Bathtub liners also known as tub liners, acrylic tub liners and tub inserts are made by vacuum forming an acrylic or PVC (poly vinyl chloride) sheet which is heated then formed in a mold by applying vacuum. Having the proper mold is important for the tub liner to fit properly. Typically a bathtub liner will fall between the cost of replacing a bathtub at the high end and bathtub refinishing at the lower end in cost when renovating a damaged or outdated bathtub.