High-Density Impact-Resistant Polymer Also called a pipe stiffener or pipe insert it is a cylindrical support tube pushed into the end of a flexible MDPE (Medium Density Polyethylene) pipe before it is connected into a compression or push-fit fitting. Reinforces the flexible plastic wall of MDPE Pipe from the inside so it retains its perfectly round shape when the fitting's compression ring or O-ring clamps down tightly. Provides a rigid surface for the fitting's internal grab ring/clip to bite into, preventing high mains water pressure from popping the pipe out of the fitting. Inserting a liner is a mandatory requirement by water regulations when joining MDPE pipes with compression or push-fit fittings for underground drinking water mains. Leaving it out is one of the most common causes of underground plumbing leaks over time.