Low density foams (in this work, foam density refers to apparent density) are materials of interest for fusion experiments. Low density poly(4-methyl-1-pentene) (commercial name TPX) foams have been produced for [approximately]30 years. TPX foams have been shown to have densities as low as 3 mgcm-3, which is very close to air density (1.2 mgcm-3). Around this density foams are very light and highly fragile. Their fabrication is thus a real technological challenge.

However, shrinking always appears in ranges ranking from 25% to almost 200%. As a result, the apparent density of the final foam never matches the expected value given by the precursor solution concentration. Besides, even if the mold dimensions are precisely known, shrinkage is never linear, and foams have to be machined for precise density measurement.

In our work we present a fabrication process for TPX foams and discuss machining and density measuring issues.

Particularly, we have found that there are volume and weight limits for a determination of density within the range of 3% uncertainty. This raises the question whether density should rather be determined directly on millimeter-sized targets or should be performed on a bigger scale sample prepared from the same batch.