Some materials cannot be bonded because they do not react or adhere to the glue. They can then generally be welded as is done for metals. This is extrusion welding process which involves adding molten material to the intersection of the two pieces to be welded. When the molten material has cooled, the two pieces are then welded. The main materials that can be welded are: HDPE, polypropylene and PVC. This process is mainly used for the custom-made manufacture of tanks or waterproof containers of irregular shapes.