Individual or groups of cavities. Mostly large with smooth walls.
Incidence of the defect
Gases entrapped by solidifying metal on the surface of the casting, which results in a rounded or oval blowhole as a cavity. Frequently associated with slags or oxides.
The defects are nearly always located in the cope part of the mould in poorly vented pockets and undercuts. The formation of blowholes is more predominant in grey iron castings than in SG iron.
- Inadequate core venting
- Excessive release of gas from core
- Excessive moisture absorption by the cores
- Low gas permeability of the core sand
- Moisture content of sand too high, or water released too quickly
- Gas permeability of the sand too low
- Sand temperature too high
- Bentonite content too high
- Too much gas released from lustrous carbon producer
- Compaction of the mould too high
Gating and pouring practice
- Casting temperature too low
- Metallostatic pressure too low when pouring
- Improve core venting, provide venting channels, ensure core prints are free of dressing
- Reduce amounts of gas. Use slow-reacting binder. Reduce quantity of binder. Use a coarser sand if necessary.
- Apply dressing to cores, thus slowing down the rate of heat-ing and reducing gas pressure.
- Dry out cores and store dry, thus reducing absorption of water and reducing gas pressure.
- Reduce moisture content of sand. Improve conditioning of the sand. Reduce inert dust content.
- Improve gas permeability. Endeavour to use coarser sand. Reduce bentonite and carbon carrier content.
- Reduce sand temperature. Install a sand cooler if necessary. Increase sand quantity.
- Reduce bentonite content. Use bentonite with a high mont-morillonite content, high specific binding capacity and good thermal stability.
- Use slow-reacting lustrous carbon producers or carbon carri-ers with higher capacity for producing lustrous carbon. In the last instance, the content of carbon carriers in the moulding sand can be reduced.
- Reduce compaction of the moulds. Ensure more uniform mould compaction through better sand distribution.
Gating and pouring practice
- Increase pouring temperature. Reduce the pouring rate as appropriate.
- Increase metallostatic pressure by changing the gating sys-tem. If possible raise the cope flask.
The occurrence of gas cavities and blowholes is dependent on the gas volumes present and their pressure. If it is not possible to discharge the gases from the mould cavity, they will become trapped in the liquid metal.
There is a great danger of surface pitting on cores because they are surrounded by liquid metal and the gaseous reaction products are primarily removed through core prints. Blowholes are more frequently observed with smaller cores. It is recommended to use coarser sands and a corresponding application of mould dress-ings.1 Cores with an unfavourable shape should contain waste gas channels. The necessary channel cross-sections for gas dis-charge from cores in relationship to core binders and geometry are thoroughly investigated in.2 Obstruction of gas discharge results in bubbles being trapped in the metal. This problem also occurs with large gas discharge cross-sections when using pheno-lic resins. Hygroscopic binders such as sodium silicate require large cross-sections for gas discharge. Conversely, drying the cores can combat the occurrence of blowholes. Use of cold cores in hot moulds can lead to water adsorption with hygroscopic bind-ers. This can result in explosive vaporization and the associated defects.