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This article was taken, with permission, from the May/June 1997 issue of The Blueprinter, a magazine published by The Ertl Company, Dyersville, IA

HOBBY HISTORY: PLASTIC MOLD INJECTION Mainstay of the Model Kit Industry

This year, AMT will use approximately five million pounds of polystyrene plastic to create the industry's most popular model kits. To the casual observer, the process employed to create model kits may seem like a more recent invention, but the advent of plastic mold injection can be traced back 125 years. In 1872, Smith & Lock patented the earliest injection molding machine to cast the first commercially successful plastic: celluloid. A variety of products - including billiard balls, combs and photographic film - were created with celluloid. After its initial use with the injection molding technology, however, it was realized that this tough new material - invented in 1870 by American chemist John W. Hyatt - was highly flammable and, consequently, unstable in this application. Smith & Lock's machine would have to wait nearly 50 years for a suitable molding medium. Until then, plastic products continued to be machined, formed and assembled by craftsmen in a much more labor- and time-intensive environment. Thermosetting would temporarily remain the mode of the day.

The first fully-synthetic plastics - PVC, or polyvinyl chloride, being one of several - had been developed in the early 1800's. French chemist Henri Regnault created PVC in 1838 by solidifying this new resin in sunlight, a rather lengthy technique. This thermosetting process had been the traditional forming method utilized throughout the 19th century. During manufacture, the thermosetting plastic was poured into a mold and "cured" by heat or the addition of chemicals. After curing, however, the plastic could no longer by shaped, even if extreme heat was applied. The arduous production of the plastic and this forming method made high output, commercial manufacture virtually impossible.

By 1919, development efforts by German scientists resulted in a more thermally stable plastic called cellulose acetate. Accompanying this new material was an injection molding machine (the U.S. version was developed by Leo Baekeland) that successfully molded the plastic. In 1909, Baekeland had already patented his phenol-formaldehyde resin, Bakelite. It was one of the first commercially successful synthetic plastics, and is still in use today. Injection mold machine design and plastic chemistry have progressed through the subsequent decades, but the concept remains the same.

Plastic mold injection involves heating and high-pressure injection of a thermoplastic into a water-cooled mold. A thermoplastic contains properties that allow it to soften when heated and harden again when cooled. Polystyrene, a rigid material used by AMT to mold model kit parts, is one such plastic. The mold injection process applied to producing model kits has several advantages over other methods. The thermally durable qualities of the polystyrene and quick-cooling manner of the injection technique make it extremely time- efficient. The time required to complete one mold, or "shot," can be measured in mere seconds. The curing time for the thermosetting method employed by many resin kit manufacturers can span minutes if heat cured, or hours if cold cured. The time-consuming nature of the cold method makes it ill-suited to a it manufacturer who needs to produce large quantities of plastic parts over a relatively short time span.

AMT has utilized plastic since 1949, when the model hobby industry was revolutionized by this new technology. Suddenly, a new level of authenticity was achieved in the promotional models, and later the kits, that were being produced by AMT and its competitors. Previously, hobbyists and manufacturers worked with less pliable media, such as balsa wood and metals. Plastic offered greater detail in kit manufacturing, placing a new emphasis on accuracy. As a result, product development and research became central to creation quality model kits. It also removed much of the drudgery from the building process. With this new man-made material, all that was needed to complete a realistic replica of a favorite car or aircraft was a tube of glue and some paint. For the modeler who was not satisfied with building "out of the box," plastic could be easily modified with sandpaper emery board or a knife. In one fell swoop, the average builder could achieve the kind of customizing that previously required a higher level of manual skill.

The same efficient principles that make it the medium of choice for model kits apply to other consumer products as well. It has become a standard for any manufacturer that needs to mass-produce plastic components quickly and cost- efficiently. Since its inception, plastic mold injection has touched just about every industry, from pharmaceuticals to automotive. Everyday products like utensils and clothes hangers are included on the extensive list.

Modelers today will continue to reap the benefits of this established method as it accompanies AMT into the next millennium, ensuring the highest quality model kits in the future.


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