Crack formation and warped products are the scourge of curved plywood producers. The problems often go undetected until the product has reached the customer, by which time considerable claim costs are involved. To counteract this problem Casco Adhesives, along with a number of Swedish companies in curved plywood, has participated in a research project spanning several years at the Department of Forest and Wood Technology, Växjö University.
In recent years Swedish curved plywood producers have seen major changes in the market, and large production series are a mere memory. Production now tends to focus on small series of 100 to 1,000 details, and the products are becoming increasingly complex in terms of shape and structure. At the same time there are increasing demands for short delivery times, more product versions and a focus on price and quality. As a result, demand for high output of the best product in production is necessary.
The project has identified several factors that contribute to a lack of shape stability in curved plywood products, cross grain and moisture deviations being the most important causes. Incorrect layering of the veneer, which results in different moisture ratios in a veneer "batch", caused many of the randomly occurring rejects in seat shells. Veneers with different moisture ratios are glued into a structure, and when the moisture evens out, tensions arise, which lead to deformation.
This is particularly evident when the veneer has cross grain. Cross grain, i.e. when the wood fibres do not lay in the same direction as the length wise direction of the veneer, causes increased shrinkage and swelling in the finished product, in directions where this is not wanted. The degree of cross grain varies within the same type of wood, although some woods have more tendencies to cross grain than others. The incorrect layering of veneer in a structure can also cause the same negative effect as cross grain.
The apparent increase in shape stability problems seems to be due to the increasing use of very "low quality" veneer in glued structures. These veneers often have a haphazard fibre structure. Cross grain mainly leads to warping of the product, but in some cases it can also cause cupping. In some circumstances the deformation is so severe that the final product is not fit for use. Different glue systems have also been tested, and the perennial question of whether it is better to use a powder or a liquid hardener to achieve shape stability in the product has now been answered (see figures).
The practical difference between the hardening systems is marginal, and the following conclusions could be made from the tests:
Increased moisture content in the glue does not necessarily produce increased warping in the products.
The choice between powder or liquid hardener is not crucial to shape stability. The collaboration between Casco Adhesives and Växjö University is continuing, even though the project financed by the companies and the Knowledge Foundation has come to an end. For further information, contact Niclas Berg or Micael Manford at Casco.