FI94106C - Method and apparatus for heating a plastic tube from an extruder - Google Patents

Description

94106

The present invention relates to a method according to the preamble of claim 1 for heating a plastic pipe coming from an extruder for crosslinking.

The invention also relates to an apparatus for applying the method according to the preamble 10 of claim 5.

When making a plastic pipe with an extruder, the production line usually consists of an extruder, a pipe calibration and cooling equipment, a drawing device, and a sawing device 15 and a winding device. In this case, calibration and cooling follow the extrusion head of the extruder almost immediately, and there are no problems in this respect.

Some pipe manufacturing processes, such as the production of crosslinked polyethylene pipes, require the pipe product to be heated after the extruder. This is because the crosslinking reaction requires a higher temperature than the plastic mass at the end of the extruder, because the crosslinking additive must not decompose before the forming step at the press end of the product.

One method of providing additional post-extruder heating is disclosed, for example, in SE 7116207-7. In this method, the tube is supported 30 internally by a core extending the tool torpedo and:. . from the outside with a steel pipe the outside diameter of the plastic pipe. Both the steel pipe and the core are similar in size to a plastic pipe and can both be heated.

35 However, such an arrangement creates strong friction on the one hand between the inner surface of the plastic tube and the core 2 H i 06 and on the other hand at the interface between the outer surface of the plastic tube and the inner surface of the steel tube. The increasing back pressure, in turn, impairs the ability of the extruder to push the tube at high speed. Attempts have been made to eliminate this disadvantage by pump-5 racking of, for example, silicone oil or other lubricant to these interfaces, but in this case difficulties are encountered in cleaning the pipe.

Another known method of heating a pipe is an oil-10 bath in which the pipe is heated to the temperature required for crosslinking. Such an oil bath has to be made very long because the plastic is a poor heat conductor, which makes the heating time long. Since the length of the oil bath sets a limit to the manufacturing speed of the pipe, the length of the bath 15 must be increased as the speed is increased.

Unlike the previous method, the heating required for crosslinking can also be performed as a separate step in an autoclave after tube fabrication. However, this makes the pipe manufacturing process slow and expensive.

In general, it can be said that the need for additional heating required for cross-linking causes many problems that do not occur in the manufacture of a normal * 25 pipe.

Finnish publication 63540 describes a method by means of which the disadvantages of the previous methods can be eliminated and a process for the manufacture of a crosslinked PE pipe 30 can be obtained, which corresponds to the normal production of pipe 1. This method is based on the idea that, after the die of the extruder, a plastic tube is threaded into the grooves of the heating wheels of the crosslinking unit, which are heated by means of heating resistors placed on the inner surfaces of the wheels. As the tube rotates the heating wheel, half of the outer surface of the tube is against the wheel and heats up efficiently, and the opposite side is in the ambient atmosphere, so that no heating occurs. Thus, the heating of the tube is intermittent on its opposite surfaces and the tube does not heat as evenly as would be desirable.

5

Another disadvantage of this heating method is the rather complicated and expensive structure of the heating device. The heating wheels are heated by electrical resistors placed inside the wheels, which are arranged in several pieces on the circumference of the wheel 10. In order to cause the circumference of the wheel to heat up fairly evenly, the distance between the resistors must not be very large, so that their number naturally becomes large. Attaching the resistors to the surface of the wheels must be done carefully, as the resistor must be tightly against the surface of the wheel 15 to ensure good thermal conductivity.

Also, no thermal stresses must be generated between the resistors and the wheel that could detach the resistor from the wheel. The electrical power required by the resistors must be applied to the rotating heating wheel through a sliding contact, which naturally wears out and becomes dirty, thus increasing the need for maintenance of the device.

For the heat treatment of web-like materials, special. electrically heated as described above. Instead of electric heaters, gases or liquids are also used to heat the rollers. However, these devices as such are not suitable for pipe handling.

It is an object of the present invention to provide an improved method and apparatus for operating with heating wheels. to heat the pipes.

The invention is based on the fact that the heating wheels are heated by introducing a hot inert gas inside them, which is further introduced into the enclosed space surrounding the heating wheels in order to create a shielding gas atmosphere.

94106 4 More specifically, the method according to the invention is characterized by what is set forth in the characterizing part of claim 1.

The apparatus according to the invention, in turn, is characterized by what is set forth in the characterizing part of claim 5.

The invention provides considerable advantages. Thus, due to the method 10, the heating takes place evenly and the pipe to be treated is surrounded by a constant temperature throughout the heating. A shielding gas atmosphere is automatically created around the pipe and no separate shielding gas equipment is required. Conducting gas on a rotating heating wheel is considerably easier than the structure of electricity and heating wheels is simplified because the heating elements are omitted. There is an advantage to a simpler structure, especially when the diameter of the pipe to be manufactured changes so much that the heating wheels have to be replaced. The heating unit-20 requires little space, because the pipe to be manufactured travels a long distance through the heating wheels and the heating time is made even at a fast extruder production speed.

The invention will now be described in more detail with reference to an exemplary embodiment according to the accompanying drawing.

Figure 1 shows an apparatus according to the invention in a side view.

20

Figure 2 shows a top view of the apparatus of Figure 1.

Figure 3 shows on an enlarged scale a tube in one heating groove in a cross-sectional view.

In the exemplary case according to the drawing, the plastic tube 2 coming from the extruder li 35 94106 5 1 is threaded through two cylindrical heating wheels 4, 5. These heating wheels, which belong to the crosslinking unit 3 of the plastic tube 2, are each provided with four circumferential parallel grooves 8 in their outer circumference and the wheels are placed in a closed chamber 3. The plastic tube is threaded through the heating wheels 4, 5 so as to alternately rotate the first wheel. 4 in one first groove 8. After rotating the wheel 4 by about 90 'on its upper circumference 10, the tube 2 in turn moves in a corresponding manner with the second wheel 5. The tubes 2 begin to rotate in the groove 8 of the wheel 5 at the lower circumference of the wheel 5, rotating by about 270 ° to move from the upper circumference of the wheel 5 back to the groove 15 adjacent to the inlet groove of the wheel 5 in the groove 8. It is understood that directions.

The heating wheels 4, 5 are mounted on parallel axes 70 and 6 at a distance of less than one wheel diameter from each other. The cross-section of the grooves 8 corresponds approximately to the cross-section of the side of the plastic pipe 2 to be heated (in the example case, a semicircular cross-section), whereby due to the surface contact the heat transfer is * 25 efficient.

The number of heating grooves 8 depends on the size of the production and, on the other hand, on the temperature used to achieve a sufficient degree of crosslinking. The number of parallel grooves 8 in each wheel 4, 5 can, for example, vary in the range 2 ... 10.

According to the invention, the heating wheels 4, 5 are heated by an inert hot gas. The heated gas is introduced into the heating wheels 4, 5 via their hollow support shafts 35 6. Inside the heating wheels 4, 5, gas flows into the enclosed chamber 3 through holes 7 6 94106 drilled in the bases between the grooves 8 of the wheels, whereby a hot inert gas atmosphere is formed inside the chamber. The gas atmosphere sphere prevents the surface of the plastic pipe from oxidizing in the heat and keeps the surfaces of the pipe facing outwards from the grooves 8 of the heating wheels 4, 5 warm. This enhances the heating effect and the heat distribution in the cross-section of the pipe is more even. From the heating wheels 4, 5, the pipe 2 passes through the guide wheels 9 for calibration and cooling 10.

Due to the structure of the heating equipment, different heating wheels 4, 5 heat the opposite sides of the pipe 2 so that even heating is achieved. This uniformity of heating is further enhanced by the fact that the pipe 2 moves from one wheel 4, 5 to another several times (the number of grooves 8 is in the case 4 of Example-15) and the pipe is constantly at a constant elevated temperature.

Within the framework of the invention, it is also possible to consider solutions that differ from the • embodiment described above. Thus, the number of heating wheels 4, 5 may be more than two, but for natural reasons it is preferable to be even.

The cross-section of the grooves 8 can, of course, deviate from the shape of the semicircle. It is essential that the pipe 2, when in the groove 8, follows the wall of this. The wheels 4, 5 can be freely rotatable due to the friction effect of the tube 2 or, for example, one of them can be motor-driven. The heating gas is most preferably introduced inside the heating wheels through their shafts or alternatively through the end faces of the wheels. In the enclosed space 3, the gas can further be led through the ends of the wheels instead of the casing surface. or through holes formed in the parts of the support shafts remaining inside the housing forming the closed space 3. The support shafts can advantageously be mounted only at one end, in which case the other side 35 of the housing 3 of the device can be opened to facilitate maintenance.

li 94106 7

It should be mentioned that an inert gas can also be used inside the pipe 2 to prevent oxidation. It is preferred to use nitrogen as the heating and shielding gas, but other inert gases, for example noble gases or carbon dioxide, are also well suited for this purpose.

Claims (9)

A method of heating a plastic tube (2) coming from an extruder (1) by contacting its outer surface with a solid material heater (4 - 8), by which method the plastic tube (2) is brought by means of at least two rotating heating wheels (4, 5), the sheaths of which are provided with latches (8) in the circumferential direction, so that the plastic tube (2) is rotated in turn with one wheel (eg 4) in a groove (8), wherein the plastic tube (2) is substantially heated on the side which lies against the groove (8), and is alternately moved to rotate correspondingly with the second wheel (e.g., 5) rotating in the opposite direction; the opposite side of the tube (2), in turn, coming into contact with a corresponding second spar (8), and in which process the heaters (4, 5) are heated by the introduction of heated inert gas, characterized in that the heating gas for effecting of a heated protective gas atmosphere is passed from the interior upwards the heating means (4, 5) into a closed space, which surrounds the heating means (4, 5) and the part of the pipe (2) to be heated. Method according to claim 1, characterized in that the inert gas is fed into the heating means (4, 5) via the hollow axes (6) of the means and further into the closed space via openings (7) in the men of the heating means ( 4, -5). 30 Method according to claim 1, characterized in that the inert gas is fed into the heaters (4, 5) via openings in the end surfaces of the heater, and further into the enclosed space via openings (7) in the men of the heaters (4, 5). Process according to any one of claims 1-3, 94106 characterized in that nitrogen is used as gas. Apparatus for heating a plastic tube (2) coming from an extruder (1), comprising 5 - at least two sprocket wheels (4, 5), which are stored for rotation in opposite directions at the eatmint tone in the near parallel axis. (6) at least substantially opposite to each other at a distance from each other and whose sheaths are provided with circumferential latches (8), which latches (8) to their average at least approximately correspond to the average of the side of the plastic tube (2) to be heated, and 15. a housing (3) surrounding the heaters (4-5) and the portion of the tube (2) to be heated forming a closed space, and - means (6) for introduction of a heated inert gas 20. the heating means (4-5) for heating them, characterized by means (7) for introducing the inert gas from the interior of the heating means (4, 5) in the enclosed space (3) for formation of a hot protective gas atmosphere inside this closed space. Device according to claim 5, characterized by the hollow shaft (6) of the heater (4, 5), through which the gas can be led into the heater. Device according to claim 5, characterized by openings in the end surfaces of the heater (4, 5), through which the gas can be led into the heater (4, 5). y 41 06 Device according to any of claims 5-7, characterized in that the openings (7) formed (7) for the passage of the gas from the interior of the heater (4-5) into the casing of the heater (4 - 5) enter into it. enclosed space (3), which surrounds the same. Device according to any of claims 5 to 7, characterized by openings formed in the end surfaces of the heater (4-5) for conducting the gas from the interior of the heater (4-5) into the enclosed space (3), which surrounds the same.