JPH0885154A - Electric fusion bonded joint, manufacture thereof and injection mold thereof - Google Patents

Abstract

PURPOSE: To obtain an excellent electric fusion-bonded joint in which no shrinkage, etc., occurs by forming the cylindrical member layer of the entire joint of three or more layers of multilayer resin layers including an inner cylindrical member layer in the joint in which a thermoplastic resin is molded on the outer surface of the inner cylindrical member layer in which an electric heating wire is embedded in its inner periphery to provide an outer cylindrical member layer. CONSTITUTION: Cavities A-C are provided in one set of molds, a mandrel 10 is mounted in the cavity A in which a melted resin injected from an inlet 11 can be charged in the cavities A-C via a runner 12 and a gate 13 to inject the resin. After the resin for molding an inner cylindrical member 21 is solidified, the member 21 with the mandrel 10 is removed from the molds, and an electric heating wire 22 is wound on the outer peripheral groove 23 of the member 21. Then, the member 21 with the wire 22 is mounted in the cavity B, the melted resin of an intermediate member layer 25 is injected to obtain an intermediate member 27. Thereafter, the member 27 is mounted in the cavity C, the resin of a final outer cylindrical member layer 26 is injected to obtain an electric fusion bonded joint 28.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric fusion joint in which a heating wire is embedded in an inner peripheral portion of a thermoplastic resin joint body for fusion splicing thermoplastic resin pipes and the electric fusion joint. And a metal mold for injection molding thereof, and more particularly to a thick and large-diameter electric fusion-bonded joint that can be efficiently manufactured without internal defects.

[0002]

2. Description of the Related Art Conventionally, an electric fusion type plastic pipe joint disclosed in JP-A-57-69010 has been used as a pipe joint for electrically connecting a thermoplastic resin such as a polyethylene pipe by heat fusion. is there. As shown in the partial cross-sectional view of FIG. 8, this is provided with a groove 34 extending from one end to the other end, and an inner cylinder member 31 having a heating wire 33 wound around the groove 34 and an outer surface of the inner cylinder member 31. It is an electric fusion type plastic pipe joint provided by the outer member 32 integrally formed with the same resin. This pipe joint is fused and connected integrally with the resin pipe inserted in the pipe joint by energizing the heating wire 33 buried in the inner peripheral portion.

In the above-mentioned plastic pipe joint, the outer member 32 is injection-molded on the outer surface of the inner cylindrical member 31 around which the heating wire 33 is wound, and the inner cylindrical member 31 and the outer member 32 are integrated. Since the heating wire 33 heats the boundary between the connection resin pipe inserted in the joint and the inner peripheral surface of the joint to melt them, it is necessary to provide the heating wire 33 as close to the inner peripheral surface of the joint as possible. Therefore, the wall thickness t of the bottom of the groove 34 that accommodates the heating wire 33 of the inner tubular member 31 is t.
Is a thin wall having a thickness of 1 mm or less. On the other hand, when the nominal diameter of the pipe joint increases, the wall thickness of the pipe joint itself also increases, so the wall thickness T of the outer member 32 also increases. As an example, the wall thickness dimensions of the inner tubular member and the outer member according to the nominal diameter of the two-layer polyethylene pipe connecting pipe for gas are shown in FIG.
Shown in

[0004]

When the outer member 32 having a large wall thickness is injection-molded on the outer surface of the conventional thin inner cylinder member 31, the inner cylinder member 31 is produced by the large injection pressure and heat capacity of the outer member 32. Is melted, the heating wire 33 wound around the groove 34 of the inner cylinder member 31 moves irregularly, and the heating wires 33 come into contact with each other during the connection work with the pipe, causing a short circuit and normal heating is not performed. There is a problem that causes it. In general, the thermal conductivity of synthetic resins is very poor, and the volume contraction from the melting to the solidification is quite large. This shrinkage ratio may reach 4% to 10% depending on the resin material. For this reason, depending on the mold injection molding method, if there is an unsolidified resin part in the molded product in the cavity even after the gate and runner resin has solidified, resin injection pressure will continue to be applied. However, the molten resin cannot be replenished to the non-solidified resin portion in the molded product, and a shrinkage occurs inside the wall thickness of the resin layer due to volume contraction during solidification.

In order to prevent sink marks, the gate and runner may be made thicker and the injection pressure of the resin may be increased to apply the injection pressure for a long time. In such a case, the thin inner cylinder member 31 is melted. Therefore, there is a problem that the arrangement of the heating wire 33 is moved or the time for the thick molten resin in the thickness T portion to be completely solidified is long, resulting in a long molding tact and a marked deterioration in efficiency. As a result, injection molding a thick resin into a thin inner tubular member is difficult to control in manufacturing,
The quality is not stable. This problem remarkably occurs for a joint having a maximum wall thickness T of 10 mm or more and a nominal diameter of 75 A or more. The present invention is to solve the above problems, defects such as sink marks do not occur, an electric fusion joint excellent in quality and a method of manufacturing the electric fusion joint that can efficiently manufacture this electric fusion joint and its injection A molding die is provided.

[0006]

DISCLOSURE OF THE INVENTION The gist of the present invention is to provide an inner cylinder member layer having a heating wire embedded in its inner peripheral portion, and an outer cylinder member formed by molding a thermoplastic resin on the outer surface of the inner cylinder member layer. In the electrofusion-bonding joint including layers, the cylindrical member layer of the entire joint is made up of three or more multilayer resin layers including the inner tubular member layer. This multi-layer may be provided with resins having different resin materials and colors. Further, in a joint having a nominal diameter of 75 A or more, an electro-fusion joint characterized in that the cylindrical member layer of the entire joint is composed of 3 to 5 multilayer resin layers including the inner cylindrical member layer. is there. Further, regarding the joint in which the thickness of the outer cylindrical member layer of the joint is 10 mm or more, the cylindrical member layer of the entire joint is composed of 3 to 5 multilayer resin layers including the inner cylindrical member layer. The electric fusion-bonded joint has a wall thickness of 15 mm or less per layer.

Further, there is provided a method for obtaining an electric fusion-bonded joint in which a heating wire is embedded in an inner peripheral portion of a thermoplastic resin joint body, wherein an inner tubular member accommodating the heating wire forming an inner peripheral surface of the joint body is first formed. The inner cylinder member is mounted in the cavity of the molding die, a thermoplastic resin layer is molded on the outer surface of the inner cylinder member, and an intermediate member integral with the inner cylinder member is provided. It is installed in the cavity of the mold, the thermoplastic resin layer is molded on the outer surface of the intermediate member to provide a molded member integral with the intermediate member, and the thermoplastic resin layer is sequentially molded on the outer surface of the molded member that is molded below. A method for manufacturing an electric fusion joint, which comprises obtaining an electric fusion joint having a target shape.

This method for manufacturing an electric fusion joint has one or a plurality of sets of the molding dies, and a step of obtaining an inner cylinder member and a step of obtaining an intermediate member in each cavity portion of the dies are sequentially performed. It is characterized in that the step of obtaining a molded member is performed in synchronism with each other to obtain an electric fusion joint having a final target shape.

Further, it is an injection molding die for obtaining an electric fusion joint in which a heating wire is embedded in an inner peripheral portion of a thermoplastic resin joint body, and a plurality of cavities capable of injection molding molten resin using the same mandrel. The cavity portion has a first-layer cavity portion that forms an inner cylindrical surface of the joint and that forms a groove for accommodating the heating wire, and a cavity portion that is one size larger than the cavity portion. A second layer cavity portion capable of injection molding a thermoplastic resin layer on the outer surface of the tubular member, and third to fifth cavity portions capable of sequentially injection molding the thermoplastic resin layer on the molded outer surface, An electrofusion-joint injection-molding die, characterized in that the thickness of each layer is 15 mm or less.

[0010]

The present invention has the above-described structure, and the conventional thick outer member layer is injection-molded on the outer surface of the thin inner cylindrical member layer a plurality of times to form a cylindrical member layer of the entire joint. It is a multi-layered electric fusion joint having three or more layers. In particular, it is effective for joints with a nominal diameter of 75 A or more, and a large effect can be obtained by using three or five layers.
Further, for a joint in which the thickness of the outer tubular member layer is 10 mm or more with respect to the inner tubular member layer having a heating wire, the entire tubular portion of the joint is formed of three to five multi-layered resin layers and By reducing the thickness of the product to 15 mm or less, the quality can be improved and the molding time can be greatly shortened.

To this end, an inner cylinder member which forms the inner peripheral surface of the joint and accommodates the heating wire is provided in the inner peripheral portion, and the inner cylinder member with the heating wire is attached to the cavity portion of the molding die. Injection molding a thermoplastic molten resin layer on the outer surface to provide an intermediate member integral with the inner cylinder member, and further injection molding a thermoplastic resin layer on the outer surface of the intermediate member to provide a molding member integral with the intermediate member, A resin layer is sequentially molded on the outer surface of the molded member molded in this manner, and finally an electric fusion-bonded joint in which the resin layer of the joint is a multilayer of 3 to 5 layers is obtained.

The number of layers from 3 to 5 is an optimum number in consideration of the shape of the electric fusion joint and the handling time of each layer during the molding work. If the number of layers is three or more, the thick outer member layer of the joint having the outer member layer having a wall thickness of more than 10 mm is divided into a plurality of layers, and the difference in wall thickness from the inner tubular member is reduced. Since the wall thickness and the resin capacity per layer are reduced, the injection pressure may be small. Therefore, problems such as melting of the thin inner cylinder member and movement of heating wires do not occur, and the cooling rate after injection is fast and the cooling time is short, so that internal defects such as sink marks do not occur. In this case, since the forming operation of each layer can be performed in synchronization, the longest forming time per layer is the whole forming tact, and the thick portion of the joint including the thick outer member layer of 10 mm or more is plural. By being divided into layers, the thickness of each layer is 15 mm or less and the resin capacity is reduced, and the cooling rate is increased and the cooling time is greatly shortened due to the reduced thickness. For this reason, the tact for molding the entire electric fusion joint is greatly shortened.

Generally, the cooling time of the molten resin in the mold is
Although theoretically dependent on the temperature at which the product is taken out from the mold, it is theoretically proportional to the square of the wall thickness of the molded product. Therefore, if the thickness per layer in one cavity portion is halved, the cooling time will be 1/4. Further, since the volume of the injected molten resin per layer in one cavity portion is significantly reduced, the cooling rate of the injected molten resin is increased and the injection molded resin portions are uniformly cooled. For this reason, the cooling time is greatly shortened and defects such as sink marks are not generated inside.

Therefore, it is no longer necessary to continue to apply a large injection pressure or a long injection pressure to prevent sink marks, and even a resin molding machine having a smaller injection capacity and a smaller injection pressure than the conventional one has a large diameter of electricity. A fusion splice can be molded. In general, if the volume of injection resin in one layer per cavity is large, the injection molding conditions such as the resin temperature during molding and the temperature of the mold will greatly affect the quality after molding. By doing so, the wall thickness per layer is 15 mm or less, and the injection resin capacity is small and the wall thickness is uniform. Therefore, the cooling time becomes short, the injection molding conditions are not so much affected, the molding operation is easy, and an electric fusion-bonded joint of good quality without internal defects can be obtained.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. 1 to 3 are cross-sectional views of an electric fusion joint injection mold and an electric fusion joint showing an embodiment of the present invention. 4A and 4B are views showing a molding state in which an embodiment of the present invention is described in comparison with the prior art. FIG. 4A shows a conventional molding state having two layers, and FIG. It is a figure which shows a molding state. In FIG. 1, three cavity portions A, B and C for molding molten resin are provided in a set of molds,
The cavities each have a larger diameter in the radial direction. Further, a common mandrel 10 is attached to each of the cavities so that they can be molded. The molten resin injected from the mold inlet 11 passes through the runner 12 and the gate 13 and is filled in each of the cavitation parts A, B, and C.

When the mandrel 10 is attached to the cavity portion A and the molten resin is injected, FIG. 2, FIG. 3 or FIG. 4 (b)
The thin inner cylindrical member 21 of the electric fusion joint shown by is molded.
After the resin injection-molded in the cavity portion A is solidified, the inner cylinder member 21 with the mandrel 10 is taken out of the mold, and the heating wire 22 is attached to the outer peripheral groove 23 of the thin inner cylinder member 21 with the mandrel 10 attached. Is wound. The heating wire 22 is connected to the connector pin 24 by inserting and fixing the connector pins 24, 24 at the end of the inner tubular member 21.

The inner tubular member 21 with the heating wire 22 forming the inner surface of the joint thus formed is then attached to the B cavitating portion with the mandrel 10 attached, and the intermediate member layer 25 of the B cavitating portion is attached. The inner cylinder member 21 is formed by injection molding a molten resin.
The attached intermediate member 27 is obtained. Even during the injection molding of the intermediate member layer 25, the thin inner cylinder member 21 is molded at the cavity portion A. After the intermediate member 27 with the inner cylinder member is injection-molded in the cavity portion B, the intermediate member 27 formed with the mandrel 10 is taken out from the mold as in the above. Then, the resin of the final outer cylindrical member layer 26 is injection-molded on the outer surface of the intermediate member 27. After molding the final outer cylinder member layer 26 on the outer surface of the intermediate member layer 25, the molded product is taken out from the mold with the mandrel 10 attached, and the mandrel 10 is removed from the molded product as shown in FIG. 2, FIG. 3 or FIG. The electric fusion-bonded joint 28 shown in FIG.

The inner cylinder member layer 21 thus formed into three layers
The resin layers of the intermediate member layer 25 and the final outer tubular member layer 26 are
The electric fusion-bonded joints 28 in which their contact surfaces are heat-fused by the heat of the molten resin injected to the outer surface of each layer and are integrally joined
Is molded. The inner cylinder member layer 21 and the intermediate member layer 25 may be made of a pure polyethylene resin material, and the final outer cylinder member layer 26 may be made of a resin material containing carbon black. Even while the final outer member layer 26 is being injection-molded in the cavity portion C, the thin inner cylinder member 21 in the other cavity portion A and the intermediate member layer 25 in the cavity portion B are synchronously formed. There is. Therefore, as the entire injection molding tact, the longest molding time of any of the cavities A, B, and C becomes the entire tact, and if the injection molding time of each layer is shortened, the entire molding tact is greatly shortened. .

FIG. 5 shows a socket type electric fusion joint (nominal 150A) (outer diameter 207 mm, length 248 mm) shown in FIG.
The cooling time in the mold of the outer member 32 having a wall thickness of 22 mm and the final outer cylinder member layer 26 having a wall thickness of 11 mm of this embodiment shown in FIG. It is a cooling diagram which measured the relationship between and cooling temperature. The cooling line a in the figure represents the cooling line of the conventional outer member 32 having a wall thickness of 22 mm,
B represents a cooling line of the final outer cylinder member 26 of this embodiment having a wall thickness of 11 mm. As can be seen from this cooling diagram, for example, if the temperature for taking out from the mold is 95 ° C., in the conventional method of (a), the time from the injection of the molten resin to the cooling in the mold to 95 ° C. is 450 seconds, In this embodiment (b), the time until cooling to 95 ° C. is 150 seconds. Therefore, if the temperature for taking out from the mold is set to 95 ° C., the molding tact can be simply shortened to 1/3 in the present embodiment (2), as compared with the conventional method (1).

FIG. 6 is a temperature diagram of the temperature taken out from the mold, which was obtained by investigating the relationship between the wall thickness of the joint and the cooling temperature as described above.
It is a figure explaining the relationship between resin molding thickness and injection molding time (cooling time). In the case of a socket-type electric fusion joint having a nominal diameter of 150 A, in the conventional method shown in FIG. 4 (a), as shown in FIG. 8, the wall thickness t of the groove of the inner tubular member layer 31 (the wall thickness minimum portion). Is 0.6 mm, and the maximum thickness of the groove is 5.
It is 5 mm. The maximum thickness portion T of the outer member layer 32 formed on the outer surface of the inner tubular member layer 31 is 22 mm. The cooling time for injection molding the inner tubular member layer 31 was 90 seconds, and the cooling time for injection molding the outer member layer 32 was 360 seconds. The take-out temperature from the mold at this time is 135 ° C. The point E in FIG. 6 is a plot of the molding state of the outer member layer 32. In this embodiment shown in FIG. 4B, the inner cylinder member layer 21 has the same size as the conventional inner cylinder member 31, and the thickness of the conventional outer member layer 32 is set to the intermediate member layer 25.
The final outer cylinder member layer 26 was divided into two layers, and the thickness of each layer was reduced to 11 mm, which is half the conventional thickness. As a result, the intermediate member layer 2
5 and the final outer cylinder member layer 26 have an injection molding time of 100, respectively.
Seconds, the temperature of taking out from the mold at this time is 130 ° C
Met. Moreover, an electric fusion-bonded joint having no defects such as sink marks was obtained. This state is plotted at point F in FIG.

In the case of a socket-type electric fusion-bonding joint (outer diameter 278 mm, length 321 mm) having a nominal diameter of 200 A,
In the conventional method shown in FIG. 4A, the minimum thickness part t of the inner tubular member layer 31 is 0.8 mm and the maximum thickness part is 5.5 mm. The thickness T of the outer member layer 32 injection-molded on the outer surface of the inner cylindrical member layer 31 is 28 mm. The injection molding time of the inner cylinder member layer 31 was 100 seconds, and the injection molding time of the outer member layer 32 was 900 seconds as indicated by the point J in FIG. The take-out temperature from the mold at this time is 120 ° C. Meanwhile, FIG.
In the present embodiment shown in (b), the inner tubular member layer 2 is similar to the above.
1 has the same size as that of the conventional inner tubular member layer 31, and the thickness T of the conventional outer tubular member layer 32 is the same as that of the intermediate member layer 25 and the final outer tubular member layer 2.
It is divided into 2 layers of 6 and the thickness of each is half that of the conventional one, 14 mm.
And As a result, the injection molding time of each of the intermediate member layer 25 and the final outer cylindrical member layer 26 is 1 as shown by the point K in FIG.
It was drastically shortened to 50 seconds. The take-out temperature from the mold at this time is 115 ° C., which is lower than that of the conventional method. Moreover, an electric fusion-bonded joint having no defects inside and outside the sink mark was obtained.

Next, an embodiment in which the wall thickness of the socket type electric fusion splicing joint having the nominal diameter of 200 A is divided into four layers in order to further shorten the molding time will be described. FIG. 7 is a cross-sectional view of each layer of a joint formed by molding a joint having a nominal diameter of 200 A into four layers. The first layer 41 has a heating wire 22 wound in a groove like the inner tubular member layer 21 of the above-described embodiment. , The second layer 42 and the third layer 4 on the outer surface as described above.
The third and fourth layers 44 are sequentially injection-molded to obtain an integral electric fusion joint 45. Also in this case, the thickness of each of the second to fourth layers is 1 mm, which is the thickness of the conventional outer member layer 32 of 28 mm.
/ 3 of about 9.5 mm.

In this embodiment, unevenness is also formed on the outer surfaces of the second layer 42 and the third layer 43 to increase the area of contact with the inner surface of the mold cavity portion during injection molding in the cavity portion, and the injection is performed. The cooling efficiency of the resin is increased to reduce the cooling time. Further, the contact area between the injected resin layers is increased to increase the melt adhesion strength for melting integrally. The time taken out from the mold after injection molding of this example was 115 seconds for the second layer 42 having the longest cooling time, and the temperature at this time was 100 ° C. Therefore, in the case of the four-layer molding of this example, compared with the conventional molding time of 900 seconds, it was possible to significantly reduce the time from 900 seconds to 115 seconds, and the temperature for taking out from the mold was also lowered. In addition, compared with the case of the above three-layer embodiment,
It was possible to reduce the time from 0 seconds to 115 seconds by 35 seconds.

In the above embodiment, the thickness of the joint is three layers or four.
Although the embodiment of forming into layers has been described, it may be divided into five layers and formed depending on the shape and size of the joint and the specifications of the forming machine. The plurality of layers may be made of the same resin material, or may be made of resins having different resin materials and colors.
Although it is possible to have any number of layers, 3 to 5 layers are most suitable in practical use in consideration of the shape of the joint and the handling time of the molding work of each layer. Further, as in the case of the electric fusion-bonded joint shown in FIGS. 3 and 7, the joint may be divided into 3 or 5 layers in the radial direction as well as in the longitudinal direction of the joint. In addition, although a mold in which a plurality of layers of cavity portions A, B, and C are provided in one mold has been described, the cavity portion A for injection molding each layer,
As a matter of course, it is possible to sequentially perform multi-stage molding using a plurality of sets of dies each having B and C individually.

[0025]

According to the present invention, since the thickness of the resin injected into the joint body can be made sufficiently small and the volume of the molten resin injected at one time can be reduced, the molten resin injection-molded in each layer stage can be reduced. The cooling rate is faster, the molding tact is greatly shortened, and the molding work efficiency is improved. Further, since each part is cooled uniformly, internal defects such as sink marks of the related art do not occur.
Therefore, it is possible to efficiently obtain an electric fusion joint having excellent quality.

[Brief description of drawings]

FIG. 1 is a plan view of an electric fusion joint injection molding die according to an embodiment of the present invention.

FIG. 2 is a sectional view of an electric fusion splicing joint according to an embodiment of the present invention.

FIG. 3 is a cross-sectional view of an electric fusion splicing joint according to another embodiment of the present invention.

FIG. 4 is a diagram showing a molded product in each step for comparing and explaining the conventional method (a) and the method of the present embodiment (b).

FIG. 5 is a diagram showing the relationship between the cooling time and the cooling temperature in the mold of the conventional method and this embodiment.

FIG. 6 is a diagram showing the relationship between the resin wall thickness and the injection molding time (cooling time) with respect to the time taken out from the mold, which will be described by comparing the embodiment of the present invention with the prior art.

FIG. 7 is a cross-sectional view of a four-layer molded electric fusion-bonded joint according to an embodiment of the present invention.

FIG. 8 is a partial cross-sectional view of a conventional electric fusion splicing joint and a view showing wall thicknesses T and t of each layer.

[Explanation of symbols]

A, B, C Each cavity part in the mold 10 Mandrel 11 Inlet 12 Runner 13 Gate 21 Inner cylinder member layer 22 Heating wire 23 Groove 24 Connector pin 25 Intermediate member layer 26 Final outer cylinder member layer 27 Intermediate with inner cylinder member Member 28, 45 Electric fusion-bonded joint after molding 41 First layer 42 Second layer 43 Third layer 44 Fourth layer

Claims (6)

[Claims] 1. An electric fusion joint comprising an inner tubular member layer having a heating wire embedded in its inner peripheral portion and an outer tubular member layer formed by molding a thermoplastic resin on the outer surface of the inner tubular member layer, The cylindrical member layer of the entire joint including the inner cylindrical member layer is 3
An electric fusion-bonded joint, characterized by comprising a multilayer resin layer of at least one layer. 2. A joint having a nominal diameter of 75 A or more, wherein the cylindrical member layer of the entire joint is composed of 3 to 5 multilayer resin layers including the inner cylindrical member layer. 1. The electric fusion joint according to 1. 3. The wall thickness of the outer cylinder member layer of the joint is 10 mm.
In the joint having the above size, the cylindrical member layer of the entire joint is composed of 3 to 5 multilayer resin layers including the inner cylindrical member layer, and the thickness of each layer is 15 mm or less. The electric fusion joint according to claim 1 or 2. 4. A method for obtaining an electric fusion joint in which a heating wire is embedded in an inner peripheral portion of a thermoplastic resin joint body, wherein an inner tubular member accommodating a heating wire forming an inner peripheral surface of the joint body is firstly provided. And install this inner cylinder member in the cavity of the molding die,
A thermoplastic resin layer is formed on the outer surface of the inner cylinder member to provide an intermediate member integral with the inner cylinder member, and the intermediate member is mounted in the cavity of the molding die,
A thermoplastic resin layer is molded on the outer surface of the intermediate member to provide a molded member integrated with the intermediate member, and then the thermoplastic resin layer is sequentially molded on the outer surface of the molded member to obtain an electric fusion joint having a final target shape. A method for manufacturing an electric fusion splicing joint, comprising: 5. The molding die has one set or a plurality of sets,
It is characterized in that the step of obtaining the inner cylindrical member, the step of obtaining the intermediate member, and the step of successively obtaining the molded member are performed in synchronization with each other in the cavity portion of the mold to obtain the electric fusion-bonded joint having the final target shape. The method for manufacturing an electric fusion joint according to claim 2. 6. An injection molding die for obtaining an electric fusion joint in which a heating wire is embedded in an inner peripheral portion of a thermoplastic resin joint body, wherein a plurality of cavities capable of injection molding molten resin using the same mandrel. The cavity portion has a first-layer cavity portion that forms an inner cylindrical surface of the joint and that forms a groove for accommodating the heating wire, and a cavity portion that is one size larger than the cavity portion. A second layer cavity portion capable of injection molding a thermoplastic resin layer on the outer surface of the tubular member, and third to fifth cavity portions capable of sequentially injection molding the thermoplastic resin layer on the molded outer surface, An electrofusion-joint injection-molding die, characterized in that the thickness of each layer is 15 mm or less.