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Fig. 6 Effect of vibration frequency on product strength for
various blends and frequencies
Fig. 7 Effect of vibration frequency on pressure profile: ( a )100/0, (b) 75/25, and( c) 50/ 50
however, for both the 50/50 and 75/25 material blends the actual frequency at which the vibration was applied did not appear to have a dramatic effect. Some frequency dependence was found with the 100% new material, which exhibited a slight peak in strength enhancement at 6 Hz. Thus, based on these findings the effect of vibration frequency appears to be less critical for virgin/recycled PS blends than for pure virgin materials.
Figures 7 (a) –7 (c) compare the effect of vibration frequency on the pressure-difference profiles ~pressure difference between P1 and P2! for a 0.4 s period, approximately 6 s after the cycle started (during the packing stage ) for 100/0, 75/25, and 50/50 virgin/recycled PS blends, respectively. These resultant pressure difference-profiles correspond to the tensile strength results shown
Fig. 8 Effect of vibration duty cycle on product strength for
50 /50 virgin / recycled blend
in Fig. 6. The significant difference in the pressure profiles was the amplitude of the pressure-difference curves. As the frequency increased, the amplitude decreased.
in Fig. 6. The significant difference in the pressure profiles was the amplitude of the pressure-difference curves. As the frequency increased, the amplitude decreased.The observed effects of the fourth vibration-related parameter duty cycle, on product tensile strength are illustrated in Fig. 8. The results shown are for the 50/50 blend processed at a number of vibration frequencies. The delay time to begin vibration and vibration duration levels were kept constant for each frequency. It should be noted, however, that under such conditions changing the duty cycle causes a corresponding change in the actual stroke amplitude of the oscillating injection screw. Nevertheless, from Fig. 8 it is clear that the product strength decreases as duty cycle increases. This was uniformly observed with each of the materials and blends tested.
A final vibration-related parameter that deserves some attention is vibration amplitude, which during the present study was monitored in the form of the oscillating injection screw stroke amplitude. Figure 9 illustrates the effect of this variable on tensile strength, again with data presented for the 50/50 blend processed at a number of vibration frequencies. The x-axis of the figure is labeled in a relative scale, where a 1.0 corresponds to the maximum stroke amplitude achievable with a duty cycle of 1.0 at each frequency. The other vibration parameters were kept constant for the data presented. From the Fig. 9, it is clearly seen that larger screw stroke amplitudes uniformly produced stronger products. As was observed with other effects, this same trend was observed for both the 75/25 blend and the 100% virgin PS material.