Preformed thermoplastic tube including at least one bend

A method for making a preformed thermoplastic tube having a tube wall having a wall thickness and including at least one bend formed by thermoforming, wherein on the inner curve of the at least one bend the tube wall is provided with a pattern of corrugations with selected dimensions in terms of number of and heights of corrugations such that a wall length reduction resulting from the corrugations along the inner curve is kept at a minimal value necessary to absorb overlength on the inner curve resulting from forming the bend, and in that on the outer curve of the at least one bend the tube wall is smooth and free of corrugations, in which method a straight thermoplastic tube is provided,

• characterized in that said method comprises:
• holding the thermoplastic tube at a temperature that is below a thermoforming temperature;
• bending and forcing the thermoplastic tube, at the temperature below the thermoforming temperature, against a concavely curved surface of a forming tool to shape the thermoplastic tube to obtain the at least one bend, wherein during the bending a pattern of corrugations is formed on an inner curve surface of the at least one bend to absorb an overlength resulting from a length reduction of the inner curve surface due to the bending, wherein the pattern of corrugations is formed by a corresponding pattern of depressions in the concavely curved surface of the forming tool against which the thermoplastic tube is forced when the thermoplastic tube is being bent around the forming tool;
• after bending and forcing the thermoplastic tube against the concavely curved surface, heating the thermoplastic tube to the thermoforming temperature while the thermoplastic tube is still bent around the forming tool;
• and
• forming the pattern of corrugations on the inner curve of the thermoplastic tube received in the pattern of depressions of the forming tool.