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Injection Molding Machinery 101

Injection molding machinery forms the foundation of an industry with a global value of $258.2 billion, approximately $7.2 billion of that attributed to the U.S. injection molding market.

Injection molding is the most common method of mass-manufacturing plastics products. An amazingly wide range of products are made using injection molding machinery, including:

  • Chairs
  • Toys
  • Cases for consumer electronics
  • Disposable cutlery
  • Bottle caps
  • Automotive components
  • Pocket combs
  • Storage containers
  • Gears

The basic premise behind injection molding is simple: melted plastic is injected into a mold, which is cooled and then opened to release the molded product. In practice, injection molding is a complicated process involving often complex molds.

How it Works

To understand how injection molding works, it helps to understand how injection molding machinery is constructed. The typical plastic injection molding machine is made of three main components: the injection unit, the mold and the clamp. The injection unit is comprised of a hopper that feeds plastic pellets into a barrel equipped with a reciprocating screw.

The screw achieves several goals: heating the pellets, mixing the melting material and moving the material to the front of the barrel. Reciprocating screws provide 60% to 90% of the heat required to melt the plastic; the rest of the heat is supplied by external heat sources like heater bands.

Once enough melted plastic is moved to the front of the barrel, the screw acts like a plunger, forcing the melted plastic into the mold. As this happens, the injected plastic displaces the air in the mold cavity. The air escapes through vents that are narrower than the width of a human hair – too narrow for the viscous melted plastic to enter.

Clamping is Key

As the plastic moves into the mold, the clamp keeps the two parts of the mold in place. The clamping pressure is designated in tons and is generally used to describe the capacity of injection molding machinery. A small injection molding machine might offer 5 tons of clamping pressure, while larger machines can provide up to 60,000 tons of pressure.

Before the clamp can separate the two mold halves and eject the molded part, the material must be cooled, often by circulating water or oil through the mold. After the part is cooled and ejected, the sprue – the length of plastic that connected the mold to the injection unit – must be removed.

Injection molding machinery can operate on a number of systems – hydraulic, variable displacement pumps, servo motor or all-electric. Depending on the level of technology and clamping tonnage required, the cost of an injection molding machinery can vary drastically. A small injection molding machine might cost around $13,000, while a larger machine could run into the hundreds of thousands or even millions of dollars.

Visit Polysort's Injection Molding Machinery category for a list of injection molding machine manufacturers. 

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