DE2327540A1 - Mixing unit for screw extruder - comprising coaxial rotary mixing sleeve, driven at differential angular velocity - Google Patents

Abstract

A mixing unit at the screw tip of a screw extruder with plasticising and conveying screw rotating ain a cylindrical barrel, and a rotating cylindrical kneeding unit, features a freely rotating mixing sleeve arranged coaxially with the screw shaft and consisting of several baffle plates which are provided with openings and connected by bushings, each bushing forming, with the screw barrel, an outer chamber and with the screw shaft, an inner chamber. The mixing sleeve is driven at an angular velocity different from that of the screw shaft. The claimed construction eliminates excessive mech. stresses on the polymer melt, with consequent risk of polymer degradation.

Description

Mixing element at the screw tip of a thermoplastic processing screw press The invention relates to a mixing element at the screw tip a screw press processing thermoplastics with one in one cylindrical screw housing revolving plasticizing and conveying screw as well a rotating, cylindrical kneading element devices for mixing, kneading and Homogenizing thermoplastic plastic melts in screw presses are already happening known in numerous embodiments. What they all have in common is the temporary narrowing and then widening the flow cross-section for the plasticized one Plastic on its way through the screw press. By narrowing the flow cross-section the pressure in the melt before the constriction is increased and therefore imperfect Melted plastic particles plasticized0 In addition, occurs in the area of the Constriction an additional heating of the melt due to shear stress, which supports the homogenization process.

In addition to the so-called internal mixing effect due to the shear rate in a narrow cylindrical gap is often an additional external kneading effect exploited by damming elements.

It is known, for example, to have baffle plates in the area of the screw head to be arranged, which are either connected to the screw and rotate with it or stationary are arranged in the worm housing. The disadvantage of these embodiments that the plastic melt in the gap between the stationary and rotating machine part is subjected to high shear stress. Too much shear stress but should generally be avoided, as they can damage the plastic in particular can lead to degradation to a low molecular weight product.

There are also various gears arranged at the tip of the worm or rollers or other rollers arranged in the enlarged working gap at the end of the screw Rolling elements used. These also work directly with the rotating screw The plastic melt often becomes mechanically too strong when the tools are located claimed without a sufficient mixing effect can be achieved.

The invention was therefore based on the object of providing a mixing element find that without geometrical enlargement of the mixing zone compared to the mixing intensity the known embodiments significantly improved. This task is carried out with a Screw press of the type described in the introduction solved according to the invention by a coaxial to the worm shaft arranged and freely rotatable mixing sleeve, which consists of several with There are openings provided with baffle plates that are connected to one another via bushings, each sleeve having an outer chamber with the worm housing and with the worm shaft forms an inner chamber. According to a further feature of the invention, the mixing sleeve driven with a different angular speed compared to the worm shaft.

These measures in particular improve the mixing effect by dividing and spreading out the plastic melt several times at a lower level Shear stress reached.

Furthermore, the temperature homogeneity of the melt is more targeted as a result Changing places improved. Another advantage is that for a good mass quality required low energy requirement of the mixing sleeve, with an increase in the melt temperatures was not found in the mixing section.

The mixing zone can be easily adjusted by installing another mixing sleeve are changed and thereby the shearing, kneading, mixing and homogenizing effect each plastic to be processed can be adapted. That simple, symmetrical and Mixing element, which is designed to be fluidically favorable in terms of its shape, also has an effect favorable, for example, when changing the color of the one used Plastic or when processing temperature-sensitive products.

An exemplary embodiment of the device according to the invention is - in the drawings - Figure 1 to Figure 3 - shown schematically and in the following explained in more detail.

It shows Figure 1 a longitudinal section of the screw press with screw conveyor, Kneading element and mixing element, FIG. 2 shows a cross section (FIG. 1) in plane A. - B and Figure 3 a cross section (Fig. 2) in the plane C - D.

According to Figure 1 follows the first two screw sections, the plasticizing or conveying zone (3) and the shear part (4), the mixing element in the Area of the screw tip (5). The mixing element consists essentially of a mixing sleeve (6), which consists of several, advantageously 2-10, baffle discs (7) and a corresponding number of bushes (8) and placed on the worm shaft (2) is freely rotatable. Baffle plates (7) and bushings (8) can be shaped and be non-positively connected to one another but also made in one piece. the both outer sockets (8 ') and (8 ") are each end with an annular groove (9) or (10) for receiving the bearing elements, for example ball bearings without a cage. The axial deep groove ball bearing (11) at the screw tip (5) takes the entire Axial load, while the ball bearing (12) on the cylindrical shaft end exclusively serves for centering.

The annular baffle plates (7) point on their end faces an equal number of openings (13) and - (14), for example holes or Slits in the area of the outer and inner edge of the disc that oppose each other are offset, i.e. between two holes (13) on the outer edge of the pane a bore (14) is provided on the inner edge of the disc at a radial distance. These Breakthroughs (13) and (14) - connect the inner (17) and the outer (18) Chambers among each other. In addition, an equal number are sloping Breakthroughs (15) and (16), which correspond to the number of openings (13) and (14) and which connecting an inner chamber (17) to an outer chamber (18) and vice versa.

The total number of breakthroughs can be varied within wide limits. Depending on the nominal size of the screw press and the viscosity of the thermoplastics to be processed 9 - 36 bores, slots, etc. have proven to be useful overall.

The inside width of the sockets (8) is generally dimensioned so that inner and outer chambers are formed of approximately the same size. However, it is too conceivable to use chambers with different volumes.

The plasticized plastic conveyed in the direction of the nozzle (19) is broken down by the first baffle plate (7 ') into a large number of partial flows, which merge behind the baffle to form two main streams - inner and outer chambers. This process is repeated according to the number of baffle plates. In the inner Chamber (17) is the rotational movement of the rotating screw shaft (2) over the plastic melt partially transferred to the freely rotatably mounted mixing sleeve (6), while the in the outer chamber (18) located plastic melt because of the neighboring stationary screw housing (1) brakes the rotary movement of the mixing sleeve (6). As a result, the mixing sleeve (6) runs at a speed in the direction of rotation of the worm shaft (2) um, which is less than the speed of the worm shaft (2).

In tests with the same radial expansion of the inner and outer Chambers as well as with the same gap widths in columns (20) and (21) became a speed ratio Worm shaft (2) to mixing sleeve (6) observed from about 2 to 1.

According to the invention it is also possible not to free the mixing sleeve to be rotated, but by suitable and known ~ facilities a different, fixed angular velocity compared to the worm shaft to drive.

The plastic melt is on the way through the mixing element a subject to low and precisely definable shear and by the mixing element well mixed and homogenized. The two main streams of the plastic melt reunite behind the last baffle plate to form a single flow of plastic, which is discharged from the screw press via the nozzle.

Claims (2)

Patent claims Mixing element at the screw tip of a thermoplastic processing screw press with one in a cylindrical screw housing revolving plasticizing and conveyor screw as well as a revolving cylindrical one Kneading element, characterized by one arranged coaxially to the worm shaft (2) and freely rotatable mixing sleeve (6), which consists of several, with openings (13, 14, 15, 16) and baffle plates (7) connected to one another via sockets (8), each bushing (8) having an outer chamber (18) with the worm housing (1) and with the worm shaft (2) forms an inner chamber (17). 2) mixing element according to claim 1, characterized in that the Mixing sleeve (6) with an angular velocity that differs from that of the worm shaft (2) is driven. Sign. Blank page