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Die casting is a metal casting process where molten metal is forced under high pressure into a mold cavity machined into the desired shape. Most die casting is limited to low melting points, non-ferrous metals, specifically zinc, copper, aluminum, magnesium, lead, pewter, and tin-based alloys. Once the die cavity has been filled with the molten metal, coolant (usually water) is circulated around the die to cool the part. After cooling, the die halves are separated, and the finished part is ejected.
At the beginning of the die-casting sequence, lubricant is sprayed onto the die cavities, the mechanical plunger, and the die guide bars. Due to the elevated temperature of the dies, some of the die lubricants evaporate, creating a plume of mists and fumes.
Benefits of filtering the mist, fumes and smoke
- Protecting worker health
- Reduced exhaust air make-up requirements up to 80% through recirculated conditioned air
- Extended machine life
- Reduced operational costs through reclamation of lubricants
- Improved part and product quality
- Reduced housekeeping costs
- Compliance with even the strictest federal, state and local environmental standards
Contaminant Characteristics
The contaminants from die casting processes fall into two categories:
Metal Oxides: As the molten metal is injected into the die cavities and when the dies are opened and closed, a certain amount of metal oxides will escape. The amount of metal oxides will vary based on the type of metal being cast and the temperature of the die cavity.
Die Lubricants: The majority of die-casting emissions come from lubricating the main plunger, die cavities, and die guide bars. This operation creates submicronic smoke, which is harmful to workers who can breathe in the hazardous particles during the operation of the die-casting machine. The molten metal temperatures can exceed 1,000º F. Therefore, normal lubricating oils and greases cannot be used. Instead, a mixture of graphite and carrier oil (or water) is commonly used. During each die-casting operation cycle, a plume of this lubricant is driven off due to these high temperatures. This plume consists of a combination of light and heavy oils, graphite, carbon, and other materials. The resulting contaminant is typically a black, semi-conductive, grease-like substance.
Recommended Approaches for Die Casting Mist and Fume Control
Source Capture: Source capture should always be the primary means for capturing die-casting emissions. Source capture involves utilizing fixed capture hoods or extraction arms proximate to the die cavities. Capture hoods need to be as close as practical to extract the airborne contaminants at or near the generation source to protect the machine operator and prevent fumes from migrating elsewhere in the facility. Source capture is the most effective means of capture and requires the least amount of energy and initial investment to accomplish.
Ambient Air Collection: When used with source capture, ambient air collection can help remove the ambient haze caused by airborne pollutants. However, ambient air collection is not an effective approach for controlling the fume and mist emissions from a die casting machine. It does not protect the machine operator’s breathing zone. Therefore, ambient air collection is not recommended unless used with source capture.