Process and Apparatus for Degassing Plastic Melt in a Screw Extruder

The invention relates to the treatment of plastic melt in a screw extruder and particularly to a process and a degassing unit for degassing the plastic melt treated in the screw extruder, specifically by a mode of heating the degassing unit to prevent accumulation of plastic particles thereon especially at corner or edge regions thereof.

The degassing of volatile components from a polymer melt or solution is an important process step in the manufacture and treatment of plastic materials. Of decisive significance for the correct degassing of a specific type of plastic material and for maintaining operational safety is the arrangement and configuration of degassing openings in the housing of the screw extruder. It is known (H. Werner, Degassing of Plastics, Kunststoffe 71 (1981) 1, p. 23, FIG. 11), that degassing housings of various configurations can be utilized with or without ancillary degassing units for the screw extruders, depending on the vapor velocities that are produced and the tendency of the product to foam in the degassing zone in the extruder. The ancillary units can have a configuration which completely or partially covers the "wedge" region of two overlapping extrusion screws. Even if the ancillary degassing units are specially shaped to correspond to the prevailing flows in the degassing zone of the extruder, separation of particles from the plastic melt being processed cannot be avoided, particularly during start-up and when fluctuations in throughput occur. These separated particles adhere to the outer surfaces of the degassing unit and housing as well as on the inner wall of the degassing dome to form solid residues thereat. This produces a constricting of the cross section resulting in higher gas velocities which cause the entrained particles in the gas current to be increased.

The thus formed residues decompose under the effect of temperature and drop, almost decomposed, onto the screws after a certain period of time, and adversely affect the quality of the melt product. In addition, depositions at corners or edges of the degassing unit can become detached and drop into the melt at any time and cause melt contamination.

In order to eliminate the formation of depositions in the degassing dome, it is known from U.S. Pat. No. 3,078,512, to employ a displaceable unit, which has exactly the same contour shape as the degassing dome. The depositions in the degassing dome are removed and reintroduced into the screw extruder by periodic displacement of this unit by a hand lever system.

This system is relatively expensive and even if the degassing dome remains free of depositions by precise monitoring of the formation of depositions and corresponding operation of the displaceable unit, depositions on the surface of the unit facing the screws cannot be eliminated. Unfortunately, this is where most of the plastic particles are deposited.

An object of the invention is to provide a process and apparatus for degassing plastic melt in a screw extruder, which effectively prevents deposition of separated plastic particles on the surfaces of a degassing unit mounted in the extruder housing and especially at the corner or edge regions of the degassing unit.

The above and further objects are satisfied by the process of the invention which comprises heating the degassing unit to a temperature at least as high as the melting point of the plastic melt so that plastic from the melt which contacts the degassing unit will liquefy and return to the melt. The process of the invention is particularly addressed to heating the degassing unit at corners or edges thereof whereat the particles of plastic melt accumulate.

In the case of double screw extruders, the degassing unit is heated at the surface thereof directly facing a wedge-like region of the surfaces of the overlapping threads of the two screws of the extruder. Hereat, the surface of the degassing unit has two corners or edges in the form of cusps and the heat is supplied in the region of the cusps.

The above and further objects are further satisfied by apparatus of the invention according to which the degassing unit is mounted in the housing of the screw extruder with a surface facing the extrusion screw and the degassing unit is provided with means for heating the degassing unit so that at said surface the temperature will be at least as high as the melting point of the plastic melt.

In a particularly advantageous embodiment, the heating means comprises means for conveying a heated medium, preferably a fluid, through the degassing unit over the length of the opening in the extruder housing at which gas escapes from the melt. Advantageously, the fluid is conveyed in a channel in the degassing unit located in proximity to the surface thereof facing the extrusion screw and especially in proximity to a corner or edge of the degassing unit. When two screws with overlapping threads are utilized in the extruder, the degassing unit faces a wedge region formed by the surfaces of the overlapping screw threads and the surface of the degassing unit is formed with an additional corner or edge region in the form of a cusp, the channel being disposed in the degassing unit proximate said cusp.

Hence, a deposition of entrained plastic particles on the degassing unit is effectively prevented. Even at the time of start-up of the process, the formation of depositions is prevented as it is contemplated that the degassing unit is already preheated. In this way, it is assured that a constant cross section of the removed gas is always present in the course of the extrusion process, and there will no longer be any reduction in the quality of the plastic melt due to the presence of decomposed depositions.

The invention contemplates heating the degassing unit at its surface facing the extrusion screw. This has the advantageous feature of being most effective while saving heating energy by confining the heating to the region most at risk of accumulating separated melt particles.

FIG. 1 shows a simplified representation of a screw extruder partially in section.

FIG. 2 is a sectional view taken along line I--I in FIG. 1.

FIG. 3 is similar to FIG. 2 for a double screw extruder unit.

In the drawings is seen a screw extruder for a plastic melt which extruder comprises a housing 1 in which two adjacent extrusion screws 2 are rotatably driven in the same direction by a drive means 11. The housing 1 is preferably heated. The screws 2 have overlapping threads as evident from FIGS. 2 and 3. Plastic material to be treated is introduced at an inlet 12 and the plastic material is longitudinally advanced in successive treatment zones along the extruder and exits as a treated product at discharge outlet 13.

The invention is concerned with the apparatus and process of treatment of the plastic melt in a degassing zone Z. The degassing zone Z can be one of a number of such degassing zones located along the length of the extruder. In the degassing zone Z, the housing 1 is provided with an opening 9 for escape of gases from the melt and above the opening 9 a degassing dome 3 is mounted on the housing 1. A degassing unit 4 is mounted on the housing 1 beneath the degassing dome 3. The degassing unit 4 extends over the length L.sub.1 of the opening 9. A channel 5 is provided in the degassing unit and the channel 5 is connected to inlet 6 and outlet 7 for conveying a fluid heating medium through the channel. The channel 5 extends over a length L.sub.2 representing the distance between inlet 6 and outlet 7 and corresponding substantially to the length of the degassing unit 4. The channel 5 extends in proximity to a surface of the degassing unit which lies closely adjacent to the outer surface of the threads of a screw 2. In FIG. 2 the degassing unit 4 is disposed above the screw 2 at the right and its lower surface faces the threads of the screw and conforms in shape thereto. The degassing unit 4 cooperates with the screws to advance and guide the plastic melt in its longitudinal advance in the degassing zone while also providing a passage via opening 9 for escape of gas from the melt to the dome 3. The surfaces of the degassing unit 4 are polished in order to reduce adhesion forces.

The dome 3 has an outlet 8 for discharge of gases from the degassing zone Z. A suction pump (not shown) can be connected to outlet 8 to facilitate the discharge of gas from the dome.

In FIG. 3, the degassing unit 4' extends completely over the screw 2 at the right and partially over the screw at the left. The overlapping threads of the adjacent screws 2 form a wedge-like contour at the nip where the threads begin to overlap. The degassing unit 4' has a surface facing the threads which is of corresponding contour and defines an edge or corner region A in the form of a cusp and an edge or corner region B at the boundary of the opening 9 for escape of gas. In FIG. 2, the surface of the degassing unit facing the screw is formed with a single edge or corner region B. In both embodiments, the channel 5 is located in proximity to the edge or corner regions A and B to heat them and prevent accumulation of plastic melt particles. It is at these edges or corners where the plastic melt particles tend to agglomerate as these are the locations of maximum polymer and gas flow.

In operation, before the screw extruder is supplied with plastic material at inlet 12, the degassing unit 4 or 4' is heated by the heating medium, for example, heated oil conveyed through heating channel 5, to a temperature which is at least as high as the melt temperature of the plastic material to be treated. For example, if the melting point of the plastic material to be treated is 280.degree. C., then before the treatment process is begun, the edge or corner regions of the degassing unit 4 or 4' are preheated to approximately 300.degree. C. During the treatment process, the heating of the degassing unit continues so that the surface of the degassing unit facing the screws, especially the corners or edges thereof are at a temperature which is equal to or higher than the melting point of the respective plastic material.

Thereby depositions of separated plastic material from the melt can no longer form on the outer surface of the degassing unit 4 or 4' even during start-up when most deposits were previously formed on the degassing units, since the outer surface of the degassing unit is now hotter than the melting point of the plastic particles.

Since particles entrained with the gas flow occur first, for the most part, at the corners or edges of the degassing unit, the particles are liquified and are returned from the corner or edge regions into the plastic melt whereby the degassing dome remains essentially free of depositions.

Although the invention has been described with reference to specific embodiments thereof, it will become apparent to those skilled in the art that numerous modifications and variations can be made within the scope and spirit of the invention as defined in the attached claims.