Injection Molding

Flash Percentage on a Blow-Molded Container, and What It Costs

Flash on a blow-molded part is treated as recoverable because it goes back in the hopper. Every gram of it was heated, extruded, pinched off, ground, and reheated, and none of that was free.

Flash in extrusion blow molding is the material pinched off outside the cavity: the tail, the dome, the handle slug, and the trim around the neck. It is calculated as a percentage of total shot weight, and because it is usually reground and fed back, most operations treat it as a rounding error. That treatment is why it rarely gets attacked, and why it is often one of the larger recoverable costs in the process.

The calculation

Flash percentage is flash weight divided by total shot weight. Take a 45-gram finished container with 12 grams of flash, which is a 57-gram shot: flash is 21.1% of everything extruded. Across 500,000 parts a year that is 6,000 kilograms of resin passing through the machine solely to be cut off and reprocessed. Weigh the actual shot and the actual trim rather than taking the tool designer's figure, because flash grows as pinch-off edges wear.

• Flash share of shot weight: 21.1%
• Annual flash at 500k parts: 6,000 kg
• Resin PPI, Jun 2026: 310.75

Regrind is not free, and here is why

Four costs that survive recycling the flash

• Energy. Every gram of flash was melted, extruded, and then ground and melted again. That energy is spent and not recovered.
• Degradation. Each heat history reduces molecular weight, so regrind ratios are capped by property requirements, and above the cap the excess becomes genuine waste or a downgrade.
• Throughput. Machine time is consumed producing material that will not be sold, which on a capacity-constrained line is lost output rather than merely wasted resin.
• Handling. Grinders, conveying, and the labor of managing regrind streams are real overheads attributable to flash.

Regrind recovers the resin. It does not recover the energy, the machine time, or the molecular weight, and those are the parts you actually paid for.

Where flash actually comes from

Parison programming is the largest lever: a parison wall profile matched to the part geometry puts material where it is needed and reduces the excess pinched away. After that come pinch-off design and its condition, since worn edges leave heavier flash and eventually produce a seam quality problem as well, and mold layout, where the space between cavities on a multi-cavity tool is flash by definition. Because resin cost tracks a published index, currently at 310.75 index (1982=100) (Jun 2026), the value of a flash reduction rises and falls with the market, and a project that did not pencil at a low resin price may pencil now.

Use the blow molding resin cost calculator to convert shot weight and flash into cost per bottle. Price the resin

Published 2026-08-08.