- Check the welder parameters and settings to ensure that the machine is providing the required squeeze pressure and current.
- Check the compressed air source to confirm required welder squeeze pressure is achieved.
- Check welder power source to confirm required welder current is achieved.
Collins outlines the top challenges faced in spot welding:
Problem: The Squeeze Type Resistance Spot Welder (STRSW) is welding poorly or will not weld at all.
Solution:
- Ensure that there is clean, bare metal where the welder contact tips make direct contact to the flange. Also, remove the E-Coat from between the flanges before applying the seam sealer or adhesive.
- Shunt the current. The adhesive acts as an insulator between the panels and can cause poor current flow, particularly on the very first weld which acts as the electrical conduit for subsequent welds. To shunt the current use a shunt clamp (see figure 2) or clean locking pliers to act as a conductor between the panels. The clamp must be in contact with clean, bare steel on both panels so that the current will travel through the clamp (see figure 1) to allow current flow through both layers of steel. This is most critical on the first weld, after which the current will use the previous weld to conduct through. The clamp must be as close to the first weld as possible – 1 to 2 inches is ideal. If you skip to a new location away from the initial welds, reposition the clamp near the new weld site. Failure to do this will result in a very poor first weld and will negatively affect current flow through subsequent welds.
- Make sure the contact tips are very clean and correctly shaped so you get good flat contact to the steel. The 3M™ File Belt Sander with a 3M™ Cubitron II™ or Scotch-Brite™ belt works well to occasionally clean the tips as you continue to make welds. Even the highest-quality machine available will not make proper welds with dirty or mis-shaped tips. Most welding manufacturers have a pneumatic tip dressing tool available for when the tips need re-shaping.
Problem: Blowing adhesive or sealer out of the weld joint.
Solution: Avoid welding too close to the edge of the flange. The current and heat radiates outward from the center of the weld. If welding close to the edge, the heat and current are confined to a smaller space and may overheat the adhesive/sealer in the joint resulting in blow-out.
Problem: Poor welds or blowing holes in the steel.
Solution: Tight panel fit-up. If the fit-up of the flanges isn’t adequately tight, it increases the resistance at the weld location, which creates excessive heat, causing blow-out of the adhesive or holes in the flange. Make sure fit-up is tight and if necessary, on areas with gaps, use a vise grip clamp very near each weld location. Never rely on the welder contact tips to pull the panels together because as the weld area becomes molten, the pressure of the panels will force them apart creating a hole around the contact tip.
Problem: Excessive sparking or overheated welds.
Solution: Use grade 80 or finer abrasive to sand the flanges prior to application of the adhesive. A Scotch-Brite™ belt also works excellent. If you use a coarse abrasive (coarser than grade 80) it forms air pockets between the scratches that create resistance which creates excessive heat that can cause sparks, a poor weld and/or blow-out.
Problem: Poor adhesion.
Solution: Apply bonding adhesive to only clean bare metal. Do not to use a weld-through primer where the adhesive will be applied. Weld-through primer has poor adhesive properties. Apply the adhesive to clean bare metal.
Please visit 3MCollision.com for more product and process information on trouble-shooting weld-bonding and weld-sealing.
IMPORTANT NOTE: There are many factors that can affect an individual repair, so the technician and repair facility need to evaluate each specific application and repair process and determine what’s appropriate. 3M recommends referring to relevant vehicle repair and OEM guidelines prior to starting all repairs.
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