Plastic Tips » Metal-to-Plastic

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Plastics are finding their way more and more into applications traditionally held by metals. Today's stronger more consistent engineering polymers, and better control of the molding process now make it possible to produce components that can easily replace ones made of metal.

One such application is that of gears used for power transmission. Historically, they were limited to very low power designs as in clocks, printers and lawn sprinklers. With the advances in polymer engineering, designs now see gears in higher horsepower equipment such as washing machines.

When designed for a particular polymer, using design fundamentals adapted for polymers, plastic components have realized advantages in;

• Reduced weight – specific gravity of steel = 7.8 vs polymers typically 0.9 - 1.4
  
• Reduced noise as in the application of gears
  
• Reduced cost – plastic injection molded parts take less time to make and require no secondary machining. Secondary operations such as painting, texturing, assembly of several components
  
• Reduced wear – as in the case of gears for power transmission
  
• Increased design opportunities – designs for injection molded parts are not as limited as for metals. Possibilities arise for molding complex shapes in plastic rather than having several components assembled together when made in metal
  
• Greater product structural strength – through the use of engineering grade materials. Also the ability to mold in strength providing features such as ribs, bosses and gussets makes for a stronger product – no assembly required

Many of today's engineering resins provide the necessary performance for parts generally initially considered in metal. They also have the consistent melt viscosity, additive concentrations and other qualities essential to consistent, accurate injection molding.

Properties characteristic of plastics such as heat deflection temperatures, tensile modulus and impact strength have been greatly improved through the introduction of new polymers and additives and now make it possible for plastic to step in where metals were norm.

Definitions and Examples

Deflection Temperature (HDT) – is a measure of a polymer's resistance to distortion under a given load of either 66 psi or 264 psi, at elevated temperatures. Typically, this property is used to consider maximum service temperature for a material. The value obtained for a specific polymer grade will depend on the base resin and on the presence of reinforcing agents. Deflection temperatures of glass fiber or carbon fiber reinforced engineering polymers will often approach the melting point of the base resin
(see Table 1).

Table 1. Typical Deflection Temperatures and Melting Points of Polymers

Tensile Modulus – is the ratio of stress to strain in a material that is elastically deformed.

The higher the modulus, the more loading the material will take in deforming a set amount.

Impact Strength (Izod) Notched – is a measure of a polymer's ability to absorb energy from impact at a point of high stress concentration such as sharp corners or cutouts without breaking.

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