Cork, foams, composite, laminates, adhesive
As a rule, denser materials support better and last longer, but require greater skill and care to avoid hot spotting and complaints.
Computer machined blanks or vacuum bag molded footbeds are usually monolithic, which means one uniform type of material comprises the entire footbed. Most other footbeds are laminated, which is to say, some sort of thermo-formable blank is molded to the shape of the foot; posting foam is glued to the underside of this blank, and then the excess material is ground off. In the case of some 3/4 length walking orthotics, the posting material at the heel is actually made out of an epoxy. My experience has been that the laminate approach allows the fabricator to use a variety of posting material to achieve a desired result. In many instances, for specific pre-existing forefoot hotspots, it is possible to grind the posting foam away down to the contoured blank, and then glue in a properly sized piece of softer foam. Another advantage to laminates is the use of different blank materials for different applications. Some materials are too stiff to flex repeatedly without cracking, but work extremely well for situations where flexing is not an issue. The bottom line is that you can choose a blank material that contours well, hardens/cools in a timely manner, grinds easily, and lasts a long time. Some materials cool very quickly, which means you can mold more blanks in a given amount of time, but they have a very short ‘drape interval,’ which is to say that the fabricator has very little time between reaching working temperature and becoming too cool to contour.
As far as posting materials are concerned, I usually build with a foam that ranks somewhere around 60-65 durometer. This seems to provide enough support without being unruly to work with. When I was learning how to do this, I was told that nobody needed anything stiffer than around 30-35 durometer. I think that was so mistakes made on substandard footbeds would not be so apparent. The hardest posting foam I have used grinds just like the tire on a mountain bike. It works really well, but to get it to contour to the blank, it needs to get really hot, and then it can bond, without glue, to the blank. Not a lot of room for error. I suppose I could use cork, or a nylon/cork composite known a Birko-cork as a posting material, but the expense does not seem worth it, as it also comes in around 55-60 durometer, and doesn’t seem to offer anything more than the eva/cork blend I currently use.
Some in the industry use composites for the contoured part of the footbed, but those materials are better suited for plaster cast molding, and 3/4 length footbeds. Besides, I don’t want to deal with any additional gluing products.
Speaking of glue, a high tack type contact adhesive is used to attach the posting foam to the molded blank. This adhesive works like rubber cement, in that you apply a layer to each material, let the carrier solvents ‘flash off’ and dry, and then squeeze the two layers together. If the glue is not too old, and sufficiently tacky, you can proceed immediately with grinding, with no additional cure time. This is not to say that curing is not helpful, as with time, it is nearly impossible to pull the posting foam from the blank. The glue I like to use comes from Austria, and is formulated to resist degradation from, among other things, sweat. Due to its flammable nature, it costs more to ship than to purchase. In a pinch I have used a high-grade automotive trim adhesive to good effect.
Needless to say, I have built an externally vented glue box to avoid breathing toxic glue fumes. In the event of glue thickening, I use toluene as a solvent. This is not a friendly material and should be treated appropriately.