Our Members

Title
Logo Atlantic Veneer Corporation

Members login

Events

There are no up-coming events

Dimensional variability of wood based material and

interaction with digital and highly automated wood processing equipment.

 

Pierre Walsh LeBel

Director, Engineering, R&D

Perfecta Plywood

 

More than ever, wood manufacturers are acquiring computer assisted panel-machining equipment (CNC, double end tenoners, grooving machines,etc). The 1980’s computer revolution combined with the development of efficient servomechanism gave birth to a new generation of cutting tools and wood processing machines. Nevertheless, such equipments were initially developed mainly for processing metal, glass and plastics. Applications for wood based materials came later.

 

The challenge is that wood is characterized with an attribute called “the dimensional variability”. This characteristic was not common to the material for which these equipments were originally designed to process.

 

 

Hardwood plywood and furniture manufacturers need to adapt such equipment but also simultaneously learn to condition their raw material. They need to learn to cope with dimensional variability caused by warping, thick and thin, flatness, and shrinkage, which characterized hardwood plywood, when it’s sawn, machined, conveyed, transferred and manipulated within variable environments. Remember that wood is a porous and fibrous material such as textile. Whatever you do, it will tend to restore its hygroscopic equilibrium condition with the environment. In other terms, it will adapt to the surrounding air moisture content. This is the main condition responsible for shrinkage and warpage. Also each wood species has its own density which can even vary within a given piece of wood.

All these variable factors will definitely give a headache to a CNC machine operator if the equipment has not been selected specifically to adapt to the dimensional variability of wood.

 

The variability of wood is mainly caused by its “anisotropy[1] “a characteristic specific to certain raw material like decorative minerals (granites, marbles).

Wood anisotropy becomes important when it’s time to select, design or adapt computerized or automated wood processing equipment.

 

When selecting such equipment and their affectation to a given type of raw material (veneer, particleboard, MDF or plywood) three groups of factors have to be taken into account and carefully evaluated.

 

 

1 Dimensional variability of raw material

 

Manufacturers of computer-assisted or highly automated wood processing equipment should always consult their customers about the variability of their raw materials. The user must also pay special attention when measuring the variation of their panel characteristics. Typical examples are minimum-maximum allowance for thickness, width and length. Special attention should also be paid to “Extreme values” that this minimum/maximum dimensional variable can produce and at which frequency. Neglecting this aspect could lead to costly equipment failure.

 

The variability of the wood surfaces characteristics is another factor that has to be taken into account. Vacuum handling tools like robots, palletzer, etc, must be adaptable to the specific surface characteristic of each species. Repetitive test are essential on porous wood species such as fuma and/or wood with non-diffused porosity grain like oak and ash.

 

 

2 Expected tolerances of finished products.

 

Even if this factor is an integrated part of standard exchanges between a machine buyer and a seller, the dimensional variability of wood must be considered when setting expected tolerance. Often plywood manufacturers will limit the discussion to “expected tolerances” and neglect the impact of the variability of wood. However they are facing variability not only in their finished products but in their raw material as well. It is well known that the physical property of a given species of wood will vary depending on the growing condition of a tree. Ex: A birch growing on a steep terrain will adapt to gravity and wind. These conditions will produce a veneer with ‘’tensioned wood’’ which can lead to excess warping of a plywood panel.

 

 

3 Environmental conditions of warehousing and transformation.

 

Experienced woodworkers will say that wood needs to ‘’rest’’ before you start processing it. Today, automated wood manufacturing processes and computer driven equipment has shortened the manufacturing cycle. Conditioning period is now very brief. For this reasons end user shall consider the following factors.

 

  • Hygrometric equilibrium stability of the wood.
  • Stability of surrounding temperature.
  • Stability of air relative humidity.

 

The above variables do not create major problems in mild temperate regions. But in severe cold and dry winter conditions, shrinkage of certain type of species may be spectacular if stable environmental conditions are not respected.[2] Sudden moisture loss may generate cracks in veneer surfacing. Similarly machining and finishing plywood at very low or high temperature may be risky. Panels, which have travelled at –35 C during 12 hours in winter, may create the same problems as panels freshly pressed. Conditioning and “rest” is the secret.

 

Conclusion

 

Acquisition of CNC and computerized wood processing tools is different than the acquisition of conventional wood machining equipment. Conveying and handling component of these equipments were originally developed to process dimensionally stable material. Their performance with wood-based material could be deceiving.

However using these machines could be beneficial to our industry if we learn to deal with the environmental conditions impact, the veneer species behaviour and the variability of wood.



[1] Anisotropy: characteristics specific to natural materials like wood, where physical-mechanical properties are not uniforms in all directions. By opposition, material like glass, steel and most of transformed solids and/or reconstituted materials are “isotropic “in other words, their properties are similar whatever the direction.

[2] As an example, red oak, a relatively known stable wood has a tangential shrinkage of 8,6% compared with 18% for bamboo. Tangential shrinkage is fundamental in plywood face and back balancing.

rep_membres_en

infolettre_en