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Release time：2026-10-20 Visits：55
An unscrewing mold releases threaded plastic parts by rotating the part off the core inside the tool. It is the only fully automatic way to mold internal threads with no draft angle and no secondary machining — bottle caps, medical connectors, valve bodies, threaded bosses on power-tool and appliance assemblies. It is also the mold type where small design decisions move your tooling budget the most. This guide explains how the mechanism works, where the cost sits, and what to check before you approve steel. It is written for buyers sourcing threaded plastic parts, and reflects how our mold shop in Dongguan runs every month.
Every unscrewing injection mold solves the same physics problem: a molded internal thread wraps around the core, so the core cannot pull straight out. The tool rotates the part relative to the core until the thread disengages, then ejects. Three subsystems decide whether it works reliably:
Once the thread clears, a stripper plate or ejector pins push the part free. The whole sequence runs inside the molding cycle, which is why a well-built unscrewing mold produces threaded caps at production rates with no operator intervention.
Buyers usually compare four routes for a threaded feature. Each trades tooling cost against per-part cost and thread quality.
| Factor | Unscrewing mold | Collapsible core | Slide / side action | Hand-loaded insert |
|---|---|---|---|---|
| Thread quality | Full form, no draft | Full form, parting-line witness on thread | Partial; thread broken on one side | Full form |
| Relative tooling cost | Highest | Medium-high | Medium | Lowest |
| Cycle time | Longer; rotation adds seconds | Short | Short | Long; manual load each shot |
| Automation | Fully automatic | Fully automatic | Fully automatic | Manual |
| Best fit | Caps, connectors, sealed threads | Short threads, moderate volume | External threads or loose-tolerance threads | Prototypes and low volume |
Choose an unscrewing mold when the thread must seal, carry load, or pass a torque test, and volumes justify the tool. Choose a collapsible core for short threads at moderate volume. Choose slides only if a witness line across the thread is acceptable. Below a few thousand parts, hand-loaded inserts are cheaper than any automatic solution.
Five drawing-level decisions decide most of the cost:
Fixing any of these on a drawing costs a revision. Fixing them in hardened steel costs a cavity rework or a new core.
Two factors dominate. First, rotation time: fine pitch over a long thread is direct cycle-time cost, paid on every shot. Second, drive reliability: a worn rack or mis-timed servo shows up as incomplete thread release, and the scrap lands in your unit cost. Ask the molder to state the added seconds per cycle from the unscrewing mechanism separately, so you can compare quotes on the same basis.
Material matters too. Glass-filled resins and high-temperature engineering plastics (PEEK, PPSU) wear threads and cores faster; a hardened or coated core extends tool life and holds the thread profile longer. For PVC and acidic resins, the shop needs a dedicated screw and barrel — worth confirming before the RFQ, not after.
Send these six questions with your RFQ. A supplier who answers all six with specifics has actually built unscrewing tools:
Plan around the T1 report, not the PO date. Our standard response on a complete 3D/2D package is DFM feedback in 1–3 working days and a quote in 1–3 working days. T1 samples run 2–4 weeks on quick tooling; production steel tools with an unscrewing mechanism typically run 4–8 weeks, because the drive system is built, timed and tested before the tool ships. Complex or large tools run longer — get that stated in the quote, not discovered at T1.
On certification, references to ISO, CE, FDA or ASTM compliance require manual verification against current certificates before you print them in a specification sheet. Ask the supplier for the certificate, not the claim.
Volume planning: unscrewing tools earn their cost back through automation. Once annual volume is above roughly the low tens of thousands, automatic thread release beats manual loading on unit cost — and it removes an operator from every shift.
A supplier who sends photos but no gauge numbers has not finished the trial.
An unscrewing mold is an injection mold that releases threaded plastic parts by rotating the part off the core inside the tool. A drive system — rack and pinion, hydraulic motor, or servo — turns the core or the stripper plate until the internal thread disengages, then the part ejects automatically. It produces full-form internal threads with no draft and no secondary machining.
Use an unscrewing mold when the thread must seal, carry torque, or hold a tight profile over long runs, and when the thread is long enough that a collapsible core's parting-line witness or size limit becomes a problem. A collapsible core is cheaper and faster for short threads at moderate volume. Ask both options to be quoted side by side for your thread spec.
Pitch sets the number of rotations: thread length divided by pitch. A fine pitch over a long thread means more rotation per shot, which adds cycle time on every part for the life of the tool, and it demands a more precise drive. Coarse pitch and short threads are the cheapest to mold automatically.
Often yes. External threads can be formed by two slides meeting at the parting line, which leaves a witness line across the thread. If the application accepts that witness — many closures do — a slide tool is significantly cheaper than an unscrewing mold. If the thread must be continuous and sealed, unscrewing or a hand-loaded insert is required.
Plan 4–8 weeks for a production steel tool with the unscrewing mechanism, counted from DFM confirmation and mold deposit, because the drive must be built, timed and tested before shipping. Quick tooling without full automation can reach T1 in 2–4 weeks. Send a complete 3D/2D package and DFM feedback returns in 1–3 working days.
An unscrewing mold is a machine inside a mold. The thread spec decides the rotations, the rotations decide the drive, and the drive decides both your tooling budget and your cycle time. Get the anti-rotation feature and the thread form settled on the drawing, ask the six checklist questions, and judge suppliers by gauge data at T1 rather than by photos.
We design and build unscrewing (thread-release) molds in-house — 30 mold-shop machines across EDM, CNC and grinding, feeding 50 injection presses from 140T to 500T, with monthly output of 50–60 mold sets. Send us the 3D model and the thread callout. We will return DFM feedback with the rotation count, added cycle time and tooling cost spelled out.
Request a Quote or Get a Free Sample of a comparable threaded part before you commit to tooling.
