Hot Plate Welding Coatings

There has been a lot of discussion over the years about which materials or coatings to use for hot plate surfaces to minimize sticking and stringing of the plastic material. If one can minimize sticking, one can minimize plate cleaning activities. Every now and again someone will claim a miraculous coating that eliminates sticking and stringing, but quite often it proves to be less than completely miraculous and require regular recoating. Hot plate surfaces are usually made of copper or possibly aluminum. Using bare aluminum hot plates is generally not recommended, and some kind of coating is needed. Bare copper is a somewhat better situation for most materials, but if the material is filled or reinforced, bear in mind that fillers and reinforcements are quite abrasive and neither copper nor aluminum stand up well long term without a tough coating. Either copper or aluminum can be plated, but quite often at hot plate temperatures the plating will begin to crack and peel, often from differential thermal expansion rates. Be aware that some high-temperature thermoplastics will require high plate temperatures that may actually result in heat treating (annealing) of copper, aluminum, and possibly even steel structures in the plate assembly– a good reason to avoid hot plate welding high temperature materials. Some have used fluoropolymer (i.e. Teflon TM) coatings to some success, but care must be exercised to avoid overheating the coatings, and they are generally only effective with polyolefin materials because almost every other thermoplastic requires plate temperatures that will quickly destroy fluoropolymer coatings. An approach that often yields success is to oil the hot plates repeatedly over several hours of heating to build up what could be termed a seasoned surface, much like what develops on an iron skillet after cooking with it several times (one usually tastes quite a lot of iron in the first few meals prepared with a brand-new skillet). Some of the lower temperature thermoplastics can be processed with a fluoropolymer-coated paper covering the hot plates, and often machines are built that have either manual or automatic paper-changing systems to ensure reasonably fresh surfaces are used at all times. This eliminates the problem of coating breakdown but adds a consumable to the operation. Finally, sooner or later the plates will have to be scrubbed, and this activity repeated many times will result in surface damage to the plates. For most operations, the plates are mounted on a plate carrier, the carrier being part of the machine itself, and the plates considered renewable to a point but ultimately consumable. Many times when coatings are used, multiple sets of plates will be made so that one set is in production, one is being recoated, and one is waiting to go into the machine. Ultimately, choice of plate material and coating is on a case-by-case basis and depth of experience along with some experimentation will determine the right direction to go.