Flash

Clamp force (clamping force or tonnage) is the force the machine applies to hold the mold halves shut against the pressure of the melt during injection and pack. If it is lower than the force trying to push the mold open, the parting line separates and the part flashes — that is why sizing it correctly is a first-step machine-selection decision. ## How to calculate clamp tonnage The standard estimate: Clamp force = projected area × clamp factor - projected area is the part-plus-runner area seen along the mold-opening direction (in² or cm²). - The clamp factor (see tonnage factor) is an empirical pressure in tons per in² (or bar of cavity pressure). Example: a 50 in² projected area at 3 tons/in² needs 50 × 3 = 150 US tons; add ~10 % margin and pick a press rated near 165–200 tons. ## Typical clamp factors | Resin / situation | Clamp factor (tons/in²) | |---|---| | Easy-flow commodity (PE, PP) | 2–3 | | General engineering (ABS, PA, PC) | 3–5 | | Thin-wall, long-flow, glass-filled | 5–8 | Metric rule of thumb: clamp force in kN ≈ projected area (cm²) × cavity pressure (bar) ÷ 10. ## Why it matters - Too little: flash, dimensional drift and the mold opening during pack — see flash. - Too much: crushed vents and shutoffs, faster mold wear, wasted energy and ruling out otherwise-suitable presses. Most shops size the press from the estimated tonnage required plus a safety margin, without overspending on an oversized machine. ## Related terms - See also: projected area, tonnage factor, estimated tonnage required, flash, injection molding machine imm ## What is clamp force in injection molding? It is the force that keeps the mold closed against injection pressure, quoted in tons (or kN). Too little lets the mold open and the part flashes. ## How do you calculate clamp tonnage? Multiply the projected area by a clamp factor (tons/in²) and add ~10 % margin. A 50 in² part at 3 tons/in² needs about 150 tons, so you would choose a press around 165–200 tons. ## What happens if clamp force is too low? Melt pressure forces the parting line open, giving flash, heavier and dimensionally unstable parts, and eventually damage to the shutoff faces of the mold.

Parting Line is the line or surface where the two halves of the mold (cavity and core) meet when closed. It defines how the mold opens to eject the part and leaves a linear mark on the plastic that is usually visible. ## Importance in design - Determines the mold's opening plane - Defines which surfaces are cosmetic and which can accept the mark - Conditions the location of gates, vents and ejectors - Affects mold complexity and cost ## Parting-line types - Flat: simplest, single horizontal or vertical plane - Stepped: with offsets to accommodate geometries - Curved / 3D: follows the part contour, requires 5-axis machining - Multiple: when there are slides for undercuts ## Design rules - Place the parting line on the part's "natural" edges (corners, flanges) - Avoid crossing cosmetic surfaces - Guarantee accessibility for machining and polishing - Maintain minimum draft of 0.5° on both sides - Plan venting through the parting line ## Associated defects - Flash: the most common, material exits through a poorly sealed line - Visible mark: noticeable on cosmetic parts; mitigated with texture or hidden location - Asymmetric wear: if pressure is poorly distributed across cavities - No venting: if the line is polished perfectly with no air paths ## Maintenance - Visual inspection every 100,000 cycles - Re-grind plates if the line shows recurring flash - Vent cleaning: 0.02 – 0.05 mm standard depth - Plate flatness check: <0.01 mm in precision molds

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.