EXTRUDER MIXER

Abstract: A process and apparatus for producing an extrudate having a predetermined temperature. Material to be extruded is delivered by an extruder to a cavity transfer mixer and the output of the mixer is delivered to a die. The rate at which material is fed to the mixer and the rate at which energy is supplied to the material by the mixer is controlled such that the temperature of the material is increased to the predetermined temperature.

Description: EXTRUDER MIXER The present invention relates to a process and apparatus for producing an extrudate having a predetermined temperature. In order to bring about crosslinking or vulcanization of polymeric materials the application of heat is normally required. It is known practice to carry out the extrusion of hose, cable covering or other profile and to heat the extruded section to achieve crosslinking. The extruded section can be heated by passing it through steam, molten salt mixtures, hot gas, or heated fluidized beds, or by exposing it to radiant heat. The disadvantage of these systems is that polymeric materials are normally poor conductors of heat and therefore it takes a considerable amount of time for heat to penetrate to the interior of the extruded section and bring about the required crosslinking reactions. Improvements in production rates and/or economies in the size of the equipment used for the crosslinking process can be achieved if heat can be generated within the interior of the extrudate and two known systems are available which achieve this result. One system uses microwaves which dissipate energy within the extrudate after its formation into the required shape. The other system uses a so-called "shear head" which heats the polymeric material by dissipation of mechanical energy, prior to the material being formed into its final shape. Microwave heating systems have the disadvantage that the dissipation of energy is very dependent on the properties of the processed material which must contain substances of a polar nature. Problems also arise in the application of this form of heating to extruded sections containing a metallic component such as a cable. Shear heads suffer from the disadvantage that cooling has to be applied to the metal parts of the head in order to prevent premature crosslinking of material in contact with those parts. This cooling results in temperature differences within the extrudate which are undesirable for rapid and uniform crosslinking. In a typical shear head polymeric material is sheared in an annular space between surfaces rotating with a speed differential between them. The nature of the heat generation process and the need to keep the metal surfaces relatively cool mean that higher temperatures are generated in the centre of the annular space than at the peripheries. There is little or no material movement in a radial direction and insufficient time for significant conduction of heat to occur. Therefore this temperature differential is carried through to the die at the outlet of the shear head and hence - into the product being formed. As a result of work carried out to improve distributive mixing of polymeric materials processed in a single screw extruder, a device has been developed known as a cavity transfer mixer. Examples of such mixers are described in British Patent No. 930 339 and European Patent No. 0,048,590. Cavity transfer mixers cut and blend a flow of polymeric material to effectively mix layers of for example different pigment or other additive concentrations and blend layers with different temperatures to assist in producing a thermally homogeneous extrudate. One side effect of the cavity transfer mixer is to generate heat in the material being processed through the cutting and shearing actions which take place. Generally this is thought of as an undesirable effect of the cavity transfer mixer. It is an object of the present invention to provide a process and apparatus for producing an extrudate having a predetermined temperature which avoid the problems associated with the systems for heating extrudates outlined above. According to the present invention, there is provided a process for producing an extrudate having a predetermined temperature, wherein material to be extruded is delivered by an extruder to a cavity transfer mixer and the output of the mixer is delivered to a die, the rate at which material is fed to the mixer and the rate at which energy is supplied to the material by the mixer being controlled such that the temperature of the material is increased to the predetermined temperature. As the temperature of the material within the mixer is increased it is mixed so that the material is heated homogeneously. The invention also provides an apparatus for carrying out the above process, comprising an extruder, a cavity transfer mixer connected to the extruder outlet, a die connected to the mixer outlet, means for determining the rate at which material is fed to the mixer by the extruder, and means for controlling the rate at which energy is supplied to the material by the mixer, whereby the temperature of material fed to the die can be determined. The cavity transfer mixer can comprise an inner member or rotor driven at the same speed as a feed screw of the extruder, or may be independently driven at a different speed to. that of the extruder screw. The mixer rotor may rotate within a fixed outer member or stator, or the outer member within which the mixer rotor is mounted may itself be rotatable. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 illustrates a first embodiment of the present invention comprising a cavity mixer having an inner member driven at the same speed as a feed screw; Fig. 2 illustrates an alternative embodiment of the invention in which the cavity mixer is separately driven from the extruder screw; and Fig. 3 illustrates in detail an embodiment of the present invention of the general type illustrated in Fig. 2. Referring to Fig. 1, the illustrated arrangement comprises an extruder screw 1 supported in a conventional manner within a housing 2 so as to feed extrudate in the direction of arrow 3. The cavity transfer mixer comprises a rotor 4 secured to the end of the extruder screw 1 so as to rotate therewith and an outer member 5 supported within a housing 6. The outer member 5 is rotatable about the axis of the extruder screw and rotor assembly. Material forced through the mixer by the extruder screw 1 issues from outlet 7. The rate at which material is delivered to the cavity transfer mixer is largely determined by the rate of rotation of the extruder screw 1. The rate at which energy is imparted to the material within the mixer is determined by the speed of the surface of the rotor 4 relative to the surface of the outer member 5. Thus by appropriate control means the speed at which extrudate is fed to the mixer and the rate at which energy is imparted to that extrudate can be independently controlled so as to produce the appropriate extrudate temperature at the outlet 7. Referring now to Fig. * 2, in the illustrated arrangement the cavity transfer mixer comprises a rotor 8 supported within the stator 9. Extrudate is fed to the cavity transfer mixer by an extruder screw 10 rotating within an extruder casing 11. Thus the rate of extrudate supply is a function of the speed of rotation of the extruder screw 10 whereas the rate at which energy is imparted to the material within the transfer mixer is a function of the speed of the rotor 8. In addition to its function in raising extrudate temperature the extruder mixer of the invention can also be used to give good distributive mixing of additives introduced into the extruder itself or injected at or near the beginning of the mixing shear head. In particular crosslinking agents can be added to the extruder or into the mixing shear head and will be effectively mixed with the polymeric material before it is raised to crosslinking temperature. A further application of the system extruder mixer of the invention is in the grafting of silane type materials on to polymers using the chemical reaction described in British Patent Nos. 1 234 034, 1 286 460, 1 346 588, 1 412 628 and 1 406 680. Here the essential action required is to obtain good mixing of reactive chemicals into the polymer, then to heat that material to a temperature where grafting occurs. In this case crosslinking does not occur primarily through heat but takes place subsequently through chemical reactions normally involving the presence of moisture. Referring now to Fig. 3, an embodiment, of the invention of the general type illustrated in Fig. 2 will now be described. In the embodiment of Fig. 3, an extruder 12- feeds material through a breaker plate 13 to an inlet 14 defined by a stator 15. The extruder 12 is secured to the stator 15 by bolts 16. The stator comprises a body within which a lining 17 is secured, the lining 17 defining cavities 18. A rotor 19 defining peripheral cavities 20 is supported within the stator to rotate about axis 21. The rotor 19 defines an axial bore within which cooling passages are defined, the cooling arrangement comprising helical passageways through which a heat transfer medium is pumped in the direction of arrows 22 and 23. Thus the rotor is effectively temperature controlled and maintains a substantially constant temperature. Bolts 24 secure a stator retainer 25 on the output end of the stator and bolts 26 secure a die retainer 27 on the stator retainer. A wedge ring 28, die holder 29 and die 30 complete the die assembly. • Using an arrangement as shown in this embodiment cause for cable production or mandrels in hose production can be coated directly with polymeric material emerging from the mixing device. A heating and cooling jacket 31 is provided for maintaining the stator assembly at a predetermined temperature. In use, extrudate is fed to the cavity transfer mixer defined by the cavities 18 and 20 in the rotor and stator structures by the extruder 12. The rate of extrudate supply is a function of the speed of rotation of the extruder whereas the rate at which energy is imparted to the material within the transfer mixture is a function of the speed of the rotor relative to the stator. The two variables (rate of extruder rotation and rate of rotor rotation) may be controlled in accordance with the invention such that the temperature of the material being processed is increased to but does not exceed a predetermined desired temperature.

Claims: CLAIMS : 1. A process for producing an extrudate having a predetermined temperature, wherein material to be extruded is delivered by an extruder to a cavity transfer mixer and the output of the mixer is delivered to a die, the rate at which material is fed to the mixer and the rate at which energy is supplied to the material by the mixer being controlled such that the temperature of the material is increased to the predetermined temperature. 2. An apparatus for carrying out the process according to claim 1, the apparatus comprising an extruder, a cavity transfer mixer connected to the extruder outlet, a die connected to the mixer outlet, means for determining the rate at which material is fed to the mixer by the extruder, and means for controlling the rate at which energy is supplied to the material by the mixer, whereby the temperature of the material fed to the die can be determined. 3. An apparatus according to claim 2, wherein the cavity transfer mixer comprises an inner member driven at the same speed as a feed screw of the extruder. 4. An apparatus according to claim 2, comprising means for independently driving an inner member of the cavity transfer mixer on a feed screw of the extruder, whereby the rate of rotation of the inner member of the cavity transfer mixer and of the feed screw of the extruder may be different. 5. An apparatus according to claim 3 or 4, wherein the mixer comprises a rotor which is rotatable within a fixed stator. 6. An apparatus according to claim 3 or 4, wherein the mixer comprises a rotor which is rotatable within an outer -member, the said outer member being rotatable about the mixer axis. 7. An apparatus according to any one of claims 2 to 6, wherein the extruder comprises a rotor 5 rotatable about a first axis and the cavity transfer mixer comprises a rotor rotatable about a second axis, the first and second axes being mutually inclined. 8. A process for producing the extrudate having ° a predetermined temperature substantially as hereinbefore described with reference to the accompanying drawings. 9. An apparatus for carrying out a process to produce an extrudate having a predetermined 5 temperature substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: LOVEGROVE JOHN GEORGE ARNOLD
Applicants: SHAW FRANCIS & CO LTD, FRANCIS SHAW & CO LTD
Application year: 1986
Main classification: B29C47
IPC codes: B29C47, B29K96, B29K105, B29L23