8.4.1 Typical application errors

Typical application errors

  • Drag errors
  • Dosing too late, dosing too early
  • Initial blobs
  • Overhanging strands of adhesive at the end of the dosing
  • Hollow beads
  • “Wavy” beads

Drag errors

The application of the adhesive does not follow the pre-programmed path

Dependent on:

  • Speed of travel
  • The radians to be travelled
  • Distance of the dosing nozzle to the component
  • Adhesive (wetting, viscosity)
  • Surface state of the substrates

Remedy:

  • Fine adjustment (programming)

Dosing too late, dosing too early:

Start of the bead application dependent on:

  • The viscosity
  • The pre-pressure
  • The distance of the dosing nozzle to the component

The control must “wait” until the start of the bead reaches the component.

Likewise, the dosing procedure must be stopped before reaching the end point.

Initial blobs

Cause: Pressure drop at the start of dosing

At the end of the dosing process the outlet valve closes and the feed pump stops. There then follows pressure equalisation along the length of the line. The moment the outlet valve is opened again, the adhesive there is under a higher pressure than during the rest of the application process, causing initially “too much” adhesive to be applied. The effect is in some cases intensified by delayed start of the robot.

The effect is also intensified by air in the system (inadequate de-aeration on changing the container) or hoses/lines having inadequate dimensional stability.

Remedy:

  • Minimise the length of line between the pump and outlet
  • Start of dosing during movement

Overhanging strands of adhesive at the end of the dosing:

Remedy: Snuff-back valve:

The closing action creates a slight vacuum in the outlet needle
→ The adhesive is drawn back into the dosing nozzle
→ Cleaner cut-off

Hollow adhesive beads

Cause: Entrapped air at too high application rates; especially for high viscosity adhesives

Remedy: Lower the application rate

“Wavy” beads

Cause:

  • Too large a distance from the dosing nozzle to the substrate
  • Possibly a contaminated dosing nozzle

Remedy:

  • Reduce the distance (reprogram the path)
  • Inspect dosing nozzles regularly and change if necessary