Co-injection molding of reinforced polymers

Co-injection Molding of Reinforced Polymers G. AKAY

School of Zndustrial Science Cranjield Institute of Technology

Cranfield, Bedford MK43 OAL, England

Co-injection molding of calcium carbonate filled polypropylene, short glass-fiber-filled polypropylene, or unfilled high-density polyethylene melts is studied using a medium- size injection-molding machine and a center-gated disc mold. Injection molding is carried out under non-isothermal conditions. Order of injection of the melts, injection speed, and mold temperature is changed in order to understand the mold filling in general and to investigate the type of skinicore structure and mechanical interlocking of the phases in the moldings. It is found that the order of injection is not significant in obtaining a skidcore structure but it is important in obtaining extensive phase interlocking, which is reduced if the flow rate and the mold temperature are low. Presence of fillers appears to result in more mechanical interlocking of the phases.

INTRODUCTION here has been considerable technological interest in T the stratified two-phase flow of polymer melt sys-

tems because of its application to the co-extrusion of sheets, films, bicomponent fibers, and sandwich injection molding (1). Basic experimental studies of two- phase melt flow in dies and in molds (1-16) have been studied recently, although the stratified two-phase Aow of Newtonian fluids has been utilized since the early 1950s, for the pipe-line transport ofoil (17-19). It is known that during flow through a cylindrical die, an initially flat interface between two polymer melts of different viscosity becomes distorted with the low-viscosity melt encapsulating the high-viscosity melt. The completion of encapsulation depends on the relative viscosity of the melts and on the length of the flow path. Although the application of the encapsulation effect to co- extrusion and bi-component fiber formation has been studied in detail, little attention has been given to its application in injection molding. It appears that most of the published work on sandwich injection molding has been done by White, et al. (7, 9, 11, 16).

In sandwich injection molding, it is possible to obtain moldings with skins and cores consisting of different polymers. The advantages offered by such a structure can be viewed to be the same as those offered by the co- extruded sheets and films and bi-component fibers. Co-extrusion avoids many of the manufacturing ste,ps re- quired by conventional lamination and coating pro- cesses, combining the desired properties of two or more polymers by melt fabrication.

There are two important problems encountered during co-extrusion or sandwich injection molding. These

are interfacial adhesion between the phases and obtaining uniform skin/core structure.

(a) Interface adhesion: When a co-extruded sheet or film is cooled and the fluid layers solidify, the solidified layer may hold together tenaciously or they may peel apart easily. Polymer-polymer adhesion (20) may be due to the following reasons; (i) polymers may be miscible or partially miscible therefore causing interdiffusion at the interface; (ii) polymers with com- plementary polarities; or (iii) interface distortion or formation of interpenetrating networks may result in mechanical interlocking and partial adhesion.

A number of techniques exist in order to improve interface adhesion. One such technique is to use a “glue layer,” which consists of a third polymer capable of good adhesion with both polymers. This is, in fact, similar to the use of silane agents used in the coating of glass fibers or other reinforcing agents in filled polymers. In such systems, interface adhesion between the phases may be improved by the fibers or the filler particles. Mechani- cal interlocking may be more significant and filler particles coated with silane agents suitable for both polymers may provide better interfacial adhesion. For example, silane agent 26032 produced by Dow Corning can couple with inorganic materials and with phenolic, olefinic, styrenic, and esteric polymers.

(b) Un$omity of the skinlcore structure: White and his colleagues (7,9, 11, 16) have studied the influence of melt viscosity on skidcore structure formation during co-injection molding. It is found that in isothermal injection molding, the relative viscosity of the two melts is the primary variable determining the phase distribution in the moldings. Generally the most uniform skidcore

256 POLYMER COMPOSITES, OCTOBER, 1983, Vol. 4, No. 4

Co-injection Molding of Reinforced Polymers

structure occurs when the second melt entering the mold has a slightly higher viscosity than the first melt injected (16). Temperature fields are found to be very important if the first polymer melt injected has a greater activation energy of viscosity.

There appears to be no published work on the co- injection molding of reinforced polymers under non- isothermal conditions. In this study we examine the skidcore structure in the moldings obtained by co- injection of two incompatible polymers in which at least one phase is filled with short glass fibers or calcium carbonate particles. The effect of injection speed, mold temperature and order of injection are examined.

EXPERIMENTAL Materials

The materials used in this study are: HW60 GR/30: A commercially available short glass-

fiber-filled polypropylene (PP) produced by ICI. It is supplied in granular form and contains 30 percent by weight of well-dispersed short glass fibers.

CC -PP/60: A commercially available calcium carbonate filled PP containing 60 percent by weight calcium carbonate. It is produced by Plascoat Systems Limited (W-

Rigidex XGR791: A commercially available injection- molding grade high-density polyethylene (HDPE) produced by BP Chemicals.

Co-injection Molding Approximately one-half of the full injection shot

(material-A) is placed in front ofa specially designed barrel (Fig. 1 ) in the form of a cylindrical plug produced by compression molding at 180°C. The gap between the solid plug and the tip of the nozzle is packed with the same material in granular form. This barrel assembly is then fitted to a medium-size injection-molding machine equipped with a closed-loop adaptive controller. The second component (material-B) which is in the rest of the injection-molding machine, is screwed forward so that it is in contact with the solid plug in front of the barrel. Material-A in the barrel and the nozzle is melted at 210"C, the rest of the heating zones are also kept at this temperature. After 45 minutes (which is sufficient to melt the solid plug), injection molding is carried out

Heate i b a n d s - I I I

I z = 0

Fig. I. Barrel attachment used in co-injection molding experiments.

using a center gated 6 millimeter thick disc mold with a diameter of 170 millimeters (Fig. 2). The length of the sprue is 10 centimeters. When molding is complete, barrel and nozzle assembly are removed and cleaned for use in the next injection molding. We use four injection speeds, corresponding to the flow rates, Q,,, 160 cchec, 80 ccfsec, 20 ccfsec, and 4 cdsec. In all cases mold temperature (TM) is either 25" or 65°C. Nozzle temperature is denoted by TI .

Hold time of the moldings in the mold after the completion of the injection is 20 seconds and the packing pressure is 5500 psi. A pressure transducer is fitted to the barrel 75 millimeters from the nozzle tip and the development of the nozzle pressure; PN is recorded together with the plunger position, z . At the beginning of the injection z = o and the maximum plunger displacement is given byz = z'. Details of this injection-molding machine are available (21, 22).

Depending upon which material is in front and which material is at the back of the barrel, we obtain a different skinkore structure. In Table 1, we give the materials combination in which the material occupying the front of the barrel (material-A) is written first.

Structural Examination Thin slices (-100 micrometers thick) are cut from the

sprue or from the molding along a radius from the gate. The slices cut from the sprue are L = 5 millimeters away from the gate, where L is the distance measured from the mold surface (on the sprue side) along the sprue as shown in Fig. 2. Four slices are cut from the mold itself in order to evaluate the fiber orientation development along a radius. The first slice cut from the molding in- cludes the gate and part of the sprue as shown in Fig. 2.

These slices are laid on a fine grain photographic plate and exposed to a beam of X-rays which casts "shad- ows" of the fibers on to the plate. This technique is known as contact micro-radiography (CMR), and has been described by Darlington and McGinley (23).

RESULTS AND DISCUSSION Development of Nozzle Pressure during Co-injection Molding

Transient nozzle pressure development during co- injection molding may be expected to reflect the pressure development patterns of the individual polymer

F L P W @ C H R l o c a t i o n s

I * - 85mm-I Fig. 2 . Centergated disc mold and the locution of slices cut from the moldings for contact micro-rudiography ( C M R ) inuestiga- tion. Plunger position ut the sturt of injection is z = o and the muximum plunger displacement is Z.

POLYMER COMPOSITES, OCTOBER, 1983, Vol. 4, No. 4 257

G . Akay

Table 1. Material Combinations in Co-injection Molding Experiments.

Combination Material-A Material-6

I HW60 GR130 CC-PPIGO . . . . - - -. ., - - II. CC-PPIGO HW60 GRl30 111. HW60 GRl30 Rigidex XGR791 IV. Rigidex XGR791 HW60 GRI30

melts. It has been shown that the transient nozzle pressure of filled polypropylene (for example HW60 GW30 and CC-PP/Go) yields large pressure overshoots during mold filling (22), whereas the pressure overshoots in the unfilled (Rigidex XGR791) or filled HDPE (Holstalen GC VP7620 GV 1/20 which contains 20 percent glass fiber) and nylon 6.6 (Maranyl A100 and Maranyl A190, ICI trade names for unfilled nylon 6.6 and filled nylon 6.6 containing 32 percent by weight of short glass fibers respectively) or polycarbonate (Lexan 141R and Lexan 3412, where Lexan 141R is the unfilled base polymer used in the filled grades) a re not significant (21). Since high-speed injection provides the most significant difference between various types of base materials and fillers, we therefore present the nozzle pressure development when the flow rate is Qo = 160 cchec. In Fig. 3, through 6 we reproduce the transient nozzle pressure together with plunger position for all the polymer pairs.

The maximum nozzle pressure reached during injection molding of CC-PP/60 is higher when compared with that of HW60 GW30, as shown in Ref. 22. InFig. 3, there is an initial pressure overshoot (due to the start-up flow of HW60 GW30), which is followed by the second pressure over-shoot due to the start-up flow of CC-PP/Go. In Fig. 4, there is only one pressure over-

4

0 0 t i m e 1 ( s e c ) 2

Fig. 3. Development of nozzle pressure PN during the CO-

injection molding of short glass-fiber-filled polypropylene (HW60 GR130; material-A) and calcium curbonate filled polypropylene (CC-PPI60; material-B) when the nozzle and barrel temperatures (TI are 210"C, and mold temperature (TJ is 25"C, und $ow rate (QJ is 160 cclsec.

258

z = o h

0 t i m e 1 (sec) 2 Fig. 4. Development of nozzle pressure P , during the co- injection molding of calcium carbonate filled polypropylene (CC-PPI60; materiald) and glass-jiber-filled polypropylene (HW60 GRl30; material-8) when T , = 21O"c, T , = 25°C and Q. = 160 cclsec.

0 0 t ime 1 b e d 2

Fig. 5. Development of nozzle pressure P R during the co- injection molding of glass-fiber-jilled polypropylene (HW60 GR130; materiald) and unfilled high density polyethylene (RigidexXGR791; material-€3) when T I = 210@, TM = 25"C, and Q,, = 160 cclsec.

POLYMER COMPOSITES, OCTOBER, 1983, Vol. 4, No. 4


z = o+ I \ , lxlo'ty

(psi)

t = i 0 0

0 timekec) 1 2 Fig. 6. Development of nozzle pressure P,v during the co- injection molding of unfilled high-density polyethylene (Rigidex XCR791; material-A) and glass-jiber-filled polypropylene (HW60 GRl30; material-B) when TI = 21O'C, T M = 25C, and Q0 = 1 6 0 cclsec.

shoot due to the start-up flow of CC-PP/GO and the pressure overshoot due to HW60 GR/30 is probably overtaken by the initial overshoot. In Fig. 5 , pressure overshoot due to HW60 GW30 is still present while it is reduced considerably in Fig. 6, in which the front material is unfilled HDPE, which does not yield any pressure overshoot. Therefore the presence of low-viscosity melt or melt which does not yield pressure overshoot helps to reduce the maximum injection pressure. It is shown by Lee and White (8) that total pressure loss during co-extrusion of polymers is much closer to that of the less viscous component (which forms the skin) at high length to diameter ratio of the capillary.

Mechanism of Mold Filling during Co-injection Molding

Among the materials used in co-injection-molding calcium carbonate filled PP (CC-PP/60) has the highest viscosity at all shear rates followed by glass-fiber-filled PP (HW60 GW30). Viscosity of the unfilled HDPE (Fiigidex XGR791) is the lowest. The rheology of various filled and unfilled polymer melts, including those re- ported here, have recently been studied (24-26). These studies include flow instability (24), entrance and exit pressure loss (25), and the effects of pressure and temperature on the steady viscosity (26).

It has been shown by Young, et al. (16) that, in an isothermal mold filling of two melts, the most uniform skidcore structure occurs when the second melt entering the mold has a slightly higher viscosity than the first melt injected. It is found that the relative viscosity of the melts is the primary variable determining phase distribution in the molding.

The present co-injection molding experiments are substantially different from those used by White and his colleagues (7, 9, 11, 16). In our case, the mold filling is not isothermal, injection rate is very high and the total

POLYMER COMPOSITES, OCTOBER, 1983, Voi. 4, No. 4

length of the flow path is small. Therefore direct com- parison may not be possible but yet their results may in general be applicable to our experiments.

Investigation of the moldings by contact microradi- ography and the melt in the barrelattachment after each co-injection molding indicates, that material-A (which occupies the front of the barrel) always forms the skins and is not removed from the walls of the barrel and the sprue by material-B, irrespective of the relative viscosity of the melts. In the mold, material-B may burst through material-A and also wet the walls of the mold. This behavior depends on the relative viscosity of the melts, injection speed, and the temperatures ofthe melt and the mold. Burst-through is more pronounced if material-B has lower viscosity, as shown by Younget al. (16). It is found that if the injection speed is high, burst- through is not observed in any of the co-injection experiments even if material-B has lower viscosity. At high injection speeds, material-A appears to fill the mold and subsequently be pushed forward without any substan- tial freezing at the wall. As a result, the edges of the moldings are richer in material-A. If however, the flow rate and the mold temperature are low, material-A will solidifL readily and will not be able to reach the end of the mold. As a result, hot material-B will burst through the center of material-A and fill the rest of the mold. Hence the concentration of material-B will be higher at the edge of the moldings. This type of behavior is more pronounced if the mold temperature is low and material-A is filled (i.e. high viscosity and high heat conductivity). If however, material-A is unfilled, (low viscosity, low conductivity) skin may extend more uniformly further towards the edge of the moldings.

These observations are illustrated in Figs. 7-12. In these figures, contact micro-radiographs of the slices cut from the moldings along a radius are shown in order to illustrate the development of skidcore structure. In Figs. 7 a,b,c, the development of the skidcore structure in co-injection molded CC-PP/60 and HW60 GW30 is shown when the flow rate is high (Qo = 160 cdsec). The effect of injection speed on the formation of skidcore structure is illustrated in Figs. 8 a, b, c, where the flow rate is Qo = 4 d s e c . The order of injection is reversed in obtaining the moldings illustrated in Figs. 9 and 10. In Figs. 9 a,b,c the injection speed is high (Qo = 160 cc/sec) and it is reduced to 4 cdsec in Figs. 10 a,b. At low injection speeds, skin thickness is more uniform initially but a burst through may occur especially ifthe high viscosity fluid occupies the skin. The effect of injection order can also be seen Figs. 11 and 12.

Interface Stability In co-injection molding, interface instability is some-

thing desired since it will result in stronger interfacial strength. Inspection ofFigs. 7 through 1 1 indicates that there is only a small amount of interface instability during the co-injection molding of HW60/30 (occupying the front of the barrel) and CC-PP/60 (occupying the back of the barrel) compared with co-injection molding in the reverse order. The same is true in the case of Fiigidex XGR791 (material-A) and HW60 GW30 (material-B).

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Fig. 7 . Skinlcore structure, fiber orientation, and interface instability in co-injection molded calcium carbonate filled polypropylene (CC-PPl60) und glass-fiberjilled polypropylene (HW60 GRl30;) when T I = 21O"c, T.w = 25V, and Q,, = 160 cclsec. (u) slice-1; (h) slice-3; (c ) slice-4.

Interface instability is more pronounced near the edge of the moldings and when the flow is fast.

The equilibrium configuration of two immiscible liquids of different viscosities is such that the high-viscosity fluid is surrounded by the low-viscosity fluid. Before the start of flow, if this configuration exists, the fluid will keep it. If however, the initial configuration is different, fluids will try to achieve stable equilibrium configuration during flow, provided sufficient flow length and time are allowed. Further complications will arise in polymeric liquids due to the presence of normal stresses, which will result in interface distortion or rip-

Fig. 8. Skinlcore structure, f iber orientation, and interface instability in co-injection molded CC-PPI60 and HW60 GRl30 when Q,, = 4 cclsec. All other condtions ure the same as in Fig. 7. (a ) slice-1; (b) slice-3; ( c ) slice-4.

ple at the interface even when the equilibrium configuration is achieved. The interface distortion observed in our experiments is probably due to a tendency by the melts to achieve a stable skidcore structure. If material-A has higher viscosity or its viscosity increases during mold filling due to cooling, then it tends to move towards the center, while the low-viscosity melt (material-B) tends to surround it. These conditions are achieved towards the edge of the moldings due to cooling of material-A, even if the viscosity difference of the phases at the same temperature is not significant enough to form any instability under isothermal conditions. Another important reason why instability occurs

260 POLYMER COMPOSITES, OCTOBER, 1983, Yo/. 4, No. 4


Fig. 9. Skinlcore structure, fiber Orientation, and interface instability in co-injection molded HW60 GRl30 (material-A) and CC-PP160 (material-B) when T I = 210T:, T M = 25C, and Qo = 160 cclsec. (a) slice-1; (b) slice-& (c) slice-4.

away from the gate is the absence of entrance effects in these regions.

In glass-fiber-filled materials, fiber orientation is along the stream lines in the sprue (except at the center of the sprue where the fiber orientation is perpendicu- lar to the flow direction) and also near the gate. There- fore, the viscosity of material-A is low in these regions. When the fiber orientation becomes random or perpen- dicular to main flow direction with increasing distance from the gate, its viscosity increases, in addition to viscosity increase due to cooling. Hence, interface instability starts towards the edge of the moldings.

POLYMER COMPOSiTES, OCTOBER, 1983, Vol. 4, No. 4

‘Fig. 10. The effect o f f o w rate on skinlcore structure,fiber orientation, and interface instability in co-injection-niolled HW60 GRl30 (muterial-A) und CC-PPI60 (material-B) when T I = 210C, T , = 25C, and Q n = 4 cclsec. (a) slice-3; (b) slice-4.

Fiber Orientation and Variation of Skin Thickness Fiber-orientation pattern in co-injection molded

parts is very similar to that encountered in fiber- reinforced PP melt (HW60 GW30) when injection is carried out using one component only. In the sprue and at the gate, fibers are highly oriented parallel to the main flow direction (parallel orientation) near the wall. This parallel orientation is still maintained even if the skin is occupied by another material (i.e., CC-PP/60 or Rigidex XGR791) as shown in Figs. 7a, 8a, and 12a. However, parallel orientation is soon destroyed in the mold if the fiber-reinforced material is in the core. When the fiber-reinforced material forms the skin, parallel alignment is maintained for some distance (approximately 4 centimeters from the gate) in the mold as seen in Fig. l l a , 12a. Whenever the fiber-reinforced PP occupies the core, fiber orientation appears to be transverse to the main flow direction (transverse orientation).

When the core material has higher viscosity (i.e. co- injection of HW60 GW30 and CC-PP/60 in Fig. 9 and10 or co-injection of Rigidex XGR791 and HW60 GW30 in Fig. 12) the thickness of the skin is small initially, especially on the opposite side to the sprue, but the skin thickness increases gradually if the injection speed is high, as shown inFigs. 9 andl2. Ifthe core material has

26 1

G . Akuy

F i g . 1 1 . Skinlcore structure,jiber orientution, und interfuce instubility in co-injection molded short gluss-fiber-filled polypropylene (HW60 GRl30; muterial-A) und unfilled high- density polyethylene (Rigidex XGR791; muterial-B) when T , = 21O"c, TM = 25T, und Q n = 160 cckec. ((I) slice-1; (h) slice-4.

lower viscosity (i.e. co-injection of CC-PP/GO and HW60 GW30 in Fig. 7 and 8 or co-injection of HW60 GW30 and Rigidex XGR791 in Fig. 11) core thickness is high initially and may extend uniformly along a radium, although a burst-through is possible in this case. In general, skin on the sprue side is always thicker and fibers form parallel orientation, while on the opposite side, skin thickness is low and fibers form transverse orientation. This is due to the entrance effect and the flow is not symmetrical about the midplane of the disc. Co- injection molding using short shots clearly confirms this conclusion, as seen in Figs. 13 a-c. After injecting part of material-A into the mold, material-B emerges from the center of the sprue and impinges on to the opposite wall and curves back towards the sprue side (Figs. 13a and b). The impingement of material-B removes material-A from the skin and hence the thickness of the skin de- creases (Fig. 13c). It appears that if material-A has higher viscosity, removal by material-B is not as effec- tive, i.e., compareFigs. 7a, 8a, andl la withga andl3a.

Fiber orientation at the HDPE-PP interface appear to have a wavelike structure, as seen in Fig. 12a. Similar structure is also found in the short shots as seen in Fig. 13. In short shots at high injection speeds, a large number of voids appear to be present at HDPE/PP interface on the sprue side. Fiber orientation at this free surface

262

Fig. 12. The effect of injection order on the skinlcore structure, j iber orientation and interjuce instubility in co-injection, und interfuce instability in co-injection-molded Rigidex XGR791 (muteriul-A) and HW60 GRl30 (material-B) when TI=210"C, T,,, = 25"C, und Q. = 160 cclsec. ((I) slice-1, sprue side; (h) slice-4.

resembles that in Fig. 12a. It is therefore conceivable that during the packing stage after the completion ofthe injection, voids are eliminated and the two phases are contacted, resulting in a wavelike fiber orientation at the interface. The shape of the wavy interface somewhat resembles that predicted by Khan and Han (27, 28) and wavelike fiber oreintation may also be the result of the wavy interface. Interface distortion and flow instability during co-extrusion of polymers have been discussed by Lee and White (8).

Void formation in fiber-reinforced PP results in a foamed core which can be observed visually. Under identical molding conditions, unreinforced PP does not yield voids, and voiding increases with increasing injection speed and mold temperature. These voids can not be observed in contact micro-radiographs but can be easily detected in transmission optical micrographs (21). If the mold temperature is high (SSOC), the foamed core can extend to the edge of the moldings. When the mold temperature is reduced to 25"C, foamed core is only confined to a small region at the entrance (21).

We note that fiber-free layers (21,22) are also present in the sprue and near the gate as seen in Figs. 9a and b and l l a ) . Once again, the fiber-free layers are sand- wiched between two regions where fiber orientation is parallel to in the main flow direction. Particle migra-

POLYMER COMPOSITES, OCTOBER, 1983, Vol. 4, No. 4


Fig. 13. Skiidcore structure, fiber orientation, and interface instability during the co-injection molding of Rigidex XGR791 (material-A)andHW60GR/30(material-B)when TI =21O“c, T M = 25C, and Q,, = 80 cc/sec. Sequence of moldfilling: (a) slice-1; ufter injecting one-half of the full shot; (b) slice-2; after injecting one-half of the full shot; (c) slice-3; ufter injecting three quarters of the full shot. No packing pressure is present. Fiber orientation pattern ut the gate in the sprue gives the impression of melt fracture in the core &id.

tions is also observed in CC-PP/60 phase as seen in Fig. 8. Particle-depleted regions are in the form of fine layers near the mold cavity wall when the flow is slow, as observed previously (22). If the unfilled material occupies the skin (i.e., co-injection of Rigidex XGR791 and HW60/30), then differential crystallization during flow in the sprue results in concentric rings of varying transparencies, similar to those obtained during injection molding of the unfilled PP and HDPE (21, 22).

CONCLUSIONS Co-injection molding of filled or unfilled polymers is

studied using a center-grated disc mold in order to understand the mold filling mechanism in general and to

determine the type of skidcore structure in the moldings. It is found (not surprisingly) that the flow is not symmetrical and the entrance length is large when the injection speed is high. This non-symmetrical flow af- fects the uniformity of the skin along a radius and results in substantially different skin thickness on the opposite surfaces of the moldings.

Conditions for obtaining a certain type of skidcore structure under non-isothermal flow conditions are substantially different from isothermal flow conditions. Order of injection is not significant in obtaining a skidcore structure. Important variables in obtaining uniform skin/core structure are injection speed and the temperature of the melt and the mold. Interface instability appears to be more pronounced if the high- viscosity fluid occupies the skin and if the injection speed is high.

Fiber orientation pattern during co-injection molding is similar to that in single-phase injection molding. Fiber-free or particle-depleted regions are also present in co-injection molded parts and their occurrence fol- lows the general pattern discussed previously (21, 22). The presence of fillers appears to enhance mechanical interlocking of the phases.

This process may be adapted for industrial applica- tions by feeding the two components concentrically using an additional extruder and for a given mold geom- etry by programming the injection speed and temperature of the melts and the mold, so that a uniform skin/core structure with enhanced mechanical interlocking is obtained.

ACKNOWLEDGEMENTS This research has been supported by the National

Physical Laboratory (NPL) and I would like to acknowl- edge the financial support and help I have received from them. My thanks are due to Mr. K. Thomas (NPL) for helpful discussions; Mr. A. J. Scott for his invaluable help in the experiments; and Professor D. W. Saunders and Dr. M. W. Darlington for their help and continued interest in this research. I am grateful to ICI, BP Chemi- cals, and Plascoat Systems Limited for the donation of the materials used in this study.

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Co-injection molding of reinforced polymers