GB2171951A - Improvements in extruder screws - Google Patents

Abstract

An extruder has a barrel 1 with screw section 9 of large diameter and coarse pitch and screw section 10 of smaller diameter and finer pitch, both separately rotatable therein in the same sense to move material from solid inlet 6c to melt outlet 8c. Sections 9 and 20 are separate, coaxial screws having either separate gear control from a single motor, or separate motor drives through motors 4, 5 separately controlled by gearboxes 20, 21 respectively. Thus feed section 6 of the barrel, fed through orifice 6c with non-uniform scrap polymer is controlled separately from metering section 8 feeding outlet 8c whereby non-uniformity of feed does not overload the extruder. Separate control can be effected by feedback of pressure signals to the motors or gearboxes. End seal 19 can be water cooled. <IMAGE>

Description

SPECIFICATION Improvements in extruder equipment This invention relates to polymer melt extrusion and more particularly to extruder equipment for such extrusion.

The basic principle of such equipment is well known. Solid polymeric materials, usually as granules but possibly chopped reclaimed scrap, are fed from a hopper into a heated barrel occupied by a coaxial screw. This screw both conveys material along the barrel and mixes up the material to ensure good heat-transfer. Thus, the material progressively melts as it moves along the barrel and is extruded in molten form from a downstream end.

It is recognized that mixing and flow conditions near the point of entry of solids should preferably differ from those near the point of exit of melt. It is therefore well known to have an internally stepped barrel fitted with a screw of different diameters or pitch, etc one either side of the step (the step is usually smoothed or part conical). It is also well known to use a conical screw within a conical housing, with pitch varying along the screw.

Both of the above expedients manifest the same operating problem, which is that the larger diameter portion (the feed portion) can if carelessly used greatly overload the smaller diameter portion (the metering portion) leading to excess back pressure, which conventionally triggers an "overload" switch and turns off the motor. This problem is particularly notable with scrap material feed which is typically non-uniform.

The present invention sets out to overcome this problem.

In one aspect the invention consists in an extruder of the type in which solid polymeric material is conveyed by screw means along within, and progressively melted in contact with, the internal walls of an elongate heated housing, prior to extrusion as a melt: wherein the screw means comprises at least two separately driven sections pitched in the same sense but differing in diameter and/or screw pitch distance whereby melt and flow conditions at the point of entry of solid may be varied from those at point of exit of melt.

It will be found that two such screw-sections, preferably coaxial are usually enough. Each section is preferably cylindrical. The sections preferably differ both in diameter and in pitch from each other.

Usually, of course, the larger diameter section will be upstream of the smaller diameter section.

The drives to the sections are most preferably constituted by separate electrical motors and respective gear boxes. It is of course possible that a single motor and a more elaborate gear-box providing separately controllable offtakes, one for each section, might be used, but in practice we have established that it is much to be preferred to use two motors.

In embodying this invention in a practical form, we have tried an arrangement in which one screw section is mounted on a shaft extending coaxially through, and rotatable in relation to, the other screw section: e.g. a smaller diameter section of finer pitch, useful as a metering section closer to the outlet is mounted on the end of a shaft extending back through a larger diameter section of suitable pitch useful as a feed section closer to the inlet. In such a case, two separately controllable drive means will be provided at the "upstream" ends only, i.e. one for the internal shaft and one for the external threaded section.

For convenience, such an arrangement can be referred to as a "tandem" screw. We have found, however, that while operable, it requires certain ancillary features for extended trouble-free use.

Thus, the internal shaft should be supported on bearings within the external screw section; however, such bearings operate in contact with the molten polymer, in a region which is under high pressure (typically 6000 p.s.i. i.e about 400-450 kg/cm2 or occasionally much more if an overload of polymer to the finer metering screw takes place) and are therefore prone to failure as hot molten polymer is forced into the bearing structure.

Accordingly, the practical form of the invention with which this Patent Application is particularly concerned utilises separate screw sections in which the whole rotary structure of one screw section extends coaxially with, extends away from and is rotatable in relation to the other screw section.

For example, a smaller diameter section of a suitablefiner pitch, useful as a metering section closer to the outlet is mounted to be coaxial with but extend only as a totally separate continuation of, a larger diameter section of coarser pitch useful as a feed section closer to the inlet. In such a case two separately controllable drive means will be provided, one at the "upstream" end and the other at the "downstream" end, i.e. one for each separate threaded section. Also, because of the "downstream" drive means location it will be generally desirable to position the extrusion outlet at the side of the downstream end of the smaller diameter section.

For convenience such an arrangement can be referred to as a "head-on" arrangement. Since no internal bearings are required between two rotary parts other than shaft end bearings which present a different and easier technical problem this embodiment is preferable as more reliable over long periods to the earlier embodiment. The shaft and bearing near the outlet is not subject to pressure overload since such load as is transferred from the larger section, around the screw threads of the smaller section is of course relieved by the proximity of the extrusion outlet.

The mode of operation of this embodiment is such that if the (downstream) metering zone starts to become overstuffed and the load on its motor starts to become excessive, an electronic signal may be given to vary the speed of the other motor controlling the feed (upstream) sections to slow down the feed. Similarly, the motor load will vary with the use of thicker or larger reclaimed polymer pieces, or with filmy and insubstantial reclaimed pieces, but the equipment can still be automatically controlled.

While the invention has been generally described above in relation to extruder equipment it will be apparent to the man skilled in the art that the screws themselves; a method of polymer melt-extrusion utilising the equipment; and the extruded polymer all constitute aspect of the present invention; The invention will be further described with reference to the accompanying drawings in which: Figure 1 is a longitudinal section through a "head-on" embodiment of extrusion machine, and Figure 2 shows a watercooling detail near an end seal.

The polymer melt extruder shown in Figure 1 consists in general terms of a barrel region 1, two separate screw means 2a and 2b within the barrel region, two gear box sections 3a and 3b at one at each end of the barrel region for driving the separate screw means, and motors 4 and 5.

Barrel 1 can be considered in three externally cylindrical, parts. The first of these parts is the feed and compression section 6, having end flanges 6a and 6b and a top orifice at 6c beneath a feed hopper (which is not shown) for scrap or granular polymer.

The second part of the barrel is a transitional portion 7, which internally conical, and flanged at 7a for attachment to flange 6a. The third part of the barrel is a compression and metering portion 8, flanged at a forward end 8a and at a rearward end 8b which is attached to the conical portion 7. This compression and metering portion 8 has a transverse outlet at the outermost (lefthand) end 8c.

The screw means 2a and 2b comprises two screw members at 9 and 10, having different diameters and pitches and of completely separate rotary structure.

Screw 9 lies within the feed and compression section 6 of the barrel. It is generally speaking a cylindrical member having an external screwthread 11, a wind-back groove 12, anda forward somewhat conical end 13 lying within transitional portion 7.

The screw 10 faces in the opposite or head-on direction to screw 9, and is completely separate therefrom. Its pitch is of course in the same sense. It lies within the compression and metering section. It is a solid cylindrical body with screw threads 14, and with a wind-back groove 15, located at the rearward end of the screw 10 in flange 8a. The exact shape of the body of screw 10 is such that the spacing with the surrounding walls at the rearward end of the final metering section gives a smooth transition between the two screw portions. End seal 19 prevents flow of molten polymer into the interior of the gearbox 3a with consequent interruption of rotation.

Gear box 3 sections 3a and 3b again comprise two parts generally indicated as gearbox 20 and 21.

Gear box section 20 is provided generally for the drive of screw 9. It comprises gear wheel 22 keyed to the rearward end of shaft 9, which shaft is also provided with thrust bearings 23 and support bear ings24. Gearwheel 22 meshes with gearwheel 25 rotatable in bearings 26.

Gear box 21 is generally provided for the drive of screw 10. Gear wheel 27 is keyed for this purpose to a part of an end shaft 10a, which is supported against thrust bearings 28 and upon support bearings 29. Gear wheel 27 is driven by gearwheel 30 rotatable in bearings 31.

The motors 4 and 5 are similar in nature. Motor 4 drives by virtue of belt drive 32, the wheel 33 keyed on to the shaft gear of wheel 25. Similarly, motor 5 drives, by belt drive 34 the wheel 35 keyed upon shaft of gear wheel 30.

In operation, therefore, scrap material entering the orifice 6c in the heated barrel section 6 and is conveyed along within this section by the action of screw 9. The solid material feed becomes progressively melted and compressed together by this screw. This may not happen completely uniformly, since it depends upon the rate of feed and uniformity of bulk density of feed material which, if scrap, can be of considerable variability from thin film to thick lumps of polymer.

The molten polymer is compressed within the conical section and progresses forward through the metering section under the action of screw 10. If there is an overload of material at 6c, the pressure will build up which can be converted to a signal governing motor 4, causing the screw 9 to rotate more slowly until the metering screw 10 carries away the excess material. By suitable feedback and control mechanisms, the feed of extrusion material can be accurately metered notwithstanding disuniformities in feed of solid scrap or other raw material, with consequent improvement in the whole extrusion process.

It is important in the practice of the present invention that the two screw portions 9 and 10 instead of being arranged one within the other are arranged in an separated and opposed or head-on fashion. This has the advantage of a considerably simpler screw construction and also the advantage that there is no danger of high pressure molten polymer arriving between two relatively rotatating cylindrical screw members with consequent interruption of motion. The seal 19 shown between the gearbox and the barrel portion is near the outlet, and only has to deal with one essentially rotary member, and is a pressure-relieved region.

Figure 2 shows the downstream or exit end of the barrel can be modified by broadening the flange 8a and incorporating therein an annular water-jacket to surround the shaft 10 whereby, even if polymer escapes past windback grooves 15, it is solidified and prevented from reaching and affecting the adjacent seal and bearings.

Claims (12)

1. An extruder of the type in which solid polymeric material is conveyed by screw means along within, and progressively melted in contact with, the internal walls of an elongate heated housing, prior to extrusion as a melt: wherein the screw means comprises at least two separately driven sections pitched in the same sense but differing in diameter and/or screw pitch distance whereby melt and flow conditions at the point of entry of solid may be varied from those at point of exit of melt.

2. An extruder as claimed in claim 1 having two such screw-sections.

3. An extruder as claimed in claim 2 in which the screw-sections are co-axial.

4. An extruder as claimed in claim 2 or 3 in which each section is cylindrical.

5. An extruder as claimed in claim 2,3 or 4 in which the screw sections differ in both diameter and pitch.

6. An extruder as claimed in claim 5 having a smaller diameter screw section of finer pitch useful as a metering section and a large diameter screw section of suitable pitch useful as a feed section with its rotary structure separate therefrom, coaxial therewith, extending away from and rotatable in relation to the smaller diameter section.

7. An extruder as claimed in claim 6 having two separately controllable drive means connected one to the finer screw section and located downsteam of the two sections and one to the suitable screw section and located upstream of the two sections.

8. An extruder as claimed in claim 7 in which the separately controllable drive means are separate electric motors connected one to each screw section by separate gear boxes.

9. An extruder as claimed in any one preceding claim having a side outlet for melt at the downsteam end of the smaller diameter section.

10. An extruder as claimed in claim 1 and substantially as herein described with reference to, and as illustrated in the accompanying drawings.

11. A method for the melt extrusion of scrap polymer in which the extruder as claimed in any one preceding claim is used.

12. Reclaimed polymer obtained by the method as claimed in claim 11.