Glass Bead-Fil led Polypropylene Part II: Mold-Filling Studies During Injection Molding*

L. R. SCHMIDT

Gen era/ Electric Cornpa n y Corporute Research and Development

C he mica1 La ho ratory Schenectady, New York 12301

Five glass I)ead-filled pol ypropyleiie coniposites, which had been rhcologically characterized, wcrc used i n mold-filling studies to investigate the iriflucnce of rigid beads (0 to 26 vol percent) in complex flow fields. The two Ixisic Bows studied \vere simple shear flow in an edge-gated, rectangular cavit) a n d stagnation flo\v leading to diverging radial How in a center-gated, rectangular cavit).. Pigmented tracer elenients were used to illustrate flow patterns a id to distinguish the skin-core structurc. which is ii consec1rience of the injection molding process.

Despite the strong dependence ofthe rheological properties (both viscous and elastic) and physical properties (density, thermal conductivity, etc.) on bead volume fraction, the basic character of the polymer dominated the flow behavior of each composite and the flow patterns were unaffected by the beads. Injection pressures were increased with increasing bead frac- tion to maintain the same, constant volumetric flow rate (2.21 in.3/~nin) for each composite. Relative injection forces follo\v closely the relative viscosity trend.

INTRODUCTION uring the past five years the number of published D mold-filling studies associated with the injection

molding process has increased significantly. The inore recent investigations differ from the earlier work (1,2) in that not only was the flow observed in a coniplex channel and pressures arid temperatures monitored, but that an analysis of the flow and/or resulting structure was at- tempted.

Kamal and Kenig (3) observed the bulk flow in 21

rectangular channel as a function of filling time to evaluate the effects oftemperature and pressure profiles on the position and shape of the advancing flow front. I n addition, these authors developed a computer simula- tion of 11101d filling to predict the flow of a power-law fluid. Similar computer studies for different channel geonietries have b c ~ e n conipletrtl hy several rliff’erent authors (4-9).

The primary ohjective of the above studies was to understand the average or bulk flow features of the non-isothermal, unsteady injection molding process. The theoretical results (computer simulations with power-law fluids) are in surprisingly good agreement with the experimental data. More recently, attention has heen given to the details of the splitting melt front (10-12) and the resulting skin-core structure o f

* i’resenrcd 1 1 1 part at the 45th Annual Meeting of the Socirt? of Rheolog\, Ovrolwr 2 1-24, 1971, Amhrr\t, hlarsachu*etts

molded parts. Kantz, Newinan, and Stigale (13) used optical microscopy and X-ray diffraction to relate niolecular orientation and the morphological features of diflerent crystalline zones to tnechanical properties of molded test bars. Tan and Katnal (14) have reported on a in o 1 d i n g study to re 1 ate i n o rp h ol og y ( p ri in aril y molecular orientation) of polyethylene to processing conditions.

This paper presents mold filling results on five glass bead-filled polypropylene cornposites which had previ- ously 11een rheologically characterized (15). The glass 1,eads were added to the polypropylene to systemati- cally alter the physical and mechanical properties of the system so that the effect of filler on processing behavior could lx, evaluatcd. An earlier injection inolding study investigated composites of only 5 vol percent glass heads i n polypropylene and 5 vol percent short glass fibers i n the same polypropylene (12). When compared to the unfilled polypropylene, the composites showed oiily sinall differelices i n rheological parameters and essen- tially 110 processing cliffet-ences. However, when the glass fil)er-filled composite was molded into large area parts with thin cross sections, severe distortion or war- page was ohserved after the part was removed from the mold. The I)ead-filletl composite did not warp in ;I simi- Iiir set of tests. Since the analysis of any glass fiber-filled melt is complicated b y the fiber-length distribution and the fiber-orietit~~tioii distrihution, this study was initi-

666 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1977, Vol. 17, No. 9

Glass Bead-Filled Polypropylene Part 11:

ated with solid glass beads (aspect ratio of 1 .O) as a logical first step to understanding complex flows ofcomposites.

Tensile and flexural test bars were also molded from each composite for a mechanical properties profile. A schematic of the cavity plate and the test results are reported in reference 15.

EXPERIMENTAL The injection molding machine employed in this

study consists ofa small mold attached to the reservoir of an Instron capillary rheometer, model MCH. The mold has six basic parts-a cavity plate sandwiched between two face plates, two end yokes and a capillary (0.225 in. ID)-all machined from 347 stainless steel. Each steel face plate has a centrally located plug which can be replaced with either a fiber optic probe for an infrared thermometer or a pressure transducer. Two similar cav- ity plates were used with the mold in this study. Both plates are '/R in. thick and have a 2% in. square hole cut out. No further machining was required for the center gated configuration since an adaptor replaced the plug in one face plate and this adaptor served as the gate (0.125 in. ID) The edge-gated cavity had a '/R in. square slot machined through one end and midway across the end. Since the mold is massive (7.5 Ibs), heater bands were not necessary. Mold temperatnres were established by placing the fixture in a natural convection oven for a snfficient period of time to allow the metal to reach the set temperature which was checked with a surface ther- mometer. During a typical run little heat was transfer- red from the mold to the surroundings since the entire molding operation took less than one minute. Details of this niold and the tracer technique described in the following paragraph can be found in references 11 and 12.

The compounding of the glass beads and polypropyl- ene powder is described in Part I of this work (15). The thick compression molded sheets which were broken up for the capillary rheometry study were also used in the molding trials. Each composite material was molded into rods, measuring % in. diameter, by compression molding the material in the reservoir of the capillary rheoineter. The temperature ofthe reservoir was held at 240°C during the compression molding then allowed to cool slowly to room temperature. The rod was then removed and five. holes (0.10 in. diaineter) were drilled pq)endicular to the axis of symmetry to a depth of '/4 in. The hole centers were spaced 1.5 in. from each other. The first hole was drilled ?h in. from the rod bottom, i.e., the surface which contacts the mold capillary. A small tracer element, preinolded into a cylinder 0.10 in. diameter and 0.12 in. long and consisting of the compos- ite plus less than 1 percent inorganic pigment was placed in each hole and positioned in the center of the reservoir rod. A different colored tracer was placed in each hole. Thc tracer order in each case was red, yellow, green, blue and black and this is the order in which the tracers entered the mold cavity. The remainder of each drilled hole was plugged with the original material (unpig- mented). Thus, the tracers occupy precise, known loca- tions in the inlet stream.

Mold-Filling Studies During lnjection Molding

Two basic molding flows were investigated-simple shear flow with the edge-gated cavity and stagnation flow leading to diverging radial flow from the center- gated cavity. A pressure transducer (Dynisco, model PT-467 with model PT-420A tip) was flush mounted at the stagnation point for the center-gated runs.

The molding procedure followed five steps. First, the rod with the tracers in place was inserted into the hot reservoir and allowed to reach the set temperature. During this heat-up period a small steel plug was po- sitioned in the capillary to prevent material from flowing under the force of gravity. Just after the rod reached 240°C (heat-up t ime determined previously with thermocouples in the melt), the plug was removed from the capillary and the warm mold (SOOC) was quickly attached. For the radial flow experiments the pressure transducer was next flush-mounted at the stagnation point. The melt was then injected into the cavity at a constant volumetric flow rate of 2.21 in.Ymin. The cross- head was stopped manually when the packing pressure reached about 3,000 psi. The mold was removed from the capillary and allowed to cool slowly to about 35°C and then disassembled to remove the plaque.

Several samples were rerun with one or both face plates replaced with a special face plate having a 1-in. thick glass window so that the flow could be recorded on movie film. The flow patterns for each tracer have been analyzed and will be presented i n a future publication (17).

RESULTS AND DISCUSSION Figure 1 shows twelve injection-molded plaques

which were molded under identical conditions but with two different types of cavity gates. The flow patterns are essentially identical for each basic flow, i .e. , the location and spacing of the color bands are not altered by the glass beads. The skin-core structure which results from the splitting melt front, has previorlsly been discussed in detail (11, 12). Briefly, the flow front is continuously being regenerated by new material flowing along the midplane. Thus, only inidplane material is found at the fluid-air interface. This behavior is a direct consequence of the zero velocity of the polymer at the mold walls (no-slip boundary condition) and the high velocity inidplane inaterial. A solid skin forms next to the rela- tively cool inold walls. The skin thickness w a s found to be independent of h a d fraction despite thc increase in th(arnial conductivity which results when glass beads are added to polypropylene (16). Hence, the skin thickness and core diinension at a point remain constant during

Fig. I lnjeclion-inolded plaques-2-112 x 2-112 x 118 in

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L. R . Schmidt

the time necessary to fill the cavity and the tracers remain distinct without build-up of one beneath the other. Since the melt flowing in the core loses relatively little heat, this flow can be treated as if it were isother- mal.

A second consequence of the no-slip boundary condi- tion is that the first material to enter the cavity is found closest to the gate. The next material to enter the mold cavity is found further from the gate and so forth. For the edge-gated plaques, the flow patterns are nearly identi- cal on both sides of the plaque. In this case, the exten- sion of the symmetry axis of the reservoir into the cavity lies on the cavity midplane. The flow symmetry estab- lished in the reservoir is preserved in the rectangular channel.

With the center-gated plaque, on the other hand, the flow patterns are not symmetric about the cavity midplane since this is a stagnation flow and under lami- nar flow conditions, streamlines cannot cross. The res- ervoir symmetry is not maintained in the rectangular channel, but midplane symmetry is again established downstream from the stagnation point. This explains why all of the tracers (Figs. 2 , 3 and 4 ) except for a portion of the red in the entry region, are found on the side opposite the gate (stagnation-point side). The red tracer is unique because it entered the cavity during the time when the complex entry region was being estab- lished. After reviewing the movie film ofthis flow, it was learned that initially the melt extrudes through the cir-

Fig. 3 . Cro,ss section of injection-molded polyprop!/lene plaque showing uctuul trucer locations. Rudiul ,P(JIC tous .from left to right.

Fig. 4 . Cross section of injection-molded, glass bead-filled poly- propylene plaque showing nctual tracer locations. RadiolJow wasfrom right to left (6 = 0.26). Note the black trucerin the core und the resin rich region.

cular gate and the extrudate remains cylindrical until contact is made with the cavity boundary opposite the gate. Next the extrudate bulges and begins to form a small flat disk as additional melt enters the cavity. At this point the red tracer is found at the melt front, i .e. , melt-air interface. The melt front then splits in two halves at the cavity midplane and the halves move to respective mold walls as more material enters the cavity. I t is only at this time that the flow field becomes estab- lished as a stagnation flow. The remaining four tracers are found on the stagnation-point side since they were initially located in the central portion of the reservoir fluid and move through a well-formed entry channel.

Since the flow removed from the complex gate region is basically a pressure-driven flow, a flattened parabolic velocity profile (power-law behavior) is established in the open core, i.e., between skin material. The splitting flow front and subsequent shear create the “V” patterns in the skin. Thus, the tracer order from the gate outward remains the same as the edge-gated plaques. Figures 3 and4 are photomicrographs of the cross section of injec- tion molded plaques (center gated) of two different ma- terials (+ = 0.0 and + = 0.26). Special note should be made of the increasing skin thickness from the stagna- tion point (as indicated by the tracer pattern) to a dis- tance equal to about 5 times the plate separation. In both pliotoiiiicrographs and F i g . 2 the Mock tracer is seen as a long line Iwcause the cavity filled, i.e., the flow stopped while the majority of this tracer was still moving in the core. Had the cavity been larger in the flow direction, the Iilack tracer would also look like a “V” mark. Also, with each composite sample there is a resin-rich region leading away from the gate (Fig. 4 ), This head migration is prohably due to a coml)ination of entrancr effects associated with the viscoelastic behavior of the melt i n a sudden expansion and rrarrangelnent of the velocity prafilc associated with the stagnation flow. A resin rich region w a s not obscrved with any of the edge-gated plaques. A more detailed analysis of this diverging flow w a s discussed i n reference 17 and will lie published in the near future. The position of the flow front and the tracers during filling (data obtained from 16 min movie film) provide inforination on the txtent and nature ofthe slirar and extensional flows crcated I)? this channel geometry.

668 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1977, Vol. 17, No. 9

Glass Bead-Filled Polypropylene Part 11: Mold-Filling Studies During Injection Molding

1000

The rheological characterization of the composites (15) indicated that increasing injection pressures would be required to injection inold resin with increasing vol- ume fraction of filler. Figures 5 and 6 show plunger

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pressure (injection pressure) and stagnation-point pressure for two different composites molded under identical conditions. The plunger pressure attains a con- stant value after the initial pressure drops through the capillary and gate. This pressure is measured by the load cell on the crosshead of the Instron. The plunger pressure reflects primarily the pressure drop through the reservoir, capillary and inold gate. The plunger pressure remains constant as the mold cavity fills be- cause the converging channel above the capillary prob- ably acts as a buffer, i .e. , the stagnant material in the entrance channel above the capillary can change geometry to reduce the net resistance to flow or energy loss. The stagnation point pressure transducer shows the expected result that as the path length increases so does the pressure drop despite an increase in cross sectional area for flow.

The plunger pressure, although relatively insensitive, provides a net resistance to flow for each material in this complex channel. Since the plunger pressure traces were similar for each composite, i .e., each showed a constant pressure region, relative injection pressures or forces were computed. These relative values are plotted along with the relative viscosity in Fig. 7. Relative force is equivalent to the relative viscosity because the geometry was the same from sample to sample and hence the shear rates will be nearly the same. The slight variation in shear rate results from the interaction of the beads in the flow field. Increasing bead fraction pro- duces an increase in flatness (more plug like) of the velocity profile. Hence, at equal volumetric flow rates, the greater the bead fraction, the greater the shear rate at the wall. The relative force values follow a similar trend to relative shear viscosity with the force values Falling below the highest shear-rate viscosity data. The niaxiinuin shear rate for the molding flows occurs where

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POLYMER ENG/N€ERING AND SCIENCE, SEPTEMBER, 1977, VoI. 17, No. 9 669

L. R . Schmidt

the cross sectional area is the smallest. This point is clearly the gate. The maximum shear rate in the gate was computed to be about 1,500 sec-’ so the trend in the force data was expected and these data are completely consistent with the viscosity data. However, a definite lack ofiiiHuence on the flow patterns was not anticipated since the rheological characterization showed that both the viscous and elastic response of the filled resins were significantly different from the unfilled resin. At low strains the storage modulus data show that the beads reinforce the molecular entanglements which permits the polymer to store inore mechanical energy. How- ever, for large strains the beads interfere with the molecular orientation of the polymer chains, and de- crease the ainoiuit of energy which can be stored elasti- cally.

CONCLUSIONS Despite the rheological and physical changes to the polypropylene produced by the glass beads, the basic character of the polymer dominates in complex flow channels. The How patterns illustrated with colored tracers verify this point and clearly display the skin-core structure i l l the cross section. The skin thickness re- mained unchanged with increasing bead fraction b u t head migration, i.e., a resin rich region, was observed with each composite sample in radial flow. The oidy observed processing change resulting from an increase in bead concentration was the increase in injection force which was required to maintain a constant volumetric flow rate.

ACKNOWLEDGMENT The author is indebted to I .M. Mella for his help in

obtaining the photomicrographs.

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REFERENCES H. S. Spcncerand G.D. Gilmore,J. Coll. Sci., 6 , 118(1951). C. E . B e ~ e r and K. S. Spencer, “Hheolog!, Theory aiid .lpplicatioii\,” \’oI. 3, p. 510, F. R. Eirich, Ed., Accideniic Press Inc. Pul)., N e w TorL (1960). \,I. R . Kamal a r i d S. Kenig,Polym. Errg. Sci., 12,294(1972). J. L. Berger and C. G. Gogos, PoIlyrn. E n g . Sci., 13, 102 (1973). P. C . WU, C. F. Huang, and C . G. Gogos, P o l y r n . Eng. Sci., 14, 223 ( 1974). G . \Villiamsaiid 11. A. Lortl,Pol!/ni. Erig. Sci., 15,553(1975). M. -4. Lordmt l G. \ ~ i l l i ~ i i i i ~ , P ~ ) ~ ! / ~ ~ i . E t i g . Sci., 15,569(1973). J . F. Stevciison, C . A. Hielwr. .I\. Galsko!, ‘i i it l K. K. \$‘ang, Tech. Piiprr, 34th AXTEC, Soc. Plastics Eng.Vol.22, p. 282, .ltluritic City, N.1. (~Ipr i l 26-29, 1976). h l . H. K:wial, Y. K i l o , ;ind P. H. Doan, Pol! /nr . Eiig. Sci., 15, 863 (1975). 2. T;idmor,/. ;\ppl. Po/ ! / tn . Sci., 8, l75:3 (1974). I,. 13, Schmidt, PolyTrt. E i i g . Sci., 14. 797 (1974). I,. R. Schiiiitlt, “.4CS Advances i n Chenii\ti? Series,” Vol. 132, S . A. J . Platzer, Ed., p. 4lFj (1975). \I. R. Kaiitz, H. L). Nrwmun, JI-., and F. 1I. Stigalr,]. .-tpp/. Po/!/tii. S c i . , 16, 12441) (1972). V. Tun :inti hl. H . Katn;tl, Tech. Paper, 34th AN’I’EC:. Soc. Plastic\ Eiig., Yol. 22, p. :339, .%ttlaiitic Cit!, N . J . (.\pril26-29, 1976). 1). L. Farilkiic.1- and I,. K . Scliinitlt, Po/!ytri, Erig. S ( , i . , 17, 651 (1971). L. E. Nielsen,]. A p p l . Pollym. Sci . , 17, 3819 11973). L,. 11. Schmidt, paper pre3rntc.d at the 46th A i i t ~ u a l \let,ting Soc. Rheolog~, St. Louis, hlo. (Oct. 37-30, 197.5) to 1)r p u b lished.

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