Hot Plate Welding Plate Cleaning

It seems that one could avoid some of plate-cleaning issues associated with hot plate welding by switching to non-contact hot plate welding. This technique has essentially the same cycle as hot plate welding using the same equipment, except that instead of pushing the parts to be joined onto the hot plate, they are simply held in close proximity– about one mm typically– to the hot plate. This is a lot harder than is sounds. First, part fit up is about ten times more critical in non-contact hot plate welding than the standard approach because the small gap between the plate and part needs to be essentially the same across the entire joint. The plate needs to be considerably hotter in order to drive the heat across the air gap, air being a reasonably good thermal insulator. Air also contains oxygen, which degrades most plastics when they melt in its presence, so the skinning effect is more pronounced than with contact hot plate welding. Degradation can be severe enough that heating time must be reduced, therefore heat is not driven as deeply into the parts, which in turn causes lower joint strength. Since the hot plate itself will often need to be 100 to 150 degrees C hotter in a non-contact process, and since the materials that are most difficult to weld in a contact process are typically those with higher melt temperatures anyway, the factor that most often makes this approach unfeasible is simply having to run the equipment at such extreme plate temperatures. While 200 to 250 dgress C as used in a contact process is one thing, 350 to 500 degrees C in an non-contact process is another thing entirely. It is very important to consider the bushings and cylinder seals and sensors and various tooling components that are used when temperatures run that high. So, while it often seems like a good idea, non-contact hot plate welding is actually a rare process.