What is different about a mould
The parts made in a mould are laminated once. The mould is laminated once and then heated by every part cured in it. That repeated service — plus the need to hold shape and surface for the whole production run — is why tooling resins exist.
The options
GP Clear Resin and ISO Polyester Resin are general laminating grades. SPR 401 TL is the catalogue’s tooling grade: an isophthalic, NPG-based, highly reactive resin, moderately thixotropic, listed for composite mould production and tooling and die preparation. It carries an SPR 401 code but is not terephthalic.
Published typical values
ISO Polyester Resin publishes cured-resin data; the tooling grade and GP Clear publish liquid-resin values only. No heat-distortion or shrinkage value is published for SPR 401 TL.
| Property | GP Clear Resin (SPR 201 GPC) | ISO Polyester Resin (SPR 301) | Tooling Grade Resin (SPR 401 TL) |
|---|---|---|---|
| Chemistry | Orthophthalic, Unsaturated Polyester | Isophthalic Unsaturated Polyester Resin | Isophthalic NPG (neopentyl glycol)-based unsaturated polyester |
| Viscosity (typical) | 350–450 cPs | 500 cPs (mPa·s) at 25 °C | 400–650 cPs (60 rpm); 1,500–2,500 cPs (6 rpm) |
| Thixotropy | Not published | Not published | 2.5–3.8 |
| Gel time / pot life (typical) | < 8 minutes | 15 minutes at 25 °C | 15–30 minutes at 25 °C |
| Specific gravity / density (typical) | Not part of the published specification for this grade | 1.12 g/cm³ at 25 °C | 1.12–1.16 |
| Source of values | GP Clear Resin TDS | ISO Polyester Resin TDS | 2026 catalogue; grade TDS |
Typical values, reproduced from each product’s own published data. They describe the liquid resin at its reference condition, not a finished part, and are not guaranteed limits.
How to decide
Count the pulls the mould must deliver, note the tolerance it must hold and the heat it will see from each part’s cure. A long-life production mould points to the tooling grade; a prototype or short-run mould may be well served by a general laminating resin. In both cases the mould face is a gelcoat, and the build schedule matters as much as the resin.