Spray Lay-Up

Spray lay-up deposits chopped fiber and resin into an open mold, ideal for low-cost, medium-volume composite shells with moderate structural demands and loose tolerances.

Overview

Spray lay-up (chop gun layup) is an open-mold composite process where chopped glass or other fibers and resin are sprayed together into a mold, then rolled to compact and de-air. It excels at producing large, relatively simple fiberglass parts with low tooling cost and fast cycle times. Surface finish comes from the gelcoat in the mold, with thickness and properties controlled mainly by operator technique.

Use spray lay-up for medium-performance parts where cost and size matter more than precise tolerances or highly repeatable mechanical properties. Expect variable fiber orientation, moderate void content, and lower fiber volume fraction than closed-mold or prepreg processes. It’s a strong fit for boat hulls, tubs, tanks, and fairings, but a poor fit for structurally critical components, thin walls, sharp details, or features requiring tight control. Design with generous radii, consistent wall thickness, and only a few critical machined features and you can keep both part cost and risk under control.

Common Materials

  • E-glass fiber with polyester resin
  • E-glass fiber with vinyl ester resin
  • E-glass fiber with epoxy resin
  • Gelcoat polyester
  • Core materials (PVC foam)

Tolerances

±0.030" to ±0.060" on as-molded features; ±0.010" to ±0.020" on trimmed edges

Applications

  • Boat hulls and decks
  • Bath and shower enclosures
  • Truck and trailer body panels
  • Industrial tanks and covers
  • Agricultural equipment shrouds
  • Recreational vehicle exterior panels

When to Choose Spray Lay-Up

Choose spray lay-up for large to medium composite shells where you need low tooling cost, moderate production volumes, and moderate structural performance. It suits parts with simple geometry, cosmetic outer surfaces from gelcoat, and where dimensional accuracy is secondary to size and cost. Avoid it for safety-critical structures or parts requiring tight, repeatable tolerances.

vs Resin Transfer Molding

Pick spray lay-up over Resin Transfer Molding when you want the lowest tooling cost for large parts and can live with higher variability in thickness and properties. It’s better suited when you expect modest volumes, frequent design changes, and mainly cosmetic or low-criticality structural requirements rather than tight process control.

vs Vacuum-Assisted Resin Transfer (VARTM)

Choose spray lay-up instead of VARTM when budget or schedule cannot support vacuum systems, sealed bagging, and more complex tooling. It’s appropriate when part performance is moderate, porosity control is less critical, and you can accept lower fiber volume and higher operator dependency to keep part price down.

vs Prepreg Layup with Autoclave

Use spray lay-up over autoclave prepreg when part count is low to medium, cost pressure is high, and aerospace-grade properties are unnecessary. Spray lay-up avoids expensive materials, freezers, and autoclave time, and works well for non-structural or semi-structural shells where weight and performance targets are relaxed.

vs Prepreg Out-of-Autoclave (OOA)

Select spray lay-up instead of OOA prepreg when you prioritize very low material and tooling cost and can accept less consistent fiber volume and properties. It’s a fit for large cosmetic or protective covers where curing under vacuum-only prepreg would be overkill and handling uncured prepreg would slow production.

vs Hand Lay-Up

Choose spray lay-up over hand lay-up when you need to cut labor time on medium to large parts and can invest in a chop gun system. Spray lay-up can build thickness faster and more consistently on simple surfaces, while still allowing manual rolling for de-airing and compaction.

Design Considerations

  • Keep wall thickness relatively uniform and avoid very thin sections; specify minimum and target laminate thicknesses
  • Use large radii and smooth transitions to allow consistent spray coverage and proper rolling without fiber bridging
  • Design critical interfaces (mounting pads, holes, sealing surfaces) to be machined after cure rather than relying on as-molded accuracy
  • Provide clear trim lines and adequate flange widths so shops can fixture and trim parts accurately and quickly
  • Limit highly loaded features; if unavoidable, define local reinforcement plies or inserts and show them clearly on the drawing
  • Define acceptable cosmetic criteria (print-through, porosity, surface waviness) and gelcoat requirements so quotes reflect the true finish level