Injection Molding

Injection Molding

Injection molding is the most common molding process after extruders. It is widely used to process thermoplastics and thermosets. There are two types of injection molding machines: injection molding machines, and reaction injection molding machines.

Injection Molding
(Copyright Milacron, Batavia, OH)

Polymer Product Manufacturing Steps

Injection molding is one of the ways you can process the polymer in the manufacture of polymer products:

General Information

The first injection molding machine was patented by John Wesley Hyatt in 1872. It was a stuffing machine that consisted of a steam-heated chamber and a hydraulically operated plunger. In injection molding today, the molding material is first heat-softened, injected into a mold, then cooled to form the desired product. Injection molding machines are typically categorized as either vertical or horizontal.

The first type of commercialized injection machine was the single-stage plunger, followed by preplasticizing machines. Today, single screw injection molding machines are the industry’s choice. In addition, multi-station machines are used to produce multi-color products.

Injection Molding
Injection Molding

(Copyright Milacron, Batavia, OH)

The polymer enters through the hopper into the screw area, where it is heat softened. A screw forces the polymer into the mold. After the polymer is cooled, the molded product is ejected and the process can begin again.

injection molding
(Copyright Milacron, Batavia, OH)

Equipment Design

The screw of a single screw injection machine functions as an extruder as well as an injection plunger. As the screw turns, the material is forced to the front of the screw, moving the screw backward. The material is injected into the mold by bringing the screw forward as a plunger. The spacing between the screw and the wall is increasingly narrow down the flow path, since the plastic viscosity will decrease as the plastic travels down the screw.

The screw operates only in an intermittent fashion and for very short times. The screw is designed to withstand high pressures and to prevent the melted material from leaking backward down the screw.

The picture below shows a clamping mechanism in a horizontal machine. In this type of injection molding machine, both the clamping and the injection units are horizontal, making access easier. This style is preferred when an automated system is desired or if regular maintenance and mold adjustments are needed.

clamping mechanism
(Copyright Milacron, Batavia, OH)

The picture below shows a clamping element of a vertical molding machine that operates in a similar fashion as the horizontal molding machine. Both the clamping and injection units are vertical. This machine setup is preferred when space needs to be preserved or a relatively heavy or large mold needs to be used.

clamp ing element of a verticle molding machine
(Copyright Milacron, Batavia, OH)

Usage Examples

Injection molding machines may be used to make almost anything out of plastic. The only exceptions are bottles and other containers, which are manufactured using blow molding. Products can range in size from small handheld containers to car bumpers.

injection molding product
(Copyright Master Molded Products Corp., Elgin, IL)
injection molding products
(Copyright Milacron, Batavia, OH)

Other examples of products that can be made using injection molding are plastic beach chairs and all-terrain vehicle components.

plastic beach chair
all-terrain vehicle components.

(Copyright Milacron, Batavia, OH)


  • Injection molding is cheaper to operate than most other polymer processing processes.
  • Versatile in types of materials that can be handled.
  • Can produce low tolerances, comparable to other molding processes.


  • Enormous pressure losses add to inefficiency due to the laminar flow.
  • Preplasticizing machines are more expensive to operate and require more floor space than single screw machines.



  • Chanda, Manas and Salil K. Roy. Plastics Technology Handbook, 3rd ed. New York: Marcel Dekker, Inc., 1998, 262-268. Print.
  • Kamal, Musa R., Avraam I. Isayev, and Shih-Jung Liu. Injection Molding: Technology and Fundamentals. Munich: Hanser, 2009. Print.
  • Macosko, Christopher W. RIM- Fundamentals of Reaction Injection Molding. New York: Oxford Press, 1989. Print.
  • Rubin, Irvin I. Injection Molding Theory and Practice. New York: John Wiley & Sons, Inc. 1972. 3-20, 63-74. Print.
  • Whelan, A. Injection Molding Machines. Elsevier Applied Science Publishers Ltd., 1984. 95-104, 289. Print.
  • Wigotsky, Victor. “Injection Molding Machinery: on to 2000.” Plastics Engineering December 1997: 24-30. Print.


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