Moulds and Parts for UAV and Homebuilt Work

Making a mould, in order

1. The pattern. It is the shape of the part, so every flaw on it is copied into the mould. It must be fully cured, sealed and prepared with a release agent before anything goes on it (Easy Composites UG1 data sheet).

2. Check the pattern surface. Fresh paint, filler primer and polyester body filler can leach solvent and spoil the release; styrene in a polyester or vinylester gelcoat can also soften a paint finish. The maker's advice is a small test patch before the real mould, and oven-cured industrial paint is the safe case (UG1 data sheet).

3. Gelcoat. The face of the mould. In the UG1 system: two coats of about 0.020 in (0.5 mm) each, each left to cure 4 to 8 hours at 20 °C before the next. Mix by weight at the catalyst percentage on the sheet (2 to 3 % MEKP for UG1) and mix only what the pot life lets you use (UG1 data sheet).

4. Backing. Glass or carbon laminate in tooling resin builds the mould's body. Tooling resins are used to impregnate fibre reinforcement to make permanent tools (FAA-H-8083-31B p. 7-20). Add flanges, for the bag tape and for trimming the part.

5. Cure the mould properly. A mould cured at room temperature is handleable early but takes at least 7 days to reach full properties in the UG1 system, and the maker recommends a stepped post cure, 24 hours after room-temperature cure, then a slow cool-down in the oven to avoid distortion. Support the mould during it (UG1 data sheet).

6. Release the pattern. Demould carefully, then release, polish and heat-cycle the mould before the first part. Hexcel advises a simple, stable tool with uniform pressure on the part, and heat cycling any new tool to relieve stress (Prepreg Technology FGU 017c, p. 13).

Laying up on the mould

Release first. Release agents let the part come off the tool or caul plate (Handbook p. 7-21). Apply with a brush, pad or cloth, and keep release off the prepreg; Hexcel's order of work is release, then layup, then bag (FGU 017c p. 18). Check the agent suits your resin: the maker's sheet says.

Then the plies on the mould face, built as the repair would be: the wet layup guide and repair plies guide give the ply order and orientation, and the laminate plies calculator the count for a thickness.

Then the bag. Peel ply, perforated film, bleeder, breather and bag are the same stack as on a repair: see the vacuum bagging guide. The cure guide covers temperature and time.

Infusion. In vacuum infusion the dry reinforcement is bagged first and the resin is drawn through it by vacuum. The Handbook describes the small-scale version, impregnating fabric inside a bag with sealing tape at least 4 in beyond the material, an edge breather 1 to 2 in wide, solid film above and below, and says it reduces trapped air and gives a better resin-to-fibre ratio than squeegeeing (pp. 7-30, 7-31). The flow media, resin lines and gate layout for a full part are not in the FAA text we use: follow the resin maker's infusion guide.

For UAV and homebuilt work

A mould earns its cost when you make more than one part, or when the finish matters. If you take a mould from an existing part, treat the part as a pattern: seal it, release it and test the paint first.

Two cautions. A styrene gelcoat and its catalyst need the precautions in the safety guide. And a laminate cured at room temperature on a low-rated tool does not match a 250 °F or 350 °F cured part; for a primary structure the repair or build data sets the process.