Scrap Risk

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

A quality system (quality management system, QMS) is the documented set of procedures, records and responsibilities a molding shop uses to consistently make parts that meet specification — and to prove it. It turns "we made good parts" into "we control the process that makes good parts and have the evidence." In injection molding it ties the molding process, the people and the paperwork together. ## What it covers in a molding shop - Process control: documented setups, target windows and monitoring of molded part dimensions, weight (cavity weight) and dimensional stability — ideally developed with scientific method scientific molding so the process is robust and repeatable. - Incoming & material control: verifying resin against the material data sheet, drying records, lot traceability and controlled regrind ratios. - Validation: IQ/OQ/PQ (installation, operational, performance qualification) to prove a new mold or process makes good parts across its window — required in medical and automotive work. - Maintenance & changeover: preventive maintenance schedules, single minute exchange die and 5 s to keep the cell capable and organized. - Records & improvement: inspection data, non-conformance and corrective action, reducing scrap over time. ## Common standards ISO 9001 is the general QMS standard; IATF 16949 adds automotive requirements; ISO 13485 covers medical devices. Certification signals a customer the shop runs a real, audited system. ## Why it matters A quality system is what makes good parts repeatable and provable — it lowers scrap and returns, satisfies regulated customers, and turns problem-solving into a documented, systematic loop rather than firefighting. ## Related terms - See also: scientific method scientific molding, molding process, material data sheet, preventive maintenance, scrap ## What is a quality system in injection molding? The documented procedures, records and responsibilities a shop uses to consistently make parts to spec and prove it — covering process control, material verification, validation (IQ/OQ/PQ), maintenance and continuous improvement, often certified to ISO 9001 or IATF 16949. ## What is the difference between ISO 9001 and IATF 16949? ISO 9001 is the general quality management standard for any industry; IATF 16949 builds on it with stricter automotive-specific requirements (PPAP, APQP, traceability) for suppliers to vehicle manufacturers. ## Why does a molder need a quality system? To make conforming parts repeatable and provable: it controls the process, documents material and maintenance, validates new tooling, lowers scrap and returns, and is usually required to supply automotive, medical or other regulated customers.

Scrap is any material or product that leaves the molding process without becoming a saleable part: defective mouldings plus non-part plastic such as runners, sprues, start-up shots and purgings. It is tracked as a scrap rate and is one of the largest hidden costs in an injection molding plant. ## Two kinds of scrap - Process scrap: runners, sprues, flash, start-up and changeover shots — usually clean and recyclable as regrind. - Reject scrap: parts that failed inspection — short shot, flash, sink marks, burns, splay, contamination or dimensional fails. ## Scrap rate Scrap rate = scrap ÷ total produced (by count or by mass). Example: 60 rejects in 2,000 shots = 3 %. It feeds the quality term of OEE; a 3 % scrap rate means 3 % of your machine time, resin and labor produced nothing to sell. ## Why it matters and how to reduce it Resin is usually the biggest cost in a molded part, so every scrapped gram is lost money plus the energy spent making it. Reduce it with a stable, documented process window (scientific molding), mold and dryer maintenance, and root-cause work on the dominant defect. Some process scrap returns as regrind, but regrind ratios are capped because reprocessing degrades the polymer. ## Related terms - See also: regrind, short shot, flash, runner, sprue ## What is scrap in injection molding? It is everything that does not ship: rejected parts plus runners, sprues, start-up shots and purge. It is measured as a scrap rate against total production. ## What is the difference between scrap and regrind? Scrap is the discarded material; regrind is scrap that has been ground up to be re-melted and reused. Clean process scrap becomes regrind, while contaminated or degraded scrap is waste. ## How do you reduce the scrap rate? Stabilize the process to a documented window, keep molds and dryers maintained, attack the top defect by root cause, and use a hot-runner or smaller-runner design to cut process scrap.

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.