Die casting is a metal casting process where molten metal is forced under high pressure into a mold cavity which has been machined into the desired shape. Most die casting is limited to low melting point, 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 lubricant evaporates which creates a plume of mists and fumes.
Some of the key benefits from filtering the mist, fumes and smoke generated by die casting processes include:
The contaminants from die casting processes fall into two categories:
Metal Oxides. As the molten metal is injected into the die cavities, as well as 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 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 & 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 located proximate to the die cavities. Capture hoods need to be located as close as practical to extract the airborne contaminants at or near the generation source to protect the machine operator and to prevent the fume 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 in conjunction with source capture, ambient air collection can help remove the ambient haze caused by the airborne pollutants. However, ambient air collection is not an effective approach for controlling the fume and mist emissions from a die casting machine and it does not protect the machine operator’s breathing zone. Therefore, ambient air collection is not recommended unless used in conjunction with source capture.