An interesting RCG note from Springer at Model Plane Foam. They're a pretty cool outfit... they asked the manufacturer of Dollar Tree foamboard for a foam more suited to model building.

Model Plane Foam is available from modelplanefoam.com. it is the same recipe as the latest Dow fff, white, no film or printing or perforations. Shipping to AZ is high, but it is still typically less than Depron.
Here's the thread with the history and info on it: 

http://www.rcgroups.com/forums/showt...del+plane+foam

Adams says that the dtf formula is different from the Dow formula which is what we get (less the pigment, of course). A grade MPF has a fine skin with almost a rubbery feel to it that is decently dent resistant. A big part of the difference in both stiffness and durability comes from that skin.When the paper is removed the dtf surface is composed of open cells (no dense skin left at all) which is fragile and dents easily. So the difference in strength/stiffness is a combination of factors. Current A grade MPF is running between .2 and .23" thick, on the low side of the thickness spec.

I'm sure that a big part of the cost difference is shipping (I find it is awfully expensive to ship mostly air!), but process may also be a factor, if I understand correctly, Depron is extruded in flat sheets (probably why it has best flatness) wheras MPF and fff are extruded in a different process that has higher throughput, but sacrifices flatness (sometimes - it varies within a run and is one of the parameters we specifically ask them to control better than they do for fff).

As far as stiffness, density, etc. MPF is targeted at 2lb/cu. ft. From what I have seen of depron, it seems like similar density. Depron is stiffer but more brittle, which I attribute to the formula difference. One customer who made 3d plane kits loved MPF for his planes because a crash that would previously result in a bunch of Depron pieces was a non issue for MPF. He was one of those who milled away foam to make a girder structure in the foam sheet so the increased toughness was important.

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