JPH10138336A - Method and apparatus for molding tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a method and apparatus for molding a tube for simply forming the tube having thickness without unevenness. SOLUTION: The apparatus for molding a tube comprises a tube molding means, a heating cylinder 1 and an orienting means. The cylinder 1 heats the tube molded by the molding means to a temperature capable of radially orienting the tube. The cylinder 1 is aligned concentrically with a tube T and disposed at an outside of the tube T, and constituted by a plurality of far infrared heaters 14 for regulating the temperatures. Before orienting by the orienting means, the heaters 14 of the cylinder 1 are regulated at their temperatures to provide temperature difference on a circumferences of the tube T. Thus, orienting amounts are altered according to the temperature difference to make thicknesses uniform.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a tube forming method and a tube forming apparatus for forming a tube by extruding a molten resin from a die.

[0002]

2. Description of the Related Art As a method of forming a tube, a raw material resin is melted and kneaded by an extruder, and the molten resin is extruded through a die (spiral die or the like) to form a resin bubble. There is a method of forming a tube having a required diameter by blowing air. Here, when another member is welded to the formed tube (for example, when a mouth member is welded to a tooth brushing tube, etc.), a large heat shrinkage characteristic (shrinkage amount) in the radial direction of the tube is required.

[0003] For this reason, when forming a tube, first, it is formed into a tube having a diameter smaller than the finally required diameter of the tube as a molded product in a presizing water tank. Then, in the heating cylinder disposed between the presizing water tank and the sizing water tank, the tube once cooled in the presizing water tank is heated again until it reaches a temperature at which it can be stretched. The reheated tube is cooled after being stretched to a required diameter in a sizing water tank, and is stabilized in a final shape as a molded article.

[0004] When a tube is formed by such a molding method, the stretching in the radial direction is mainly performed in a sizing water tank. However, when the sizing water tank is stretched, the tube may have an uneven thickness (uneven thickness). When the thickness of the tube is uneven, the die is adjusted to make the thickness uniform.
That is, the thickness unevenness is corrected by changing the amount of eccentricity between the outer member and the inner member in the die.

[0005]

However, depending on the material of the molten resin and the change in the flow of the resin in the die, there is a problem that the unevenness of the thickness cannot be corrected by the adjustment in the die as described above. Was. Further, in the die, since the outer member and the inner member are firmly fixed by the mounting bolts, there has been a problem that the adjusting operation is difficult.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a tube forming method and a tube forming apparatus capable of easily forming a tube having an even thickness.

[0007]

In order to achieve such an object, a tube forming method according to the present invention forms a tube by proceeding from a tube forming step to a drawing step through a heating step. ing. In the tube forming step, the tube is formed by extruding the molten resin from the die, and in the heating step, the tube formed in the tube forming step is heated to a state where it can be stretched in the radial direction.

Next, in the stretching step, the tube heated in the heating step is stretched (expanded) in the radial direction to form a tube having a required outer diameter and thickness. When the tube is heated to a state where it can be stretched in the radial direction in the heating step, a temperature difference can be provided on the circumference of the tube.

[0009] According to the tube forming method including such a process, the portion where the temperature is increased in the heating process has a larger amount of stretching because the resin is softer than the other portions. As the amount of stretching increases, the thickness of that portion decreases.
Therefore, when the thickness of the tube extruded from the die is not uniform (the thickness is uneven), if the temperature of the tube in the thickened portion is set higher than that of the other portions, the temperature in the stretching step is increased. At the time of stretching, the ratio of stretching the portion can be increased, and the thickness of the tube can be made uniform. That is, by providing a temperature difference on the circumference of the tube, the thickness of the tube on the circumference can be adjusted by changing the amount of stretching in the radial direction of the tube during stretching.

Further, a tube forming apparatus according to the present invention for achieving the above object is provided with a tube forming means, a heating means, and a stretching means. The tube forming means forms the tube by extruding the molten resin from the die, and the heating means heats the tube formed by the tube forming means to a temperature at which the tube can be stretched in the radial direction. The heating means is configured to have a plurality of heaters that are positioned outside the tube and concentrically with the tube and that can adjust the respective temperatures.

The stretching means is configured to radially stretch the tube heated by the heating means. In the tube forming apparatus of the present invention, the heater provided in the heating means before the stretching is performed. By performing the temperature adjustment described above, it is possible to provide a temperature difference on the circumference of the tube at the time of stretching in the molding means.

According to the tube forming apparatus configured as described above, when the thickness of the tube extruded from the die is uneven, the temperature of the heater located in the vicinity of the thickened portion is adjusted by another heater. If the height is higher than that, it is possible to increase the ratio of stretching the portion where the temperature of the tube has become high during stretching by the stretching means, and it is possible to make the thickness of the tube uniform.

The above tube forming apparatus is suitable for forming a tube having shrinkage in the radial direction. When such a tube is formed, the tube extruded from the die is once formed into a required outer diameter and thickness that is a smaller outer diameter than the final molded product, and is cooled and cured. Preferably, a pre-forming means is provided.

[0014] According to the tube forming apparatus configured as described above, after the thin-diameter tube formed by the preforming means is reheated to a temperature at which it can be drawn in the radial direction by the heating means, it is finally heated by the drawing means. Stretched to the required outer diameter for a molded article. Accordingly, the thickness of the tube can be reduced as a whole, and the heat shrinkage characteristic for welding another component to the formed tube can be increased. Further, when the thickness of the final molded product is uneven, the temperature of the heater in the heating means is adjusted, so that the thickness of the tube can be made uniform during stretching by the stretching means.

The heating means preferably comprises a plurality of heating members arranged in series in the axial direction of the tube, and it is preferable that a heater is provided for each of these heating members. It is preferable to dispose the heater with a phase shift in the circumferential direction for each member. According to the tube forming apparatus configured as described above, the temperature can be changed at a fine pitch on the circumference of the tube without bringing the heaters close to each other, so that the thickness of the formed tube is made more uniform. Can be done.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. First, a tube forming apparatus according to the present invention will be described with reference to FIGS. The forming apparatus S includes an extruder 2, a die 3, an external cooling device 4, a presizing water tank (pre-forming means) 5, a plurality of pinch rollers 6, a sizing water tank (stretching means) 7, a tube regulator 8, a surface treatment device 9, and a winding device 10, and a heating cylinder (heating means) 1 is provided between the presizing water tank 5 and the sizing water tank 7.

When forming a tube, first, a chip made of a resin material such as polyvinyl chloride or polyethylene is put into the hopper 21. The input resin material is supplied to the extruder 2
At a time, and is supplied to the die 3 as a molten resin. The molten resin supplied to the die 3 is extruded from the die 3 as a tube. At this time, compressed air is fed into the tube. The tip of the tube sent out from the die 3 is closed. For this reason, molten resin
It is extruded from the die 3 as a resin bubble. 2 and 3, only the center line CL of the tube is shown.

The extruded tube is first placed in the external cooling device 4.
Cooled by. The external cooling device 4 includes a blower unit 41 that generates a cooling wind, and a blowing unit 42 that blows the generated wind to the outer periphery of the tube. The tube cooled by the external cooling device 4 is passed to the presizing water tank 5, the first pinch roll 6a, the heating cylinder 1, and the second pinch roll 6b. When the tube is passed to the second pinch roll 6b, the second pinch roll 6b is closed, and the tube is passed to the sizing tank 7, the third pinch roll 6c, and the fourth pinch roll 6d. After passing through the fourth pinch roll 6d, the third pinch roll 6c is closed, and after the fourth pinch roll 6d, the tube is suspended for the time being.

From the die 3 to the second pinch roll 6b,
When the tube is formed with a diameter having a certain degree of stability, the first pinch roll 6a is closed. At this time, the first
When the cooling of the tube up to the pinch roll 6a is insufficient, the first pinch roll 6a is closed to close the first pinch roll 6a.
And the tube between the second pinch rolls 6a and 6b sticks up and down, but if cooled to some extent,
Air can be kept in the tube between the first pinch roll 6a and the second pinch roll 6b. Thus, the space between the die 3 and the first pinch roll 6a is completely stabilized by enclosing the air in the tube.

Thereafter, the temperature of the heating cylinder 1 is increased, and
The inside tube is made to be in a state where it can be stretched again. When the temperature of the tube rises, the second pinch roll 6b is opened and the air between the first pinch roll 6a and the second pinch roll 6b is removed from the first pinch roll 6a to the third pinch roll 6b.
c. Once the air is over,
The fourth pinch roll 6d is closed, the third pinch roll 6c is opened, and air for tube stretching is filled.

After the tube T whose shape has been stabilized by passing it through the external cooling device 4 or the presizing water tank 5 at the start of molding, the tube temperature is raised to a temperature at which the tube T can be stretched in the heating cylinder 1. This is performed using a compressed air discharging device (not shown) from the fourth pinch roll 6d side. When air is sealed, first, the tube hanging down after the fourth pinch roll 6d is cut at a location where air can be easily blown.

The fourth pinch roll 6d is provided between the third pinch roll 6c and the surface treatment device 9. 4th
Unlike the other pinch rolls, the pinch roll 6d has a groove formed in the center of the roll. Therefore, when the tube T passes through the fourth pinch roll, the tip of the tube T is reduced by the groove. Will open. Therefore, the tube T
When air is sealed in the inside, an operator enters pressure-adjusted air from a space created by using the groove by using a compressed air discharge device.

At this time, the second pinch roll 6b is closed instantaneously so that the air that has entered does not excessively enter between the first pinch roll 6a and the second pinch roll 6b. When the necessary and sufficient air is supplied between the second pinch roll 6b and the fourth pinch roll 6d, the third pinch roll 6
Close c. At this time, if the second pinch roll 6b is kept closed, the tube T heated by the heating cylinder 1 is closed.
Are stuck together in a flat state, and the mouth of the tube T after the fourth pinch roll 6d cannot be opened,
The second pinch roll 6b is kept open.

If the pressure of the air to be introduced is too low, no force for stretching the tube T is generated, and if the pressure is too high, the tube T bursts.
Is appropriately adjusted in consideration of the properties of the resin forming the ink, the temperature of the heating cylinder 1, and the like. The fourth pinch roll 6d is used to supplement air when filling the tube T with air. Therefore, the fourth pinch roll 6d is opened when not filling air.

The tube T filled with air in this way is formed and cooled by the presizing water tank 5 to stabilize the shape, and then taken up by the first pinch roll 6a. Is fed in the feeding direction. The tube picked up by the first pinch roll 6a is sent into the heating tube 1 and picked up by the second pinch roll 6b provided on the left side of the heating tube 1.

The presizing water tank 5 is formed such that a tube is formed to a required outer diameter (presizing), and then cooled to form a final shape in the sizing water tank 7 as described later. The outer diameter and thickness having shrinkage characteristics are stabilized.

The presizing tank 5 has a presizing portion formed in a substantially cylindrical shape. The inner diameter of the presizing portion is formed to be smaller than the outer diameter of the exit of the die 3, and the tube pushed out of the die 3 is fed into the presizing portion. Compressed air is blown into the tube located in the presizing part, as described in detail later. Here, since the outer periphery of the tube is in contact with the inner periphery of the sizing portion, the tube is stretched more in the flow direction (axial direction of the tube) than in the radial direction. The shape of the stretched tube is stabilized by cooling in the water tank portion of the presizing water tank 5.

The heating cylinder 1 heats the tube cooled by the presizing water tank 5 to a temperature at which stretching is possible again. Hereinafter, the heating cylinder 1 will be described with reference to FIGS. The heating cylinder 1 includes a first heating unit 1a located on the die 3 side and a second heating unit 1b located on the sizing water tank 7 side. Here, the first heating unit 1a and the second heating unit 1b are a heater 1 described in detail later.
Since the basic configuration is the same except that the arrangement position of the temperature sensor 4 and the temperature sensor O are different, the configuration will be described below taking the first heating unit 1a as an example.

The first heating unit 1a includes a case 11 formed in a rectangular box shape, and a case 2 disposed inside the case 11.
And two heater holder plates 12. The heater holder plate 12 has a through hole 12a formed at the center thereof, and cutouts 12b extending radially from the through hole 12a are formed at six locations at equal intervals. A heater holder 15 is provided for each notch 12b on an extension in the direction in which the notch 12b extends.

The heater holder plate 12 thus configured is provided with a holding fitting 11 protruding from the inside of the case 11.
By being attached to a, it is securely attached to the case 11. The far-infrared heater 14 is attached to the heater holder plate 12 attached to the case 11. The far-infrared heater 14 is formed in a columnar shape, and is attached to the heater holder plate 12 by holding both ends of the far-infrared heater with the heater holder 15.

The far-infrared heater 14 is attached by inserting the far-infrared heater 14 from the through hole 12a into the notch 12b as shown by an arrow C as shown in FIG. Done. Therefore, the two heater holder plates 12 and 12 have an interval substantially equal to the interval between both ends of the far-infrared heater 14 and the case 11
And each far-infrared heater 14 is provided with each notch 1.
2b.

After the mounting of the far-infrared heater 14 on each heater holder 15 in this manner, the through-hole 12a
The semi-discharge plate 13 is attached inside. The semi-cut plate 13 is
It is formed in a cylindrical shape by a punching plate, and has a configuration capable of being vertically divided into two parts. The semi-cutting plate 13 thus configured is screwed to the mounting flange 12c formed around the through hole 12a. Semi-cut plate 13
Prevents the heat from the far-infrared heater 14 from reaching the tube T directly and the tube T
Or the water from the tube T is directly
This is to avoid hitting.

A first pinch roll 6a is provided on the left and right sides of the case 11 in the first heating section 1a and the second heating section 1b.
A tube passage hole 11c is formed, through which the tube T drawn out of the tube can pass, and the center of the through hole 12a (the axis of the semi-discharge plate 13) is arranged so as to coincide with the center of the tube passage hole 11c. Have been.

A tube guide 16 is provided on the outer end surface of the case 11 thus formed. The tube guide 16 has an inner diameter slightly smaller than the diameter of the tube passage hole 11c, and a guide member 16a formed in a ring shape by felt, and a guide for fixing the guide member 16a concentrically with the tube passage hole 11c. Holder 1
6b.

In the second heating section 1b shown in FIG. 1B, the heater holder plates 12, 12 are attached to the case 11 similarly to the first heating section 1a. Provided heater holder 15
The far infrared heater 14 is to be attached to the
In the second heating section 1b, the heater holder plate 1
2, 12 notches 12b are formed at positions shifted by 30 °. Thus, in the first heating unit 1a and the second heating unit 1b, the far-infrared heaters 14 are arranged in a circular shape with a phase shift of 30 °.

The far-infrared heater 14 is configured so that the heat generation temperature can be adjusted by adjusting the voltage.
Further, the first heating unit 1a and the second heating unit 1b are provided with temperature sensors O for measuring the entire ambient temperature in each heating unit in order to adjust the voltage of all the heaters. The temperature of each of the far-infrared heaters 14 is adjusted by adjusting a volume V provided in the heater temperature adjustment box H so as to correspond to each of the far-infrared heaters 14.

The first heating section 1a and the second heating section 1b thus formed are mounted in a row on a base 19 by a mounting member 18. The mounting member 18 includes an outer member 18a disposed on both sides of each of the heating units 1a and 1b,
And an intermediate member 18b disposed between the heating portions 1a and 1b.

An intermediate guide 17 is mounted on the intermediate member 18b. The intermediate guide 17 includes an intermediate guide member 17a formed of felt, and an intermediate guide holding member 17b that holds the intermediate guide member 17a.
The tube T passing through the inside of the heating cylinder 1 is prevented from sagging between the two heating units 1a and 1b.

The tube T cooled by the presizing water tank 5 is fed into the heating tube 1 thus formed, and the tube T is located at a plurality of circumferential positions (after the fourth pinch roll 6d). The thickness is measured at the same position and the same number as the arrangement position of the heater 14). Then, for example, when the thickness of the portion indicated by D in FIG. 1A is thicker than the thickness of the other portions,
The temperature of the far-infrared heater 14d located closest to the part D is set higher than the temperatures of the other far-infrared heaters 14.

The tube T thus heated so that the thick portion has a particularly high temperature is fed to the sizing tank 7 by the second pinch roll 6b. In the sizing tank 7, since the amount of stretching in a portion at a high temperature is larger than that in other portions, the thickness of the tube T can be made uniform. And the tube that was fed through the sizing tank 7
Since it is taken up by the third pinch roll 6c, it is stretched and cooled in the sizing water tank 7, and a uniform thickness tube T having a required outer diameter can be obtained.

The sizing tank 7, as shown in FIG.
The sizing section 7a is configured to include a sizing section 7a and a water tank section 7b.
In the right side). The sizing part 7a is formed in a substantially cylindrical shape, and the inner diameter φ is formed to the outer diameter (required outer diameter) of the tube T finally formed.

The siding portion 7a thus formed
, The tube T extruded from the die 3 is fed by the second pinch roll 6b, and is sent as it is into the water tank 7b. Here, since air is sealed in the tube T located in the sizing portion 7a, the tube T is stretched until the outer periphery comes into contact with the inner periphery of the sizing portion 7a. Cooling water W is sprayed from the shower nozzle 7c disposed in the water tank portion 7b on the outer periphery of the tube T and the outer periphery of the sizing portion 7a extended in this manner. By spraying the cooling water W, the tube T is stabilized at a shape and a size with the inner diameter φ of the sizing portion 7a as the outer diameter.

Here, the air sealed between the second pinch roll 6b and the third pinch roll 6c to perform the stretching may escape from the third pinch roll 6c side. The tube T is flattened by the third pinch roll 6c. However, when the air in the tube escapes, the diameter of the tube T decreases, and the width of the flattened tube T decreases. For this reason, in the forming apparatus S, the tube regulator 8
, The width of the tube is measured, and if the measured width of the tube is smaller than the predetermined width, it is determined that the diameter of the tube T is small, and the third pinch roll 6c is moved to the second pinch roll 6b. This keeps the width (diameter) of the tube T constant. The width is measured using a laser sensor or the like that can measure the increase or decrease of the width of the tube T as the increase or decrease of the light blocking amount.

That is, even if the air in the tube T escapes from the third pinch roll 6c side, the tube T is flattened and closely adhered to both pinch rolls 6b and 6c. Therefore, if the third pinch roll 6c is moved toward the second pinch roll 6b, the air sealed between the two pinch rolls 6b and 6c hardly leaks, and the tube T is expanded (stretched). . Therefore, by adjusting the amount of movement of the third pinch roll 6c, the outer diameter of the tube T finally formed in the sizing tank 7 can be kept constant.

The tube picked up by the third pinch roll 6c is surface-treated by the surface treatment device 9 and then wound up by the winder 10. The tube T to be wound by the winding machine 10 is wound in a flat state. However, when the tube T is thick, even if the tube T is folded in a flat state, both ends cannot be firmly folded flat and the central part cannot be folded. Both ends are higher in shape.
Therefore, if the tube T is wound up as it is, the tube T will be wound up in a state where the central portion is largely dented.

Therefore, when the tube T is wound on the winding drum 101 of the winding device 10, the tube T is moved on the oscillating roll immediately after the fifth pinch roll 9a provided in the surface treatment device 9. It is to be wound around the winding device 10. One end of the swing roll is swingably held in a conical shape, and the other end rotates so as to draw a circle. As a result, the swing roll swings so as to draw a conical trajectory, so that the tube T moving on the swing roll is wound while moving in the left-right direction. Therefore, since the thickened portions at both ends and the thin portion at the center are wound so as to be alternately overlapped, the wound tube T can be formed into a cylindrical shape having no dent at the center. .

In the above-described molding apparatus S, the temperature of each far-infrared heater 14 in the heating cylinder 1 is set based on the measurement result of the thickness of the wound tube T by the operator. However, the present invention is not limited to this. For example, the fourth pinch roll 6
The thickness is automatically measured by measuring the thickness using a non-contact thickness sensor from d to the winding device 10 and feeding back the measurement result to the heater temperature adjustment box. It may be.

Further, the heating cylinder 1 does not necessarily need to be constituted by two heating sections 1a and 1b, but may be constituted to have one or three or more heating sections.
The location and number of the far-infrared heaters 14 provided in the heating cylinder 1 can be appropriately changed depending on the size and material of the tube to be formed. Further, the heating means for heating the tube is not limited to the far-infrared heater, but may be a heater having another configuration.

Although the above-mentioned forming apparatus S is configured to form a single-layer tube, the tube forming apparatus according to the present invention is not limited to forming a single-layer tube. Of course, it can also be used as an apparatus for forming a tube having more than one layer. In addition, when forming a tube of two or more layers, a plurality of extruders are prepared, the resin is supplied to the die 3 from each extruder, and the respective resins are merged in the die. Extrusion from the die 3 is performed.

[0050]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. This example uses polyvinyl chloride as a material,
The diameter is 50 mm and the thickness is 100 μm (hereinafter “50 mm ×
100 μm ”), 50 mm × 50 μm, 1
This is a molding apparatus S capable of molding a tube T of 50 mm × 100 μm, 150 mm × 50 μm, or the like.

When such a tube T is formed, the far-infrared heater 14 is set to 200 V AC, 500 V
A heater with a heater length of about 400 mm for W is used. Then, in each of the heating units 1a and 1b, the tube T to be heated is heated.
6mm at equal intervals with a pitch circle diameter of 210mm
They are arranged one by one. Thereby, the ambient temperature in the heating cylinder 1 can be raised to about 200 ° C., and the surface temperature of the tube T to be heated can be raised to 80 ° C.

According to the molding apparatus S having the heating tube 1 configured as described above, for example, when a tube having a thickness of 100 μm is formed, the thickness of the tube T is reduced only by the thickness deviation adjusting bolt in the die 3. If the adjustment is made, the maximum value is 1
09 μm, the minimum value was 77 μm,
By adjusting the temperature of the far infrared heater 14, the maximum value can be set to 99 μm and the minimum value can be set to 86 μm. That is, when only the thickness adjustment bolt was adjusted, the thickness difference was 32 μm (the thickness unevenness was 16%), but the adjustment in the heating cylinder 1 reduced the thickness difference to 13 μm ( (Thickness unevenness 6.5%).

[0053]

As described above, the tube forming method and the tube forming apparatus according to the present invention allow a desired portion on the circumference of the tube to be formed more than other portions before the tube is drawn to the final outer diameter. Since it can be heated, it is configured such that the amount of stretching of a desired portion during stretching can be increased, and the rate at which the thickness of the tube can be reduced can be increased. For this reason, if the present molding method and molding apparatus are used, unevenness in thickness that could not be corrected by adjustment using a die can be eliminated, and a tube having a uniform thickness can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of the heating cylinder provided in the tube forming apparatus according to the present invention, taken along a line AA and a line BB in FIG.

FIG. 2 is a front view showing the entire tube forming apparatus.

FIG. 3 is a view of the tube forming apparatus as viewed from an arrow III in FIG. 2;

FIG. 4 is an enlarged view of a heating tube in the tube forming apparatus.

FIG. 5 is an enlarged view of a sizing part in the tube forming apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heating cylinder 2 Extruder 3 Die 4 External cooling device 5 Presizing water tank 6 Pinch roller 7 Sizing water tank 8 Tube regulator 9 Surface treatment device 10 Winding device

Claims (4)

[Claims] 1. A tube forming step of forming a tube by extruding a molten resin from a die; a heating step of heating the tube formed by the tube forming step to a state in which the tube can be stretched in a radial direction; A stretching step of stretching the tube heated by the above in the radial direction to form a required outer diameter and thickness, when the tube is heated to a state in which the tube can be stretched in the radial direction. Providing a temperature difference on the circumference of the tube to change the amount of stretching at the time of radial stretching of the tube, thereby enabling adjustment of the thickness on the circumference of the tube. . 2. A tube forming means for forming a tube by extruding a molten resin from a die; a heating means for heating the tube formed by the tube forming means to a temperature at which it can be stretched in a radial direction; A stretching means for radially stretching the tube heated by the means, wherein the heating means is positioned concentrically with the tube outside the tube and adjusts the temperature of each tube. A tube forming apparatus, comprising: a plurality of heaters capable of adjusting the temperature of the tube by adjusting the temperature of the heater so that a temperature difference can be provided on the circumference of the tube during stretching by the stretching means. 3. The tube according to claim 1, wherein the tube has a radially contractible property, and has a preforming means for forming the tube extruded from the die to a required outer diameter and thickness, and cooling and curing the tube. The tube forming apparatus according to claim 2, wherein the heating means reheats the tube formed by the preforming means to a temperature at which the tube can be stretched in a radial direction. 4. The heating means comprises a plurality of heating members arranged in series in the axial direction of the tube, and the heaters are arranged with a phase shift in the circumferential direction of the heater for each heating member. The tube forming apparatus according to claim 2 or 3, wherein the tube forming apparatus is used.