We are surrounded by plastics in our daily lives, but have you ever wondered what plastic is exactly? Plastics are macromolecular connections from the elements Oxygen, Nitrogen, Hydrogen and Carbon. They consist of connected molecules called macromolecules, chainmolecules or polymers. Plastics are manufactured by the transformation of natural products (rubber, cellulose, hardened milk protein) or by synthetic production (petroleum, natural gas, coal).

Benefits of plastics:

  • Low density (about 0,8 – 2,4 g/cm3)
  • Good durability
  • Electrical insulator
  • Thermal insulator
  • Partial acid and alkali resistant
  • Competitive and cost-effective manufacturing
  • Flexible


There are 3 main categories of plastics that can be molded into a variety of parts. The characteristics of the plastics depend on the inter-connections of their macromolecules.

  • Thermoplastic (amorphous and semi-crystalline thermoplastic)
  • Thermoset
  • Elastomer
The injection molding process using thermoplastic or thermoset plastics is fundamentally different. Thermoplastic in pellet form is melted and then injected into a cool mold. After cooling the mold opens and the part drops out. In thermoset plastic molding the cold material is injected into a heated mold to make the part. Through this process the part cures.


But which plastic material should I choose for my need? Here are the most common materials used for injection molding and their properties:


  • Linear or less branched macromolecules
  • Becomes liquid and moldable through heat
  • Gets inflexible after cooling down
  • Molding process is completely reversible (only physical change, no chemical bonding)
  • Recyclable - parts can be re-melted and reused for production of more parts
  • Cannot withstand as high of temperatures as thermoset material (parts can deform or melt)
  • All-purpose

Amorphous thermoplastic

Such as PS (Polystyrene), PC (Polycarbonate), PMMA (Polymethyl Methacrylate, also known as Plexiglass)
  • Softens gradually when the temperature rises
  • Tend to be transparent, glassy and brittle
  • picture showing Polystyrene, amorphous thermoplastic, by ENGEL Polystyrene
  • picture showing Polycarbonate, amorphous thermoplastic, by ENGEL Polycarbonate
  • picture showing plexiglass, amorphous thermoplastic, by ENGEL Plexiglass

Semi-crystalline thermoplastic

Such as PE (Polyethylene), PP (Polypropylene), PA (Protactinium, also known as Nylon)
  • Stays solid until a certain quantity of heat is absorbed
  • Tends to be opaque
  • picture showing Polyethylene, semi-crystalline thermoplastic, by ENGEL Polyethylene
  • picture showing Polypropylene, semi-crystalline thermoplastic, by ENGEL Polypropylene
  • picture showing Nylon, semi-crystalline thermoplastic, by ENGEL Nylon


Such as PF (Phenolic Formaldehyde)
  • Closely interlaced macromolecules
  • The part gets a permanent strength and shape after curing
  • Cannot be melted and reformed again
  • Stiff till decomposition, no softening point
  • Dimensional stability
  • Strong resistance to high temperatures (important requirement for electrical parts)
  • Electrical and thermal insulation properties (perfect for electrical and electronic applications)
picture showing Phenolic Formaldehyde, thermoset, by ENGEL
Phenolic Formaldehyde


Such as Rubber, LSR (Liquid Silicone Rubber)
  • Natural or synthetic manufactured polymer material with elastic properties
  • Less chemical interlacing
  • Cures when heated to a certain temperature
  • After curing the part gets a permanent strength and shape
  • Cannot be melted and reformed again
  • Strong resistance to high temperatures
  • Flexible
  • Perfect for high temperature sealing functions
  • picture showing Rubber, elastomer, by ENGEL Rubber
  • picture showing Liquid Silicone Rubber, elastomer, by ENGEL Liquid Silicone Rubber