A method of blow molding a hollow article of thermoplastic material in which the newly formed article is cooled internally by explosively injecting into the article a charge of a highly pressurized and chilled mixture of air and water.     This invention relates to the art of blow molding hollow plastic articles such as bottles of thermoplastic materials and it is directed to an improved method of rapidly cooling the interior of a newly formed article. 
    In the typical method, a tube or parison of molten plastic is extruded from a die, pinched between the halves of a mold, and then pressurized internally to expand outwardly and into contact with the inner surfaces of the mold parts which give the article its finished shape. It has been the common practice to run coolant such as water through the mold parts in order to cool the newly formed article externally. This type of cooling, used alone, has proved to be slow in production, and as a result, a number of methods have been developed for cooling the article internally simultaneously with the external cooling in order to increase production. 
    A stream of air has been used. 
This increased the rate of internal cooling, but not significantly. 

Water has been sprayed into the newly formed articles. 
This method increased the cooling rate substantially over the use of air, but it left water in the articles and such residual water cannot be tolerated in many instances. 
    The above two methods have been combined, that is, water has been sprayed into the article followed by a blow-through of air to rid the article of the water. In a more sophisticated method, air has been circulated through the article under pressure for blowing purposes and thereafter water sprayed into the circulating air for cooling and drying purposes. These later methods solved the residual water problems, but they were still comparatively slow productionwise. However, when considered solely from the viewpoint of coolant costs, these methods have not been surpassed. For example, liquid carbon dioxide has been used as a coolant, and rather effectively, from the viewpoint of increased production, and it is perhaps the fastest known method in the prior art of cooling a blow molded plastic article. But, the cost of this coolant is very high compared to an air-water coolant. 
    Thus, the primary objective of this invention has been to provide a method of blow molding a plastic article wherein the article is cooled internally using an air-water coolant, for economy, but which is capable of use at a rate of production comparable to the liquid carbon dioxide method. 
    In the preferred embodiment of the invention, air at ambient temperature is used for blowing the article following known techniques. 
External cooling of the article is achieved as has been done in the past by using coolant in the mold parts. The novelty of the improvement of this invention resides in the way in which internal cooling is achieved. 
    A pressurized and chilled mixture of air and water is used. In this state the mixture consists of water that is supersaturated with air, and air. Preferably, the pressure to which the mixture is subjected is in the range of from about 1,000 to 2,000 pounds per square inch. As will appear, the limits of this range, (and particularly the upper one) are not critical to the successful practice of the method. The important consideration is that the chilled mixture be pressurized to an extent substantially greater than the pressure within the newly formed article into which it is to be injected such that a charge of the mixture released into the article literally explodes. 
    The sudden pressure drop at the time of the injection causes a substantially instantaneous, adiabatic expansion of the air of the mixture, including the air with which the water is supersaturated. The explosive nature of the injection causes an immediate dispersion of the mixture to all internal surfaces of the article so that regardless of the complexities of the internal configuration of the article, it is coated throughout. 
    It is hypothesized that the explosive injection of the mixture results in a three phase system. 
Observations show that one phase consists of minute ice crystals. The second phase consists of minute drops of cooled and probably supercooled, water. The third phase is gaseous consisting of water vapor and air. Preferably only a single injection of the charge is made per article, i.e., a "one shot" injection. 
    The amount of water required per "shot" is surprisingly small because of the great degree of dispersion achieved by the explosive nature of the injection. As an example, approximately only 1 cc of water is required for a 22 oz. capacity bottle formed from about 36 grams of plastic. It may be observed, using a clear plastic bottle, that immediately following the explosive injection of the charge into an article, the entire internal surface thereof is coated substantially evenly throughout. Apparently, the minute ice crystals melt almost instantaneously upon contact with the hot internal walls of the article. In any event, the coating is in the form of exceedingly fine droplets of water, so small that there is little tendency for the droplets to coalesce and run so that the integrity of the coating is maintained to a great degree. 
    Preferably, the charge is injected into the newly formed article while it is still pressurized with blowing air, this pressure usually being around 80- 90 pounds per square inch. This pressure condition is relatively insignificant compared to the high pressure of the charge and has little effect upon the dispersion of the components of the charge. 
    The immediate cooling effect obtained is the result of heat transferred from the walls of the article to the ice crystals, which melts them, to the minute droplets of cooled and possibly supercooled water, which heats them, and to the water vapor and air, which heats them. Of the three phases initially present (solid, liquid and gas), the heating of the solids -- the melting of the ice crystals -- is the most efficient cooling factor per unit of time. For this reason it is desired that a substantial quantity of ice crystals be present in the mixture discharged into the article. Conditions effecting the quantity of ice crystals present in the mixture, and ways to create substantial quantities therein, are discussed below. 
    At this stage, immediately following the explosive discharge of the mixture into the enclosed article, the walls of the article have been surface cooled and solidified both internally and externally, but the core area between surfaces is still molten or substantially molten. To achieve further rapid cooling, the article is opened to vent it to atmospheric pressure. Whereupon there follows a fast boil-off of the remaining droplets of water by the residual heat within the walls of the article. This cools the article to a point where it is sufficiently stable to be ejected from the mold.