Posted by Klaus Kremmin on January 04, 2007 at 19:07:50:
In Reply to: Plastic gears VS Metal Gears posted by Martin Nirschl on August 15, 2006 at 11:26:39:
K2 Plastics successfully insert molds plastic gears onto metal sleeves, shafts, bearings or hubs. The potential problem with doing so is the stress fracturing of the plastic under hoop stress over the metal. The plastic will fail at its weakest link - the weldline. This is the point where the melt flow front meets during the part filling phase of injection molding. Weldline strength can be 10-50% of the strength without a weldline depending on material choice and processing parameters. The only way to successfully insert mold plastics over metal is to mold the part with no weldlines.
You definitely want to mold the plastic over the metal and not the other way around. The plastic at the gear profile provides natural lubrication and noise reduction.
Klaus Kremmin
www.K2Plasticsinc.com