Plastic models are primarily made through a process called injection molding, where molten plastic is injected into a mold cavity. This cavity is shaped like the desired model parts. Once cooled and solidified, the parts are ejected, and the modeler then assembles them, typically using a plastic solvent-based adhesive.
Here's a breakdown of the process:
Injection Molding Process
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Material Selection: The most common material used for plastic models is polystyrene. Cellulose acetate was used in the early days, but polystyrene offers a good balance of cost, ease of molding, and paint adhesion.
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Mold Creation: The mold itself is typically made from metal (steel or aluminum). It consists of two or more parts that clamp together to form the cavity. The shape of the cavity precisely determines the shape of the model part. Molds are created through machining and/or other precise manufacturing processes.
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Injection: Polystyrene pellets are fed into a hopper of an injection molding machine. This machine heats the plastic until it melts. The molten plastic is then injected under high pressure into the mold cavity.
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Cooling and Solidification: Once the mold is filled, the plastic cools and solidifies. Cooling channels within the mold help to accelerate this process.
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Ejection: After the plastic has solidified, the mold opens, and the part is ejected, often with the aid of ejector pins. The resulting part is a near-perfect replica of the mold cavity.
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Sprue and Runner System: During injection, the plastic flows through channels called "runners" to reach the mold cavity, and a small amount of plastic (the "sprue") connects the runner to the injection molding machine. These excess pieces of plastic are removed from the molded parts. The parts are often attached to a larger frame, also molded, called a "sprue tree."
Assembly
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Part Removal: The modeler carefully removes the individual parts from the sprue tree, often using hobby knives or sprue cutters.
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Adhesion: A solvent-based plastic cement is typically used to bond the parts together. This cement melts the surface of the plastic slightly, allowing the parts to fuse together as the solvent evaporates. Epoxy can also be used, especially by more experienced modelers for greater strength or to join dissimilar materials.
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Painting and Detailing: Once the model is assembled, it is usually painted and detailed to improve its appearance.
Example
Imagine making a model airplane. The fuselage halves, wings, tail, and other parts are all injection molded separately. The modeler then carefully glues these pieces together to create the completed airplane.
