Why do some cap molds unscrew and others strip the cap off the core?

It depends on the thread and the resin. A shallow, rounded thread in a flexible resin can flex over the core and be stripped off, which keeps the mold simpler. Deeper or sharper threads, or more rigid resins, need rotating cores that unscrew the cap before ejection.

How many cavities can a LIHE closure mold hot runner serve?

Closure molds are available from 12 to 72 cavities, and LIHE closure hot runner systems are available for up to 96 cavities.

Which closure mold parts should I keep in stock?

Keep core inserts, cavity inserts and stripper rings for the cavities that wear first, plus hot runner nozzle tips, heaters, thermocouples and seals, and O-rings for the cooling circuits. Cap mold parts have a lead time of about 4 weeks, so the stock should cover at least that period.

What causes cap weight differences between cavities?

Common causes are an unbalanced or partly blocked hot runner zone, a heater or thermocouple fault, uneven cooling flow between cavities, or worn alignment parts that shift the core. Checking zone temperatures and cooling flow per cavity is the usual starting point.

Is the closure mold compatible with my machine?

LIHE closure molds are made for mainstream closure injection molding machines and SACMI compression machines. Send the machine model and interface dimensions with your request so the mold base, ejection and hot runner connection are designed to fit.

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Closure mold

Closure Mold

Multi-cavity closure molds for one-piece and two-piece caps, including EU tethered cap designs. Available from 12 to 72 cavities for 26mm, 28mm, 2925, 3025 and 38mm neck finishes, and compatible with major closure injection molding machines as well as SACMI compression machines. Used for water, juice and tea drinks, edible oil, condiments and soy sauce, cosmetics, health-care products, pharmaceuticals and food packaging.

Cavitation: 12 to 72 cavities.
Cap specifications: One-piece and two-piece caps, tamper-evident bands and EU tethered cap configurations.
Neck finishes: 26mm, 28mm, 2925, 3025, 38mm and custom finishes.
Machine compatibility: Mainstream closure injection molding machines and SACMI compression machines.
Hot runner option: Closure hot runner systems of up to 96 cavities for balanced filling across every cavity.

Quotes and technical questions are handled directly by the manufacturer's international sales department.

Closure mold technical reference: core and cavity, ejection, hot runner, cooling and wear parts

Updated October 13, 2026Author: LIHE Precision Engineering Team

A closure injection mold is a multi-cavity tool in which molten plastic is injected through a hot runner into cap-shaped cavities, cooled against steel cores and then ejected as finished screw caps. This page is a technical reference for engineers who run or buy cap molds: how the forming parts are arranged, how threaded caps are released from the core, what the hot runner and cooling circuits do, which parts wear and what to send when ordering. LIHE Precision (Foshan Lihe Precision Machinery Co.,Ltd.) is a manufacturer of PET preform molds, blow molds, closure and compression molds, hot runner systems and mold components for the beverage and packaging industry, with manufacturing in Foshan, China.

Closure mold anatomy: core, cavity and mold base

Every cavity of a cap mold is a small set of precision inserts mounted in a common mold base. The cavity side forms the outside of the cap (top panel, knurling or grip ribs, skirt); the core side forms the inside (thread, inner seal, tamper-evident band). Because the thread and seal are on the core, most of the dimensional risk in a closure mold sits on the core side.

Cavity insert: forms the outer shell and top panel; its surface finish defines cap appearance and printability.
Core insert and inner core: form the thread, the inner bore seal or olive seal and the band geometry.
Stripper ring or sleeve: pushes the cap off the core at ejection and forms the lower edge of the skirt or band.
Mold base: plates, guide pillars and taper locks that hold every cavity in alignment so that cap walls stay even.
Hot runner half: manifold and nozzles that deliver melt to each gate at the top center of the cap.
Cooling circuits: channels in cavity plates and bubblers or baffles inside each core.

Unscrewing vs stripping ejection for threaded caps

A screw cap has an internal thread, which is an undercut: the cap cannot simply be pushed straight off a rigid core without either rotating it or flexing it. Closure molds solve this in two general ways. Which one fits depends on the thread profile, the resin and the band design, and it is decided together with the cap design.

	Unscrewing ejection	Stripping (bump-off) ejection
Principle	Cores rotate and unscrew the cap while it is held against rotation, then the cap is ejected	A stripper ring pushes the cap off the core; the cap skirt flexes elastically over the thread
Drive	Rack and gear, hydraulic or electric motor drive in the mold	Ejector stroke of the machine or mold; no rotating parts
Thread profiles	Deep, sharp or long threads; rigid resins	Shallow, rounded thread profiles; flexible resins such as polyethylene
Mold complexity	Higher: gears, bearings and seals per cavity	Lower: fewer moving parts per cavity
Maintenance focus	Gear wear, bearing lubrication, rotating seals	Stripper ring wear, core polish, ejection timing
Typical use	Two-piece shells, special closures	High-volume one-piece beverage caps

Closure hot runner systems up to 96 cavities

In a multi-cavity cap mold, the hot runner keeps the plastic molten from the machine nozzle to each gate, so no cold runner is produced and every cavity fills at the same time. LIHE supplies closure hot runner systems of up to 96 cavities; LIHE hot runners use a Husky Ultra-type design with a 3-year leakage-proof guarantee.

Balance: the manifold layout gives every cavity the same flow length so cap weights stay consistent across the mold.
Gate type: valve gates close mechanically and leave a clean gate mark on the cap top panel; this is important for printed or visible top panels.
Temperature zones: each nozzle has its own heater and thermocouple, so a cold or overheated zone shows as a short or heavy cap in that cavity.
Service parts: nozzle tips, valve pins, heaters and seals are replaced as wear parts; existing hot runners can also be refurbished.

Cooling design in cap molds

Cooling usually takes the largest share of the injection molding cycle, so cooling layout has a direct effect on output. In a closure mold the core is the hard part to cool, because the heat from the cap interior must be removed through a small-diameter core.

Core cooling: a bubbler tube or baffle inside each core brings water close to the cap top and thread area.
Cavity cooling: drilled channels or cooling sleeves around each cavity insert cool the outer shell and top panel.
Flow balance: each cavity should receive the same water flow; restrictions in one branch cause oval or warped caps in those cavities.
Water quality: scale and corrosion reduce flow over time; filtered and treated water keeps cooling performance stable.
Highly conductive insert materials can be used in core tips where extra heat removal is needed.

Wear parts and spare parts for closure molds

Closure molds run at high cycle counts, so a defined spare parts set protects production. LIHE tooling is built to withstand up to 8 million shots on Husky, SACMI and Netstal platforms, and cap mold parts have a lead time of about 4 weeks.

Forming parts: core inserts, inner cores, cavity inserts and stripper rings, which wear at the thread, seal and band edges.
Hot runner parts: nozzle tips, valve pins, heaters, thermocouples and seals.
Moving parts: gears, bearings and racks in unscrewing molds; ejector components in stripping molds.
Alignment parts: guide pillars, bushings and taper locks; worn alignment shows as uneven wall thickness or flash.
Consumables: O-rings and cooling seals, replaced at every major service.

Closure mold ordering checklist

Step 1: Send the cap drawing or cap samples and the matching neck finish drawing (26mm, 28mm, 2925, 3025, 38mm or custom).
Step 2: State the cap construction, resin grade, cap weight target and whether the band is tamper-evident only or an EU tethered design.
Step 3: Give the machine model, clamp force, platen and tie-bar dimensions, ejection stroke and any unscrewing drive interface.
Step 4: Specify cavitation (12 to 72 cavities) and hot runner requirements: valve or open gate, controller type and number of zones.
Step 5: List cooling water conditions at the mold: available flow, pressure and water treatment.
Step 6: Agree the spare parts set and the acceptance criteria: sample caps, CMM dimensional reports and a process capability target.
Step 7: Plan installation and start-up support with the LIHE technical team for the first production run.

Inspection and acceptance of a closure mold

Forming parts are measured on an in-house Zeiss CMM; validation and inspection reports are supplied on request.
LIHE tooling consistently achieves CPK ≥ 1.33, a common process capability threshold for critical dimensions.
Cap samples from every cavity are checked for weight spread, thread and seal dimensions and band function.
Operational defect rates as low as 0.0064% have been achieved on LIHE tooling.

Frequently asked questions

Why do some cap molds unscrew and others strip the cap off the core?: It depends on the thread and the resin. A shallow, rounded thread in a flexible resin can flex over the core and be stripped off, which keeps the mold simpler. Deeper or sharper threads, or more rigid resins, need rotating cores that unscrew the cap before ejection.
How many cavities can a LIHE closure mold hot runner serve?: Closure molds are available from 12 to 72 cavities, and LIHE closure hot runner systems are available for up to 96 cavities.
Which closure mold parts should I keep in stock?: Keep core inserts, cavity inserts and stripper rings for the cavities that wear first, plus hot runner nozzle tips, heaters, thermocouples and seals, and O-rings for the cooling circuits. Cap mold parts have a lead time of about 4 weeks, so the stock should cover at least that period.
What causes cap weight differences between cavities?: Common causes are an unbalanced or partly blocked hot runner zone, a heater or thermocouple fault, uneven cooling flow between cavities, or worn alignment parts that shift the core. Checking zone temperatures and cooling flow per cavity is the usual starting point.
Is the closure mold compatible with my machine?: LIHE closure molds are made for mainstream closure injection molding machines and SACMI compression machines. Send the machine model and interface dimensions with your request so the mold base, ejection and hot runner connection are designed to fit.