Why a mould is a different job
An FRP production mould is laminated once and then used again and again. Every part made in it cures against its surface and gives off heat, so the mould goes through a heat cycle with each pull. Over many cycles, a mould laminate that is not up to the job loses dimension, distorts, and shows fibre pattern or cracks through its surface.
A tooling resin is chosen for that repeated service. The goal is a mould that keeps its shape and surface for as many parts as the production plan needs.
SPR 401 TL in the catalogue
The 2026 catalogue lists SPR 401 TL for composite mould production and tooling and die preparation. It describes the grade as isophthalic, NPG (neopentyl glycol)-based and highly reactive. NPG is a glycol widely used in higher-performance polyesters and gelcoats.
Its typical values are moderately thixotropic — 400–650 cPs at 60 rpm and 1,500–2,500 cPs at 6 rpm, index 2.5–3.8 — with a gel time of 15–30 minutes at 25 °C, specific gravity 1.12–1.16 and styrene 35–42% by weight. The reference cure is MEKP 1.5% with cobalt octoate 0.4%, each added separately.
The SPR 401 code does not mean terephthalic
SPR 401 is the catalogue’s terephthalic series, but SPR 401 TL is an exception. The catalogue retains it within the SPR 401 code range while stating that it is an isophthalic NPG-based resin, not a terephthalic one. That is why the site lists it with the isophthalic polyester resins.
The resin is one part of a good mould
Mould building is commonly done in stages: a tooling gelcoat on the pattern, a fine skin laminate behind it, then the bulk laminate and any stiffening frame. Building in controlled stages limits the heat each stage generates and the distortion that heat can cause. Post-cure and conditioning before first use are also part of mould practice.
The pattern quality, the tooling gelcoat and the build schedule shape the finished mould as much as the resin does. The mould size, the part it will make and the expected number of pulls are the facts that decide the approach.