EP2571669A1

EP2571669A1 - Method and apparatus for processing plastic materials

Info

Publication number: EP2571669A1
Application number: EP11730033A
Authority: EP
Inventors: Tuomo Takku
Original assignee: KWH Pipe Oy AB
Current assignee: Uponor Infra Oy
Priority date: 2010-05-18
Filing date: 2011-05-18
Publication date: 2013-03-27
Also published as: WO2011144814A1; EA201291062A1; FI20105538A0; FI20105538L

Abstract

Method and device for processing plastic. According to the method, the plastic raw material is fed to the feed zone of a single- screw extruder (1), from the feed zone it is transferred to a compression zone, in which it is melted to form a molten plastic mass, after which the molten plastic mass is led through a dosing zone to a tool, which a plastic product is formed. In the feed zone, is a grooved portion of the shaft (4) and an at least generally considered cylindrical feed sleeve ( 9 ), in which there are guides, which is moved parallel to its longitudinal axis and/or is rotated around its own longitudinal axis during the regulation of the process, when the plastic raw material is being fed to the extruder, so that an even raw-material flow is created.

Description

Method and Apparatus for Processing Plastic Materials

The present invention relates to a method, according to the preamble of Claim 1, of processing plastics.

According to such a method, plastic raw material is fed to the feed zone of a single-screw extruder, from where it is transferred to a compression zone where it is melted, after which it is led through a dosing zone to a tool in which a plastic product is formed from it.

The invention also relates to an apparatus, according to the preamble of Claim 6, for processing molten plastic.

The term "plastic" refers principally to thermoplastics, which can be repeatedly melted and shaped with the aid of heat and pressure. Typical thermoplastics are polyolefins, such as polyethylene and polypropylene, as well as polyamide, polyacetal, and poly(ethylene terephthalate). These polymers are partly crystalline. Amorphous thermoplastics are, for example, polycarbonate, acrylonitrile/butadiene/styrene, poly( vinyl chloride), polystyrene, and poly(methyl methacrylate).

Plastics can be processed using different molten processing methods, such as extrusion, injection moulding, and blow moulding. The present invention relates primarily to extrusion.

According to the operating principle of a nozzle-compression device, i.e. an extruder, the plastic raw material, typically in the form of plastic pellets, i.e. nurdles, is fed from a feed hopper to a heated cylinder, at the start of which is a feed zone. The task of the feed zone is to use an internally rotating screw to move the plastic to the tool at the other end, which, through the joint effect of the pressure it creates, the heat of the cylinder, and the power transmitted by the motor to the screw, causes the plastic to melt and homogenize. The molten plastic obtained from the pressure or compression zone is fed through the dosing zone to the tool. The device, in which there is a single feed screw, is referred to as a single-screw extruder. US Patent Specification No. 3 300 811 and EP Published Patent Application No. 71159 can be referred to as examples of publications according to the background art.

An increase in extruder melting power and in the molten viscosity of plastics materials has led development to the use of powerful grooving, with the aid of which a high pressure is created in the material, in the inner surface of the cylinder in the feed zones. The pressure causes high friction in the grooves, for which reason the grooving is generally made in a separate sleeve, in the outer surface of which powerful liquid cooling can be arranged. The use of sufficiently long grooving has increased the revolution output of extruders, i.e. the melting power created by one revolution of the screw, many times over. At the same time, the revolution output remains nearly constant over a large screw rotation speed range. The useful output range of extruders in manufacturing different types of product has thus clearly expanded.

Because a grooved feed zone, along with screw geometry, has become a very important factor, particular problems relating to the basic construction of a single-screw extruder have been encountered as the processing pressures of plastic have increased.

In the feed zone, the screw is generally manufactured in such a way that is has a single helix, which has a rise of about 1-...0.8-times the size of the screw's diameter. Its depth varies between 6 and 10 mm, according to the size and purpose of the extruder. When nurdles are fed to the screw from an overhead opening about the width of the cylinder's diameter, if the cylinder has deep grooving and effective cooling, a solidly compressed plug forms from the plastic material, which cannot rotate along with the screw, but which is propelled by the ridge of the screw. However, this plug does not always form evenly around the screw's entire circumference, but only on one side, which causes uneven wear of the screw and cylinder and a large bending force against the screw. This is because all the evenly-spaced grooves in the feed sleeve cannot be utilized, but only some of the grooves participate in the work.

An additional drawback is the variation of the moment during the rotation of the screw, which makes it difficult to tune the motor's speed regulator and causes vibration in the machine body. This is due to a change, specific to single-screw extruders, in the free size of the feed opening, according to where the ridge of the screw is in the opening at any moment. The use of a deeply grooved feed zone has also been restricted by accompanying problems in screw geometry and compatibility with the feed zone. If the revolution output of the screw is high, the term high-power extruder is used. Such machines are suitable, for example, for plastic-pipe manufacture, which uses high outputs, but does not make such great demands on the homogeneity of the molten plastic as the manufacture of, for example, plastic films.

In film manufacture, extruders with a smooth feed zone have traditionally been used, which generally operate in such a way that the revolution output varies according to the screw's rotation speed and to the counter-pressure caused by the tool. Thus, the extruder often

"adapts" to the process and achieves an equilibrium, when the low revolution output gives good homogeneity in the melt. On the other hand, a smooth feed zone can also be a drawback; at low screw revolutions, or high counter-pressures, or high viscosities in the plastic material, the revolution output does not remain constant and the product is thus not of even quality.

For the aforementioned reasons, quite different extruders must be made for different extrusion products, which makes the standardization of extruder production difficult and raises costs.

Attempts to reduce these problems have been made by, for example, making two helixes in the screw at the feed zone, which cuts output variations and body vibration. The solution is not, however, very suitable in connection with the so-called barrier screws now widely used. Its use has therefore remained limited, especially in high-power extruders.

In the literature, several different proposals have also appeared for regulating the depth of the grooves in the feed sleeve and through that the revolution output, but these proposals have not been implemented due to their technical complexity.

The present invention is intended to eliminate at least some of the problems relating to the prior art and to create a new type of solution for processing plastic in an extruder.

The invention is based on the idea that a feed device, which comprises an essentially cylindrical (more precisely, one with an annular cross-section) channel, delimited by an internal shaft and an external cylindrical surface, is arranged in the extruder at the location of the feed opening (such as a feed funnel). There are guides in both the internal shaft and the external cylindrical surface, with the aid of which a pressurized flow of the nurdles coming from the feed opening is created. The feed device is arranged to feed the flow of nurdles to the normal feed zone of the extruder. According to the invention, the mutual position of the guides in the external cylindrical surface and the feed opening can be altered, in order to regulate the flow created by the guides in the cylindrical surface.

The extruder according to the invention comprises an elongated body, inside which is a channel bounded by an internal cylindrical surface, connected at one end to the plastic raw- material feed opening and at the other end to a tool used to process plastic. A rotatable shaft is arranged inside the cylindrical surface, with a central axis that is at least roughly concentric with the central axis of the cylindrical surface. Between them, the cylindrical surface and the extruder's screw delimit a channel with an annular cross-section, which acts as the extruder's feed, compression, and dosing channel, through which a molten plastic mass can be fed to the tool.

In the feed channel, there is then a first feed zone (hereinafter, also an "additional feed zone"), which comprises a grooved portion of the extruder screw, around which a cylindrical sleeve is arranged, which is equipped with at least one guide, so that the grooved portion in question and the cylindrical sleeve surrounding it form a channel with an essentially annular cross- section. The sleeve can be moved around the shaft or axially, when an even and adjustable raw-material flow is obtained from the plastic raw material being fed to the first feed zone. More specifically, the method according to the invention is mainly characterized by what is stated in the characterizing part of Claim 1.

For its part, the device according to the invention is characterized by what is stated in the characterizing part of Claim 6. Considerable advantages are gained by means of the invention. Thus, at the feed opening, an additional feed zone is formed in the screw and feed sleeve, with the aid of which an even and adjustable raw-material flow can be produced. As stated above, a single-screw extruder's melting power and the properties of the molten plastic are set by the geometry of the screw's feed zone. These define the screw's revolution output, which has a great effect on the molten plastic's temperature. The melt temperature often determines the production line's output. These properties can be dimensioned to suit each raw material. Until now, the difficulty has been that they could not be affected while running; in the present solution, this is achieved by altering the sleeve's position in the feed zone. In the aforementioned publication US 3 300 811, the geometry cannot be affected during running. The revolution output can only be affected by rotating the feed-zone sleeve with a motor while running. The ratio of the speeds of the screw and feed sleeve will then determine the revolution output. One drawback, among others, of the solution is that no operationally durable construction could be developed from it. It is also an expensive solution. The device of EP application publication 71159 does not permit the revolution output to be adjusted while running, but only during shutdowns, by changing components.

In the present solution, the edge of the feed sleeve, or the guides or ridges (typically internal) arranged in it transport the nurdles forward in an even flow, when desired volume of pre- pressurized nurdles moves to the next part of the extruder, which is, for example, the grooved feed zone of a traditional high-power extruder. The screw, together with the grooved feed zone then transports the material through the screw, melts it and presses it through the tool to form a plastic product. Thanks to the new extruder's integrated feed device, a plug using all the grooves evenly and extending around the screw is formed from the plastic.

In the following, the invention will be examined with the aid of a detailed description and with reference to the accompanying drawings, in which

Figure 1 presents a partial cross-section of one embodiment of the extruder according to the invention,

Figure 2 presents a cross-section of the grooved portion 6 of the screw, and

Figure 3 presents the cylindrical portion of the sleeve, when spread open. As stated above, in the solution according to the invention the plastic raw material, usually plastic pellets or nurdles, is fed to the new feed device of the extruder through a feed opening made in the extruder body. The feed device then forms an additional feed zone in the extruder. Here, the term "additional feed zone" refers to a zone that is arranged before the actual, i.e. traditional, feed zone. In the additional feed zone, an even flow of nurdles is created, in the traditional feed zone, the plastic is melted and homogenized through the combined effect of the pressure, the heat of the cylinder, and the power transmitted by the motor to the screw. The surface of the feed device receiving the raw material mainly comprises the shaft of the extruder, in which grooves have been formed. This grooved portion of the shaft is surrounded by a generally cylindrical sleeve, thus forming an annular feed channel between the grooved feed surface and the inner surface of the sleeve surrounding it. In the sleeve, there is typically a slanting or slantingly-cut edge on the feed-zone side, which forms the sleeve's pushing edge, as will be described in greater detail in the following.

Hereinafter, the edge in question will also be referred to as the front edge.

The sleeve can be moved, i.e. its position can be adjusted, to regulate the raw-material flow being led to the extruder's traditional feed zone. Particularly, the sleeve can be rotated around its own longitudinal axis. Alternatively, it can be moved axially. In one preferred embodiment, the sleeve is moved with the aid of an operating element. The position of the sleeve can be changed while feeding plastic raw material. However, the position of the sleeve is typically arranged to be fixed, once an even plastic raw-material feed flow has been created.

The sleeve has two principal tasks: firstly, it can be used to restrict the free feed opening, i.e. part of the feed opening can be covered on the external surface of the sleeve, to limit the plastic raw material flow flowing from the feed opening. Generally, 0 - 95 %, especially 0 - 75 %, most suitably 0 - 70 %, and particularly 0 - 50 % of the feed opening (calculated from the feed opening's perpendicular section at the shaft) can be covered using the sleeve's external jacket surface. Most suitably, the sleeve is arranged primarily on the motor side of the shaft, so as to create a portion of the feed opening to be left free on the extruder's tool side. The sleeve's second task is, with the aid of the shaft's grooves, to create a pressurized plastic raw material/nurdle flow. For this reason, there is at least one pushing edge or similar guide or ridge in the sleeve. Preferably, the moveable feed sleeve has one or more ridges, which run helically around the feed sleeve's external surface. According to one alternative, the rise of the ridge is at least 45° and its helix is preferably in the opposite direction to the rise of the extruder screw's ridges.

The example described below is of a generally cylindrical sleeve, in which the tool-side edge (i.e. the front edge) is cut at a slant, so that this edge will act as a guide pushing the plastic raw material during feeding.

After the feed sleeve in the direction of the flow of plastic material is a traditional feed zone, in which grooves or guides are formed in the cylindrical surface surrounding the screw, which, together with the ridges of the screw create a pressure high enough to melt the plastic material.

In one alternative, the number of grooves/guides in the screw is equal to or even greater than the number of guides in the cylindrical surface. By arranging the feed device according to the invention in front of the feed zone, and by moving the feed sleeve axially, or especially by rotating it around its own longitudinal axis, an even raw-material flow is created when plastic raw material is fed to the extruder. Compared to the traditional solution, in which the integrated feed device shown is not used, the mutual position of the guides on the external cylindrical surface and the feed opening can be flexibly changed always according to the extruder's feed and operating parameters.

According to one preferred embodiment, the additional feed zone is kept at a lower temperature than the actual feed zone, so that the melting of the plastic raw material takes place essentially only after the additional feed zone.

Figure 1 shows a single-screw extruder, with its feed zones, intended for melting plastic (but without the tool used to shape the plastic melt). In the figure, the extruder is given the reference number 1. It generally comprises a screw 4 and an external jacket 7 surrounding the screw 4, when the jacket is attached to a gearbox 3, which rotates the screw from the grooved shaft 5.

The plastic material coming from the feed hopper 2 fixed to the jacket flows into the grooved part 6 of the rotating screw, which takes the material with it, when the edge of the sleeve 9 moves the plastic forwards to the jacket's grooved portion 8. More specifically, in the embodiment according to the figure the screw's straight ridges push the plastic granulate towards the circumference and the rising surface of the adjustable sleeve for its part guides the granules parallel to the shaft of the screw, thus forcing the granules to travel to the helical portion of the cylinder and screw.

From here, the screw 4 carries the material forwards in a known manner right up to and through the tool to become the desired plastic product.

The sleeve 9 can be rotated around its longitudinal axis by a worm 10, either manually or by a motor, thus adjusting the flow of material. At the grooving 8, the jacket is equipped with a cooling channel 11.

Figure 2 shows a cross-section at point 6 of the grooved screw 2 while Figure 3 shows the cylindrical part of the sleeve 9. Figure 2 shows the ridges 6 forming the grooved portion, and the grooves 6'. A sleeve with a slanting front edge is created from the plate 9 shown in Figure 3 by attaching the edge on the left of the figure to the right-hand edge, in such a way that the axis marked by the broken line in the figure forms the central axis of the joined cylinder.

The sleeve can be rotated around its axis. In one embodiment, the outer cylindrical surface of the sleeve covers part of the feed opening. Typically, in the operating position, the sleeve's cylindrical surface is about half-way, i.e. 50 % of the feed opening is closed. The adjustable sleeve is then in the "central position". It will be obvious to one skilled in the art that the various embodiments of the invention are not restricted to the example shown, but can vary widely within the scope of the

accompanying Claims.

Thus, it is possible that the grooves in the screw can be either axial or helical and their shape can vary considerably. Correspondingly, the pushing edge of the sleeve can replaced with different numbers of grooves or ridges of different shapes, either machined directly into the jacket or formed in an auxiliary part or parts. The adjustment of the sleeve, i.e. the delimitation of the effective area of the pushing edge, can also be made by moving the sleeve axially or by using various types of shut-off components to prevent the material flow from meeting the pushing edge.

Claims

Claims:

1. Method of processing plastic in an extruder, according to which method

- plastic raw material is fed from a feed opening (2) to a single-screw extruder (1), - the plastic raw material is melted in the feed and compression zones into a molten plastic mass, after which

- the molten plastic mass is led through the dosing zone to a tool, in which a plastic product is formed from it,

characterized in that

- the plastic raw material is led from the feed opening (2) to the feed zone, through an additional feed zone which comprises a channel, which is formed between an internal, grooved shaft (5) and an external, cylindrical surface (9), which at least partly surrounds the shaft, and with the aid of which a pressurized flow of the plastic granules coming from the feed opening is created.

2. The method according to Claim 1, characterized in that the cylindrical surface at least partly surrounding the shaft consists of a moveable sleeve (9), in which there are one or more guides, which run around the feed sleeve helically.

3. The method according to Claim 2, characterized in that the rise of the ridge is at least 45° and it is preferably handed in the opposite direction to the rise of the ridges of the extruder's screw (4).

4. The method according to any of Claims 1-3, characterized in that the number of ridges in the shaft (5) is at least greater than the number of guides on the cylindrical surface.

5. The method according to any of Claims 1-4, characterized in that the position of the sleeve forming the cylindrical surface (9) can be altered, e.g., with the aid of an operating element, in order to change the flow of plastic granules.

6. A single-screw extruder, which comprises

- an elongated body ( 1 ), - a cylindrical surface (7) inside the body, which at one end is connected to the plastic raw-material feed opening (2) and at the other end to a tool used to process plastic,

- a rotatable screw (4) arranged inside the cylindrical surface, the central axis of which is at least more or less concentric with the central axis of the cylindrical surface, and around which at least one ridge runs helically, so that

- the cylindrical surface (7) and the screw (4) equipped with a ridge between them form a cylindrical channel, which acts as the extruder's feed, compression, and dosing channel, through which the molten plastic mass can be fed to the tool,

characterized in that

- in the feed channel there is a feed zone, which comprises a sleeve (9) arranged around the grooved portion (6) of the extruder screw (4), which is equipped with guides, and which can be moved in order to form an even and adjustable raw-material flow of the plastic raw material brought to the feed sleeve.

7. The extruder according to Claim 6, characterized in that there is at least one guide in the surface of the feed sleeve (9), which runs helically around the feed sleeve, preferably forming an opposite-handed helix to the ridges of the extruder's screw.

8. The extruder according to Claim 7, characterized in that the rise of the guide is at least 45° and the number of grooves in the screw is the same or preferably greater than the number of guides in the cylindrical surface.

9. The extruder according to Claim 7 or 8, characterized in that the guide in the surface of the feed sleeve (9) comprises the edge of the sleeve.

10. The extruder according to any of Claims 6-9, characterized in that the flow created by the guides in the external cylindrical surface can be adjusted by altering the mutual positions of the guides and the feed opening.