The present invention relates to a reaction injection machine utilizing a plurality of reactant liquids, in which in view of the fact that the mixed liquids are cured rapidly, the reactant liquids are mixed immediately before their injection into a mold and the mixed liquids are required to be injected under pressure into the mold.
Conventionally, a reaction injection machine of this kind is known from, for example, U.S. Pat. No. 4,473,531 as shown in FIGS. 1 and 2. The known machine includes supply vessels 1A and 1B for containing reactant liquids A and B, respectively and a mixhead 2 which is connected to the supply vessels 1A and 1B by flow lines 3A and 3B, respectively. Metering pumps 4A and 4B are, respectively, provided in the course of the flow lines 3A and 3B. The liquids A and B are continuously supplied at a predetermined pressure from the supply vessels 1A and 1B to the mixhead 2 by the metering pumps 4A and 4B. A piston 5 is provided in the mixhead 2 and is formed with passages 6A and 6B. In response to downward and upward movement of the piston 5 in the mixhead 2 in the directions shown by the arrows, communication of the passages 6A and 6B of the piston 5 with the flow lines 3A and 3B is established and cut off, respectively. Namely, at the time of injection, communication of the passages 6A and 6B with the flow lines 3A and 3B is cut off as shown in FIG. 2 and the liquids A and B are shot at high pressure and at high speed into a mixing chamber 7 disposed below the piston 5 in the mixhead 2 so as to impinge against each other. Through impingement of the liquids A and B, the liquids A and B are mixed. Subsequently, the mixed liquids A and B are injected at a predetermined injection pressure into a mold (not shown).
On the other hand, at the time of completion of injection, the piston 5 is moved to its lower stroke end as shown in FIG. 1 and the passages 6A and 6B of the piston 5 are, respectively, communicated with the flow lines 3A and 3B such that the liquids A and B are, respectively, circulated by the metering pumps 4A and 4B as shown by the arrows. Clearance between the piston 5 and an inside diameter of the mixhead 2 is set at a quite minute value. Thus, during travel of the piston 5, the mixed liquids A and B adhering to the wall of the mixing chamber 7 is mechanically scraped therefrom through sliding contact of the piston 5 with the wall of the mixing chamber 7.
However, the above described known machine of the impingement mixing type has such a drawback that the liquids A and B adhering to the wall of the mixing chamber 7 are not sufficiently scraped therefrom during the travel of the piston 5 and therefore, the remains on the wall of the mixing chamber 7 to some extent. Therefore, the mixed liquids A and B remaining on the wall of the mixing chamber 7 are required to be washed off periodically and may adversely affect molded items.
Furthermore, in order to prevent improper mixing of the liquids A and B in the known machine, since the liquids A and B are required to be injected into the mixing chamber 7 at high pressure and at high speed so as to impinge against each other, a high pressure of about 170 to 220 kg/cm.sup.2 should be applied to the machine. Hence, the known machine of the impingement mixing type has the inconvenience that an apparatus for generating high pressure is required to be provided and that even if the liquids A and B are injected into the mixing chamber 7 at high pressure and at high speed, the mixing degree of the liquids A and B is unstable as compared with a case in which the liquids A and B are stirred and mixed by a stirrer.
Meanwhile, at the time of injection molding, it is desirable for achieving quality of molded items as shown in FIG. 3 that after the mixed liquids A and B have been injected at an injection pressure (primary pressure), a holding pressure (secondary pressure) higher than the primary pressure is applied to the injected mixed liquids A and B. However, in the above mentioned known machine of the impingement mixing type, since it is difficult to perform such two-stage pressure control of the primary and secondary pressures, it is next to impossible on the following ground to apply a secondary pressure larger than the primary pressure. Namely, since the flow lines 3A and 3B open into the mixing chamber 7, the mixed liquids A and B will flow back to the flow lines 3A and 3B from the mixing chamber 7 if a high secondary pressure is applied.