WO1981000525A1

WO1981000525A1 - Dispersion device for plastic or colouring material

Info

Publication number: WO1981000525A1
Application number: PCT/SE1980/000218
Authority: WO
Inventors: T Akesson
Original assignee: Axon Plastmaskiner Ab; Master Plast Ab; T Akesson
Priority date: 1979-08-27
Filing date: 1980-08-27
Publication date: 1981-03-05
Also published as: JPS56501194A; AU6226280A; EP0034159A1; SE426785B; SE7907120L

Abstract

Dispersion device for plastic or colouring material comprising means (11) for feeding the material under pressure. In front of the said means a dispersion valve is arranged with a spring washer (18; 18'; 29) arranged as a non-return valve engaging a seat (14; 14'; 24, 26).

Description

DISPERSION DEVICE FOR PLASTIC OR COLOURING MATERIAL

The invention relates to a dispersion device for plastic or colouring material, comprising means for feeding the material under pressure, preferably a plasticising screw of the type which has apertures in the screw thread for the return flow of the material. Conventional plasticising screws in extruders or injection moulding machines have a specific finishing zone, the volume of which is less than the thread volume at the material intake. This difference in thread volume is specified as the screw compression ratio, and this ratio varies for different thermoplastics in dependence of melting properties, viscosity, and the volume ratio between the material in the cold granulated form and in the form of a homogeneous melt. Thus, the compression ratio can vary e.g. from 1.5:1 to 5:1. The higher the compression ratio of the plasticising screw, the lower will be the capacity of the screw, and this is connected with the fact that the finishing zone becomes too small with respect to its volume, so that a smaller quantity of material is given off per rotation of the screw as compared with the quantity of material of a plasticising screw which has a low compression ratio. Plasticising screws can be designed in different ways so as to give a certain compression ratio, but the object of compression is always, inter alia, to create a certain back pressure in the plasticising screw, which affects the incipient melt during its feed towards the finishing zone where the material should have reached a homogeneously molten state. However, this is not always the case if the compression ratio is too low for a certain material, and hence the plasticising screw has been developed, which has a low compression ratio and apertures in the screw thread for the return flow of the plasticised material so as to give the material a longer heating-up time and to bring about better mixing of cold and hot material which flows forwards and backwards in the plasticising screw. A screw of this type is shown and described in the Swedish laid-open specification 328,400.

In an injection moulding machine the back pressure can be influenced by hydraulic means, but this facility is not available in an extruder. Instead a so called screen plate is placed in front of the end of the plasticising screw, this screen plate being provided with fine-mesh screen cloths which are able to raise the back pressure in the plasticising screw. The group of screens obtained thus influences homogenisation. It is also possible to provide in an extruder a displacement insert, which restricts the flow of material to the extruder nozzle whilst increasing the rearward pressure in the screw because of the throttling effect thus brought about. These methods are not however satisfactory solutions to the problem of homogenisation, especially in the case of low-compression plasticising screws, e.g. plasticising screws with apertures in the screw thread.

In an injection moulding machine use is normally made of a non-return valve at the end of the plasticising screw so as to prevent the material flowing backwards in the screw thread during the actual injection process. In an injection moulding machine there is also a sel f closing nozzle so as to prevent injection of the material from the machine during the plasticising operation proper, during which the plasticising screw retracts in position so as to bring about a new injection stroke during which the plasticising screw is pressed forward like a piston. The self-closing nozzle can consist for example of a movable valve nozzle or a spring-loaded piston inside the injection nozzle and it can also consist of a ball valve or a hydraulically operated piston . However, none of these arrangements is envisaged to provide increased dispersion and homogeneity of the material and they cannot be employed for this purpose. Furthermore, they are of such nature that they cannot be modified for use in an extruder.

With the aim of replacing screen plates, throttling nozzles, self-closing nozzles and non-return valves of the conventional type in extruders or injection moulding machines, arranged in the manner as described above, in accordance with the present invention a device is provided of the type specified in the introduction with the characteristics as indicated by claim 1. To illustrate the invention this will be described in greater detail by reference to the accompanying drawings, in which

FIG 1 is an axial sectional view of the front portion of an extruder with a dispersion valve accord ing to the invention,

FIG 2 is a diagrammatic view which illustrates the flow of material with the dispersion valve open, FIG 3 is a view corresponding to FIG 1, with a modified dispersion valve, FIG 4 is an axial sectional view of the front portion of an injection moulding machine with dispersion valve in accordance with the invention, FIG 5 is a fragmentary axial sectional view of a cylinder with a plasticising screw provided with a dispersion valve in accordance with the invention at its front end,

FIG 6 is an axial sectional view of an adaptor which is designed for incorporation in an injection head of an injection moulding machine and is provided with a dispersion valve in accordance with the invention,

FIG 7 is an axial sectional view of a dispersion valve with spring loaded washer of modified design, and FIG 8 is an axial sectional view of a dispersion valve which is mounted in an extruder die.

FIG 1 illustrates the front portion of an extruder which comprises an extruder cylinder 10 with a plasticising screw 11 arranged rotably therein, preferably of the type specified, above with apertures in the screw thread for the reverse flow of the material being plasticised. A conventional extruder head 12 for extrusion of tube or pipe is provided on the machine.

Between the plasticising screw 11 and the extruder head 12 a displacement sleeve 13 is provided. Between the end of this sleeve, which faces the plasticising screw, and the cylinder 10 a dispersion valve in accordance with the invention is clamped, at the location where a conventional screen plate is usually mounted. This valve incorporates a seat 14 with a flow aperture which in this case is smaller than the actual screw diameter but which can also be made larger than the screw diameter. The aperture can be tapered so as to facilitate flow. Furthermore, the valve comprises a screen washer 15 which has no hole in the centre and which instead is provided with two small torpedoes 16 and 17. These torpedoes are pointed and are directed axially. On the torpedo which faces the screw 11 a truncated conical spring washer 18 is located, contacting the seat 14 at a certain bias. The bias is adjusted to the force which is required to produce an adequate specific pressure in the plasticising screw or to achieve an adequate dispersion in the valve proper. The spring washer is replaceable. As such it does not need to be truncated and conical but right from the outset can be. plane and be given the truncated conical form by the washer being clamped when assembled on the supporting torpedo. Dependent on the choice of spring washer diameter, thickness, convexity, spring path and preloading, different pressures can be produced. It is also possible to increase the pressure by arranging a group of two or more spring washers. The conventional screen plate which can be regarded as a common low-pressure arrangement in the extrusion process has thus been replaced by a more modern high pressure equipment.

The screen washer 15 can be provided with one or more screen cloths 19 in the conventional manner.

When the plasticised material leaves the plasticising screw 11 it passes through the aperture in the seat 14 but after this it cannot escape freely through the screen washer 15, because the preloaded spring washer (or spring washers if several are provided) 18 closes against the seat 14. Thus, the plasticising screw has to work up or "pump up" a pressure corresponding to the spring bias before the material is allowed to pass through. This, however, is not the case when employing conventional plasticising principles.

The pressure thus generated by the screw can vary from a few atmospheres to several hundred atmospheres gauge pressure, and it can be adjusted to the maximum positive pressure which the screw is able to generate by its conveying force.

When the plasticising screw has generated the internal pressure which corresponds to the bias of the preloaded spring washer against the seat, the spring washer opens by elasticity and leaves a thin gap between the seat and the contacting edge of the spring washer, the plasticised material flowing through this gap as a thin film at high flow velocity to pass subsequently through the screen cloths 19, the screen washer 15 and the displacement sleeve 13 to the extruder head 12.

FIG 2 shows diagrammatically the flow of material through the dispersion valve by arrows F. It can be seen that material flows approximately radially outwards through the gap.

Since the material plasticised by the plasticising screw 11 must spread out to form a thin film in the manner described above, excellent dispersion is obtained from a thick layer with low flow velocity to a thin layer with high flow velocity, and then, as the material continues to flow, there is a return to a thick layer with low flow velocity when the material has passed the dispersion valve. This alternation between thicker and thinner layers results in an excellent homogenisation of the material. The greater the diameter of the spring washer 18, the narrower becomes the gap between the washer and its seat 14 at constant material flow. Furthermore, the principle is that the larger the diameter and thickness of the dispersion washer, the higher is the pressure applied.

The dispersion valve can be so designed that the screen washer 15 with the screen cloths 19 is located on the other side of the spring washer than that illustrated here, or the dispersion valve can be provided with a screen washer and associated screen cloths on either side of the spring washer.

It should be pointed out that the displacement sleeve 13 can be designed in different ways and can also be completely dispensed with, e.g. if a profile die is to be mounted instead of the tube head illustrated, without the dispersion being less effective thereby.

As shown in FIG 1 the spring washer 18 is placed in such a way that the large end thereof, i.e. the concave side, faces the screw, i.e. the direction from which the pressure is applied. However, it can also be rotated half a turn so that the concave surface faces the unpressurized outlet and the convex surface of the spring washer faces the pressure side, i.e. the plasticising screw 11. This embodiment is illustrated in FIG 3 which otherwise corresponds with the design shown in FIG 1. As a result of the spring washer being located as shown in FIG 3, a seal is obtained by means of the spring washer acting as a non-return valve, e.g. when there. is a requirement that propellant gases during an expansion process should not penetrate into the plasticising screw 11, interfering with pi astici sation and cooling down the melt there. This provides a guarantee that the expansion process will take place only on the unpressurized side of the dispersion valve. Both with chemically-reactive solid propellants and with liquid and gaseous propellants it is necessary to be able to control the internal pressure in the plasticising screw so that the gases do not escape before the material passes out from the cylinder. During extrusion of expanded thermoplastics it has hence not been possible hitherto to incorporate sufficient gas into the raw material in the plasticising screw, and this also applies to the injection moulding process. During extrusion of expanded thermoplastics it is customary that the gas "spits" out at a density of 0.7 g/cm³ or an even lower density. It is thus not possible to deliver a lighter product from the machine.

On the other hand the dispersion valve in accordance with the invention retains the gas at a high internal pressure and if the valve is combined with a plasticising screw of the type which has apertures in the screw thread, also a better mixing and homogenisation of gas and melt is obtained. Thus, the dispersion valve prevents expansion in the plasticising screw proper. The gas is released only when the material passes through the gap between the spring washer 18 and the seat 14, i.e. at atmospheric pressure on the unpressurized side. All available types of propellant are gaseous at atmospheric pressure and at room temperature. The expansion of the gas in the plasticising screw is thus impeded by an internal overpressure corresponding to the plasticising temperature.

This is particularly important if it is required to obtain an expansion down to a weight of 10 kg/m³. Here it is necessary to employ propellants which are essentially completely gaseous at room temperature and at atmospheric pressure. These propellants should not leave any residual products behind after changing to the gaseous form; however, any residual products present during gas formation after the dispersion passage can be redispersed in a second dispersion valve. Here inter nal pressures of 150-200 kp/cm² in the plasticising screw must be anticipated but fundamentally the higher the internal pressure, the better is the expansion obtained. The dispersion valve in accordance with the invention makes possible the maintenance of a high pressure in the plasticising screw.

What has been stated above regarding the extrusion method also applies appropriately for the injection moulding method. This is illustrated in FIG 5 which shows the front portion of an injection moulding machine with an injection head 20 and an injection nozzle 21 mounted therein. The plasticising screw 11 is provided with a conventional non-return valve 22 at its front end.

The non-return valve 22 can be replaced by a dispersion valve in accordance with the invention in the manner illustrated in FIG 5; this dispersion valve can be provided in combination with a conventional head or be combined with a head with a dispersion valve as shown in FIG 4. If the dispersion valve in FIG 5 is combined with the dispersion valve in FIG 4, two dispersion processes take place, independently, and in many cases this can be advantageous. Two dispersion processes separated from each other can also be arranged in an extruder by locating a dispersion valve in accordance with FIG 1 in front of the screw and a dispersion valve in the die proper or in connection with the head. In such a case the two dispersion valves are stationary, whilst on the other hand in the injection moulding method one is stationary in accordance with FIG 4 whilst the other is movable together with the plasticising screw shown in FIG 5.

The dispersion valve as shown in FIG 5 includes a pin 23 which is screwed into the plasticising screw 11 and supports the spring washer 18 at the large end thereof, i.e. with the concave side facing the plasticising screw. The spring washer 18 rests against a ring 24 at one end edge thereof said ring being displaceable in the cylinder 10, whilst the other end edge of the ring 24 rests against the seat 14 which is mounted by means of the pin 23 on the plasticising screw 11. Under the pressure of the plasticised material a gap is thus formed between the spring washer 18 and the ring 24 on one hand and between the ring 24 and the seat 14 on the other hand, whereby two thin films of the plasticised material are formed. In the manner described above these thin films can be matched, by suitable choice of spring washer 18 and by combination of two or more spring washers. As a result of two films being obtained, the flow velocity is extremely high and a dispersion is achieved which greatly surpasses that which has been obtainable previously.

In an injection moulding machine it is customary to maintain a hydraulic counter-pressure in the plasti cising screw by throttling the rearward movement which the screw tends to perform automatically in conjunction with the plasticising process which precedes the next injection stroke. As a result the pressure rises in the plasticising screw to the same extent as this natural backward movement is impeded and counteracted.

The disadvantage of applying a hydraulic back-pressure to the plasticising screw is that this back-pressure influences the melt in front of the screw and is propagated rearwards through the melt in the finishing zone without bringing about any actual dispersion. Semi-molten or non-dispersed material particles or particles of colouring of filling agents can as a result, even under high pressure, penetrate the finishing zone, this resulting in poor plastici sation, non-homogenised mixing and poor dispersion of the material. A hydraulically-applied counter-pressure which is effective from the front in other words only reverses the screw under the set pressure whilst at the same time non-molten particles, unmixed colouring agent and unmixed filler are passed through.

The advantage of the dispersion valve in accordance with the invention, when this is arranged as shown in FIG 4, is that on one hand it brings about a counter-pressure inwards in the screw and on the other hand brings about simultaneous dispersion in the manner described here, preventing semi-molten or poorly dispersed particles making their way past the non-return valve, because the narrow gap does not permit this. The high internal pressure and the narrow gap, in combination, force the non-molten particles to disperse with the remaining melt. The non-molten particles which by chance reach the dispersion valve are thus forced to change into the plastic state if they are to be able to pass through the narrow gap in the dispersion valve. During regeneration and recovery of plastic scrap it is advantageous to employ a plasticising screw with apertures in the screw thread, because the low compression in this type of plasticising screw, purely geometrically, permits a deeper thread in the finishing zone. Small stones, particles of metal and other non-meltable impurities can be collected at the front of the valve, which in such a case is provided with a special pocket for this purpose. During the regeneration of mixed plastic scrap consisting of many different types of plastic with a large number of differing melting ranges and flow or temperature intervals, it is possible to control the homogenisation of the melt by means of the dispersion valve according to the invention, which is not possible with the designs known hitherto. In the embodiments shown in FIG 1 - 5 the dispersion valve is arranged as an insert in the actual extruder or injection moulding machine. However, it can also be arranged as an adaptor as shown in FIG 5. In the embodiment shown in this figure the dispersion valve is located in a nipple 25, the seat being formed by a conical surface 26 in the nipple. This has a threaded hollow stud 27 so that it can be screwed into the head, whilst the nozzle 21 is screwed into the nipple. The nozzle here serves to hold the screen washer 15 inside the nipple 25 and it is thus possible to remove the screen washer with the spring washer 18 for cleaning purposes or maintenance merely by unscrewing the nozzle 21. Outside the nipple 25 an electrical heater strip 28 is provided.

The spring washer in FIG 7 is more stream-lined in shape and provides a larger contact surface against the seat than do the spring washers previously described. The stream-lined shape can be important with material which cannot withstand the formation of so called flow pockets, e.g. PVC, and the advantage of a larger contact surface is that a wider gap is obtained. The washer in FIG 7 is generally indicated at 29 and is made from spring steel. It consists of a conical section 30 which has its point facing towards and inserted into the aperture in the seat 14, whilst the curved surface of the section sealingly engages the seat. The large end of the tapered portion 30 joins, with a gentle radius, to the large end of a truncated conical ring-shaped section 31, whereby the washer is threaded on the torpedo 16. A washer of this type functions in principle as two conical spring washers placed against each other.

Furthermore, it can be mentioned that the dispersion valve can also be arranged in the die proper whether an extruder or an injection moulding machine is concerned, and can also be designed as an in-feed bush to a die. Thus, FIG 8 illustrates a die which fundamentally can be used as a film blowing head or alternatively as a tube head for producing plastic foil or plastic tube, but the die shown in FIG 8 can also represent a bottle blowing die. A die of the type shown in FIG 8 always has a central mandrel holder 32 to which mandrels can be replaceably attached for different diameters of the tube or pipe which is to be produced. Either a number of bores 33, as shown in the lower portion. of FIG 8, or a number of supporting webs 34, as shown in the upper portion of FIG 8, are arranged in the mandrel holder. The bores 33 or the interstices between the webs 34 permit the through-flow of the material with the central portion of the mandrel holder located in the centre of the flow of material. This section supports a mandrel 35 which is detachably fastened thereto, the latter being surrounded by a sleeve 36 and together with this it confines the nozzle gap through which the material is to be extruded. Both the material between the bores 33 and the web 34 can however deposit undesirable flow stripes (cords) in the finished product. To eliminate this it happens, for example during film blowing, that the die is allowed to rotate slowly, for which complicated drive means is required which renders the die complicated. It also occurs that threaded grooves are provided both on the mandrel 35 and on the sleeve 36, so that the flow of material is divided both radially and axially in the direction of flow so as to eliminate flow stripes or other marks from the mandrel holder.

A dispersion valve in accordance with the invention with a spring washer 18 which interacts with a seat 14 is arranged on the mandrel holder 32 in FIG 8 by being fastened to the mandrel holder by means of a screwed-on torpedo 37. The spring washer 18 is a high pressure washer and is adapted to bring about a high pressure backwards in the screw of the plasticising device. In the lower portion of FIG 8 there is shown however also a further spring washer 18' which interacts with a seat 14' formed by the sleeve 36 and which is attached to the mandrel holder 32 by being clamped between this and the mandrel 35 which is threaded thereon. The spring washer 18 ' is smaller than the spring washer 18 and serves t o eliminate flow stripes and other marks after the mandrel holder. As a consequence thereof the complicated and rather unsatisfactory arrangements for elimination of such flow edges and marks mentioned above can be dispensed with.

By making the opening pressure for the washer 18' lower than the opening pressure for the washer 18, an intermediate pressure chamber is obtained between the two washers. During the manufacture of expanded products, expansion occurs in this intermediate pressure chamber. As mentioned above an extra spring washer may be required for dispersion of possible residual products left because of chemical reaction at the gas generation during expansion, and in the embodiment shown in FIG 8 the spring washer 18' carries out this function.

Summarizing, there are two things which bring about homogenisation and dispersion in the dispersion valve in accordance with the invention:

One is the thin gap which forms between the spring washer and its seat and which provides an essentially radially-directed flow of material. The other is the high specific pressure to which the material is subjected in the narrow gap. This latter pressure is also transferred rearwards in the plasticising screw and thus contributes to a better circulation in the groove section and to an improved homogenisation. If the spring washer has a diameter of for example

50 mm it can produce alone an internal pressure of 100 kp/cm² depending on the preloading. Two washers one on top of the other can bring about twice the pressure etc. If the gap between this washer and the seat is 0.5 mm, this corresponds to a surface through which the material can flow which is: π x 5 x 0.05 = 0.8 cm².

This corresponds to a circular hole 10 mm diameter.

Thus, if the washer by elasticity opens a gap relative to the seat of 0.5 mm, this corresponds to the material flowing through a 10 mm hole. The difference is, however, that in accordance with the invention this takes place under a high pressure in a thin film-like layer providing a dispersion not obtained so far which is incomparably better than if the material had to flow unpressurized through the corresponding 10 mm aperture.

The invention has been described here with respect to the processing of plastic material in extruders and injection moulding machines, but in its widest sense can also be employed in the paint industry for the dis persion of paint materials. Then, colouring pigments can be dissolved in oil, varnish or water and forced through the dispersion device in accordance with the invention for homogenisation and fine distribution of the colouring pigment.

It is not essential that the device by means of which the material is fed under pressure be a screw, as this device can also be designed as a piston with a reciprocating motion. It should also be mentioned that the spring washer can be designed in one piece with the supporting torpedo.

Claims

1. Dispersion device for plastic or colouring material comprising means (11) for feeding the material under pressure, preferably a plasticising screw of the type which has apertures in the screw thread for the return flow of the material, c h a r a c t e r i s e d in that in front of said means there is provided a dispersion valve with a spring washer (18; 18'; 29) arranged as a non-return valve, which rests against a seat (14, 14'; 24; 26) to expose a ring-shaped gap which permits an essentially radial flow of the material as a result of the elastic yielding of the spring washer when subject to a predetermined pressure from the fed material.

2. Device as in claim 1, c h a r a c t e r i s e d in that the spring washer (18; 18') as such, or as a result of the mounting thereof, has a truncated conical shape.

3. Device as in claim 2, c h a r a c t e r i s e d in that the spring washer (18; 18') is arranged with its large end against the feeding means (11).

4. Device as in claim 1 or 2, c h a r a c t e ri s e d in that the spring washer (18) is arranged with its small end facing the feeding means (11).

5. Device in accordance with any of claims 1 to 4, c h a r a c t e r i s e d in that the spring washer (18; 18'; 29) is passed with a central aperture on a supporting member (16, 17; 23; 32) arranged in the passage for the plasticised material.

6. Device in accordance with any of claims 1 to 5, c h a r a c t e r i s e d in that the dispersion valve is arranged stationary between the feeding means (11) and a screen washer (15) located in the passage for the material.

7. Device as in any of claims 1 to 5, c h a r a c t e r i s e d in that the dispersion valve is located at the front end of a plasticising screw (11) which is arranged as feeding means.

8. Device as in claim 7, c h a r a c t e r i s e d in that the spring washer (18) rests against a ring (24) which can move axially and which can be pressed against the seat (14; 24; 26).

9. Device as in any of claims 1 to 5, c h a ra c t e r i s e d in that the dispersion valve is arranged in an adaptor (25), which is screwed into the head and has a holder for a nozzle (21).

10. Device in accordance with any of claims 1 to 9, c h a r a c t e r i s e d in that two dispersion valves (18, 18') are arranged one after the other for confining an intervening chamber, the valve (18) nearest the feeding means (11) having a higher opening pressure than the second valve (18').