Injection Molding
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PP jetting marks (snake patterns)
Problem Phenomenon
When the gate faces an open cavity, snake-like or earthworm-like twisted patterns appear on the product surface, mostly extending from the gate into the cavity, unacceptable on high-gloss parts.
Cause Analysis
After the melt passes through a small gate at excessive speed, it jets directly into the cavity without contacting the cavity wall; the melt strand folds and tumbles in the air and cools and sets, forming twisted patterns; excessive initial injection speed, a gate that is too small, and the gate facing an empty cavity are the three main conditions.
Solution
①Change the gate angle or position so that the melt first hits the cavity wall or core after entering the cavity and then spreads; ②use a 'slow-fast-slow' injection curve, with a low-speed starting segment through the gate; ③appropriately enlarge the gate cross-sectional area to reduce the jetting speed; ④switch to a lap-type or fan gate when necessary; ⑤appropriately increase the mold temperature to improve front contact.
Key Takeaway
The mantra for jetting marks is 'let the material hit the wall as soon as it enters': through gate design and the speed curve, make the melt front cling to the cavity wall as soon as it enters; once the melt spreads along the wall, the snake patterns naturally disappear—more effective than simply reducing speed.
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