Dust and Fume Collection for Resistance Welding
Solutions for Resistance Welding Fume and Dust Control
RoboVent offers robust dust and fume control solutions for manufacturers using thermal spray processes. RoboVent Senturion is a rugged, high-powered dust collector that minimizes maintenance and operating costs for resistance welding applications.
A RoboVent solutions engineer can design an effective dust control system for your resistance welding application that fully meets ACGIH recommendations.
We can help you evaluate the health and combustion risks of your resistance welding emissions and design a comprehensive air quality solution, including resistance welding dust containment and collection, ductwork design, and ventilation and make-up air.
Considerations in Dust and Emission Control for Resistance Welding
ACGIH has created recommended capture procedures for resistance welding that should be followed by manufacturers. For low-volume manual resistance welding, a good industrial ventilation system that pulls air up and away from the breathing zone of the welder may be enough to keep weld fumes, gases and vapors in check. However, in most cases, a dedicated dust collection and filtration system is recommended for resistance welding. When welding galvanized steel or painted, coated or lubricated parts, it is essential to take the gas and/or liquid phases of the emissions into consideration when designing the solution.
There are several considerations for designing an effective dust collection solution for resistance welding.
- For robotic resistance welding, keep processes confined under a hood or in an enclosure where possible. This will allow for efficient source capture of fumes. The hood or enclosure is ducted to an industrial dust collector for capture and filtration of dust and particulate.
- For manual resistance welding, use fume arms, hoods or backdraft tables that pull weld fume up and away from the welder’s breathing zone. These should be ducted to an industrial dust collector to filter contaminants out of the air before returning air to the facility.
- When welding galvanized metals, care must be taken to collect the gas phase of zinc oxide as well as solid particulate. Allowing a longer duct run will allow zinc oxide to transition back into a solid state for collection in the dust collector filters.
- Gases and odors produced by resistance welding of coated materials may require a specialized solution. If the total emissions are low, it may be possible to vent gases to the outside. For higher-production applications, the dust collector should be fitted with an activated carbon after-filter to adsorb gaseous emissions. Gases will not be captured by standard cartridge filters, so air should not be returned to the facility unless the dust collector is fitted with the appropriate after-filters.
- When welding metals contaminated with metalworking fluids or other oils, clean the area to be welded first wherever possible. The process should be evaluated to determine if oil mists are created. When collecting oily particulate, use an oleophobic filter media to prevent dust from sticking to the filters. (Very oily applications will require a specialized oil mist collector fitted with a coalescing filter, but this is not typical for resistance welding applications.)
- Depending on the volume and combustibility of fumes collected by resistance welding, the dust collector may need to be equipped with a deflagration system. This system may include explosion vents for safe venting of the energy from a dust collector explosion, isolation valves and a rotary airlock to prevent dust in the collection bin from being used to fuel an explosion.
Air Quality Issues with Resistance Welding
Uncontrolled fumes and gases produced by resistance welding of galvanized or coated materials can create several problems, including:
- Health impacts for exposed workers, especially for manual resistance welding.
- Build-up of particulate in large robotic resistance welding cells, which can cause equipment short-outs.
- Problems with HVAC or make-up air units due to excessive loading of filters.
- Combustion risk if weld fume is allowed to build up in an enclosed space.
- Housekeeping issues as fumed particulate settles out of the air and onto horizontal surfaces.
Resistance welding galvanized or coated metals produces additional challenges due to the way these materials react when exposed to high temperatures. The zinc coating on galvanized metals sublimates at high temperatures, going directly from a solid to a gas phase. This gas phase is not visible to the eye but can produce many adverse effects when inhaled. Paints and lubricants may also produce gases, vapors or oil mists rather than dry particulate. The air quality solution must be able to collect both the dry particulate in weld fume and gases, vapors or mists emitted by the coatings or lubricants.
Emissions from Resistance Welding vs. MIG or Stick Welding
Resistance welding uses a combination of pressure and heat created by an electric current to bond two pieces of metal. Force is applied to push the pieces of metal together. Then, a strong electric current is passed through the metal at the joint. This process generates enough heat to melt the edges of the joint, which are then bonded together as the metal resolidifies. Resistance welding processes include:
- Spot welding
- Seam welding
- Butt welding
- Projection welding
Unlike MIG or stick welding, resistance welding does not require any additional material to create the weld joint. MIG and stick welding require a wire electrode to be constantly fed onto the workpiece to create the joint. Resistance welding, by contrast, simply melts the edges of the two materials to be joined together. Since no extra material is needed, it is a cost-effective welding method.
In comparison, the majority of MIG and stick weld fumes are generated from the filler material, not the base metal. The absence of a filler material in resistance welding not only simplifies the process, but also lowers dry particulate generation. However, if there are any coatings, lubricants or adhesives on the metal being joined, they will vaporize. The fumes and gases produced by resistance welding of coated or lubricated materials may not be as visible as the particulate produced by a welding wire.
Still, the fumes, vapors and gases given off when heating coated or lubricated metals to high temperatures are dangerous—even if they can’t be seen.
Resistance Welding with Galvanized Metals
Galvanized metals are commonly used in the metalworking industry. Galvanization is the process of applying a protective zinc coating to metal. The zinc coating provides long-lasting protection against rust for products made of iron or steel. Galvanization works because zinc oxidizes more readily than the base metal. In essence, it corrodes first to prevent oxygen from reaching the metal underneath. Sometimes, chromate is added to the zinc coating to increase its oxidation potential. Chromating makes galvanized parts more durable.
Galvanized steel can be produced using the hot-dip method or through electrogalvanization. When the metal is galvanized, it results in a distinctive “spangle” pattern with varying shades of grey. Galvanized steel is widely used in the auto industry, for structural steel and other applications where steel will be exposed to the elements.
The zinc coating on galvanized steel has a much lower melting point than the steel underneath. When welding galvanized steel, the coating quickly vaporizes, forming zinc oxide fumes. At very high temperatures, the zinc will sublimate directly from a solid to a gas phase. Galvanized coatings may also contain lead, chromate and other materials that are dangerous when vaporized.
Health Risks of Resistance Welding with Galvanized, Coated or Lubricated Metals
Resistance welding with galvanized, coated or lubricated metals creates special health and safety concerns. Almost all coatings and lubricants vaporize to some degree during the welding process. As with all welding applications, thermal processes generate very fine particulate, vapors, fumes and gases that are inhaled deeply into the lungs. The exact health risks will depend on the composition of the coatings or lubricant in use.
- Galvanized steel: Most of the health risks associated with resistance welding of galvanized steel come from exposure to vaporized zinc oxide. Short-term exposure to zinc oxide fumes can cause an acute illness called “metal fume fever.” Metal fume fever causes flu-like symptoms, including nausea, headaches, fever, chills and extreme thirst. Most symptoms will go away within 48 hours of eliminating the exposure. Long-term exposure to zinc oxide has not been thoroughly studied but may be associated with developmental problems for a developing fetus. For this reason, pregnant women should take extra precautions when working with galvanized metals. Galvanized coatings also commonly contain small amounts of lead that will vaporize along with the zinc and generate lead oxide fumes. Even low-level exposure to lead oxide is dangerous, leading to irritation of the skin, eyes and respiratory tract. Chronic exposure may damage the kidneys, reproductive system and central nervous system and is associated with lung and brain cancer. Because lead bioaccumulates in biological tissue, it is very important to reduce exposure as much as possible.
- Chromated metals: Chromate conversion coating is used on galvanized steel as well as other metals, including aluminum, cadmium, copper, silver, titanium and tin alloys. The chromate coating may be used as a primer for other paints and adhesives, a corrosion inhibitor or a decorative finish. Welding chromated metals produced fumed hexavalent chromium and other metal oxides. Exposure to fumed hexavalent chromium may cause irritation and damage to the eyes, skin and respiratory tract. Long-term exposure may lead to lung cancer, sores on the skin or in nasal tissue, kidney or liver damage, pulmonary diseases and perforated eardrums. Some employees develop an allergy to hexavalent chromium.
- Paints and coatings: Many paints and coatings used on metals contain heavy metals such as chromium, cadmium, nickel, lead, cobalt, manganese, copper and molybdenum. Coatings and adhesives may also have organic solvents and other chemical compounds that are dangerous when inhaled. Long-term exposure to these chemicals may be associated with damage to the liver, kidneys and central nervous system and an increased risk of cancer.
- Lubricants: Welding lubricated metals (e.g., from metalworking fluids (MWFs) or other lubricants) results in oil mists and vapors that can be inhaled. Exposure to fumed hydrocarbon-based lubricants may cause irritation to the eyes, nose and lungs. Extensive long-term exposure may cause problems with the lymphatic systems, nervous system, reproductive system and hematopoietic (blood-forming) system. Extensive exposure may also cause problems for a developing fetus. Vaporizing hydrocarbons also releases benzene, a known carcinogen. Synthetic lubricants may release phosphate esters, which may have adverse effects on the central nervous system.
Regulations for Resistance Welding and Galvanized Coatings
The Occupational Safety and Health Administration (OSHA) has issued specific regulations for many different metallic and non-metallic dusts, fumes and vapors. These are expressed as Permissible Exposure Limits (PELs), which are measured and averaged over a worker’s 8-hour shift. Manufacturers are responsible for ensuring that workers are not exposed to hazardous dusts in levels above the PELs. Failure to control dust in areas where workers may be exposed can result in serious fines and legal action. For some highly toxic substances, OSHA also sets a separate action limit; exposures above this limit may require specific compliance activities such as blood testing.
These are PELs for some of the elements and compounds commonly found in dust from resistance welding of galvanized steel or resistance welding of coated or lubricated metals:
- Zinc oxide: 5.0 mg/m3 (for zinc oxide fume or respirable dust)
- Lead: 50 μg/m3 (PEL)/action limit 30 μg/m3
- Hexavalent chromium: 5 μg /m3
- Nickel: 1.0 mg/m3
- Cadmium: 0.005 mg/m3
- Manganese: 5.0 mg/m3
- Cobalt: 0.1 mg/m3
- Mineral oil mists: 5.0 mg/m3
- Benzine: 1 part per million (5 ppm for short-term exposure limit)
- Triphenyl phosphate (a phosphate ester): 3.0 mg/m3
In addition, manufacturers must consider the following regulations.
- Combustible dusts: Metallic dusts produced during resistance welding are combustible. Combustible dusts are regulated under OSHA’s General Duty Clause (Section 5(a)(1)) with additional requirements under the Hazardous Locations (§1910.307), Hazard Communication (§1910.1200) and Housekeeping (§1910.22) standards. In addition, manufacturers dealing with combustible dusts must follow National Fire Protection Association (NFPA) standards for prevention of fires and explosions.
- Nuisance dusts: Manufacturers using resistance welding must consider the general limit for exposure to particulate of any kind. The limit for “Particulates Not Otherwise Regulated” (PNOLs) is 15 mg/m3 (8-hour TWA limit) for total particulate and 5 mg/m3 for respirable particulate. Employers must also follow general Housekeeping standards (OSHA 1910.22, Walking-Working Surfaces) to prevent accumulation of dust on surfaces.