Inexpensive flat moulds for wet-layup carbon fin blades

Context

The intended project was to make carbon-fibre freediving fin blades by wet layup. Because the blade mould was essentially flat, the question was which inexpensive sheet materials could provide a suitably flat and smooth working surface. Acrylic or plexiglass was then considered specifically.

No mould material, layup configuration, or successful result was selected in the conversation.

Advice received

Glass

Glass was described as very flat and smooth, capable of producing a clean surface finish and easy to prepare with a suitable release system. Its disadvantages were brittleness, risk of breakage during handling or clamping, and hazardous edges if damaged.

Melamine-faced board

Melamine-coated particle board was presented as a cheap and accessible option for one-off or short-run work. It is reasonably smooth and rigid over a short span, but exposed edges and holes need sealing against resin and moisture. The advice was to use release wax and optionally PVA.

Glass with PET or Mylar film

Putting replaceable PET or Mylar film over flat glass was suggested as a way to protect the mould and obtain a glossy working surface. The film needs to be secured without wrinkles, trapped air, or contamination.

Metal plate

Steel or aluminium plate was described as durable, reusable, and compatible with clamping or heat when appropriately selected. It may be heavy or expensive, and its surface needs cleaning, preparation, and a reliable release agent.

Sealed MDF

MDF was described as cheap, flat, and easy to work, but highly absorbent and prone to swelling or warping. It would require thorough sealing, smoothing, and release preparation, making it most appropriate for very low-cost, short-term tooling.

Acrylic or plexiglass

Acrylic was considered workable for low-temperature, small-run layups. Its smoothness and transparency could help with surface finish and layer alignment, and it is easy to cut or drill. The cautions were that it scratches readily, can bow without continuous support, may crack around stressed edges, and is unsuitable for temperatures beyond the selected sheet's service specification. A replaceable PET or Mylar film was suggested to protect its surface.

The practical recommendation received was to use float glass or melamine-faced board for value, glass with PET or Mylar for an easily renewed glossy surface, or acrylic if a suitable sheet was already available and could be fully supported.

Editorial notes

• The response said aluminium could “react with epoxy.” The more relevant practical concerns are epoxy adhesion, the metal's oxide and surface condition, and preparation of a reliable release layer.
• PET or Mylar should not be assumed to release cleanly with every resin and process combination. A test coupon would be needed.
• Vacuum bagging is not inherently unsuitable for acrylic. Suitability depends on sheet thickness and span, continuous backing support, temperature, sealing, and the applied pressure differential.
• The response's approximate acrylic temperature threshold should not replace the sheet manufacturer's service-temperature specification.

Unresolved process choices

• Whether the layup would be vacuum bagged, pressed, or cured under another loading arrangement.
• Whether heat or a post-cure would be used.
• Required blade dimensions, mould thickness, and backing support.
• Resin system and compatible release method.
• Expected number of parts and acceptable mould life.