Views: 0 Author: Site Editor Publish Time: 2025-12-15 Origin: Site
When you need millions of identical threaded parts, injection molding is almost always the answer. But there’s a catch: how do you eject a spiral-shaped part from a mold?
How it works: The mold has fixed threads, and after cooling, the part is literally pushed off the core. The plastic (usually soft like PE or PP) stretches slightly and “pops” free.
Best for: Low-cost, shallow-threaded items like water bottle caps.
Limitation: Only works with flexible plastics and simple thread profiles.
How it works: The part of the mold that forms the threads (core or cavity) can rotate. After injection, it spins out of the part while the plastic part is held in place (using anti-rotation features).
Best for: Deep, high-precision threads—think medical devices or fuel connectors.
Why it’s cool: Leaves perfect threads with no damage, but adds complexity and cost to the mold.
How it works: The threaded section of the mold splits into two or more sliding blocks. They pull sideways to release the threads, then the part is ejected.
Best for: External threads that aren’t fully circular, or where a parting line is acceptable.
Downside: You’ll see witness lines on the part.
Sometimes, it’s easier to add threads after the part is made.
Process: A plastic-compatible tap cuts threads into a pre-molded hole.
Use case: Low volumes, prototypes, or when design changes are frequent.
Watch out: Cutting plastics can create burrs and weaken the structure by severing fibers.
Process: Threads are cut from solid plastic rod or a blank on a lathe or CNC.
Use case: Custom one-offs, giant threads, or when material can’t be injection molded.
Reality check: High cost, slow, and material-wasteful—not for mass production.
How it works: Like metal thread rolling, a hardened die presses into a plastic shaft, displacing material to form threads without cutting.
Advantage: Excellent strength (fibers remain continuous), fast, and no chips.
Common in: Plastic screws and fasteners made from extruded rod.
How it works: Threads are built layer-by-layer using FDM, SLA, or SLS.
Perfect for: Prototyping, highly complex integrated threads, or custom dental/medical implants.
Limitation: Layer lines can affect thread engagement and strength.
Ask yourself:
Volume: Millions? → Injection molding. Hundreds? → Machining or tapping.
Thread design: Full-depth precision thread? → Unscrewing mold. Shallow, rounded thread? → Force ejection.
Material: Flexible (PE, PP)? → More options. Brittle (PS, PMMA)? → Careful with force ejection.
Budget: High upfront cost for a complex mold is okay if spread over millions of parts.
Start with a lead-in chamfer to help engagement and prevent cross-threading.
Add a run-out section at the thread end to allow clean mold disengagement.
Avoid sharp corners—use rounded roots and crests to reduce stress and improve flow.
Account for shrinkage: Mold threads must be oversized based on your resin’s shrinkage rate.
Talk to your mold maker early! Thread design is a partnership.
Next time you twist open a soda bottle, take a second to appreciate the engineering in that simple cap. Whether it popped off a core, unscrewed from a rotating mold, or was rolled into shape, that tiny thread is a masterpiece of manufacturing ingenuity.
Plastic threads keep our world connected—literally. And understanding how they’re made helps designers and engineers create better, smarter, and more reliable products.
