Injection Molding Basics & History

What is Injection Molding?

Injection molding is considered to be the most common way to consistently and accurately mass-produce large quantities of identical components ranging from hundreds to 100,000+ units at competitive prices.

The basic process cycle is as follows:

  • The polymer resin is first dried and placed in the machine hopper where they are mixed with coloring pigment and/or other reinforcing additives.

  • Then the resin mix is fed into the barrel, where the screw mixes and heats the resin, and moves it towards the mold.

  • Melted resin is then injected into the mold under high pressure through the runner and sprue system, filling the entire mold cavity.

  • The cooling cycle then starts, reducing the temperature of the mold and liquid plastic so that it solidifies into the shape of the desired part.

  • Finally, the mold opens and the resulting solid part is pushed out of the mold by the ejector pin system.

  • The mold then closes and repeats this cycle.  A single tool cycle can range from a few seconds to minutes, depending on part complexity, size and number of cavities in the mold.


Materials used in the injection molding process are thermoplastic polymers that can be custom colored or filled with additives such as glass or talc.  The most commonly used materials are:

  • Polypropylene (PP): approximately 1/3 of global production

  • ABS: approximately 1/4 of global production

  • Polyethylene (PE): approximately 1/8 of global production

  • Polystyrene (PS): approximately 1/10 of global production

Advantages and Disadvantages:

Injection molding is very popular in large part because of the comparably low cost per piece when manufacturing high quantities of parts.  The process also offers repeatable and dependable accuracy, along with a high degree of flexibility in materials (ie. rigid and durable nylons, flexible PET and polyethylenes, heat resistance, etc) and design options (ie. snap fits, living hinges, threads, etc).

The main disadvantages to molding typically center around the high investment cost and lead time for the mold.  Molds can run anywhere from $5,000 to over $100,000, depending on size and complexity of part, part material and tool material, annual volume, and required accuracy.

Plastics are all around us:

If you look around, you will begin to notice that just about everything you touch is at least partly built with plastic injection molded components.  They can be found in lighter duty consumer and household products to highly engineered automotive/transportation and electronics applications.​

Brief History of Injection Molding:

Where it got started:

Plastic injection molded is widely regarded to have roots all the way back to the late 19th century.  The Hyatt brothers, John and Isaiah, wanted to develop a material that could replace ivory for the production of billiard balls.  So they invented celluloid along with the first injection molding machine which was patented in 1872.  This machine used a barrel to heat up the plastic and a plunger to inject the material into the mold.  This quickly kick-started a manufacturing revolution that soon led into other simple items molded out of plastic.

Another important step for the injection molded plastics industry:

In the early 1900’s, two scientists from Germany (Arthur Eichengrun and Theodore Becker) created soluable forms of cellulose acetate that were much less flammable than earlier resins.  By the time the 1930’s came around, many popular resins were invented such as PVC , polystyrene, and polyolefins.

Affects of the World Wars on the industry:

The wars created high demand for manufacturing with all the military equipment requirements.  They wars also resulted in the destruction of global distribution channels for rubbers and metals.  The plastics industry had developed enough by then that it could step in to fill the demand gap, and grew so significantly that it became a primary, affordable and scalable manufacturing process by the middle of the 20th century.

More revolutionary inventions:

In the mid 1950s, American inventor James Watson Hendry invented the reciprocating extrusion screw injection machine that revolutionized the plastics industry again. This invention addressed key problems with uneven resin heating, and unlocked new potential for further growth.  Hendry later developed the first gas-assisted injection molding process, which would allow manufacturers to produce long, hollow and complex components out of plastic.

Injection molding today:

Plastic injection molding soon developed into a cost-effective and efficient manufacturing method for precise high-quality parts that it is now used by practically every industry in the world.  The injection molding market is now considered to be in excess of $300 billion globally, with millions of tons of plastic parts being produced each year.  In addition, the advancement of 3D design software technology continues to evolve and improve, making design of parts and molds easier and more reliable.


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