JPS6314819B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming connections or terminals for rubber or plastic insulated cables, and more particularly to a method for removing residual strain in a mold by thermal crosslinking.

In the formation of a mold part that includes a heating crosslinking process, the mold part thermally expands due to heating, and thermally contracts in a cooling process after completion of crosslinking. If this thermal contraction does not occur freely, so-called residual strain occurs in the molded portion, causing problems in electrical and mechanical properties. As the size of the mold section increases, this problem becomes more serious, so it is desirable to eliminate residual strain.
Conventionally, this residual strain has been removed by slowly cooling the entire mold part, but this method requires a long cooling time, which increases the overall working time.

The purpose of the present invention is to solve the problem of working time due to residual strain removal, which was conventionally performed by slow cooling, by employing forced cooling of the bull part of the mold part, and to eliminate residual stress that cannot be obtained by conventional slow cooling. The objective is to provide a method that allows complete removal.

According to the present invention, the above object is achieved by selectively heating and cooling the mold for a plurality of sections depending on the wall thickness of the mold part. That is, cooling conditions are determined to minimize residual strain in the mold portion, the mold is divided based on the cooling conditions, and cooling is performed by giving independent cooling conditions to each of these divisions. In addition, residual strain is often caused by freezing the strain at a temperature near the melting point of the mold material, but according to the present invention, for this reason, in the cooling process after the completion of the crosslinking reaction, each part of the mold is brought to a temperature near the melting point as quickly as possible. The mold is cooled to a temperature distribution that minimizes distortion near the melting point, and after the mold temperature has decreased to the complete solidification temperature, the entire mold is cooled again.

Ethylene-propylene copolymer, polyethylene, etc. are generally used as the heat-crosslinking material used for molding the end portion or connection portion of rubber/plastic insulated cables, and the molded portion is formed by wrapping it with tape, extruding it into a mold, or injecting it. be done. This is the same in the present invention, but the temperature distribution in the mold part is controlled using various heaters and cooling pipes provided in the mold, and the temperature control of the cable conductor, which is the central part, is performed using the conventional method. This is done using an induction heating coil and cooling device placed outside the mold. In addition, it is effective to use forced air cooling for forced cooling of the entire mold.

The method of the present invention will be explained in detail below using an apparatus for implementing the method of the present invention shown in the drawings.

FIG. 1 is a partial cross-sectional view of an apparatus in which the present invention is applied to forming connections for rubber-plastic insulated power cables. In FIG. 1, the insulating layer 3 at the end of the cable is peeled off to expose the conductor 1, the conductors are connected to each other by a connecting tube 2 to form a connection part, and this connection part is placed in a mold 4, made of polyethylene, for example. An insulator 8 is placed in the mold. The mold 4 has an axially symmetrical and laterally symmetrical structure, and is divided into A and B sections depending on the thickness of the molded part to be formed. Each section has a heating device 5 and a cooling device 6, respectively.
are arranged and these heating and cooling devices are controlled separately from those of the other sections.

In the outer section C of the mold 4 there is an induction heating coil 7 for heating the cable conductor 1 and a device 5 for cooling it.
are provided, and these are controlled separately from those of sections A and B.

First, suppose that the mold 4 and the conductor 1 are heated to 180° or more as usual with the heating device 5 and the induction coil 7 to complete the crosslinking of the mold material. The power to the coil 7 is cut off to stop heating. At the same time, the cooling device 6, which is a water-cooled pipe for example, is activated throughout the mold, and cold air is blown from the outside of the mold 4 to forcibly cool the entire mold to about 120°C, which is close to the melting point of the mold material made of polyethylene. Make it so that

Once this temperature has been reached, only the cooling device 6 in section A is deactivated, and cooling of sections B and C continues while maintaining the temperature of that section appropriately at this value. By cooling the C section, the conductor 1 of the cable in that section will be cooled, and therefore the conductor 1 in the A and B sections will also be cooled, and the molding material in those sections will also be cooled from the inside, especially in the B section. In the section, cooling progresses rapidly together with cooling from the outside by the cooling device 6, causing the material 8 to shrink and solidify. At this time, material 8 of category A
Since the material 8 is still kept in a molten state, the shrinkage of the material 8 in section B is compensated by the inflow of material from section A and is solidified without distortion.

After the solidification of the mold material 8 in section B is completed, cooling of section A, where the mold material has a smaller wall thickness and therefore has a smaller amount of shrinkage, is restarted by the cooling device 6. As a result, the mold material in category A also shrinks and solidifies, but as mentioned above, the amount of shrinkage is small, so residual strain can be suppressed to a small level.

When the overall temperature drops to 100°C, which is the solidification temperature, the entire mold is again forcedly cooled to room temperature, completing the connection formation.

As described above, by solidifying the thick part from the center and compensating for the shrinkage during solidification from the periphery, a molded part with extremely low residual strain can be formed. In this case, since the solidification shrinkage of the thicker portion is supplied from the thinner portion, the thickness of the thicker portion tends to be smaller than planned. Therefore, it is effective to set the thickness of this portion somewhat larger in advance. Further, in order to smoothly compensate for the shrinkage from such thin portions, it is preferable to heat section A when cooling section B. Further, it is extremely easy to automatically perform heating and cooling of each part according to a preset program. Further, when the molding material is supplied to the mold by extrusion or injection, it is preferable to provide a supply port in section A and replenish the shrinkage from there.

In the connections or terminals formed in this way, no cracks, voids, or peeling at interfaces due to residual strain were observed, and electrode non-heating of the internal and external semiconducting layers did not occur. Also, no other deterioration in insulation performance due to residual strain occurs.
The molded part produced by the method of the invention is therefore extremely reliable and also extremely time-efficient.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram of an apparatus for carrying out the method of the present invention. 1... Cable conductor connection part, 2... Cable conductor,
3... Crosslinked polyethylene insulator, 4... Mold, 5... Heating device, 6... Cooling device, 7... Induction heating coil, 8
...Mold material.

Claims (1)

[Claims] 1 Place the molding material in the mold in which the cable terminal or cable connection part is placed, heat the entire mold and the cable part near the mold part to the crosslinking temperature of the molding material, and then stop heating the entire mold and the cable part. Then, the entire mold and the cable part are forcibly cooled to a predetermined temperature near the melting point of the mold material, and the temperature of the part of the mold corresponding to the thinner part of the mold is maintained at the predetermined temperature, while that part is forcibly cooled. As the cable portion cools, the thicker part of the mold is further cooled from the inside through the conductor of the cable to solidify the mold material in that part, and then the thinner part of the mold is cooled. A method for removing residual strain in a mold part by heating crosslinking for forming a cable terminal or cable connection part, the method comprising restarting forced cooling of a mold part corresponding to the part to solidify the mold material in that part.