NO20200657A1

NO20200657A1 - Method and apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end

Info

Publication number: NO20200657A1
Application number: NO20200657A
Authority: NO
Inventors: Tom Rønning Timeland; Lars Sigurdsen
Original assignee: Pipelife Norge As
Priority date: 2020-06-03
Filing date: 2020-06-03
Publication date: 2021-12-06
Also published as: NO346307B1

Description

Method and apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end

The present invention is related to a method for manufacturing a molded high-density polyethylene body for production of at least one stub end, according to the preamble of claim 1.

The present invention is also related to an apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end, according to the preamble of claim 9.

The present invention is especially related to a method and apparatus for manufacturing a molded high-density polyethylene (HDPE) body in one piece that may be machined to produce at least one stub end.

Background

Tubes, pipes or pipelines of larger diameter are commonly produced in high-density polyethylene (HDPE) by an extrusion process. The tubes, pipes or pipelines could have a diameter of up to several meters and lengths up to several hundred meters long. At connection of the mentioned tubes, pipes or pipelines there are welded stub ends at the ends of the tubes, pipes or pipelines before a flange is arranged over the stub ends to hold them together. The stub ends are typically mainly L-shaped tube elements, which at one end has the same diameter and thickness as the tube, pipe or pipeline, and at the other end is considerably larger in material thickness for providing attachment to the flange, which is to hold the tubes, pipes or pipelines together.

In US 4,290,456 is described several examples of thermoplastic pipe stub ends for welding to thermoplastic pipes. The stub end has a flange portion integrally joined to a neck portion having inner and outer surfaces and terminates at a welding face. In US 4,290,456 is claimed an improved stub end permitting higher strength heat welds to be made by butt fusion between the thermoplastic stub end and a thermoplastic pipe to be joined thereto.

The most common method to manufacture stub ends is by building up a very thick pipe by adding layer by layer of polyethylene, where the material is extruded against a rotating drum. The pipe is next cut in desired lengths before surplus material is machined away, such that the result is the original pipe diameter at the one end.

The manufacturing of stub ends for large pipe diameters is presently performed by building up an entire pipe, as described above, to achieve a pipe with the required diameter of the stub end, followed by cutting of the pipe and machining of the cut pipe down to the desired thickness of the stub end.

A further disadvantage with prior art solutions is that they result in weaknesses in the produced stubs end due to formation of voids (vacuum holes) and delamination due to the layer-by-layer production method.

It is accordingly a need for a method and apparatus enabling production of stub ends that require fewer production/manufacturing steps, hereunder removing the need for manufacturing a pipe to be able to produce a stub end.

It is further a need for a method and apparatus enabling production of stub ends using a lower amount of material in the production process.

A further need is to provide a method and apparatus enabling production of a required number of stub ends when needed.

It is further a need to provide a method and apparatus avoiding void formations and delamination in the stub ends.

Object

The main object of the present invention is to provide a method and apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end partly or entirely solving the mentioned drawbacks of prior art and needs identified above.

It is an object of the present invention to provide a method and apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end by few production steps.

An object of the present invention is to provide a method and apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end avoiding voids and delamination in the stub ends.

A further object of the present invention is to provide a method and apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end requiring lower investment costs, compared to prior art solutions.

An object of the present invention is to provide a method and apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end enabling production of a required number of stub ends when needed, and thus using only the required amount of material in the production process.

It is an object of the present invention to provide a method and apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end of large diameter.

An object of the present invention is to provide a method and apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end that is space-saving compared to prior art solutions.

Further objects of the present invention will appear from the following description, claims and attached drawings.

The invention

A method for manufacturing a molded high-density polyethylene body for production of at least one stub end according to the present invention is defined by the technical features of claim 1. Preferable features of the method are described in the dependent claims.

An apparatus for manufacturing a molded high-density polyethylene body for production of at least one stub end according to the present invention is defined by the technical features of claim 9. Preferable features of the apparatus are described in the dependent claims.

The method and apparatus according to the present invention is related to the manufacturing of a molded high-density polyethylene body for production of at least one stub end, and especially large diameter stub ends, i.e. with a diameter larger than 1000 mm.

The manufacturing process according to the present invention has two phases in the form of a casting process followed by a curing/hardening process.

The method and apparatus according to the present invention is based on molding a high-density polyethylene body in one piece, followed by a curing process, providing a high-density polyethylene body that by a machining process provides at least one stub end of desired dimension, tolerances and finish.

The casting process according to the present invention is based on the use of a casting mold. The casting mold is formed by at least two parts detachably arranged to each other. The at least two parts, when assembled together, form a complete casting mold of desired shape and size, such as mainly disc-shaped. The cross-sectional shape of the casting mold may be circular, rectangular, square, elliptic, triangle, polygonal or similar. The two parts are provided with corresponding fastening means for detachable attach the at least two parts together.

According to the present invention, the casting mold is arranged on edge, i.e. in an upright position, and provided with an inlet opening at one side thereof, wherein the inlet opening is arranged at a lower part of the casting mold, when the casting mold is arranged in upright position. The casting mold is preferably secured at this position.

According to the present invention, the inlet opening of the casting mold is arranged in fluid connection to an extruder by corresponding connection means. The extruder is arranged for filling molten high-density polyethylene with a desired temperature, in the range of 180-230 degrees Celsius, even more preferable in the range of 200-225 degrees Celsius, into the casting mold.

The extruder is according to the present invention designed to provide continuous supply of molten high-density polyethylene of a desired volume to the mentioned casting mold.

Accordingly, with the casting mold on edge and connected to the extruder, a continuous flow of molten high-density polyethylene is supplied to the casting mold from the bottom and upwards.

The supplied molten high-density polyethylene will, as it is supplied into the casting mold, hit the opposite side of the casting mold, rear wall seen from the inlet side. When the supplied molten highdensity polyethylene hits the rear wall, this will create a counter pressure, which results in that a balloon of molten high-density polyethylene is formed interior in the casting mold. This results in that the supplied molten high-density polyethylene is not allowed to twist (like a snake or spaghetti) in the casting mold. Due to the creation of the mentioned balloon of molten high-density polyethylene, further supplied molten high-density polyethylene will be supplied into the mentioned balloon in the casting mold, which results in that the flow of molten high-density polyethylene in the casting mold will evenly and uniformly expand both in transversal and longitudinal direction of the casting mold.

As the casting mold is filling up with molten high-density polyethylene, the self-weight of the supplied molten high-density polyethylene in the casting mold will result in sufficient compression of the molten high-density polyethylene. As there will be no need for high follow-up pressure or compression pressure, the casting mold may be designed more plain and with lower material thickness.

The use of a continuous flow of molten high-density polyethylene further results in that there will be no weld lines in the formed molded high-density polyethylene body.

According to a further embodiment of the present invention, the casting mold will be provided with at least one opening close to upper end of the casting mold, when arranged in upright position, i.e. at the opposite end of the inlet opening, to allow air to escape from the casting mold as it is filled with molten high-density polyethylene.

The mentioned opening and/or further inspection opening arranged in longitudinal and/or transversal direction of the casting mold may be used for inspection of the casting progress, as well as for reducing the flow of supplied molten high-density polyethylene when the casting mold is close to full and stopping the supply of molten high-density polyethylene when the casting mold is full.

In an alternative embodiment the casting mold is provided with at least one sensor or sensor system reading the filling degree of molten high-density polyethylene in the casting mold, as well as capable of providing a signal for automated reduction of the flow of molten high-density polyethylene from the extruder when the casting mold is close to full and a stop signal for automated stop of the flow from the extruder when the casting mold is full.

The size of the casting mold will be is adapted to the circumference of the stud end to be manufactured, both as regards diameter and thickness. To reduce the interior size/volume of the molded high-density polyethylene body and thus reduce the amount of required high-density polyethylene, the casting mold may be provided with at least one insertion element, which is adapted to be arranged in the center of the casting mold, and designed for reduction of interior size and volume thereof and thus the molded high-density polyethylene body to be manufactured.

The degree of constriction in the casting mold provided by the at least one insertion element will be dependent on the viscosity and flow properties of the high-density polyethylene that is supplied.

After the casting mold has been filled with molten high-density polyethylene, the casting mold is ready for the curing process.

According to the present invention, the extruder is disconnected from the casting mold and the casting mold is moved from the upright position to a horizontal position.

High-density polyethylene is a material that shrinks a certain amount during cool-down, after the casting process. A well-known challenge with casting/molding of thick bodies is that the body first cools down and hardens at the surface, while the core of the body is still hot. This results in that the core, as it gradually becomes colder and shrinks, will increase the tensions and one will risk having voids (vacuum holes). Such voids create weaknesses and are not allowed in stub ends. To avoid this, the cooling of the molded high-density polyethylene body will according to the present invention is performed in a controlled manner and over a long time, and wherein the temperature differences between the core and the surface the molded high-density polyethylene body is held so low, that the high-density polyethylene body is allowed shrinking freely.

As the casting mold is made with low material thickness one avoids a (too) rapid heat transfer and cool-down of the molded high-density polyethylene body. By the present invention, one achieves a slow cool-down of the molded high-density polyethylene body such that one avoids tensions from arising/building up in the molded high-density polyethylene body, as well as avoids voids from being formed in the high-density polyethylene body.

According to one embodiment of the present invention, the cool-down of the molded high-density polyethylene body is performed in room temperature, i.e. from 18 – 25 degrees Celsius. Tests made by the applicant has shown that this provides a sufficiently long cool-down time to avoid void formations in the molded high-density polyethylene body.

In an alternative embodiment of the present invention, the cool-down process/curing process is performed in a heat-controlled environment, such as a heating stove or similar, where one is able to control the environment temperature.

According to a further embodiment of the present invention, it comprises providing the casting mold with at least one sensor or sensor system for monitoring the cool-down of the molded high-density polyethylene body. According to one embodiment of the present invention, the casting mold is provided with sensors or sensor systems for temperature monitoring of both the surface and core of the molded high-density polyethylene body.

After the temperature has lowered to a desired temperature and the surface of the molded highdensity polyethylene body has hardened during an initial curing period, the casting mold is removed from the molded high-density polyethylene body for further cool-down and curing. This will typically take 1-2 days.

Before starting the machining process for production of at least one stub end, the molded highdensity polyethylene body will have to be entirely cooled down during a final curing period in horizontal position. In practice, this means 3-7 days, depending on the thickness of the molded highdensity polyethylene body.

According to a further embodiment of the present invention, it comprises using a heat-controlled environment for removing possible tensions in the molded high-density polyethylene body. By using a heating stove or similar the environment temperature and time may be controlled. E.g. by using a temperature between 40 and 100 degrees, more preferably between 70 and 90 degrees, over a time of 12-78 hours, more preferably 20-30 hours.

In a further embodiment of the present invention, it comprises rough cutting of the molded highdensity polyethylene body, before the molded high-density polyethylene body is processed in a heat-controlled environment for removing possible tensions in the molded high-density polyethylene body.

From the molded high-density polyethylene body at least one stub end may be produced by the use of prior art machining equipment, such milling, cutting or sawing or similar equipment. A molded high-density polyethylene body manufactured by the present invention may provide several stub ends of different dimensions (diameters), inside each other.

The stub ends are produced depending on dimension, tolerances and finish.

Further preferable features and advantageous details of the present invention will appear from the following example description, claims and attached drawings.

Example

The present invention will below be described in further details with reference to the attached drawings, where:

Fig.1 are principle drawing of an apparatus according to the present invention,

Fig.2a-b are principle drawings of a casting mold according to the present invention, and

Fig.3 are principle drawings of a molded high-density polyethylene body and a produced stub end.

Reference is made to Fig.1 showing principle drawings of an apparatus 100 according to the present invention.

The apparatus 100 according to the present invention comprises an extruder 110, known per se, and a casting mold 200 according to the present invention, as shown in Fig.2a-b.

The extruder 110 comprises an extrusion screw 111 arranged in a heated barrel 112, closed at one end and open at the other end, wherein the extrusion screw 111 is powered by at least one controllable electric motor 113 connected to the extrusion screw 111 at the closed end of the heated barrel 112, and further comprising baker plate 114 arranged at the outlet end of the heated barrel 112.

The apparatus further comprises a hopper 120 arranged above the barrel 112 and in fluid communication with the heated barrel 112 close to closed end thereof. The hopper 120 is feeding high-density polyethylene material in the form of powder, beads, pellets or similar, commonly referred to as resin, by gravity into the heated barrel 112 of the extruder 110, where it comes into contact with the extruder screw 111.

The extruder screw 121 is rotated by the at least one electric motor 113 at a desired rpm forcing the high-density polyethylene material forwards in the heated barrel 112. The desired extrusion temperature is rarely equal to the set temperature of the heated barrel 112 due to viscous heating and other effects. For solving this, it is common to use a heating profile set for the heated barrel 112 in which independently controlled heater zones gradually increase the temperature of the heated barrel 112 from the rear (closed end where the high-density polyethylene material enters) to the front (outlet). This allows the high-density polyethylene material to melt gradually as it is pushed through the heated barrel 112 and lowers the risk of overheating which may cause degradation in the high-density polyethylene material.

The extruder 110 may also be provided with cooling means to keep the temperature below a set temperature value if too much heat is generated.

At the front of the heated barrel 112, i.e. outlet, the molten high-density polyethylene leaves the heated barrel 112 via at least one screen (not shown) to remove any contaminants in the melt. The screens are reinforced by the breaker plate 114, which is a thick metal puck with many holes drilled through it. The at least one screen/breaker plate 114 also serves to create backpressure in the heated barrel 112. Backpressure is required for uniform melting and proper mixing of the highdensity polyethylene material, and how much pressure is generated can be altered by varying the number of screens, their wire weave size, and other parameters. The mentioned breaker plate 114 and at least one screen combination also eliminates the rotational movement of the molten highdensity polyethylene and creates a longitudinal movement.

After passing through the breaker plate 114 the molten high-density polyethylene in a continuous material flow enters the casting mold 200 according to the present invention directly or via a connecting pipe 115, as shown in Fig.1.

Reference is now made to Fig.2a-b showing a principle drawing of a casting mold 200 according to the present invention, wherein Fig.2b shows a cross-sectional view of the casting mold 200 halffilled with molten high-density polyethylene, as in Fig. 1. The casting mold 200 according to the present invention is formed by at least two parts 200a-b, in the form of at least one upper part 200a and at least one lower part 200b, respectively. The upper 200a and lower 200b part are provided with corresponding fastening means 210, such as circumferentially extending attachment flanges provided with holes for receiving fastening bolts, for detachable attachment to each other. The at least two parts 200a-b, when assembled together, form a complete casting mold 200 with a desired shape and size, i.e. desired cross section, diameter and thickness (volume). In the shown embodiment, the casting mold 200 is disc-shaped, which is the shape of most of the stub ends 400 to be produced.

The casting mold 200 is further provided with an inlet opening 220, arranged at lower part thereof, i.e. close to the lower end, and preferably at a center axis of the casting mold 200. In the shown embodiment, the inlet opening 220 is arranged close to the circumference of the lower part 200b.

According to the present invention, the casting mold 200 is further provided with at least one opening 230 arranged at the upper end of the casting mold 200, i.e. at the opposite end of where the inlet opening 220 is arranged. In the shown embodiment of Fig. 2a, there are arranged two openings 230 at the circumference of the upper part 200b of the casting mold 200. The mentioned at least one opening 230 will allow air to escape from the casting mold 200 as it is filled with molten high-density polyethylene. The mentioned at least one opening 230 may further be used as inspection holes for controlling the filling of the casting mold 200 with molten high-density polyethylene.

According to a further embodiment of the present invention, the casting mold 200 is further provided with one or more inspection openings 240 arranged in longitudinal direction and/or transversal direction of the casting mold 200 for inspection of the filling of the casting mold 200 with molten high-density polyethylene. As the filling of the casting mold 200 is approaching full, it may be desirable to reduce the supplied flow of molten high-density polyethylene. By manual inspection, the operator of the extruder 110 may thus reduce the supplied flow for filling of the last few presents of the casting mold 200. When the operator observes that the casting mold 200 completely filled, the operator stops the extruder 110, and thus stops the supply of molten high-density polyethylene to the casting mold 200.

Alternatively, or in addition to the manual inspection openings 240, the casting mold 200 may be provided with at least one level sensor 250 or sensor system, such as ultrasonic-, optical- or microwave-based or similar sensors or systems, for reading the filling degree of molten high-density polyethylene in the casting mold 200.

By connecting the at least one sensor 250 or sensor system to a control unit 116 (Fig.1) controlling the extruder 110 automated control of the extruder 110 is achieved. Based on readings of the level of molten high-density polyethylene in the casting mold 200 by the mentioned at least one level sensor 250 or sensor system, automated reduction of the supplied flow of molten high-density polyethylene from the extruder 110 to the casting mold 200 is achieved when the readings show that the casting mold 200 is close to full. Similarly, automated stop of the supplied flow of the molten high-density polyethylene is achieved when the readings of the level of the molten high-density polyethylene in the casting mold 200 shows that it is full.

The casting mold 200 is further preferably provided with lifting brackets 260 at upper end thereof, enabling movement of the casting mold 200 by suitable lifting equipment.

According to a further embodiment of the present invention, the casting mold 200 is further provided with support legs 270 at lower end thereof, enabling the casting mold 200 to be arranged in an upright direction.

The upper 200a and lower 200b parts of the casting mold 200 is preferably made with thin material thickness, but still with the sufficient strength. It will be preferable to have low material thickness to avoid high and rapid heat transfer from the molded high-density polyethylene body 300 during the curing process. The casting mold 200 may be made of e.g. stainless steel, cast iron or similar.

Accordingly, the casting mold 200 is arranged in an upright position and wherein the molten highdensity polyethylene is supplied into the casting mold 200 at lower part thereof via the inlet opening 220, i.e. the casting mold 200 is filled from the bottom and up.

After the casting mold 200 is filled with molten high-density polyethylene, the casting mold 200 is disconnected from the extruder 110 and moved to horizontal position for curing. After an intimal curing period the casting mold 200 is disassembled and the at least two parts 200a-b is removed from the formed molded high-density polyethylene body 300. After the casting mold 200 is removed, a final curing period in horizontal position is performed as described above, as wells as possible steps for removing possible tension in the molded high-density polyethylene body 300.

Reference is now made to Figure 3, which to the left shows a molded high-density polyethylene body 300 after an initial curing process/cool-down of the body 300 has been performed and where the casting mold 200 has been removed from the body 300. To the right in Figure 3 is shown a produced high-density polyethylene stub end 400, which has been machined out from the highdensity polyethylene body 300 by machining equipment, known per se for a skilled person, after the final curing period, and possible steps for removing possible tension in the molded high-density polyethylene body 300.

Claims

1. Method for manufacturing a molded high-density polyethylene body (300) for production of at least one stub end (400) comprising

casting the molded high-density polyethylene body (300) in one piece by using a casting mold (200) formed by at least two parts (200a-b) detachably arranged to each other, by, with the casting mold (200) arranged in upright position, filling the casting mold (200) with molten high-density polyethylene from the bottom and upwards via an inlet opening (220) arranged in lower part of the casting mold (200),

after the casting mold (200) is filled with molten high-density polyethylene, moving the filled casting mold (200) with the molded high-density polyethylene body (300) from the upright position to a horizontal position for an initial curing period of a curing process of the molded high-density polyethylene body (300),

removing the casting mold (200) from the molded high-density polyethylene body (300) after the ending of the initial curing period for a final curing period of the curing process of the molded highdensity polyethylene body (300) in horizontal position.

2. Method according to claim 1, wherein initially supplying the molten high-density polyethylene into the casting mold (200) such that a balloon of molten high-density polyethylene is formed interior in the casting mold (200), and wherein subsequently supplied molten high-density polyethylene is supplied into the formed balloon.

3. Method according to any preceding claim, wherein comprising continuous supply of molten highdensity polyethylene to the casting mold (200).

4. Method according to any preceding claim, wherein manually or automated reducing of supplied flow of molten high-density polyethylene when the casting mold (200) is close to full and/or manually or automated stop of supplied flow of molten high-density polyethylene when the casting mold (200) is full.

5. Method according to claim 1, wherein performing the initial and/or final period of the curing process in a heat-controlled environment.

6. Method according to any preceding claim, wherein using a heat-controlled environment for removing tensions in the molded high-density polyethylene body (300).

7. Method according to claim 6, wherein rough cutting of the molded high-density polyethylene body (300) before removing tensions in the heat-controlled environment.

8. Method according to any preceding claim, wherein machining the molded high-density polyethylene body (300) for production of at least one stub end (400).

9. Apparatus for manufacturing a molded high-density polyethylene body (300) for production of at least one stub end (400), wherein the apparatus comprises

- a casting mold (200) formed by at least two parts (200a-b) detachably arranged to each other, wherein the casting mold (200) is arranged in a upright position and provided with an inlet opening (220) at lower part thereof,

- an extruder (110) adapted for connection to the inlet opening (220) of the casting mold (200), wherein the extruder (120) is adapted for continuous filling of molten high-density polyethylene into the casting mold (200) via the inlet opening (220), and

wherein the casting mold (200) is provided with at least one opening (230) at upper end of the casting mold (200) allowing air to escape from the casting mold (200) as it is filled with molten highdensity polyethylene.

10. Apparatus according to claim 9, wherein the casting mold (200) is designed to provide a counter pressure to the supplied of molten high-density polyethylene from the extruder (110) enabling the forming of a balloon of the supplied molten high-density polyethylene in the casting mold (200).

11. Apparatus according to claim 9, wherein the casting mold (200) is provided with one or more inspection openings (240) arranged in longitudinal direction and/or transversal direction of the casting mold (200).

12. Apparatus according to claim 9, wherein the casting mold (200) is provided with at least one level sensor (250) or sensor system for reading filling degree of molten high-density polyethylene in the casting mold (200).

13. Apparatus according to claim 9, wherein the casting mold (200) is provided with at least one insertion element for reduction of interior size or volume of the molded high-density polyethylene body (300).

14. Apparatus according to claim 9, wherein the extruder (110) is adapted to supply a continuous flow of molten high-density polyethylene to casting mold (200).

15. Apparatus according to claim 9, wherein the casting mold (200) has low material thickness.