---
type: process
name: "DMC and SMC Molding"
category: "Forming"
subcategory: "Composites"
materials: ["Composite", "Plastic"]
tolerances: "About ±0.1–0.3% of dimension, roughly ±0.005 in/in, thanks to the near-zero mold shrinkage of low-profile compounds. Tighter than most thermoplastic molding, and dimensionally stable with temperature."
volumes: "1,000–100,000 parts per year"
lead_time: "10–20 weeks for steel tooling; cycle times of 60–180 seconds per part in production."
url: https://manufacturingprocesses.org/processes/forming/dmc-and-smc-molding
---

# DMC and SMC Molding

DMC and SMC molding compression-molds a pre-mixed dough or sheet of chopped fiber and thermoset resin into a stiff, dimensionally stable part.

- **Category**: [Forming](https://manufacturingprocesses.org/processes/forming.md)
- **Family**: Composites
- **Materials**: Composite, Plastic
- **Typical tolerances**: About ±0.1–0.3% of dimension, roughly ±0.005 in/in, thanks to the near-zero mold shrinkage of low-profile compounds. Tighter than most thermoplastic molding, and dimensionally stable with temperature.
- **Surface finish**: Molded surfaces replicate the tool. Low-profile SMC with a polished tool and a coating step reaches Class A automotive paintability; standard compounds show a fine fiber pattern and are normally primed and painted.
- **Typical volumes**: 1,000–100,000 parts per year
- **Lead time**: 10–20 weeks for steel tooling; cycle times of 60–180 seconds per part in production.

## Overview

SMC (sheet molding compound) and DMC (dough molding compound, also called BMC) are compression molding compounds of chopped glass fiber, thermosetting polyester or vinyl ester resin, mineral filler, and a thickening system. SMC arrives as a tacky sheet with roughly 1 in (25 mm) fibers at 25–30% glass by weight; DMC arrives as a bulk dough with shorter 0.12–0.5 in (3–12 mm) fibers at 15–20% glass and more filler.

Both are compression molded in matched heated steel tools at roughly 285–320 °F (140–160 °C) under 700–1,500 psi (50–100 bar), curing in 60–180 seconds depending on wall thickness. Low-profile additives bring mold shrinkage to 0–0.2%, which is what makes Class A automotive body panels possible in a thermoset.

The result is a stiff, dimensionally stable, thermally and electrically stable part at 1.8–2.0 g/cm³ — heavier than a thermoplastic but far stiffer at temperature. Truck hoods, electrical switchgear and breaker components, headlamp reflectors, battery trays, and shower trays are typical. Volumes run 1,000 to 100,000 a year against steel tooling.

## How it works

1. **Compound.** For SMC, resin paste containing filler, catalyst, low-profile additive, and a thickener is doctored onto carrier film, chopped roving is rained onto it, a second filmed layer is laid on top, and the sandwich is compacted. For DMC, all constituents including the short fiber are mixed in a bulk mixer.
2. **Mature.** SMC is held for roughly 2–7 days at controlled temperature while the magnesium-oxide thickening system raises viscosity by orders of magnitude, turning a sticky paste into a leathery, handleable sheet. Under-matured SMC runs resin-rich; over-matured SMC will not flow.
3. **Cut the charge.** Sheet is cut into blanks and stacked to a mass that matches the part, and the carrier film is stripped. Charge mass is controlled tightly — the process has no runner to absorb error.
4. **Place the charge.** The stack covers roughly 30–70% of the cavity area. Placement determines flow direction, which in turn determines fiber orientation, knit line position, and where the part will be weakest. This is the single most important process variable.
5. **Close and cure.** The press closes and holds at roughly 700–1,500 psi (50–100 bar) with tool surfaces at 285–320 °F (140–160 °C). Material flows to fill the cavity and cures in place; cure time runs roughly 30–40 seconds per millimeter of wall thickness.
6. **Eject and finish.** The part is ejected hot and dimensionally stable — thermosets do not need cooling in the tool. Flash is removed, and painted parts go through a wash and prime cycle. Metal inserts can be molded in place, which is common in electrical hardware.

## Design guidelines

### Wall thickness
0.060–0.250 in (1.5–6 mm) is the working range, with 0.100 in (2.5 mm) typical for panels. Cure time scales with thickness, so a thick section costs cycle time everywhere in the part.

### Uniform sections and ribs
Keep the nominal wall uniform and add stiffness with ribs rather than thickness. Rib base thickness should be about 60–75% of the adjoining wall to limit sink, though thermoset shrinkage is low enough that SMC tolerates thicker ribs than a thermoplastic would.

### Draft
1–3° on all vertical faces, more on textured surfaces. Thermoset parts are ejected hot and rigid, so they do not shrink onto cores the way a thermoplastic does, but glass fiber makes the surface abrasive and low draft galls tooling.

### Radii
0.060 in (1.5 mm) minimum inside radius, 0.125 in (3 mm) preferred. Sharp corners create resin-rich, fiber-poor regions where the compound cannot carry fiber around the turn.

### Charge placement and knit lines
Where two flow fronts meet, fibers do not cross the boundary and the knit line is a strength defect. Design so the knit line falls in a low-stress region, and expect the tool trials to move it.

### Molded-in inserts
Threaded inserts, brackets, and grounding hardware can be placed in the tool and molded in. This is one of SMC and DMC's strongest advantages over sheet metal fabrication for electrical enclosures.

### Class A surfaces
Achievable with low-profile SMC and a polished tool, but it requires shrinkage control at 0–0.2%, careful charge placement, and usually an in-mold or post-mold coating step. Do not assume Class A comes free with the material.

### Electrical performance
DMC/BMC is a standard material for arc-resistant and tracking-resistant electrical components, which is a functional reason to choose it over a filled thermoplastic rather than a cost one.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Wall thickness | 0.100 in (2.5 mm) | 0.060–0.250 in (1.5–6 mm) | Cure time scales with thickness |
| Rib base thickness | 60–75% of wall | 100% of wall | Sink and cure differential |
| Draft | 2° | 1° | Abrasive glass galls tooling |
| Inside radius | 0.125 in (3 mm) | 0.060 in (1.5 mm) | Fiber cannot turn a sharp corner |
| Charge coverage | 50% of cavity | 30–70% | Sets flow, fiber orientation, knit lines |
| Molded-in insert | Knurled, with resin shoulder | — | Prevents pull-out and reduces stress |

## Cost drivers

Steel compression tooling is the entry cost, comparable to an injection mold of similar size, and it is the reason the process needs 1,000 parts a year or more to make sense. Compression tools are somewhat simpler than injection tools — no runner system, lower clamp requirements per unit area than injection — but they are still matched steel.

Cycle time is set by cure, which is set by wall thickness: roughly 30–40 seconds per millimeter. A 3 mm panel cures in around 90–120 seconds, so a single press produces on the order of 20–30 parts an hour. Press tonnage requirements are substantial, since molding pressure applies over the full projected area.

Material is inexpensive relative to other composites — filler is a large fraction of the compound by weight — but SMC has a finite shelf life after maturation and must be stored at controlled temperature, which is an operational cost.

Volume breakpoints: below roughly 1,000 parts a year, hand [composite laminating](/processes/forming/composite-laminating) or RTM is usually cheaper. From 1,000 to 100,000, SMC and DMC compression molding is the sweet spot. Above that, and where the material allows, injection-molded BMC or a filled thermoplastic can be faster.

1. Hold the nominal wall thin and uniform — cure time is the cycle.
2. Mold in inserts and hardware rather than assembling them afterward.
3. Consolidate a sheet metal assembly into a single molded part; that is where the process pays.
4. Place knit lines deliberately by controlling charge placement.
5. Specify Class A only on the surfaces that are actually visible.

## FAQ

### What is the difference between SMC and DMC?

SMC is supplied as a matured sheet with roughly 1 in (25 mm) chopped fibers at 25–30% glass by weight, giving higher strength and suiting large panels. DMC (also called BMC) is a bulk dough with 0.12–0.5 in (3–12 mm) fibers at 15–20% glass and more filler, which flows into intricate geometry and can also be injection molded.

### Why does SMC need to mature before molding?

A magnesium-oxide thickening system raises the compound's viscosity by orders of magnitude over roughly 2–7 days, turning a sticky paste into a handleable leathery sheet that will carry fiber with it as it flows. Under-matured compound runs resin-rich; over-matured compound will not fill the tool.

### Can SMC produce a Class A automotive surface?

Yes, and that is one of its historical justifications. It requires a low-profile additive package that holds mold shrinkage to 0–0.2%, a polished tool, controlled charge placement, and normally an in-mold or post-mold coating step. Class A is an engineered outcome, not a property of the material.

### What molding pressure and temperature does SMC need?

Roughly 700–1,500 psi (50–100 bar) over the projected area, with tool surfaces at 285–320 °F (140–160 °C). Cure time runs about 30–40 seconds per millimeter of wall thickness, so a 3 mm panel is in the tool for approximately 90–120 seconds.

### Why is BMC used for electrical components?

Thermoset polyester compounds have high comparative tracking index and arc resistance, retain stiffness at elevated temperature, and do not soften or drip. That combination — plus the ability to mold in metal contacts and inserts — makes DMC/BMC a standard material for breakers, switchgear, and terminal blocks.

### How does SMC compare with hand-laid composite?

SMC uses short chopped fiber and reaches lower specific strength than a continuous-fiber laminate, but it molds in minutes rather than hours, holds tighter tolerance, and gives two finished faces. Choose continuous-fiber laminating for structural efficiency and SMC for repeatable production geometry.

## Alternative processes

- [Compression Molding](https://manufacturingprocesses.org/processes/forming/compression-molding.md): Compression molding presses a measured charge of rubber or thermoset into an open heated mold that closes to cure it into its final shape.
- [Injection Molding](https://manufacturingprocesses.org/processes/forming/injection-molding.md): Injection molding forces molten thermoplastic into a steel mold under high pressure, producing high-volume identical plastic parts.
- [Composite Laminating](https://manufacturingprocesses.org/processes/forming/composite-laminating.md): Composite laminating lays reinforcing fiber into a mold and consolidates it with resin, curing into a stiff, light monocoque part.
- [Sheet Metal Fabrication](https://manufacturingprocesses.org/processes/forming/sheet-metal-fabrication.md): Sheet metal fabrication cuts, bends and joins flat stock into a finished assembly, typically laser cutting followed by press braking, hardware and welding.
- [Thermoforming](https://manufacturingprocesses.org/processes/forming/thermoforming.md): Thermoforming heats a plastic sheet until pliable and pulls or presses it over a single-sided mold, producing large thin-walled parts with low tooling cost.

## Related processes

- [Compression Molding](https://manufacturingprocesses.org/processes/forming/compression-molding.md): Compression molding presses a measured charge of rubber or thermoset into an open heated mold that closes to cure it into its final shape.
- [Composite Laminating](https://manufacturingprocesses.org/processes/forming/composite-laminating.md): Composite laminating lays reinforcing fiber into a mold and consolidates it with resin, curing into a stiff, light monocoque part.
- [Injection Molding](https://manufacturingprocesses.org/processes/forming/injection-molding.md): Injection molding forces molten thermoplastic into a steel mold under high pressure, producing high-volume identical plastic parts.
- [Pultrusion](https://manufacturingprocesses.org/processes/forming/pultrusion.md): Pultrusion pulls continuous fiber through a resin bath and a heated die, curing it into a constant-section composite profile of unlimited length.
- [Filament Winding](https://manufacturingprocesses.org/processes/forming/filament-winding.md): Filament winding wraps resin-impregnated continuous fiber onto a rotating mandrel in a controlled pattern, producing tubes and pressure vessels.

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*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/forming/dmc-and-smc-molding)*

*Last updated: August 11, 2026*