So I had some success locating thicker (4”) XPS Foam Board. I wanted to compare it to similar and lower density EPS in regard to how well it machined and finished since EPS is also available in thick billets of various density. The XPS sample is Owens Corning Foamular 250 NGX. The NGX just designates that is was made with a more eco friendly non-HFC blowing agent. The OC specifications state the density for 250 should be 1.55 lb/ft3 and Foamular 150 should be 1.3 lb/ft3. When I measured and weighed the Foamular 250 NGX, it calculated as 1.6lb/ft3. However when I weighed a half dozen different samples of Foamular 150 from 1”, 1 ½”, and 2” thick boards I had in my shop, it all weighed 1.5lb/ft3, not 1.3 lb/ft3. Hmmmmm? Now I used a triple beam scale and measured my blocks diligently and though I could be off by a couple percent but not 20%, and not on six different samples. So I mounted some thin samples, sanded, and machined them. The first picture are the samples (XPS and two EPS Densities) sanded with 150 grit. Though the surface finish does improve on all samples, you can still see some modeling and small voids in the EPS samples. If I used finer grit abrasive, the XPS finish would continue to improve up to 320 grit. The size of each respective sample is 3.5" x 1.75" The next pictures are the samples exactly as they came off my CNC router. The sanded plaque was surfaced with a .25D bottom cutting bit at 26krpm and 45ipm. The details were machined with a 1/32"D 2-flute spiral bit at 26krpm and between 30-45 ipm. You can see the XPS has markedly higher retention of detail as machined. The EPS has some flaws and the modeled/beaded EPS surface finish would be visible on the castings with the use of lost foam refractory coating. This XPS has more of a sparkle in appearance on the machined surfaces that differs from the Foamular XPS I get at the retail stores. Don’t know why. Though the slightly higher XPS density does give it some advantage, I don’t think very much so. I do have some 2 lb/ft3 EPS but I’d rather not pursue that high of density for my pattern foam. It just confirms my previous experience that XPS finishes better than EPS and lessens my interest in pursuing thick EPS billets for machining larger patterns......at least if XPS is available. The difference in the measured vs published densities of Foamular 150 and 250 is a bit surprising, but given I’ve been using the 150 forever, I have less concern about using 250. Best, Kelly
