JPH08203358A - Method for extruding crosslinked-polyethylene admixture - Google Patents

Abstract

PURPOSE: To provide a method for extruding a crosslinked-polyethylene admixture, most suited for use in extrusion molding the insulator of an extra-high voltage CV cable. CONSTITUTION: In an extruding machine, in which a screw having a supply portion S, a compressing portion C, a measuring portion M1 and a mixing device portion MD is placed within a barrel, when the screw diameter is D the length Ls of the supply portion S of the screw is 5 to 8D and multiple threads with a pitch P of 1.0 to π D are formed therein. A heating zone sectioned longitudinally into 4 to 8 zones is provided on the barrel side, these heating zones Z1 to Z5 are independently temperature-controlled, and the screw thus heated and held at or above the melting point of a crosslinked-polyethylene admixture is rotated at low speeds of about 6 to 16rpm to extrude the crosslinked-polyethylene admixture.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for extruding a polyethylene mixture for cross-linking, which is suitable for extrusion molding of an insulator of an ultra high voltage CV cable.

[0002]

2. Description of the Related Art A cross-linked polyethylene cable uses a cross-linked polyethylene (XLPE) having a three-dimensional network structure formed by a cross-linking reaction to improve heat resistance as a cable insulating layer. Since it has a large capacity, excellent physical and mechanical characteristics, and disaster prevention, and is easy to handle and maintain, it is widely used for power cables of hundreds of volts to hundreds of thousands of volts. Among them, the chemically cross-linked polyethylene, which cross-links polyethylene with a cross-linking agent such as organic peroxide, can be applied to thick-walled insulators, so an ultra-high voltage CV cable using this is a conventional OF cable. Instead, it is being adopted one after another in the mains of electric power. The ultra-high voltage CV cable used for such an important power trunk line will have a serious impact on consumers and society in the event of dielectric breakdown, so the cable quality will be reliable. Is required,
In the manufacturing, it is necessary to adopt the optimum technology and strictly control the process. Usually, an ultra-high voltage CV cable is obtained by extrusion-coating an inner semiconductive layer on a cable conductor, extruding and coating a polyethylene mixture for cross-linking on the outer side thereof to form an insulating layer, and further extruding and coating an outer semiconductive layer on the outer side thereof. After that, these are heated in a cross-linking tube, the cross-linking agent added to the inner / outer semiconductive layer and the polyethylene mixture for cross-linking is reacted to cross-link the polyethylene, and then a shielding tape is wound around the outer side of the cable, It is manufactured by coating a sheath and an anticorrosion layer.

There are many possible causes for deterioration of the ultra-high voltage CV cable in use, resulting in dielectric breakdown.
Particularly, the most important factor related to the step of extruding the insulating material is caused by foreign matters and burns mixed in the insulating material. Concerning foreign matter, it is possible to prevent foreign matter from entering by properly managing foreign matter during the material manufacturing and use process, and even if foreign matter is present in the feed material, the breaker of the extruder can be used. By making the mesh of the screen attached to the front surface of the plate fine to about 500 to 1,000 mesh, it is possible to surely remove the foreign matter having a size larger than a certain value.
On the other hand, burns are likely to occur when the cross-linking polyethylene admixture stays in the extruder for a long period of time, or when temperature control or pressure distribution is inappropriate, and if this mixes in the cable insulation, voids and Since it causes microcracks and impairs the insulation characteristics of the cable, it is necessary to prevent it as much as possible.

A single-screw extruder with one screw in the barrel is usually used for extrusion coating the polyethylene blend for cross-linking in ultra high voltage CV cables. There are various types of this single-screw extruder, but those generally used are: screw length L = 22 to 38D, screw pitch P = 1.0D, supply section length Ls = 5, where D is the screw diameter. .About.8D Compressing portion length LC = 10 to 15D Measuring portion length Lm = 7 to 15D Measuring portion groove depth hm = 5 to 12 mm Compression ratio C = about 2.5 to 3.5. These dimensions appear to have been determined based on long experience.

[0005]

As described above, in order to prevent foreign matter from entering the extruder, a screen of about 500 to 1,000 mesh is attached. The machine head pressure is 100 to 200 including the resistance of the crosshead.
It reaches about bar. When the head pressure of the extruder is high,
The backflow in the extruder increases, the shear strain rate increases, the resin temperature rises, the screw torque increases, and the resin temperature rises even after leaving the extruder. As a result, the resin is scorched, and not only does it cause discoloration, but also it may cause problems such as separation of the components of the compound. On the other hand, since the cross-linking polyethylene admixture contains a cross-linking agent such as DCP, the resin temperature has an allowable upper limit, and above this temperature, the resin is burned. Therefore, it is desirable that the polyethylene mixture for cross-linking be slowly extruded at a temperature near the melting poit. Moreover, long-term retention in the extruder must be absolutely avoided.

The prior art has not sufficiently examined these problems. As a result, burns may be mixed in the extruded cross-linked polyethylene mixture, and the cable in use may deteriorate in a relatively short period of time, leading to dielectric breakdown. As a result of various experiments and theoretical studies on the above problems, the present inventors have found an extrusion method of a polyethylene mixture for cross-linking, which is optimal for extrusion molding of an insulator of an ultrahigh voltage CV cable.

[0007]

A method for extruding a polyethylene mixture for cross-linking according to the present invention comprises a supply section S, a compression section C, and
In an extruder in which a screw equipped with a measuring unit M1 and a mixing device unit MD is arranged in a barrel, when the screw diameter is D, the length Ls of the screw supply unit is 5 to 8D and the pitch is Is 1.0 to πD
Multiple heating screws are formed on the barrel side, and heating zones divided into 4 to 8 zones are provided on the barrel side in the lengthwise direction. These heating zones are temperature-controlled, and the screws are rotated at a low speed to mix polyethylene for crosslinking. The main feature is to extrude a product.

[0008]

According to the present invention, it is possible to extrude a cross-linked polyethylene admixture free from burns and foreign matter.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. The extruder used in the present invention is a so-called single-screw extruder in which one screw is arranged in the barrel, and the screw has a feeding portion S from the gland portion 1 to the tip portion 2 as shown in FIG. , A compression section C, a measuring section M1, a mixing device section MD, and a boosting section M2 are formed in this order. The dimensions of each of these parts are as follows.
The preferred size range is shown in parentheses. Screw diameter D = 175 mmφ Screw length L = 24 D (22 to 28 D) Screw pitch P = 1.2 D (1.0 to πD) Groove depth hm1 = 8 mm (7 to 12 mm) hs = 22 mm (17.5 to 40 mm) ) Supply section length Ls = 6D (5-8D) Compressing section length LC = 9D (6-12D) Measuring section length Lm1 = 5D (4-7D) Mixing device section length LX = 2D (2-3D) Boosting section length Lm2 = 2D (1- 3D) Further, on the barrel side, from the inlet of the supply section S to a pitch of 1 to 2 or more, and from the outlet end of the booster section M2 to the heating zone Z1 to
Z5 is provided, and each set temperature is controlled independently. In the present invention, a screen of 500 to 1,000 mesh is attached to the front surface of the breaker plate of the extruder, and the screw is heated and held at a temperature not lower than the melting point of the polyethylene mixture for crosslinking, specifically about 110 ° C., Rotate at a low speed of 6 to 16 rpm and slowly extrude the cross-linking polyethylene admixture at a temperature slightly higher than the melting poit.

Next, the function and configuration of each part of the screw and the temperature control on the barrel side will be described. (1) Supply unit S The supply unit S pushes pellets of the polyethylene mixture for crosslinking sent from the hopper into the extruder and presses the pellets against the barrel wall to dissolve them. However, in this case, only the pellet that contacts the barrel wall will melt. The melted resin adheres to the barrel surface in a film shape, but if this exceeds a certain thickness, the screw flight will be scraped off.
This molten resin goes around the screw channel and becomes a heat source for melting the pellet. In the present invention, the supply section S has a length Ls of 5 to 8D and a screw pitch PS of 1.0 to .pi.D.
And the screw is a multiple screw of 2 to 5 layers. Originally, the resin has poor thermal conductivity, and even if the resin is heated on the barrel surface, only the temperature of the resin in the vicinity thereof rises. Therefore, the resin having a high temperature scraped off is convected to raise the temperature of the resin in other portions. In the present invention, as described above, the screw pitch PS is made long, and the screws are multiplexed and scraped off to shorten the pitch, so that the heat exchange with the barrel becomes active and the convection of the resin becomes active. As a result, the resin temperature rises faster. The temperature on the barrel side is not heated at the inlet of the supply section, and a heating zone Z1 is provided from a position separated by 1 to 2 pitches. Set temperature is 110
Approximately 130 ° C. Except for a large-sized extruder, this portion does not necessarily have to be heated by a heat medium such as oil, but may be heated by an electric heater.

(2) Compressing part C Here, the air that fills the space between the pellets while the semi-molten pellets are melting is extruded rearward by the compressing action, and degassing and melting of the pellets are performed concurrently. It is ideal that all the pellets are melted in this portion, but XLPE cannot keep the heating temperature of the barrel high, so that it may remain slightly in the next measuring unit. In particular, in the present invention, the screw rotation speed of the extruder is kept low at about 6 to 16 rpm,
Since the shear heat generation is small, the pellet burn-through tends to be delayed. Therefore, in order to compensate for this, the compression portion C has a length Lc.
Is set to 6 to 12D and the pitch PC is set to 1.0 to πD, and the screw screw in the front stage about 1/3 part of the compression section C is set to 2 to 5 multiple screws to reduce the scraping pitch of the barrel surface. There is. The screw in the latter half of the compression section C, which is about 2/3, is a single screw. On the barrel side, a heating zone Z2 with a heating medium is provided over the entire length of the compression section C, and heating is performed at about 120 to 130 ° C. It should be noted that this part allows for finer temperature control,
It may be divided into three or three blocks, and temperature control may be performed separately.

(3) Measuring unit M1 Here, the extrusion pressure is increased and the extrusion amount is controlled. Since the channel depth is shallow, the resin is repeatedly subjected to shear deformation and kneaded well, but since the heat generation is large, scorch tends to occur. In the present invention, this portion has a length of 4 to
7D, the screw is a multiple screw of 2 to 4 layers, pitch P
By setting m1 to 1.0 to πD and enhancing the cooling effect from the barrel wall, the generation of scorch is prevented and the appropriate extrusion temperature, pressure and extrusion amount are adjusted. On the barrel side, the heating zone Z by the heat medium is provided over the entire length of the measuring section M1.
3 is provided and the set temperature is set to about 110 to 125 ° C. In addition, since this part also enables finer temperature control,
It may be divided into two blocks in the length direction and the temperature may be controlled separately.

(4) Mixing device part MD In the present invention, the screw rotation speed of the extruder is kept low, and the kneading of the measuring part M1 is not sufficient, and the resin temperature raised in the measuring part M1 is sufficient. In order to reduce the noise, the mixing device section MD is provided. This mixing device unit MD has a length of 2 to 3D and a pitch of 1.
5 to 5D, as shown in FIG. 2, the inlet groove X1 and the outlet groove X
Two are provided adjacent to each other. In FIG. 2, the grooves X1 and X2 are shown parallel to the screw axis for the sake of simplicity, but they are actually spiral and the total number of grooves is 10 or more on the inlet side and the outlet side. This is to activate the rotational flow of the resin to increase the temperature control effect. In the mixing device MD, the pitch is preferably small in order to facilitate the passage of the resin from the inlet groove X1 to the outlet groove X2, but it is preferably large in order to improve the temperature control effect. Therefore, in the present invention, the pitch of the mixing device unit MD is set to 1.5 to 5D as described above to achieve the balance between kneading and cooling. A heating zone Z4 by a heating medium is provided on the barrel side over the entire length of the mixing device MD, and the set temperature is set to about 110 to 125 ° C.

(5) Boosting unit M2 The boosting unit M2 is also called a second measuring unit and has a function of boosting and rectifying the resin. In the present invention, it is provided as necessary. This booster M2 has a length Lm2 of 1 to 3
D, the pitch Pm2 is 0.4 to 1.6D, and the screw screw is single. The flow rate of the resin is adjusted by the depth of the screw channel, but when the depth of the channel increases, the shear rate decreases and the heat generation of the resin decreases. A heating zone Z5 with a heating medium is provided on the barrel side over the entire length of the booster section M2, and the set temperature is set to about 110 to 125 ° C.

In the above, the temperature difference in the circumferential direction in each part of the barrel is controlled to be as small as possible (preferably 0.5 ° C. or less). FIG. 3 shows the set temperature on the barrel side and the barrel temperature in each of the above-mentioned screw parts, and the temperature distribution of the polyethylene mixture for crosslinking that passes through the barrel under these influences. As is clear from this figure, the resin temperature of the polyethylene mixture for cross-linking gradually rises from the middle of the supply section S to the measuring section M1.
Although the maximum temperature is reached at the exit end of, the temperature of each part is set so that it reaches a temperature slightly above the melting point of the resin. As described above, in the present invention, the feeding section S, the compression section C and the measuring section M1 are attached to the screw of the extruder for extruding the polyethylene mixture for crosslinking.
, The mixing device MD, and the booster M2 as required, and the optimum pitch and number of screw screws are provided for each part, so that the amount of resin extruded increases and kneading is sufficiently performed. Moreover, since the heat exchange between the resin and the barrel is actively performed, the quality is stable. Further, the barrel side is also divided into heating zones of 4 to 8 parts so that the temperature of each zone is controlled independently. Therefore, in order to guarantee the size of foreign matter and dust, 5
Even if a filter of 00 mesh or more is used, the rotation speed of the extruder can be suppressed low. When the rotation speed is reduced, the shear strain rate decreases, but the heat generation is proportional to the square of the shear strain rate, and the temperature rise is inversely proportional to the extrusion rate (proportional to the rotation rate). ,
The temperature of the resin also drops. Therefore, the polyethylene mixture for cross-linking can be easily extruded at a temperature slightly lower than the melting point of the resin, and the polyethylene mixture for cross-linking with less foreign matter or burns and less flow marks can be stabilized. It is possible to perform extrusion molding for a long time while maintaining the extruded amount.

In the present invention, the heating zone Z1
The dividing positions of Z5 to Z5 are, as described above, the screw supply section S, compression section C, measuring section M1, mixing device section M.
It is not always necessary to provide them corresponding to the respective boundary portions of D and the boosting portion M2, and they may be provided at positions appropriately shifted.

[0017]

EFFECTS OF THE INVENTION According to the present invention, a cross-linking polyethylene admixture free from foreign matters and burns and having few flow marks can be extruded for a long period of time while maintaining a stable extrusion amount. It can contribute to the quality improvement of the cable.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of each part of a screw of an extruder used in the present invention and an arrangement example of a heating zone on a barrel side.

FIG. 2 is an explanatory diagram showing a configuration example of a mixing device portion of a screw of an extruder used in the present invention.

FIG. 3 is a graph exemplifying a barrel-side set temperature and a barrel temperature in each part of the screw and a temperature distribution of the cross-linking polyethylene mixture passing through the barrel.

[Explanation of symbols]

 1 ... Gland part 2 ... Chip part S ... Supply part C ... Compression part M1 ... Measuring part MD ... Mixing device part M2 ... Boosting part X1 ... Inlet groove X2 ... Outlet groove Z1 to Z5 ... Heating zone.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H01B 9/00 A // B29K 23:00

Claims (8)

[Claims] 1. An extruder in which a screw including a supply unit, a compression unit, a measuring unit, and a mixing device unit is arranged in a barrel, and a screw diameter is D,
The length of the screw supply part is 5 to 8D, and the pitch is 1.0 to πD (where π represents the circular constant).
same as below. ) Is formed, and a heating zone divided into 4 to 8 zones is provided on the barrel side in the length direction thereof, the heating zones are temperature-controlled, and the screw is rotated at a low speed to crosslink polyethylene. A method for extruding a polyethylene mixture for cross-linking, which comprises extruding the mixture. 2. The length of the compression part of the screw is 6 to 12D.
The pitch is 1.0 to π in the 1/3 part of the previous stage.
Form multiple threads of D, and pitch is 1.0 in the rest.
It is characterized by forming a single screw of ~ πD, providing a heating zone on the barrel side with a heating medium over the entire length of the compression section, and extruding the polyethylene mixture for crosslinking at a preset temperature of about 120 to 130 ° C. A method for extruding the polyethylene mixture for crosslinking according to claim 1. 3. The length of the measuring portion of the screw is set to 4 to 7D, a multiple screw having a pitch of 1.0 to πD is formed therein, and a heating zone by a heating medium is provided on the barrel side over the entire length of the measuring portion. The method for extruding a polyethylene mixture for cross-linking according to claim 1, which is provided and extrudes the polyethylene mixture for cross-linking at a preset temperature of about 110 to 125 ° C. 4. The length of the mixing device portion of the screw is 2 to 3D, the screw pitch is 1.5 to 5D, and the barrel side is provided with a heating zone by the heating medium over the entire length of the mixing device portion, and the set temperature thereof. To 110
The method for extruding a cross-linking polyethylene admixture according to claim 1, wherein the cross-linking polyethylene admixture is extruded at about 125 ° C. 5. A step-up portion having a length of 1 to 3D is provided at the tip of the mixing device portion of the screw, a single screw having a pitch of 0.4 to 1.6D is formed there, and a step-up portion is provided on the barrel side. 2. A cross-linking polyethylene admixture is extruded by providing a heating zone with a heating medium over the entire length of the section and setting the set temperature to about 110 to 125.degree.
A method for extruding the polyethylene mixture for cross-linking according to 1. 6. The screw supply section has a length of 5 to 8D, a multiple screw having a pitch of 1.0 to πD is formed therein, and the compression section has a length of 6 to 12D. / 3
Form a multiple screw with a pitch of 1.0 to πD at
A single screw with a pitch of 1.0D to πD is formed in the remaining portion, the length of the measuring unit is set to 4 to 7D, and multiple screws with a pitch of 1.0 to πD are formed there, and the mixing device unit is formed. 2 to 3D and the screw pitch is 1.
5 to 5D, a heating zone with a heat medium is provided on the barrel side at a pitch of 1 to 2 or more from the inlet of the supply section, and the set temperature of the heating zone outside the supply section (1 to 2 pitch or more from the inlet) is 110 to 130. ℃, set the temperature of the heating zone outside the compression section to about 120 to 130 ℃, set the temperature of the heating zone outside the metering section to about 110 to 125 ℃, set the heating zone outside the mixing device section The polyethylene mixture for cross-linking according to claim 1, wherein the cross-linking polyethylene admixture is extruded at a temperature of about 110 to 125 ° C. and a set temperature of a heating zone outside the pressurizing section at about 110 to 125 ° C. 3. How to extrude a product. 7. The cross-linking polyethylene admixture is extruded by heating and holding the screw at a temperature above the melting point of the cross-linking polyethylene admixture and rotating at a low speed of 6 to 16 rpm. The method for extruding a polyethylene mixture for crosslinking according to the item 1. 8. The method for extruding a polyethylene mixture for cross-linking according to any one of claims 1, 2, 3, 6 and 7, wherein the multiple screws of the screw are 2 to 5 double screws.