Contoured Mold Layup: Hull Section Build

Contoured Mold Layup: Hull Section Build

Building a laminate over a contoured hull-section mold is where classroom knowledge becomes a real part. Every decision you make — how carefully you release the mold, how thoroughly you wet out each ply, how you handle the bilge radius — shows up permanently in the finished laminate. This lab walks you through the full sequence from bare mold to demolded, audited part.

Mold Preparation and Release

A female fiberglass mold is an expensive, precision tool. Damaging it wastes shop time and money. Your first job is applying a reliable release system.

Paste wax is applied in thin, even coats. Buff each coat to a haze-free shine before applying the next. Three coats are required on this mold; each coat adds insurance. Work wax into all radii and corners — anywhere resin could lock the part in place. Do not skip inside corners. After waxing, apply PVA (polyvinyl alcohol) release film in smooth, overlapping strokes with a foam applicator or soft brush. PVA dries to a water-soluble barrier that prevents any residual resin contact with the mold surface. Let it dry fully — a milky wet PVA will trap solvents and cause surface defects. Inspect the mold under a raking light before instructor sign-off. Any holidays (missed spots), runs, or tackiness must be corrected.

Safety: Wear nitrile gloves and safety glasses during mold prep. PVA is low-hazard but keep it off your eyes.

Understanding Your Laminate Schedule

The printed laminate schedule is your legal document for this part. It specifies:

• Gel coat — pigmented polyester, sprayed or brushed as the show surface
• Skin coat CSM (1.5 oz) — chopped strand mat bonds to gel coat and begins building bulk
• Two plies WR (24 oz woven roving) — the structural plies; provide tensile and flexural strength
• Finishing CSM (1.5 oz) — closes the laminate, improves surface for secondary bonding or painting

Deviations from the schedule must be logged immediately. If a ply is accidentally skipped or a section is left dry, document it — hiding a defect is a safety and liability issue.

Laminating Over Contours and the Bilge Radius

Flat panels wet out easily. Contoured molds require deliberate technique.

Cutting and darts: Before laminating, dry-fit each CSM and WR ply. At tight curves, cut relief darts — short radial cuts into the fabric edge — so the ply lies flat without bridging. Bridging (fabric spanning across a concave curve instead of conforming to it) traps air and creates voids that weaken the laminate and fail tap testing.

Wet-out sequence: Brush catalyzed resin onto the mold surface first, then lay the fabric and stipple resin through from above. Work from the center outward toward edges and into the bilge radius. Use a ribbed consolidation roller firmly in the radius — push toward the corner, not away from it. Air bubbles must be worked out before each ply gels.

MEKP catalyst: GP polyester resin is catalyzed with methyl ethyl ketone peroxide (MEKP). Follow the manufacturer's recommended ratio precisely — typically measured by weight or calibrated drops. Over-catalyzing accelerates cure and may cause exothermic cracking; under-catalyzing leaves a soft, tacky laminate. Record the shop temperature at each ply: warmer temperatures speed cure and require less catalyst or faster work pace.

Safety — MEKP is a serious hazard. It is a strong oxidizer and can cause severe eye injury including blindness on contact. Wear chemical-splash goggles (not just safety glasses), nitrile gloves, and a chemical-resistant apron when handling MEKP. Keep a dedicated eyewash station within ten seconds of your work area. Store MEKP away from acetone and other flammables — contact between them can cause fire. Review the SDS before the lab begins.

Resin vapors: GP polyester resin releases styrene vapor. Work only in areas with mechanical ventilation. Wear a respirator with organic vapor cartridges — a dust mask does not protect you from styrene.

Instructor Checkpoints

Between each ply, pause for an instructor checkpoint. The instructor will check wet-out (no dry, white fiber visible), consolidation (no lifting or bridging), and radius quality. Do not proceed until cleared. If a defect is found, correct it before the ply gels.

Demolding and Quality Audit

After full cure (follow the posted cure schedule — do not rush), demold the part using wooden wedges only. Metal tools will gouge the mold. Tap wedges gently at the flange and work around the perimeter evenly. If the part resists, do not force it — check for any locked area, add a wedge location, and try again.

Trim flash (excess laminate at the flange edge) with the trim saw. Wear a dust mask or half-face respirator with P100 particulate filter, safety glasses, and hearing protection during trimming — cured fiberglass dust is a respiratory hazard.

Thickness measurement: Use the digital thickness gauge at all twelve marked locations. Record each reading on your quality audit form. Compare to the target thickness from the laminate schedule.

Tap test: Tap the entire surface methodically with a coin or small hammer. A clear, sharp ring indicates a sound bond. A dull thud indicates a void or delamination. Mark and measure any dull areas. Unbonded areas larger than 0.5 inch are a reject criterion.

Quality audit form: For every defect found — void, dry spot, print-through, thickness deviation — document the location, likely cause, and corrective action. This is professional practice and required for your grade on this lab.