JP2002234066A - Method for manufacturing resin tube having three-layer structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a resin tube having an inner layer and an outer layer formed of a non-foamed foam thermoplastic resin composition and an intermediate layer formed of a foamed thermoplastic resin composition so that a thickness of the tube at its product finishing time always becomes uniform. SOLUTION: A method for manufacturing the resin tube having a three-layer structure comprises the step of forming the resin tube by setting an adding amount of a foaming agent to be added to a thermoplastic resin for forming the intermediate layer to that of a theoretically adding amount for obtaining an expansion ratio of a predetermined product dimension when the tube is formed without specifying a radial spread of the tube and sizing in a predetermined finishing dimension against a foaming pressure by using a tube outer surface forming tube and a tube inner surface forming core.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an unsolidified tubular three-layer structure in which a foamed thermoplastic resin composition serving as an intermediate layer is interposed between a non-foamed thermoplastic resin composition serving as an inner layer and an outer layer. The present invention relates to a method for producing a three-layer resin tube for coextruding a body from a mold, foaming a foamed thermoplastic resin composition in a co-extruded unsolidified tubular body, and cooling and solidifying the thermoplastic resin.

[0002]

2. Description of the Related Art It is desired that a resin tube made of a synthetic resin be lightweight while maintaining rigidity and impact resistance to fracture. Various methods have been proposed, such as a tube provided with a resin and a tube using a hard foamable thermoplastic resin. In particular, a resin tube in which the inner layer and the outer layer are formed of a non-foamed thermoplastic resin composition and the intermediate layer is formed of a foamed thermoplastic resin composition has attracted attention as a resin tube capable of achieving weight reduction while maintaining rigidity and impact fracture strength. Have been. As a manufacturing apparatus for manufacturing this three-layer resin tube, there is, for example, one described in Japanese Patent Publication No. 7-51308.

In the manufacturing apparatus described in Japanese Patent Publication No. 7-51308, in order to form an inner layer, an outer layer, and an intermediate layer, a plurality of cylindrical mold members having different inner and outer diameters are superimposed on a coat hanger. By assembling it, we propose a mold without a spider. By manufacturing a resin tube using this apparatus, the appearance of the surface of the resin tube can be improved, and the thickness of the tube when discharged from the mold can be made substantially uniform.

[0004]

However, when the composition or the mixing ratio of the thermoplastic resin composition for forming the outer layer and the inner layer and the foamable thermoplastic resin composition for forming the intermediate layer are changed, or when the finished outer diameter is changed. The resin viscosity may change when the extrusion conditions are changed in order to obtain products having different dimensions. If the resin viscosity changes in this way, the amount of resin discharged from the mold will fluctuate, and the discharge flow velocity distribution will increase, making it difficult to obtain a molded product having a uniform thickness. In particular, when using recovered resin (recycled resin recovered from discarded resin products) as the foamed resin in the intermediate layer, it often becomes impossible to follow the change in resin viscosity, and the thickness of the foamed resin layer varies greatly. There are many.

When a chemical foaming agent is used as a foaming agent, the amount of gas generated varies by changing various conditions such as the type and amount of the resin used and the additive. There is also a problem that the magnification distribution becomes large and it becomes difficult to obtain a uniform thick product after discharging the mold.

The present invention has been made in view of the above-mentioned problems in the conventional method of manufacturing a three-layer resin tube, and has been made in consideration of the problems described above. To provide a method of manufacturing a resin pipe having a three-layer structure capable of manufacturing a resin pipe so that the thickness of the resin pipe formed by the intermediate layer formed at the time of finished product is always uniform. Aim.

[0007]

In order to achieve the above object, the invention according to claim 1 is directed to a foamed thermoplastic resin composition which forms an intermediate layer between a non-foamed thermoplastic resin composition which forms an inner layer and an outer layer. The co-extruded unsolidified tubular body having a three-layer structure with a material interposed therebetween is extruded from a mold, and the foamed thermoplastic resin composition in the co-extruded unsolidified tubular body is foamed, and the thermoplastic resin is cooled and solidified. In the method of manufacturing a three-layer resin tube, the amount of the foaming agent to be added to the thermoplastic resin forming the intermediate layer is adjusted to a predetermined product size when the resin tube is molded without restricting the radial expansion of the resin tube. A resin tube is formed by sizing to a predetermined finished size against the foaming pressure using a tube for forming the outer surface of the tube and a core for forming the inner surface of the tube, in addition to the theoretical amount for obtaining the expansion ratio. I did it.

According to the production method of the present invention, even if the foamed thermoplastic resin layer tries to expand radially inward and outward by foaming, the foamed thermoplastic resin is formed by the tube for forming the tube outer surface and the core for forming the tube inner surface. Attempts to expand the layer are suppressed, and the foamed thermoplastic resin layer is in a pressurized state, so that a constant thickness according to the finished dimensions is obtained. In addition, when using the recovered resin recovered from waste resin products, viscosity changes occur, and even if the discharge amount from the mold fluctuates, the resin tube is formed by the tube for forming the outer surface of the tube and the core for forming the inner surface of the tube. Since the surface can be smoothed by sizing in a compressed state, it is not necessary to adjust the flow path in accordance with the change in viscosity. In addition, as a method of cooling and solidifying at the time of performing sizing, there are a forced cooling method such as water cooling and a natural cooling by natural leaving.

As the thermoplastic resin used in the production method of the present invention, vinyl chloride, polystyrene, polypropylene, polybutene, chlorinated polyethylene, ethylene-propylene copolymer, ethylene-ethyl acrylate copolymer, polyethylene terephthalate, ABS resin And extrudable thermoplastic resins such as acrylic resins. These may be used alone or in combination of two or more.

The blowing agent used in the present invention is not particularly limited as long as it does not deteriorate the above-mentioned thermoplastic resin. For example, pyrolytic blowing agents such as azodicarbonamide, benzenesulfonylhydrazide and toluenesulfonylhydrazide, and dichlorotetrafluoroethylene Physical foaming agents such as fluoroethane, monochlorodifluoroethane, butane, pentane, carbon dioxide, nitrogen, air, and argon. These may be used alone or in combination of two or more.

The addition amount of the foaming agent does not deviate from the amount usually used, but it is a theoretical value for obtaining a foaming ratio of a predetermined product size when the resin tube is molded without restricting the radial expansion of the resin tube. It is necessary to add more than the addition amount. However, if too much is added, there is a problem in cost, and it is preferable that the amount is 1.5 to 3 times the theoretical addition amount.

[0012] In the resin composition of the present invention, a stabilizer, an antioxidant, a processing aid, a lubricant, a foaming aid, a filler, a pigment, a flame retardant and the like can be added as required.

According to a second aspect of the present invention, in the method for manufacturing a three-layer resin tube according to the first aspect, the thermoplastic resin used for the intermediate layer is a recovered resin. What is recovered resin?
Resin that collects discarded resin products and crushes the resin part, scraps of molded products in factories, and crushed scrap generated at the start, stop, or trouble of molding, etc. Means

According to a third aspect of the present invention, in the method for producing a three-layered resin tube according to the first or second aspect, the thermoplastic resin is a vinyl chloride resin. In the case of vinyl chloride resin, when using a pyrolysis type foaming agent, the amount of gas generated varies depending on the type of stabilizer and the like, and the foaming ratio fluctuates. Although problems such as fluctuation and discharge distribution are likely to occur, according to the manufacturing method of the present invention, a resin tube having stable finished dimensions can be obtained even if such a problem occurs.

The production apparatus for carrying out the production method of the present invention includes a mold for co-extruding a resin tube having a three-layer structure in which an intermediate layer is formed of a foamed thermoplastic resin layer, and a tube outer surface for cooling the outer layer. A molding tube is provided, and a part of the mandrel provided in the mold is projected toward the tube for forming the outer surface of the tube to form an inner core (core for forming the inner surface of the tube), and at least a part of the inner core is formed for forming the outer surface of the tube. It can be configured by providing it in a tube.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for manufacturing a three-layer resin tube according to the present invention will be described with reference to the drawings. 1 and 2
FIG. 1 is an overall configuration diagram of a manufacturing apparatus for manufacturing a three-layer resin tube of the present invention, wherein an inner / outer layer extruder 11 in which a non-foamable thermoplastic resin composition 21 is melt-kneaded; Intermediate layer extruder 12 in which plastic resin composition 22 is melt-kneaded
And the non-foamable thermoplastic resin composition 2 of the inner and outer layer extruder 11
1 and foamable thermoplastic resin composition 22 of intermediate layer extruder 12
A cooling water tank 4 to which a tube 5 for forming the outer surface of the resin tube 2 for forming the outer surface of the resin tube 2 having a three-layer structure, and a resin tube 2 sent from the cooling water tank 4 are formed. It comprises a receiving machine 6 for receiving and a cutting machine 7 for cutting the resin tube 2 sent from the machine 6 into a predetermined length.

As shown in FIG. 3, a support rod 32 projecting toward the tube forming tube outer surface 5 is provided at the tip of the mandrel 31 in the mold 3. At the side end, a tube inner surface forming core 8 for forming the inner surface of the resin tube 2 having a smaller diameter than the inner diameter of the tube outer surface forming tube 5 and having a three-layer structure is provided. The tube inner surface forming core 8 is disposed so that at least a part thereof is located in the tube outer surface forming tube 5.

First, as shown in FIGS. 1 and 2, a non-foamable thermoplastic resin composition 21 melt-kneaded by an inner and outer layer extruder 11 and a foamable thermoplastic resin composition melt-kneaded by an intermediate layer extruder 12 The object 22 is injected into the mold 3 and merges inside the mold 3. When the unsolidified tubular body having the three-layer structure is discharged from the mold 3, it foams and is inserted as it is between the tube 5 for forming the outer surface of the tube and the core 8 for forming the inner surface of the tube. Cooling water tank 4
The resin tube 2 is cooled inside. Further, the cooled resin pipe 2 is delivered to a take-off machine 6 and sent to a cutting machine 7, where it is cut into a predetermined length.

In this embodiment, as shown in FIG. 3, a predetermined space is provided between the resin outlet of the mold 3 and the inlet of the tube 5 for forming the tube outer surface. The outlet and the inlet of the tube outer surface forming tube 5 may be brought into contact with each other. When an interval is provided as shown in FIG. 3, molding can be performed stably without causing resin clogging, and the interval is preferably set to 50 mm to 100 mm.

Further, as shown in FIGS. 3 and 4, the manufacturing apparatus of this embodiment is provided with one core 8 for forming the inner surface of the tube, and the shape of the core 8 for forming the inner surface of the tube is as follows. As shown in FIG. 3 or FIG. 4, a taper surface is formed on the side of the extruder in order to smoothly feed the unsolidified resin tube 2 or
Alternatively, it is preferably formed on a curved surface. In addition, the inlet side of the outer surface forming tube 5 is also formed with a tapered surface in order to smoothly feed the unsolidified resin tube 2.

Further, in this embodiment, in order to cool the core 8 for forming the inner surface of the tube, a cooling medium is sent into the core 8 for forming the inner surface of the tube. Specifically, as shown in FIGS. 3 and 4, a cooling medium inflow passage 34 communicating with a cooling medium sending passage 33 formed inside the support rod 32 is formed in the mold 3. As shown in FIG. 4, a cooling medium return passage 35 extending parallel to the cooling medium feed passage 33 is formed inside the rod 32.

Then, inside the core 8 for forming the inner surface of the tube,
As shown in FIGS. 3 and 4, a plurality of cooling passages 81 extending radially outward from the center communicating with the cooling medium feed passage 33 and the cooling medium return passage 35 are formed. Although not shown, the mold 3 is provided with a cooling medium recovery passage formed in parallel with the cooling medium inflow passage 34 for collecting the cooled cooling medium.

In the tube inner surface forming core 8 shown in FIG. 4, the cooling medium sent from the cooling medium inflow passage 34 of the mold 3 to the cooling medium feed passage 33 of the support rod 32 is supplied to the tube inner surface forming core 8.
It is sent to the cooling passage 81 in the inside, and the pipe inner surface forming core 8 is cooled. Then, the cooled cooling medium is sent to the cooling medium return passage 35 of the support rod 32 and collected in the cooling medium collection passage provided in the mold 3.

As the cooling medium, a material such as air, water, oil, polyethylene glycol or the like which does not corrode the mold 3 and flows in the passage and has good heat exchange is used. Of these, air and water that do not require cleaning and recovery of the refrigerant are preferred.

As shown in FIG. 5, one end of the cooling passage 81 formed in the core 8 for forming the inner surface of the pipe is opened to the surface of the core to form a jet port 82. May be ejected into the tube 5 for forming the outer surface of the tube.

In the embodiment shown in FIGS. 4 and 5, one core 8 for forming the inner surface of the tube is provided. However, as in the other embodiments shown in FIGS. May be set to a plurality (three in FIGS. 6 and 7).

The core 8 for forming a pipe inner surface shown in FIGS.
In this case, the cooling medium inflow passage 3 communicated with the cooling medium feed passage 33 formed inside the support rod 32 in the mold 3.
4, only the cooling medium feed passage 33 is formed inside the support rod 32, and the cooling medium feed passage 33 is formed on the outer surface of the support rod 32 exposed between the adjacent pipe inner surface forming cores 8 respectively. Is formed with a discharge port 36 that communicates with the nozzle. The cooling medium is ejected from these discharge ports 36 toward the product.
In this case, no cooling passage is formed in the core 8 for forming the inner surface of the pipe.

Hereinafter, the operation of the present invention will be described in detail. The foaming agent is added to the molten resin at an amount not less than the theoretical amount to obtain a foaming ratio of a predetermined product size when the resin tube 2 is molded without restricting the radial expansion of the resin tube 2 in the intermediate layer extruder 12. Dissolve.

The resin of the intermediate layer starts foaming as soon as it is discharged from the mold. The foamed resin tube 2 enters the gap between the tube outer surface forming tube 5 and the tube inner surface forming core 8, and the expandable thermoplastic resin composition 22 serving as an intermediate layer of the resin tube 2 moves radially outward. Even if it tries to spread, the tube 5 for forming the outer surface of the tube and the core 8 for forming the inner surface of the tube suppress the expansion in the radial direction against the foaming pressure, and become a pressurized state.
The resin tube 2 can have a wall thickness having a predetermined product size.

As a result, in the portion where the discharge flow rate is low, when the tube enters between the tube 5 for forming the outer surface of the tube and the core 8 for forming the inner surface of the tube, the foaming is sufficiently performed, and the reduction in wall thickness due to insufficient flow is compensated for. The wall thickness is obtained, and foaming is suppressed by the tube 5 for forming the outer surface of the tube and the core 8 for forming the inner surface of the tube, so that a constant thickness is obtained. It will be constant.

As described above, according to the production method of the present invention, the expandable thermoplastic resin composition 22
In addition, since a foaming agent having an addition amount equal to or more than a theoretical addition amount for obtaining a foaming ratio of a predetermined product size when the resin tube 2 is formed without restricting the radial expansion of the resin tube 2 is added,
After the mold is ejected, the foam tends to foam to a thickness greater than the predetermined product size. However, the foaming is suppressed by the tube 5 for forming the outer surface of the tube and the core 8 for forming the inner surface of the tube against the foaming pressure. The resin tube 2 having a thickness is obtained.

Further, it is possible to forcibly mold a resin pipe having a uniform thickness without making the flow rate of the intermediate layer strictly uniform. In particular, even in the case of a recovered resin having a large discharge flow rate distribution, a final product having a uniform wall thickness can be obtained. Further, since it is not necessary to strictly adjust the discharge flow rate of the intermediate layer, the resin of the intermediate layer can be easily changed.

Further, since the outer and inner surfaces of the resin tube 2 are molded by sizing with the tube 5 for forming the outer surface of the tube and the core 8 for forming the inner surface of the tube, the smoothness of the inner and outer surfaces of the resin tube 2 is improved and the transferability is improved. And the resin tube 2 having a three-layer structure having a good surface appearance can be easily obtained. In particular, a product having a good surface condition of the inner layer can be obtained by the core 8 for forming the inner surface of the tube.

[0034]

Next, the present invention will be described in detail with reference to examples and comparative examples, but the present invention is not limited to these examples. Raw materials used in the following Examples and Comparative Examples will be described.

[Raw Materials Used] As a thermoplastic resin for forming the inner and outer layers, a vinyl chloride resin for a rigid vinyl chloride pipe specified in JIS K 6741 was used.

The thermoplastic resin forming the intermediate layer includes: A: a recovered vinyl chloride resin used in a sewage vinyl chloride pipe mixed with a lead-based stabilizer B: a tap water mixed with a tin-based stabilizer Recovered vinyl chloride resin used in a vinyl chloride pipe for use C: A vinyl chloride resin having a degree of polymerization of 800 (TS800E manufactured by Tokuyama Sekisui Kogyo Co., Ltd.) was used, and those shown in Table 1 and the following Formulations 1 to 3 were prepared.

The following three compounding ratios were used for Compounds 1, 2, and 3. Formulation 1: 50 parts by weight of raw material A, 50 parts by weight of raw material C Formulation 2: 50 parts by weight of raw material B, 50 parts by weight of raw material C Formulation 3: 25 parts by weight of raw material A, 25 parts by weight of raw material B, 50 parts by weight of raw material C

Further, as shown in Table 1, 2 parts by weight of a lead stabilizer, 5 parts by weight of an acrylic processing aid, and 0.6 parts by weight of a lubricant were added to the above-mentioned Formulations 1 to 3.

[Table 1]

Example 1 In Example 1, a foamable thermoplastic resin composition obtained by adding 0.3 parts of an ADCA (azodicarbonamide) foaming agent to Table 1 and Formulation 1 shown above was used. And the resin pipe manufacturing apparatus shown in FIG.
A resin tube was formed by using the mold 3, the tube 5 for forming the outer surface of the tube, and the core 8 for forming the inner surface of the tube, as shown in FIGS. Then, the non-foamable thermoplastic resin composition has a temperature of 190 ° C.
And the mixture is kneaded with the inner and outer layer extruder 11 and injected into the mold 3 at an extrusion rate of 40 kg / h.
The mixture was kneaded at 0 ° C. and injected into the mold 3 at 60 kg / h. Mold 3
Used a mold having an outer diameter of 89 mm and an inner diameter of 77 mm.

Example 2 Example 2 shows that a foamable thermoplastic resin composition obtained by adding 0.3 parts of an ADCA (azodicarbonamide) foaming agent to Table 1 and Formulation 2 shown above was used. Except for using the same manufacturing apparatus as in Example 1,
A resin tube was molded under the same conditions.

Example 3 Example 3 shows that a foamable thermoplastic resin composition obtained by adding 0.3 parts of an ADCA (azodicarbonamide) foaming agent to Table 1 and the composition 3 shown above was used. Except for using the same manufacturing measures as in Example 1,
A resin tube was molded under the same conditions.

[Comparative Example 1] In Comparative Example 1, a resin tube was formed using a foamable thermoplastic resin composition obtained by adding 0.1 part of ADCA (azodicarbonamide) foaming agent to the same formulation 1 as in Example 1. Each of Examples 1 to 3 was molded.
In the manufacturing apparatus used in (1), sizing was performed with the tube inner surface forming core 8 removed, that is, only with the tube outer surface forming tube 5 to form a resin tube.

When the expansion ratio for obtaining the finished product size of the intermediate layer in the case where the resin tube is formed without restricting the radial expansion of the resin tube is set to about 2 times, usually an ADC is used.
About 0.1 part of A is added.

Comparative Example 2 Comparative Example 2 was the same as Example 2 except that a foamable thermoplastic resin composition was used in which 0.1 part of an ADCA (azodicarbonamide) blowing agent was added. Then, a resin tube was molded under the same manufacturing conditions as in Comparative Example 1 under the same manufacturing conditions.

Comparative Example 3 Comparative Example 3 was the same as Example 3 except that a foamable thermoplastic resin composition was used in which 0.1 part of an ADCA (azodicarbonamide) blowing agent was added. Then, a resin tube was molded under the same manufacturing conditions as in Comparative Example 1 under the same manufacturing conditions.

[Sample Evaluation Method] The sample evaluation was made based on the thickness variation of the resin tube. The extruded product was cut every one minute, and the thickness at four places at the top, bottom, left and right of the cross section was measured. Forty data were taken and their standard deviation was calculated. In Table 2,
The evaluation result of the thickness fluctuation | variation of the resin pipe of a three-layer structure performed in the Example and the comparative example is shown.

[Table 2]

[Results] As shown in Examples 1 to 3, according to the production method of the present invention, even if the type of stabilizer and the degree of polymerization of the raw materials are different, a stable thickness can be obtained. A three-layer resin tube with improved smoothness on the inner surface was obtained.
According to the method of the comparative example, the change in wall thickness was increased due to the change in the degree of polymerization and the change in the stabilizer.

[0048]

According to the method for producing a three-layer resin tube of the present invention, a foamed thermoplastic resin composition as an intermediate layer is interposed between a non-foamed thermoplastic resin composition as an inner layer and an outer layer. Co-extruded the unsolidified tubular body of the three-layered structure from the mold, foams the foamed thermoplastic resin composition in the co-extruded unsolidified tubular body, and cools and solidifies the thermoplastic resin. In the method for manufacturing a resin tube, the amount of the foaming agent to be added to the thermoplastic resin forming the intermediate layer is obtained to obtain a foaming ratio of a predetermined product size when the resin tube is molded without restricting the radial expansion of the resin tube. In addition to the addition amount that is greater than the theoretical addition amount, a resin tube was formed by sizing to a predetermined finished size against foaming pressure using a tube for forming the outer surface of the tube and a core for forming the inner surface of the tube. Predetermined product dimensions after mold discharge Since the foaming is suppressed by the tube for forming the outer surface of the tube and the core for forming the inner surface of the tube, it is necessary to manufacture a resin tube having a predetermined thickness and a uniform thickness. Can be.

As a result, it is not necessary to strictly adjust the discharge flow rate of the intermediate layer, and the resin of the intermediate layer can be easily changed. Further, even if a recycled resin containing many kinds of additives is used for the intermediate layer, a resin pipe having a stable thickness and a three-layer structure can be easily formed.

Further, since the inner and outer surfaces of the resin tube are molded by sizing using the tube for forming the outer surface of the tube and the core for forming the inner surface of the tube, the smoothness of the inner and outer surfaces of the resin tube is improved, and the transfer of the inner and outer surfaces is performed. A resin tube having a three-layer structure that can improve the properties and also has a good inner surface appearance can be easily obtained.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a manufacturing apparatus for manufacturing a resin tube having a three-layer structure according to the present invention as viewed from above.

FIG. 2 shows an overall configuration diagram of the manufacturing apparatus shown in FIG. 1 as viewed from a side.

FIG. 3 is a cross-sectional view of one embodiment showing a mold, a tube for forming an outer surface of a tube, and a core for forming an inner surface of a tube used for manufacturing a resin tube having a three-layer structure of the present invention.

FIG. 4 is a partial cross-sectional view of one embodiment of a tube inner surface forming core used in the present invention.

FIG. 5 is a partial cross-sectional view of another embodiment of the core for forming a pipe inner surface used in the present invention.

FIG. 6 is a cross-sectional view of another embodiment showing a mold, a tube for forming an outer surface of a tube, and a core for forming an inner surface of a tube used for manufacturing a resin tube having a three-layer structure of the present invention.

FIG. 7 is a sectional view of the core for forming a tube inner surface shown in FIG. 6;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Inner and outer layer extruder 12 Intermediate layer extruder 2 Resin tube 21 Non-foamable thermoplastic resin composition 22 Foamable thermoplastic resin composition 3 Die 5 Tube outer surface forming tube 8 Tube inner surface forming core

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) B29L 9:00 B29L 9:00 23:00 23:00

Claims (3)

[Claims] 1. An unsolidified tubular body having a three-layer structure in which a foamed thermoplastic resin composition serving as an intermediate layer is interposed between a non-foamed thermoplastic resin composition serving as an inner layer and an outer layer, is formed from a mold. Extrusion, while foaming the foamed thermoplastic resin composition in the co-extruded unsolidified tubular body, in the method of manufacturing a three-layer resin tube cooling and solidifying the thermoplastic resin, the thermoplastic resin forming the intermediate layer The amount of the foaming agent to be added should be not less than the theoretical amount for obtaining a foaming ratio of a predetermined product size when the resin tube is molded without restricting the radial expansion of the resin tube, and the outer surface of the tube is formed. A method for producing a resin tube having a three-layer structure, wherein a resin tube is formed by sizing to a predetermined finished size against foaming pressure using a tube for forming a tube and a core for forming an inner surface of the tube. 2. The method according to claim 1, wherein the thermoplastic resin used for the intermediate layer is a recovered resin. 3. The method according to claim 1, wherein the thermoplastic resin is a vinyl chloride resin.