CH618381A5 - Process for extruding thermoplastic material and machine for implementing this process - Google Patents

Abstract

The external part of an extrusion channel (12') is surrounded by a heat-storage member (26) maintained at a temperature above the softening point of the thermoplastic material to be extruded. This storage member (26) constitutes a heat source from which each individual extrusion channel can be heated above the range of softening temperatures of the polymer, so as to assist the melting of the solidified block of polymer which could sometimes obstruct this channel. An air space (29) is used to slow down the heat transfer from the storage member (26) to the individual extrusion channels (12') in order to allow savings in heat whilst still maintaining the high temperature source available for melting solidified plugs. <IMAGE>

Description

The present invention relates to a method of extruding thermoplastic material; it also relates to a machine for implementing this process.

Already known, from US patent disclosure No. 3981959, a thermoplastic material extrusion die, known die which is shown in FIG. 1. This die, designated by the general reference sign 10, is approximately circular in shape and has several extrusion conduits 12 connected to an extrusion machine or a extruder. This die comprises a central zone for supplying thermoplastic resin such as polypropylene, polyethylene or the like, made fluid by heating and forced into several of these conduits 12. This initial zone is indicated by the brace 14 on the fig. 1. The material then passes into an intermediate zone 16 where a pressurized cooling fluid, for example water, is introduced through an orifice 15 so as to come into direct contact with the external surface of the resin filling the conduit d extrusion 12 which passes through a body 18 advantageously made of metal with an open porous structure as shown. As soon as the coolant comes into contact with the extruded material, due to a pressure decrease occurring at this time, part of this fluid vaporizes immediately and therefore quickly eliminates a significant part of the heat from the passing material in the body 18 by vaporization and by heat transmission, causing a drop in temperature. Although the device is hydraulically open, that is to say although the extrusion duct 12 freely ends at the face 20 of the die 10, part of the coolant is kept in the liquid state in s' flowing along the extrusion pipe towards the outlet face 20, as a pressure gradient is established in the pipe 12, from the high pressure end situated in the initial zone 14 towards the low pressure end or at ambient pressure, located at the outlet face 20. This results in the establishment of cooling gradients on the surfaces of the polymer wire and the wall of the duct and, consequently, a cooling effect on these surfaces. These pressures, greater than the ambient pressure, prevailing in the extrusion conduit 12 allow this liquid phase to be maintained, despite the maintenance in the conduit 12 of temperatures much higher than those allowing the existence of this liquid phase at pressure normal atmospheric. It is therefore likely that a mixture of water vapor and liquid phase water exists between the exterior surface of the

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extruded material and the inner surface of the conduit 12 and forms a film or a medium assuming both and effectively a cooling function and a lubrication function for the material which passes through this medium. It seems that this explanation corresponds to the phenomenon occurring in the extrusion pipe 12, although it is possible to give other explanations concerning the effective passage of the material in the pipe 12 and the formation of a solidified film and cooled surrounding a liquid core of material when the latter, in the form of a rod or wire, leaves the face 20 of the die.

The porous element 18 is fixed to the die 10 by means of an attached member 22 which is screwed into an outer part 24 of the body of the die. Seals 25 are mounted between the element 18 and the die. The part of the die located downstream of the body 24 constitutes a final zone 28 of extrusion situated downstream of the intermediate zone 16 in which the material is contained in the form of a rod or a wire having a solidified external surface. during its passage in the sector. At the outlet of the material at the face 20 of the die, a knife cuts the rod or the wire formed by said material into separate pellets or granules.

In such a device, one or more of the extrusion conduits 12 can be closed or blocked by solidified or cooled masses of materials, temporarily preventing the exit of the wire or the rod of these conduits due, inter alia, to the significant cooling effect produced by the cooling fluid under vacuum, and particularly due to the boiling (effect of the heat of vaporization) which occurs on the aforementioned surface of this material. Although the closure of one or even more of these extrusion conduits 12 in the case of a die having numerous conduits does not constitute a particularly serious problem, there may come a time when the efficiency of the operation decreases. It is obviously desirable to locate these plugs and to heat them directly and specifically in order to melt them. However, this method is impractical due to the large number of conduits which are generally used, at least in the current state of the art. In addition, it is known that maintaining a heat source around the conduits at a temperature substantially higher than that of the latter harms the control of the wires or rods produced by the extrusion machines known up to now.

The object of the invention is to reduce the frequency of solidifications and to allow resumption of extraction to be more effective and safer following these solidifications.

According to the invention, this object is achieved by the presence of the characters set out in claim 1, for the method, and in claim 7, for the machine implementing the method.

The appended drawing illustrates, by way of example and compared with what the prior art knew, embodiments of the subject of the invention; in this drawing:

fig. 1, already considered because illustrating the prior art, is a partial schematic cross section of a known extrusion head,

fig. 2 is a partial axial section, on a larger scale, of an extrusion head constituting an embodiment of the object of the invention and in which the high-temperature heating ring and the extrusion bores of the supply chain are arranged to reduce energy consumption, and fig. 3 is a partial section of a variant of the extrusion head.

In the extrusion head partially shown in FIG. 2, an attached sleeve 22a, delimiting a conduit 12 ′, is partially introduced into a plate 24a forming the outer surface of the die 10, in order to project therefrom. Juxtaposed with this plate 24a, an outer plate (or a reservoir) of heat 26 is positioned by stops or other conventional members which can be located at its periphery, so as to be placed at a certain distance from the outer surface of the plate 24a for

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form with it an air space or gap 29. This interval 29 not only extends over the entire front face of the plate 24a, but also surrounds a large part of the attached sleeve 22a, so as to reduce the speed at which the heat from the high temperature source or reserve plate 26 thermal is transmitted to the extrusion pipe 12 'by providing at least partial insulation between the two. However, the continuous exchange of heat by radiation between the reservoir or radiator 26 and the attached sleeve 22a, through the air space 29 in contact with a large part of the added sleeve 22a, makes it possible to melt the polymer plug. solidified and resume the extrusion operation although this heat transmission takes place at a speed lower than that which would be obtained with a conduction transmission. The presence of this high-temperature radiator also makes it possible to preheat the extrusion pipe to a temperature higher than that which can be obtained until now when the extrusion machine is started.

It should be noted that separate heating elements (not shown) are housed in the plate 26 so as to maintain its temperature independently of that of the remaining part of the die 10 at a substantially higher value. The plate 26, as shown in FIG. 2, has a boss 30 in contact with the added sleeve 22a, so as to carry out a heat transmission by the latter with conduction and thus increase the speed of this heat transmission if necessary and advantageously close to the face 20 exit from the die, since this zone tends to be subjected to variations in ambient temperature and that it can therefore cause large variations in the characteristics of the polymer thread passing through it. The boss 30, due to its small dimension compared to the overall dimension of the extrusion conduit 12 'in the final zone 28, limits the efficiency of the thermal bond between the elements of the die and, consequently, reduces the passage of heat from the plate 26 to the extrusion sleeve 22a, which has the effect of reducing the increased load to be absorbed by the lubrication and cooling film without these lubrication and cooling effects on the extruded material are significantly reduced.

Fig. 3 shows a heating plate 26b, similar to the plate 26 described above. This plate 26b is kept at a certain distance from the attached sleeve 22a by a ceramic spacer 32 which, as shown, consists of two parts, that is to say of a flat part intended to be applied against the plate 24a, and several cylindrical projections intended to surround the added sleeves 22a. In this embodiment, the plate 26b may also include an internal boss 30 providing efficient heat transmission by conduction between the heat source and the extruded material. The common feature of these two embodiments is the insulation which reduces the speed of heat transmission in order to save energy.

The following example relates to the extrusion process described above, in its application to a polypropylene having a melt flow index equal to 5 and a midpoint of the melting temperature range, allowing the work of this material, about 230 ° C. This material is extruded at a flow rate of about 1135 kg / h in a two-stage extruder, with a power of 300 kW, produced by the company Johnson Plastics Machinery Co. , Chippewa Falls, Wisconsin, EUA, and connected to a cutting device mounted opposite the die, as described in the US patent No. 3981959 cited above. The outer plate, that is to say the heat tank, is maintained at temperatures between 260 and 315 ° C. Cooling water is introduced under high pressure by attached sleeves made of sintered metal with open cell structure, surrounding each orifice, as described in the aforementioned patent, the water supply rate being adjusted so that the polymer rods leaving the die have a cooled outer casing

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and hardened and that they can be cut into pellets or granules which do not stick together.

The machine and the process implemented above for the polypropylene were used with a commercial acrylonitrile / butadiene / styrene copolymer, having an average melting temperature for extrusion of approximately 220 ° C. same flow rates as those indicated above are used, but the heat reservoir is maintained at a temperature of approximately 290 ° C. As previously, polymer rods are thus obtained comprising a cooled outer shell and which, when they are cut, produce separate granules (or pellets) which do not adhere to each other.

2 sheets of drawings

Claims (15)

618,381 1. A method of extruding thermoplastic material, characterized in that it consists in heating the material to make it fluid, in passing it through the initial zone of an extrusion die having at least one extrusion duct, to discharge by force this material in the conduit and to exit it from the die in the form of a rod or a wire comprising at least a solidified outer envelope which surrounds a liquid inner core, to direct a coolant under pressure in an intermediate zone of the extrusion duct, downstream of the initial zone, so that this liquid comes into contact with the external surface of the material while at least partially vaporizing so as to simultaneously form the solidified external envelope and a film lubrication between the inner wall of the duct and the material, this film facilitating the passage of the material in the duct, and passing the material through a final area of the duct it extrusion, downstream of the intermediate zone, the temperature of the final zone being higher than the softening temperature range of the extruded material. 2. Method according to claim 1, characterized in that the final zone is maintained at a temperature above 27 to 110 ° C at the material softening temperature range. 2 3. Method according to claim 1, characterized in that a lubrication film is maintained consisting of the partially non-vaporized coolant, so as to isolate the solidified outer surface from the material passing through the final extrusion zone to prevent a new softening of this material. 4. Method according to claim 1, characterized in that the rods are cut at their exit from the outer face of the die, so as to form separate pellets or granules. 5. Method according to claim 1, characterized in that one regulates the speed of the heat transmission between the material and the final zone of extrusion by a heat exchange occurring on most of the final zone, this heat exchange determining the rate of heat transmission to the extruded material. 6. Method according to claim 5, characterized in that the heat is transmitted by convection and radiation. 7. Machine for implementing the method according to claim 1, characterized in that it comprises an extrusion die having at least one extrusion duct, a device placed in an initial zone upstream of the duct introducing a material thermoplastic made liquid by heating in the conduit, under a pressure causing the passage of this material in the conduit and its exit from the die in the form of a rod or a wire having at least a solidified outer envelope which surrounds a core liquid interior, an element placed in an intermediate zone located downstream of the initial zone, applying a pressurized coolant through a part of the wall of the duct in direct contact with the outer surface of the material passing through this duct, a retaining element placed in a final zone situated downstream of the intermediate zone, comprising an extension of the duct, and a heat reservoir which surrounds the retaining element enue. 8. Machine according to claim 7, characterized in that the heat tank consists of a plate attached to the die. 9. Machine according to claim 8, characterized in that it comprises an element placed between the duct and the heat reservoir and intended to reduce the speed of the heat flow between the duct and the reservoir. 10. Machine according to claim 9, characterized in that the element reducing the speed of the heat flow is an insulating element. 11. Machine according to claim 9, characterized in that the element reducing the speed of the heat flow comprises an air space which completely separates the heat reservoir and the retaining element and which extends from the intermediate zone near the outlet face of the die. 12. Machine according to claim 11, characterized in that the plate constituting the heat reservoir comprises a part which comes into direct contact with the retaining element, between the downstream end of the air space and the face of outlet of the die, so as to ensure a passage of heat by conduction through this contact zone. 13. Machine according to claim 10, characterized in that the insulating element is a ceramic insert placed between the plate and the retaining element. 14. Machine according to claim 8, characterized in that the die has several extrusion conduits, and in that the die and the plate which constitutes the heat reservoir, are of annular shape. 15. Machine according to claim 14, characterized in that it comprises a cutting device mounted movably near the outlet face of the die and intended to cut the rod or the material wire into pills or separate granules when this rod or this wire leaves the die.