Why do you need to cool under weight?
Cooling under weight isn't
always necessary. Trouble is, most framers have no way to know when it is and when it is not. Mostly it depends on the type of heat-activated adhesive, and the types of materials in the pressed stack. So, the only safe bet is to cool under weight every time.
Thermosetting adhesives, which bond when heated, may be OK if cooled in open air, but if any part of the finished mount cockles or warps, the bond was probably weakened while the adhesive was still hot and before it was fully bonded. These adhesives bond only once, and can not be re-melted if there's a problem.
Thermoplastic adhesives bond during cooling, and need to be cooled under weight. Every time. These are the adhesives that can be re-melted with re-application of heat.
When a dry mount fails because it was not cooled under weight, the bubbles do not show up right away, which leads some framers to incorrectly conclude that their dry mounts were successful. It takes some time to reinvest air to the materials after pressing under heat, so in the worst cases, where some of the mounted area never bonded at all, bubbles might show up in just a few days - but almost never immediately.
In a dry mount that has bonded over the entire area, but with weak spots, normal expansion & contraction cycles over time can disintegrate the bond in the weak spots. In this case, there might be no hint of bond failure until it happens, and that could be months or even years later.
Eventually, the cheaper high-temperature heat-activated adhesives deteriorate and fail, but that takes years and is not associated with faulty procedures. If you want your dry mounts to endure, avoid cheap and older-technology adhesives.
Dry mounting consists of three dissimilar materials stacked together, which
expand during heating and
contract during cooling, but
at different rates:
1. The
art paper is typically thin and, especially since it is on top of the stack of materials, would cool quickest in open air.
2.
Heat-activated adhesive is also thin, and it might cool quicker than the art paper if it were not sandwiched between the art and the board substrate, where it is insulated. So, it would hold heat longer than the art paper, but not as long as the board substrate.
3. The
board substrate is usually foam board, heavy paperboard, or some other thick, dense material that will hold heat longer than any other part of the stack. This layer cools slowest.
The stacked materials expand when heated in the press, but at different rates. Presumably, when the heat of the press permeates all of the layers, they will all have expanded in nearly-equal proportions, but it doesn't matter, since they are being pressed together during heating.
Cooling under weight serves to (A) press all of the materials tightly together during the cooling/contraction process, and to (B) slow the cooling/contraction process by some number of seconds.
If the cooling occurs in open air with no weight, the art paper cools fastest, contracting more than the adhesive and board layers of the stack. As the art paper cools & contracts, it can cockle so slightly that you can't see it happen, but it takes only a few ten-thousandths of an inch of deflection to weaken the bond. When this happens, the adhesive might go with the paper, or might stay with the board substrate, but the bond is weakened either way. The cooling generally occurs at the edges first, which is why most dry mounts remain fully bonded at the edges, even if the mid-section has completely failed.
Sometimes the dry mounted assembly will become slightly (but visibly) warped for a short time, then flatten out again, as the cooling/contraction process causes temporary deflection of the materials in the stack. That usually means the bond is weaker in some parts of the mounted area.
No matter what conditions might cause a dry mount bond to be weak in spots, cooling under weight is the solution. Come on, folks - it usually takes less than 30 seconds under weight to do it right.
