Wood working09.12.2016

Drying veneers for gluing

Series of articles on plywood manufacturing, part 2

Veneers are fed into the roller dryer in sheets.

Veneers are fed into the roller dryer in sheets.

Plywood manufacturing involves the drying of veneer ribbon peeled from a log in massive drying machines to enable the gluing of the veneer into plywood. At Koskisen the veneer is dried using a roller dryer, which means that the veneer ribbon is cut into sheets after peeling. Another option for drying would be a mesh veneer dryer, in which the veneer is fed into the drying machine as a continuous ribbon directly from the peeling line. The benefit of roller drying is that flawed pieces can be removed from the process even before drying, in which case the veneer’s quality and drying capacity remain good. 

“We have four roller dryers, basically one for each peeling line and a reserve one for redrying and maintenance shutdowns; after all, the drying machines are cleaned thoroughly every two weeks or so for fire safety reasons,” says the plywood mill’s Production Manager Miikka Lehtinen

The veneers move through the drying machine in several layers along rollers, as the machine’s name indicates. The target moisture level after drying is around 6–7% which is precisely the right moisture level for gluing veneer.  

“A sufficient drying temperature is 175–180 °C and the veneers pass through the machine in about six minutes. Drying conditions and the moisture of the veneers are monitored continuously to make sure that the drying quality remains even,” Lehtinen describes the process. 

The drying machines consume the lion’s share (approx. 50–60%) of the heat production of the power plant located adjacent to the plywood mill, where the heat is produced ecologically using the mill’s by-products.

Dried veneer is sorted according to quality into compartments.

Dried veneer is sorted according to quality into compartments.

Veneer for a variety of applications

Towards the end of the drying machine, machine vision is used to sort the veneer according to quality into compartments as face and core veneers and further subclasses. This ensures the right veneer properties for customer-specific products.  

“For example, plywood needed for furniture requires beautiful veneers to guarantee an attractive surface. Plywood that will be coated, on the other hand, requires a solid surface and plywood that carries a load requires strong veneer,” Lehtinen illustrates.

Marina Keningi feeding veneers into the drying machine.

Marina Keningi feeding veneers into the drying machine.

Mill’s long-standing veterans

Although plywood manufacturing is often considered to be a male-dominated sector, there are a lot of women in the field too. At Koskisen, around a third of the plywood mill’s personnel are women. Marina Keningi, who works at the drying machine and in veneer sorting, has been with Koskisen for more than 25 years.  

“The best thing about working at the drying machine is the variety; during our shift, we switch back and forth between the feeding end and veneer sorting, every two hours,” Keningi says.

Paula Hongisto monitoring the veneer sorting at the machine’s dry end.

Paula Hongisto monitoring the veneer sorting at the machine’s dry end.

Paula Hongisto also praises the variety and independence that the job offers. She has worked in drying for 22 years and she applauds her employer for ensuring that employees have work despite the tough times. She explains that working at the drying machine requires precision and a good reaction speed, for example, to stop the line quickly if the veneers are not moving as they are supposed to. In addition, she emphasises the importance of work partners. 

“It’s easy to work at the drying machine when you’ve had the same partner for almost 20 years and you know the other person’s habits like the back of your hand,” Hongisto says.  

Veneer drying and sorting in a nutshell

  • -Veneers are sorted into different face and core veneers and their subclasses
  • -After drying, the moisture level of veneer is 6–7%
  • -3/7 shifts with approx. 30 people
  • -4 roller dryers
  • -50–60% of the mill’s power plant’s heat production used for drying