Common Mistakes

Composites Estimating and Process Mistakes That Wreck Your Numbers

The specific errors that make composite estimates and layups go wrong, each with a symptom, root cause, and a numeric fix you can check today.

Symptom: your laminate comes out thicker than the drawing and parts weigh 12 to 18 percent over target. Root cause is almost always a fiber volume fraction assumption that does not match the process. Hand layup rarely beats 45 to 50 percent Vf, vacuum bagging lands near 50 to 55 percent, and only autoclave or infusion reaches 55 to 60 percent. If you size ply count in the Laminate Thickness calculator using 60 percent Vf but shop-floor wet layup delivers 48 percent, each ply cures roughly 0.20 mm instead of 0.16 mm, so an 8-ply skin overshoots by about 0.3 mm. Match Vf to the actual process before you trust any thickness or areal weight number.

Symptom: you run out of resin mid-part or throw away half a mixed batch. Root cause is quoting resin at the ideal resin-to-fiber ratio while the floor works wet. A 300 gsm cloth at 50:50 needs 300 g/m2 of resin, but hand layup routinely runs 60:40 or worse, pushing consumption to 450 g/m2, a 50 percent miss. Add roughly 10 to 15 percent for pot-life waste and roller loss. Size the Fiberglass Resin Usage calculator at your measured ratio, not the datasheet ideal, and log actual mixed versus applied mass for three parts to calibrate.

Symptom: infusion stalls and a dry spot forms two thirds of the way across the part. Root cause is flow length scaled wrong, since infusion time grows with the square of distance, not linearly. Double the flow path and fill time roughly quadruples, so a 45-minute fill over 1 m becomes about 3 hours over 2 m. Feed resin that gels in 90 minutes and you race the clock. Check the Resin Infusion Time estimate against resin pot life with a 30 percent margin, and add a flow channel or second inlet before you push a longer path.

Symptom: prepreg parts pass but material spend is 25 to 40 percent over the theoretical part weight. Root cause is nesting and shelf-life scrap that never entered the estimate. Prepreg nesting yields 60 to 75 percent on curved plies, so 30 percent of roll goes to the floor, and any roll past its out-time or 6 to 12 month freezer life is a total write-off. Track it explicitly in the Prepreg Scrap Cost tool. A shop buying 500 kg per month at 80 dollars per kg that ignores 30 percent scrap under-quotes by 12,000 dollars monthly.

Symptom: an autoclave or oven run costs far more per part than budgeted. Root cause is loading the vessel at 40 percent of capacity and charging one part for a full cycle. A 5-hour cure amortized over 4 parts is 1.25 hours of vessel time each, but the same cycle over 20 parts is 15 minutes each, an 80 percent swing in machine cost. Use Cure Cycle Capacity to confirm how many parts share a run, and never quote autoclave time per part from a single-part trial. Batch small parts and stage tooling to hit 70 percent load or better.

Symptom: laminates cure with porosity above 2 percent or partial cure that fails coupon tests. Root cause is a ramp rate or dwell that ignores exotherm and vessel lag. Thick sections above 10 mm can exotherm 30 to 50 C past setpoint if ramped at 3 C per minute, while thin skins undercure if the vessel lags the program. Verify the actual part-temperature ramp with an embedded thermocouple, hold ramps near 1 to 2 C per minute for thick stacks, and confirm dwell time meets the resin degree-of-cure spec, not just the oven clock.

Symptom: your labor hours per part drift 30 percent above the estimate. Root cause is treating layup time as flat per ply while ignoring geometry and debulk. A flat panel ply may take 4 minutes, but a tight-radius or draped ply runs 10 to 15 minutes, and every debulk cycle adds 15 to 20 minutes. Build the Composite Layup Labor estimate from ply category counts, not an average, and add debulk frequency explicitly. On a 20-ply part, missing three debulks alone hides an hour of labor.

Symptom: two quotes for similar parts differ by 40 percent and you cannot explain why. Root cause is inconsistent consumable and bagging accounting across estimates. Vacuum bag film, breather, peel ply, tacky tape, and tubing run 8 to 20 dollars per square meter of bag and get forgotten on complex shapes where bag area is 1.5 to 2 times part area. Standardize with the Vacuum Bagging Cost and Composite Part Cost calculators so every quote carries the same consumable basis, then reconcile estimate versus actual on every job to catch the drift early.

Published 2026-07-01.