Sheet moulding compound, step by step
SMC is made in a continuous line. A paste of resin, mineral filler and other compound ingredients is spread onto a carrier film, chopped glass roving is scattered onto it, a second pasted film is laid on top and the sandwich is compacted. The sheet is then rolled up and held while it matures — the paste thickens chemically until the sheet can be handled without being sticky.
In the press, pieces of the matured sheet are cut, stacked into a heated matched-metal tool and compressed. The compound flows to fill the tool and cures in minutes.
What the resin contributes
The resin is the binder of the compound and the matrix of the moulded part. It has to accept the filler loading, respond to the thickening system during maturation, flow under pressure in the tool and cure rapidly at tool temperature. Its chemistry also carries through to the moulded part’s behaviour in service.
The compound formulation — filler, thickener, initiator, release agent and glass content — belongs to the compounder. The resin is one ingredient chosen to suit it.
Where SPR 301 SMC fits
The 2026 catalogue offers SMC resins in all three polyester families: orthophthalic (SPR 201), isophthalic (SPR 301) and terephthalic (SPR 401). Its selection guide chooses between them by the chemistry the compound needs, not by treating one as a universal upgrade.
SPR 301 SMC is the isophthalic option, with typical viscosity 800–1,400 cPs at 25 °C, specific gravity 1.10–1.14 and styrene 31–36% by weight. It shares a combined catalogue entry — and therefore its typical specification — with SPR 301 DMC, but the two are separate products for different compound forms.
Reading the gel time correctly
The typical gel time of 12–18 minutes at 25 °C is measured at room temperature with the reference cure, MEKP 1.5% with cobalt octoate 0.4%. It is a quality-control figure that confirms batch-to-batch consistency. SMC in production cures in a hot tool with the initiator the compounder builds into the paste, on a very different timescale.