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Fig.3 sample cavity pressure profile of VAIM (2Hz):(a)pressure at P1,(b)pressure at P2,and (c)pressure difference between P1 and P2
Vibration-Assisted Injection Molding Results. The addition of vibration to the otherwise optimized molding conditions gave rise to significant product strength improvements. The effects of npidual vibration parameters will be first discussed, followed by the presentation of the best strength enhancements realized through VAIM processing.
Effects of Inpidual Vibration Related Parameters. A representative sample of the observed effect of the delay time to begin vibration on product strength is shown in Fig. 4. The data in the figure is for a 50/50 blend exposed to vibration at various frequencies for a fixed duration after the delay. From the figure, it can be seen that a maximum strength was obtained when the delay time
Fig. 4 Effect of vibration delay time on product strength for
50Õ50 virginÕrecycled blend
Fig. 5 Effect of vibration duration time on product strength for
50 /50 virgin / recycled blend
was 0 s, or, in other words, when the vibration was introduced immediately upon the start of injection. This same trend was observed for both the 75/25 blend and 100% virgin PS.
Cavity pressure traces for the molding conditions of Fig. 4 (not shown ) indicate that melt vibration is initiated prior to the cavity being filled. Melt manipulation during fill alters the flow behavior during this portion of the cycle, supporting the concept that the shear stress/strain applied to the melt is modified, which could alter the skin formation in the molded parts. Control over skin formation within the mold is most likely more effective early in the process when relatively low viscosity levels are present and the polymer is more easily manipulated.
The information presented in Fig. 5 focuses on the effects of vibration duration on resultant product tensile strength. Again,data is presented for a 50/50 blend of recycled and virgin PS which was processed at various vibration frequencies for specified duration times. For the data shown, the delay time to begin vibration was 0 s and a duty cycle level of 1.0 was uniformly applied.As can be seen, a vibration duration time of 10 s led to the strongest products at each of the frequencies. This was also found for the cases of the 75/25 blend and 100% virgin PS materials, which were processed into the strongest products when vibration duration levels between 9 and 11 s were applied. Thus, it was concluded that a critical amount of exposure to the applied oscillatory energy ~in the present case 10 s! is necessary to optimally affect the molecular structure within products. Too little or too much exposure to the vibrational energy, however, can reduce the beneficial impacts.