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Material Use

In general, thermoplastics and thermosets form a comprehensive range of materials used in plastic injection molding.

A ranking of 100 means that the material is excellent for use with this process.

Thermoplastics

Thermoplastics don't have strong bonds with each other. Therefore, they tend to be soft and ductile. When heated above glass transition temperature, the bonds weaken and a reduction in strength results. That's why they can be easily molded under shaping forces but when cooled, the plastic returns to its original strength and hardness making the process reversible. They have a low thermal and electrical conductivity making then good insulators. At last, they have a high coefficient of thermal expansion, otherwise known as the shrink rate (see Metalcast Engineering's shrink rate table).

Thermosets

Thermosets have strong covalent bonds with each other. That's why they are rigid and stiffer but tend to be more brittle. They also have a low electrical and thermal conductivity making them excellent insulators. Unlike thermoplastics, their properties are not very much dependent on the temperature or strain rate. But if the temperature is high enough, they burn up and disintegrate.

Additional Properties

Other properties can be imparted to both types of plastics by adding fillers, plasticizers, colorants and lubricants.

• Fillers improve the strength and stiffness of plastics
• Plasticizers make plastics more flexible
• Colorants are added to give plastics different colors or to make them transparent
• Lubricants reduce the friction of the plastic to the mold

Two Examples

Toothbrush:

Design Considerations

• It needs to be stiff and must be okay to use for a broad range of temperature and humidity conditions.
• It should be chemically inert to water and toothpaste . It should not be brittle rather it should be impact resistant.
• It should be available in different colors and last at least 3 to 6 months.
• Its surface should be smooth and since it's made in large production volume, the scrap should be recyclable.
• The toothbrush handle should have a thick cross section with non complex geometry.
  

Material Selected: A thermoplastic: Nylon.

Computer Casing:

Design Considerations

• It should have good electrical insulating properties and should have the ability to withstand elevated temperatures without a loss in mechanical strength;
• It should be impact resistant, and have good dimensional stability.
• The casing should have a smooth finish and intricate details for aesthetic purposes.
• The service life should be at least 4 to 5 years. Although it is produced in high production volume, there is no restraint on recyclability.