Draft and undercuts: how a threaded cap leaves the mould
A cap forms around a steel core, then has to come off it. That sounds simple until you look at its thread: it is designed precisely to stop it from sliding. The whole mould design starts from this contradiction.
By Tecmom editorial team · 2 min read
Draft, an almost invisible angle
As it cools, the material shrinks and grips the mould core; see shrinkage. If the walls were perfectly parallel to the opening direction, the part would have to be torn off: friction, scratches, distortion.
These walls are therefore given a slight taper, the draft angle. A Protolabs design guide recommends at least 0.5° on all vertical faces, and 1 to 2° in most cases. A textured surface needs more: 3° for a light texture, 5° or more for a heavy one. See surface finish.
Undercuts, what catches
An undercut is a feature that prevents the part from leaving in the mould's opening direction: a snap bead, a tamper-evident band, and above all a thread. On a cap, the internal thread hooks onto the core that formed it.
A way to release the part is then needed, and it is one of the weightiest choices in mould design.
Three ways to release a thread
Stripping pushes the cap off the core: the material flexes just long enough to clear the thread, then recovers its shape. It suits flexible materials, such as polyethylene and polypropylene, and shallow threads.
A collapsible core is made of segments that fold inwards and release the thread without rotation. An unscrewing mould rotates the core, by gears or a motor, to unscrew it from the part: this is the solution that best preserves the thread profile, at the cost of a more complex mould.
A choice paid for in steel
Each solution has its consequences. Stripping is simple but limits thread depth; a collapsible core leaves witness lines; an unscrewing mechanism adds moving parts, maintenance and cost. See what a mould costs.
The thread itself follows from the bottle neck finish, whose profile it must match. For a manufacturer that designs its own moulds, this choice is made at the drawing stage, long before the first part; see our expertise.
A cap holds on its bottle thanks to what makes it hard to eject. The mould has to solve this paradox at every cycle.
Glossary
- Draft angle
- A slight taper on walls parallel to the mould's opening direction, which eases ejection.
- Undercut
- A feature of the part that resists its removal in the mould's opening direction.
- Core
- The part of the mould that forms the inside of the part.
- Stripping
- Ejecting a part that flexes momentarily to clear an undercut.