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Vented barrel injection molding of PC and a PC/ABS blend.

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Products molded from undried PC and a PC/ABS blend by vented barrel injection molding have mechanical properties comparable to., or better than, those molded from Predried materials.

Most engineering plastics are hygroscopic. Reducing the absorbed moisture to an acceptable level, which is of great importance in injection molding, is accomplished either by pre-drying the material or by using a vented barrel machine.

The problems that arise from processing insufficiently dried hygroscopic materials fall into two categories. First, the entrained moisture may be released from the melt as it advances across the mold cavity, resulting in product with surface or interior defects such as streaks, sprays, and voids. Stress concentrations around these physical defects may result in reduced mechanical properties. Second, residence times at high temperatures in the barrel prior to molding may be long enough to allow materials that are hydrolically degradable to react with the moisture present. When a vented barrel machine is being used. particular attention must be given to residence time considerations. The use of auxiliary dryers to predry the material is a well-established practice, but it is subject to economic and operational penalties, The alternative approach-using a vented barrel injection molding machine without predrying the material-is also widely accepted, but it is often lacking in substantive performance data. In the absence of such data, many molders are reluctant to use vented barrel machines, and instead accept the penalties of predrying.

Vented barrel injection molding (for details, see PE, February 1980, p. 35, and December 1984, p. 25) involves the use of a two-stage screw to first feed and melt the material, then decompress and devolatilize the melt, and finally recompress and pump the material into the shot reservoir downstream of the check valve prior to injection into the mold. Most commercial two-stage screws for this process utilize a single screw channel in the devolatilization region. However, some improvement might be found in particularly demanding applications by providing more than one channel in parallel in this region.

This article presents the results of an experimental study of vented barrel injection molding of a polycarbonate (PC) and a polycarbonate/acrylonitrile-butadiene-styrene(PC/ABS) blend conducted with the following objectives: 1) to compare the tensile and impact properties of product molded from undried materials in a vented barrel machine with product molded from predried materials; 2) to investigate the effects of feed rate and screw speed on the mechanical properties of the molded product; and 3) to compare the performance of a standard two-stage vented molding screw with that of an experimental two-stage vented molding screw having twin channels in the devolatilization region.

Experimental

Makrolon FCR 2405 and Bayblend FR 1439, commercial injection molding grades of PC and PC/ABS, respectively, were provided by Mobay Corp., Pittsburgh. These materials were selected because of the widespread use of PC and its derivatives in injection molding applications, particularly vented barrel molding. In addition, these materials are potentially hydrolytically degradable, and hence, the mechanical properties of samples would be expected to be sensible-underlined letters are the abbreviations for the test parameters used in the threeletter legends in Figs. 2 through 13.

After the process had stabilized at each condition, 50 samples molded from wet material and 20 samples molded from predried material were collected. Five samples from each batch were randomly chosen for tensile testing (ASTM D638) and five for impact testing (ASTM D3763). An Instron Model 6025 tensile testing machine and a Rheometrics Model RDT-5000 instrumented drop-dart impact testing machine were used.

Results and Discussion - Little variation was shown in the recovery rates calculated for the PC trials Fig. 2), but Fig. 3 shows some variation in recovery rates for the PC/ABS trials. This most likely indicates more sensitivity of the feed rate to experimental error in reproducing the starve feeder setpoint for PC/ABS. In one instance, a drift in feed rate was evident, possibly resulting from machine vibration.

Figures 4 and 5 show the expected increase in extruder motor hydraulic pressure (corresponding to screw torque) with increasing screw speed. Differences between low feed and high feed conditions for either screw were small, probably indicating that the first stage of the screw was running well starved and that a relatively large proportion of the melting energy was supplied by conduction from the heated barrel, as would be expected for the relatively small diameter (1.50 in), relatively long (26:1 L/D) screws used. The operating characteristics of both screws appear quite similar in Figs. 2 through 5, requiring comparable torques at comparable outputs. However, the twin vent channel screw generally tended to surge more than the standard screw. In one instance, during processing of wet PC at the lowest screw speed, intermittent vent bleed necessitated abandonment of the test. This may have been the result of the lower degree of fill compared with the single vent channel screw-each parallel channel being expected to convey one half the total output. The anticipated benefit of twin channels is the potentially greater devolatilizing capability provided by the additional free melt surface and increased recirculation in each of the two channels.

In Figs. 6 through 13, in which impact yield energy and tensile yield stress results are presented, for ease of identifying significant trends the vertical coordinate span is 12 times the mean standard deviation (Y) of the data.

Figures 6 and 7 show that no significant difference was evident in impact yield energy between either screw or between wet and predried PC. In addition, no significant variation was shown with screw speed, indicating that no significant shear-induced degradation was present, even at higher screw speeds.

For PC/ABS, shown in Figs. 8 and 9, the impact yield energy obtained with wet feedstock was significantly higher (i.e., >3 [omega] than for predried material. At present, no firm explanation of this phenomenon is proposed. One possible explanation lies in the usual foaming of the melt in the devolatilization region of the screw. As the material foams and foam cells subsequently rupture, very high effective melt surface area is generated, possibly sufficient to reduce the moisture level in the melt, and resultant molecular degradation, to a value even lower than that obtained with a predried (and hence nonfoaming) feedstock. Again, no significant difference was observed between the two screws, nor was any significant variation found with screw speed.

At low feed rates Fig. 10) no significant difference in tensile yield strength was observed between wet and predried PC or between either screw. However, for both screws at high feed rates Fig.11), wet PC is significantly improved over predried PC. Again, this may be a result of possible foam-assisted devolatilization coupled in this instance with the shorter residence time (in the first stage of the screw prior to devolatilization) at the higher feed rates. At both feed rates, tensile yield strength increased significantly with increased screw speed, possibly indicating enhanced devolatilization resulting from the concomitant decrease in channel fill and increase in flow recirculation.

Figures 12 and 13 show that the tensile yield strength for wet PC/ABS was significantly higher than for predried PC/ ABS for both screws at both feed rates. This may be the result of the foam-assisted devolatilization process.

Conclusions

For the process conditions used in this study, believed to be representative of typical industrial injection molding practice, the tensile and impact strengths of products molded from an undried PC and an undried PC/ABS on a vented barrel injection molding machine were comparable to, or higher than, those obtained from molding predried materials.

In some cases, properties were improved with increasing screw speed, and in other cases, they were unaffected. Although no explanation for these results has been established, one possible mechanism may be foam-assisted devolatilization.

The twin channels in the devolatilization region of the screw generally gave a slight, but statistically insignificant, improvement in mechanical properties over the standard single vent channel screw. However, the twin vent channel screw was more susceptible to surging, and therefore it is not recommended for general-purpose applications.
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Title Annotation:polycarbonate, acrylonitrile-butadiene-styrene
Author:Hemmati, Majid; Nunn, Robert E.
Publication:Plastics Engineering
Date:Dec 1, 1990
Words:1344
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