The SMC process and the resin’s part in it
SMC production runs in two stages, and the resin matters in both. On the sheet line, a paste of resin, filler, thickener, initiator, release agent and pigment is metered onto carrier films, chopped glass is added, and the sandwich is compacted and rolled. During maturation the paste thickens until the sheet is dry to handle.
At the press, cut charges of sheet are placed in a heated matched-metal tool and compressed. The compound must flow to fill the cavity and then cure quickly at tool temperature. The resin has to wet the filler and glass, respond to the thickener, flow under pressure and cure fast when heated.
Questions that decide the family
Compound and part questions
- What is the moulded part, and where is it used?
- Does it face heat, moisture, chemicals or weathering in service?
- What surface finish is required — painted, pigmented or Class A appearance?
- Is there an electrical, flammability or customer material specification?
- What filler system and glass content is the compound designed around?
The three SMC resin families
The 2026 catalogue offers SMC resins in all three polyester families. The orthophthalic resin is supplied as the DMC/SMC Resin (SPR 201 SMC · DMC), with a published TDS. SPR 301 SMC is the isophthalic option. SPR 401 SMC is the terephthalic option, which the catalogue describes as having high heat-resistant capacity, good resistance to corrosive chemicals and excellent mechanical and impact properties.
The catalogue chooses between them by the chemistry the compound requires rather than ranking them. A dry interior part, a part facing water and a part facing heat may each point to a different family.
Published typical values
| Property | DMC/SMC Resin (SPR 201 SMC · DMC) | Isophthalic SMC Resin (SPR 301 SMC) | Terephthalic SMC Resin (SPR 401 SMC) |
|---|---|---|---|
| Chemistry | Orthophthalic unsaturated polyester resin | Isophthalic unsaturated polyester | Terephthalic unsaturated polyester |
| Viscosity (typical) | 600 cPs (at 25°C) | 800–1,400 cPs at 25 °C | 800–1,500 cPs at 25 °C |
| Gel time / pot life (typical) | 12–18 minutes (at 25°C; review current TDS for test context) | 12–18 minutes at 25 °C | 12–18 minutes at 25 °C (ambient QC) |
| Specific gravity / density (typical) | 1.12 g/cm³ | 1.10–1.14 | 1.11–1.15 |
| Styrene content (typical) | 35% | 31–36% by weight | 32–38% by weight |
| Source of values | DMC/SMC Resin TDS | 2026 catalogue; grade TDS | 2026 catalogue; grade TDS |
Typical values from each product’s own published data. Gel times are ambient 25 °C quality-control figures; SMC cures in a heated tool with the compound’s own initiator.
What the compounder controls
Filler type and loading, thickener and maturation, initiator selection, release, low-profile additives and glass content are the compounder’s formulation. Moulded-part properties — mechanical, electrical, heat and flammability — come from that formulation and are proven on moulded specimens. The resin supplier’s role is to provide a consistent base resin and help match the family to the requirement.
Trials
Run the candidate resin through the full sequence: paste mixing at production filler loading, sheet making, maturation to the handling point, and moulding on the production tool. Check paste viscosity development, sheet handling, flow and fill, cure cycle and moulded surface.