GB1599591A

GB1599591A - Screw extruder apparatus

Info

Publication number: GB1599591A
Application number: GB1795/78A
Authority: GB
Original assignee: Sussex Plastics Engineering Inc
Current assignee: Sussex Plastics Engineering Inc
Priority date: 1978-01-17
Filing date: 1978-01-17
Publication date: 1981-10-07

Description

(54) SCREW EXTRUDER APPARATUS (71) We, SUSSEX PLASTICS ENGINEER ING Inc., of 454 Peach Tree Street, Andover, New Jersey, 07821, United States of America, a corporation existing under the laws of the State of New Jersey, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a screw extruder apparatus.

Degradable materials, such as poly(vinyl chloride), presently are extruded through annular die passages to form tubes, tubular film or parisons for subsequent blow molding. Both single and multiple screw extruders are employed. The annular passage for plastics extrusion is generally formed between an inner mandrel supported in concentric relation to an outer die member by a plurality of streamlined spider legs. The plastics stream must flow around these spider legs and then weld downstream in the annular area. These spider legs present an area for stagnation and subsequent degradation of degradable materials such as poly(vinyl chloride). The problem is most severe on single screw extruders, because of the high stock temperatures employed, and is less pronounced on multiple screw extruders.

However, on both types of equipment, the length of run often is limited by material degradation on the spider legs.

It is known that high molecular weight poly (vinyl chloride) provides superior impact strength, higher elastic modulus and higher tensile strength. However, such material has a higher melt viscosity which results in poor processability on either single or multiple screw extruders. This is due to non-uniform shearing patterns found in both single and multiple screw extruders, resulting in differences in the degree of mixing and final stock temperature within the melt.

A screw extruder apparatus made in accordance with this invention is particularly adapted for processing high molecular weight resins to produce smooth, uniform extrudates having fully developed physical properties.

According to the invention there is provided a screw extruder apparatus for melt extrusion of a polymer, comprising at least one screw for advancing the polymer through the extruder under conditions of elevated temperature and pressure, an extrusion die at the outlet of the extruder having an annulus formed between an inner mandrel member and a surrounding outer die member for flow of the polymer therethrough, and polymer pumping and shear ring means spaced from the outlet of said extrusion die comprising shear ring means mounted on at least one of said members for rotation about the common longitudinal axis of said members, said shear ring means having a surface facing said annulus and spaced from and opposed to a surface of the other of said members, and at least one helical pumping groove is provided in at least one of said surfaces, the outer die member and the inner mandrel member being stationary at the outlet of said extrusion die.

In operation, the rotating shear ring in combination with the helical groove or grooves subjects the material to a pumping and shearing action as the material passes through the die annulus, thereby providing a uniform mixing action and melt, which results in a smooth extrudate, and a pumping action, which reduces the pressure on the extruders. This results in a reduction of mechanical work on the material which,in turn, reduces the stock temperature of the material.

There have been numerous proposals to incorporate grooves or protrusions in one or both of the surfaces facing the annulus formed between the inner mandrel member and the surrounding outer die member of an annular melt extrusion die. See e.g. U.S. Patents 3 281 897; 3008 187; and 2937402 and German Patent 1 218 706. However, these prior proposals are all directed to providing circumferential orientation of mixing. They do not provide the means according to the invention for pumping molten plastics resulting from a combination of a rotating shear ring and a helical groove in a surface facing the annulus.

Moreover, the rotataing devices described in U.S. Patents 3 281 897 and 3 008 187 employ rotation at the outlet of the die which can result in wrinkling, corrugating and even tearing of viscous extrudates. In contrast, the outlet of the extrusion die used in the present invention is stationary so that these difficulties are avoided.

The present invention is illustrated in terms of a preferred embodiment in the accompanying drawings, in which: Figure 1 is a side elevational view, in simplified form, of a screw extruder apparatus according to the invention; Figure 2 is a side crossffectional view taken through the axis of the die; Figure 3 is an end elevational view of a modified form of the shear ring; Figure 4 is a sectional view taken along the line 44 of Figure 3; Figure 5 is a side corss-sectional view, similar to Figure 2, of another embodiment of the invention; Figure 6 is a side-sectional view of another form of the shear ring; Figure 7 is a fragmentary view, in section, of another embodiment of the invention; and Figures 8 and 9 are schematic representations of the pressure and temperature gradients, respectively, through an extruder apparatus of the prior art and of the invention.

Referring to Figure 1, there is shown an annular extrusion die 10 connected to an extruder 11. In accordance with this invention, the die includes a die shear ring 12, mounted for rotation about the die axis. The die shear ring 12 is mechanically coupled to a variable speed drive motor 13 by a drive chain 14 passing around a sprocket 15 connected to the output shaft of a gear reduction unit 16.

As shown in Figure 2, the extrusion die 10 comprises an outer die member 18 and an inner mandrel 19, said mandrel being secured to the die member by a plurality of radially-spaced streamlined spider legs 20. The die shear ring 12 is positioned in a cylindrical groove formed in the inner surface of the die member 18 and seal rings 22 are provided to prevent material leakage out of the die. A chain sprocket 23 is secured to the outer surface of the die shear ring. In this embodiment of the invention, helical grooves 24 are formed in the surface of the mandrel, whereas the facing surface of the die shear ring is smooth. Alternatively, the mandrel surface may be smooth and helical grooves may be provided in the die shear ring, as shown in Figures 3 and 4 wherein the numeral 24' identifies the grooves formed in the die shear ring 12'. Also, the mandrel 19 may be used with the die shear ring 12'. Also, the mandrel 19 may be used with the die shear ring 12' having helices of opposite hand.

In operation, the polymer flows through the die inlet orifice 25, Figure 2, around the spider legs 20 and into the region of the rotating shear ring 12 or 12'. The relative rotation between the shear ring 12 or 12' and the mandrel 19 pumps the polymer out through the outlet of the die 10.

Figure 5 shows an alternative embodiment of the invention in which the relative rotation between the mandrel and the die is effected by having the mandrel rotate and the die remain stationary. Thus, with reference to Figure 5, the extrusion die 30 may be used in place of die 10 shown in Figure 1. Extrusion die 30 has an offset inlet 31 in outer die member 32 adapted to be connected to the outlet of extruder 11 in a conventional manner. Outer die member 32 accommodates an adjustable terminal member 33, which is adjustable radially by means of adjusting screws 34 to ensure that member 33 is coaxial with the axis of the mandrel tip member 40.

Mandrel tip member 40 threadedly engages back member 41, which in turn carries mandrel shear ring 42 having gear teeth 43 that mesh with the teeth of the spur gear 44 carried by drive shaft 45. End cap 46 is bolted to outer die member 32 and to member 41. Bearings 47 and 48 are provided to support the shaft 45 for rotation. Any suitable motor (not shown) may be used to rotate drive shaft 45. Seal rings 49 seal the rotating mandrel shear ring 42.

The die member 32 is provided with helical grooves 50 in the surface facing mandrel shear ring 42. Upon rotation of drive shaft 45, mandrel shear ring 42. will rotate while the die 30 remains stationary.

Figure 6 shows a further modification of the invention, wherein mandrel shear ring 42' is provided with helical grooves 50'. Shear ring 42' may be substituted for mandrel shear ring 42 in Figure 5 so that both the die and mandrel will thus be provided with helical grooves, in which case the grooves will be of opposite hand. Alternatively, mandrel shear ring 42' may be substituted for shear ring 42 and the inner surface of the die member 32 can be smooth.

Figure 7 represents a further alternative embodiment of the invention, wherein rotating shear rings 12' and 42 are provided in the die and mandrel, respectively, the helical groove thus being in the die shear ring 12'. Shear ring 12' (Figure 7) may be rotated by connection to the motor 13 and chain 14 of Figure 1.

By the use of the die shear ring 12 or 12' and the mandrel shear ring 42 or 42' in the embodiment shown in Figure 5, it is possible to have the die shear ring and the mandrel shear ring both rotating, with the helical groove being in the die shear ring, the mandrel shear ring, or both. As mentioned above, when the die and mandrel both employ shear rings with helical grooves, the grooves are of the opposite hand.

The embodiment of the invention shown in Figure 5 operates in the same manner as the embodiments of Figures 14. Thus, drive shaft 45 is rotated and polymer is introduced from the extruder into inlet 31. Here again, the shear ring or rings (12, 12', 42, 42') will pump the polymer out through the outlet of die 30.

Figures 8 and 9 show pressure and temperature profiles, respectively, in prior art extrusion apparatus and in the apparatus according to the invention. Using the present invention, it is possible, by using sufficient lubrication in the extrusion compound and by controlling the extruder temperature, to hold the melt temperature in the range of 300-360 F on entering the region of the rotating shear ring or rings. Due to the work input required to shear the material, a controlled temperature rise can be achieved in the region of the shear ring or rings. This also provides uniform mixing and uniformity of melt which results in a smooth extrudate. The pumping action of the rotating shear ring or rings in combination with the helical grooves reduces the pressure acting on the extruder (Figure 8), which results in a reduction of the stock temperature of the polymer in the extruder (Figure 9). The apparatus according to the invention acts to maintain the polymer at the lowest possible temperature (Figure 9) until it passes through the shear ring or rings where the pumping and shearing action causes a rapid temperature rise to bring the polymer to its optimum outlet temperature just prior to exiting from the die.

Since degradation of poly(vinyl chloride) is a function of time and temperature, it is possible to heat PVC to relatively high temperatures for short time periods without encountering excessive degradation. In the proper operation of the described device, the polymer temperature rises quickly in the region of the shear ring and the material exits through the annular orifice where the extrudate is cooled and sized.

Consequently, the material is at a temperature above the normal range of 300-360"F for only a short period of time.

The utilization of a rotating shear ring in a pipe extrusion die provides the following advantages: 1. It provides a smooth, uniform extrudate with high molecular weight PVC resins, thus allowing the processor to utilize the optimum physical properties exhibited by these materials.

2. By shearing the material at an angle of substantially 90 degrees to the material flow path, flow discontinuities caused by the spider legs are eliminated, thus providing a more uniform wall thickness with resultant material savings.

3. Since the high temperature residence time is reduced, lower levels of costly stabilizers are needed. This advantage applies to resins of conventional as well as high molecular weight.

The above advantages for pipe extrusion can also be realized in the extrusion of other annular products such as film and parisons for blow molded objects as also with extrusions of other degradable materials such as chlorinated poly(vinyl chloride) and poly(vinylidene chloride). Poly(vinyl chloride) resins normally used for pipe extrusions are classified as GP415343 by the ASTM Specification for Poly (vinyl chloride) Resin, ASTM D 1755. The described die allows GP5-15443 resins, or resins of even higher molecular weight, to be processed into pipe with excellent properties.

A pipe extrusion run was made with the apparatus Figures 2 and 3 using poly(vinyl chloride) resin conforming to ASTM GP5-l 5443 requirements, stabilized with 0.2 per cent organotin mercaptide and employing a 2 inch 20/1 length to diameter ration single screw extruder using a screw with an 0.090 inch metering depth having a 3.5/1 compression ratio. The die consisted of a standard 1 inch Schedule 40 pipe die fitted with a rotatable shear ring 2 inches long and having an internal diameter of 1.280 inches, and forming an annular gap of 0.090 inch with the internal mandrel. The shear ring 12' had 8 helical grooves sith a helix angle of 600 between the flight lands and the axis of the ring and with a channel depth of 0.050 inch.

The facing surface of the mandrel 19 was smooth.

With the shear ring 12' stationary and the extruder screw operating at 17 RPM, output was 20.8 lbs.ihour at a pressure of 2600 psi and a stock temperature of 3500F. The extrudate was lumpy and unfused and a satisfactory pipe could not be formed. With the extruder operating at the same conditions, but with the shear ring 12' rotating at a speed of 33 RPM, output was 25.5 lbs./hour, pressure was 2100 psi and the stock temperature was 385 F. Both the inside and outside surfaces of the pipe were smooth and uniform and the material was com- pletely fused. Power consumed in rotating the shear ring was 800 watts.

While the preferred embodiment of the invention described above employed eight helical grooves in the shear ring, as few as one groove or substantially more that eight grooves can be used. For smaller pipe, it will generally be suitable to use from about 4 to about 12 grooves. For larger pipe, as many as 24 or more grooves may be needed. What is desired is that there be effective mixing and pumping without areas of stagnation, and through the use of conventional extruder screw technology the optimum number of grooves can be readily detennined.

If the rheological and thermal properties of the polymer are known, the design of the helices themselves can be calculated from existing procedures. Since it is common to produce a range of tube sizes on any given die head, the optimum operating speed will depend on the diameter and annular opening for the particular size being extruded. Optimum operating conditions can be readily found by first operating the extruder with the shear ring rotating slowly, then gradually increasing the speed of the shear ring until the proper extrudate temperature is reached. The condition of attaining a low operating temperature within the extruder is verified by locating a melt thermocouple in the entry region of the die.

WHAT WE CLAIM IS: 1. A screw extruder apparatus for melt

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. extrusion apparatus and in the apparatus according to the invention. Using the present invention, it is possible, by using sufficient lubrication in the extrusion compound and by controlling the extruder temperature, to hold the melt temperature in the range of 300-360 F on entering the region of the rotating shear ring or rings. Due to the work input required to shear the material, a controlled temperature rise can be achieved in the region of the shear ring or rings. This also provides uniform mixing and uniformity of melt which results in a smooth extrudate. The pumping action of the rotating shear ring or rings in combination with the helical grooves reduces the pressure acting on the extruder (Figure 8), which results in a reduction of the stock temperature of the polymer in the extruder (Figure 9). The apparatus according to the invention acts to maintain the polymer at the lowest possible temperature (Figure 9) until it passes through the shear ring or rings where the pumping and shearing action causes a rapid temperature rise to bring the polymer to its optimum outlet temperature just prior to exiting from the die. Since degradation of poly(vinyl chloride) is a function of time and temperature, it is possible to heat PVC to relatively high temperatures for short time periods without encountering excessive degradation. In the proper operation of the described device, the polymer temperature rises quickly in the region of the shear ring and the material exits through the annular orifice where the extrudate is cooled and sized. Consequently, the material is at a temperature above the normal range of 300-360"F for only a short period of time. The utilization of a rotating shear ring in a pipe extrusion die provides the following advantages: 1. It provides a smooth, uniform extrudate with high molecular weight PVC resins, thus allowing the processor to utilize the optimum physical properties exhibited by these materials. 2. By shearing the material at an angle of substantially 90 degrees to the material flow path, flow discontinuities caused by the spider legs are eliminated, thus providing a more uniform wall thickness with resultant material savings. 3. Since the high temperature residence time is reduced, lower levels of costly stabilizers are needed. This advantage applies to resins of conventional as well as high molecular weight. The above advantages for pipe extrusion can also be realized in the extrusion of other annular products such as film and parisons for blow molded objects as also with extrusions of other degradable materials such as chlorinated poly(vinyl chloride) and poly(vinylidene chloride). Poly(vinyl chloride) resins normally used for pipe extrusions are classified as GP415343 by the ASTM Specification for Poly (vinyl chloride) Resin, ASTM D 1755. The described die allows GP5-15443 resins, or resins of even higher molecular weight, to be processed into pipe with excellent properties. A pipe extrusion run was made with the apparatus Figures 2 and 3 using poly(vinyl chloride) resin conforming to ASTM GP5-l 5443 requirements, stabilized with 0.2 per cent organotin mercaptide and employing a 2 inch 20/1 length to diameter ration single screw extruder using a screw with an 0.090 inch metering depth having a 3.5/1 compression ratio. The die consisted of a standard 1 inch Schedule 40 pipe die fitted with a rotatable shear ring 2 inches long and having an internal diameter of 1.280 inches, and forming an annular gap of 0.090 inch with the internal mandrel. The shear ring 12' had 8 helical grooves sith a helix angle of 600 between the flight lands and the axis of the ring and with a channel depth of 0.050 inch. The facing surface of the mandrel 19 was smooth. With the shear ring 12' stationary and the extruder screw operating at 17 RPM, output was 20.8 lbs.ihour at a pressure of 2600 psi and a stock temperature of 3500F. The extrudate was lumpy and unfused and a satisfactory pipe could not be formed. With the extruder operating at the same conditions, but with the shear ring 12' rotating at a speed of 33 RPM, output was 25.5 lbs./hour, pressure was 2100 psi and the stock temperature was 385 F. Both the inside and outside surfaces of the pipe were smooth and uniform and the material was com- pletely fused. Power consumed in rotating the shear ring was 800 watts. While the preferred embodiment of the invention described above employed eight helical grooves in the shear ring, as few as one groove or substantially more that eight grooves can be used. For smaller pipe, it will generally be suitable to use from about 4 to about 12 grooves. For larger pipe, as many as 24 or more grooves may be needed. What is desired is that there be effective mixing and pumping without areas of stagnation, and through the use of conventional extruder screw technology the optimum number of grooves can be readily detennined. If the rheological and thermal properties of the polymer are known, the design of the helices themselves can be calculated from existing procedures. Since it is common to produce a range of tube sizes on any given die head, the optimum operating speed will depend on the diameter and annular opening for the particular size being extruded. Optimum operating conditions can be readily found by first operating the extruder with the shear ring rotating slowly, then gradually increasing the speed of the shear ring until the proper extrudate temperature is reached. The condition of attaining a low operating temperature within the extruder is verified by locating a melt thermocouple in the entry region of the die. WHAT WE CLAIM IS:

1. A screw extruder apparatus for melt

extrusion of a polymer, comprising at least one screw for advancing the polymer through the extruder under conditions of elevated temperature and pressure, extrusion die at the outlet of the extruder having an annulus formed between an inner mandrel member and a surrounding outer die member for flow of the polymer therethrough, and polymer pumping and shear ring means spaced from the outlet of said extrusion die comprising shear ring means mounted on at least one of said members for rotation about the common longitidinal axis of said members, said shear ring means having a surface facing said annulus and spaced from and opposed to a surface of the other of said members, and at least one helical pumping groove is provided in at least one of said surfaces, the outer die member and the inner mandrel member being stationary at the outlet of said extrusion die.

2. Apparatus according to Claim 1, wherein said shear ring means is mounted on said die member.

3. Apparatus according to Claim 1, wherein said shear ring means is mounted on said mandrel member.

4. Apparatus according to Claim 1, wherein a said shear ring means is mounted on said die member and on said mandrel member is provided with said helical groove.

5. Apparatus according to Claim 2, wherein the opposed surface of said mandrel member is provided with said helical groove.

6. Apparatus according to Claim 2, wherein the surface of said shear ring means is provided with said helical groove.

7. Apparatus according to Claim 6, wherein the opposed surface of said mandrel member is provided with said helical groove of opposite hand to said helical groove of said shear ring means.

8. Apparatus according to Claim 3, wherein said opposed surface of said die member is provided with said helical groove.

9. Apparatus according to Claim 3, wherein the surface of said shear ring means is provided with said helical groove.

10. Apparatus according to Claim 9, wherein said opposed surface of said die member is provided with said helical groove of opposite hand to said helical groove of said shear ring means.

11. Apparatus according to Claim 4, wherein said surface of said shear ring means on said die member is provided with said helical groove.

12. Apparatus according to Claim 4, wherein said surface of said shear ring means of said mandrel member is provided with said helical groove.

13. Apparatus according to Claim 12, wherein said surface of said shear ring means on said die member is provided with said helical groove of opposite hand to said helical groove of said shear ring means.

14. Apparatus according to any of the preceding claims in which said extruder is a single screw extruder.

15. Apparatus according to any of the preceding claims, in which said extruder is a multiple screw extruder.

16. A screw extruder apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.