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Structure of PP Structural foamMoldings Made by the Gas-Counterpressure ProcessS. Djoumaliisky a , N. Touleshkov a & G. Kotzev aa Institute of Metal Science , Bulgarian Academy ofSciences , Sofia, 1574, BulgariaPublished online: 15 Aug 2006.
To cite this article: S. Djoumaliisky , N. Touleshkov & G. Kotzev (1997) Structureof PP Structural foam Moldings Made by the Gas-Counterpressure Process, Polymer-Plastics Technology and Engineering, 36:2, 257-271, DOI: 10.1080/03602559708000618
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P0LYM.-PLAST. TECHNOL. ENG., 36(2), 257-271 (1997)
STRUCTURE OF PP STRUCTURAL FOAM MOLDINGS MADE BY THE GAS-COUNTERPRESSURE PROCESS
S. DJOUMALIISKY, N. TOULESHKOV, and G . KOTZEV
Institute of Metal Science Bulgarian Academy of Sciences Sofia 1574, Bulgaria
Abstract
An experimental study was carried out to investigate the gas- counterpressure process by egression of a part of the polymer melt from the core of the molded body towards the accumulator. A systematic study of bubble morphology development and struc- tural parameters of structural foam moldings is reported. The structural foam samples were produced on a two-stage molding machine (SIEMAG Structomat 2000/70) with passively transport- ing accumulator and on an in-line injection molding machine (KuASY 800/250) with FIFO-type accumulator, the melt tempera- ture being varied in the range 473 to 533 K. The polymer used was isotactic polypropylene into which chemical blowing agent (azodicarbonamide) was added. The structural properties studied were overall density, local density, and density distribution. It was found that the distance from the sprue to the extreme of flow and the melt temperature have a profound influence on the bubble sizes and their distribution. It was established that the use of the two types of accumulator causes remarkable differences in the structural organization of the structural foam moldings.
257
Copyright 0 1997 by Marcel Dekker. Inc.
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INTRODUCTION
The macrostructure of structural foam moldings produced by various injection-molding processes includes three morphologically defined components: relatively solid integral skin, cellular core, and a transition layer. The gas-counterpressure process by egression of a part of the polymer melt from the core of the molded body is particularly suitable among the processes known for building such integral structures be- cause the gas-counterpressure controls the skin thickness and the den- sity and structure of the cellular core affected by the quantity of the egressed polymer melt.
The principle of this process was developed at the end of the 1960s by Allied Chemical Corp. (TMF process) [ l ] and by the Institute of Metal Science at the Bulgarian Academy of Sciences (TM process) [2]. It is realized through the following operations:
0 Injection of gas containing polymer melt into a mold cavity preli- minarily pressurized with nitrogen or air (gas-counterpressure) Foaming by egression of a part of the polymer melt from the core of the molded body back towards the accumulating cylinder (accumulator)
0 Compression of the egressed and foamed polymer melt in the accumulator Addition of measured fresh polymer melt in the accumulator
The macrostructure and surface quality of the structural foam mold- ings are dependent not only on the processing parameters but also on the arrangement of the compressed and fresh gas containing polymer melt in the accumulator measured for the next cycle [3]. That is a fact until now inadequately studied and analyzed but the one that explains some of the shortcomings of the TMF and TM processes.
The purpose of the present study is to investigate the effect of two paths of polymer melt flow in the system plasticizing cylinder-mold cavity on the macrostructure of the structural foam moldings made by gas-counterpressure process by egression.
EXPERIMENTAL
The polymer used was isotactic polypropylene BUPLEN 7523, a Bul- garian product, (p = 901 kg/m3, MFI = 3.5 g/10 min at 503 K, and
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load 21.6 N). Into this was added 1% (by wt) of the azodicarbonamide blowing agent Genitron EPA.
Object of investigation was a center-gated cylindrical plate “disk” (180 mm diameter and 11 mm height). The test specimens were pro- duced in a tempered and pressurized (with nitrogen) mold cavity at the following processing variables: melt temperature T, = 473. 493, 503, and 533 K; mold temperature Tf = 293 K; cooling time T = 5 min; gas- counterpressure P , = 0, 1 MPa.
A
J
a
B
U
- b -
d
n
FIG. 1. Schematics of the gas-counterpressure process with two types of accumulators: A-passively transporting accumulator; B-FIFO-type accu- mulator. a-injection; b-egression: c-compression; d-addition.
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Before egression of a part of the polymer melt the volume of the mold cavity was filled completely with gas containing polymer melt (maximum shot).
Two processing variants of flow in the system plasticizing cylin- der-mold cavity were studied, where various thermomechanical treat- ments on the polymer melt are used (Fig. 1).
Processing variant A illustrates in principle the TM process. It pre- sents the intricate Z-shaped path of flow of gas containing polymer melt from the plasticizing cylinder to the accumulator at a relatively low level of mechanical action (q = 150 to 250 sec- ’). The material egressed from the core locates just to the piston front pushed by the incoming gas containing polymer melt of the next shot; i.e., it enters the accumu- lator first and goes out last, which subjects it to additional thermal action. This processing variant is performed on a two-stage injection- molding machine, SIEMAG Structomat 2000/70, modified in compli- ance with Ref. 4.
The other processing variant, B, illustrates in principle the TMF process. It provides a linear flow path at higher mechanical action (9
,90°
FIG. 2. Center-gated structural foam specimen and structural parameters.
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= 1300 to 1500 sec- ') as well as a shorter period of heating the material egressed, which in this case locates at the front of the shot for the next molding (first in, first out-FIFO). This processing variant is performed on an in-line injection molding machine, KuASY 800/250, fitted with a hydraulic shut-off nozzle.
The following structure parameters were investigated: overall den- sity p, local density in sectors of the test specimen pn, (i = 1 to 4); local density from the sprue to the extreme of flow p , ( j = 1 to 4); density distribution across test specimen p4 = Ap/At ( q = I to IV cross section, f = thickness of cross section) as schematically shown in Fig. 2.
The overall density p of the test specimen and local density ( p , and p,) of prisms cut from the test specimen were determined from mea- surements of their weight and volume.
RESULTS AND DISCUSSION
Bubble Morphology
The formation of cellular structure in structural foam moldings made by the gas-counterpressure technique by egression is a dynamic process resulting from rapid growth of microbubbles in the polymer melt as the surrounding pressure is lowered and the gas-counterpressure after mold filling is decreased. This process can be divided into two stages, differ- ent in the conditions of their run.
The first stage, mold filling, is carried out at relatively high injection pressure and high wall shear rate, together with fast cooling of the gas containing polymer melt in the cavity walls. Initially, in the fully filled mold cavity, in the condition of restricted foaming, the bubble growth compensates the thermal shrinkage and the shrinkage as a result of the continuing crystallization of the polymer melt.
After increasing the cavity volume the combined growth of all bub- bles in the system creates a flow directed from the mold cavity back to the accumulator. That is the second stage, real foaming, which runs in a condition of continuously decreasing pressure and temperature of the gas containing polymer melt and of low shear rate of the foamed polymer melt. After interrupting the communication between the accu- mulator and mold cavity, the final density reduction is fixed, while the formation of the structure continues to the final solidification of the
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molded body by diffusion, coalescence, and migration of the gas bubbles.
Figure 3 compares the bubble morphology from the sprue to extreme of flow in the test specimens produced by processing variant A at melt temperature 473 K . Obviously, because of continuously changing pres- sure, shear stress and temperature fields, there is a great variety not only of bubble sizes but also of bubble configuration in the volume of the molded body [5] . Close to the cavity walls the viscosity of gas containing polymer melt is higher than inside of the flow, and as a result the nucleation of gas bubbles is suppressed. In the areas close to the sprue, elliptical bubbles predominate that are oriented in the direction
FIG. 3. Photomicrograph of bubble morphology vs. the distance from the sprue to the periphery of the test specimen at melt temperature Tm = 493 K.
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of egression. The internal layers in this area contain strongly deformed and broken bubbles. The bubble growth in the areas away from the sprue can be considered as running in relatively undisturbed polymer melt, and because of that the bubbles acquire a shape near to spherical. In these areas in the transition layer there are also elliptically stretched bubbles exceeding by several times the size of surrounding bubbles and oriented perpendicularly to the skin plane. Their occurrence can be explained both by migration of gas bubbles from the outer areas towards the inside and by the effect of mechanical deformation due to the fast shrinkage of the surface layer. The coalescence of the gas bubbles in the transition layer is probably favored by the secondary heating during the exothermic crystallization of the polymer matrix that retains for a longer time the conditions suitable for the development of the above- said diffusion and deformation processes.
It is well known that the melt temperature plays an important role in determining the bubble dynamics during mold filling mainly because of the effects of temperature on diffusivity and viscosity [6]. Simultane- ously, the time allowed for expansion before final solidification in- creases and, hence, influences the bubble size.
Figure 4 shows the effect of the melt temperature on the bubble morphology in the areas away from the sprue. It is seen that not only the bubble size increases with the increase of the melt temperature but also the relative part of bubbles of irregular shape obtained as a result of deformation or coalescence.
Overall Density
The overall density is the only structure parameter that can be specified quantitatively. The overall density of structural foam moldings made by the gas-counterpressure process by egression depends first of all on the egression grade. i.e., on the quantity of foamed polymer melt which flows out from the core of the molded body towards the accumulator.
Figure 5 shows the effect of the melt temperature on the egression grade and overall density of structural foam samples produced by the two processing variants studied. The data support the well-known fact of decrease of the overall density with increasing melt temperature. This tendency is more strongly expressed in processing variant A in the whole temperature range (curve I ) , while in processing variant B the overall density above 473 K tends to one constant value (curve 2). The decrease of the overall density is explained in general by decrease
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FIG. 4. Photomicrographs showing the influence of the melt temperature on the bubble morphology at the furthest distance from the sprue: (a) 473 K; (b) 493 K; (c) 513 K; (d) 533 K.
of the viscosity of the polymer melt and increase of the system internal energy which results in creation of a more favorable condition for egression of polymer melt from the core towards the accumulator. It is only natural, then, that the egression grade increases for both pro- cessing variants, as in the cases of free foaming or free flow of the polymer melt from the core under the action of expansion (curves 5 and 6). However, unexpectedly the values of egression grade in processing variant B (curve 3) do not change considerably at high melt tempera- ture. Thoroughly analyzing the path of the flow of gas containing poly-
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500 473 4 93 9 0 513 T,. K 533
FIG. 5. Effect of the melt temperature T , on the overall density p and egression grade E .
mer melt in the system plasticizing cylinder-mold cavity, it has been found that the overall density is affected also by the mechanism of mold filling and the arrangement of fresh and egressed polymer melt in the accumulator as well as its location in the final moldings.
In processing variant A the egressed and degased polymer melt is pushed by the fresh incoming melt in front of the injection piston. In the next cycle it goes into the core of the molded body, probably forming in it an undesirable section of increased density. In processing variant B the egressed polymer melt stands in front of the fresh melt in the accumulator. Thus it comes into the mold cavity first, forming a part of the body skin.
Local Density
Nonuniformities in the state of the compressed and fresh gas containing polymer melt (density, viscosity, molecular characteristics) as well as
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in the bubble morphology, inevitably result in local density variations throughout the molded bodies. It was found in previous studies that in the gas-counterpressure process by egression the local density in- creases with the distance from the sprue [7] while in the low-pressure structural foam process it slightly decreases [8], irrespective of the processing parameters.
Figure 6 shows the effect of the melt temperature on the local density in some sectors of the test specimens made by processing variants A and B. It is seen that in processing variant A the deviations of the local density in the separate sectors compared to the overal density are in wider limits than in variant B. Besides, in variant A these deviations decrease with the increase of melt temperature, which is related to the disappearance of the section of increased density above 513 K. In var- iant B just the opposite tendency is observed, namely increase of the deviation of the local density from the overal density with the increase of the melt temperature, which is probably a result of the increase of the skin thickness of the molded body.
Figure 7 gives plots of local density versus distance from the sprue to the extreme of the flow in sectors I and 3 of the test specimens at melt temperature 493 K. The results presented in the figure for the
&& -5 -1 ol
.lo1 473 493 51 3 533 Tm , K
FIG. 6 . Effect of the melt temperature T , on the deviation of the local density p., from the overall density p.
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I Tm= 493 K
I I I J -80 -60 - 4 0 -20 0 2 0 4 0 ~ . m m 8 0
FIG. 7. Variation in local density ps, vs. the distance from the sprue to the periphery L in the opposite sectors, 1 and 3, of the test specimen.
processing variants A and B are in agreement with the published data. Characteristically, there is a local density increase at the extreme of flow as compared to the sprue. The maximum of the curve confirms the formation of a section of increased density in the samples made by processing variant A.
Density Distribution Across Structural Foam Moldings
An idea of the processes of foaming and structure formation could be given by density profiles across the sample thickness, i.e., from skin to skin. The curves of density distribution were obtained by measuring, with a Carl Zeiss G-3 photometer, the amount of light transmitted through an x-ray photograph of a prism cut from the test specimens. The measurements were made on four cross sections of the prisms cut from two opposite sectors (1 and 3 ) of the test specimens.
The analysis of the diagrams presented in Fig. 8 shows that for speci- mens made by processing variant A, a more irregular run of density profiles is characteristic in the temperature range examined. In sector 1 the formation of dense, unfoamed areas is confirmed and the determi- nation of the basic morphological parameters (skin and core thick- nesses, core density) is of no use. In the opposite sector 3 there is a
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300 u
900
600
300
FIG. 8. Density distribution across structural foam sample made by pro- cessing variant A at various melt temperature in cross section: I , -; 11, ---; 111, -.-.-; IV, - x - x -.
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300-
FIG. 9. Density distribution across structural foam sample made by pro- cessing variant B at various melt temperature in cross section: I , -; 11, ---; 111, -.-.-; IV, - x - x -.
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correlation between the overall density and its distribution across the section of the specimen that is expressed in decrease of skin thickness and core density with increase of the melt temperature. At 533 K the density profile changes unevenly, which explains the rapid density re- duction and low quality of the macrostructure: lack of solid integral skin, uneven distribution of bubbles in size and configuration, cavities.
Figure 9 shows uniform density distribution in variant B having smooth transition between the surface layer and core in all cross sec- tions examined. At first glance the increase of the skin thickness with increase of the melt temperature seems illogical and in contrast to the results discussed above for variant A, but as already stated, this is determined by the formation mechanism of the skin. In this case the increase of the skin thickness results in increase of the transitional layer thickness and in decrease of core thickness and core density with not very great differences in the overall density of the molded body.
CONCLUSIONS
The results obtained by bubble morphology in the gas-counterpressure process with egression of a part of the polymer melt from the core of the molded body can be summarized as follows.
The bubbles are of different size, configuration, and orientation. The bubble size increases with the melt temperature and decreases with the distance from the sprue to the sample periphery. The bubbles near to the sprue are nonspherical and oriented parallel to the skin, and those furthest from the sprue become spherical. In the areas away from the sprue in the transitional layer, nonspherical bubbles also occur. They are oriented perpendicularly to the skin, which is explained by the combination of deformation and diffusion phenomena.
This study shows that the use of different types of accumulator causes remarkable differences in the structural organization of the structural foam moldings.
It has been proved that when using a passively transporting accumu- lator, the temperature dependence of the overall density is more strongly expressed than it is with an FIFO-type accumulator, which is accompanied by irregular changes of the other structure parameters: local density, density profile, skin, transitional layer, and core thick- nesses.
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A. Spaak and C. L. Weir, U.S. patent 3, 773, 873, Method and apparatus for injection moulding plastics articles (1970). A. Balevsky, I. Dimov, A. Antonov, and S. Semerdjiev, Bulg. patent 18, 242, Method and apparatus for the production of thermoplastic bodies with a solid skin and cellular core (1972). N. Touleshkov, S. Djoumaliisky, and G. Kotzev, Some properties changes in injection moulded PP structural foam, Polym. Degrud. Stabil., 26, 327 (1989). S. Semerdjiev and N . Popov, Bulg. patent 23, 063, Distributing device for machine for injection moulding of thermoplastic part with solid skin and cellular core (1976). J. L. Throne, Structural foam moulding parameters, J . Cellular Plast., 12, 161 (1976). C. A. Vilamisar and C. D. Han, Studies of structural foam processing. 11. Bubble dynamics in foam injection molding, Polym. Eng. Sci., 18, 699 (1978). N. Popov and S. Semerdjiev. Technologische Moeglichkeiten des Strukt- urschaum-Spritzgiessens rnit Gasgegendruck, Kunststoffaerater, 9, 55 (1980). S. Semerdjiev and N . Tuleschkov, Uber die Gasphase in Strukturschaums- toff-Formteilen, Plusre und Kuutschuk, 1 , 32 (1980).
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