Views: 0 Author: Site Editor Publish Time: 2026-11-06 Origin: Site
When designing injection molded parts with undercuts, two headaches always come up: surface scratches (drag marks) and visible parting lines. Many buyers and mold designers wonder: lifter, slide or collapsible core? There is no one-size-fits-all answer. The right selection helps you reduce cosmetic defects and hide parting lines on visible surfaces.
Scratches happen when steel components slide and rub against the plastic surface during ejection. Parting lines are unavoidable for split mold structures — they are the seam where two steel inserts meet. Our goal is not to eliminate them completely, but to move them to non-visible areas and lower friction risk.
Best for small, shallow internal undercuts, such as inner snap hooks in housing parts, where mold space is limited. ✅ Advantages: Compact structure, lower mold cost, parting lines stay on the inner hidden surface. ⚠️ Risks: The lifter slides against plastic during ejection, so it has a higher risk of surface drag marks. Not recommended for glossy, transparent or glass-filled A-surface. Tips: Increase draft angle, add radius at the undercut root, polish and nitride the lifter steel to reduce scratching.
The first choice for outer undercuts, medium & deep undercuts, and cosmetic A-surface. ✅ Advantages: The slide pulls sideways without sliding friction on the part surface, so scratch risk is much lower. This is the most reliable option for high appearance requirements. ⚠️ Risks: It will leave a parting line at the split position. Tips: Arrange the parting line along edges, step lines, assembly overlapping areas or hidden grooves. Texture finish can also weaken the visual trace of parting lines.
Designed specially for full circular internal undercuts, like round fittings, lock cylinder parts and threaded inner bores. ✅ Advantages: The outer surface of the round part can be free of parting lines. ⚠️ Risks: Multiple seam lines appear on the inner bore; easy to get scratches and trapped flash at the petal gaps. It is expensive and needs precise fitting.
Demolding Type | Best Application | Scratch Risk | Parting Line Location | Notes |
|---|---|---|---|---|
Lifter | Shallow inner undercuts, snap hooks, limited mold space | High | Inner hidden surface | Avoid on glossy / transparent / GF-filled A-surface |
Slide | Outer undercuts, medium-deep undercuts, cosmetic A-surface | Low | Part edge, step, assembly covered area | Preferred for high appearance requirements |
Collapsible Core | Full circular internal undercut, round lock parts, inner thread | Medium | Inner bore (petal seam lines) | Outer surface clean; high mold cost, high fitting requirement |
Route parting lines to non-visible positions as your first priority.
Add radius at undercut corners, avoid sharp edges.
Use surface treatment like nitriding or TD coating for friction steel.
Optimize molding parameters: proper mold temperature and lower packing pressure to reduce holding force.
If possible, redesign the part to remove the undercut for high-gloss appearance parts.
Lifter, slide and collapsible core each have their own sweet spot. The best mold design balances appearance requirement, undercut size, material property and mold budget. If you have a custom part with undercuts, send us your 3D drawing, we will work out the best demolding solution for you.
