Short Shot

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

Injection Pressure is the pressure the screw exerts on the molten material during dynamic filling, up to the transfer point. It is an output of the process, not a setpoint: it rises as much as needed to maintain the programmed injection velocity. ## Pressure types - Plastic (Ppsi): actual pressure on the material, in bar - Hydraulic (Hpsi): oil pressure in the hydraulic cylinder - Relation: Ppsi = Hpsi × intensification ratio (typically 10:1 to 15:1 depending on screw diameter) ## Typical values by resin - Commodity (PE, PP): 400 – 1200 bar plastic - Engineering (ABS, PC, PA): 700 – 1800 bar - Fiber-reinforced: 1000 – 2200 bar - High-viscosity resins (PEEK, PSU): up to 2500 bar - Modern machines: up to 2400 bar maximum ## Why it matters If pressure saturates (hits machine max), velocity drops and the part fills slower → cold part, cold weld lines, short shot. Design to not saturate: enlarge gates, runners or thickness, or reduce flow length. ## Diagnostics - Repeatable peaks shot-to-shot: stable process - Rising peaks: worn check valve, contamination, partially blocked gate - Falling peaks: mold temperature rising, gate wearing ## Optimization Raise melt temperature, enlarge gates if the restriction is there, switch to higher-MFI resin, or move to a higher-pressure machine (rarely needed with well-designed molds).

Injection Speed (Injection Velocity) is the linear speed at which the screw advances during fill, programmed in mm/s (or volumetric cm³/s). It is one of the parameters that controls fill quality, alongside temperature and hold pressure. ## Why it matters Speed determines: - Fill time: 0.3 – 3 s on technical parts - Shear on the material (faster → more shear → lower effective viscosity) - Cosmetic marks: jetting (excessive speed on small gate), flow marks (too slow or interrupted) - Molecular orientation and residual stress ## Multi-stage profile Modern machines allow 5 – 10 velocity steps along the screw stroke: 1. Slow at gate entry (avoids jetting) 2. Fast in wide cavities 3. Slow near critical vent zones (avoid air trap) 4. Slow at end of fill for smooth transition to hold ## Typical values - Commodity resins, standard wall thickness: 50 – 150 mm/s - Technical detailed parts: 30 – 80 mm/s - Very thin wall parts (<0.8 mm): 200 – 500 mm/s (high-dynamic servo machines) - Shear-sensitive resins (PVC, PMMA): moderate speed ## Optimization Moldflow / Moldex3D analysis to define theoretical profile, iterative tuning with short-shot studies, and melt-temperature monitoring at end of fill (must not rise more than 5 – 10 °C from over-shear). ## Common issues Jetting from high speed on point gates, flow marks from insufficient speed, burn marks from trapped air at end of fill, and delamination if the flow front cools partially.

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.

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.