A primer on six methods for forming plastics

injection molding machine

Globally, nearly 300 million tons of plastic are produced every year. Plastics are part of every facet of our daily lives because they are highly versatile and have varying characteristics based on composition and how they’re manufactured. As such, product designers need a keen understanding of the many ways to form plastics for commercial and industrial use to meet specific applications. These six methods have been around a while, but all of them are still undergoing continual refinement to improve speed, lower cost and enhance quality.

Plastic injection molding

The granddaddy of mass production processes, plastic injection molding is indispensable for medical equipment, aerospace, automotive and consumer goods. With this process, plastic is melted and then injected into a mold under high pressure. There, the material is cooled, solidified and afterward released by opening the two halves of the mold. This technique results in a plastic product with a predetermined, fixed form. Like other molding or casting processes, there is some reverse engineering involved.

First, the product engineer designs the ideal finished part. Then, a tooling engineer needs to design a tool that can make that part, which is finally transferred to a moldmaker who machines the mold, taking into account the variables of raw material, complexity, tolerances and so forth. Plastic injection molding is ideal for making large runs of identical parts quickly. Using plastic injection molding, a low-cost part can be made with an excellent surface finish.

Reaction injection molding

Reaction injection molding is common in the automotive industry because it produces lightweight parts. The rigid skin on these parts is easily painted to make body panels, dashboards and other components. Unlike injection molding, reaction injection molding uses thermoset—not thermoplastic—polymers. Through a variety of chemical reactions, these polymers expand, thicken and harden only after they’re injected into the heated mold, accommodating much more intricate designs than ordinary injection molding. Raw materials and molding techniques can be selected and even customized to precisely deliver desired weight, strength, density and hardness characteristics.

Unlike thermoplastics, thermosetting is non-reversible and the parts can’t be recycled; the raw materials also are more expensive. Tooling costs for prototypes are relatively low, while production tooling is moderately expensive because of the increased production time.

Rotational (roto) molding

Rotational molding is ideal for making large hollow or concave shapes, often for outdoor use like canoes and tubs. This process involves a heated hollow mold that is filled with a charge or shot weight of material. Then, the mold is slowly rotated (usually around two perpendicular axes), causing the softened material to disperse and stick to the walls of the mold. To maintain even thickness throughout the part, the mold continues to rotate at all times during the heating phase. This helps to avoid sagging or deformation during the cooling phase. The finished parts are stress-free and have no seams, ensuring high strength. The tools are also relatively simple and inexpensive to make. One caveat of this process is that tools don’t last more than a few thousand cycles before they need to be replaced, and the part finish quality is average, at best, so it’s not suited for precision forming.

Extrusion blowmolding

Extrusion blowmolding provides designers with the ability to make thin-walled, inexpensive containers such as disposable drinking cups or bottles. Using this process, plastic is melted and extruded into a hollow tube (a parison). This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container or part. After the plastic has cooled sufficiently, the mold is opened, and the part is ejected. This process is fast and simple. However, parts cannot be overly complex or require high precision.

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