Hot Plate Welding Flash

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.