Extrusion

The pellet process (pelletizing / compounding) is how raw polymer and additives are turned into the uniform pellets a molder buys. The ingredients are mixed, melted, formed into strands or sheets and cut into small granules — the upstream step that creates resin feedstock, done at the material producer or compounder, not in the molding shop. ## Typical steps 1. Compounding / feeding: base polymer is dosed with additives, stabilizers, colorants, fillers or reinforcement. 2. Melt & mix: a (usually twin-screw) extruder melts and homogenizes the blend — the extrusion heart of the process. 3. Form: the melt is pushed through a die as strands (or sheet/underwater face). 4. Cool: strands run through a water bath or the melt is cut underwater and quenched. 5. Cut: a pelletizer chops the cooled strands (strand-cut) or the die face is cut hot (hot-face / underwater) into uniform pellets. 6. Dry & screen: pellets are dried, classified to remove fines/oversize and bagged or boxed. ## Why uniform pellets matter Consistent pellet size, shape and bulk density are what let a thermoplastic feed freely and melt repeatably in the molding machine — the whole point of pelletizing instead of shipping powder. The pellet process also locks in the grade's compounded formulation (color, fillers, modifiers) so every bag molds the same. ## Pellet process vs other terms - Pellet process = making the pellets (compounding + pelletizing). - pellet = the granule itself. - emulsion polymerization = creating the polymer chemically, an even earlier step. Virgin pellets come straight from this process; reground flakes (regrind) skip it and are less uniform. ## Related terms - See also: pellet, resin, extrusion, additive, virgin resin ## What is the pellet process in plastics? The compounding-and-pelletizing process that turns raw polymer and additives into uniform pellets — mix, melt-extrude, form into strands, cool, cut and dry — producing the resin feedstock a molder buys. ## How are plastic pellets made? Base polymer is compounded with additives, melted and homogenized in an extruder, pushed through a die as strands (or cut underwater at the die face), cooled, cut into uniform granules, then dried and screened. ## Why are pellets made instead of using powder? Uniform pellets feed freely from the hopper and melt repeatably in the machine, carry the full compounded formulation, and have predictable bulk density — making molding far more consistent than loose powder would.

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.

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.