The Basics – What is Injection Molding? How Does it Work? What is it Used For?

By far the majority of high-production plastic parts today are made by injection molding. Once a mold has been constructed, the manufacturing process is simple and quick. Various plastic resins can be used to meet specific requirements. Thousands and tens-of-thousands of parts can be made at an economical price.

Typical molding system schematic 333333What is Injection Molding?

Injection molding is a manufacturing process where liquified plastic is forced into the cavity of a mold to make a product. Once the liquified plastic has cooled and solidified, the product is ejected. After ejection, more liquified plastic is forced into the mold, and the manufacturing process continues.

Typically, the time it takes to produce a part is 15 to 60 seconds. Molds can be made to produce more than one part per cycle, thus increasing production capabilities. Identical parts can be produced by the hundreds and thousands every hour.

The cost of injection molding is on the front end. Digital product design has to be made. Certain design criteria must be met so that the produced parts will be strong, uniform, and without defects. 

A metal mold of the design must then be constructed. Molds can cost between a few thousand dollars to ten-of-thousands of dollars. The minimum production volume should be around 500 units. This is the point that injection molding becomes economical. 

All thermoplastic materials, some silicone, and some thermoset resins can be used to manufacture products with injection molding. The most commonly used plastic injection molding materials are: polypropylene, polyethylene, polystyrene, and ABS.

A Brief History of Injection Molding

Injection molding started with an innovative way to make billiard balls.

Plastics replace ivory: 

Celluloid, considered to be the first thermoplastic, was patented in 1868 by John Hyatt. He used celluloid to replace ivory in the making of billiard balls. In 1872, he was the first to inject hot celluloid into a mold. He patented an injection molding machine that used a plunger to force the celluloid through a heated cylinder and into a mold. 

The revolutionary invention:

The invention of the reciprocating screw in the 1950s revolutionized the injection molding process. It solved major issues with uneven heating of the plastic. This invention opened up new possibilities for the mass production of plastic parts.

Injection molding today:

Injection molding today is about a 300-billion-dollar global market. Over five million metric tons of plastic parts are produced each year. Striving to be a green manufacturing process, the demand for the use of biodegradable materials is growing.

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