Parts that have special shapes or that are not flat on the bottom would be considered contoured parts. The shape of these parts could be machined into aluminum or stainless steel but would require programming a CNC machine to cut the detail. Another added step would be that fixture would also need to have the machine tool marks polished out. Pouring a Polyurethane fixture would eliminate the need for programming time and polishing. Although the Polyurethane would need to cure (harden) overnight, this additional time spent is still less costly than programming, machining and polishing. When cured, the Polyurethane casting just needs minor machining before being mounted to a leveling plate.
Amorphous materials and most semi-crystalline materials are good candidates for Polyurethane fixtures. The exceptions are Polypropylene (PP) and Polyethylene (PE) parts; which are considered “softer” materials. Since PP and PE already absorb much of the ultrasonic vibrations, Polyurethane is normally not recommended. If the parts have a textured surface, the texture could be damaged if using aluminum or stainless steel. Using a Polyurethane fixture instead could significantly reduce the surface damage. The reason is that the Polyurethane is poured directly onto a production part (always preferred) so that the textured surface is somewhat incorporated into the casting.
The process being used is important in that normal ultrasonic welding of two plastic components is usually successful if thought out properly. However, when inserting brass or steel inserts, there are times when this process could be problematic due to excessive heat build-up directly under the inserting area that could cause the Polyurethane material to distort and become damaged. In some cases, Brass “plugs” can be added to the fixture directly under the inserting area to eliminate excessive heat and damage to the Polyurethane material.
Thin wall sections may need extra support that the Polyurethane may not be able to provide. In this case, a combination of Polyurethane and aluminum or stainless steel can be used to stabilize the thin wall areas of the assembly.
These are just some of the basic guidelines. Every application is examined on an individual basis to determine which fixture material will produce the best results.