A METHOD OF PROVIDING CYLINDRICAL WORKPIECES WITH A COATING OF A HIGHLY VISCID MATERIAL AND A CROSS HEAD EXTRUDER FOR USE THEREWITH

Abstract: A crosshead extruder having an extruder housing with a throughlet passage for cylindrical workpieces to be coated with a highly viscid material such as unvulcanized rubber, an extruder nozzle (12) for defining the thickness of such coating, and a side inlet for supply of a pressurized coating material to the extruder housing, is provided with an interior power driven worm member (22) for receiving the pressurized inlet material and conveying it around the workpiece passage and forwardly towards the extruder nozzle (12), whereby it is ensured that the coating material is present at the extruder nozzle (12) all the way around the workpieces (10) even when these are of a large diameter. For counteracting a rotational movement of the coating material out through the extruder nozzle (12) the worm member (22) has a protruding front end core portion which is held non-rotating by being mounted on a holding pipe (34) which projects rearwardly through the central passage of the worm member and beyond the rear end thereof, where the holding tube (34) is anchored non-rotatably to the chassis (42) of the crosshead extruder.

Description: The present invention relates to a method of applying, by means of a crosshead extruder, an outer layer of an unvulcanized rubber mass or a similar coating material onto a cylindrical workpiece, e.g. for the production of stabilized rubber cylinders for use in the printing industry. The term "crosshead" refers to an arrangement whereby an elongate carrier tube is conveyed through a "crosshead" extruder device which receives the coating material under a high pressure through a side inlet thereof and compacts this material on and around the carrier tube workpiece as the latter leaves the crosshead through an extruder nozzle adjacent the front end of the crosshead. Thus, inside the crosshead the crosswise introduced coating material is distributed around the cylindrical workpiece and is forced into contact therewith adjacent the outlet nozzle of the device, whereby the workpiece may leave the nozzle coated with a surface layer of any desirable thickness as determined by the diameter of the extruder nozzle opening. However, a highly viscid rubber mass is difficult to handle, and it has been found that these simple crossheads are not usable for producing coated products when the diameter of the articles to be coated exceeds some 50 mm. One problem is that the rubber is not well distributed around the workpiece tube, and it has been endavoured, as disclosed e.g. in GB-A-1,082,538, to effect a better distribution by incorporating in the crosshead a stationary worm structure which will guide the rubber around the workpiece, but still such a system will be usable for small diameters only, as it will be practically impossible to force the rubber through a worm groove of any considerable length. In connection with less viscid materials it is known to effect an extrusion by a crosshead having a rotating worm with a central passage for the throughlet of the articles to be coated, whereby the worm operates to draw the material from a supply hopper and deliver it under pressure adjacent the nozzle end of the crosshead. Such an operation will not be applicable for highly viscid materials, not only because they cannot just be drawn from a hopper, but also because the rotating discharge end of the worm will cause the material to be almost screwed onto the article to be coated, and normally the quality of such an extrusion will be unacceptable. Generally, in order to obtain a good result the space just inside the extruder nozzle and around the article to be coated should be filled by the viscid material under a high pressure and with an entirely even distribution of the material. In a crosshead structure the correct filling of the said space can be promoted by an increased inlet pressure of the material or an increased axial distance between the material inlet port and the extruder nozzle, but such measures will be usable to a limited extent only, because for larger dimensions the required movements of the material will be connected not only with extremely high pressure requirements, but also with the development of a considerable compression heat and frictional heat in the material. This, of course, is acceptable only to a certain extent, owing already to the risk of the rubber starting to be vulcanized in an uncontrolled manner. In practice, therefore, despite many prior proposals for crosshead extruders of different kinds, it is customary to produce rubber coated cylinders of diameters larger than some 5-10 cm by entirely different and rather difficult methods, primarily by a rolling on of thin rolled rubber sheets, this with the well known risk of air blisters being trapped between the sheets in an unacceptable manner. From DE-B-1,050,849 is known a cross head extruder adapted for coating wires or cables with an insulating material, this being technically remote from the task of applying a coating of a highly viscid material to cylindrical workpieces. However, this known extruder has a feature which has been found very useful for use in the present extruder, viz. that of the said projecting tube element, the so-called nozzle core, being anchored against rotation in being connected with a holding pipe, which extends rearwardly through the central passage of the worm, closely inside the inner wall thereof and rearwardly beyond the rear end of the worm, where the holding pipe is anchored to the extruder housing in a non-rotatable manner. Hereby the foremost nozzle core will be held stationarily without any radial supporting or holding ribs being present in the flow-through space between the periphery of the nozzle core and the inner surface of the extruder nozzle, such that the flow of the coating material through this outermost space can be as undisturbed as possible, i.e. it can flow axially without being screwed by any co-rotating nozzle core and without being cut by any exterior supporting ribs holding the nozzle core. This feature of the nozzle core being mounted on an inner holding pipe having its rear end anchored nonrotatably to the extruder housing is very advantageous also for extruders adapted to work with highly viscid substances, and on that background the present invention relates to a cross head extruder having the features stated in the introductory clause of claim 1. Such an extruder with its associated feeding equipment is a rather sophisticated unit, which could be used with an improved economy if it could be easily adaptable to operate with different sizes of workpieces and different types of viscid materials, and it is the purpose of the invention to provide such an extruder having a simple design. For working with different types of viscid materials it is important to be able to adjust the effective length of the free space between the end of the nozzle core and the extruder nozzle, and a measure to that effect, of course, would be to arrange for the nozzle core to be axially adjustable. However, the nozzle core, as also known from the said DE document, should be arranged as a widened head portion tightening rearwardly against the front end of the worm in order to avoid injection of the pressurized material rearwardly in the system, and it is difficult to arrange for the entire worm to be axially adjustable. In connection with the invention, despite the desire of an axial adjustability of the nozzle core, it is preferred to maintain the feature of the nozzle core comprising an outer tube head which is provided on the foremost end of the holding pipe so as to seal against the front end of the core tube of the worm and to project forwardly therefrom, preferably in a forwardly converging manner, and according to the invention this feature is used in combination with a further forwardly projecting end portion of an inner pipe mounted inside the holding pipe and being connected, outside the rear end of the holding pipe, with means for adjusting the axial location of the inner pipe, the inner pipe being rearwardly retractable from the holding pipe, all as specified in the characterizing clause of claim 1. The added inner pipe can be used as a guiding pipe for the workpieces to be coated, and with this pipe being retractable from the holding pipe it will be possible to use any of a variety of inner pipes having equal outer diameters, but different inner diameters, whereby it is possible to adapt the extruder for operation with workpieces of different diameters. The selected inner pipe may be adjusted with respect to its axial position such that it projects more or less in front of the core pipe head, i.e. it will thus reduce more or less the effective free length between the extruder nozzle and the front end of the nozzle core as a whole, this making it possible to adjust the extruder to operate with substances of different viscosities. According to claim 2 the said inner pipe can be made as a dual part comprising an exterior pipe and an interior lining tube, whereby a large number of different inner diameters, for different workpieces, can be provided by selecting and combining pipe elements from a relatively low number of such elements. In the following the invention is described in more detail, by way of an example, with reference to the accompanying drawing, in which:- Fig. 1 is a perspective view of a crosshead extruder according to the invention, and Fig. 2 is a sectional view thereof. The system shown in Fig. 1 comprises a crosshead extruder 2 having a side inlet tube 4, which is connected to a feeding extruder 6 operable to supply an unvulcanized rubber mass received through an inlet 8 to the crosshead extruder 2 under a high pressure. The crosshead has a central passage for a cylindrical workpiece 10, which by the action of the extruded material, is drawn through the crosshead in direction of the arrow shown, whereby the workpiece 10 will leave the crosshead through a foremost extruder nozzle 12, now provided with a coating 14 of the rubber mass. So far described the system is fully conventional. However, the crosshead is provided with an internal rotatable worm which is driven by a motor 16 having a driving pinion 18 cooperating with a larger pinion 20 on the rear end of the worm behind the rear end of the crosshead 2. The said worm is illustrated in Fig. 2, and is designated 22. It comprises a tubular worm core 24 projecting forwardly from a rear block portion 26 and provided with a single, double or multiple worm winding 28 cooperating with the inside of the cylindrical main housing 30 of the crosshead 2. The cylindrical block portion 26 projects rearwardly beyond the rear end of the housing 30 and carries the pinion 20. The front end of the worm 22 is located spaced behind the extruder nozzle 12, and the worm core 24 is forwardly extended by a conical worm core head 32, also named a "nozzle core", which seals against the front end of the worm core 24 and converges towards the extruder nozzle 12. The nozzle core 32 is connected with the front end of a holding pipe 34, which projects rearwardly through the central passage of the worm back to behind the rear end of the worm, where the holding pipe has an exposed end portion 36. This portion has a fixed radial arm 38, which projects downwardly and is secured to a rear projection 40 on a carrier chassis 42 of the crosshead, whereby the holding pipe 34 and therewith the nozzle core 32 is prevented against rotation. Inside the holding pipe 34 is arranged a straight cylindrical pipe 44 having a front end 46 projecting in front of the nozzle core 32 or rather forming a front end thereof and having a rear end 48, which projects beyond the rear end 36 of the holding pipe 34 and is provided with a downwardly projecting radial arm 50 connected with an adjustment spindle 52. The latter is held and guided by two fixed brackets 54, between which an adjustment nut 56 is mounted on the spindle 52. With this arrangement the axial position of the inner pipe 44 may be adjusted by rotating the adjustment nut 56, whereby the position of the front end 46 of the inner pipe 44 is finely adjustable. This adjustability of the effective front end of the nozzle core is very important in practice, particularly when the crosshead is used for handling varying types of coating materials. The workpieces 10 to be coated are conveyed and guided through the inner pipe 44, centered directly by this pipe or by means of an innermost lining sleeve 58 made e.g. of rubber. The extruder nozzle 12 is provided in an end cap 60 screwed onto the front end of the crosshead housing 30, and of course the crosshead should comprise several end caps 60 having different nozzle diameters for the production of coated products of different exterior diameters. Alternatively different nozzle inserts may be provided for insertion in one or only a few different end caps 60. Similarly, for adapting the extruder to workpieces of a wide range of diameters the apparatus may comprise a low number of inner pipes 44 having the same external diameter corresponding to the diameter of the central passage of the holding pipe 34, but having different inner diameters, each corresponding to respective outer diameters of a series of sleeves 58 having different inner diameters. The inner pipes and the sleeves are easy to exchange, as they can be retracted and inserted from behind, without the machine having to be cleaned. In Fig. 2 the contour of the front end of a thicker inner pipe 44 is shown in dotted lines at 47. In a manner not shown the machine may be provided with e.g. hydraulically operated means for retracting and inserting the inner pipes 44. In operation the rubber mass is forced into the inlet 4 from the extruder 6 and thus into the rear end portion of the worm winding 28, and the worm 22 is rotated by the motor 16 so as to convey the pressurized mass towards the extruder nozzle, the worm hereby discharging the mass successively all the way around the nozzle core 32,46. Thus, the material is "screwed" out into the space between the worm 22 and the nozzle 12, but since the nozzle core 32 is locked against rotation the frictional engagement between the material and both the outer surface of the nozzle core and the other rigid walls of the said space will soon cause the material to stop rotating as it is forced further towards the nozzle 12. Therefore, the highly viscid material is brought to safely fill out the space all around the path of the workpiece 10 and also to be forced into contact with the workpiece with a direction of movement more or less exactly coinciding with that of the workpiece (which is not rotated). A potential disadvantage of the rotating worm with respect to the material being screwed onto the workpiece is thereby eliminated. The workpiece is advanced by the drawing action of the moved material. For the handling of rubber compounds and most other highly viscid masses it is advantageous or practically necessary to hold the material at an elevated temperature, and at least during a starting phase, therefore, it is desirable to heat the crosshead. The crosshead housing 2 is constructed with a ring cylindrical chamber 62, which is connectable with a heater unit 64 through a switch over valve 66 for preheating of the crosshead for that purpose. However, during the work the frictional heat developed inside the crosshead may be liable to effect a temperature increase which is not only sufficient in this respect, but in fact more than sufficient in that it may be too high, e.g. by causing the rubber mass to initiate a vulcanization at some 90-100°C. For enabling the crosshead to operate with a reasonably high capacity it may be found necessary, therefore, to provide for active cooling of the crosshead, and in a preferred arrangement the chamber 62 is also connectable through the valve 66 with a cooling unit 68 for effecting such cooling when required. The nozzle core 32 should be adequately sealing against the front end of the worm 22, but it will be possible to provide for a suitable sealing even if the holding pipe 44 and therewith the nozzle core 32 is arranged in an axially adjustable manner, whereby the inner straight pipe 44 could be avoided. The inner pipe 44 should not necessarily be held in a non-rotatable manner. When the coating material is a rubber mass the coated workpieces 10 should normally proceed further to a heating station in which a well controlled vulcanization is effected. The extruder housing is provided with a radially projecting cylindrical portion 70 which is closed by an outermost lid 72 secured by bolts 74, the interior space of this portion being designated 76. This space communicates with the outside of the worm member 22 near the inlet end thereof, and it serves several purposes, viz. 1) as an expansion chamber for the coating material in case the inlet rate thereof should temporarily supercede the rate of material outlet through the extruder nozzle 12, 2) as a security device in case the inlet rate supercedes the outlet rate more permanently, whereby the lid 72 will be blown off by breakage of the bolts 74 for protection of the remaining crosshead apparatus against damage by excessive internal overpressure, 3) as an inspection opening enabling visual inspection of the handled material when the lid 72 is removed, 4) as an inlet for an infill material such as raw rubber usable for the cleaning out of a previously handled coating material from the worm and from the space next to the extruder nozzle, preparatory to the handling of a different coating material, and 5) as a chamber which is well suited to be connected to a vacuum source 78 for the extraction of air bubbles from the inlet material.

Claims: An extruder crosshead for applying a coating of an unvulcanized rubber mass or a similar highly viscid coating material onto a cylindrical workpiece, comprising a cylindrical extruder housing (2) having an extruder nozzle (12) in a front wall portion (60) thereof and having, adjacent its rear end, a cross oriented supply opening (76) for the introduction of the coating material, and internally provided with a conveyor worm (22) rotated from the rear end of the housing and having a central passage for the throughlet of elongate workpieces (10) to be treated, the core portion (24) of the worm (22) adjacent the delivery end thereof being extended towards the extruder nozzle (12) through a projecting tube element (32) which is anchored against rotation together with the worm, the projecting tube element (32), the so-called nozzle core, being anchored in being connected with a holding pipe (34), which extends rearwardly through a central passage of the worm, closely inside the inner wall thereof and rearwardly beyond the rear end of the worm, where the holding pipe (34) is anchored to the extruder housing (2) in a non-rotatable manner, characterized in that the nozzle core is constituted by a combination of an outer tube head (32) which is provided on the foremost end of the holding pipe (34) so as to seal against the front end of the core tube (24) of the worm (22) and to project forwardly therefrom, preferably in a forwardly converging manner, and a further forwardly projecting front end portion (46) of an inner pipe (44) mounted inside the holding pipe (34) and being connected, outside the rear end (36) of the holding pipe (34), with means (50, 56) for adjusting the axial location of the inner pipe (44), the inner pipe being rearwardly retractable from the holding pipe. A crosshead according to claim 1, characterized in having as accessories some different inner pipes (44) fitting in the central passage of the holding pipe, but having different interior diameters, and, for each of these inner pipes, a small number of lining tubes (58), preferably made of rubber, fitting inside the respective inner pipes (44), but having different internal diameters.

Inventors: RASMUSSEN, VIGGO
Applicants: VIRADAN A/S
Application year: 1987
Main classification: B29C47
IPC codes: B29C47, B29K21