Your question is not ambitious, but I suggest you can read some articles about traditional foams, like EPS. There are many method to prepare foams but according to the condition of your lab.

You can use chemical foaming agents like ADC Azodicarbonamide.

Nowadays there are many references about foaming processes using CO2 at high pressure. It is a versatile, easy and environmentally friendly technique.

Did you check this handbook?

http://books.google.ch/books?id=O4uNJjUT3ngC&lpg=PP1&dq=polymeric%20foam&hl=it&pg=PR5#v=onepage&q=polymeric%20foam&f=false

Let me add a little bit from industry

Three well known processors

•Wet process elastomer latex foam

•Dry process microcellular products

•Instant PU foams of different harnesses

Two manufacturing processors are briefly explained below

•Wet process of making latex foams especially natural rubber latex foam for mattresses/upholstery products. It is done in small batches in open beater similar to industrial cake beater and once nitrogen gas generated in the beating process will trap in the foam and then the foam poured in to open mould and vulcanized in steam chambers. Then removed from moulds, water and serums squeezed and air dried to get the foam products of different shape

•Dry process in compounding plasticized dry elastomer with nitrogen emanating chemicals uniformly dispersed in compound and molded under pressure in closed cavity at higher temperature. Just before vulcanization takes place microcells are formed and built in to the product. This process is mainly employed in making microcellular sheets for shoe soling and beach slippers etc

If you require further details I may post after referring my hand books etc

The first thing would be to know which kind of polymer you plan to use ? Foaming a thermoset resin or foaming a thermoplastic polymer are totally distinct processes ...

I agree with the previous comments: literature is abundant about both aspects, but the first step is chosing the polymer which should be foamed.

Alain

In general, polymer foams can be classified as closed cell foams and open cell foams. In closed cell foams, the membranes which separate the individual bubbles/cells remain intact but in open cell foams, these membranes (also called cell walls) have openings/pores/pinholes thereby creating inter-connectivity between cells allowing mobile phases to flow through the foamed structure just like a porous solid monolith. Thus, it seems what you are asking is how to prepare open-cell foams with a high open-cell content. 

Luckily, preparing open-cell foams of polyurethane is well studied and easy! Polyurethane foams can be prepared by a reaction between a polyether polyol with a polyfunctional isocyanate, (see attached paper). The two reagents are readily available as Polyurethane Foaming System Kits typically labelled as System A and System B by various chemical retailers (e.g. I know Flinn Scientific used to sell them). The foam obtained through this reaction is highly crosslinked and is therefore a thermoset.

Creating open-cell foams of a non-crosslinked thermoplastic material is a lot more involving.  This is why polyurethane foams are almost exclusively used for applications which require open-cell/porous foam structures.

For the cases with recyclable thermoplastic open-cell foams, one conventional technique is the Salt Leaching method which is capable of preparing porous foams through the use of a secondary sacrificial or dissolvable domain. Solid particles such as sodium chloride (table salt) or potassium chloride crystals are blended into the polymer. The polymer is then foamed to produce closed cell foams (see attached paper reference for foaming techniques). The cell walls of the closed cell foams contain these sacrificial particles, and then can later be dissolved out by a solvent (e.g. water to dissolve the salt). Of course, the solvent must selectively dissolve the particles and not the polymer! The pores which are created in the cell walls are controlled by the size and shape of the salt particles while the porosity is mainly dependent on the polymer/salt ratios.

Another technique involves fabricating a closed cell foam, then rupturing the cell walls of the closed-cell foam either mechanically through perforations, chemically through etching in a caustic bath, or thermally through controlled gas ignition inside a pressure vessel.

Freeze drying are also used to create porous foams. In freeze drying the polymer is dissolved in a solvent, the solution is frozen, and the solvent removed (i.e. the solution is dried), leaving behind a porous or cocontinuous foam.

Similar to freeze drying, Thermally induced phase separation (TIPS) involves dissolving the polymer in  a solvent then inducing a thermodynamic instability e.g. a thermal quench, which causes phase separation and the polymer phase precipitates as a cocontinuous/open-porous phase.

In continuous foam extrusion thermoplastic open cell foams have been prepared by introducing a structurual heterogeneity to create a stiffness contrast with well dispersed arrays of rigid and soft regions in the polymer system. When foaming occurs, the rigid regions preserve the cell structure and the soft regions undergo opening. The structural heterogeneous melts have been created by blending immiscible polymers with distinct and different melting temperatures. When the polymer blend is processed at a temperature between the melting temperatures of the two components, one polymer is molten/soft and the other solid/relatively rigid. Other methods have involved using rigid nanoparticles such as nanoclay or partial crosslinking to create the structurally heterogeneous melt.  

Increasing the foams expansion ration has also been effectively used to increase the open cell content. Making the foam expand to a larger degree causes the cell walls to get thinner, and easier to rupture during foaming. One method of accomplishing this is by using a larger amount of blowing agent which can be used to blow the foam to a larger extent.

Another method to introduce open cells in thermoplastic polyolefins is to use controlled plasticization by using a co-blowing agent like pentane/butane, with the primary blowing agent (such as CO2), to make the soft regions of the polymer softer, thereby more likely to open.

 Preparation of open-cell foams in a scalable process is commercially important and an active area of research and there are very clever strategies coming up in the academic literature regularly.