foam core

[rapscallion]

I'm interested in how the foam sheets used in making composite boats are made. PVC seems to have fallen by the wayside and SAN seems to be the preferred material.

Why do I have to buy the stuff in 4x8 sheets? why can't I set up an injection mold and end up with a foam core with my hull shape?

[Roly]

I guess it is difficult to achieve a consistent density and as a one off operation is just not economically viable.
Blowing polyurethane in a compound curved shapes of size requires heavily built molds and stricttly controlled input of the reagent to get uniform cell size. Much easier to blow thin flat sheets.

[Eric Sponberg]

Foam is mixed in large vats with various chemicals, which, when combined in the vat, balloon into large buns that look like oversized bread loaves. The sides and bottom of the loaf take the shape of the vat, and the top overflows, just like a real bread loaf. These loaves are really big, bigger than 4'x8'. The density of any loaf depends on the mix of chemicals going in, and the density in the middle of the loaf is fairly constant, but changes as you go toward the surfaces. The big loaves are trimmed to shape in conveniently sized sheets, just like when Mom used to cut off the crusts of bread from your sandwich--the center of the loaf is the best part, the edges are waste. And since 4'x8' is so ubiquitous in our society, that's the size they come in. Other treatments are made to the core sheets as needed, such as pre-coating, slicing, punching, scrimming, etc.

If you were to do the molding yourself, I personally think you would end up doing a lot more work than it is worth. You would not have any control over density as the mix expanded and hardened. You would not know what you have, and you would not have any idea of the mechanical properties that you ended up with. Best to stick with the standard established products with predictable properties and quality.

Of the PVC foams, such as Divinycell and Airex, there are the cross-linked (Divinycell) and the linear (Airex). The linear PVC is heat formable--you can heat it in an oven to about 140-150 deg. F, and it gets really pliable. You then have a short time to bend it over a form, or press it into your mold, if you like. As it cools, it hardens again into the shape you have made. I am not sure if the SAN foams (Core-Cell) can do this, but you can go to suppliers and find out. Core Cell is marketed by Gurit Composite Technologies, formerly SP Systems.

Eric

[rapscallion]

That is my point! The middle of the loaf seems to be consistent. My feeling the edges of the loaf vary in density due to the rather silly process used.

Polymers can be very sensitive to things like concentration, ratios of monomers mixed, temperature, stir rate... ect.

I imagine a flexible tube where the two monomers are injected into at a very controlled rate in order to govern the consistency of the foam produced. the foam would come out the end of the tube a consistent density. (this type of reactor is used frequently) I used to work for a chemical company, and i know how much the raw materials cost for the foam and the epoxy... It makes me sick how much mark up is there... here is an example...

I was asked to develop a process for Monoxidil sulfate. Big bucks. Monoxidil was 1800 bucks a gram (what we charged) The cost to us including labor was 48 cents.

Boat builders have to get together with some chemists and bring the prices of new boats down a bit. IMHO.