Spray Lay-Up

Spray lay-up builds fiberglass composite parts by spraying chopped roving and catalyzed resin onto open molds for fast, low-cost large shapes with moderate control.

Overview

Spray lay-up (chop gun layup) deposits chopped glass fiber and catalyzed resin simultaneously onto an open mold, then the laminate is rolled/compacted and cured at room temperature. Tooling is typically low-cost FRP or composite molds, making it attractive for large parts and short-run production.

Choose it for big, non-structural to semi-structural components where surface quality on the mold side matters and you can tolerate variation in fiber content, thickness, and voids. It’s common for cosmetic shells, covers, and tanks where weight and stiffness targets are modest.

Tradeoffs: properties are generally lower and less repeatable than closed-mold or prepreg processes due to higher void content, less-controlled fiber length/orientation, and resin-rich zones. Expect more finishing (trim, bond, paint/gelcoat), more odor/VOC controls, and limited ability to hold tight tolerances or integrate high-performance cores and inserts without added steps.

Common Materials

  • E-glass chopped roving
  • Vinyl ester
  • Polyester resin
  • Epoxy resin
  • Gelcoat
  • PVC foam core

Tolerances

±0.060 in (±1.5 mm) typical; tighter requires secondary machining/fixturing

Applications

  • Shower and tub surrounds
  • Boat hull and deck shells
  • Truck fairings and body panels
  • Equipment covers and enclosures
  • Water/chemical storage tanks
  • Architectural panels

When to Choose Spray Lay-Up

Fit is best for large, simple-to-moderate geometry parts in low to medium volumes where tooling budget and lead time matter more than tight tolerances or maximum mechanical performance. Works well when one “show” surface comes off the mold and the back side can be unfinished. Plan on trimming and possible post-finishing to hit assembly interfaces.

vs Resin Transfer Molding

Choose spray lay-up when you need low-cost tooling, very large parts, or quick iteration and can accept more variation in thickness, fiber volume, and cosmetic consistency. It’s also practical when part geometry would make RTM tooling and sealing complex or expensive.

vs Vacuum-Assisted Resin Transfer (VARTM)

Choose spray lay-up when vacuum consumables, leak management, and infusion setup time don’t pencil out for the volume or part value. It’s a good fit when laminate quality requirements are moderate and you prefer faster laydown over tighter control of voids and fiber content.

vs Prepreg Layup with Autoclave

Choose spray lay-up when cost, facility requirements, and throughput outweigh high-performance laminate needs. It avoids freezer storage, debulk cycles, and autoclave capacity constraints, but you give up strength-to-weight, repeatability, and aerospace-grade quality control.

vs Hand Lay-Up

Choose spray lay-up when you need faster deposition over large areas and consistent chopped-glass distribution without manually cutting and placing mat. Hand lay-up can be better for deliberate fiber placement, localized reinforcements, and lower overspray/cleanup.

vs Compression Molding (Composites)

Choose spray lay-up when part size is too large for presses, volumes are lower, or you can’t justify matched-metal tooling. Compression molding wins on repeatability, cycle time, and dimensional control when the tooling investment is acceptable.

Design Considerations

  • Keep geometry shallow-draft and avoid deep, narrow draws that are hard to wet-out and roll without trapping air
  • Define cosmetic requirements separately for mold-side and back-side surfaces; expect print-through and texture variation on the back side
  • Call out nominal laminate thickness and allowable range by zone; add extra thickness where you need stiffness instead of relying on tight tolerance
  • Design trim lines and flanges wide enough for trimming variability and fixturing; don’t put critical datums on free edges
  • Plan hardpoints: use bonded inserts, local reinforcement patches, or potted cores; don’t rely on threads in chopped-glass laminate
  • Specify resin system and gelcoat compatibility (chemical exposure, UV, fire/smoke) early to avoid rework and requalification