Introduction
Bulk Moulding Compound (BMC) is a thermosetting composite material widely used in electrical, automotive, and consumer products due to its excellent strength, heat resistance, and cost-effectiveness.
However, improper material handling or moulding conditions can lead to surface or structural defects that compromise part quality.
This guide outlines the most common defects in BMC products — their root causes and effective corrective measures to maintain consistent production quality.
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Bubbles
Definition:
Bubbles form when trapped air or gas expands during pressing, causing surface irregularities or voids.
Common Causes:
- Dry or unevenly wetted fibreglass traps air within the BMC mix.
- Contamination from moisture, oil, or mould release agents creates steam during curing.
- Slow mould closing or low temperature traps air in the cavity.
- Misaligned moulds or poor venting restrict gas escape.
- Inappropriate viscosity or excessive initiators create pre-gel and uneven flow.
Solutions:
- Ensure raw materials are fully dried and free from oil or dust.
- Adjust mould closing speed and maintain consistent temperature.
- Verify proper air venting and mould alignment.
- Optimise pressure and viscosity for smooth flow.
- Avoid thick product sections that prevent uniform curing.
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Spider Webbing
Definition:
White, spiral thermoplastic aggregates appearing on the surface, caused by incompatibility between thermoplastic and polyester.
Causes & Solutions:
- Slow thickening: ensure compound reaches about 500,000 cps within the first hour.
- Incompatible or excessive thermoplastic additives: select compatible resins and reduce additive content while maintaining shrinkage control.
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Contamination
Definition:
Surface impurities or stains on finished parts.
Causes:
- Moisture in fillers or calcium stearates.
- Airborne dust, oil, or debris.
- Impurities left in moulds or from trimming operations.
Solutions:
- Keep fillers dry and sealed.
- Maintain a clean production environment.
- Clean and inspect moulds regularly.
- Prevent cross-contamination from tools and work surfaces.
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Surface Cracks
Definition:
Shallow cracks or micro-fractures that do not penetrate the base layer.
Causes:
- Undercuts causing stress during demoulding.
- Uneven or rapid ejector pin action.
- Incomplete curing or uneven temperature distribution.
- Excessive mould release agents or poor weld line strength.
Solutions:
- Avoid undercuts in part design.
- Balance ejector movement and reduce ejection speed.
- Extend curing time and slightly raise mould temperature.
- Use minimal mould release agents to ease demoulding.
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Brittle Cracks
Definition:
Cracks that penetrate through the entire product thickness.
Causes & Solutions:
- Severe undercuts: modify design to prevent part locking.
- Unbalanced ejector pins or excessive ejection speed: adjust to prevent stress.
- Incomplete curing or misaligned moulds: ensure even curing and proper alignment.
- Optimise curing time and mould temperature for full strength.
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Burn Marks
Definition:
Discoloured or scorched areas from trapped air or styrene ignition due to high mould temperature.
Causes & Solutions:
- Large material charge preventing air escape: reduce material area.
- Inadequate venting or narrow shear edges: modify gaps to release trapped gases.
- Temperature imbalance between hot and cold zones: ensure uniform mould temperature.
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Dull Surface
Definition:
Loss of surface gloss caused by incomplete curing or improper mould conditions.
Causes & Solutions:
- Extend curing time and ensure consistent pressure.
- Polish or clean worn mould surfaces.
- Shorten material flow distance to prevent separation and dullness.
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Edge Tears
Definition:
Micro-tears appearing at the product’s shear edge.
Causes & Solutions:
- Excess flash or rough shear edges: polish mould edges.
- High pressure or fast closing speed: reduce both to prevent tearing.
- Inadequate surface lubrication: apply proper mould release agents.
- Control material viscosity to stabilise fibre orientation.
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Visible Glass Fibre Orientation
Definition:
Exposed or aligned glass fibres visible on the surface, reducing appearance quality.
Causes & Solutions:
- Long flow paths: shorten and adjust material placement.
- Slow mould closing or low viscosity: increase both for random fibre distribution.
- Varying wall thickness: maintain consistent thickness to prevent fibre marks.
- Use additives with low shrinkage to mask fibres effectively.
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Weld Lines
Definition:
Weak lines formed where two material flows meet and fail to fuse completely.
Causes & Solutions:
- Excessive flow distance or splits in flow path: redesign part or adjust placement.
- High temperature and closing speed: reduce both for improved bonding.
- Reinforce critical weld areas with fibreglass mesh.
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Dark Spots
Definition:
Discoloured areas caused by uneven pressure, pre-gel, or temperature imbalance.
Causes & Solutions:
- Maintain even pressure across the cavity.
- Control mould temperature to ensure uniform curing.
- Minimise air exposure and pre-gel by optimising temperature and pressure.
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Incomplete Fill
Definition:
Areas where material fails to completely fill the cavity.
Causes:
- Low pressure, slow closing, or excessive viscosity.
- Poor venting or long flow distance.
- Pre-gel formation or unbalanced cavity pressure.
Solutions:
- Increase mould pressure and closing speed.
- Check venting and reduce flow distance.
- Maintain proper resin activity and prevent pre-gel.
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Phase Separation
Definition:
Separation of low-shrink additives or incompatible materials during curing.
Causes & Solutions:
- Excess air exposure or short flow distance: increase flow to improve mixing.
- Excessive pressure or temperature: reduce both to maintain material homogeneity.
- Select compatible additives and control viscosity to prevent segregation.
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Pinholes
Definition:
Small holes (<1 mm) formed by trapped air or incomplete flow.
Causes & Solutions:
- Inadequate venting or high mould temperature: optimise both to release trapped air.
- Contamination or small, scattered BMC charges: ensure clean materials and continuous placement.
- Adjust viscosity and pressure for smooth material flow.
Conclusion
By controlling key process variables — including mould temperature, curing time, pressure, viscosity, and material placement — most BMC moulding defects can be prevented.
Consistent equipment maintenance, proper storage of raw materials, and a clean production environment are equally critical.
Implementing these corrective measures ensures stable production, higher yield, and improved quality in BMC composite products.
Quick Reference Table: Common BMC Defects and Solutions
For a quick overview of BMC defects, their main causes, and corrective actions, see the table below.
| Defect | Main Cause | Corrective Action |
|---|---|---|
| Bubbles | Trapped air or moisture in BMC or mould cavity | Dry raw materials, improve venting, adjust mould temperature and pressure |
| Spider Webbing | Incompatible or excessive thermoplastic additives | Use compatible additives and ensure proper thickening |
| Contamination | Moisture, oil, or dust in raw materials or mould | Keep fillers dry and clean, maintain a dust-free environment |
| Surface Cracks | Stress during demoulding or incomplete curing | Reduce ejection speed, ensure full curing, avoid undercuts |
| Brittle Cracks | Excess stress or unbalanced ejection | Adjust ejection balance, increase curing time and mould temperature |
| Burn Marks | Trapped air or styrene ignition from high heat | Improve venting and balance mould temperature |
| Dull Surface | Incomplete curing or worn mould surface | Extend curing time, maintain mould polish and cleanliness |
| Edge Tears | High ejection pressure or rough shear edges | Polish mould edges, reduce closing speed and pressure |
| Visible Glass Fibre Orientation | Uneven flow or long flow distance | Increase mould closing speed, control viscosity, maintain uniform wall thickness |
| Weld Lines | Weak bonding where material flows meet | Shorten flow paths, lower temperature, reinforce weld areas |
| Dark Spots | Uneven curing or localised pre-gel | Balance pressure, maintain consistent temperature, reduce air exposure |
| Incomplete Fill | Insufficient pressure or pre-gel during flow | Increase pressure, check venting, and shorten flow distance |
| Phase Separation | Incompatible or excessive low-shrink additives | Use compatible additives and adjust viscosity for uniformity |
| Pinholes | Trapped air and poor venting near edges | Improve venting, optimise viscosity and pressure, ensure clean materials |
TECHNICAL QUESTIONS
Common BMC Moulding Defects
The most frequent BMC moulding defects include bubbles, cracks, burn marks, dull or low-gloss surfaces, weld lines, contamination, and incomplete filling.
Ensure uniform curing, avoid deep undercuts in the design, balance the ejector pin layout, and reduce ejection speed to minimise stress during demoulding.
Burn marks are typically caused by trapped air or styrene igniting at excessively high mould temperatures. Improving venting and maintaining balanced temperature control can effectively prevent this issue.
Spider webbing refers to white, spiral-like patterns on the surface, caused by incompatible or excessive additives within the BMC formulation.
