Hot Plate Welding of Thermoplastics Blog

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23 July 2008 -- 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.

21 July 2008 -- Hot plate welding works well with non-planar joints. That said, part fit-up is always important, and especially so when parts are relatively large as is often the case when the process is selected. A quick rule of thumb is to avoid having to hot plate weld joints that run more than 60 degress from the plane of the plate carrier, and try to hold it to 45 degrees maximum if possible.

3 July 2008 -- Success in hot plate welding depends on generation of flash. This seems a little counter-intuitive, but it makes sense if you think about it. The material is pressed againt the hot plate. It begins melting at the interface and if no force were applied to keep it against the surface of the plate, a gap would soon appear. To prevent this, clamp force is applied and the material allowed to flow naturally until the part has collapsed about 0.3 to 0.4 mm, typically controlled by tooling stops. The displaced material forms a flash dam around the melting plastic, shielding it from air and allowing the heat to penetrate into the plastic. This penetration of heat is important, because enough heat must be stored that the material in the joint area remains molten during the change over or open time. Some studies suggest that joint strength increases with heating time but cannot be improved with heating times beyond 13 seconds. Sometimes this heating time is as little as four to five seconds. Plate temperature can be the subject of some experimentation, but good results are generally obtained when the plate temperature is set at the material's highest recommended heater zone setting for molding or extruding. This will be somewhat below the degradation temperature but is usually well above the melt temperature. The change over or open time is meant to be a short as practical. What happens during this time is the parts are pulled off the hot plates, the plate gotten out of the way, then the parts brought into contact in final alignment and under clamp force. Fixtures must be designed to make sure parts are securely held because often it will take some force to get them to release from the plates. During change over time, the hot material is in contact with relatively cold air and will oxidize and skin over a bit. When the parts are brought back into contact, clamp force presses the parts together and causes flow of this damaged material out of the joint in the form of flash. During the join or weld time, the parts are allowed to collapse another 0.3 to 0.4 mm per side, for a total joint collapse of the entire assembly of about 1.5 mm, again, typically controlled by tooling stops. If heat has been allowed to soak deeply into the parts, resolidifcation will occur somewhat slowly. This is important for joint strength, because the flow of material perpendicular to the joint plane (causing flash) will cause molecular orientation to be parallel to the joint plane and therefore weaken the joint. The longer it takes the joint to cool, the less molecular orientation there will be, as the molecules are free to reorient randomly for a longer time. In hot plate welding, speed of process is always traded for joint strength. Proper flash formation indicates that the material was in fact hot enough to be melted, and that most of the material damaged by contact with cold air during changeover time has been expelled from the joint. Flash can be hidden or removed, but it is a necessary by-product of a well-fused joint. The good news is that hot plate welding does not produce particulate like the frictional processes do. The bad news is that hot plate welding is not recommended for materials that have a relatively small spread between melt temperature and degradation temperature.

5 June 2008 -- 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.

21 May 2008 -- When contemplating use of hot plate welding, a critical consideration is the thermoplastic material to be joined. Generally, one would want to join materials with relatively low melting temperatures, low thermal conductivity, a good spread between the melting and the degradation temperatures, relatively high viscousity when melted, and resistance to interaction with oxygen when melted. If it sounds like this list eliminates a lot of materials, it probably does, but it also includes a lot of materials. Materials with reinforcements and fillers complicate matters and may be better suited to another process. The reason to avoid hot plate welding filled or reinforced materials is the potential for buildup of a residue of filler or reinforcement on the hot plates. This tends to become more of a problem the more parts are run and requires frequent scrubbing or scraping of the plates. This plate cleaning can be relatively easily automated if the plates are planar, but can be complicated and expensive if the plates have complex geometry. Materials that work well with hot plate welding include olefins like polyethylene and polypropylene, fluoropolymers, polystyrene, polymethyl-methacrylate, and acrilinitrile-butadiene-styrene. Materials to avoid hot plate welding include polyamid, polyvinylchloride, and almost all of the very high temperature thermoplastics. Depending on the application, polycarbonate can be hot plate welded but its relatively high melt temperature and especially its relatively high thermal conductivity can make it difficult to work with. High material lubricity is not an issue for hot plate welding and therefore recommends it over frictional methods. Hot plate welding also handles complex joint geometries pretty well, which also recommends it over frictional processes for these materials. Part warp issues must be minimized by fixturing and can be an issue with some assemblies, especially larger assemblies.

13 May 2008 -- Hot plate welding is the oldest of the plastics welding processes. Even today, some assemblies are put together by sliding a hot piece of iron between two parts until the plastic flows and then pressing the parts together by hand. While this may be acceptable practice for some assemblies, it is hardly practical for the vast majority. Still, all of the basic principles of hot plate welding are present in this simple process. First, plastic parts are fixtured, then pressed against a hot tool until the surfaces are melted, then the hot tool is removed and the parts are pressed together until they cool sufficiently to remain joined. The key variables in the process are the heat of the plate, the pressing time, pressure, and distance traveled when against the hot plate, the speed of the change-over, that is, the time the parts are separated but not against the hot plate, and the time, pressure, and distance traveled when the parts are pressed together to effect the weld. A key feature of traditional hot plate welding is actual contact between the plastic parts and the heated tool itself. Several attempts have been made to eliminate this contact over the years, some more successful then others, but traditional hot plate welding is still a common and steady process for manufacturing a wide variety of assemblies in a dizzying array of industries.

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Last updated 14 March 2018.