AN APPARATUS FOR FEEDING AND METERING FLUID COMPONENTS TO A HIGH PRESSURE MIXING HEAD

Abstract: An apparatus for feeding and metering fluid compo­ nents to a mixing head, in particular for metering and feed­ ing reactive chemical components (A, B) to a high pressure mixing head (10). The apparatus comprises at least a first (17) and a sec­ ond (18) transfer cylinder whose control unit (25, 26) is fed with hydraulic fluid under constant pressure, and whose component pumping unit (23, 24) is connected to a tank (11, 12) for the component (A, B), respectively to an inlet aper­ ture (21a, 22a) for the component in a chamber (10a) of the mixing head (10), by means of a proportional servo-valve (21, 22) which is controlled in relation to the rate of flow of the components so as to keep both the pressures and the flow of the components to be mixed constant.

Description: This invention relates to an apparatus for feeding and metering fluid components to a mixing head, in particular for feeding and metering reactive chemical components such as polyhydric alcohol and isocyanate or others, to a high pressure mixing head. There are generally-known apparatuses for metering and mixing two reactive chemical components, comprising a component metering and feeding assembly, in which the metering and feeding unit of each component consists of a hydraulically-actuated piston pump, connected to the storage tank of the component and, respectively, to a mixing head. In particular, the German patent EP-A- 0 120 419 refers to an apparatus of the above-mentioned kind, in which the hydraulic circuits controlling the component feeding and metering units are connected to a delivery pump common to both circuits of the type controlled in such a way that each of the hydraulic circuits for feeding the control fluid is provided with a flow control valve which can be controlled according to the speed of the piston assembly of each unit, and in which the hydraulic circuits are further connected to the control inlet of the fluid delivery pump. It is also desirable to feed the chemical components into the chamber of a mixing head under controlled conditions with regard to pressure, temperature and quantity, so that the mixing takes place in the desired stoichiometric ratios and within the established reaction times, in order to prevent the mixture from solidifying before reaching the mould. In particular, in high pressure mixing heads, it is essential to have a precise adjustment of the component feeding pressure, which must be maintained at a pre-established constant value, due to the fact that this pressure in substance constitutes the energy that gives rise to the correct mixing of the two jets of components which impinge at high speed in the mixing chamber. At present, the pressure is adjusted manually at the beginning of each stage of the work, or at the start-up of the installation, by acting on the injection nozzles of the mixing heads, without any possibility whatsoever of doing so while the apparatus is working, in that it would involve stopping or interrupting the work. The above constitutes a limit in the development of the present apparatuses in that there are causes of disturbance or alteration of the mixing conditions, which can take place when the apparatus is working, about which nothing can be done immediately. The use of automatic control systems in addition to the already existing ones is rather unadvisable in the present apparatuses, in that it could cause disturbance or interference with the other existing control equipment, with the risk of causing the entire control system to resonate. The problem of keeping the pressure and flow rate of the components constant in high pressure mixing heads is consequently of crucial importance. In the known apparatuses, it has therefore been suggested to control the constancy of the flow rates of the components, in relation to their feeding speed, and to vary the flow rate of the hydraulic control fluid in relation to variations in the aforesaid feeding speed, whenever greater power is required in the metering units in order to keep the flow rates of both components constant. For this reason, with the known apparatuses, it is not possible by means of adjustment to affect the causes which tend to alter the pressure required for feeding and injecting the components into the chamber of a mixing head; causes which for the most part can be attributed to residual substances or solidified particles of components in the nozzles of the injectors which consequently give rise to variations in the component injection pressure and in the mixing conditions. A scope of this invention is to provide an apparatus for metering and feeding reactive chemical components into high pressure mixing heads, which makes use of hydraulically-controlled piston-type pumping and metering units, by which it is possible to remedy the above-mentioned problems and to ensure that the components are fed into the mixing chamber at a constant pressure and rate of flow. A further scope of this invention is to provide an apparatus as described above, which is extremely simple in structure, capable of feeding the chemical components to the injectors of a mixing head maintaining the same components at established constant feeding pressures to provide the required constant mixing energy independently from the flow-rate control system for the chemical components. A still further scope of this invention is to provide an apparatus as described above, in which the hydraulic control circuit can operate with comparatively lower pressure values, due to the fact that the causes of loss of hydraulic power have been eliminated or reduced to a minimum. The above has been made possible according to claim 1 by completely reversing the conventional control criteria, that is to say, by operating in a way completely opposite to and not envisaged by the previously known automatic control systems. In other words, according to this invention, the pressure in the hydraulic control system of the apparatus is kept unchanged so as to ensure that the individual components are always fed at a constant pressure, and constancy in the flow rate of each component is controlled by means of a proportional servo-valve interlocked with the speed of the piston assembly of the pumping unit and, more generally, with the flow rate of each component. Since the servo-valve acts directly and automatically on the valve member of the injector which controls the injection of the component into the mixing chamber, the apparatus thus eliminates all the causes which would tend to alter the feeding and mixing conditions of the components, by immediately and automatically coming into action during the actual mixing phase. This invention will be illustrated in greater detail hereunder, with reference to the figures of the accompanying drawings, in which: Fig. 1 shows a circuit diagram of the apparatus according to the invention; Fig. 2 shows the operating mode of an injector forming part of a proportional servo-valve, according to the apparatus of the previous figure. In the diagram of figure 1, the two components to be fed info a mixing head 10 of the high pressure type, for example, polyol A and isocyanate B, are contained in their respective storage tanks 11 and 12. The mixing head 10 can be of any desired type or, as described in several prior patents, for example, of the type designed to enable the individual components to be recycled towards their respective storage tanks 11 and 12 by means of separate ducts and through recycling grooves provided in the cleaning plug 10b, operated by a control cylinder 10c, as is described for example in a prior patent US-A-4332335 of the same applicant, the content of which constitutes an integral part of this description. The tanks 11 and 12 are connected by means of the ducts 13, 14 and the check valves 15, 16 to respective feeding and metering assemblies 17, 18, so that each individual component always flows in the direction of the pumping unit of each single assembly, and never in the opposite direction. Downstream of the check valves 15, 16, each feeding and metering assembly 17, 18 is connected to a respective inlet 21a, 22a for the components into the mixing chamber 10a, by means of ducts 19, 20 and proportional servo-valves 21 and 22, as shown, which can be adjusted to control the flow rates of the individual components, as explained further on; the servo-valves 21 and 22 are connected to a source P and, respec tively, to a drainage T of a common hydraulic control fluid. Each assembly 17, 18 for metering and feeding the chemical components A and B is made up, in a per sè known way, of a transfer cylinder comprising a pumping unit 23, 24, hydraulically operated by a hydraulic control unit 25, 26; each unit 25, 26 consists of a hydraulic cylinder, such as a double-acting hydraulic cylinder, whose piston 25a, 26a is mechanically connected to the piston 23a, 24a of the relative pumping unit, by means of an intermediate rod or in any other way, in order to constitute a piston assembly which is movable linearly in either direction. The two chambers of the double-acting cylinders 25, 26 in the example shown, are connected by means of the ducts 27, 28, 29, 30, the flow reversing solenoid valves 31, 32 and the ducts 33 and 34, to the delivery side of a controlled pump 35, of the constant pressure type and, respectively, to a tank 36 containing the hydraulic fluid for the automatic pump 35. This particular hydraulic circuit controlling the two transfer cylinders 17, l8, constitutes an innovatory feature of the apparatus according to this invention, in that the use of an actuating pump 35 capable of supplying, at its delivery side, a hydraulic fluid at constant pressure, for controlling the transfer cylinders 17, 18, without the need for other intermediate control members, makes it possible not only to simplify the apparatus both structurally and functionally, thereby eliminating causes of breakdown or inefficiency, but also to feed the individual components A and B to the mixing head 10 with a constant pressure, proportional to the constant pressure of the same hydraulic control fluid. Since residual substances could accumulate in correspondence with the nozzles or the inlet apertures 21a, 22a for the components in the mixing chamber of the head 10 due, for example, to solidified particles of mixture, or other accidental causes tending to give rise to variations in the flow rate could occur, with a consequent alteration in the stoichiometric ratios between the components A and B to be mixed, and in the optimal mixing conditions, according to a further feature of this invention, the servo-valves 21 and 22 for controlling the flows of components A and B have been subjected or interlocked to the rate of flow of the components to be controlled; this could be achieved for example by controlling the servo-valves 21, 22 in relation to the speed of travel of the piston assemblies 23a, 25a and 24a, 26a, so as to ensure the controlled feed of each individual component A and B, by keeping it regulated at a constant initial value. The servo-valves 21 and 22 can be controlled or interlocked to the flow rates of the components A and B in any suitable way; for example, as shown, this can be achieved by providing means 37, 38 capable of sensing the speed of travel of the piston assemblies and of transmitting an electric operative signal proportional to the flow rates. The operative signals are sent to a corresponding data input E1, E2 of a processing unit MP which processes them and compares them with nominal speed or flow rate signals, according to a pre-established program, providing control signals at its outputs U1 and U2 which are sent to the control members of the servo-valves 21 and 22. These operate by controlling the flows of components entering the mixing chamber, keeping them constant, according to the desired stoichiometric ratios. In this connection, as shown in the schematization in figure 2, the proportional servo-valves 21 and 22 operate directly, that is to say, they comprise a hydraulic member 39 for controlling the pin 40 of the injector 41, represented schematically, which varies its sectional area of flow, thereby directly acting on the causes which could give rise to an alteration in the mixing conditions in the chamber 10a. As the feeding pressures of the components A and B are kept at a constant value, the feeding rate will also as a result be constant and the degree of mixing of the components will therefore be constant and even. It will be clear, from what has been described and shown in the accompanying drawings, that an apparatus has been provided for feeding and metering chemical components to a high pressure mixing head, which makes use of piston pumping units for feeding and metering the components, which are controlled by means of a hydraulic constant-pressure source, and of only two proportional servo-valves in the component circuit, which are controlled according to the flow rates of the components themselves, thus making it possible to act directly upon the injectors and the causes which would tend to alter the conditions in which the components are injected into the mixing chamber; therefore not only is it possible to keep the values of the pressures and the flow rates of the components constant, but also to substantially simplify the apparatus itself and its operation, thereby reducing the causes of breakdown or inefficient functioning.

Claims: An apparatus for feeding and metering fluid components to a high pressure mixing head (10) comprising at least a first (17) and a second (18) feeding assembly for a first and second component respectively, each feeding assembly (17, 18) comprising a piston type pumping unit (23, 24) mechanically linked to a piston type hydraulic control unit (25, 26), each of said pumping units (23, 24) being connected to a respective storage tank (11, 12) for the respective fluid component and, respectively, to a component inlet injector (41) in a mixing chamber (10a) of the mixing head, said control units (25, 26) of said first and second feeding assemblies (17, 18) being connected to a hydraulic fluid source (35), and a flow-rate control system comprising servo-valves and means (MP, 37, 38) to control said servo-valves (21, 22) in relation to the flow-rate of a respective fluid component characterized in that said control units (25, 26) of said feeding assemblies (17, 18) are directly connected to and are fed by said fluid source with a hydraulic fluid having a constant pressure, said servo-valves being adjacently located to the mixing head and comprising a flow control valve member acting on said injectors, said fluid source being operatively independent from said flow-rate control system. An apparatus as claimed in claim 1, characterized by the fact that said servo-valves (21, 22) controlling the flows of the components to be mixed are of the proportional type. An apparatus as claimed in claim 1, characterized by the fact that said component flow control valves (21, 22) are controlled in relation to the speed of the pistons (23a, 24a) of the component pumping units (23, 24). An apparatus as claimed in claim 1, characterized by the fact that the hydraulic control units (25, 26) of the component feeding and metering assemblies (17, 18) are of the double-acting type and are connected to a single hydraulic pump (35) of the constant pressure type and, respectively, to a hydraulic control fluid tank (36), by means of solenoid valves (31, 32). An apparatus as claimed in claim 3, characterized by the fact that it comprises sensing means (37, 38) for sensing the speed of the pistons (23a, 24a) of the component feeding and metering assemblies (17, 18), and an operative connection for transmitting a flow rate signal to the data input (E1, E2) of a processing unit (MP) which compares said flow rate signals with reference signals, the signal outputs (U1 , U2) of said processing unit (MP) being operatively connected with control members of the component flow control servo-valves (21, 22).

Inventors: FIORENTINI, CARLO
Applicants: AFROS S.P.A.
Application year: 1986
Main classification: B01F15
IPC codes: B29B7, G01F5, G01F9, G01F11