Self-cleaning, mixing apparatus and method for the production of polyurethane mixtures

Abstract: A high-pressure mixing apparatus, of the self-cleaning type, for the production of polyurethane mixtures; the apparatus comprises a mixing chamber (22), wherein inlet conduits (23, 24) for feeding primary components radially open; a cleaning member (27) is reciprocable between a backward position wherein it opens the inlet conduits (23, 24) towards the mixing chamber (22), and a forward position wherein it closes the inlet conduits (23, 24) for the polyurethane components in respect to the mixing chamber (22), connecting them simultaneously to respective recycling circuits (47, 50) by means of grooves (39, 44) formed in axially spaced apart zones (27A, 27B) of the cleaning member (27). One of the primary polyurethane components, preferably an isocyanate, is fed axially into the mixing chamber (22) through a longitudinal hole (41) opening at the fore end of the cleaning member (27); moreover an annular seal (51) is provided on the cleaning member (27) in an intermediate position between the recycling zones (27A, 27B).

Description: BACKGROUND OF THE INVENTION The present invention relates to the preparation of polyurethane mixtures, and more particularly the invention is directed to a high-pressure mixing apparatus, of the self-cleaning type, for the production of polyurethane mixtures with two or more reactive components. Although the apparatus according to the present invention is suitable for preparation of any type of polyurethane mixture for making flexible, semi-rigid and rigid foams, according to some aspects disclosed herein, the invention is particularly directed to the preparation of mixtures starting from polyurethane chemical components having different viscosities. STATE OF THE ART In the preparation of polyurethane mixtures for the production of . moulded parts, use is made of formulations comprising two or more chemically reactive primary components, suitable for producing a polymer, using a high-pressure mixing apparatus of the self-cleaning type; according to conventional techniques, stoichiometrically metered quantities of the primary components, such as a polyol and an isocyanate, with various additives, are fed separately and injected radially in opposite or angularly spaced positions of a mixing chamber, wherein the various jets frontally collide with one another with kinetic energy which is sufficiently high to cause a strong atomisation and their total mixing. As schematically shown in Figures 1 and 2 of the accompanying drawings, which represent the general state of the art, in the case wherein the mixture is obtained from a polyol and an isocyanate which are premixed with various additives, the two reactant jets or streams are fed into the mixing chamber 13 of a self-cleaning mixing apparatus 10, via diametrically opposed conduits or injection nozzles 11 and 12, between which an angular spacing of approximately 180° exists; this angular spacing is represented by the double arrow 14 in Figure 2. Examples of these mixing devices can be found in a number of prior art documents, from among which the following are mentioned purely by way of an example: US-A-3.706.515, US-A-3.975.128, US-A-4.096.585 and US-A-4-332.335. In these apparatuses, during the recycling phases, the necessary seals are provided by a narrow dimensions between the external surface of the cleaning member 15 and the internal diameter of the hole defining the mixing chamber and the large angular space between the feeding conduits 11 and 12 for the two components, and between corresponding recycling grooves normally provided in opposite positions on the same cleaning member; these dimensions can approximated to a gap of a few microns. In general, the high viscosity of the primary polyurethane components used in conventional formulations contributes to improving the sealing conditions. Specifically the high viscosity makes any seepage of one component into the other through the very narrow gap between the inner wall of the mixing chamber and the outer surface of the cleaning member difficult even during the recycling phases, when the same cleaning member is moved fully forwards. Moreover effective sealing between the primary components during the recycling phases is a critical condition for continued operation of the apparatus. In fact any contact between the polyol and the isocyanate, or more in general between chemically reactive components during the recycling phases, could lead to the formation and/or deposition of solid particles on the inner surfaces of the mixing apparatus, as well as in the recycling circuits. Such solid particles can damage for the apparatus and/or require to be removed from service for cleaning and repairs. In an attempt to improve the mixing conditions, as schematically shown in Figure 3, it has likewise been proposed to slant the feeding conduits 11 and 12 for the polyurethane components to face towards the bottom or a wall of the mixing chamber 13. This technique is shown for example in US-A-3.933.312 and US-A-4.473.531. However, in this case too, the two primary components are always injected in a substantially radial plane, through conduits or nozzles which open directly in the wall of the mixing chamber. Any seepage of one polyurethane component into another, during a recycling phase, is once again prevented not only by a narrow gap between the contacting surfaces of the cleaning member and the mixing chamber 13, but also by a sufficient angular space between the same conduits 11 and 12. In other cases, in particular in the preparation of multi-component polyurethane mixtures with a chamber such as shown in Figure 4, where three or more primary components are fed at high pressure into a mixing chamber 13 through respective conduits 16, 17, 18 and 19, as schematically shown in Figure 4, the angular space between adjacent conduits is considerably reduced. In Figure 4 the angular spacing is equal to an angle of 90° or less, if account is also given to the diameters of the feed conduits themselves. In all cases this distance, measured on the contact interface between the mixing chamber 13 and the cleaning member 15, is reduced to about ten millimetres approximately, or less, and is totally insufficient for ensuring the necessary seal. Considering that the various polyurethane components are normally recycled at high pressures, of the order of 15-20 MPa (150 - 200 Atm) or higher. The jets or streams of the various primary components which collide at high speed in the mixing chamber 13 generally have comparatively high flow rates, that is there is sufficient kinetic energy for ensuring good and complete mixing. Accordingly in the cases in which the pressure of recycling of the components is considerably high, the angular spacing between the individual conduits and the corresponding recycling grooves is inadequate to ensure sufficient sealing, such as to prevent the seeping of one component into another. Use therefore has to be made of special techniques, such as for example the formation of appropriate longitudinal seals on the cleaning member of the mixing chamber 13, as schematically denoted by 20 in Figure 4. Examples of longitudinal seals on the cleaning member are represented in DE 2.117.533. According to this technology, the necessary seals between the feed conduits and the respective recycling grooves for the components are obtained by forming longitudinal slots in the cleaning member of the mixing chamber. The slots are later filled with an appropriate resin, by adopting a method which is somewhat complex and difficult to perform directly in the place of these mixing apparatuses. Moreover the resin filling technique for the sealing slots, due to the high wear, has an extremely limited life span, equal to a few working days of the apparatus. Therefore the resin filling has to be renewed frequently, with subsequent inconvenience and interruptions of the production cycle. The seal between conduits for feeding the components, between the various recycling grooves respectively, in certain cases is more critical when a total pre-polymer or a polyurethane TDI formulated foam is used, i.e. when one of the primary chemical components has a considerably low viscosity, or a viscosity that is at least lower than the viscosity of the other component, for example of the order on a few centipoises. In general therefore the background art has always been oriented towards feeding all the primary components, in radial directions, substantially in the same plane and perpendicular to the longitudinal axis of the mixing chamber, through feeding conduits or nozzles which open directly in the side walls of the mixing chamber. Consequently, the sealing during the recycling phase is critical for the reasons already explained. For certain applications it has also been proposed to feed a component axially in the mixing chamber. This is described for example in GB-A-2.036.586, US-A-4.053.283, US-A-4.608.233 or in EP-A-594 981. In particular GB-A-2.036.586 describes a mixing device for the production of polyisocyanates, or more generally for mixing chemically reactive substances, which uses a special mixing chamber with a bell shape, in which the jets or streams collide in directions substantially at right angles with one another. This mixing device is therefore totally lacking in any mechanical self-cleaning, and the conduits for feeding the various chemical components for mixing, are provided directly in the body of the same mixing device. Moreover a similar mixing device in practice is suitable for continuous mixing processes, for which the control of the stoichiometric ratios between primary components to be mixed is, initially, less critical than discontinuous mixing processes used in moulding polyurethane. Therefore a similar device is wholly inadequate for discontinuous production of polyurethane mixtures, where the stoichiometric control of the components of the mixture must be strictly ensured right from the start, in theory from the first drop of mixture which is supplied into a mould. For this purpose, for some time high-pressure mixing apparatuses of the self-cleaning type have proved to be particularly suitable. In self-cleaning appartuses the same cleaning member of the mixing chamber also performs a valve function for simultaneous opening and closing of the injection nozzles, as well as recycling of the various components. However in self-cleaning mixing devices, of the traditional type, the various nozzles for injecting the components are arranged radially or variously oriented in relation to the longitudinal axis of the mixing chamber, maintaining practically a circumpherential arrangement, in the manner discussed previously with reference to Figures 1 to 4 and shown in the numerous documents mentioned previously. Unlike GB-A-2.036.586, US-A-4.053.283, US-A-4.608.233 and EP-A-594 981 show high-pressure mixing devices, of the self-cleaning type, in which the cleaning member of the mixing chamber is provided with a longitudinal hole which opens axially at the front end in the mixing chamber, for feeding a nucleation gas or an auxiliary, not chemically reactive, constituent. In particular, in US-A-4.053.283 a hole is provided for feeding nucleation air which extends along the entire cleaning member to lead into a rear air chamber. The remaining two documents, US-A-4.608.223 and EP-A-594 981, describe once again high-pressure, self-cleaning mixing apparatus, in which the longitudinal hole of the cleaning member is only used for feeding an auxiliary component, such as a dye substance, a releasing agent or anything else for which strict batching of the quantities fed is less critical, or has no effect on the resulting mixture nor on the features of the moulded parts. In the aforementioned documents, the polyurethane components, such as a polyol and an isocyanate, are therefore always fed into the mixing chamber by radial jets, in a wholly conventional manner. From the above, it is clear that, despite the fact that it has been proposed to use an axial hole in the cleaning member of the mixing chamber for feeding a component, the conventional art relating to high-pressure mixing devices of the self-cleaning type has always been oriented towards feeding the primary polyurethane components radially through the facing nozzles which open directly in the wall of the same mixing chamber. Therefore, in cases in which it is necessary to place several injection nozzles closer together, the need arises to use special techniques for the necessary sealing to avoid the risks and disadvantages referred to previously. OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide a high-pressure mixing apparatus, of the self-cleaning type, for the production of polyurethane mixtures to be fed into a mould, which is suitable for solving or considerably reducing the above mentioned problems and ensuring effective mixing and a strict stoichiometric control of the various components from the commencement of their feed. A further object of the invention is to provide high-pressure mixing apparatus, of the self-cleaning type, which is suitable for producing multi-component polyurethane mixtures, and which also allows feeding and recycling conditions at sufficiently high pressures, with sealing suitable for preventing any seeping of one component into the recycling circuit of another. The polyurethane mixture may include for example toluenediisocyanate (TDI) based or of total pre-polymer type, that is characterised by primary components having low viscosity, of the order of a few centipoises or tens of centipoises, and/or diphenyl-methane-diisocyanate (MDI) based, characterised by primary components of greater viscosity, for example on the order of a few hundred centipoises. A further object again of the present invention is to provide a high-pressure mixing apparatus suitable for allowing a strict control of the stoichiometric ratio between the chemical components, during mixing, and a synchronism of start and stop of the various components, by the same mixing apparatus, by using a sole source of pressurised fluid to selectively control the feeding and subsequent injection into the mixing chamber of the chemicals to produce a polyurethane foam having a good quality. BRIEF DESCRIPTION OF THE INVENTION The above can be achieved by means of a high-pressure mixing apparatus for the production of a polyurethane mixture, in which at least a first and a second primary polyurethane components are fed under pressure into a mixing zone from which the resulting mixture is fed into the cavity of a mould, the apparatus comprising a mixing chamber having a longitudinal axis, and a cleaning member reciprocating into the mixing chamber. The cleaning member is provided with a longitudinal hole which opens at its front end towards the mixing chamber, and recycling grooves for the recycling of the various chemical components. The apparatus further comprises means for feeding the primary polyurethane components into the mixing chamber in which a first one of the primary polyurethane foaming components, is axially fed into the mixing chamber through the longitudinal hole of the cleaning member, and in which the cleaning member comprises a first and a second recycling zones axially spaced apart and provided with recycling grooves; and sealing means on the cleaning member, intermediate between said recycling zones. More precisely the invention is directed to a high-pressure self-cleaning mixing apparatus adapted to mix at least first and second reactive polyurethane-forming components and feed the mixed components to a mould. The apparatus includes at least a mixing structure defining a mixing chamber, a feed structure having first and second feed passages and first and second recycle passages, and a cleaning member having at least first and second longitudinally-extending recycle grooves spaced from each other. The cleaning member is movable between an open position in which the first feed passage radially feeds the first polyurethane-forming component to the mixing chamber and a closed position in which the first longitudinally-extending recycle groove communicates the first feed passage with the first recycle passage to recycle the first polyurethane-forming component instead of feeding the first polyurethane-forming component to the mixing chamber. The second feed passage may be communicated with a longitudinally-extending passage of the cleaning member to permit the second polyurethane-forming component to be fed into the mixing chamber along an axial direction of the mixing chamber. A valve member may be provided to prevent the flow of the second polyurethane-forming component to the mixing chamber by recycling the second polyurethane-forming component to the second recycle passage. Spacing between the first and second grooves seals the first feed and recycle passages from the second feed and recycle passages. According to another aspect of the invention, the cleaning member of the mixing chamber comprises a valve element slidable inside the longitudinal hole, as well as control means, which can be selectively actuated and in sequence, comprising hydraulic actuators coaxially arranged and operatively connected to a single pressurised fluid source. According to another aspect of the invention, a high-pressure self-cleaning mixing apparatus has been provided, for the production of a polyurethane mixture from first and second chemical components; the apparatus comprises a mixing chamber provided with at least one radial injection nozzle for a first chemical component, said mixing chamber opening into a delivery channel angularly arranged, with respect to the same mixing chamber. The apparatus further comprises a cleaning member movable in the mixing chamber between an advanced position to close the radial injection nozzle, and a retracted position to open the injection nozzle and permit the delivery of the first chemical component to the mixing chamber. The cleaning member comprises a longitudinal duct axially opening into the mixing chamber, to permit communication between an inlet side hole and the mixing chamber for delivery of a second chemical component. A valving member coaxially arranged in the longitudinal duct of the cleaning member permits the flow of the second chemical component to be controlled. Coaxially arranged control means operatively connectable to a pressurized fluid source, synchronously operates the valving member and the cleaning member between said advanced and retracted positions. BRIEF DESCRIPTION OF THE DRAWINGS These and further features of the mixing apparatus and method according to the invention will be made clearer by the following description, with reference to the accompanying drawings, in which: Fig. 1 shows schematically some parts of a conventional mixing apparatus; Fig. 2 shows a cross sectional view along line 2-2 of Figure 1; Fig. 3 shows schematically some parts of another conventional apparatus; Fig. 4 shows a cross sectional view similar to that of Figure 2, for yet another mixing apparatus; Fig. 5 is a longitudinal view of an apparatus according to the present invention; Fig. 6 is an enlarged view of a region of the apparatus of Figure 5. DETAILED DESCRIPTION OF THE INVENTION With reference first to Figures 1 to 4, a brief summary will be given of the general features of conventional high-pressure mixing devices, according to the prior art discussed previously. In particular Figures 1 and 2 show the fundamental parts of a known mixing device for the mixing of two components; the device comprises a body 10 having a cylindrical chamber 13 in which two nozzles 11 and 12 for injecting polyurethane components open radially in diametrally opposite positions in relation to the longitudinal axis of the mixing chamber 13. Reference numeral 15 denotes a cleaning member, connected, in a known manner, to a hydraulic drive cylinder to be moved between a backward or open condition, shown in Figure 1, in which the two nozzles 11 and 12 open towards the mixing chamber 13, and a forward closed position in which the cleaning member 15 expels the residue of the polyurethane mixture from the chamber 13, and connects the injection nozzles 11 and 12 to appropriate recycling grooves in the same cleaning member 15. As shown in Figure 2, the two nozzles 11 and 12 are arranged radially and opposite on a same diameter of the chamber 13, at an angular spacing of approximately 180°, indicated by the double arrow 14. This spacing is generally sufficient for enabling a good seal and for preventing the seepage of one polyurethane component into the recycling circuit of the other, when the cleaning member 15 is moved forwards to close the chamber 13. Figure 3 shows a device similar to that of Figure 1 except for the injection nozzles 11 and 12, instead of being perpendicular to the axis of the mixing chamber 13, are slanted towards the bottom of the same chamber, in the direction of the cleaning member 15. The slanting arrangement causes a reversion of the flow and greater turbulence condition for mixing. However, in this case too, the arrangement of the injection nozzles 11 and 12 is similar to that of the previous example. Figure 4 of the drawings shows a device in which four injection nozzles 16, 17, 18 and 19 are provided, again radially arranged, in opposite pairs. In this case the angular spacing between two adjacent nozzles, again indicated by the double arrow 14, is equal to or less than 90°. This angular spacing is inadequate for ensuring a good seal between the same nozzles in the recycling condition of the device, i.e. such as to prevent a seepage of one polyurethane component into the circuit of another, when the various components are recycled towards respective storage tanks via suitable grooves in the cleaning member 15. Therefore, it is necessary to implement some laborious techniques, for example by forming special longitudinal seals 20 on the cleaning member 15, in an intermediate position to the recycling grooves, as schematically shown. According to the embodiment of Figures 5 and 6, the present invention is again directed to a at high-pressure mixing apparatus, of the self-cleaning type, which uses a cleaning member in the mixing chamber, as a valve. The cleaning member is connected and arranged to permit selective and controlled feeding of a primary polyurethane component in an axial direction to the mixing chamber, that is in a direction at right angle or perpendicular to the radial flow of the other remaining components; according to this embodiment good seal conditions between the recycling grooves conditions are ensured and risk of seepage is eliminated. As shown in Figure 5, the mixing apparatus comprises a body 21 provided with a mixing chamber 22 into which two radial nozzles 23 and 24 open; the nozzles 23 and 24 serve to feed two opposite flows of the same component or different types of reactive components such as polyols and/or additives, according to the type of polyurethane formulation used. The mixing chamber 22 in turn opens towards a discharge conduit 25 arranged at right angle or forming an angle with the longitudinal axis of the same chamber 22; the discharge conduit 25 serves to supply the resulting mixture into a mould (not shown). The dimensions, configurations and relative arrangement of the mixing chamber 22 and the discharge conduit 25 can be modified in relation to what is shown. The mixing chamber 22 is preferably in the form of a cylindrical chamber of small volume, with a smaller diameter than that of the discharge conduit 25, and has an axial length which is preferably equal to or less than 2-2.5 times its diameter of the same chamber. The mixing chamber 22 extends to the rear (left side in Figure 6) into a cylindrical hole 26 of the same diameter, in which a cleaning member 27 slides; the cleaning member 27 also performs a valving function of opening and closing the various injection nozzles and to permit recycling of the polyurethane components. The cleaning member 27 of the mixing chamber 22 is therefore connected to a pressurised fluid actuated control device such as a piston 28 of a hydraulic cylinder 29 in order to be moved between a backward or open position, shown in Figures 5 and 6, in which it opens the various nozzles for injecting the polyurethane precursor components into the mixing chamber 22, and a forward or closed position in which the cleaning member closes the nozzles towards the mixing chamber. In a substantially similar manner, a second cleaning member 30 is operable in the discharge conduit 25; the cleaning member 30 is connected to the piston 31 of a respective hydraulic cylinder 32 in order to be moved between a backward position, shown in Figure 5, in which is free and clear of outlet of the mixing chamber 22, and a forward position in which the second cleaning member 30 closes the mixing chamber 22 and ejects the residual mixture from the discharge conduit 25. In this embodiment the two hydraulic cylinders 29 and 32 are of the double-acting type and can be connected to a source of pressurised oil by an appropriate valve system which effectively may govern the operation of the entire apparatus. Moreover, as shown in Figure 5, in order to improve mixing, the outlet of the mixing chamber 22 can be throttled so that the flow of the mixture leaving the chamber 22 is mixed further in the conduit 25. The throttling of the outlet of the mixing chamber 22 can be obtained by any suitable means. It is however preferable to use the same cleaning member 30 of the discharge conduit 25 for throttling the outlet of the chamber 22. In this respect the control cylinder 32 can be provided with an adjustable stop means for the piston 31, schematically shown by a screw which is screwed in the back wall of the cylinder 32. As referred to previously, the nozzles 23 and 24 which open radially in the mixing chamber 22, and are arranged to feed two opposing flows as indicated by A1 and A2 in Figure 6. The flows may include the same primary polyurethane component, preferably a polyol, if necessary premixed with various additives, or may include different polyols or a total pre-polymer. Another primary polyurethane component B, such as isocyanate or primary additive, which is reactive with the first component A is fed through a longitudinal hole in the cleaning member 27 and the mixing chamber 22. Each of the nozzlez 23 and 24 is connected to a respective tank 33 and 34 for storing the component, via a respective batching pump 35 and 36 in a manner which is substantially known. Reference numerals 37 and 38 in the various drawings also denote the conduits for recycling the components; the conduits 37 and 38 are connectable to respective injection nozzles 23 and 24 via respective recycling grooves 39 and 40 formed longitudinally to the cleaning member 27. In the rearward closed position the cleaning member 27 defines the bottom of the mixing chamber 22. As referred to previously, at least one additional polyurethane component B or another primary polyurethane component, preferably the isocyanate, for example an isocyanate based on TDI or MDI, or a primary additive such as chemical water, is fed through the cleaning member 27 axially to the mixing chamber 22 when the cleaning member 27 in a rearward open or backward position in relation to the nozzles 23 and 24. According to the present invention the component B is injected axially in the mixing chamber, at right angle or perpendicular to the radial nozzles 23 and 24. The component B is fed through a longitudinal hole or passage 41 in the cleaning member 27, which opens towards the mixing chamber 22 through a nozzle or a narrow orifice 43. The hole 41 for feeding the polyurethane component B extends from the front portion of the cleaning member 27, beyond a front recycling zone 27A for chemicals in which recycling grooves 39 and 40 are provided, towards and slightly beyond a second recycling zone 27B, to the rear of the previous one, where a further longitudinal groove 44 is provided; the groove 44 is used both for feeding the chemical component B to the mixing chamber 22 through the longitudinal hole or passage 41; the groove 44 is connected to the passage 41 by a radial hole 45; the groove 44 also serves to recycle the component towards a storage tank 46, through a conduit 47 which may communicate with the hole 41 of the cleaning member 27, via a second radial hole 48 on the opposite side of the radial hole 45. Other connections and arrangements are possible, without prejudice to the general principles described above. For example, instead of the single groove 44, two grooves and/or separate holes could be provided, for feeding and for recycling the component. Finally reference numeral 49 denotes a batching pump which feeds the polyurethane component B from the tank 46 towards the feed conduit 50 which opens into the groove 44. As referred previously, according to the present invention, valve means are provided for controlling both the feeding and recycling of all the polyurethane components. For example the front zone 27A recycles the polyurethane component A which is fed directly into the mixing chamber via the radial nozzles 23, 24; a second zone 27B, to the rear and axially spaced from the first zone 27A includes the groove 44 formed and arranged for feeding and recycling the polyurethane component B which is fed via the cleaning member 27, axially to the mixing chamber 22. The recycling grooves 39, 40 and 44 of the two recycling zones 27A and 27B are separated one from the other by an intermediate sealing zone 27C, which extends axially for a short length. The sealing zone 27C prevents any risk of seepage of the component B in the recycling circuit with one or both components A1 and A2. Therefore, in order to improve the sealing conditions, at the intermediate zone 27C one or more annular gaskets 51 can be provided, for example at least one O-ring which allows for a good seal between the groove 44 of the second recycling zone 27B, and the grooves 39 and 40 of the first recycling zone 27A. In this way a perfect seal is ensured for a long period of operation of the mixing apparatus. The cleaning member 27 of the mixing chamber 22 also performs a valving function also for the component B, which is injected axially into the mixing chamber 22. In the longitudinal hole or passage 41 of the cleaning member, a coaxially arranged valve element 52 slides and extends axially between the recycling zones 27A and 27B. The valve element 52 has a smaller diameter than that of the hole 41. The same valve element 52 and the cylindrical wall of the hole 41 define an elongated annular chamber 53 therebetween which opens towards the injection nozzle 43. The valve element 52, as shown in Figure 6, ends at its front end with a cylindrical tip 54 having the same length and the same diameter as the nozzle 43; in this manner, insertion of the tip 54 into the nozzle 43 cleans the nozzle 43. The valve element 52 can be reciprocated in the hole 41, and in tandem with the cleaning member 27 of the mixing chamber 22, by means of its own hydraulic actuator, which can be driven synchronously with the hydraulic actuator 29 of the cleaning member 27. In this respect, as shown in the drawings, the valve element 52 extends at the rear to define a stem 55 connected to the piston 56 of a second hydraulic actuator formed coaxially and inside the piston 28 of the hydraulic actuator 29. In this matter both of the actuators are connected to a single source of pressurised fluid for in-phase control of the cleaning member 27 and the internal valve element 52. In particular, as shown, the second hydraulic actuator comprises a chamber 57 directly inside the piston 28, in which the piston 56 reciprocates. The second hydraulic actuator 56, 57 is therefore arranged coaxially to the inside of the first hydraulic actuator 28, 29 so that both hydraulic actuators can be actuated in the same phase, via a connection to a single fluid pressure source. In practice this is accomplished by connecting the inner actuator 56, 57 of the valve element 52 to the source of pressurised fluid indirectly via the same chamber 29' of the hydraulic actuator 29. In this respect the two sides of the chamber 29' of the hydraulic actuator 29 can be connected alternatively to the pressure source via inlet/outlet apertures 58, 59; the two sides of the chamber of the second hydraulic actuator 56, 57, can in turn be connected indirectly to the same pressure source, through passages which open on corresponding sides of the chamber 29' of the first actuator. More precisely, the chamber of the second hydraulic actuator 56, 57 can be connected on one side to the aperture 58 of the first actuator, through a central hole of an annular element 60 screwed in the rear part of the piston 28, while it is connected to the other aperture 59, through the same chamber 29' and a hole 61 in the wall of the piston 28. The operation of the apparatus is described below when the valve element 52 and the cleaning member 27 are in their respective forward or advanced positions, the injection nozzles 13, 24 and 43 are closed, allowing the recycling of the polyurethane components to the respective storage tanks 33, 34 and 46. Further, any seepage or leakage of a polyurethane component into the recycling circuit of the other is prevented by the seal in the intermediate zone 27C of the cleaning member 27. When a mixing phase and feeding of the resulting polyurethane mixture into the cavity of a mould is to be performed, the pressurised oil is fed to the aperture 59 of the chamber of the cylinder 29, while the aperture 58 is connected to the drain. The pressurized oil is immediately transmitted inside the chamber of the second hydraulic actuator provided by the same piston 28 of the actuator of the cleaning member 27. Since the piston 56 has an extremely light weight, the valve element 52 is immediately moved backwards, and simultaneously the backward stroke of the piston 28 of the cleaning member 27 will also start, until the cleaning member 27 moves into the retracted condition shown in Figure 6. The sequence of the movements, both in one direction and in the other, will occur in any case in phase and in a controlled manner, enabling the opening or simultaneous closure over the various injection nozzles, and a strict control on the stoichiometric ratios of the various polyurethane components right from the start of their injection and mixing phases. The foregoing detailed description of the preferred embodiments of the invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art.

Claims: High-pressure self-cleaning mixing apparatus, for the production of a polyurethane mixture, wherein at least a first and a second primary polyurethane components are fed under pressure and injected into a mixing zone to form a mixture which is fed into the cavity of a mould, the apparatus comprising: a mixing chamber (22) having a longitudinal axis; nozzle means (23, 24, 43) for feeding the polyurethane components, opening into the mixing chamber (22); a reciprocating cleaning member (27) for the mixing chamber (22), said cleaning member (27) being provided with a longitudinal hole (41) opening at its fore end to axially feed a first one of said polyurethane components into the mixing chamber (22); and recycling means for recycling the polyurethane components, characterized in that said recycling means comprises first and second recycling zones (27A, 27B) provided with axially spaced and longitudinally extending recycling grooves (39, 44) on the cleaning member (27); and sealing means (51) on the cleaning member (27) in an intermediate sealing zone (27C) between said first and second recycling zones (27A, 27B). Apparatus according to claim 1, wherein said intermediate sealing zone (27C) extends axially. Apparatus according to claim 1, wherein the sealing means comprises at least one annular sealing member (51) on the cleaning member (27) between said recycling zones (27A, 27B). Apparatus according to claim 1, wherein the longitudinal hole (41) of the cleaning member (27) comprises a narrow orifice (43) defining an injection nozzle at its fore end. Apparatus according to claim 1, wherein a valve means (52) for controlling the flow of the component is provided into said longitudinal hole (41). Apparatus according to claim 5, wherein said valve means comprises an axially movable valve element (52) in the longitudinal hole (41) of the cleaning member (27), and wherein the cleaning member (27) and the valve element (52) are connected to respective pressure fluid actuators (28, 29; 28, 56) selectively connectable to a single pressure fluid source (58). Apparatus according to claim 6, wherein said pressure fluid actuators comprise a first double-acting hydraulic actuator (28, 29) having a piston chamber (29') and a reciprocating piston member (28) connected to the cleaning member (27), and a second double-acting hydraulic actuator (28, 56) for the valve element (52) coaxially arranged in the piston member (28) of the first hydraulic actuator (28, 29), said second actuator (28, 56) having fluid passages (60, 61) opening on opposing side into the piston chamber (29') of the first actuator (28, 29), for the connection to the pressure fluid source. Apparatus according to claim 6, wherein the valve element (52) comprises a stem portion having a smaller diameter than the longitudinal hole (41) of the cleaning member (27), and in which an annular chamber (53) is provided between said stem portion of the valve element (52) and the inside surface of the hole (41), said annular chamber (53) longitudinally extending between said recycling zones (27A, 27B). Apparatus according to claim 1, wherein the polyurethane mixture is produced by mixing an isocyanate with at least one polyol, in which one of said components fed through the longitudinal hole (41) of the cleaning member (27) is the isocyanate. Apparatus according to claim 1, wherein the mixing chamber (22) opens into an angularly oriented discharge duct (25). Apparatus according to claim 10 comprising means (30) for throttling the outlet opening of the mixing chamber (22). Apparatus according claim 11, wherein said throttling means (30) can be adjusted. Apparatus according to claim 12, wherein said throttling means comprises the cleaning member (27) of the discharge duct (25).

Inventors: BONANSEA ALBERTO, FIORENTINI CARLO, BONANSEA, ALBERTO, FIORENTINI, CARLO
Applicants: AFROS SPA, AFROS S.P.A.
Application year: 1998
Main classification: B29B7
IPC codes: B29C45, B29K75, B29C39, B01F5, B01F15, B29C67, B29B7, B01F3