Insights · Design
Unscrewing molds vs collapsible cores
Both mechanisms exist to solve one problem: how do you get a thread out of a mold without dragging steel across it? They are not interchangeable, and choosing wrong costs either cycle time for years or a tool that cannot produce a clean thread at all.
Use an unscrewing mold when the thread is deep, continuous, fine-pitch or cosmetic — the core rotates out and the thread is never split. Use a collapsible core when the thread is shallow to medium, internal and less than roughly 7–10% of the part diameter in depth — segmented fingers collapse inward before ejection, giving a shorter cycle but limiting thread depth and pitch. For external threads, a split cavity or side action is often the practical answer. If the part allows both, quote both and decide on cycle time.
How each mechanism works
Unscrewing mold
The thread core is mounted on a rotating mechanism — geared racks, a chain drive or a hydraulic motor — driven by the machine or the mold itself. Before ejection, the core rotates out of the molded thread at a controlled speed, exactly as a screw is undone. Because the thread is formed by an unbroken steel surface, the result is a clean, continuous thread with no segment lines.
Strengths: any thread depth and pitch, multi-start threads, fine cosmetic threads, reliable sealing threads, and a consistent result over millions of cycles. Costs: additional mechanism, longer cycle time (the unscrewing stroke), more maintenance attention, and typically lower cavity counts per press tonnage because of the mechanism footprint.
Collapsible core
The core is built from segments that are held expanded during injection to form the undercut or thread, then collapsed inward before the part is ejected. The thread is therefore formed by discrete segments, and it will show fine witness lines where the segments met.
Strengths: shorter cycle than unscrewing, simpler press requirements, well suited to internal threads on closures and shallow undercut rings, and easier to run on lower-tonnage machines. Costs: thread depth and pitch are limited; witness lines are visible on close inspection; the mechanism demands good cooling and careful maintenance.
Side-by-side comparison
| Criterion | Unscrewing mold | Collapsible core |
|---|---|---|
| Thread type | Internal or external, continuous | Internal threads and undercut rings |
| Thread depth | Unlimited by mechanism | Practically limited (roughly 7–10% of diameter or less) |
| Pitch / multi-start | Fine pitch and multi-start are straightforward | Coarse, simple threads only |
| Appearance of thread | Clean, no segment lines | Fine witness lines at segment joints |
| Cycle time | Longer (unscrewing stroke) | Shorter |
| Cavity count | Commonly 1, 2, 4 or 8 | Scales to higher cavity counts more easily |
| Mold complexity | Higher — gears, racks, motor or chain drive | Lower mechanism, but segments need precise fitting |
| Maintenance | Regular lubrication and wear inspection of the drive | Segment wear and cooling discipline |
| Best for | Cosmetic tubes, sealing closures, deep or fine threads | Shallow internal threads, undercut snap rings, fast cycles |
Deciding: five questions in order
- Is the thread internal or external? External threads usually point to a split cavity or rotating cavity rather than a collapsible core.
- How deep is the thread? Beyond roughly 7–10% of the diameter, collapsible cores become impractical and unscrewing takes over.
- Is the thread a sealing or cosmetic feature? If the customer will see it, or it must seal, choose the mechanism with no witness lines.
- What is the annual volume? High volume makes cycle time the dominant cost, which favours collapsible cores when the thread allows it.
- What press will run the tool? The mechanism determines the mold height and tonnage requirement, especially on unscrewing tools.
Design decisions that remove the mechanism entirely
Before accepting either mechanism, it is worth asking whether the thread is needed at all. Many closures can use a snap bead, a bayonet, an interference rib or a separate threaded insert, all of which eliminate the mechanism, shorten the cycle and reduce the tool price. This is exactly what a DFM review is for: we check whether the geometry that forces a costly mechanism can be replaced without changing how the product works.
What CENTS brings to threaded parts
Threaded and cosmetic tube tooling is a long-standing specialty here. We have built lipstick tube and inner-tube molds for a well-known European cosmetics brand for more than 15 years; those tools were specified for a 1,000,000-shot life and validated at 5,000,000 shots. Unscrewing units and collapsible cores are specified from recognised suppliers, while the mold plates, cores, cavities and all motion design are machined in our own plant to ±0.01 mm. When a part supports both solutions, we quote both with cycle times so the choice is made on data.
Threaded part?
Send the thread detail — we will quote both mechanisms
Include the thread form, depth, pitch and the resin. You will get a mechanism recommendation, a cavity layout and priced alternatives.