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.

(Copyright Milacron, Batavia, OH)

Process Map

Injection Molding

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.

General Information

The first type of a 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.

(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.

(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 backwards. 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 flowpath, 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 backwards down the screw.

The picture below show s 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.

(Copyright Milacron, Batavia, OH)

The picture below shows a clamp ing element of a verticle 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.

(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.

(Copyright Master Molded Products Corp., Elgin, IL)

(Copyright Milacron, Batavia, OH)

Other examples of products that can be made using injection molding are plastic beach chairs and 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.


Bayer MaterialScience AG , Fribourg, Switzerland

EPW , Elkhart, IN

Graco Inc. and Subsidiaries

Master Molded Products Corp. , Elgin, IL

Milacron , Batavia, OH

Nonferrous Products, Inc. , Franklin, IN


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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.

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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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