Custom Rubber Silicone Molds uses a rubber end caps for round tubing is a stopper that is meant to be put into the end of a pipe in order to halt the stream of water, wastewater, rainwater, or sewage so that maintenance teams may operate on the pipeline. rubber bumpers lowes often referred to as grommet bumpers, are tiny bits of rubber material created to shield other items from harm. They often have a circular form with a little hole in the centre, however, this is optional.