DE4232616A1 - Extruder combination for gas free extrudate - has first single screw extruder, twin screw extruder and second single screw extruder or gear pump - Google Patents

Abstract

Plant comprises initial single screw extruder followed by second, twin screw extruder. The first machine underfeeds the second extruder which has initial zone forming vacuum chamber and screws with the same rotational direction. Further single screw extruder or alternatively gear pump is positioned downstream from the second extruder and has controlled speed to give constant pressure. USE/ADVANTAGE - The extruder arrangement produces thermoplastic extrudate for use in spinning of microfibres where gas-free material is essential to avoid breakageI

Description

The invention relates to a cascade extruder for gas-free Extruding a thermoplastic according to the Preamble of claim 1.

For gas-free extrusion of molten Schnec polymers to connect the ken extruder in a cascade design, is known. For example, DE-PS 22 29 171 a generic device with two consecutively switched single-shaft extruders, with between them an evacuable vessel for the removal of gases from the Polymer melt, which is from the first extruder is fed to the second extruder. The fill level of this evacuable container is essentially con kept constant that the extruder feeding the container is regulated. Plants of this type have been found in general proven. However, they are less suitable for use in spinning systems for microfibers. As a result of their extremely fine Titers are the microfibers emerging from a spinneret particularly at risk of breakage, especially due to the Melt contained or suspended gas bubbles.

Therefore, the invention has for its object a to indicate direction, in particular for the spinning of Highly degassed polymer melts suitable for microfibers can prepare.  

The object is achieved by those stated in claim 1 Characteristics.

By using a twin screw extruder as the second Stage of a three-stage extruder cascade particularly favorable results when degassing polymer melts been achieved. The cited in claim 2 Simultaneity of the two screws of the twin screw extruders has the further advantage that the Schnec clean themselves in a manner known per se.

The basic values noted in further subclaims Working pressures and temperatures have changed at Processing of polyesters, polyamides, polyolefins and Polystyrenes well proven.

The third stage of the extruder cascade can be a wide one rer extruder, preferably a single-shaft, or else also a discharge gear pump according to common practice.

Both the twin-screw machine that performs the degassing in second place of the cascade as well as the actual one Extrusion of the melt, third extruder or the gear pump will be regulated, essentially constant speed driven. As a drive For this purpose, aggregates are preferably DC motors or frequency controlled three-phase motors for application, whereby only the first Ex used to melt the polymer truder is driven at speeds by the to be promoted by the second and third cascade levels Amounts of melt are dependent. That is, the exit of the doubles screw machine and the exit of the last extruder are carried out with regard to the melt quantities passing through them Pressure control is monitored and the actual values determined compared with specified target values, with any Deviations caused by changing the speed of rotation the first extruder screw, corresponding to the sign of the  Deviation, in the sense of an increased or decreased Melt production can be compensated. Preferably the Output amount of the melt extruder controlled so that the the degassing double screw machine below is feeding.

Further advantages of the invention result from the following ing description to which with reference to the drawing is referred to.

It shows:

Fig. 1 consists of three screw extruders Kaska de together with the associated, schematically illustrated control circuit for the control of the first extruder and

Fig. 2 is a schematic representation of a cascade formed from two screw extruders and a gear pump according to the invention.

The system 10 illustrated in Fig. 1 consists of a first single-screw extruder 20 with a screw 21 , which can be driven by a drive unit, preferably a DC motor 22 , with variable speed or speed.

The extruder 20 is provided with a feed hopper 23 , via which it with a bulk material, for. B. granulated polye, polyamide, a polyolefin or polystyrene can be loaded. The extruder 20 serves to melt the polymeric granulate, the heat required partly from the pressure - this may be between 50 and 200 bar - in the screw cylinder, which is produced when the extruder 20 is operated, from the shear force generated by the rotating screw 21 (friction ) and partially supplied via a cylinder heater from the outside. The extruder 20 is designed for a melt temperature range between 200 and 300 ° C., it being possible to work for polyesters with approximately 270 °, for polyamides with approximately 250 to 270 ° and for polyolefins and polystyrene with approximately 200 ° C. If necessary, the screw cylinder is also seen with a heating sleeve 24 ver. In order to prevent overheating of the melt and possible damage to the polymer caused thereby, the extruder 20 can be provided with cooling devices known in the art which keep the temperature of the melt in the range of an appropriate limit.

In the vicinity of its upstream end, ie in the vicinity of the feed hopper 23 , the screw 21 can be surrounded by a cooling ring 25 , which prevents the melt from flowing back in a known manner. The discharge end of the ex-extruder 20 is connected via a transfer line 26 to the inlet of a second screw extruder 30 , a throttle 27 being provided in the line 26 for relieving the melt flow.

The throttle 27 is used to keep the pressure at the inlet of the extruder 30 below 1 bar absolute. The second extruder 30 is a twin-screw extruder of the short design. The provided in the extruder 30 Schnec ken 31 and 32 are driven by a drive unit, preferably a DC motor 33 , in the same direction and at gere gelter, constant speed. Preferably, the screw flights are designed so that they interlock, so that the rotation of the screws 31 , 32 simultaneously leads to their cleaning.

The twin-screw extruder 30 serves to degas the polymer melt fed to it under reduced pressure from the first extruder 20 . For this purpose, the extruder 30 is provided with a degassing nozzle 34 leading into its screw cylinder, which leads via a vacuum device, for example a suction pump 35, into a disposal chamber 36 . The degassing 34 is seen in the conveying direction of the polymer melt in front of the line 26 coming from the first screw machine. In this way, penetration of polymer melt into the degassing pipe is prevented. The comparatively higher degassing effect of the twin-screw extruder 30 can be attributed to the good kneading action of the screw pair and to the enlarged free surface of the melt.

Following its degassing, the melt can be fed to a third single-screw extruder 40 at a pressure between 20 and 50 bar via a line 38 in the embodiment of the invention shown in FIG. 1. The third extruder is provided with a single screw 41 and serves as a hot melt extruder for homogenizing and discharging or metering the melt, for example via an outlet 43 , through a tool (not shown) or a spinneret. For this purpose, the pressure in the extruder 40 may be set up to up to 200 bar, wherein the screw extruder 40 operates in the temperature range 220-290 ° C. This temperature range has proven itself for the processing of polystyrene, polyamides and polyesters, the lower limit for polystyrene and the upper limit for polyester being used and polyamides being processed at temperatures around 280 ° C. Otherwise, the extruder 40 is essentially identical to the extruder 20 except for the formation of the filling section.

Like the twin-screw extruder 30 , the extruder 40 is driven by a drive unit, which is preferably a frequency-controlled three-phase or a direct current motor 42 , at a constant speed.

FIG. 2 shows an alternative embodiment of the invention, in which a melt discharge pump, preferably a gear pump 50 , as is generally used for the spinning of endless synthetic fibers, is used as the third stage of the cascade.

It is important for the spinning of synthetic filaments tig, the output of the third extruder stage, be it one Worm machine or be it a gear pump, constantly hold. For this reason, the extruder or tooth wheel pump regulated to constant pressure at the outlet (Pressure-speed control).

The twin-screw extruder 30 must also be kept at a constant pressure. For this purpose, regulation of the first extruder 20 is provided, which uses the melt pressure at the outlet of the second extruder 30 as a parameter for the extruded melt quantity. The measured value is fed via a measuring line 62 to a comparison circuit 63 , in which it is compared with a target value V2.

In the event of any deviations, a change in the drive speed of the screw 21, which is due to the sign, is carried out and the screw speed is monitored if necessary.

As soon as the melt pressure at the output of the twin-screw machine 30 deviates from a predetermined target value V2, a corresponding signal appears at the output 65 of the comparison circuit 63 . A signal is only generated in the comparison circuit 61 when the filling pressure at the input of the twin-screw machine 30 , which is tapped via a measuring line 60 at the filler neck, exceeds the comparison value V1. If a signal is present at the control device 66 , be it from the comparison circuit 63 or 61 , the speed of the extruder screw 21 is readjusted via control line 27 until the deviation signal disappears.

Claims (15)

1. extruder cascade ( 10 ) for gas-free extrusion of thermoplastic with two series-connected screw machines ( 20 ; 30 ) and a vacuum device, characterized in that the second screw machine ( 30 ) is a twin-screw machine and that its first screw area serves as a vacuum chamber. 2. extruder cascade ( 10 ) according to claim 1, characterized in that the screws ( 31 ; 32 ) of the twin-screw machine ( 30 ) are driven in the same direction. 3. extruder cascade ( 10 ) according to one of claims 1 and 2, characterized in that the first screw machine ( 20 ) is operated so that the twin screw machine ( 30 ) is relined. 4. extruder cascade ( 10 ) according to any one of claims 1 to 3, characterized in that a throttle ( 27 ) is provided between the output ( 26 ) of the first screw machine ( 20 ) and the twin screw machine ( 30 ). 5. extruder cascade ( 10 ) according to any one of claims 1 to 4, characterized in that the first screw machine ( 20 ) is designed for operation at pressures between 10 and 200 bar, preferably 50 and 100 bar. 6. extruder cascade ( 10 ) according to any one of claims 1 to 5, characterized in that the pressure at the input of the twin-screw machine ( 30 ) is below 1 bar absolute. 7. extruder cascade ( 10 ) according to any one of claims 1 to 6, characterized in that the first screw machine ( 20 ) can be driven at a variable speed. 8. extruder cascade ( 10 ) according to any one of claims 1 to 7, characterized in that the twin-screw machine ( 30 ) is driven at an adjustable, constant speed. 9. extruder cascade ( 10 ) according to any one of claims 1 to 8, characterized in that sensors are provided at the inputs and outputs of the twin-screw machine ( 30 ), the signals corresponding to the pressure of the input and output melt ben. 10. extruder cascade ( 10 ) according to claim 9, characterized in that the signals compared with predetermined comparison values and in the event of deviations the rotational speed of the screw ( 21 ) of the first screw machine ( 20 ) is changed according to the sign of the deviation. 11. extruder cascade ( 10 ) according to any one of claims 1 to 10, characterized in that a third melt conveyor ( 40 ; 50 ) is connected downstream of the twin-screw machine ( 30 ). 12. Extruder cascade ( 10 ) according to claim 11, characterized in that the third conveyor ( 40 ; 50 ) is driven for a constant pressure at a controlled speed. 13. extruder cascade ( 10 ) according to claim 12 or 13, characterized in that the third conveyor device is a single-shaft screw machine ( 40 ). 14. extruder cascade ( 10 ) according to claim 12 or 13, characterized in that the third conveyor is a discharge pump ( 50 ). 15. extruder cascade ( 10 ) according to claim 15, characterized in that the pump ( 50 ) is a gear pump.