a method of making a one - piece , rigid thermoplastics body for a discrete , releasable push - in tube coupling device of the collet type comprises initially injection moulding the body about one or more rigid cores . each rigid core is then withdrawn axially while the moulding is still in a plastic state whereby the outer , narrower bore portions of the body expand radially . the expanded portions are then radially contracted back to their as - moulded configuration using a jaw - like tool and finally the body is allowed to cool .