Molded parts welded hot off the press will weld differently than if they are allowed to cool. Since these parts are not fully cured, their size and shape can change significantly in the first few minutes out of the molding machine.
Freshly molded parts may also retain quite a bit of heat, which can be beneficial in some welding processes. It’s important to realize that the set up for welding warm parts must be different than that for cold parts. Otherwise, finished part consistency will suffer. For instance, polycarbonate is one material which reacts well when warm, for two reasons. First polycarbonate is one of the most thermally conductive of the common thermoplastics. Welding warm polycarbonate eliminates one of the problems in welding this material cold. It is often difficult to retain heat in the joint long enough to accomplish the desired weld. Second, since polycarbonate requires a relatively high temperature to flow, less energy will be required to make it flow, if the part is already warm. Stress in the joint area may also be reduced by having the surrounding material somewhat warmer, allowing the weld to cool more slowly.
Of course, not everything behaves like polycarbonates. Materials such as olefins (polypropylene and polyethylene), which are not very stiff and have relatively low thermal conductivity, usually weld better when allowed to cool. On the other hand, hygroscopic materials should almost always be welded while they are still warm to reduce absorption of moisture.
Many factors must be considered in determining whether welding hot off the press is appropriate for a particular assembly. Therefore, it is important to process sample parts using this method to define as many parameters as possible before going into full production.