Two-stage investment casting processes can be used to make open-cell metal foams which replicate the shape of polymeric foams. The processing steps to produce polymeric foams are straightforward, with good control over the cellular nature of the foams.

Figure 1: Replication of a polymer foam structure by the production and infiltration of a plaster mould.

The process is shown in Figure 1. A polyurethane foam is first infiltrated with plaster. The assembly is then heated to 700ºC, to fire the plaster and remove the polyurethane foam. Molten metal is then poured onto the mould - again, combinations of vacuum and high pressure can be used to ensure full infiltration. The plaster is then dissolved, to give a net-shape metal foam with an identical structure to the original polymer foam.

This process produces relatively small quantities of expensive, high-quality foams with reliable material properties. An example is shown in Figure 2. The porosities can be as high as 98%, with pore size between one and several millimetres. The shape of the end product can easily be tailored to the desired use, and solid wax areas can be included in the polymer preform to create perfectly bonded solid regions within the final foam. Any metal or alloy can be used. This method is used commercially to produce DUOCEL™ foam, for use as heat exchangers, filters and acoustic absorption materials. The main obstacle to widespread use lies in the complexity, cost and difficulty of scaling up the process.

Figure 2: A DUOCEL™ aluminium foam, produced by the production of a plaster mould of of a polymer foam, and subsequent infiltration of the mould with molten metal.