Scrap

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.

Flash is the molding defect in which material escapes along the parting line, vents, ejector-pin clearances or between inserts, forming a thin film stuck to the part. It signals that internal mold pressure exceeded the clamp force locally or that sealing is poor. ## Common causes - Insufficient clamp force (actual tonnage < required tonnage) - Excessive injection or hold pressure - Injection velocity too high at end of fill - High melt temperature (resin too fluid) - Mechanical clearance: worn parting line, vents too deep - Misaligned mold or platen flatness out of tolerance ## How to detect - Visual inspection of part, especially around the parting line - Typical flash thickness: 0.03 – 0.3 mm - In multi-cavity tools, only some cavities may flash → imbalance ## Systematic fix 1. Verify actual clamp force (tie-bar sensor) 2. Reduce pressure / transfer velocity 3. Lower melt temperature 5 – 10 °C 4. If it persists: repair mold mechanics (re-grind plates, adjust vents) ## Cost of flash - Manual deflashing secondary op: $0.01 – 0.05 per part - Scrap if flash falls in a critical area - Accelerated mold wear due to poor sealing - Safety risk from sharp edges on technical parts

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.

Scrap risk is the estimated quantity of molded parts a job is expected to lose to scrap for normal process reasons — start-up purge, first-article samples, validation shots, setup adjustment and ordinary reject rate. It is a planning allowance: molders add it on top of the good parts ordered so the run starts with enough material and machine time to still ship the full quantity. ## Where the scrap comes from - Start-up & changeover: the first shots after a cycle time or color change are off-spec until the process stabilizes. - Samples & validation: first-article inspection, capability studies and approval samples are consumed, not shipped. - Process rejects: the ongoing baseline rate of short shots, flash, sink, dimensional or cosmetic defects. - Component-insertion / complex jobs carry higher risk than a simple single-cavity part. ## How it is used - Material & quoting: scrap risk feeds the extra resin in the total-weight-required calculation and the part price; under-estimating it eats the margin. - Scheduling: it sets how many shots and how much machine time to plan so the customer quantity is met on time. - Improvement target: scrap risk is also a number to drive down — better setup (scientific method scientific molding), a robust quality system and reusing rejects as regrind all shrink the real loss and its cost. ## Why it matters Treating scrap as a planned, estimated figure — not a surprise — is what lets a molder commit to a delivery quantity and a price with confidence. A realistic scrap risk protects the schedule and the margin; tracking actual vs estimated scrap is a continuous-improvement signal. ## Related terms - See also: scrap, molded part, quality system, regrind, scientific method scientific molding ## What is scrap risk in injection molding? The estimated number of parts a job will lose to scrap for normal reasons — start-up, samples, validation and baseline rejects — added on top of the order quantity so enough material and machine time are planned to still ship in full. ## How do you reduce scrap risk? Develop a robust, documented process (scientific molding), stabilize start-up and changeovers, run a real quality system to catch causes early, and reuse rejects as regrind — each lowers the actual scrap and its cost. ## Why include scrap risk in a quote? Because some loss to start-up, samples, validation and rejects is unavoidable; pricing and material planning that ignore it run short of parts or margin, so a realistic scrap allowance protects both delivery and profit.

Short shot is the injection-molding defect in which the cavity is not completely filled and the part comes out incomplete — typically missing material in the zones furthest from the gate, or in bosses, ribs or thin walls. ## Common causes - Insufficient shot size (low dosing) - Injection speed too low: the flow front freezes before filling - Melt or mold temperature out of range (resin too viscous) - Blocked vents: trapped air prevents the melt from advancing - Injection pressure saturating due to upstream restriction (gate, runner, worn check valve) ## Parameters to check Compare actual shot vs. nominal size, multi-stage velocity profile, transfer position, back pressure, barrel temperatures by zone, and vent cleanliness. Cushion must be stable; a cushion at zero means missing material or pressure. ## Systematic fix Increase shot size, ramp velocity in stages, raise melt temperature 5 – 10 °C, open vents, inspect the check valve and verify no restriction in hot runner or gates.