Threaded caps
Stripping, unscrewing, or collapsing the core.
Threaded capsProcess guide
Unscrewing Molds
Once a threaded part has to be unscrewed off its core, the mold gains a small gearbox. How that gearbox is driven, and how well it is looked after, decides a lot about how the tool runs.
Scope
What happens after the decision to unscrew
Whether to strip, collapse, or unscrew a thread is covered on the threaded caps page. This page starts after that choice.
In an unscrewing mold, each threaded core sits on a shaft with a gear or sprocket at its base. When the mold opens, a drive turns those gears and the cores rotate out of the parts, or the parts are rotated off the cores. The outside of the part has to be held still while this happens, usually by grip features in the cavity.
As the thread turns, it also moves along its axis by one lead per turn. The mold has to allow that: either the core retracts at the same rate it turns, or the part is pushed forward by a stripper plate in step with the rotation. Get the rate wrong and the thread is dragged or crushed on its way off.
Drives
Ways to turn the cores
The choice depends on how many turns are needed, the press, and how much control the process needs.
| Drive | How it works | Strengths | Limits |
|---|---|---|---|
| Rack driven by mold opening | A rack fixed to one half turns a pinion as the mold opens | No extra power supply; self contained tool | Turns are tied to opening stroke, so only a few turns are practical |
| Hydraulic cylinder and rack | A cylinder on the mold drives the rack independently | Timing is separate from mold opening | Stroke still limits total turns; needs press hydraulics |
| Hydraulic motor | A motor on the mold turns a gear train continuously | Any number of turns; strong torque | Speed and position control are coarse; oil near the tool |
| Electric servo | A servo motor drives the gear train under its own controller | Precise speed, position, and repeatable timing; clean | Higher cost; needs a controller and wiring at the press |
Design points
Four problems every unscrewing tool has to solve
These are what separate a tool that runs from one that keeps coming off the press.
- 1
Following the lead
Core retraction or stripper plate travel has to match the thread lead exactly. A lead screw on the drive shaft is the usual way to lock the two together.
- 2
Sharing one drive
In a multi cavity tool, one drive turns a central gear that meshes with every core. Backlash and gear wear show up as cores that stop at slightly different angles.
- 3
Cooling a spinning core
The core is the hottest steel in the tool, and it has to turn. Water reaches it through rotary unions or a stationary bubbler inside a hollow core.
- 4
Stopping in the right place
Cores must come back to the same start angle every cycle, or the thread start and any features indexed to it move around.
Upkeep
What wears, and what to check
An unscrewing mold has more moving steel than almost any other tool. It needs its own maintenance plan, not the plan for a standard mold.
- Gear teeth and bearings, which wear faster when grease is thin or washed out by condensation
- Rotary union seals, which leak water into the gear housing when they fail
- Thread crests on the cores, which round over with wear and change how the part fits
- Anti rotation features in the cavity, which wear and let parts spin with the core
Keep reading
Related guides
Deciding how to release a thread, designing it, and the parts that need it.
Molded in threads
Thread design rules for molded parts.
Molded in threadsMolded threads vs inserts
When a metal insert is the better thread.
Molded threads vs insertsUndercuts
Slides, lifters, and other moving steel.
UndercutsBottle caps
Neck finishes and tamper bands.
Bottle capsClosures
Caps, hinges, and dispensing tops.
ClosuresStack molds
More parting lines on the same clamp.
Stack moldsProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about unscrewing molds
Does unscrewing add to cycle time?
Yes. The cores have to turn out before the next shot, and that time depends on the number of turns and the drive speed. Servo drives can start turning while the mold is still opening, which hides part of it.
Can external threads be unscrewed too?
They rarely need to be. External threads can usually be formed by cavity halves or side actions that open away from the part. Unscrewing is mainly for internal threads.
Why does the cap spin with the core?
The grip features in the cavity are missing, worn, or too shallow, or the part is still soft enough to twist. Fixing it can mean changing the part's outer surface, which is why grip features are best settled early.
Is an unscrewing mold always the most expensive route to a thread?
It is usually the most complex, but not always the most expensive in the end. A threaded insert adds a part and a loading step, and stripping limits the resin and thread form. Volume and function decide which costs more over the program.
Next step
Send the thread, the resin, and the torque
Upload CAD with the thread spec and assembly torque. The review compares a molded thread with a threaded insert set by insert molding. NetProto manufacturing is ISO 9001 certified.