Regrind

A cold runner is an unheated channel system in the mold that carries the melt from the sprue to each cavity. Because the runner is not heated, the plastic in it cools and solidifies along with the parts every cycle, so the runner and sprue are ejected as a connected skeleton and become scrap (usually reclaimed as regrind). It is the simpler, cheaper alternative to a hot runner. ## How it works Each shot fills the cold runner first, then the cavities through the gates. When the part solidifies, the runner does too; the whole runner-and-part assembly is ejected, then the runner is degated, separated and reground. Layouts are kept balanced so every cavity fills evenly. ## Cold runner vs hot runner - Cold runner: unheated; runner freezes and is ejected each cycle → runner scrap/regrind, but low tooling cost, simple, easy color/material changes, tolerant of many resins. - hot runner: heated manifold keeps the runner molten → no runner scrap, faster cycles, automation-friendly, but higher mold cost, more maintenance and harder color changes. ## Trade-offs and use Cold runners suit lower volumes, frequent color/material changes, and shops that can regrind their runner economically. The downsides are the recurring runner scrap, extra material per shot, degating labor, and the heat-history hit each time the runner is reground. Good cold-runner design minimizes runner volume while keeping fill balanced. ## Related terms - See also: runner, sprue, hot runner, regrind, cavity ## What is a cold runner in injection molding? An unheated runner system that delivers melt from the sprue to the cavities; the plastic in it solidifies with the part each cycle and is ejected as runner scrap, usually reground and reused. ## What is the difference between a cold runner and a hot runner? A cold runner is unheated and freezes into scrap every cycle (cheap, simple, easy color changes); a hot runner is heated to stay molten, eliminating runner scrap and speeding cycles, but costs more and is harder to maintain. ## What happens to the cold runner after molding? It is ejected attached to the parts, then degated (separated from the parts) and typically granulated into regrind that is blended back with virgin resin at a controlled ratio.

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.

The regrind process is the procedure a molder follows to turn in-house plastic scrap — runners, sprues, rejected parts and purge — into reusable regrind flakes that can be blended back with virgin resin. It is the workflow; the material it produces is regrind, the equipment that does it is the regrind system, and how many times material has been through it is the regrind generation. ## Typical steps 1. Collect & sort: keep scrap clean, dry and separated by resin and color — contamination cannot be undone later. 2. Granulate: a granulator cuts the scrap into flakes sized close to a pellet so they feed and melt like virgin. 3. De-dust / screen: remove fines and oversize; dust and long slivers cause feeding and quality problems. 4. Blend: meter the regrind into virgin resin at a controlled ratio (often 10–30 %), usually with a dosing unit. 5. Dry & re-mold: regrind re-absorbs moisture quickly, so it is dried with the virgin before going back to the machine. ## Why control it Each pass through the process adds a heat history that shortens polymer chains, so an uncontrolled regrind process degrades parts and destabilizes the cycle. A documented procedure — clean handling, fixed blend ratio, drying, limited generations — is part of a real quality system and is what lets regrind cut cost and waste without hurting the molded part. ## Related terms - See also: regrind, regrind system, regrind generation, virgin resin, scrap ## What is the regrind process in injection molding? The workflow of collecting in-house scrap (runners, sprues, rejects), granulating it into flakes, de-dusting, blending it with virgin resin at a controlled ratio, drying and re-molding — so usable material is recovered instead of discarded. ## What are the steps to reprocess plastic regrind? Collect and sort clean scrap by resin and color, granulate it into pellet-sized flakes, screen out dust and oversize, blend it into virgin at a set percentage, then dry and re-mold it with the virgin resin. ## Why must the regrind process be controlled? Because every reprocessing cycle adds heat history that degrades the polymer; controlling cleanliness, blend ratio, drying and the number of generations keeps part quality and process stability acceptable.

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.