In injection molding production, it is often encountered with thick PVC injection molded parts with a wall thickness of more than 50mm. Because the internal space of the cavity is too large, when the melt is injected from the gate into the wide cavity, the squeezed melt will be injected into the cavity from the gate in a folded wave or spiral manner. After these linear melts converge, they will leave water ripples on the injection part at the gate position and weld marks at other positions.
In addition, in a wide cavity, the flow velocity of the melt in a middle part must be faster, and the surrounding speed is reduced due to the cooling and friction of the cavity wall, which will cause the melt to shunt It also needs to be merged, so that obvious weld marks are formed at the junction. If there are obstacles such as various inserts in the cavity, the melt flow will also cause the melt to shunt and produce weld marks.
Due to the poor fluidity of PVC melt, the welding ability is not very good, so after the PVC melt is shunted, it will usually form a relatively obvious thick welding mark.
According to the formation mechanism of streaks and weld marks, the faster the injection speed, the larger the cavity space, the more obvious the folding or spiral filling and shunting, and the more serious the streaks and weld marks. This is why streaks and weld marks are extremely difficult to eliminate.
Therefore, to solve these two problems, the injection speed must be reduced so that the melt no longer fills the cavity in a folded or spiral form. At the same time, it is necessary to ensure that the flow of the melt is stable and no shunting occurs. Various effective measures should also be taken to ensure that the melt can be filled smoothly in the case of slow injection to prevent irregular filling of the injection molded parts.
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