Why plastic perfume caps have parting lines?
Why sometimes the plastic perfume caps have parting lines?
The mold of plastic perfume caps consists of two part: the moving mold and the fixed mold, which close together for molding. Micro-scale gaps inherently exist at their mating parting surfaces. When molten plastic is injected under high pressure, a small amount of material seeps into these gaps. After cooling, a raised thin ring line forms — this is the parting line.
Factors that make the parting line thicker and more visible:
1. Insufficient polishing precision, scratches or uneven steps on the mold parting surface widen the gaps;
2. Long-term mass production causes mold wear and minor edge collapse, gradually enlarging gaps;
3. Insufficient clamping force fails to hold the parting surfaces tightly, pushing them apart during high-pressure injection;
4. Excessively high injection pressure or speed creates strong impact from molten resin, worsening flash along the parting line;
5. Perfume caps are round with a full-circle parting line, making the circular mark highly noticeable visually.
