Hey Surfteach,
Thanks! now I know why coffee cups are waterproof. who would have thought that I’d find this out in a surfboard design forum.
Rio
PS- your spelling is horrible! Soe beter nut powst aghen yuu heer?! 
Quote:
Daklaw,
I say its the pressed surface beads straight out of the mold which means a styro coffee cup won’t take water.
Also they’re way denser than the foam we shape out of…
I’m not sure it is denser, nor that it doesn’t absorb water. First of all, it doesn’t usually get exposed to water for a long time as it’s thrown away pretty soon (compared to a surfboard). Secondly, who cares if the cup gains 5% water by volume as long as it doesn’t leak? (5% is a typical test number for EPS if you look at the manufacturers specs). I think you could put a pretty high column of water on an EPS blank for hours without it leaking through.
But the fusion between the beads in the cups are much better than for the big blocks. In fact, I think all molded EPS products have better bead fusion than the large blocks of construction foam.
regards,
Håvard
but if you do leave it sitting for a while it’ll soak through…
Just wanted to clarify my earlier comment.
The heat and small bead size of the cups fuse the surface so that it a barrier to the liquid seeping thru and leaking all over your suit & tie. (The typical shapers garb)
As all polystyrene bead is molded with live steam, there always is space between the fibers of polystyrene for water. (Think of a bead as a ball of string) However if the molding process is complete with 100% fusion, the take up of water is slow. That’s what we want, not a material that wicks water like a paper towel.
Block fusion is only a function of the molder and the market. If we want 100% fusion in a block, we must pay for it as it takes longer to mold the block and more energy. (still cheaper than poly) The building industry wants a cheap insulation and durability (abrasion resistance) is not a factor. That’s why Home Depot foam is not the stuff to use!