In this category are close-fit linings that use both heat and pressure to change the shape of the liner pipe to allow the installation to occur and then facilitate the expansion of the liner to fit tightly within the host pipe. Both modified PVC pipe formulations and polyethylene pipe are used in the different methods. These methods have been termed “fold-and-form” and “deform-reform” according to the materials and specific processes used. In this description, only the basic method principles will be described.

For the fold-and-form and deform-reform approaches, the pipe is first configured into a U-shape. It may be extruded in this configuration or folded into a U-shape after extrusion as a circular pipe. With this reduced geometry, the pipe can be pulled through the host pipe. Typically, the pipe is also heated during this insertion process to allow the pipe to be inserted through tight spaces such as existing manholes. Once the deformed liner pipe is in place within the host pipe, the liner pipe is expanded to a circular configuration in close contact with the host pipe using both heat and pressure. Once expanded, the pipe is allowed to cool while the internal pressure is maintained. When the pipe is cool and has regained its structural strength, the close-fit lining process is complete.

An alternate approach can be used to install fusible PVC pipe within an existing host pipe – especially for larger diameters and long sections of pipe. In this case, the liner pipe is manufactured at a reduced diameter and with a thickened wall. The PVC formulation both allows sections of pipe to be fused together to create a smooth sliplining pipe and allows the pipe, once installed, to be expanded using heat and pressure to fit tightly inside the host pipe.