a mushroom - type hook strip for a hook - and - loop fastener has a flexible backing of thermoplastic resin and , is integral with backing , an array of upstanding stems distributed across at least one face of the backing , each having a mushroom head . the stems have a molecular orientation as evidenced by a birefringence value of at least 0 . 001 . the mushroom - type hook strip can be produced continuously by injecting resin into cavities of a cylindrical mold while evacuating and cooling the cavities so that the cooled resin becomes molecularly oriented , thus affording to the stems excellent stiffness , durability , and tensile and flexural strength .