What is a Bump-Off in Injection Molding?
A bump-off is a type of undercut—an intentional design feature that temporarily interferes with the mold’s opening direction. It’s used when a part needs to flex slightly to release from the mold without requiring complex side-actions or cams.
Think of the snap on your jacket. With just the right force, it pops into place and releases cleanly. That same mechanical principle is at play in plastic components like twist-off caps, closures, and fit-to-lock features molded into polypropylene (PP) or HDPE parts.
Why Bump-Offs Matter in Part Design
Materials like polypropylene (PP) and high-density polyethylene (HDPE) are flexible enough to deform slightly during ejection and return to their original shape. That’s why these plastics are great candidates for bump-off features—they avoid the need for extra tool actions like unscrewing cores or side cams.
The payoff?
- Simplified tooling
- Faster cycle times
- Lower production costs
A well-placed bump-off can be the difference between over-engineering a mold and getting clean, repeatable parts at a fraction of the cost.
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Optimize Your Part Design for the Real World
At Onit MFG, we help customers take advantage of design strategies like bump-offs early in the process—so you don’t get stuck with avoidable costs or production delays later.
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