Potential problems with hot melt adhesive bonding
Surfaces come unstuck
- Cooling of adhesive during application preventing from formation of a strong bond. Bond may look like a high quality one, but may fail once force’s applied. Cooling of adhesive can be caused by one of the following factors:
- adhesive open time too short for application;
- application temperature too low;
- too little adhesive applied;
- air flowing around the application area;
- ambient temperature too low;
- materials to be bonded too cold.
- Contents of formed package influencing the bond:
- contents physically put additional pressure on the joint;
- chemical substances or solvents soften the adhesive joint;
- Uneven application causes high pressure on the adhesive joint.
- Pressing problems:
- insufficient pressure;
- movement of bonded surfaces during hardening of adhesive seam.
- Replacement of the material that the adhesive failed to bond:
- re-used material;
- more difficult material to be bonded;
- coated (wax, varnish) or impregnated material.
- Wrong choice of adhesive for this material and application:
- thermal stability of hot melt adhesive too low;
- poor resistance to low temperatures causing cracking;
- poor adhesion to these types of materials;
- residual heat inside of the package shifting the setting point by up to 24 hours in some cases.
Flap comes open when the box or carton comes out of the packaging machine.
- Open time too short – adhesive cools down before pressing.
- Adhesive overheated – bonding does not happen in time.
- Too much adhesive applied – adhesive does not set in time.
- Package surfaces poorly aligned which increases pressure on the «raw» seam.
- Ambient temperature too high – adhesive does not set in time.
- Temperature of surfaces to be bonded:
- too high – adhesive does not set in time;
- too low – adhesive cools down before the surfaces are pressed together.
- Poor pressing.
- Wrong choice of hot melt adhesive.
Stringing of adhesive from the applicator
- Application temperature too low.
- Draughts from open doors in the working area.
- Nozzle positioned too far from the application surface.
- Wrong choice of adhesive for this application method.
- Applicator malfunction.
Lack of hot melt in the applicator head
- Machine capacity insufficient to melt down the adhesive.
- Temperature of melting system too low.
- Low melting speed.
- Adhesive level in the tank went too low before re-filling.
- Voltage on the applicator too low.
- Adhesive blockage in the pre-melter or feed system; possibly due to wrong size or shape.
Excessive odor and vapor formation
- Adhesive overheated:
- can be caused by wrong settings.
- thermostatic control element may not be working properly.
- possible malfunction of heating element.
- adhesive tank may need cleaning.
Adhesive color change during operation
- Contamination of hot melt adhesive.
- Thermal degradation due to operation temperature being higher than recommended. This may have the following consequences:
- carbonization with formation of hard particles.
- carbon build up on the tank walls, feeding hose and nozzle.
- change of hot melt adhesive viscosity.
- hardening of adhesive in the whole system.
- nozzle blockage.