Definition
A plastic is a synthetic or semi-synthetic material whose backbone is a Polymer — long chains of repeating molecular units — usually combined with Additives that tune color, stability, flow and strength. The word "plastic" refers to its plasticity: when heated it can be shaped and then set into a solid part, which is exactly what injection molding exploits.
Plastic vs polymer vs resin
- Polymer: the pure long-chain molecule (e.g. polyethylene).
- Plastic: the usable material = polymer + additives, the everyday term for the finished compound.
- Resin: in a molding shop, the Pelletized plastic fed into the machine. In practice molders use "resin," "plastic" and "material" almost interchangeably.
The two families that matter for molding
- Thermoplastic: softens when heated and re-solidifies when cooled, reversibly — so it can be melted, molded, reground and re-melted. Virtually all injection molding uses thermoplastics.
- Thermoset: cures into a permanent crosslinked network and cannot be re-melted (e.g. epoxy, phenolic). Molded by different processes.
Structure that drives behavior
Thermoplastics are either Amorphous (random chains — PC, ABS, PS: gradual softening, lower shrink, often clear) or semi-Crystalline (ordered regions — PP, PA, POM: sharp melting, higher shrink, chemical resistance). This structure sets melt behavior, Viscosity, shrinkage and where the Molding Process window lives.
Related terms
- See also: Polymer, Thermoplastic, Thermoset, Resin, Additive
What is plastic made of?
A base polymer — long repeating molecular chains, usually from petrochemicals or increasingly bio/recycled sources — blended with additives such as stabilizers, colorants, lubricants and reinforcements to reach usable properties.
What is the difference between a plastic and a polymer?
A polymer is the pure long-chain molecule; a plastic is the usable material made from that polymer plus additives. In molding, the pelletized plastic fed to the machine is usually called resin.
What kinds of plastic are used in injection molding?
Almost always thermoplastics, which melt and re-solidify reversibly — split into amorphous grades (PC, ABS, PS) and semi-crystalline grades (PP, PA, POM); thermosets cure permanently and use other processes.