The process of dispensing heated plastic material into product molds has long been known in the plastics industry. In recent years, however, reaction molding, in the case of injection molding sometimes being referred to as Reaction Injection Molding, RIM, has become increasingly popular, in part due to the inherently shorter cycle times made possible through use of the technique. In RIM processes, for example, several components reactive with each other are combined in a mixing device or "head" immediately prior to their being introduced into a mold. The mixed material thereafter gels in the mold and forms the desired product.
The mixing head commonly takes the form of a housing in which a piston mounted within a conforming bore moves reciprocally back and forth during successive molding cycles, in turn forcing a rod mounted on the end of the piston to travel through a second bore in the housing. This latter bore constitutes a chamber in which the several components mix and begin reacting with each other prior to being discharged into, and shaped in a mold. Mixing heads of the type described can be used in conjunction with either injection-type, or open mold processes.
In one mixing head design, described for example in U.S. Pat. No. 4,332,335, two inlet nozzles are located directly opposite each other in the wall of the second bore; these discharge streams of reacted material toward each other along a common axis, thereby resulting in the mixing of the two components at their point of impact within the bore. The mixture is thereafter dispensed into an open mold, or alternatively, injected under pressure into the desired mold.
The reciprocal movement of the rod results in the sequential opening and closing of the inlet nozzle entry points communicating with the second bore. In one direction of rod travel, the nozzles remain in an open or bore-communicating mode, permitting the reactants to flow freely into the chamber and mix therein before being dispensed into the mold. In the rod's reverse travel direction, the reactive materials are blocked from entry into the chamber and recycled, while any residual material remaining in the bore chamber is simultaneously displaced by the rod into the mold.
The quality of the products thus molded, however, depends to an important degree on the quality of the mixing achieved in the chamber, poor mixing resulting in imperfectly combined, non-homogeneous mixtures that give rise to unreacted material. This manifests itself in products containing so-called "wet spots," i.e., portions of unreacted material, and also results in non-uniform gelling in the molds. As a consequence, it gives rise to objectionable soft or hard areas in the products produced, undesirably weakening them. In addition, it can detrimentally lengthen reaction times, cause slower cycle speeds, and further increase costs due to the higher incidence of rejects, as well as the increased processing times.
The problems of poor mixing are commonly encountered in the molding industry, and in fact it has even been suggested that "after-mixers" be incorporated in mixing head assemblies to remedy the problem. However, resorting to such an expedient adds to overall manufacturing expense, and the use of such devices can occasion premature gelling of the mixed material, resulting in excessive shutdowns of the production line for cleaning of the equipment in order to avoid plugging difficulties.
Among other suggestions for overcoming the problem of poor mixing, for example, is that involving the use of the device disclosed in U.S. Pat. No. 4,840,556. The device there taught involves the positioning of mixing plates across the orifices of the inlet nozzles through which the reactive components are introduced into the mixing chamber. This is said to create desirable turbulence, and therefore, better mixing. Such a design adds to the complexity of the device, however, as well as to the pressure drop across the nozzles. It might also increase the vulnerability of the nozzles to plugging, and thus would be objectionable from that standpoint as well.
In view of the preceding, therefore, it is a first aspect of this invention to provide an improved mixing head for a molding machine.
It is a second aspect of this invention to provide a mixing head for a molding machine in which superior mixing is achieved.
It is a further aspect of this invention to provide a mixing head for a molding machine in which injection nozzles direct reactant streams toward a closed end of the mixing chamber at an acute angle relative to the longitudinal axis of the chamber.
It is an additional aspect of the this invention to provide a mixing chamber for a molding machine mixing head having a ratio of chamber length to chamber diameter that assures improved mixing and optimum residence time for initial reaction.
It is another aspect of this invention to cause reactant streams mixed by their impact together in the mixing chamber of a reaction molding machine to be further mixed as a consequence of being thereafter again forced to pass through the turbulent point of impact of the streams, prior to being dispensed into a mold.
It is still a further aspect in one embodiment of this invention to provide a mixing head for a molding machine that avoids the necessity of partially closing the discharge passageway or installing downstream valves in order to assure complete interreaction of the reactive components.
It is yet another aspect of this invention to provide a mixing head for a molding machine that facilitates the mixing of reaction molding components without objectionable, unreacted wet spots.