What Mold Actions Are
Most plastic parts have features the mold cannot form with a straight open-and-close motion: side holes, undercuts, internal threads, snap-fit hooks. Mold actions are the moving mechanisms inside the tool that form these features and then get out of the way before ejection.
The three families are slides, lifters (angle lifters), and unscrewing mechanisms. Each solves a different geometry problem, and each adds cost, lead time, and maintenance to the mold. Understanding them helps you design parts that need fewer actions — or accept the ones you truly need.
Slides: For Side Holes and External Undercuts
A slide is a steel block that moves sideways, perpendicular to the mold opening direction. An angle pin drives it as the mold opens. It carries the core detail that forms a side hole, a window, or an external undercut. When the mold opens, the angle pin pulls the slide back, clearing the undercut. Then the part ejects straight.
Slides are the most common mold action. A typical slide assembly includes the slide block itself. The angle pin (usually 15–25° from the opening direction). A wear plate. A gib to guide it. And a heel block that locks it against injection pressure during the shot.
Design rules for slides:
- Keep the slide travel short. Long slides need long angle pins, which bend under load. If the undercut is deep, consider two smaller slides instead of one long one.
- The heel block must take the full injection pressure on the slide face. An undersized heel lets the slide back off a fraction of a millimeter — enough to flash on every shot.
- Cool the slide if it carries significant steel mass. An uncooled slide runs hot, extends cycle time, and causes sink near the side feature.
- Put the parting line where the slide meets the main steel carefully. A visible witness line on a cosmetic surface is the price of a side hole. Position it where the customer will not see it.
Lifters: For Internal Undercuts
A lifter (angle lifter) forms an undercut on the inside of the part. A snap hook, an internal barb, a recessed clip. It sits at an angle in the ejector assembly. During ejection, it moves forward and sideways simultaneously, following the angle. This pulls it out from under the undercut as the part lifts off the core.
Lifters are elegant but demanding. The lifter angle is typically 5–12° from the ejection direction. Too steep and the lifter binds; too shallow and it does not clear the undercut before the part is pushed past it.
Design rules for lifters:
- Keep the undercut shallow — 1 to 3 mm of hook depth is the comfortable range. Deeper undercuts need longer lifter travel, which means a taller mold and more ejection stroke.
- Give the lifter head a generous draft (3–5°) on the non-undercut sides so it releases from the plastic cleanly.
- Watch the lifter footprint. Each lifter needs space in the ejector plate and clearance in the core. A cluster of lifters close together weakens the core steel between them.
- Polish the lifter in the direction of its travel, not the ejection direction. Scratches across the travel path become drag marks on every part.
Unscrewing Mechanisms: For Threads
Internal or external threads that must be molded — not tapped afterward — need the threaded core to rotate out. This happens before ejection. The classic solution is a rack-and-pinion unscrewing mechanism. A motor or hydraulic drive turns a gear train. The threaded cores spin. The part walks itself off the threads.
Unscrewing molds are the most expensive action type. They need the drive system, the gear train, timing control, and often a collapsible or multi-stage ejection sequence. They also run slower — the unscrewing cycle adds seconds to every shot.
When to avoid molded threads: if the thread is a standard size, molding the boss plain is almost always cheaper. Tap it as a secondary operation, or use a threaded insert. Reserve unscrewing mechanisms for threads that cannot be post-machined. Deep internal threads in a blind hole, for example. Or threads in a part too small to fixture for tapping.
What Actions Cost You
Every action adds tooling cost and risk. A rough guide (broad industry ranges — your actual quote depends on mold size, complexity, and region):
- One simple slide: adds roughly $800–$2,000 to the mold, depending on size.
- Multiple slides or lifters: each additional action adds cost. But the bigger penalty is schedule. Fitting and timing four lifters takes days longer than fitting one.
- Unscrewing mechanism: adds $5,000–$15,000+ and extends both build time and cycle time.
Actions also add maintenance. Slides wear. Lifters gall if not lubricated. Gear trains need periodic inspection. A mold with six actions needs a maintenance schedule; a straight-pull mold mostly needs cleaning.
Designing to Minimize Actions
Before accepting actions in your mold quote, check whether the design can avoid them:
- Side holes: can the hole go through the parting line instead? A hole on the parting line needs no slide — the two mold halves form it directly.
- Snap hooks: can the hook flex enough to bump off the steel without a lifter? Shallow hooks in flexible resins (PP, TPE) sometimes can. This is called “bump-off” and it eliminates the lifter entirely. But verify with the mold maker first. It depends on resin, hook geometry, and draft.
- Internal undercuts: can the part be redesigned with a window or slot that lets a straight core pass through? Core-through designs are the cheapest way to form internal features.
- Threads: standard thread → tap it or use an insert. Only mold threads you cannot make any other way.
What We Check in DFM
Our mold design review maps every undercut on your part and proposes the action strategy. Which features get slides. Which get lifters. Which can be bumped off. Which should be redesigned out. We flag actions that are marginal before steel is cut. A lifter at the edge of its travel range. A slide with inadequate heel support. The cheapest mold action is the one the design does not need.