Method and installation for making plastic tubes with bi-axial drawing, and resulting plastic tube

A method for manufacturing plastic tubes, according to which a tube blank brought to a temperature near its molecular orientation temperature is subjected to biaxial stretching as a result of the radial expansion of the tube blank within a forming barrel, the inner diameter of the forming barrel being slightly larger than the diameter of the plastic tube, with the exception of thermal expansion, with formation of a blister at one end of the tube and controlled displacement of the blister as far as the other end of the tube, and with longitudinal stretching,

• wherein the radial expansion comprises a first phase and a second phase;
• wherein, during the first phase, the tube blank is inflated uniformly to an intermediate diameter, at which the circumferential elongation remains below or equal to the yield point of the material of the tube, the first phase taking place virtually without any longitudinal stretching;
• and
• wherein the second phase changes to the inside diameter of the forming barrel, with longitudinal stretching.

A method for manufacturing plastic tubes, according to which a tube blank brought to a temperature near its molecular orientation temperature is subjected to a biaxial stretching as a result of the radial expansion of the tube blank within a forming barrel, the inner diameter of the forming barrel being slightly larger than the diameter of the plastic tube, with the exception of thermal expansion, with formation of a blister at one end of the tube and controlled displacement of the blister as far as the other end of the tube, and with longitudinal stretching,

• wherein the radial expansion comprises at least two phases, during a first phase, the tube blank being inflated to an intermediate diameter determined by a double-wall casing introduced into the forming barrel, the casing having a hot fluid circulating between the two walls of the casing;
• and, during a second phase, the casing being progressively taken out of the forming barrel to allow radial expansion of the plastic tube to the inner diameter of the forming barrel and longitudinal stretching of the tube;
• wherein the forming barrel is cooled externally;
• and
• wherein the end of the plastic tube which is distant from the blister formation zone is pushed towards this zone when the blister is formed to bring about an increase in the thickness of the material making it possible to obtain a blister having a thickness which is substantially equal to that of the remaining part of the tube.

A method for manufacturing plastic tubes, according to which a tube blank brought to a temperature near its molecular orientation temperature is subjected to a biaxial stretching as a result of the radial expansion of the tube blank within a forming barrel, the inner diameter of the forming barrel being slightly larger than the diameter of the plastic tube, with the exception of thermal expansion, with formation of a blister at one end of the tube and controlled displacement of the blister as far as the other end of the tube, and with longitudinal stretching,

• wherein the radial expansion comprises at least two phases, during a first phase, the tube blank being inflated to an intermediate diameter determined by a double-wall casing introduced into the forming barrel, the casing having a hot fluid circulating between the two walls of the casing;
• and, during a second phase, the casing being progressively taken out of the forming barrel to allow radial expansion of the plastic tube to the inner diameter of the forming barrel and longitudinal stretching of the tube,
• wherein the forming barrel is cooled externally;.
• wherein a socket with a groove for receiving a gasket is formed on the plastic tube during the formation of the blister at one end of the tube;
• and
• wherein, when the wall of the blister comes into contact with the wall of a socket mold, the adjacent end of the tube is pushed towards the socket to obtain a wall thickness in the region of the groove, the wall thickness being substantially equal to the thickness of the rest of the socket and of the tube.

A method for manufacturing plastic tubes, comprising:

• biaxially stretching a tube blank at a temperature near its molecular orientation temperature as a result of a radial expansion of the tube blank within a forming barrel, the inner diameter of the forming barrel being slightly larger than the diameter of the plastic tube, with the exception of a thermal expansion,
• forming a blister at one end of the tube and controlled displacement of the blister as far as the other end of the tube, and with longitudinal stretching,
• wherein the radial expansion comprises first and second phases;
• wherein, during the first phase, the tube blank is inflated uniformly to an intermediate diameter, at which the circumferential elongation remains below or equal to the yield point of the material of the tube, the first phase taking place virtually without any longitudinal stretching;
• and
• wherein, during the second phase, the tube blank is radially expanded to the inner diameter of the forming barrel and is longitudinally stretched.

An installation for manufacturing a plastic tube, comprising:

• a forming barrel, in which a tube blank is engaged;
• a double-wall casing slidably mounted in the forming barrel and having an inner diameter for determining an intermediate expansion diameter of the plastic tube, the casing surrounding a first end of the tube blank;
• a cylinder member extending beyond one end of the double-wall casing;
• means for driving the double-wall casing and the cylinder member in a sliding manner;
• means for clamping first and second ends of the tube blank respectively, wherein:
• the clamping means at the first end of the tube blank forms a piston slidably mounted in the double-wall casing and integrated with the cylinder member;
• and
• the clamping means at the second end of the tube blank forms means for shutting off the forming barrel and defines a mold portion for forming a socket on the plastic tube, the mold portion comprising at least first and second parts which are slidably mounted relative to each other and define a groove portion for a gasket, the first part being located axially towards an outside of the plastic tube and approaching the second part to bring a material into the region of the groove portion for the gasket;
• means for detecting the coming of a wall of the socket against the second part of the mold portion to trigger the displacement of the first part of the mold portion when contact is made;
• means for introducing a fluid into the tube blank and for varying the pressure of the fluid;
• and
• means for circulating a hot fluid in the double-wall casing.