In this study, we carried out investigations on weld-line generating and vanishing process under injection molding conditions ranging from low speed to ultra-high speed using the modified Glass-Inserted Mold. Specifically, the investigations mainly focused on the shape of the weld-lines generated in thin cavities installed with obstacle pins of various shapes, and the correlation between the flow-front meeting angle and melt pressure. The results showed that for all obstacle pin cavity shapes, the depth and width of the weld-line decrease as the set-up injection rate increases. On the other hand, it was also confirmed that the weld-line length increases, and the weld-line vanishing angle increases as the set-up injection rate changes from low to high.
Based on the above results, we proposed a new weld-line generation and vanishing process model where the weld-line generating and vanishing phenomenon is influenced by the flow-front meeting angle, melt pressure in the vicinity of the flow-front meeting area, and the skin layer thickness in a broad range of set-up injection rates from low to ultra-high.

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In this study, the melt-flow behavior inside the sprue during the ultra-high-speed filling process of PP and GF-LCP was clarified using a modified glass-inserted mold. In particular, with the GF-LCP, melt-flow behavior within 3 ms was observed clearly and models of residual gas trapped in the resin were proposed in detail. The following results were obtained.
1) With the PP, the melt-flow behavior did not change at any injection rate in the range of 24.1 cm³/s to 804 cm³/s.
2) With the GF-LCP, jetting was generated at all injection rates and the jetted resin folded while filling by resin newly flowing in from the nozzle when the injection rate was less than 241 cm³/s.
3) When the injection rate was higher than 482 cm³/s with the GF-LCP, the jetted resin reached the sprue-end while maintaining a wave-form pattern and then filled back to the nozzle side while increasing the contact area with the cavity and glass surface from the sprue-end, without folding phenomenon.
4) At this time, V-notch shape resins were found to generate and disappear intermittently between the jetted resin and resin filling into its periphery. Based on these results, the mechanism of residual gas in the V-notch area trapping resins in was proposed in detail.
5) With the GF-LCP, resin pressure at the nozzle was reduced due to the jetting phenomenon, and temperature of the jetting resin gradually increased.

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