GB2090763A - Mixing reactive chemical components - Google Patents

Abstract

Apparatus for mixing reactive chemical components, to be fed into a mould and which must be recycled before each mixing operation, comprises a mixing head (1) provided with injectors (6, 7) for feeding the individual components, which injectors are axially slidable and are hydraulically controlled between a forward position for the injection into the mixing chamber (1), and a backward position for recycling the individual components through the injectors (6, 7) themselves. Control members (8, 10) for the mixing chamber (1) and the injectors (6, 7) are operated by devices in the form of double-acting piston-and-cylinder assemblies (9, 9a, 11, 23, 20). <IMAGE>

Description

SPECIFICATION Apparatus for mixing reactive chemical components, having mobile injectors This invention concerns apparatus for mixing and delivering reactive chemical components, especially, but not exclusively, suitable for polyurethane and similar materials, which react at high speed to form a synthetic resin.

Known forming apparatus is provided with a head for mixing reactive chemical components, in order to form a synthetic resin mixture to be fed into the cavity of a mould. The head usually comprises a cylindrical mixing chamber provided with inlet apertures, into which nozzles of injectors for the individual components are placed; the individual reactive components are fed in at high pressure for example, 100 to 200 atm., in order to achieve the desired degree of nebulization or atomization which ensures a mixing, in the correct stoichiometric ratios, between the chemical components, before the mixture is fed into a mould.With such apparatus before each mixing and moulding operation it is necessary to carry out a recycling phase of the individual chemical components, at a high pressure, equal or substantially close to that of the feeding for the mixing; in fact, in order to have proper control of the stoichiometric ratios between chemical components, it is advantageous to feed said components, both during the mixing and during the recycling, with substantially constant and identical pressures.

This control of the pressures is especially critical in the transient conditions or initial stages, that is to say, at the moment of changeover from a recycling phase to a mixing phase. In fact, during this period of transition, which lasts for an infinitesimal space of time, operative conditions may be created which are such as to cause considerable uncontrolled changed in the feeding pressures of the reactive components.

Consequently, any variation in the feeding pressure of the components, during the first few moments of their mixing can lead to even extremely serious defects in the moulded article.

The apparatus known to date present various systems for recycling the components, which include recycling valves separate from the injectors, or recycling valves placed in series with the injectors themselves, having for example, separate channels for circulation of the components in a control member within the mixing chamber. The use of separate recycling valves makes it highly improbable that homogeneous component feeding conditions will be obtained, that is to say, such conditions as will enable the recycling to be carried out with component feeding pressures equal or substantially similar to those achieved during a mixing phase. The use of recycling channels in series with the injectors, formed in control members within the mixing chamber, has not as yet made if possible to solve the problem adequately, especially in the switch-over transient condition from one phase to the next.

In fact, due to the limited transversal dimensions of the mixing chambers, it is impossible to form the control member inside the chamber, with recycling channels for the components having a sectional area of flow wide enough to avoid introducing additional, overpressure problems; moreover, the impurities which could possibly form along the recycling channels, after the injectors, and also in correspondence with the nozzles of the injectors themseles, could alter the recycling conditions, with respect to the feeding conditions during a mixing phase.

These variations are especially felt during the changeover from a recycling phase to the subsequent mixing phase due to a phenomenon related to the resilient behaviour of the hoses which usually connect the mixing head to the circulating pumps of the single components. In fact, in systems of the aforementioned type, the mixing heads must be able to move along the same mould or must be moved to and from the moulds which move along a moulding line; therefore the hoses must be flexible, that is to say, capable of accompanying the head during its movements.As such hoses usually consist of suitably reinforced rubber hose, that is to say, are elastically expansible at the high working pressures, and as the metering pumps for feeding the components are of the constant-delivery type, a variation in the feeding pressure of the reactive component upstream of the injector, calls for a variation in the volume of the hoses, that is to say, a variation in their capacity, resultling in an alteration in the stoichiometric ratios between components. These facts are especially critical at the moment of switching from a recycling phase to a mixing phase, as uncontrollable overpressures are created during the first few moments, even if of short duration.This problem is increased considerably, if, at the moment of deviation of the flow from the recycling circuit to the mixing chamber, there is also a variation in the feeding pressure of the reactive component.

In order to reduce the aforementioned problems, it would be desirable therefore for the recycling path of each component to present the same characteristics as the feeding path towards the mixing chamber so as to enable the feeding, the mixing, and the recirculation of the components to be carried out with substantially constant pressure conditions. This could be achieved, for example, by always making the components circulate, both during the recycling and during the mixing, through the feeding injectors and by providing recycling ducts with large sectional areas of flow so as to always have substantially identical feeding pressure conditions.

According to this invention therefore, injectors are provided which are axially mobile, in such a way that, in their forward position, they feed the components directly through the apertures in the mixing chamber, whereas in their backward position, and with the apertures of the mixing chamber closed, they feed the reactive components, separately along their respective recycling ducts.

However, merely feeding the components through the same injectors, both during the mixing phases and the recycling phases, is not yet in itself sufficient to ensure that the feeding pressures will always be identical, and to considerably reduce the phenomena connected with the variations in the capacity of the hoses during the switching from one operative phase to the next. In fact, in order to achieve a correct mixing of the components, in the desired stoichiometric ratios, it is necessary, from the very first instants of the switching over, for the injectors to be moved simultaneously, in a positively controlled way, and at a high speed, that is to say, within a switching time in the region of fractions of a second.

The object of this invention is to provide an apparatus provided with a mixing head of the previously mentioned type, capable of overcoming the aforementioned problems, that is to say, which makes it possible to achieve feeding conditions during the recycling of the single components which are similar or substantially identical to those which are achieved during the mixing phase, and which at the same time enables the changeover from one operative phase to the next, within an extremely short period of time and in a strictly controlled mode for all the components, so as to ensure a correct stoichiometric ratio in the mixing of the components right from the very first instant of the mixing.

According to the present invention, therefore, apparatus for mixing reactive chemical components, comprises: a mixing head provided with a mixing chamber having opposing inlet apertures for the individual components, and an outlet for dispensing the mixture; means for injecting each component through the respective inlet aperture in the mixing chamber and at least one control member connected to an operating device, which control member slides along the mixing chamber from a first forward position in which it closes the aforementioned apertures to a second backward position to the rear of said apertures; said injecting means being axially movable in a crosswise direction relative to the mixing chamber, from a forward position in which the nozzle of an injecting means communicates directly with the respective inlet aperture for the component in the mixing chamber, to a backward position in which, with the inlet apertures of the mixing chamber closed, the nozzle of the injecting means communicates directly with a recycling duct, the injecting means being moreover connected to a respective control device operated by a source of fluid under pressure coordinately with the operating device for the control member sliding within the mixing chamber.

According to a preferred embodiment, the control devices for the injectors are of the hydraulic type, suitably connected to a source of fluid fed at high pressure, so that they are operated both rapidly and simultaneously in order to ensure the best possible working conditions right from the very first instants of the mixing.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure lisa cross-section of a mixing head in accordance with the invention, along the line 1-1 of Figure 2, showing one injector in section and in the forward position; Figure2 is a longitudinal section of the mixing head, along a plane at right-angles to that of Figure land Figure 3 is a fragmentary view of the cross-section of Figure 1 with the injector in the backward position.

Figures 1 and 2 show a mixing head 1 for reactive chemical substances to be mixed and fed into a mould (not shown); the head 1 in the example shown comprises an L-shaped mixing chamber, defined by a first portion of chamber 2, of small dimensions and of small diameter, the length of which is substantially equal or slightly greater than the diameter itself, and by a second portion of chamber 3, of greater diameter and length than the first, arranged at an angle with respect to the first portion 2; nevertheless, it is obvious that the principles and characteristics of this invention are also utilizable with other types and shapes of mixing chambers.The mixing chamber comprises, diametrically opposed, lateral apertures 4 and 5, in a portion of its lateral surface, for example, made in the smaller-sized portion of chamber 2, in orderto allow the feeding, under high pressure, of at least two chemical components which must be mixed and fed into a mould, where they chemically react to form a moulded article in plastic material. References 6 and 7 indicate two devices for injecting the reactive components, more simply referred to as injectors (and of which only the injector 6 is shown in a sectional view in Figure 1), the nozzles of which can communicate with the inlet apertures 4 and 5 of the mixing chamber.

Sliding within the portion of chamber 2 is a first control member in the form of a plug 8 operated by a hydraulic device which comprises a piston 9, provided with a short stroke, in order to move the plug 8 rapidly from a forward position in which it simultaneously closes the inlet apertures 4,5 for the components and the outlet 2' of the chamber portion 2, and at the same time emptying out the portion of chamber 2 upon completion of each mixing operation, and a backward position flush with the rear of the apertures 4, 5 as shown, in which it frees said inlet apertures 4,5 for the components and the outlet 2' at the beginning of each mixing phase.

In the embodiment shown, the head 1 comprises a second control member 10, in the form of a plug controlled by a second piston 11 provided with a long stroke; in this case the plug 10 is also movable lengthwise in the portion 3 of the mixing chamber, from a forward position in which it closes both the outlet aperture 12 for feeding the mixture into a mould, and also the outlet 2' of the portion of chamber 2, at the same time emptying the portion of chamber 3 itself, to a backward position in which it frees both the outlet aperture 12 for the mixture and the outlet 2', as shown in Figure 2.

The structure of the injector 6 will now be described in detail, the other injector 7 being wholly identical to the latter. As can be seen from Figures 1 and 3, the injector 6 comprises a body 13 within which is situated a needle 14 provided with a conical point which penetrates into the front nozzle 15 in line with the outlet aperture 4 for the chemical components. The body 13 of the injector is cylindrical in shape and is supported axially mobile, in a direction perpendicular to the portion 2 of the mixing chamber, by means of its own hydraulic control device made up of a double-acting piston-and-cylinder assembly, described hereafter, fed by an appropriate source of fluid under pressure.

The sliding body 13 of the injector defines, at its front end, an inner chamber 16 which, at one end, is provided with the injecting nozzle 15, and which also presents lateral apertures 16a, constantly in communication with the inlet duct 17 for the component, through a radial passage 18 and an annular section 19 made in the inner surface of a body 20 defining the operating cylinder of the injector. Consequently, the reactive component contained in a tank 32 can be continuously fed through a filter 33, a metering pump 34 and a flexible hose 35, to the injector 6, irrespective of the position of the latter.Reference 21 indicates an outlet duct for the component, which can be connected to the injector 6 in a manner two be described presently, whenever each chemical component must be made to recycle to the tank 32, through a flexible duct 36, keeping the feeding pressure of the component upstream of the injector substantially unchanged.

The body 13 of the injector is screwed into a sleeve 23a provided with a piston 23 which slides axially within a cylindrical chamber in the body 20 and defining with it the double-acting hydraulic control device. As shown schematically in Figure 3, the front chamber, that is to say, the one nearest the mixing chamber, of each control cylinder of the injectors 6,7 and, correspondingly, the rear chamber of the cylinder 9a operating the plug 8, are connected by means of flexible hoses 24, 25 and 26 and a switchovervalve 27, to a source 28 of fluid under pressure (or respectively to the discharging side), whereas the rear chamber of each of the control cylinders of the injectors 6,7 and the front chamber of the control cylinder of the plug 8 are connected to the discharge side (or respectively to the source 28) of fluid under pressure, by means of flexible hoses 29,30 and 31; therefore the forward movement of the injectors 6,7 takes place contemporaneously, and in coordination with the backward movement of the plug 8 and vice versa.

The mixing head described works in the following manner: assuming that a mixing phase of the chemical components has been completed and that the plugs 8 and 10 are in the forward position, whereas the injectors 6 and 7 are in the backward position; in these conditions, the apertures 4 and 5 of the mixing chamber are closed by the plug 8, and the body 13 of the injector 6 or 7 opens the radial passage 22 as shown in Figure 3, placing the inlet duct 17 for the component in communication with the outlet duct 21, through the passage 18, 19, the chamber 16, the nozzle 15 of the injector and the passage 22, in order to recirculate the component to the tank.Each component can therefore be made to recycle, feeding it at the same pressure which is achieved in the mixing phase, as this pressure is always determined by the injector 6 or 7 itself, the setting of which has remained unchanged.

In fact, by having arranged the recycling ducts directly inside the body 1 of the mixing head, it is possible to maintain sufficiently large sectional areas of flow of the components downstream of the injectors 6,7 and considerably larger than those of the injection nozzle; thereby avoiding causes which could give rise to substantial variations in the feeding pressure of the components during the recycling.Upon completion of a recycling phase of the components, and now proceeding to a mixing phase, the plug 10 is first made to move backwards, the injectors 6 and 7 are then made to move forward and the plug 2 is made to move backwards, completely freeing the mixing chamber and the inlet apertures 4 and 5 for the components; and it is possible to use high pressures for controlling the hydraulic cylinders of the injectors, also foreseeing the use of suitable pressure accumulators, and due to the very short stroke of the injectors themselves, it is possible to achieve a controlled changeover of the flow of the reactive components, within extremely short periods of time. Moreover, additional causes of increase in the pressures are avoided, due to the fact that the thrusts of the reactive component, exerted in both directions upon the body of each injector, substantially balance each other.As the individual chemical components continue to be fed through the same injector, the feeding pressure during the mixing phase remains substantially unchanged and identical to that during the recycling. In this way, it is possible to avoid or reduce to a minimum the problems due to the switching from one operative phase to another, phenomena which, as mentioned previously, give rise to a resilient expansion and a variation in the capacity of the flexible hose connecting them to the head. A positive control is thus obtained over the stoichiometric ratios between the components and thereby a high quality of the mixture, from the very first instants.

Upon completion of a mixing phase, the injectors 6, 7 are once again made to move backwards, as shown in Figure 3, in order to recycle the components, whereas the plugs 8 and 10 are made to move forward in succession in order to close and clean the mixing chamber of any remaining mixture.

Claims (5)

1. Apparatus for mixing reactive chemical components comprising a mixing head provided with a mixing chamber having opposing inlet apertures for the individual components, and an outlet for dispensing the mixture; means for injecting each component through the respective inlet aperture, and at least one control member connected to an operating device, which control member slides along the mixing chamber from a first forward position in which it closes the aforesaid apertures, to a second backward position flush to the rear of the aforesaid apertures; said injecting means being axially movable in a crosswise direction relative to the mixing chamber, from a forward position in which the nozzle of the injector communicates directly with the respective inlet aperture for the component, to a backward position in which, with the inlet apertures of the mixing chamber closed, the nozzle of the injector is in direct communication with a recycling duct; the injecting means being moreover connected to a respective operating device connected to a source of fluid under pressure, in coordination with the operating device of the control member sliding within the mixing chamber.

2. Apparatus as claimed in Claim 1, in which the operating devices are in the form of hydraulic cylinders.

3. Apparatus as claimed in Claims 1 and 2, in which the injecting means is interchangeable provided in a sleeve defining the piston of the hydraulic control cylinder.

4. Apparatus as claimed in Claim 3, in which the injecting means comprises a cylindrical body sliding within an extension of the body of the hydraulic control cylinder, and in which the extension is provided with lateral passages for feeding and recycling the chemical component through the injector.

5. Apparatus for mixing reactive chemical components substantially as hereinbefore described with reference to the accompanying drawings.