The Problem with Blue and Pink Foam
Update 12/15/2016: Since this article was written Dow has discontinued Surfboard, aka Spyderfoam. We will leave this entry for the history books.
Note, when we say Blue Foam we do not include Surfboard (aka spyderfoam, although not technically correct). Read to the bottom to find out the difference so we don’t spoil it for you up front.
It should also be noted that Owens Pink F250 has met the same fate as Dow Blue Hi-load here at our shop. All for the same reasons listed below, but mostly due to high amounts of debris in the foam. Back to Blue…
Blue foam, or Hi-load 40/60, has been around for years, and is readily available in Big Box home improvement stores in many parts of the country. It has been a common choice for gliders and other high stress applications.
So, why do we hate the stuff for airfoils?
Blue foam is produced by an extrusion process, and has a slick surface on the sheets. This slick surface makes it easier to handle the sheets, and allows adhesives to stick well. Both are desirable properties for building contractors where Blue foam is used as an insulation material.
The problem with the slick surface is that it causes a lot of stress in the material. When wing cores are cut from the stuff, the beds will warp like a potato chip. The stress can also cause the foam blocks to move while the material is being hot wire cut, causing other problems.
The sheets are not flat, which aggravates the stress problems.
When any foam is hot wire cut, the wire melts the foam out of its way. This path of the wire is called the “kerf.” That melted material has to go somewhere – usually into the surface of the core – but not Blue foam!
The melted Blue foam beads up on the wire and falls off in the form of droplets. These droplets form dimples in the core’s surface, and in many cases, make the cores unusable.
Blue foam typically has a lot of fine dirt particles embedded in the material that cause wire marks when cut. This problem alone can cause scrap rates as high as 50%. That means we have to cut 2 or 3 cores to get 1 good one in many cases.
Otherwise, we like Blue (and Pink) foam a lot!!!
As a result of these problems, we quit purchasing Blue (and now Pink) foam some time ago.
So what are the options?
Surfboard is an excellent replacement if you need the +60 psi compression strength and fine “grain” of an extruded styrene. Surfboard and Blue Hi-load foam are both products of Dow Chemical. However, Surfboard is manufactured using a different process than Blue foam sheet, and has fewer of the problems mentioned above. Surfboard is 2.3# density, Dow Hi-Load 40 is 2.1# density and Hi-Load 60 is 2.3# density. In general, we recommend using Surfboard for aircraft applications that call for Hi-Load 40 or Hi-Load 60 Blue foam. Surfboard is also a popular choice for our boat builders.
2# EPS works well if you are using a thicker layup schedule. 2# EPS has a 25 psi compression rating, which is simlar to Owens Pink F250, but lacks the fine “grain” of XPS. 2# EPS is a popular choice for our car wing builders.