METHOD OF PROCESSING SOLID POLYMER PARTICLES OF A POLYCONDENSATE BY MEANS OF A MULTI-ROTATION SYSTEM

Abstract

A method of processing solid polymer particles of a poly condensate by a multi-rotation system. Polymer particles are melted in a first extruder section having an extruder screw that rotates. The partly molten polymer mass containing between 5% by volume and 50% by volume of unmolten polymer particles is passed into a second extruder section with a poly-rotation unit and multiple satellite screws that rotate therein. A diameter of the poly-rotation unit is increased compared to the screw diameter of the first extruder section and a transition cone is formed between the extruder sections and a conical gap is formed with respect to the housing. Ambient pressure plastification of the remaining polymer particles is performed by passage through a drive zone. The polymer mass is guided completely molten in the drive zone onward through a venting zone under reduced pressure.

Claims

1. A method for processing solid plastics particles from a polycondensate via a multi-rotation system, the method comprising: drawing in of the plastics particles and at least partial melting of the plastics particles in a first extruder section with at least one extruder screw rotating in a housing recess of a housing; transferring the at least partially melted plastics composition into a second extruder section, which is designed as a multi-screw extruder section with a poly-rotation unit and a plurality of satellite screws rotating therein, a diameter of the poly-rotation unit being increased compared with the screw diameter of the first extruder section and a transition cone being formed between the extruder sections and a conical gap being formed in relation to the housing, the partially melted plastics composition as supercooled plastics melt contains between 5% by volume and 50% by volume of non-melted and non-dehumidified plastics particles; forwarding the plastics composition which has been completely melted in the drive zone through a degassing zone in which a vacuum is applied; removing volatile constituents from the plastics melt in the degassing zone; transferring the plastics melt into a discharge extruder section; subjecting the remaining plastics particles to pressureless plasticization by at least parts of the plastics composition being passed through a drive zone that is located downstream of the transition cone in a flow direction and which has exposed drive pinions of the satellite screws; and reheating, very shortly before entering the vacuum zone, the supercooled plastics melt to such an extent that the remaining plastics particles melt and thereby release the stored residual moisture.

2. The method as claimed in claim 1, wherein the solid plastics particles are melted by shock heating when they are passed through the drive zone.

3. The method as claimed in claim 1, wherein the partially melted plastics composition contains between 10% by volume and 40% by volume of non-melted residual particles when it is transferred from the first to the second extruder section.

4. The method as claimed in claim 1, wherein the feed and metering zone of the extruder screw is temperature-controlled by a fluid flowing in the interior thereof which has an supply temperature that lies between the glass transition temperature and the melting temperature Ts of the plastic of which the plastics particles are composed.

5. The method as claimed in claim 1, wherein the filling level in the multi-screw extruder section is less than 100%.

6. The method as claimed in claim 1, wherein a width of a conical gap between the transition cone of the extruder screw and the housing recess is adjusted via an axial displacement of the extruder screw in relation to the housing.

7. The method as claimed in claim 6, wherein the width of the conical gap is adjusted as a function of the pressure at the end of the metering zone of the first extruder section, a high pressure leading to an opening of the conical gap and a low pressure leading to a narrowing of the conical gap.

8. The method as claimed in claim 6, wherein the axially displaceably arranged extruder screw is supported on an upstream spring element on the housing and the extruder screw is damped by the viscosity of the melt in which it is mounted.

9. A multi-rotation system for carrying out the method as claimed in claim 1, the system comprising: at least one housing with a housing recess which has at least one housing opening in a degassing zone in which a vacuum is applied; an extruder screw which is rotable in the housing recess; a first extruder section with at least one feed zone and metering zone on the extruder screw; a second extruder section, which is designed as a multi-screw extruder section with a poly-rotation unit and a plurality of satellite screws rotating therein, a diameter of the poly-rotation unit being increased compared with the screw diameter in the first extruder section; a transition cone which is formed between the extruder sections on the extruder screw; a conical gap formed between the transition cone and the housing recess, the conical gap being adjustable via an axial displacement of the extruder screw in relation to the housing; a drive zone which is located downstream of the transition cone in the flow direction and which has exposed drive pinions of the satellite screws; and a discharge extruder section.

10. The multi-rotation system as claimed in claim 9, wherein a ratio of the length of the pinions of the satellite screws to the axial extent of the degassing zone is 1:40 to 1:6.

11. The multi-rotation system as claimed in claim 9, further comprising: at least one pressure sensor which is arranged upstream of the transition cone in the metering zone; an adjusting device via which the extruder screw is displaceable axially in relation to the housing; and a control unit which is connected to the pressure sensor and the actuating device.

12. The multi-rotation system as claimed in claim 9, wherein provided upstream of the feed zone is a spring element via which the extruder screw is supported on the housing.

13. The multi-rotation system as claimed in claim 9, wherein the extruder screw is adapted to be temperature-controlled at least in the first extruder section by a fluid flowing in an inner flow channel.

14. The multi-rotation system as claimed in claim 9, wherein the housing is adapted to be temperature-controlled at least in the first and second extruder sections.

15. The multi-rotation system as claimed in claim 9, wherein a discharge zone of the extruder screw has a diameter that is reduced compared with the poly-rotation unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

[0031] FIG. 1 shows a detail of a multi-rotation system in section; and

[0032] FIG. 2 shows a side view of an extruder screw and a pressure and temperature profile over the length thereof.

DETAILED DESCRIPTION

[0033] FIG. 1 illustrates a detail of a multi-rotation system 100. Arranged in a housing recess 51 in a housing 50 is an extruder screw which is divided into different zones. A poly-rotation unit 20 is arranged between a metering zone 12, which serves to homogenize the previously drawn-in and at least partially melted plastics particles, and a discharge zone 30, in which the completely treated plastics melt is conveyed away.

[0034] A transition cone 21 is formed at the transition from the metering zone 12; a conical gap 52 forms towards the housing 50.

[0035] This is followed by a drive zone in which pinions 23 of satellite screws 26 run in a rotary ring 24 which is connected to the housing and which has an internal toothing 24. Passages 25 are located between the pinions 23.

[0036] The satellite screws 26 rotate themselves, while the entire extruder screw rotates, and thus also the rotor in which they are mounted. They extend over the substantial part of the length of the poly-rotation unit 20 and are guided past housing windows 54, to which a vacuum is applied.

[0037] The satellite screws 26 are mounted with their front tips in a bearing carrier 27, in which a cone is again provided in order to return from the widened diameter of the poly-rotation unit 20 to the smaller diameter of the discharge zone 30. A further conical gap 53 is correspondingly formed there.

[0038] The structural design of the multi-rotation system 100 differs according to the invention in that the width of the conical gap 52 can be adjusted by way of an axial displacement of the entire extruder screw in relation to the housing 50 in order to utilize the gap width specifically for pressure control and thus to influence the proportion of solids fractions that have not yet melted and are flushed out via the transition cone 21.

[0039] For understanding of the method according to the invention, FIG. 2 illustrates the qualitative profile of the pressure p and the temperature T with respect to the axial extent of the extruder screw 101 with its various sections 1, 2, 3.

[0040] In a feed and metering extruder section 1, solid matter is first drawn in in a feed zone 11. Pressure is built up in a compression zone 13. In the following metering zone 12, the drawn-in plastic is at least partially melted and homogenized. According to the invention, however, only part of the solid matter is melted and homogenized, while another part of 5% to 50%, in particular 10% to 40%, remains as solid matter in the plastics melt.

[0041] In the temperature profile of FIG. 2, an average melt temperature is illustrated, that is to say approximately the average of the respective temperature of proportions of the melted plastic that are in direct contact with the extruder screw and those portions that are in contact with the inner wall of the housing. According to the invention, however, solid mass fractions with a correspondingly lower core temperature are still contained therein, such that the result is that the average melt temperature of the processed plastic in the feed and metering extruder section 1 lies below a melting temperature Ts.

[0042] The method is advantageous in particular for processing polyester. Here, the melting temperature is 235° C. to 260° C., depending on the degree of crystallization.

[0043] In order to obtain such a supercooled plastics melt, the extruder screw 101 is cooled at least in the feed and metering extruder section 1. For this purpose, the heat carrier used is in particular oil with a supply temperature between 90° C. and 130° C. At the same time, the housing wall (not illustrated in FIG. 2) is heated, for example to 280° C. The simultaneous heating and cooling in the same section 1 is not contradictory. The internal cooling serves to dissipate the partial heat output introduced by the rotation of the extruder screw 101, said heat output usually being higher at this point than required for the procedure. This is because the screw rotation speed must be matched to the rotation speed required in the multi-shaft extruder section 2 and can therefore not be reduced for the extruder section 1. By contrast, the heating on the housing serves to generate a lubricating film of molten plastic independently of the proportion of solid matter in the plastics melt conveyed.

[0044] The temperature rises slightly due to the heat input as a result of the rotation of the extruder screw 101 during the transition to the multi-shaft extruder section 2, but the average temperature of the plastics volume conveyed is preferably still slightly below the melting temperature Ts. Only in the drive zone, that is to say during passage through the region of the drive pinions 23, does the temperature rise abruptly, specifically significantly above the plastics melt temperature Ts. The plastic is therefore only completely melted and brought to a temperature level exactly where moisture and contamination can be extracted by means of the applied vacuum and the intrinsic viscosity can be increased by promoting the polycondensation reaction.

[0045] The further temperature profile in the discharge extruder section 3, downstream of the multi-shaft extruder section 2, is no longer important for the quality of the processing, but is constantly above the melting temperature Ts.

[0046] In addition, the pressure profile of the plastics melt in the extruder is plotted over the length of the extruder screw 101 in FIG. 2. The example shown is an extruder screw 101 in the case of which the feed zone 11 is not grooved, such that a pressure which rises only gradually from there to the transition cone 21 results.

[0047] Downstream of the transition cone 21, there are no longer any conveying elements on the extruder screw 101, with the result that a pressure drop occurs immediately. The pressure drops to a vacuum level of virtually zero in the case of the satellite screws 26. In the drive zone with the pinions 23 which is immediately upstream thereof in the flow direction, there is already no longer any significant pressure, such that the shock heating of the plastics composition, which takes place there and which causes the plasticization of the remaining solids fractions, takes place virtually in a pressureless manner.

[0048] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.