Calculations

How to Calculate Thermoforming Sheet Usage, Cycle Time, and Oven Dwell

The core thermoforming formulas worked end to end, from sheet blank sizing to oven dwell and vacuum pump capacity, using real gauges, cavity counts, and cycle inputs.

Start with sheet usage per part, because material sets 40 to 60 percent of cost and every other number depends on it. Take the part footprint plus draw-height allowance, add clamp-frame grip (typically 1.0 to 1.5 in per side) and web spacing between cavities (0.5 to 1.0 in). A tray 8 x 10 in on a 4-up tool at 2-in web needs a blank of about (2x8 + 2 + 2x0.5) x (2x10 + 2 + 0.5) = 19 x 22.5 in = 427.5 in². Divide by 4 cavities: 106.9 in² per part. The Sheet Usage Per Part calculator handles the frame and web geometry so you are not doing this by hand each quote.

Convert area to mass, since resin is priced by weight not area. Mass per part = area x gauge x density. For 0.060-in HIPS at 0.0378 lb/in³: 106.9 in² x 0.060 x 0.0378 = 0.242 lb, but that is the blank draw, not the part. Because thermoforming thins material as it draws, quote against blank mass, not finished-part mass. A 3:1 areal draw ratio thins nominal 0.060 in to roughly 0.020 in in the deepest corner. Track the draw ratio (surface area of part divided by footprint area) to predict whether the thinnest wall clears your 0.015-in minimum spec.

Trim scrap is the gap between blank mass and part mass, and it is larger than most operators assume. Skeleton web, clamp grip, and sprue-free edges commonly run 25 to 45 percent scrap on thin-gauge roll-fed lines and 30 to 55 percent on heavy-gauge cut-sheet. Compute it as (blank area minus net part area) divided by blank area. Using the tray above, if the net formed part is 78 in², scrap = (106.9 - 78) / 106.9 = 27 percent. The Trim Scrap Percentage calculator lets you test tighter web spacing and see the yield gain before you cut a new tool.

Forming cycle time is the sum of index, heat, form, cool, and trim stages, but heat and cool dominate. Heating time scales with gauge squared for conductive-limited sheet: doubling from 0.040 to 0.080 in roughly quadruples soak time. A practical heavy-gauge 0.187-in ABS cycle might be heat 55 s, form/vacuum 4 s, cool 22 s, index/trim 9 s = 90 s total. Roll-fed thin-gauge PP at 0.030 in runs 8 to 14 s station-to-station. Use the Forming Cycle Time calculator to sum stage times and see which stage caps your parts per hour.

Oven dwell time is the heat portion isolated, and it drives both throughput and part quality. Target the forming window: HIPS forms near 280 to 360 F, ABS near 300 to 380 F, PET near 190 to 250 F. Dwell = required heat input divided by radiant flux delivered. For thin-gauge you often set dwell empirically, then trim: if sheet sags past 1.5 in mid-frame it is over-soaked, if it whitens or forms with poor detail it is cold. The Oven Dwell Time calculator estimates dwell from gauge, material, and heater watt density so you start close instead of scrapping ten sheets.

Vacuum pump capacity determines how fast you evacuate the mold cavity, which sets form time and detail pickup. Required pumping speed depends on cavity volume and target evacuation time: to pull a 0.9 ft³ cavity from atmospheric to 26 in Hg in 1.5 s you need a surge tank sized 6 to 10x cavity volume plus a pump moving roughly 15 to 40 CFM at working vacuum. Undersized systems show as slow, soft detail and webbing. The Vacuum Pump Capacity calculator sizes both pump CFM and surge-tank volume from your cavity count and target form time.

Plug-assist speed matters on deep draws to pre-stretch material and even out wall thickness. Set plug travel so it contacts sheet at 60 to 80 percent of forming temperature, moving at 0.5 to 2.0 in/s depending on draw depth and material melt strength. Too fast chills contact zones and thins the walls next to the plug; too slow lets the sheet sag and thin at the base. The Plug-Assist Speed calculator converts stroke length and desired contact timing into an in/s setpoint you can dial into the servo or valve.

Close by pressure-checking your form. Forming pressure margin compares available differential (atmospheric vacuum gives about 12 to 14 psi, pressure forming adds 30 to 90 psi) against the pressure needed to reproduce your smallest radius and texture. A sharp 0.020-in radius or fine grain may need 40 to 60 psi, which vacuum alone cannot deliver. The Forming Pressure Margin calculator flags when a feature exceeds vacuum capability so you specify a pressure box before tooling is cut. Nesting Yield ties it together by confirming how many blanks fit a master sheet or roll width.

Published 2026-07-01.