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)

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 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.



(Copyright Milacron, Batavia, OH)

The picture below shows a clamping 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)

ADVANTAGES

DISADVANTAGES

  • 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.

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REACTION INJECTION MOLDING


Reaction injection molding (RIM), is used to fabricate thermosetting polymer objects directly from monomers. This occurs by reacting the monomers within a mold of the desired shape.



(Copyright EPW, Inc., Elkhart, IN)

GENERAL INFORMATION

Reaction injection molding (RIM) was developed in the late 1950s at General Motors Corporation for the production of the bumper cover for the Corvette.


RIM is a batch process similar to injection molding, but the polymer itself is not directly injected into the mold. Instead two or more monomers are injected into the mold and polymerized. The mold acts as a chemical reactor.



(Copyright Graco Inc. and Subsidiaries)

EQUIPMENT DESIGN

The reactants are kept in overhead feed tanks. A loop that includes circulation pumps and heat exchangers ensures that the reactants are maintained near the reaction temperature. Melting pumps provide additional heating and deliver the reactants to the mixing head. There the reactants are mixed and pushed into the mold where the mixture can react and be molded simultaneously.



(Copyright Graco Inc. and Subsidiaries)

The mixing head shown on the left is typical of those used in reaction injection molding. The picture on the right is a reaction injection for polyurethane roofing tile.



(Copyright Nonferrous Products, Inc.,
Franklin, IN)

(Copyright EPW, Inc., Elkhart, IN)

The following animation depicts the reaction injection molding process. The hot reactants are injected into the mixing head, where they are forced to mix by impingement. The monomer mixture is then injected into the mold. There it reacts to form a polymer, while being molded into the desired shape. Once the product cools, the mold is emptied and the process is repeated.



USAGE EXAMPLES

RIM is used to make products in wide range of fields, from sports equipment, to home constructions and biomedical applications.


Reaction injection molding is used to produce several kinds of structural foams in a process called Foam Injection Molding (FIM). RIM foams are produced by introducing a blowing agent, such as nitrogen gas, to the monomer mix. The blowing agent helps make the foam more flexible than other RIM plastics. The bike seat shown below is made of polyurethane integral skin foam, meaning it has a high-density outer skin and low-density inner core.



(Copyright Bayer MaterialScience AG, Fribourg, Switzerland)

Structural reaction injection molding (SRIM) is the same as RIM except for the addition of a reinforcement to the mold prior to monomer injection. The monomers are infused into the reinforcement, made of fabric, metal, or plastic, to produce a more rigid structure than those of RIM products. Shown below is a picture of a mold for a SRIM truck door panel.



(Copyright EPW, Inc., Elkhart, IN)

SRIM was developed by General Motors Corporation and used to make spare tire well covers. SRIM has since been used to produce many other car parts, such as door panels, instrument panels , and sunshades. In addition to automotive parts, SRIM is commonly used to produce satellite dishes and door panels, such as the one pictured below.



(Copyright EPW, Inc., Elkhart, IN)

ADVANTAGES

DISADVANTAGES

  • Lower energy requirements to pump monomers rather than polymers.
  • Relatively inexpensive.
  • Process can be repeated as many as 1 million times on the same mold.
  • Can produce a variety of different products.
  • Hardness and other physical properties can be adjusted.
  • Produces low weight products.
  • RIM parts are more paintable than parts produced through other methods.
  • Inconsistent moldings.
  • Reactants must be fast reacting.
  • Rates of reaction must be synchronized with molding times.
  • Requires high pressures.
  • Temperature must be precisely controlled throughout process.
  • Poor performance at high humidity.

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ACKNOWLEDGEMENTS


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


REFERENCES


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.


DEVELOPERS


Daniel Viaches

Rob Kendrick

Amber Ratliff

Steve Wesorick

Christy Charlton

Joseph Palazzolo

Kelsey Kaplan

Henry Chen

Jackie Priestley


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