GB2030512A

GB2030512A - Feed device for an extruder

Info

Publication number: GB2030512A
Application number: GB7929550A
Authority: GB
Original assignee: Bischof and Klein SE and Co KG
Current assignee: Bischof and Klein SE and Co KG
Priority date: 1978-08-25
Filing date: 1979-08-24
Publication date: 1980-04-10
Also published as: FR2434020B1; SE7907039L; ES483590A1; NL7906159A; DE2837135C2; SE436403B; BE878360A; FR2434020A1; GB2030512B; DE2837135A1; DK350379A

Abstract

The device feeds waste plastics material, particularly film, to an extruder (1, 19). A comminutor (3) chops up the waste, which is delivered to a storage hopper (8) through which part of the comminuted material is continuously re-circulated via a conveyor (10), an extruder precompressor (12), a suction line (15, 14, 16) and a cyclone separator (17) to improve the homogeneity of the mix. The suction line (15, 14, 16) maintains a predetermined level of fill in the feed hopper (11) of the precompressor (12), which is supplied by the conveyor (10), such as to maintain the discharge pressure of the extruder constant. This feature, together with the better homogeneity of the mix supplied to the extruder, allows the direct reproduction of blown plastics film (18) from waste film material. <IMAGE>

Description

SPECIFICATION Feed device for an extruder The invention relates to a feed device for an extruder for thermoplastic plastics materials and concerns such a device comprising a comminuting device for plastics material particularly foil, a storage hopper for the comminuted foil and a precompressor to be disposed at the inlet of an extruder, the precompressor having a feed hopper arranged to receive comminuted foil from the storage hopper.

In the processing of plastics there is considerable interest in the processing of old and waste material, to make possible the re-cycling of cuttings and waste from newly formed plastics parts, particularly plastics foils, and to save raw material.

A known feed device of the kind referred to at the beginning is part of a device for recovering raw materials for thermoplastic plastics materials in granulated or agglomerated form and in which plastics foils are first comminuted and after temporary storage in a hopper are supplied via a precom pressorto an extruder which delivers the granulated or agglomerated raw product. This raw product is suitable, with restriction, basically for further proces- sing, for example, for the production of blown film, there being restrictions, in particular, with regard to the merely agglomerated raw material.For one thing, inequalities in properties of the material, such as appear from fluctuations in the bulk weight of the material in a range from 80 to 400 girl, for example, and accordingly, large fluctuations in density in the processing, are critical for the use of the raw material for new finished products. On the other hand, the repeated reshaping of the material and, in particular, its repeated thermal treatment, leads to unwanted alternations in the properties of the material. The use of re-cycled material for the direct production of plastics foils using the previously known feed device is particularly interesting as a source of cheap foil but the re-cycled material produced by the feed device is insufficiently homogeneous.

It is an object of the invention to provide a device by the use of which an economical processing of old and waste plastics material, particularly plastics foil, is rendered possible and an improved, processed product is obtained.

According to one aspect of the invention there is provided a feed device for an extruder for thermoplastic plastics materials comprising a comminuting device for plastics material, particularly foil, a storage hopper for receiving comminuted foil from the comminuting device and a precompressor for plastics material to be disposed at the inlet to an extruder, the precompressor having a feed hopper arranged to receive comminuted foil from the storage hopper and means for re-circulating comminuted foil from a lower region of the storage hopper into an upper region thereof.

With a feed device of the invention, as defined in the immediately preceding paragraph, the storage hopper becomes also a mixer, to which is supplied not only the comminuted foil from the comminuting device, which fluctuates greatly in its bulk weight, but also mixed comminuted foil material from its output. Re-circulation in this manner has proved extremely effective in reducing fluctuations in bulk weight of the material and enabling the production of an extremely homogeneous and dense, mixed product. The intermingling of different comminuted foil is no longer a disadvantage in any way but instead becomes favourable to the density and uniformity of the output mixture. The mixed material is not exposed to any appreciable mechanical or thermal loading and remains unaffected in its physical properties.In the result, a uniform, mixed material is supplied to the precompressor, largely independent of fluctuations in the properties of the waste or re-cycled material supplied to the comminuting device, and accordingly, decisive improvements in the product result, which make it possible to dispense with the intermediate production of granulates or agglomerates and manufacture foils directly by means of the extruder. This possibility is decisive in view of the pressure on cost in the production of plastics foils, and can be realized without significant expense.

The re-circulation mixing is not a processing stage which slows down the operation of the feed device and it does not involve any great expenditure on power. Also, the requirements regarding operating equipment are low and can be met with known means, cheaply, and without occupying much extra space. For example, an external or internal conduit equipped with a material conveying blower may connect the lower region of the storage hopper with the upper region of the hopper. Also, a connection may be provided from the lower region of the storage hopper to an existing conduit conveying comminuted material from the comminuting device to the storage hopper, for example at the suction side of a conveyor blower.

Preferably, the extraction of comminuted foil for re-circulation through the storage hopper is effected via the feed of the precompressor, from the feed hopper of which a return conduit leads to the upper region of the storage hopper. Thus, not only does a simple extraction from the storage hopper result, but also the possibility of including the precompressor in the re-circulation mixing, and of stabilizing its adjustment, as will be explained in more detail later.

A conveyor, the conveying capacity of which is a multiple of the precompressorthroughput is conveniently interposed between the storage hopper and the precompressor.

The invention also provides a feed device for an extruder for thermoplastic plastics materials comprising a comminuting device for plastics material, particularly foil, a storage hopper for receiving comminuted foil from the comminuting device, a precompressor for plastics material to be disposed at the inlet to an extruder, the precompressor having a feed hopper arranged to receive comminuted foil from the storage hopper, means for re-circulating comminuted foil from a lower region of the storage hopper into an upper region thereof and means for regulating the level of comminuted foil in the feed hopper.

The regulating means likewise represents an im portant step in improving the output product, which is indicated in particular when it is a question of producing products, such as blown film, which are critical to process with the extruder. Conventionally, the fill level of the feed hopper fluctuates greatly which leads to severe pressure fluctuations in the precompressor, and in the extruder, which impair the extrusion product, even if the materials supplied to the comminuting device are uniform in nature.

Regulation of the level of fill of the feed hopper has proved a surprisingly effective means of improving the working operation and the working result of the extruder, since it permits the direct production of blow film sufficiently free of disturbance. The equipment design for carrying out this aspect of the invention may be effective to maintain a predetermined level of fill of comminuted foil in the feed hopper or a predetermined pressure level at the output of the precompressor by regulating the level of fill of comminuted foil in the feed hopper.

Preferably, the means for regulating the level of comminuted foil in the feed hopper is arranged to operate by re-circulating comminuted foil as aforesaid via an opening into a re-circulating conduit which is adjustable in height in relation to the feed hopper.f A specific embodiment of the invention embodying both aspects thereof will now be described, by way of example, and not by way of limitation, with reference to the accompanying drawings in which figure 1 is a diagrammatic view of a feed device according to the invention in both its aspects, shown feeding an extruder; Figure 2 is a vertical section showing the precompressor of the feed device of Figure 1; and Figure 3 is a horizontal section on line Ill-Ill in Figure 2 showing the precompressor.

With reference now to the accompanying drawings, the feed device illustrated in Figure 1,feeds an extruder 1 and is designated as a whole at 2. The feed device comprises a comminuting device or cutting mill 3 to which old or waste plastics material, for example, foils, cuttings, trimmings and the like are supplied, at 4. The waste plastics material is comminuted in the cutting mill 3 and the commi nuted material is supplied through a pipeline made up of pipes or conduits 5 and 6 with an interposed conveyor blower 7, to a storage and mixing hopper 8, the hopper 8 being preceded by a cyclone separator 9 connecting the conduit 6 to the hopper.

At the output side, the hopper 8 leads at its lower end, into a screw conveyor 10 which conveys the comminuted foil taken from the hopper 8 into a feed hopper 11 of a precompressor 12. The precompressor 12 compresses the comminuted foil and forces it down into the extruder 1.

The feed device is equipped with a re-circulation mixing device 13 which has, as additional parts, a conveyor blower 14, a suction pipe 15 communicat ing with the suction side of the blower and entering the precompressor 12, a further pipe 16 leading from the pressure side of the blower 14 into an upper region of the hopper 8 via a separatorcycline 17.

The screw conveyor 10 has a capacity which is a multiple of the extruder 1 to supply the circulation mixing device with an excess of comminuted foil some of which is then drawn off by the re-circulation device 13.

The precompressor has a regulating device for the level of fill of comminuted foil in the feed hopper 11, which will be explained in more detail with reference to Figures 2 and 3. In the embodiment illustrated, the extruder 1 is designed for the production of blown film which is upwardly extruded and expanded by a blast of air through the extruder nozzle 19 and, after solidification, withdrawn continuously as a film tube, between delivery rollers 20, 21.

The precompressor 12 which is illustrated to a larger size in Figures 2 and 3 connected to the screw conveyor 10 through afeed pipe 22. Belowthe discharge end of the feed pipe is a magnetic plate 23 which separates any iron or steel particles from the comminuted foil being delivered to the hopper 11.

The feed hopper 11 tapers downwardly to an outlet pipe 24 which leads into the extruder (not illustrated in Figures 2 and 3) and a precompressor worm 25 is disposed co-axially in the outlet pipe 24 and is driven via a shaft 26 from a precompressor drive 27 at the top of the feed hopper 11.

Disposed on the shaft 26 is an ejector arm 27 which rotates with the shaft 26 to mix the material contained in the feed hopper 11 and level the material in the feed hopper. In particular, however, the ejector arm 27 serves the function of transferring material from the feed hopper into an ejector box 28 which is disposed at the side of the feed hopper 11 and which is connected to the interior of the feed hopper 11 through an ejector slot 29. The suction pipe 15 of the re-circulation mixing device 13 reaches downwardly, obliquely from above, into the ejector box, to remove comminuted foil therefrom.

The suction pipe 15 is connected to the ejector box 28 through a supporting sleeve 30 and the supporting sleeve permits an adjustment of the suction pipe 15 in the direction of the longitudinal axis of the suction pipe between an upper end position, shown in continuous lines in Figures 2 and a lower end position shown in broken lines in Figure 2.

This adjustability of the suction pipe 15 enables the level of fill of the feed hopper 11 to be preset in a simple and obvious manner since the ejector arm 27 maintains the level of fill of the hopper at the level of fill of the ejector box. The suction pipe 15 removes all the comminuted foil near to its end section 31, that is to say to within a certain small suction distance the pipe picks up the comminuted material, and, since the ejector box 28 is connected to the interior of the feed hopper 11 through the ejector slot 29 and the ejector arm 27 maintains the level of fill in the hopper and the ejector box equal the height of the end section 31 of the pipe 15, determines the level of fill of the feed hopper 11. The required level of fill of the hopper 11 is predetermined for current operation. It will be understood that a variable level of fill, for example, to maintain a constant pressure to be measured at the output of the precompressor 12, can be accomplished by any known or conve nient means.

Altogether, in the manner described, there results a feed device 2 in which the comminuted plastics material is introduced into the top of hopper 8 and is extracted from the bottom of the hopper by the screw conveyor 10 in an amount which is a multiple of the amount to be processed by the precompressor 12 or the extruder 1. The feed hopper 11 comprises means for regulating its level of fill which consists, in particular, of the adjustable end section 31 of the suction pipe 15 supported by the action of the ejector arm 27. This regulating means excludes the possibility that considerable fluctuations in the level of fill of the hopper 11 will occur such as to adversely affect the work of the precompressor and extruder from occurring in the conventional manner.Instead the regulating means makes it possible to obtain a level of fill and a pressure level in the precompressor which provides continuous optimum results.

The excess of comminuted foil supplied by the conveyor 10 leaves the precompressor 12 through the suction pipe 15 and re-enters the top of the hopper 8 with the result that the contents of the hopper 8 are exposed, in part, to a continuous and repeated recirculation and mixing. Severe fluctuations in the comminuted particle size of the plastics material is thereby avoided so that, in operation, a more homogeneous mixture is continuously supplied to the precompressor 12, which thanks to its pressure level regulation, passes on this mixture, with a stabilized pressure to the extruder 1. In the result, it is possible that plastics foils can be produced again, directly, by the extruder 1, in an economical and rapid operation, without deterioration of the properties of the material, even from unequal thicknesses of foil waste and other plastics material residue.In other words, apparatus forthe utilization of waste for manufacturing an end product, such as blown film, which is critical in its production, is disclosed.

The above analysis relates to continuous operation of the apparatus after starting-up. During starting-up, the feed device may appropriately provide a preceding mixing operation. For this purpose, the feed device is first operated in the manner described above, but the precompressorworm 25 is driven backwards so that although the ejector arm 27 rotates, the worm does not pass on any comminuted foil to the extruder 1. With the re-circulation mixing device 13 switched on, a continuous mixing of the material accumulating in the storage hopper 8 results. The preliminary mixing can be terminated when a fill gauge 32 of the hopper 8 indicates that a first predetermined level of fill has been reached.

Then, the precompressor 12 can take up its normal operation. the input of waste material through the cutting mill 3 can be continued unless a second, upper fill gauge 33 on the hopper 8 responds, this being provided to prevent overfilling of the storage hopper.

Claims

1. A feed device for an extruder for thermoplastic plastics materials comprising a comminuting device for plastics materials, particularly foil, a storage hopper for receiving comminuted foil from the comminuting device and a precompressorfor plastics material to be disposed at the inlet to an extruder, the precompressor having a feed hopper arranged to receive comminuted foil from the storage hopper and means for re-circulating comminuted foil from a lower region of the storage hopper into an upper region thereof.

2. A feed device as claimed in claim 1, wherein the re-circulating means draws comminuted foil from the feed hopper of the precompressor.

3. A feed device as claimed in claim 2, wherein a conveyor having a conveying capacity which is a multiple of the precompressorthroughput is arranged to feed the precompressor feed hopper from the bottom of the storage hopper.

4. A feed device for an extruder for thermoplastic plastics materials comprising a comminuting device for plastics foil, a storage hopper for receiving comminuted foil from the comminuting device and a precompressorfor plastics material to be disposed at the inlet to an extruder, the precompressor having a feed hopper arranged to receive comminuted foil from the storage hopper, and means for regulating the level of comminuted foil in the feed hopper.

5. A feed device for an extruder for thermoplastic plastics materials comprising a comminuting device for plastics material, particularly foil, a storage hopper for receiving comminuted foil from the comminuting device, a precompressor for plastics material to be disposed at the inlet to an extruder, the precompressor having a feed hopper arranged to receive comminuted foil from the storage hopper, means for re-circulating comminuted foil from a lower region of the storage hopper into an upper region thereof and means for regulating the level of comminuted foil in the feed hopper.

6. A feed device as claimed in claim 4 or 5, wherein the regulating means is adjustable to maintain the comminuted foil in said feed hopper at a predetermined level.

A feed device as claimed in claim 4 or 5, wherein the regulating means is adjustable in dependence upon the pressure at the outlet of the precompressor to maintain said pressure constant.

8. A feed device as claimed in claim 6 or 7 and either one of claims 2 and 3, wherein the regulating means is arranged to operate by re-circulating comminuted foil as aforesaid via an opening into a re-circulating conduit which is adjustable in height in relation to the feed hopper.

9. A feed device as claimed in claim 8, wherein said opening is formed at the end of a donwardly directed suction line which is adjustable in height.

10. A feed device as claimed in claim 9, wherein said suction line communicates with the inlet side of a conveying blower.

11. A feed device as claimed in claim 9, wherein the suction line communicates with a conduit connecting the comminuting device with the storage hopper at the suction side of a conveying blower in said conduit.

12. A feed device as claimed in any one of claims 8 to 11, wherein a rotatable ejector arm is disposed in the feed hopper for ejecting comminuted foil laterally therefrom through a discharge opening.

13. A feed device as claimed in any one of claims 8 to 12, wherein the feed hopper has an associated ejector box communicating laterally with the inside of the hopper through a discharge opening and said suction line draws comminuted foil from the ejector box.

14. A feed device for an extruder for thermoplas- tic plastics materials constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawings.