Calculations

How to Calculate Parison Weight, Cycle Time, and Cavitation Output in Blow Molding

The core blow molding formulas with worked examples: parison and shot weight, cycle time, cavitation output, cooling time, and clamp force.

Start with part weight, because everything downstream depends on it. For an extrusion blow molded HDPE bottle, the Parison Weight equals finished part weight divided by material yield. A 32 g bottle with 12 percent flash needs a parison of 32 / 0.88 = 36.4 g. In injection blow molding, use Shot Weight instead: preform weight times number of cavities, plus 3 to 5 percent for runner and sprue. Eight cavities at 22 g each give a shot of 8 x 22 x 1.04 = 183 g. Both feed directly into resin cost and machine sizing, so get them right first.

Cycle time governs output more than any other number. Bottle Cycle Time sums extrusion (or injection) time, mold close, blow, cooling, and eject. Cooling dominates: for HDPE at 1.2 mm average wall, Cooling Time runs roughly proportional to wall thickness squared, near 8 to 14 seconds. A typical single-station EBM cycle is 2 s parison drop, 1 s clamp, 0.5 s blow, 10 s cool, 1.5 s eject and transfer, totaling 15 seconds. That single number sets your hourly rate before cavitation, so measure it at the machine with a stopwatch across 20 cycles, not from the nameplate.

Convert cycle time to units with Cavitation Output. Parts per hour equals 3600 divided by cycle time, times cavities, times an uptime factor. A 15 second cycle in a 4-cavity mold at 92 percent uptime yields 3600 / 15 x 4 x 0.92 = 883 bottles per hour. Over a 7.5 hour production shift after breaks, that is about 6,620 good parts. Always separate nameplate cavitation from effective cavitation: a blocked cavity on an 8-cavity tool drops you to 7 active, an instant 12.5 percent output loss that no cycle tweak recovers.

Clamp force keeps the mold shut against blow pressure. Clamp Force Estimate equals projected part area times blow pressure times a safety factor of 1.1 to 1.3. A 90 mm diameter round bottle has a projected area near the parting line of about 64 cm2 per cavity. At 8 bar (0.8 kN/cm2 is wrong here, use 0.08 kN/cm2), force per cavity is 64 x 0.08 x 1.2 = 6.1 kN. Four cavities need about 24.6 kN, well within a small EBM machine. Undersized clamp opens the mold and flashes the part, so never run below the calculated minimum.

Blow pressure must sit inside a defined window. The Blow Pressure Window for HDPE and PP typically spans 6 to 10 bar; PET stretch blow runs far higher, 25 to 40 bar preblow and pressure. Too low and the parison fails to reach mold detail and thin spots form; too high and you blow through thin walls or lock in stress. Set target pressure from wall thickness and resin melt strength, then verify with a wall thickness map: a 500 ml HDPE bottle should hold 0.4 to 0.6 mm in the panel and 1.0 mm at the neck.

Flash and scrap correct your material math. Flash Scrap Rate equals flash and reject weight divided by total extruded weight. If each 36.4 g parison yields a 32 g bottle, flash is 4.4 g, a 12.1 percent flash fraction before any rejects. Reground and reused in-house, that flash is not lost material, but it does inflate melt throughput: your extruder must push 36.4 g per part, not 32 g, so extruder output in kg per hour must cover the full parison mass times parts per hour, here 36.4 g x 883 = 32.1 kg per hour.

Quality throughput ties it together with Leak Test Capacity. If leak testing takes 2.5 seconds per bottle on a single-head tester, capacity is 3600 / 2.5 = 1,440 bottles per hour, comfortably above the 883 per hour molding rate, so one tester keeps up. If your cycle drops to 8 seconds and cavitation rises to 1,650 per hour, one head becomes the bottleneck and you need a 2-head tester at 2,880 per hour. Size test capacity against effective cavitation output, not nameplate, or inspection quietly caps your line.

Chain the calculators in order: Parison Weight or Shot Weight, then Cooling Time, then Bottle Cycle Time, then Cavitation Output, then Clamp Force Estimate and Blow Pressure Window for setup, and finally Flash Scrap Rate and Leak Test Capacity to reconcile material and inspection. A worked example: 36.4 g parison, 10 s cool, 15 s cycle, 4 cavities, 883 per hour, 24.6 kN clamp, 8 bar blow, 12.1 percent flash, 1,440 per hour test. Each output is an input to the next, so a stopwatch error on cycle time cascades into every rate downstream.

Published 2026-07-01.