Remnant of (4) ajter burning+# showing a sharp decrease in- length in the melt

of the screw flights may contribute to the mechanical work required during the solid/melt transition, since the competing requirements are a steep angle to resist back flow and a shallow angle to provide the least tor- tuous path for the drag flow. A compromised value of 17.66 (25) is accepted for the thermoplastic screw. It is not known whether this criterion has been applied to the thermosets, or how this angle may affect fiber length degradation.

Fiber breakage in the screw and barrel was examined by twice extruding the dried phenolic dough molding compound (PDMC), with fibers 6.4 mm in length, using the injection molding machine. The fiber length distributions (FLD) of one-pass and two-pass extrusion compounds are shown in Fig. 4. The average fiber length decreased from 6.4 to 1.05 mm after the first extrusion, and further to 0.53 mm after the second extrusion, while fiber length degradation by injection from nozzle to the mold was much less, from 0.53 to 0.43 mm. It can also be seen that the longer the glass fibers, the more severe the fiber degradation. This may be explained by the fact that during melting, longer fibers remain anchored to solid granules over a

longer period of screw travel and during flow are less mobile to adopt to changing flow transients.

The development of 'Verton" technology by I.C.I. (39), which showed substantial improvements in mechanical properties, is based on the principle that "the longer the glass fibers in the moldmg compound, the longer the fibers in the moldings." This also holds true with a fourfold increase in the average fiber length for phenolic molding compound (PMC): in comparison, the maintenance of fiber length is not as efficient as in Verton, where a tenfold increase in fiber length per- sists (see Table 4 and m. 5).

After the molding compound was plasticized in the barrel, fiber length degradation through the gate to the mold was also investigated by injection molding Perstorp A2630. The results are presented in FYg. 5. By comparing FYgs. 4 and 5, it can be seen that for short fiber filled compounds, fiber length degradation is less severe. For the two sizes of gate employed, the extent of fiber length degradation is affected to only a small degree. The weight average fiber length is strong- ly influenced by the presence of long fibers, which are only found in FLD of the larger-gate moldings.

The constriction at the gate increases the shear rate of the melt. For pseudoplastic melts it lowers the viscosity to ease filling of complex shapes (6). In the narrow runners of an injection mold, the shear rate is - lo3 s-l and at the even narrower constriction of the gate it is -lo5 s-l. Gibson et aL (36) have also shown that the high shear forces associated with convergent path, such as the entry region of a nozzle, lead to fiber alignment. This helps to preserve fiber length during subsequent flow through runners. However, fiber ahgn- ment is disrupted at the gate constriction by the sudden change in shear rate, which leads to a "bottle- neck" and an increase in shear. Since the gates are only a few millimeters in length, the effect of flow through the gate on the fiber breakage is minimal.

Although some thermosets, such as DMC, are pseudoplastic, no rheological information is available on the phenolic resins (40). If they behave pseudoplasti- caUy, the benefit to flow in the gate region of a thermoset may be offset by the effect of shear heating, which promotes cure and an increase in viscosity. This raises questions concerning the use of narrow gates that are based on criteria for thermoplastic injection molding.

Fiber breakage during plasticization in the screw and barrel occurs mainly during melting at the front two flights of the screw, as demonstrated in Rg. 6. After the plasticization that occurs during normal injection molding, the nozzle was removed from the barrel, and a helical ribbon of glass fiber filled phenolic was obtained. This procedure was carried out for both of the A2710 molding compound and the laboratory synthesized PMC.

The helical ribbon (Fig. 34 and the residue after "bum-off" (Fig. 3b) was visually examined. It was found that most fiber breakage occurred where the phenolic granules were being melted. This was con-

POLYMER COMPOSITES, FEBRUARY IS#, Yo/. 19, No. 1 41

R. Sirgh, F. Chen, and F. R. Jones

- 8 Y

% 6 8 4

Nunber Average Length - 186 pfn

Weight Average Length - 411 )un Y

U

1 2 A -.. ...... - - ..- Y

Fibre Length Distribution of the First - Time Extrudate

- 8 E 9 6

Number Average Length - 130 pn

Weight Average Length - 165 pm

U

i :

Average Length - 1050 ) ~ m

Weight Average Length - 1810 Irm

Fibre Length Distribution of the Second - Time Extrudate

l5 Nunber Average Length - 525 IJJTI

Weight Average Length - 800 wn Moulding Compound 1st-time extrudate

Fibre L q I h (Wn)

Fibre Length Distribution of the Injection Mouldings

Nunber Average Length - 430 )Lm

Weight Average Length - 660 pm Mwlding Compound: 1st-time extrudate

5! '0 1500 3000 4500 6000 7500

Fibre L q l h (w)

Flg. 4. Comparison of- length distribution after one-pass and two-pass extrusion of dried dough molding compound

firmed by fiber length measurements on sections taken from the helical ribbon of A2710 molding compound, as shown in Fig. 6.

It can be seen from the helix of an experimental molding compound (Fig. 3) that before melting, feather- like tailings of long fibers are seen at the rear edge of the helix. This suggests either that those fibers at the barrel surface travel trans-flight between the screw flight and the barrel wall, or that this effect is created during extraction of the helix.

The melting action of thermoplastics in the screw compression zone of the extruder and the injection molding machine is well established. Certain aspects of melting of the phenolic molding compounds are found to be similar to the thermoplastics, to the extent that pre-melting for both materials constitutes compaction of granules and a gradual formation of a thin film of melt at the screw-barrel interface. How- ever, for these phenolics, the transition to a completely melted and plasticized state occupies the front two flights of the screw only, as opposed to 10 to 12 flights for thermoplastics. kom the onset of melting, the con- version of the solid granules to the melt occurs over

Large Injection Gate: 1.5 mm x 6 mm I

8 s g 6

a 4

Nunber Average Length - 129 pfn

Weight Average Length - 179 )un Y

e

L -- - 500 1000 1500 2000

Face Length (w)

Small Injection Gate: 0.5 mm x 6 mm

Fibre Lwgth (p)

~ig. 5. Comparison of- length distribution with di&ent injediongates (bar- width = 10 ptn).

-10 mm flight path. It can be seen from Rg. 6 that the FLD measurement of sections removed from the front two fights of the screw showed significant fiber breakage. The probable mechanisms of fiber breakage are likely to be from a complex array of flexural failure of anchored fibers in solid granules by the swirling action of the molten resin, to failure through abrasion and crushing against other granules and the barrel and screw surfaces. Further loss of fiber length may occur by post-buckling deformation, brought on by the interaction between fibers and the laminar shear motion of the polymer during subsequent flow of the material along the screw and through the nozzle. How- ever, since the phenolics melt in the h n t few flights of the screw, the extent of flow-induced damage along the screw may be nominal.

The effect of fiber fraction on the degree of fiber length degradation for thermoplastic composites has been well studied, although no single, clear trend is

42 POLYMER COMPOSITES, FEBRUARY 1998, Vol. 19, No. 1

Injection Molding of Glass Fiber Reinforced Phenolic

40

30

ul 2 D

0 2 20

s.

10

0

A2710 UC. tlight 1 tlight 2 7 flight 3

@J. 6. Fiber length analysis during conveying and melting of A271 0 mdding compound

0 100 200 300 400 500 600 700 800 900 lo00 1100 1200

Fibre Length (jun)

apparent. Some suggest that the higher the fiber fraction, the shorter the average fiber length (11, 12); by virtue of hgher viscosity, which imposes high shear on the fibers. Others suggest the opposite, since the higher fiber content leads to greater psuedoplastic behavior, which localizes the fiber damage (27).

To study the effect of glass fiber fraction on fiber length degradation, two moldings were produced by mixing GB72. This is a wood flour/minera.l filled grade, containing no glass fiber, to which phenolic coated glass fiber tows 12.7 mm in length have been added. One molding contained 2.8 ~ 6 % of glass fibers and the other 15.2 wt??. The extent of fiber damage in each molding is shown in Table 3. Longer fibers were found in moldings containing the higher fiber fraction. Since the injection molding conditions were identical for both moldmgs, the fiber content in each molding compound is consequently responsible for its level of fiber breakage. The fiber content affects the melting range and the rheology of the material (27). Since the molding compound that contains 2.8 wt% of fibers con-

Table 3. Comparison of Fiber Length Distribution in the Moldings with Different Weight Fractions (the Original Fiber Length was 12.7 mm).

Fiberweight /,, /, Shortesttiber LongemtFiber

2.8% 355 445 81 1543 15.2% 1434 1844 290 5532

- - Fraction(%) ( ~ m ) (cLm) ( ~ m ) (Pm)

'Note: $ + Number average fiber lenglh I, + Weigh average fiber length

tains more resin, it is more likely to have a longer melting range during plasticization than the molding compound, which contains 15.2 wto/6 of fibers. This leads to a prolonged shearing of the glass fibers. As mentioned above, it is likely that with increased

fiber content, the flow properties are more pseudoplastic, which localizes the shear stress to the outer regions of the melt and, thus, more fibers retain their length.

Phenolic compounds are conventionally pre-cured to raise their viscosity during processing, to reduce the overall process cycle, and to remove some of the condensate prior to mold e g . The effect of pre-cure of phenolic resin on fiber breakage and mechanical performance of moldings was examined by using two versions of PMC, one with a level of pre-cure by heat- inginanwenat 1lO"Cfor lominandtheotherwas injection molded with no pre-cure. Narrow "unidirectional" bar moldings were produced using both versions of PMC. As shown in Table 4, the compound with pre-cure was found to have a lower average fiber length. The viscosity of the compounds with and without pre-cure was qualitatively measured by weighing the melt injected into air over a set period of time. It was found that the weight of the melt without pre-cure was higher, because of its lower viscosity. The compound without pre-cure will melt faster and inflict less damage on the fibers, by reducing the shear forces on the fibers during plasticization. The lower melt viscosity will also mean easier flow through the nozzle and the runners, further reducing fiber damage.

Despite the higher average fiber length of the mold-


R. Sin& F. Chen, and F. R. Jones

Table 4. Mechanical Performance of Injection Moldings From Pultruded Molding Compounds With and Without Pre-cure (Unidirectional Mold, Standard Deviation in Brackets).

Molding Flexural Flexural lnterlaminar Average Fiber Compound Strength Modulus Shear Strength Length

No pre-cure 133.7 (19.4) 14.8 (1.6) 21.1 (2.2) 81 5 With pre-cure 190.5 (19.0) 17.0 (0.5) 27.3 (3.7) 520

(MPa) (GPa) (MPa)

Rg. 7. Tangential- orientation at the mold entry point (the shaded region corresponds to hdf of the gate exit cross section).

ing compound without pre-cure, its mechanical performance is poor, as shown in Table 4. The lower degree of final cure resulted in a lower matrix strength and lower interfacial shear strength, due to reduced fiber-matrix interaction. Thus the poorer mechanical performance can be atmbuted to the incomplete cure of the matrix and poor fiber-matrix interaction.

S.2. Fiber Orientation

The effect of mold geometry on fiber orientation was studied using a narrow "unidirectional" mold geometry, confining the flow regime to simple shear, and a disc mold geometry to study the divergent flow over a range of injection speeds. FYgure l b shows the loca- tions from which cross-sectional photomicrographs were produced to make comparative studies of skin- core structure of disc moldings, produced over a range

Fig. 8. Skin/wre uariations aamding to location in the molding and the injection speed used Microsections 1-3 arejtom gate to remote end of the molding. (a) 5% iqjection sped (bl50% injection speed. (c) 95% hjection speed


Iy'ection Molding of Glass Fiber Reinforced Phenolic

Shn core Skin Flg. 9. ?he effect of injection speed upon the shear pro@, which deterrmne ' s the skin core arrangements.

of injection speeds. The microsection in Rg. 7 illus- trates the rotation of fibers perpendicular to the flow direction as they enter the cavity. This micrograph suggests that the flow front entering the mold diverges and advances across the mold in a "fountajn flow." A series of short shot moldings further confirmed that this aspect of the filling mechanism proposed by Tadmor (41) for unfilled thermoplastics is also applic- able to thermosets.

In Rg. 8, microsections la and 3a show increase in skin regions from using high to low injection speed. The injection speed determines the shear and velocity profiles. Although no specific rheological information is available on the phenolic range of molding compounds, it has been investigated (42) and understood that addition of fibers to polymer melts can lead to pseudoplasticity. This behavior changes the flow of molten composites, at low injection speeds, to in- creasingly non-Newtonian flow at higher injection speeds, shown schematically in Rg. 9. Therefore, a high injection speed promotes a high shear rate, but narrower shearing zones. Consequently, this gives rise to reduced skin thickness and a larger core. At low injection speeds, the shear zone extends across a far

greater proportion of the flow cross section, which leads to larger skin thickness. In thermoplastics, a low shear rate at low injection speed leads to low alignment in the skins (43). However, in thermosets, since the mold temperature is set at the elevated cure temperature, low injection speed will promote cure to raise its viscosity, resulting in pronounced fiber algn- ment in the skin regions. The effect of cure is best seen by comparing la- lc with 3a-3c in Rg. 8, where the skin growth from the gate region to the remote end of the molding for low injection speed is considerably greater than that for high injection speed. Tad- mor's mold filling mechanism for thermoplastics describes the formation of a frozen skin, which at low injection speed advances in the vicinity of the gate and progressively decreases with its location further into the moldmg. Darlington (32) and Folkes (33) confirmed similar findings using glass fiber reinforced polypropylene. As seen in microsections 3a-3c in Rg. 8, our observations are in contrast to those with thermoplastics, and the skin grows from the gate region to the remote end of the molding. This shows that Tadmor's proposed mechanism for f r m n skin formation in thermoplastics does not apply to thermosets. Based on our experimental observations, the following events during mold filling are postulated to be more appropriate to these thermosetting systems:

Material deposition in the gate region during mold filling does not occur. The material that first enters the mold cavity travels to furthest point of the cavity. The viscosity gradually increases during mold filling but the material does not gel until the mold is completely filled, unless an unrealistically low injection speed is used. Solidification prior to com- plete mold filling will break up the crosslinked resin, and affect the physical properties of the molding. The variation in skin thickness is due to the pro- gressive modification to the shearing zones, as determined by the injection speed and the curing kinetics.

3.3. lldechpnical Ropertiea

The effect of skin/core structure on mechanical properties with injection speed was studied by flexural testing the central region of the disc moldings. The boundary method" was used to represent the mechanical performance of disc moldings by d e w the

Tabk 5. Mechanical Properties of Disc Moldings Made From 6887 Using a Range of Injection Speeds.

Injection Flexural Flexural Anisotropy Anisotropy SkinlCore Ratio Strength (MPa) Modulus (GPa) of Strength of Modulus of Microsection 2 0" 90" 0" 90"

speed (%I (5) (5)

95% 123 97 14.3 13.3 1:1.26 1:1.075 0.25 50% 127 81 14.2 12.0 1:1.56 1:1.83 0.375 5% 152 62 16.1 10.5 1 :2.42 1 :1.53 0.65



flow direction as the upper boundary and the transverse direction as the lower boundary, as shown in Flg. lb. Flexural testing of test pieces sectioned parallel and transverse to the flow direction gave a measure of flexural anisotropy, which increased with decreasing injection speed. Table 5 compares skin-core ratios with the degree of anisotropy, both expressed as di- mensionless quantities. Since flexural testing confines the applied load to the outer regions of the test pieces, the increase in anisotropy at low injection speeds can be attributed to thicker skins and more pronounced fiber alignment.

4. CONCLUSIONS

Fiber degradation mechanisms during injection molding of phenolic compounds have been identified, and are shown to differ from other thermosets and thermoplastics. The level of fiber breakage depends on the fiber fraction, the initial fiber length, and the extent of resin pre-cure.

The higher fiber fraction yields a longer average fiber length, which is related to the melting range during plasticization.

Injection molding of pultruded compound produced only a fourfold increase in length over the short fiber reinforced molding compounds. Average fiber length in the injection moldings is <1 mm, and is by no means close to its 4 of 5 to 8 mm for glass fiber reinforced phenolic composites. Therefore, to achieve 95% reinforcing efficiency. two improvements are required: increasing the interfacial shear strength, so as to lower Z,, and modifjmg the process to permit transport of longer glass fibers into the moldings so as to increase the average fiber length.

The injection speed has a profound influence on fiber orientation, since it affects the flow of the melt and the extent of cure during mold filling. At high injection speed, the skin/core ratio is determined by the apparent viscosity of the melt, and generally remains uniform from the gate to the remote end of the molding. However, at low injection speed, the cure starts before the mold is completed filled, which modifies the apparent viscosity of the melt as it travels through the mold, resulting in significant growth of the skin region as the melt travels away from the gate. Since this behavior cannot be explained by Tadmois flow mechanism for the thermoplastics, a separate flow mechanism for phenolics is postulated. Rheological characterization of phenolic melts is currently under way to test these observations.

The skin/core arrangements produced anisotropy in flexural strength, which decreased with decreasing skin/core ratio. High injection speed minimized flexural anisotropy, but may produce high tensile anisotropy. Further study is required to find an optimum injection speed for the desired combination of flexural and tensile properties.

ACKNOWLEDGlyLElpTs

This work was financed by a cooperative SERC/ PEG award in collaboration with T&N Technology Ltd., Arthur Lee and Sons plc, and TJ3A Composites. Discussions with Dr. David Newman, Dr. Roger Hoop- er, and Dr. AUan Lord, respectively, are gratefully ac- knowledged.

REFERENCES

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Injection molding of glass fiber reinforced phenolic composites. 2: Study of the injection molding process