EXTRUSION HEAD WITH ADJUSTABLE WIDTH, FOR THE EXTRUSION OF FAST THERMODEGRADABLE POLYMERS, AND EXTRUSION PROCESS MAKING USE THEREOF

Abstract: Extrusion head and extrusion process for molten plastic materials in sheets, wherein a laminar flow of molten materials is introduced into an inlet port (3) of the head (T), in which said materials in uniformly spread up to reaching a straight elongated outlet port (B) thereof. The width of said port (B) can be reduced by a pair of movable blocks R positioned at the opposite ends of said port and apt to be fixed thereto in the wanted position. Two independent pipes (2), wherein the molten material is circulated by pumping means (4), connect the inlet port (3) f the extrusion head (T) to the two opposite closed angle zones (A) formed inside the head in correspondance of said movable blocks (R).

Description: "EXTRUSION HEAD WITH ADJUSTABLE WIDTH, FOR THE EXTRUSION OF FAST THERMODEGRADABLE POLYMERS, AND EXTRUSION PROCESS MAKING USE THEREOF" » * * * * The present invention concerns a flat extrusion head for the extrusion of thermo-plastic polymers and, in particular, an extrusion head of this type which is particularly suited for use in the extrusion of polymers subject to a fast thermal degradation. In the extrusion process of thermoplastic materials in sheets, the solid plastic material in granules is charged into the hopper of an extruder which normally comprises a screw housed into a thermostatic cylinder, apt to draw the plastic material from the charging hopper and convey it to the extrusion head. While moving across the screw, the material is kneaded, melted, mixed and compressed, so as to reach the extrusion head in the required conditions of flowability and homogeneity. In the production of thermoplastic materials in sheets, the extrusion head applied at the outlet end of the extruder substantially consists of a hollow element having the general shape of an isosceles triangle, whose apex is connected to the outlet of the extruder and whose base consists of an elongated slit of constant high, from which the molten material comes out in the form of a continuous sheet. The inner part of the extrusion head is formed so that the extrusion channel may comprise a widening in the horizontal direction and a narrowing in the vertical direction, both being as even as possible, namely without any steps and/or sharp changes of inclination, and also so that said channel is positively free from stagnation or swirl zones of the flow of molten material. The need to form an extrusion head with the aforementioned flow characteristics is determined by the fact that the high temperature which the thermoplastic materials are apt to reach inside the extruder can cause therein the start of a partial thermal degradation, which may even lead to the carbonization of the material, with the consequent deterioration of the optimal physical characteristics, typical of each single material being treated, in the event that its stay time inside the extrusion head should exceed a preset length, variable according to the type of polymer being extruded. In fact, the response to high temperatures is not the same for all the materials, but it is evident that the flow characteristics of the extrusion heads should be planned according to the most critical requirements among those of all the different materials apt to be extruded with such heads. For the above reasons, it is evident that each extrusion head is planned and produced with a very precise width of its outlet port, and that any variations of said width - that is, in particular, narrowings thereof - are necessarily apt to alter, in a negative sense, the conditions of the flow of molten material inside the extrusion head. It is thus equally evident that, at least from a theoretical point of view, a producer of plastic materials in sheets should dispose of a number of extrusion heads having an outlet port of width corresponding to each possible width of the sheets of plastic material he needs to produce, and that moreover the extrusion head should be replaced at any change of width of the sheet of material being extruded. Such a solution has however been discarded in practice, both due to the costs involved and to the amount of space required to position a large number of extrusion heads having an outlet port of different width, and (perhaps above all) due to the fact that the operation to replace the extrusion head is long and toilsome, as well as involving a considerable waste of material. In fact, the removed extrusion head must be totally and accurately cleaned in short times, namely while the plastic material is still soft; subsequently, the newly mounted head must be filled up to reach an even flow, discarding moreover all the material which has been left into the extruder while replacing the head, seen that such material normally undergoes an at least partial thermal degradation. To overcome said drawbacks, the technique has spread on the market the use of extrusion heads extremely provided, in correspondence of the lateral ends of the outlet port, with small blocks having a substantially parallelepiped elongated shape which are apt to slide in front and along the outlet port of the extrusion head and be blocked thereon in the required position, with the interposition of suitable gaskets, so as to reduce the width of said port. A solution of this kind is illustrated in figs. 1 and 2 of the accompanying drawings which diagrammatically represent, in a plan and respectively front view, a conventional type extrusion head with the outlet port apt to be reduced in width. The drawings show the body of the extruder E with the extrusion head T, the straight outlet port B of said head T and the blocks R which allow to reduce the width of said outlet port B. This type of extrusion head solves the problem of the different widths of the sheet of material having to be produced, as it allows to adjust - very easily and in extremely short times - the width of the outlet port B according to the width of the sheet of material having to be produced. Nonetheless such a solution, for the very reasons set forth above, can be adopted only to extrude polymers subject to a very slight thermal degradation, namely polymers which can stand even quite long stay times at the high extrusion temperatures, without undergoing any deterioration of their chemical and physical characteristics. It is evident in fact that, in correspondence of the lateral ends of the outlet port B of the extrusion head T, the presence of the blocks R leads to the forming of closed angle zones A, wherein the flow of molten material - as diagrammatically illustrated in the drawings by arrows - is subject to swirls which determine backflows or stagnation points, and slow down the outlet of the material in respect of the previously planned laminar flow; this finally leads to an uncontrolled increase of the stay times, in the extrusion head T, of portions of molten material. This solution is hence acceptable and actually applied only in the extrusion of particularly stable polymeric materials, which do not undergo irreversible degradations in their own molecular chain due to a prolonged stay at high temperatures, as for example polystyrenes (PS), high impact polystyrenes (HPS), acrylonitrile-butadiene-styrene (ABS) copolymers, polymethyl methacrylate (PMMA). Whereas, for the materials which are more sensitive to the problem of thermal degradation, this solution cannot be adopted since the material which has degraded to an indefinite extent, in the zones A, would end by fouling to a variable and unforeseeable extent the lateral portions of the flow of molten material. Among such fast thermo-degradable materials we may cite, by way of example, polyesters, polyvinyl-chloride (PVC), cellulose esters, polyethylene (PE), ethylene-vinyl-acetate (EVA) copolymers, ethylene-vinyl-alcohol (EVOH) copolymers, polyvinyl-iden-chloride (PVDC). The polyester class forms a group of materials of special interest for the market, whereby there is a high request for an extrusion process allowing to extrude sheets of different width without having to replace the extrusion head and without being faced with the drawbacks mentioned heretofore. A first attempt to solve this problem has actually been made by producing extrusion heads allowing to introduce therein small blocks which, besides reducing the width of the outlet port B to the required extent, are also apt to suitably modify the flow lines inside the head T by partially occupying the zones A, so as to eliminate the stagnation of material in said zones. This technical solution has however not been particularly appreciated by the producers of plastic materials in sheets, whereby it has been scarcely adopted also because it involves considerable drawbacks as fas as its application and, on the other hand, it is not apt to fully solve the problems for which it was conceived. The drawbacks concerning the application of such inner blocks are substantially determined by the fact that the times of application and replacement thereof, including those required for the cleaning, practically correspond to the times required for the replacement of the whole extrusion head, whereby they do not allow to change the width of the sheet of plastic material without interrupting the production, as can instead be done with the outer blocks used in the extrusion of non-thermodegradable polymers. On the other hand, the introduction of the blocks into the extrusion head can easily create gap areas upon contact of the blocks with the inner wall of the head, whereby particles of material may temporarily stop in said areas undergoing an undesired degradation and the consequent problems as far as quality of the extruded material, once said particles break away from such areas and are extruded. Thus, in the extrusion of fast thermodegradable polymers, particularly polyesters, there is a high requirement for an extrusion process allowing to promptly and easily change the width of the sheet of extruded material, preferably without having to interrupt the production and without being faced with the drawbacks tied to longer stay times of the molten material inside the extrusion head, which are characteristic of the heads having an outlet port apt to be reduced in width with external blocks, already widely used in the extrusion of not fast thermodegradable materials. The object of the present invention is to thus supply a newly conceived extrusion head allowing to reach the above purposes. According to the present invention, said object is reached by means of an extrusion head for molten plastic materials in sheets, of the type in which a plug flow of molten material is introduced into an inlet port of the head, wherein said material is evenly spread up to reaching a straight elongated outlet port thereof, the width of said port being reducible through a pair of movable blocks positioned at the opposite ends of the port and apt to be fixed thereto in the wanted position, characterized in that it moreover comprises at least two independent pipes, which connect the inlet port of the extrusion head to the two opposite closed angle zones formed inside the head in correspondence of said movable blocks, and pumping means apt to allow the molten material to circulate into said pipes. The present invention also concerns an extrusion process for molten plastic materials in sheets, of the type in which a plug flow of molten material, fed from an extruder, is evenly spread into an extrusion head up to reaching a straight elongated outlet port thereof, the width of said port being reducible through a pair of movable blocks positioned at the opposite ends of said port and apt to be fixed thereto in the wanted position, characterized in that means are provided to continuously draw flows of molten material from the two opposite closed angle zones formed inside the head, in correspondence of said blocks, and introduce it into said plug flow of molten material, or viceversa. The invention is now described in further detail with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic plan view of an extruder with a flat extrusion head according to known technique, the outlet port of which can be reduced in width by external blocks; Fig. 2 is a front elevation of the extrusion head according to known technique, illustrated in fig. 1 ; Fig. 3 is a plan view similar to fig. 1 , illustrating the extrusion head with adjustable width according to the present invention; and Fig. 4 shows, on an enlarged scale, the detail of the angle zone A of the extrusion head, enclosed into the circle of fig. 3. Figs. 1 and 2 illustrate - as specified further above - an extrusion head T, the width of which can be adjusted by means of known type external blocks R; said head can be used only for the extrusion of not fast thermodegradable polymers. The different elements of this type of extrusion head have already been described in the introductory part of the present description. The extrusion head according to the present invention is instead illustrated in fig. 3. Its general structure is fully identical to that of the known type extrusion head shown in figs. 1 and 2. This structure comprises two ports 1 in correspondence of the closed angle zones A, formed at the lateral ends of the head T when its outlet port B is reduced in width by the blocks R, said ports 1 being apt to connect the inside of the head T with respective pipes 2 which both flow into the channel 3 feeding the head T with the molten material coming out of the extruder E. Circulation pumps 4, for example of the positive-displacement type, are moreover provided along said pipes 2. From the above description, the working of the extrusion head with adjustable width according to the present invention appears quite evident. In fact, when the width of the outlet port B is reduced, the pumps 4 are operated so as to draw from the angle zones A a certain amount of molten material and introduce it again into the feeding channel 3. The pipes 2 are obviously insulated and controlled in temperature, so as to keep the molten material at a constant temperature, preferably equal to that existing inside the head T, and thereby avoid any lack of homogeneity when the material drawn through the ports 1 is reintroduced into the channel 3. The flow rate of the pumps 4 is regulated so as to prevent any stagnation of molten material inside the zones A, and it is thus gradually increased as the width of the outlet port B gets reduced. The flow of molten material inside the head T - as illustrated by the arrows in fig. 3 - is no longer subject to any swirls or stagnation zones and it parts, in correspondence of the inner edge of each of the two blocks R, into two flows, namely a first flow which regularly reaches the outlet from the port B and a second flow which is sucked into the pipe 2 by means of a pump 4. Appropriately, the pumps 4 are kept in working conditions, at a minimum flow rate, also when the outlet port B has its full width, namely when the blocks R are totally removed. This actually allows not only to prevent the material left in the pipes 2 from undergoing a thermal degradation, but also to eliminate the problem of having to provide for valves closing the ports 1 , in order to restore the inner profile of the head T when the pumps 4 are not working. Such valves could in fact create small gaps or misalignments which could cause the temporary stopping of the molten material, with the drawbacks mentioned heretofore. The configuration of the pipes 2, illustrated in figs. 3 and 4, is of course purely schematic and indicative. According to a preferred embodiment of the invention, the pipes 2 can be housed into suitable lateral cavities formed inside the head T, and the pumps 4 can be positioned next to the feeding channel 3, so that the entire extrusion head T has a more compact design and the thermal control of the pipes 2 is facilitated. In the previous description, and in the drawings, reference has always been made to a flow of molten material moving in the pipes 2 from the outlet port B towards the inlet port of the extrusion head T. This is the flow direction most widely and generally used, even in the case of extruding a multi-layer material; normally, in fact, the unhomogeneous material (i.e. the material comprising the products forming the different layers) drawn from the ports 1 is reintroduced into the feeding channel 3 of the head T in correspondence of the central layer which is apt to withstand a certain variableness in the composition. Nevertheless, in the case of extruding sheets formed of a single material, it is equally possible to invert the flow direction inside the pipes 2, by drawing material from the feeding channel 3 and introducing it into the closed angle zones A of the head T. In both cases, in fact, it is possible to prevent swirls and stagnation of molten material inside the zones A, and the consequent possible thermal degradation of said material. From the previous description it can be seen how the present invention has fully solved the drawbacks typical of the known type extrusion heads having a reducible width, thus fully reaching the intended object, namely to supply an extrusion head with adjustable width and a respective process for the extrusion of plastic materials in sheets of adjustable width; such a process can be indifferently adopted with any type of polymeric thermoplastic material, particularly also with fast thermo-degradable polymers. In fact, the molten material is caused to circulate inside the head T in a perfectly uniform manner, allowing to keep a laminar flow with no swirls and stagnation zones, independently from the position taken up by the movable blocks R and, therefore, from the width of the sheet being extruded. The possibility of a very easy adjustment of the extrusion width allows to obtain great economical advantages in the production. In fact, the possibility to adapt the width of the extruded sheet exactly to that required for the necessary production allows to reduce the width of the side scraps to that strictly required for the trimming operations, contrary to what happens with the present production systems. In fact, in such systems, in order to try and reduce the overall stopping times of the machine - required for the replacement of the inner blocks in the extrusion heads of known technique - instead of producing each time sheets of the required width, one operates on the basis of a preset number of standard widths, thereby giving rise to a very high average amount of scraps. It is evident that, since the production involves thermoplastic materials, the scraps - when they are not fouled by thermodegraded particles - can be recovered by milling and be newly fed to the extruder. Nevertheless, this trimming, milling and remelting operation involves high costs as far as energy and logistics of the materials, which costs are positively eliminated with the extrusion head according to the present invention. The present invention has been described with reference to a particular schematic embodiment thereof, but it is evident that its protection scope extends to any possibile variants and improvements of the same, within reach of a technician skilled in the art, provided that they fall within the definitions given in the accompanying claims.

Claims: CLAIMS 1 ) Extrusion head for molten plastic materials in sheets, of the type in which a plug flow of molten material is introduced into an inlet port (3) of the head (T), wherein said material is evenly spread up to reaching a straight elongated outlet port (B) thereof, the width of said port (B) being reducible through a pair of movable blocks (R) positioned at the opposite ends of the port (B) and apt to be fixed thereto in the wanted position, characterized in that it moreover comprises at least two independent pipes (2), which connect the inlet port (3) of the extrusion head to the two opposite closed angle zones (A) formed inside the head in correspondence of said movable blocks (R), and pumping means (4) apt to allow the molten material to circulate into said pipes (2) . 2) Extrusion head as in claim 1 ), wherein said pumping means (4) are apt to draw molten material from said closed angle zones (A) and introduce it into said plug flow flowing into the inlet port (3) of the head. 3) Extrusion head as in claim 1 ), wherein said pumping means (4) are apt to draw molten material from said laminar flow flowing into the inlet port (3) of the head and introduce it into said closed angle zones (A). 4) Extrusion head as in claims 1 ), 2) or 3), wherein said independent pipes (2) are housed into a hollow space, controlled in temperature, formed inside the head (T) . 5) Extrusion head as in claims 1 ), 2) or 3), wherein said pumping means (4) consist of positive-displacement pumps. 6) Extrusion process for molten plastic materials in sheets of the type in which a plug flow of molten material, fed from an extruder (E), is evenly spread into an extrusion head (T) up to reaching a straight elongated outlet port (B) thereof, the width of said port (B) being reducible through a pair of movable blocks (R) positioned at the opposite ends of said port (B) and apt to be fixed thereto in the wanted position, characterized in that means (4) are provided to continuously draw flows of molten material from the two opposite closed angle zones (A) formed inside the head (T), in correspondence of said blocks (R), and introduce it into said plug flow of molten material, or viceversa. 7) Extrusion process as in claim 6), wherein the flow rate of said flows is gradually higher as the width of said outlet port (B) gets reduced. 8) Extrusion process as in claim 6), wherein the flow rate of said flows is at a minimum level, different from zero, when said outlet port (B) has its full width.

Inventors: SECHI MARIO, SECHI, MARIO
Applicants: RPC COBELPLAST MONTONATE S R L, SECHI MARIO, RPC COBELPLAST MONTONATE S.R.L., SECHI, MARIO
Application year: 2001
Main classification: B29C47
IPC codes: B29C47