What could be the reason for PVC separation from the edges of the work piece after the press operation?

In the new generation presses, the PVC is heated by the high temperature elements. The thermometers which are installed on the press machine shows the surface temperature of the MDF. While there is a temperature difference between the edges and the surface of the MDF. For example, in some cases the machine temperature indicators show the temperature of the MDF surface 90 °C, while the edges temperature is 40°C. In fact, the edges temperature is lower than the adhesive activation temperature that causes the PVC separation from the edges. It is recommended to use a laser thermometer to measure the temperature of the edges and set the elements temperature according to it.

Why does the workpiece surface become inflate after vacuum pressing?

The main cause of the inflating is the presence of water molecules on the surface of the MDF. Therefore, before spraying the adhesive it must be shaken several times to obtain a homogeneous solution.

The using pistol nozzle should be 1.2 to 1.8 mm in order to ensure uniform spraying and preventing adhesive loss. The edges should be sprayed twice and make sure that the adhesive thickness won’t be excessive.

The waiting time for drying the adhesive varies in different areas, but it should be such that no moisture felt by touching the surface.

What should be done to improve adhesion quality?

It is best to sand the MDF surfaces before gluing, and then use air pressure to clean the surface and parts of the tool from the wood pollen.

In some cases, in the preheating operation, the PVC veneer becomes flexible which causes wrinkles on the surface of the workpiece. As a matter of fact, it should be considered and it is better to blow air by reversing the pump operation under the PVC veneer to prevent covering the workpiece and enhance the quality of coating.

In vacuum machines there is no compressed air pressure from above and only vacuum is provided from under the tray. After vacuuming the PVC, the surface of the veneer must be cooled. Because the memory effect of the veneer makes it to expand as a result it will return to its original state. In professional machines cold air blows between the veneer and the upper section, and in the workshop or vacuum machines usually a fan has been embedded above the drain tray. In the summer, the condition of the veneer is stabilized by a wet wipe and fan on the work surface.

One of the most important factors is  an optimal PVC laminate adhesion which is provided with a primer coating. The primer is usually white in color on the back of the PVC coating that is applied by the manufacturer and acts as the intermediate ring between the adhesive and the PVC. It gives the polarized state to the coating to create suitable and long-lasting bond between the work pieces..

Gluing conditions

  • Spray the edges and work surface carefully (usually gluing the edges twice.)
  • Allow the adhered surfaces to dry completely. Examine the glued areas with a UV lamp.
  • The using amount of the past generation of adhesives has decreased from 130 to 90 g/m2 and in single component adhesives this amount has decreased to 60 to 80 g/m2.

 

 

 

Discharge conditions after press:

  • Separate panels to cool.
  • Carefully clean the corners of the overlay.
  • Cut boards from the inside
  • Clean the glue sprayed on the lower part of the panel with a damp cloth.

Press cycle:

Adhesives require heat to activate:

  • PVC > 60°C
  • Polyolefin Coated (PP) > 90°C
  • Press time between 60 and 90 seconds
  • Press cycle time between 150 and 190 seconds