Methods and installations for producing a biaxially oriented tube from thermoplastic material

An installation for producing a biaxially oriented tube from thermoplastic material comprising:

• an extruder configured to extrude a tube in preform condition from thermoplastic material, which extruder is provided with an extruder die head having an inner die member, the inner die member forming, in operation, a lumen in the tube in preform condition;
• a temperature conditioning means configured to subject the tube in preform condition to a temperature conditioning, so that a tempered tube in preform condition is obtained having an orientation temperature which is suitable for the thermoplastic material;
• a cooling device;
• and
• an expansion device and a drawing device which is arranged downstream of the expansion device, wherein the expansion device comprises:
• a non-deformable expansion part having a gradually increasing diameter to a maximum diameter at a downstream end thereof;
• a run-on part which is located upstream of the expansion part, the run-on part having an upstream sealing member arranged upstream of the expansion part;
• a first gas supply duct, the first gas supply duct having one or more first gas outlet ports in an outer surface of the expansion device downstream of the upstream sealing member;
• and
• a first gas discharge duct, the first discharge duct having at least one first inlet port in an exterior surface of the expansion part of the expansion device, the first inlet port being open or closed or partly closed dependent on whether or not the first inlet port is covered and closed, or partly closed, by the tempered tube;
• wherein, in operation, a first gas volume is present between the expansion device and the tempered tube, the first gas volume being limited at an upstream end thereof by sealing contact between the tempered tube in preform condition and the upstream sealing member of the run-on part and at a downstream end thereof by sealing engagement between the tempered tube and at least a downstream portion of the expansion part;
• wherein, in operation, a first gas flow is effected from a first gas supply source via the first gas supply duct and the one or more first gas outlet ports thereof into the first gas volume and, dependent on whether or not the first inlet port is covered and closed, or partly closed, by the tempered tube, out of the first gas volume via the first inlet port and the first discharge duct;
• wherein the pressure of the gas in the first gas volume is used to cause gradual expansion of the tempered tube already before the tempered tube contacts the expansion part of the expansion device;
• wherein the drawing device, in operation, is used for drawing the tempered tube in preform condition over the expansion device;
• wherein the tempered tube is transformed from the tempered tube in preform condition into a biaxially oriented tube with thermoplastic material which is oriented in an axial direction and in a circumferential direction of the biaxially oriented tube;
• wherein the cooling device is configured to cool the biaxially oriented tube;
• and
• wherein the installation is configured so that the first gas flow that is supplied from the first gas supply source via the first gas supply duct and the one or more first gas outlet ports thereof into the first gas volume is maintained at a gas pressure independent constant first gas flow rate.