Recently I have made a couple of boards with EPS for the core material instead of using a polyurethane foam. In doing some research for these boards I had some observations that I think are worth discussing.
EPS for surfboards comes in typically 2 or 3 densities 1.2, 1.5, and 2 pounds per cubic foot. It is possible to get other densities like 1lb. In metric densities are given as kilograms per cubic meter 1lb/cu ft.= 16 kg/m3, 1.2=19, 1.5=24, and 2 = 32. (I put those in there for quick reference).
I was researching boards shapes and designs for the wing foil boards I am going to build next and came across a manufacturer’s website where they claim they use a high density EPS core in their boards and specified that it is a 15kg/m3 foam…. wait, what 15kg/m3 is high density? For those of us that are metrically challenged that is…. 0.93 lbs per cubic foot. Wow that is light!
So what is the big deal you ask? Well thanks for asking. As you may know in my other life I distribute and sell surfboard blanks and we handle a lot of foam. One of the things we noticed when we began carrying eps blanks is that the difference in compression strength between 1.5 and 2lb foam is distinctly noticeable. So I jumped on the web and did a nominally exhaustive research session that didn’t make it past the first page of search results that yielded the following information:
0.9 lb eps has a compressive strength of 10psi
1.15lb (what we would call 1.2lb) has a compressive strength of 13psi
1.35lb (what we would call 1.5lb) has a compressive strength of 15psi
1.8lb (what we could call 2lb) has a compressive strength of 25psi
Wow so going up to 2lb/cu ft eps gives us a much higher compressive strength and if you know anything about surfboards you would know that most breakage and damage is caused by deformation of the foam from crushing.
So why doesn’t everybody use 2lb or even higher density EPS. Well in a surfboard I don’t see a reason why not to. The volume of the board is so low and even at 2lb the density is much lower than that of urethane foam that the board will be lighter even if glassed heavier. When we get into SUP and Wing Foil boards the volume of the board is much larger and there is a weight penalty for going with the higher density.
It is somewhat difficult to quantify what that penalty is because in order to compensate for low density foam wing board, sailboard, and sometimes SUP board manufacturers will use bamboo or high density foam veneers over the low density EPS to create a surface skin that is dent resistant. This adds back to the weight of the board and makes the manufacturing process more complex. This also requires significant reinforcing of the fin or track box areas. In addition to making the building process more complex adding wood veneers into the board also introduces future issues as we have often seen bamboo or wood veneers wick water through a board when the fiberglass skin is punctured (keep in mind that that skin is only a couple sheets of paper thick).
To recap compression strength of EPS goes up exponentially in relation to density allowing for a surfboard builder to make a higher quality board that will be more dent resistant with a minimal increase in overall weight of the board. When it comes to high volume boards one needs to calculate in the whole laminate schedule of the board to determine what the difference in board weight would be between a thin skin laminate over dense foam vs a cored skin laminate over low density foam.