Everyone in moulding is under pressure to reduce costs, improve productivity and increase capacity; all on restricted capital budgets. Perhaps moulders aren't as aware as they should be of the potential productivity benefits given by stack moulds, which can double or treble production yields for each closure of the moulding press. Stack mould techniques can greatly improve productivity on a wide variety of components, large or small, for a relatively modest increase in total tool cost.

Stack moulding covers components such as caps, closures, automotive trim, trays, and high aspect ratio containers or covers.

Essentially, the stack mould tool provides a means of increasing production volumes of injection moulded products or components from a single press.

Productivity is a vital concern when producing thousands of low unit cost components, and capital investment in presses or tools is a key consideration.

When volumes or delivery rates are high enough to consider a duplicate tool set, or even another machine, the stack mould often provides a far more cost-effective total investment route to meet production demands.

This benefit is especially valuable on large parts with relatively long cycle times, and can greatly improve machine utilisation and availability.

The basic concept is very straightforward: have two parting lines instead of one.

This gives three mould levels, the two parting plates and a central plate.

The central plate, set between the parting plates, carries one wall of the moulded component on each side.

The two parting plates each carry the other component wall(s).

When the mould opens, a gear and pinion set drives the two partings to open at the same rate to release two moulded components.

Depending upon the design the plates can produce many more than two components each time the press opens , multiple cavities can be mounted on each plate, so that is possible to produce a set of dissimilar parts in a single cycle, ready for assembly or direct to a kit pack.

While this simple concept provides a reliable route to major productivity gains, it is not always straightforward in practice.

Standard single parting tools normally incorporate the runners, gates, hot manifolds and injection nozzles on one fixed plate, while the other plate moves to create the single parting.

In the stack mould double, or even multiple, parting lines require deliveries of melt to cavities on various sides of the parting plates.

Runners, gates and other melt delivery and control systems must therefore be carried on the moving plates.

As the multi-part line plates must move, this does add some cost and additional complexity, but often results in investment costs far below the costs of duplicating tooling, not to mention the cost of additional presses.

Stack mould designs provide solutions will achieve substantial cost benefits when compared with tool duplication, extreme use of multi-cavity techniques or an additional machine.

Use of stack tools does, of course, depend to some degree upon the press specification.

Naturally, much greater 'machine daylight' is required as the press opens.

The majority of machines can accommodate the stack tools to produce caps, closures, packaging, fan blades, automotive trim parts, garden furniture, electronic enclosures and many types of small containers.

All these products have elements that dictate the size and type of machine, some by clamp tonnage and projected area, some by cavitation and required platen space.

Provided that the combination of plate thickness' and parting gap space doesn't exceed the normal 'daylight' of the machine, there are no significant problems with the standard injection moulding machine, where, for the same pressures and cycle times, productivity is increased! Obviously, many key decisions about tool design and selection depend on volumes and delivery rates.