How the Sheet Moulding Compound (SMC) Process Works, Step by Step

The sheet moulding compound (SMC) process has two halves. First, the compound is made: a resin paste and chopped glass fibre are combined between two films, compacted into a sheet and left to mature until it thickens. Second, the part is moulded: the sheet is cut into a charge, placed in a heated steel mould and pressed, so it flows, fills the cavity and cures into a strong, rigid part in a few minutes or less.

SMC is behind many of the electrical enclosures, meter boxes, manhole covers and automotive panels around you. At Prime Tools we build the compression moulds for SMC parts, and we trial every one before it leaves our tool room. This guide walks through each step and what it means for part quality.

Part 1: Making the sheet moulding compound

1. Mixing the paste

The paste is a blend of unsaturated polyester (or vinyl ester) resin, mineral filler (often calcium carbonate, or ATH for flame retardant grades), low-profile additive for shrinkage control, catalyst, internal release agent, pigment and a thickener (often magnesium oxide). The recipe sets the grade’s strength, flame rating, colour, flow and surface finish.

2. Spreading the paste on film

On the SMC machine, the paste is spread evenly onto a moving carrier film using a doctor blade. A second film is coated the same way.

3. Adding the glass fibre

Continuous glass rovings are chopped into short lengths, typically around 25 mm, and dropped evenly onto the paste layer. The fibre content and its even spread are critical to part strength.

4. Compaction

The second paste-coated film is laid on top, making a sandwich. Rollers knead and compact the sandwich so the fibres are fully wetted out by the resin and air is squeezed out.

5. Maturation

The sheet is rolled up or festooned into boxes and left to mature, often for a few days in controlled conditions. The thickener raises the viscosity, turning the sticky paste into a leather-like sheet that can be cut and handled, while still flowing under heat and pressure in the mould.

Part 2: Moulding the SMC part

6. Cutting and stacking the charge

At the press, the films are peeled off and the sheet is cut into pieces. These are stacked into a charge. The charge typically covers only part of the mould area (often roughly 40 to 70 percent, depending on the part), so the material flows outwards and pushes air ahead of it.

7. Placing the charge

The charge is placed in the lower half of the heated mould, at the position agreed during mould design. Poor charge placement causes knit lines, non-fill and uneven fibre distribution.

8. Pressing and flow

The press closes, quickly at first and then slowly as the upper mould touches the charge. The material flows, fills the cavity and reaches the shear edge, where the tight clearance between the two halves stops it and allows only a thin flash.

9. Curing

The mould, usually around 140 to 160 °C for polyester grades (check the supplier datasheet), cures the resin while the press holds pressure. A thin-walled part can cure in around a minute; thick parts take longer.

10. Ejection, deflashing and finishing

The press opens, ejectors push the part out, and it is placed on a cooling fixture if it is large and flat. Flash is removed, holes may be drilled, inserts fitted and gaskets added. Some parts are painted, though many SMC parts are moulded in colour.

What the mould must get right

The SMC process only works as well as the mould. Here is what we focus on when we design and build SMC tools:

• Shear edge: the clearance and hardness of the shear edge around the whole part perimeter decide flash thickness. On large enclosures, this edge runs for metres.
• Even heating: cartridge heaters placed so the whole moulding face stays at an even temperature. Uneven heat means uneven cure and warpage.
• Stiffness: SMC needs high pressure over large areas, so plates and supports must not deflect.
• Venting: air and curing gases must escape, especially at the end of flow.
• Ejection: large thin parts need well-spread ejection so they come out flat.
• Surface finish: polish, and chrome plating for long runs or high-gloss faces.

Our SMC moulds page shows more of our work, and the DFM and mould design page shows how we study charge placement and flow before cutting steel.

Typical SMC process problems and fixes

SMC process vs other composite processes

SMC is a closed-mould, high-pressure process. Compared with hand lay-up or spray-up (open mould, used for boats and tanks), SMC gives two finished faces, much more consistent parts and far faster cycles, but needs a steel mould and a press. Compared with resin transfer moulding (RTM), SMC is faster for medium and high volumes. That is why SMC dominates electrical enclosures and utility products.

If you are deciding between compounds, read SMC vs BMC. For the wider process view, read compression moulding explained.

Frequently asked questions