Stack Mold: Double Your Output on the Same Press
A stack mold places two or more levels of cavities in line, so you mould twice the parts for roughly the same clamp force. For high-volume shallow parts — boxes, cases, lids, Petri dishes — it is the most cost-effective way to produce enormous quantities without moving to a bigger machine.
- 2+ cavity levels in line
- Same tonnage as one level
- Hot-runner for fast cycling
- For very high-volume shallow parts
Why stack moulds exist
Many simple standard components — shallow boxes, cassette and video cases, Petri dishes — are sold in the tens or hundreds of millions, so competition is fierce and the moulder must adopt a highly efficient, cost-effective method to compete.
These parts often have large projected areas that would normally force the use of a large, expensive machine. If the moulder can run them on a smaller machine instead, the cost of production drops considerably. That is exactly what a stack mould achieves. See the injection moulding machine basics for context.
The clamp-force principle
Normally the machine locking force is the projected area of the parts multiplied by the injection pressure. Stacking changes the maths in your favour.
The projected area is the area of the moulding seen looking down the direction of injection. When one component sits directly beneath another, their projected areas coincide — so the projected area for moulding two components in line is the same as for one. The opening forces on both cavities are equal and opposite and cancel, leaving a net force the same as a single part. Any parts can share a stack provided their in-line projected areas are equal. See clamping-force basics.
How a stack mould is built
Figures below show a typical stack mould — an eight-impression tool moulding polypropylene covers with a full hot-runner system.
- Scissor links, reversed side to side, keep stripper-plate actuation balanced and prevent out-of-balance forces.
- Very long guide pillars locate the tool throughout its long opening stroke.
- High-quality alloy tool steels are used throughout, because the tool runs huge quantities.
- Continuous running demands an adequate, preferably automatic, lubrication system.
Hot vs cold runner in stack moulds
Hot runner (usual)
With very large quantities, the hot runner gives faster cycling, fewer rejects and more consistent production. It is the standard choice for stack tooling.
Cold runner
The upper cavities feed via a sprue in the normal way; the lower set feeds through a sprue extension whose interface breaks away from the upper sprue as the tool opens. Parts are ejected by air ejectors and returned by springs.
Running huge volumes of a shallow part?
A stack mould could halve your press size and cost per part. Send the part and annual volume for a stack-vs-single comparison.
- 2-level and multi-level stack tooling
- Hot-runner design for fast cycling
- High-quality alloy steel throughout
Stack mold FAQ
What is a stack mold?
A stack mold has two or more levels of cavities stacked one behind the other in the direction of injection. Because the projected areas line up, moulding parts on two levels needs roughly the same clamp force as one level — so you double output on the same size press.
Why does a stack mold not need more clamp tonnage?
Clamp force is the projected area times injection pressure. When cavities are stacked in line, their projected areas coincide — the opening forces on each level are equal and opposite and effectively cancel, so the tonnage needed is the same as for a single level. Different parts can share a stack provided their in-line projected areas match.
When should I use a stack mold?
For very high volumes of shallow parts with large projected areas — boxes, cassette and video cases, Petri dishes, lids and thin-wall packaging. Stacking lets you run them on a smaller, cheaper machine than their footprint would otherwise demand.
Do stack molds use hot or cold runners?
Both exist, but hot runner is far more usual. With the large quantities stack moulds are built for, a hot runner gives faster cycling, fewer rejects and more consistent production. Cold-runner designs feed the lower cavities through a sprue extension that breaks away as the tool opens.
What construction does a stack mold need?
High-quality alloy tool steels throughout, very long guide pillars to locate the tool across its long opening stroke, balanced scissor-link actuation of the centre section, and — because they run continuously — an adequate, preferably automatic, lubrication system.
