Pellet Process

Additive is a substance added in small amounts (typically 0.05 – 10 %) to a base polymer to modify its properties, improve processability or extend service life. The plastics industry relies on additives to meet each application's requirements. ## Main families - Antioxidants: primary (hindered phenols) + secondary (phosphites) stabilizers. Prevent oxidative degradation - UV / light stabilizers: HALS, UV absorbers. Protect against photodegradation - Lubricants / release agents: stearates, waxes. Improve processing and demolding - Antistatics: ethoxylated amides. Dissipate electrostatic charge - Flame retardants (FR): brominated, phosphorus, halogenated, synergists. UL94, V0/V2 - Plasticizers: phthalates, adipates, citrates. Flexible PVC - Nucleants / clarifiers: speed up crystallization (PP), improve clarity - Pigments and dyes: inorganic (TiO2, oxides), organic (azo, phthalocyanines), masterbatches - Fillers and reinforcements: talc, CaCO3, glass fiber, carbon fiber - Impact modifiers: EPDM, MBS, acrylics ## Addition format - Masterbatch: concentrated additive (20 – 50 %) in pellet form, let down 1 – 10 % into virgin - Compounded grade: already supplied with additives integrated (FR PP, glass-filled PA) - Liquid dosing: liquid additives injected directly into the screw ## Considerations - Compatibility with the base resin - Blooming or bleeding when concentration exceeds solubility limits - Migration into food contact (FDA / EU regulation) - Cost: TiO2 can be 30 – 50 % of a colored compound's cost - Recyclability: many additives survive regrind, others degrade

Extrusion is a continuous process in which a thermoplastic polymer is melted by a screw inside a heated barrel and forced through a die with the desired cross-sectional shape. A continuous profile (tube, sheet, profile, filament) exits the die, is cooled, and cut to length. ## Extrusion vs. injection molding Injection molding produces discrete parts with 3D geometry; extrusion produces continuous products of constant cross-section. They share the plasticizing stage —screw, barrel, heater bands— but injection adds a mold, injection pressure, and a cycle. ## Common extrusion types - Profile extrusion (PVC, PE, PP) for construction and furniture - Pipe extrusion (PE, PP, PVC, PEX) - Sheet extrusion (PS, PET, PP) for thermoforming - Filament extrusion (PLA, ABS, PETG) for 3D printing - Cable extrusion and blown film ## Typical parameters - Screw speed: 30 – 150 rpm - Melt temperature: 180 – 280 °C depending on resin - Die exit pressure: 100 – 500 bar - Screw L/D ratio: 24:1 to 36:1 - Pelletizer or cooling calender downstream ## Common defects Sharkskin from excessive line speed, melt fracture from high shear, contamination from incomplete purging, and out-of-tolerance dimensions from a poorly set sizer/calibrator.

A pellet (granule) is a small, uniform piece of resin — typically a 2–5 mm cylinder, lens or sphere — and the standard form in which thermoplastic is supplied to an injection molder. Pellets pour freely from the hopper, feed evenly into the screw, and melt consistently, which is exactly why raw plastic is pelletized rather than sold as powder or random chunks. ## Why plastic comes as pellets - Free flow & metering: uniform size gives steady, bridge-free feeding and repeatable melt in the barrel. - Built-in formulation: most pellets are already compounded — base polymer plus stabilizers, additives, reinforcements or color. Color can also be added as a masterbatch (highly pigmented pellets) blended with natural pellets. - Handling & storage: predictable bulk density makes dosing, drying and conveying repeatable; pellets are easier to dry than powder. ## Pellet vs other feed forms - virgin resin: first-use pellets straight from the producer, never melted. - regrind: reclaimed sprues/runners/scrap ground into flakes — irregular vs uniform pellets, so it flows and melts less consistently and is usually blended at a controlled ratio. - Powder/flake/repro: used in some processes but harder to feed evenly than pellets. ## What it means for molding Because pellets are the input, their dryness (moisture), bulk density and consistency drive shot-to-shot stability. Mixed pellet sizes, dust or excess regrind upset feeding and melting; that is why molders control storage, drying and regrind ratio. See pellet process for how pellets are made. ## Related terms - See also: resin, virgin resin, regrind, pellet process, hopper ## What is a plastic pellet? A small, uniform granule of thermoplastic resin — usually a 2–5 mm cylinder, lens or sphere — that is the standard feedstock for injection molding because it flows and melts consistently. ## Why is plastic supplied as pellets? Uniform pellets feed freely from the hopper into the screw and melt evenly, they can carry a complete compounded formulation (additives, reinforcement, color), and their predictable bulk density makes drying and dosing repeatable. ## What is the difference between a pellet and regrind? A pellet is a clean, uniform virgin granule; regrind is reclaimed scrap ground into irregular flakes. Regrind flows and melts less consistently, so it is usually blended with virgin pellets at a controlled percentage.

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.

Virgin resin is plastic pellet that has never been melted or processed before — first-use material straight from the polymer producer, with no regrind or recycled content. It is the baseline against which a molder judges every other feedstock, because its properties match the material data sheet exactly. ## Why molders use virgin resin - Known, full properties: chains are at their original molecular weight, so strength, color and flow are as specified — no thermal history has degraded them. - Consistency: lot-to-lot behavior is predictable, which stabilizes the process and reduces scrap. - Regulated parts: medical, food-contact, optical and many cosmetic or safety parts often require 100 % virgin for traceability and purity. ## Virgin vs regrind vs recycled - Virgin: first use, never melted. - regrind: the shop's own runners/sprues/rejects ground up and re-fed — one extra heat history, usually blended with virgin at a controlled ratio (often 10–30 %). - Post-consumer recycled (PCR): reclaimed from used products; variable quality, frequently blended with virgin to hit a recycled-content target. ## The virgin/regrind balance Each reheating shortens polymer chains and can shift color and viscosity, so adding regrind saves cost and waste but too high a ratio degrades the part. Molders pick a blend percentage the part can tolerate, keep regrind clean and dry (it absorbs moisture fast), and run pure virgin where regulations or cosmetics demand it. ## Related terms - See also: resin, regrind, pellet, material data sheet, molded part ## What is virgin resin? Plastic resin in its first-use state — pellets from the producer that have never been melted, ground or recycled — so its mechanical, optical and flow properties match the data sheet exactly. ## What is the difference between virgin resin and regrind? Virgin resin has no prior heat history; regrind is the shop's own scrap (runners, sprues, rejects) ground and re-fed, carrying one extra melt cycle. Regrind is usually blended into virgin at a controlled percentage. ## Why use virgin resin instead of regrind? For full, predictable properties and lot consistency, and because medical, food-contact and optical parts often require 100 % virgin for purity and traceability; regrind saves cost but degrades slightly with each reheat.