The principle is to melt the two surfaces is heated to a specified temperature, and then fused together so that by applying pressure. The pressure causes the flow of material covering the metal so that it melted mixed together. When polyethylene pipe is heated, its molecular structure is converted from a crystalline state non-crystalline state. When pressure is applied, the molecules of each end of the pipe to fuse together. When connecting the end of the cooling, the molecules back to the crystalline state, the initial interface was gone, two pipe has become a pipe with nature. Pipe interfaces will eventually get as strong or stronger than the two respective pipe itself.