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Selected Molding Conditions.Prior to studying the effects
of the VAIM process, a large number of specimens were molded
and mechanically tested. Through an optimization process a full
set of conventional molding conditions that yielded the strongest
products was determined. As it turned out, the optimal molding
conditions for the virgin material exactly matched those for the
recycled material. These conditions are presented in Table 1.
Since the optimal molding conditions for 100% virgin and 100%
recycled PS were found to be identical, it was concluded that these conditions would also be best for the blended mixtures.Thus, throughout the remainder of the investigation, the processing conditions shown in Table 1 were applied regardless of whether or not vibration assistance was utilized.
Table 1 Optimized molding conditions utilized throughout the
investigation
Injection time 0.5 s
Holding time 19.5 s
Cooling time 15 s
Injection pressure 45 Mpa
Holding pressure 45 Mpa
Mold temperature 70°C
Nozzle temperature 210°C
Barrel temperature ( front) 205°C
Barrel temperature (rear )200°C
Range of Vibration Parameters Studied. The hydraulic system of the BOY 15S molding machine is capable of producing suitable injection screw oscillations at frequencies up to 9 Hz.Therefore, the frequency range studied during the present effort was from 1 to 8 Hz. The delay times to begin vibration ranged from 0.0 to 1.0 s, and vibration duration levels ranged from 4.0 to 12.0 s. Lastly, duty cycles between 1.0 and 3.0 were investigated.
Within the window of the parameter levels specified above, an experimental optimization process was carried out that focused on arriving at sets of VAIM processing conditions that would yield the strongest products for each of the material blends studied.During this process, approximately 900 sample specimens were manufactured and tested for each material combination. The results that were generated are presented throughout the remainder of this paper.
Results and Discussion
Baseline Results. To study the effects of VAIM processing,results with conventionally molded products needed to be obtained first for comparison purposes. Along this line, 30 test specimens were produced from both the 100% virgin (100/0 hereafter),100% recycled (0/100 hereafter), 75%/25% virgin/recycled (75/25Hereafter), and 50%/50% virgin/recycled (50/50 hereafter) PS blends using the optimized conventional molding conditions discussed earlier. These samples were then mechanically tested, and the ultimate tensile strength (UTS) distributions that resulted are shown in Fig. 1.