Unscrewing Molds: Injection Molding Threaded Parts

Molded threads are a design decision before they are a tooling decision. If you specify a thread on a plastic part, you are also specifying how that part leaves the mold — and that choice drives both tool cost and cycle time more than almost anything else on the drawing.

Figure 1. Unscrewing Molds: Injection Molding Threaded Parts.

This guide covers the four ways to demold a thread, the geometry that makes a thread moldable, and how to size an unscrewing drive so it does not stall.

1. The Four Demolding Routes

Decision rules:

• Shallow + flexible resin → strip-off. Cheapest by far. Keep thread depth under 0.5 mm and use a radiused thread profile so the resin can deform instead of being sheared.
• Internal thread, moderate depth, moderate volume → collapsing core. No rotation, no drive, predictable cycle.
• Fine or deep thread, or thread used as a functional seal → unscrewing. The only route that will not damage the thread.
• External thread, large part → split thread (a slide carrying the thread form), because unscrewing a large external thread requires enormous torque.

2. Thread Geometry for Plastics

Plastic threads are not metal threads. Design them for molding first.

• Use a rounded or truncated profile. Sharp V-threads create stress concentrations and are hard to fill at the root. A rounded crest and root (like a buttress or a modified trapezoidal form) fills better and demolds more cleanly.
• Pitch: coarse pitches demold more easily and tolerate shrinkage better. Fine threads (below ~0.8 mm pitch) demand unscrewing or collapsing cores.
• Depth: as a working figure, keep thread depth at or below 0.6 × pitch for unscrewing and 0.5 mm for strip-off.
• Flank angle: use a slightly asymmetric flank where the thread must carry load in one direction.
• Shrinkage allowance: the molded thread will not match the steel. Allow for shrinkage in both the pitch diameter and the pitch itself — over a 20 mm long thread, a 1.5% shrink difference moves the thread by roughly 0.3 mm along its axis.
• Lead-in: always add a chamfer or a lead-in at the thread entry. It guides the mating part and protects the thread from cross-threading.
• Run-out: terminate the thread into a relief groove, not into a sharp shoulder. The groove gives the thread core somewhere to end and prevents a razor-thin edge.
• Wall thickness behind the thread: at least 1.5 × thread depth, or the part will bulge as the thread fills.

3. Unscrewing Mechanisms

An unscrewing tool rotates the thread core while holding the part against rotation. The part is held by a non-round feature — a knurl, a flat, a spline, or the mold itself.

Drive options:

Key design requirements:

• Rotational stop for the part. Something must prevent the part from spinning with the core. Common solutions: a non-round outer profile held in the cavity, a stripper plate with gripping features, or friction from a controlled ejector pre-load.
• Thread pitch and lead. For a single-start thread, one revolution advances the core by one pitch. For a multi-start thread, by one lead (= pitch × number of starts). This determines the required rotation count:

> Revolutions required = Thread engagement length / Lead

Example: a 12 mm long engagement on a 2 mm pitch, single-start thread needs 12 / 2 = 6 revolutions to fully unscrew. The same thread with 2 starts has a 4 mm lead, so it needs only 3 revolutions — half the cycle time.

• Rotational clearance: provide axial clearance so the core can retreat as it unscrews. The core must be free to move axially, else it will strip the thread it just formed.
• Cycle sequence: (1) mold opens, (2) unscrew drive engages and rotates the core out, (3) core retracts fully, (4) ejector pushes the part off. Any overlap between steps 2 and 4 tears threads.

4. Sizing the Drive Torque

Unscrewing torque is not trivial to calculate analytically, but it scales with three things: thread engagement area, friction, and radial pressure from shrinkage.

A planning approach:

> T ≈ k × A × p × r > > where A = thread engagement area, p = radial pressure from shrinkage (typically 5–20 MPa for rigid resins), r = effective thread radius, and k is a friction/geometry factor (use 0.3–0.5 for a first estimate).

Practical guidance:

• Always add a safety factor of 2 to the calculated torque, and specify a motor with a torque limiter. A stalled unscrewing drive strips the thread, and a stripped thread means the part is scrap.
• Reduce required torque before increasing motor size. Practical levers: shorten the thread engagement, raise the mold temperature slightly at the thread, increase draft on the thread flanks, or use a multi-start thread.
• Measure it. The most reliable method is to mold the thread and measure breakaway torque on the first samples, then size the drive. Ask your moldmaker to build the thread core so it can be tested before the drive is finalized.
• Account for temperature. Unscrewing a part that is still hot requires less torque but risks deformation; unscrewing a cold part needs far more torque. Define the mold temperature at which unscrewing occurs.

5. Common Problems and Fixes

Figure 2. Quick reference: the key checks and reference values from this guide.

6. Design Checklist

• [ ] Demolding route chosen deliberately (strip / collapsing / unscrewing / split)
• [ ] Thread profile rounded or truncated, not a sharp V
• [ ] Thread depth and pitch compatible with the chosen route
• [ ] Shrinkage allowance applied to the molded thread dimensions
• [ ] Lead-in chamfer at the thread entry
• [ ] Relief groove at thread run-out
• [ ] Wall thickness behind the thread ≥ 1.5 × thread depth
• [ ] Rotational stop for the part defined and verified
• [ ] Revolutions required computed from engagement length and lead
• [ ] Axial clearance provided so the core can retreat as it unscrews
• [ ] Drive torque estimated, with a safety factor of 2 and a torque limiter
• [ ] Cycle sequence interlocked so ejection cannot overlap unscrewing
• [ ] Mold temperature at the moment of unscrewing specified

Conclusion

Threaded plastic parts are a systems problem: the thread profile, the demolding method, the rotation count and the drive torque all have to be decided together. Get the route right — strip, collapse, unscrew or split — and the rest follows. Get it wrong and you will be stripping threads, scrapping parts and adding seconds to every cycle for the life of the tool.

Send us a threaded part and we will recommend a demolding method, a thread profile and a rotation count before the tool is designed.