Polypropylene

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.

Polyethylene (PE) is the world's highest-volume thermoplastic, made by polymerizing ethylene. Semi-crystalline, chemically inert and inexpensive, it is processed by injection, extrusion, blow molding and rotomolding. The PE family includes several grades with very different properties. ## Main families - HDPE (high-density): 0.94 – 0.97 g/cm³, stiff, opaque; caps, drums, pipe - LDPE (low-density): 0.91 – 0.94 g/cm³, flexible, translucent; film, soft packaging - LLDPE (linear low-density): high tear strength; stretch film - UHMWPE (ultra-high molecular weight): up to 6 M g/mol; gears, prosthetics, armor - PEX (crosslinked): PEX-A; hot-water pipe ## Key properties - Excellent chemical resistance (acids, bases, salts, water) - Food-contact approved (FDA, EU 10/2011) - Service temperature: -50 to 80 °C (HDPE), -70 to 60 °C (LDPE) - High permeability to oxygen and aromas (no barrier) - Non-hygroscopic (no drying required) ## HDPE molding parameters - Melt temperature: 200 – 280 °C - Mold temperature: 20 – 60 °C - Shrinkage: 1.5 – 3.0 % (high) - Speed: moderate; very fluid resin tends to flash - No pre-drying ## Common defects Significant warpage from directional shrinkage, visible weld lines (PE is hard to self-weld at the flow front), waxy odor during processing, and degradation above 300 °C with smoke.

Polymer is a macromolecule formed by the repeated covalent bonding of many small units called monomers. It is the molecular foundation of all plastics, rubbers, fibers and many biological materials (proteins, cellulose, DNA). ## Classification by origin - Natural: cellulose, starch, proteins, natural rubber, lignin - Synthetic: PE, PP, PVC, PS, PET, PA, PC, ABS… (most of the market) - Semi-synthetic: rayon, cellulose acetate, modified natural rubber derivatives ## Classification by architecture - Linear: straight chains (HDPE, PA 66, PS) - Branched: chains with branches (LDPE, ABS) - Crosslinked: PE-X, vulcanized rubber, thermosets - Dendritic: tree-like structures (specialty) ## Classification by thermal response - Thermoplastics: melt and reshape reversibly (PP, PE, PA, PC) - Thermosets: chemically cured, do not remelt (epoxy, phenolic) - Elastomers: flexible, recover shape after deformation (rubber, TPE) ## Classification by composition - Homopolymers: single monomer type (PE, PP-H) - Copolymers: two or more monomers (ABS = acrylonitrile + butadiene + styrene) - Blends: two polymers physically mixed (PC/ABS, PA/PPS) ## Key properties governed by structure - Molecular weight: stiffness and processability - Molecular-weight distribution: process window and toughness - Crystallinity: stiffness, opacity, shrinkage - Chain polarity: chemical resistance, adhesion, transparency

Thermoplastic is a polymer that softens and re-melts when heated above its melting or glass-transition temperature and solidifies again on cooling — with no permanent chemical reaction. This reversibility is what enables injection molding, extrusion and mechanical recycling of most plastics. ## Thermoplastic vs. thermoset - Thermoplastic: linear or branched chains without chemical crosslinks. Melts and can be remolded (PP, PE, ABS, PC, PA, PET, POM). - Thermoset: crosslinks chemically during cure (phenolic, epoxy, melamine resins). Cannot be remelted; reheating only degrades it. ## Classification of thermoplastics - Commodity: PP, PE-HD/LD, PS, PVC, PET → high volume, low cost - Engineering: ABS, PA (nylon), PC, POM, PMMA, PBT → improved mechanical properties - High-performance: PEEK, PPS, PSU, PEI, LCP → high service temperature, high cost - By structure: amorphous (PC, PS, ABS) vs. semi-crystalline (PP, PE, PA, POM) ## Processability Almost any thermoplastic can be injection-molded, extruded, thermoformed, blow-molded and roto-molded. Semi-crystalline grades require precise mold temperature to control crystallinity; amorphous grades tolerate wider windows. ## Recyclability and reuse Thermal reversibility allows scrap (regrind) to be ground and reprocessed up to 20 – 30 % mixed with virgin resin with no major property loss, depending on the polymer. Additives, cross-contamination with other resins, and accumulated thermal degradation limit the number of cycles.