Category

BMC (Bulk Molding Compound)

SMC (Sheet Molding Compound)

Phenolic Resin

Basic Definition

A bulk molding compound consisting of unsaturated polyester resin reinforced with short chopped glass fibers, fillers, and additives; supplied in bulk form.

A sheet molding compound consisting of unsaturated polyester resin reinforced with long chopped glass fibers, fillers, and additives; supplied in sheet form.

A thermosetting polymer based on phenol-formaldehyde resin, often mixed with fillers and fibers to form molding compounds.

Composition

Unsaturated polyester resin + chopped glass fibers (6–30 mm) + fillers (e.g., calcium carbonate) + curing agents + release agents + colorants.

Unsaturated polyester resin + long chopped glass fibers (25–50 mm) + fillers + curing agents + low-shrink additives.

Phenolic resin (resol or novolac) + curing agent (hexamethylenetetramine) + fillers (wood flour, mineral powder, asbestos, etc.) + lubricants.

Curing Mechanism

Cures via peroxide-initiated crosslinking of polyester resin at elevated temperatures, forming a three-dimensional network.

Same curing mechanism as BMC (peroxide-initiated polyester resin crosslinking).

Cures through condensation reaction between phenolic resin and curing agent under heat, releasing water and formaldehyde, forming a crosslinked network.

Key Properties

Good flowability for complex shapes, high dimensional accuracy, good heat and chemical resistance.

Higher mechanical strength than BMC due to longer fibers, suitable for large surface parts, excellent surface finish.

Excellent heat resistance (>200°C), inherently flame-retardant, high chemical resistance, good electrical insulation.

Molding Processes

Compression molding, injection molding (special BMC injection).

Compression molding.

Compression molding, transfer molding, injection molding (special grades).

Matching Mold Structure

Rigid steel molds with wear-resistant steel, venting grooves, mold temperature control (130–160°C); injection molds for BMC require high wear and corrosion resistance.

Large rigid steel molds, optimized venting, uniform heating.

High-hardness corrosion-resistant molds, good venting, mold temperature control (150–180°C), resistant to water and formaldehyde corrosion.

Typical Applications

Automotive lighting reflectors, electrical insulation components, pump housings, appliance handles.

Automotive exterior panels (hoods, roofs), electrical housings, large structural components.

Electrical switches, brake pistons, cookware handles, transformer components, brake pad back plates.

Advantages

Low shrinkage, dimensional stability, heat resistance, corrosion resistance, electrical insulation.

High strength, excellent surface quality, capable of molding large parts.

Flame retardant, high-temperature resistant, chemically resistant, good insulation, cost-effective.

Customizability

Adjustable glass fiber content, filler ratio, color, and flame-retardant level.

Adjustable fiber length, color, and surface properties.

Adjustable filler type, color, and curing speed.

Molding Process Flow

BMC: Material preparation → Mold preheating → Charge placement → Compression under heat and pressure (130–160°C, 2–5 min) → Cure → Demold → Deflash.

SMC: Sheet cutting → Layer stacking → Mold loading → Compression molding (130–160°C, 3–6 min) → Cure → Demold → Finishing.

Phenolic: Mold preheating (150–180°C) → Charge loading → Compression/transfer molding → Cure → Demold → Post-curing (optional).

Common Issues & Solutions

Porosity/flow marks – Improve venting, raise mold temperature, extend cure time.
Excess flash – Control charge weight, check mold fit.
Brittleness – Adjust fiber content or resin toughness.
Dimensional instability – Control moisture content, stabilize mold temperature.
Demolding difficulty – Use appropriate release agents, polish mold surface.

Same as BMC; focus on controlling sheet size and fiber distribution.

Improve venting frequency, reduce charge weight, ensure mold temperature uniformity; apply mold coatings to reduce sticking.