Resistance Welding Problems: Expulsion or Burn-Through

Expulsion is the forceful ejection of molten metal from the weld zone — and it’s one of the clearest signals that something in your weld schedule needs attention. Think of it like squeezing a tube of toothpaste too hard: when heat and pressure exceed what the joint can contain, molten metal comes shooting out. Severe expulsion can eject enough material to leave a hole completely through the workpiece, which is what gives rise to the term “burn-through.”

It’s worth being direct here: expulsion is not a sign of a powerful or aggressive weld. It’s a sign that current, weld time, and electrode force are out of balance — and that weld quality is suffering for it.

Where It Occurs and What It Looks Like

Expulsion can occur at any interface in the weld stack — at the tip-to-workpiece interface or at any faying surface between the sheets being joined. The root cause is excess heat combined with insufficient electrode containment of the expanding molten nugget. When the molten pool can’t be held in place, it escapes.

Solidified spikes of expelled material extending from the weld perimeter are called whiskers or burrs, and these secondary effects carry their own set of problems. Burn-through — the most severe form — occurs when so much material is expelled that a through-hole remains in the finished weld.

Why It Matters

Expulsion affects quality, maintenance, cost, and throughput, often simultaneously. Here’s what to watch for:

– Quality: Expulsion at the weld interface can displace or damage adhesives and sealers. Whiskers and burrs may prevent proper seal installation, or damage seals during assembly — increasing the risk of corrosion over the life of the part.

– Maintenance & Equipment: Expelled material accumulates on tooling, fixtures, and surrounding equipment. Over time, this steel grit can cause machine malfunctions and contaminate downstream processes — including paint shop operations.

– Throughput & Cost: Inconsistent welds drive rework, increase inspection burden, and can take equipment offline unexpectedly.

Detection

Expulsion is usually identifiable in real time — watch for visible molten material ejected during the weld. Post-weld inspection should look for holes, burrs, or whiskers at the weld perimeter.

Possible Causes

Strong possibilities — check these first:

  • Excessive sealer (creates pockets that vaporize explosively during the weld)
  • Poor electrode follow-up (electrodes can’t track the expanding nugget)
  • Poor or varying part fit-up (gaps allow molten metal to escape the joint)
  • Weld current too high (too much heat generated too quickly)
  • Weld flange too small (insufficient material to contain the weld pool)

Also worth investigating:

  • Damaged part or incorrect workpiece selected
  • Defective air or hydraulic system
  • Dirty or incorrect material/coating
  • Electrode faces not parallel to the workpiece
  • Electrode skidding, sliding, or excessive wear
  • Inadequate electrode alignment or incorrect electrode dressing
  • Hold time or squeeze time too short – Weld force too low or weld time too long
  • Incorrect cylinder, hoses, shank, tips, or transformer
  • Poor mechanical connection
  • Welds incorrectly located
  • Wrong transformer tap or secondary configuration

Possible Causes

Strong Possibilities

Weak Possibilities

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