how to design a unscrewing mold?

 

Quick overview (what you must decide first)

1. Part analysis — thread type (internal/external), pitch, lead, depth, helix direction, undercuts, wall thickness, critical features.

2. Material — what plastic (PE, PP, ABS, PC, POM, medical grade, etc.). Different resins shrink and behave differently (affects unscrew torque).

3. Production volume & cycle time — drives mechanism choice (mechanical vs hydraulic vs servo).

4. Surface finish & tolerance — cosmetic & functional thread tolerance requirements.

5. Ejection strategy — unscrew then eject vs unscrew + stripper ring, plus any back-pulling or side-slides.

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Step-by-step design process

1) Fully define the part

• Capture: 3D CAD, critical dimensions, thread spec (ISO/TR, custom), material grade, expected shrinkage, required surface finish, and inspection tolerances for threads (pitch, major/minor diameter, runout).

• Note: specify helix direction (R or L). If mold unscrew drive is from fixed side, helix direction matters.

2) Evaluate manufacturability & DFMA

• Avoid deep thin walls around threads. Add fillets, uniform wall thickness.

• If internal threads are deep, consider split-core designs or collapsible cores (more complex).

• Add draft where possible — for internal threads you often need to form threads on a rotating core, so draft on the outer cavity is still helpful.

3) Choose unscrewing mechanism

Options (choose based on volume, precision, budget):

• Rack & pinion (mechanical)

  • Pros: low cost, uses mold opening motion to convert linear to rotational; reliable for medium volumes.

  • Cons: limited speed control; wear on gears.

• Cam & roller (mechanical cam track)

  • Pros: simple, robust; good for synchronous operations.

  • Cons: cam wear, limited flexibility.

• Hydraulic motor

  • Pros: high torque, good for heavy shrinkage or large parts.

  • Cons: more plumbing, maintenance, cost.

• Electric servo motor

  • Pros: precise speed/angle control, programmable, energy efficient; great for high-precision or multi-stage unscrewing.

  • Cons: higher initial cost, needs controller and wiring.

• Clutch + gear drive (common) — motor/gearbox with a torque-limited clutch to avoid damage.

4) Core & cavity design

• Core rotates (most common for internal threads). Design the rotating core as a replaceable insert for wear and polishing.

• Thread relief: add small clearance between thread crest and mating surface to accommodate shrink and avoid jamming. Typical relief depends on pitch — 0.05–0.2 mm is common but check thread size.

• Stripping features: depending on thread strength, use stripper ring or ejector pins after unscrewing to push part off the rotating core. For delicate threads, use a stripper ring that moves axially once unscrewed.

• Vent & finish: vents near thread root are tricky — use micro-vents or venting grooves away from critical surfaces.

5) Cooling layout

• Threads are often on core inserts — ensure internal cooling channels in the core and the cavity to keep uniform temperature across threads. Slow cooling increases shrinkage and torque.

• Design conformal cooling if high volume & tight cycle needed.

6) Ejection and sequence control

• Typical sequence:

  1. Mold opens to unscrewing start position (or unscrews while opening).

  2. Unscrewing motor/gear engages & rotates core to free part.

  3. Stripper ring / ejector pins push part off core.

  4. Part removed by robot/operator.

• Include sensors (proximity/home) to confirm unscrew complete; interlocks to prevent opening during rotation.

7) Strength, wear & material selection for tooling

• Use hardened steels for threads area (H13, S7, or P20 with hardened inserts) depending on shot counts and abrasive materials.

• Polishing: threads often need fine finish — specify Ra target. Hardened chrome plating sometimes used for wear resistance.

8) Safety & maintenance design

• Safety covers for motors/gears, easy access to clutch, grease points, replaceable wear parts (gears, rack).

• Provide ports for service (hydraulic/servo). Include a torque limiter to avoid jamming and part damage.

9) Tolerances & inspection

• Specify thread inspection method (GO/NO-GO plug, CMM measurement).

• Include acceptance criteria for thread runout, pitch, major/minor diameters.

10) Prototype & validation

• Make an aluminum rapid prototype mold or 3D print sample parts to validate thread fit/clearance before steel tool.

• Test cycle: measure unscrew torque, cycle time, part temperature, part dimensions across runs.

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Concrete design tips & recommended values

• Draft: 0.5°–1° where possible on non-thread surfaces. Threads have no draft — they're formed by core rotation.

• Thread root clearance: start with 0.05–0.15 mm depending on pitch and part size; increase for high shrink resins.

• Stripper clearance: 0.1–0.3 mm between part and stripper to avoid scraping.

• Unscrew speed: slow enough to avoid thread damage — typical 5–60 RPM depending on size and resin. Servo gives best control.

• Torque sensing: use a torque limiter or sensor to detect jamming. Set limit based on measured torque plus safety margin.

• Cooling: try to keep temperature variation across the core <5 °C to reduce inconsistent shrinkage.

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Common mistakes to avoid

• Underestimating unscrew torque — especially with high shrink resins (e.g., POM, nylon).

• Poor cooling causing differential shrinkage → jamming.

• Not providing maintainable/replacable core inserts — threads wear quickly.

• Trying to unscrew while part still soft — wait until semi-cooled.

• No safety interlocks for drive — risk of damage and injury.

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Example unscrewing mechanism selection guide

• Low volume (<10k/yr): mechanical cam/rack if budget tight.

• Medium (10k–200k/yr): rack & pinion or hydraulic clutch.

• High (>200k/yr) or high precision: servo motor with torque sensing and programmable motion.

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Design spec template (copy into your CAD/technical doc)

Part & Process

• Part name:

• Material (grade):

• Shot weight:

• Cycle time target:

• Thread: type (internal/external), major Ø, minor Ø, pitch, lead, helix (L/R), standard/ custom.

Mold

• Cavities: (1 / 2 / multi)

• Cavity plate material / hardness:

• Core insert material / hardness:

• Thread finish Ra target:

• Unscrew mechanism: (rack/pinion | hydraulic | servo | cam) — specify model or torque rating

• Unscrew torque limit: (specify)

• Cooling: channels per core, diameter, flow rate target

• Ejection: stripper ring / ejector pins — stroke & forces

• Sensors: unscrew home, torque sensor, safety interlock

• Maintenance: replaceable core insert yes/no; spare insert qty.

Tolerances & inspection

• Thread major Ø tolerance: +/-

• Pitch diameter tolerance: +/-

• Runout: max mm

• Inspection method: CMM / GO/NO-GO.

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Testing checklist (on first try)

• Measure unscrew torque at mold temp (3–5 shots after warm-up).

• Check thread fit on mating part — GO/NO-GO and functional assembly.

• Inspect thread surface for flashes, pull marks, or cracking.

• Confirm cycle time, cooling effectiveness, and part dimensional stability across shots.

• Verify safety interlocks and emergency stop.