Regrind Process

Continuous recirculation is the practice of continuously feeding reclaimed material back into the production stream — reintroducing in-house scrap (and sometimes recycled content) into the feed so resources are used to the fullest and waste is minimized. It is the circular-economy principle that the regrinding cycle puts into practice on the molding floor. ## How it works in a molding cell - In-house loop: runners, sprues and rejected parts are ground into regrind and metered straight back into virgin resin at a controlled ratio, shot after shot — the regrinding cycle. - Closed-loop, near real time: with beside-the-press granulating, the reclaimed flakes return to the same machine's feed continuously, not in separate batches. - Steady-state balance: the recycled fraction settles to an equilibrium set by how much scrap each shot makes versus the dosing ratio. ## Why it matters - Resource efficiency & cost: less virgin resin bought and less waste hauled away for the same number of good parts. - Lower carbon footprint: keeping carbon circulating in the process beats landfill/incineration plus new fossil feedstock. - Lean & sustainability: continuous recirculation is a pillar of lean manufacturing (eliminating material waste) and of a plant's sustainability goals. ## The control caveat Recirculation must be managed, not unlimited: every pass adds a regrind generation of heat history that degrades the polymer, so molders cap the blend ratio, limit generations and cascade higher-generation material to lower-spec parts. Where properties or regulations forbid recyclate, the loop runs on pure virgin resin and chemical recycling (depolymerization) becomes the circular route instead. ## Related terms - See also: regrinding cycle, regrind, regrind generation, carbon footprint, depolymerization ## What is continuous recirculation in injection molding? Continuously feeding reclaimed in-house material (regrind) back into the production stream at a controlled ratio so resources are maximized and waste minimized — the circular-economy principle behind the regrinding cycle. ## How is continuous recirculation different from the regrinding cycle? Continuous recirculation is the broad principle of always feeding reclaimed material back into production; the regrinding cycle is the concrete closed loop — grind scrap, blend with virgin, re-mold — that carries it out on the floor. ## What limits continuous recirculation? Polymer degradation: each reprocessing pass adds a regrind generation that lowers properties, so the blend ratio and number of generations are capped, and regulated or high-spec parts may require virgin resin or chemical recycling instead.

Regrind is plastic material recovered by grinding runners, sprues, defective parts or purges, then blended with virgin resin and reintroduced into the process. It is a key sustainability and cost-reduction tool in injection molding. ## Why use regrind Recovers the ~20 – 30 % unavoidable scrap from cold runners and cuts raw-material cost by 5 – 25 %, with a lower carbon footprint. On non-critical parts, pure or near-pure regrind is fully viable. ## Typical proportions (regrind/virgin blend) - Cosmetic / engineering parts: 10 – 20 % - Non-visible structural parts: 20 – 50 % - Internal / non-critical: 50 – 100 % - Some resins (PVC, PE): up to 100 % in approved applications ## Regrind system equipment - Granulator: rotating blades, sizing screen - Pellet size: 3 – 8 mm for homogeneous mix with virgin - Beside-the-press: granulator next to the machine, regrind returns to the hopper via blower - Magnet + metal detector: mandatory to avoid screw damage - Volumetric or gravimetric blender: meters regrind and virgin in a defined ratio ## Limitations and issues - Cumulative thermal degradation each pass (lowers viscosity, properties) - Cross-contamination with another resin causes delamination or breakage - Yellowing or graying on unpigmented resins - Property loss in multi-generation regrind - FDA / medical / automotive restrictions forbid uncertified regrind ## Disallowed applications FDA food-contact, class II/III medical devices, structural crash parts in automotive, and certain children's toys under specific regulation.

Regrind generation is how many times a given batch of plastic has been melted and reground through the regrind process. virgin resin that has never been molded is "zero generation"; the first time its runners and rejects are ground and re-fed, that material is first-generation regrind; grind and re-mold it again and it becomes second-generation, and so on. It tracks the cumulative heat history, not the amount of regrind. ## Why generations matter Each melt-and-grind cycle adds thermal and mechanical stress that shortens polymer chains (chain scission) and can oxidize the resin. With every generation: - mechanical strength, impact resistance and elongation drop; - viscosity and flow shift, making the process harder to hold; - color can yellow and surface defects (splay, black specks) increase. The rate of decline depends on the resin — PC, PET and PA are sensitive; PP and PE tolerate more generations. ## How molders manage it - Limit generations: many specs allow only first-generation regrind, sometimes none for critical parts. - Cap the blend ratio: keeping regrind at, say, 10–30 % of virgin resin dilutes the high-generation fraction each cycle. - Cascade use: route higher-generation material to lower-requirement parts instead of the original part. - Document it: tracking allowed generation and ratio is part of a quality system, protecting the molded part. ## Related terms - See also: regrind, regrind process, virgin resin, regrinding cycle, quality system ## What is regrind generation in injection molding? The number of times a plastic has been melted and reground — first-generation regrind has one extra heat history beyond virgin, second-generation two, and so on. It measures cumulative thermal degradation, not quantity. ## How many times can plastic be reground? It depends on the resin and the part's requirements: sensitive resins (PC, PET, PA) may allow only one generation, while PP or PE can tolerate several; critical or regulated parts often require zero regrind. ## Why does each regrind generation reduce properties? Every melt-and-grind cycle adds heat and shear that break polymer chains and can oxidize the resin, lowering strength, impact and elongation and shifting flow and color with each successive generation.

A regrind system is the equipment that carries out the regrind process — the granulator and its supporting hardware that turn molding scrap into reusable regrind flakes and feed them back to the machine. Where the regrind process is the workflow, the regrind system is the physical line of machines that performs it. ## What it includes - Granulator: the core unit — a rotating cutting chamber with blades and a screen that sizes the flakes. Beside-the-press granulators sit at one machine; a central granulator serves many. - Screen / classifier: sets flake size and removes fines and oversize. - Metal separator & de-dusting: protect the screw and keep flakes clean. - Conveying & dosing: vacuum loaders, a blender or gravimetric hopper doser that meters regrind into virgin resin at a set ratio. - Sound enclosure & granulator type: low-speed/screenless units for brittle or heat-sensitive resins, high-speed for general use. ## Why the system design matters - Flake quality: blade sharpness, screen size and cutting geometry control flake consistency and fines — poor flakes feed and melt unevenly. - Contamination & heat: a clean, cool granulator avoids adding regrind generation damage and keeps metal and dust out. - Integration: matching the system's throughput to the press and the secondary equipment keeps regrind flowing without choking the cell or letting flakes pile up. ## Related terms - See also: regrind, regrind process, secondary equipment, regrind generation, virgin resin ## What is a regrind system in injection molding? The equipment that grinds and handles scrap — a granulator plus screens, separators, de-dusting and dosing — turning runners and rejects into clean regrind flakes and metering them back into virgin resin. ## What is the difference between a beside-the-press and a central granulator? A beside-the-press granulator serves one machine and reground material can go straight back into it; a central granulator handles scrap from many machines in one location, suiting higher volumes and mixed jobs. ## What makes a good regrind system? Sharp blades, the right screen size, effective de-dusting and metal separation, low heat and noise, and dosing matched to the press — so flakes are uniform, clean and blended at a controlled ratio.

The regrinding cycle is the closed loop in which scrap is ground, blended back with virgin resin and re-molded — over and over — while a job runs. One turn of the loop is: mold a shot → the runner and sprue (and any rejects) become scrap → the regrind system grinds them → the flakes are dosed back into the feed → they are molded again. Each completed loop is what advances the regrind generation. ## How it works in practice - Closed-loop recovery: with a beside-the-press setup, runners drop straight into a granulator and the flakes return to the same machine's feed throat in near real time — a continuous continuous recirculation of material. - Per molding cycle: because every molding cycle produces fresh runner/sprue scrap, the regrinding cycle runs in step with production, not as a separate batch. - Steady-state ratio: the regrind fraction in the feed settles to an equilibrium set by how much scrap each shot makes versus the dosing ratio. ## Why it must be managed Left unchecked, a tight regrinding cycle keeps re-melting the same material, pushing it to higher regrind generation and degrading it. Molders break or dilute the loop by: - capping the blend ratio so fresh virgin resin keeps entering; - routing some regrind to lower-spec parts instead of straight back; - limiting how many generations the loop may reach for a given part. A controlled regrinding cycle recovers nearly all in-house scrap with minimal waste; an uncontrolled one quietly degrades quality shot after shot. ## Related terms - See also: regrind process, regrind generation, regrind system, continuous recirculation, virgin resin ## What is the regrinding cycle in injection molding? The repeating closed loop of grinding scrap, blending it back with virgin resin and re-molding it as a job runs; each pass through the loop adds a regrind generation to the recovered material. ## How is the regrinding cycle different from the regrind process? The regrind process is the step-by-step procedure for one batch; the regrinding cycle is that procedure repeating continuously in a closed loop alongside production, so material is recovered shot after shot. ## How do you keep a regrinding cycle from degrading parts? Cap the regrind blend ratio so virgin keeps entering, limit the number of generations, and cascade higher-generation material to less critical parts instead of recirculating it indefinitely.