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Drying and Re-conditioning of Pre-twisted Boards Laboratory and Industrial Tests Report on WP 2.4 and WP 2.7 of the STRAIGHT project
SP - Sveriges provnings- och forskningsinstitut, Trätek.
SP - Sveriges provnings- och forskningsinstitut, Trätek.
SP - Sveriges provnings- och forskningsinstitut, Trätek.
2005 (English)Report (Refereed)
Abstract [en]

Twist is currently one of the main quality problems affecting softwood timber for construction. Several methods to minimise this problem were investigated in an EU project. The results for one method, drying boards in a pre-twisted position, are presented in this paper. It has generally been found that boards (Norway spruce) can be deformed towards straightness using this method. The tendency of a board to twist during drying is primarily dependent on the grain angle of the board. Laboratory tests have shown how much a board should be pre-twisted during drying to achieve a straight result. In practice, almost all studs tend to twist in the same direction. Pre-twisting an entire kiln stack equally gives a better average result than the conventional process. By measuring the grain angle for each board (“tracheid effect” with a laser beam), boards can be grouped and then treated “individually”. The force exerted by a board on its holders during drying in a pre-twisted position has been measured. The force decreases considerably during the heating phase, remains constant during the drying phase and decreases during cooling. When the variation in torsional stiffness as a function of temperature and moisture content is accounted for, it is found that creep deformation of the board occurs both during the drying phase and the cooling phase. Deformation due to changes in temperature seems therefore to take place in addition to mechano-sorptive creep. This has been proved by heat treating boards (wet or dry) wrapped in plastic to avoid changes in moisture content. Boards that still show excess twist after drying can therefore be “corrected” using a heat treatment process. It was also shown that boards exhibit a certain degree of memory effect. The wood material seems to remember processes which the board was exposed to at earlier stages. This memory fades over time. The results obtained in the laboratory have been verified by a series of industrial tests, including the testing effect of pre-twist on an entire kiln stack. Recommendations for industrial implementation and an economic assessment are finally presented. Key words: Twist, drying, pre-twist, grain angle, creep, torsional stiffness, memory effect, heat treatment, Norway spruce

Place, publisher, year, edition, pages
2005. , 48 p.
Series
SP Rapport, ISSN 0284-5172 ; 2005:14
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:ri:diva-4668Local ID: 5735ISBN: 91-85303-45-3 (print)OAI: oai:DiVA.org:ri-4668DiVA: diva2:962306
Available from: 2016-09-07 Created: 2016-09-07Bibliographically approved

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CiteExportLink to record
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Citation style
  • apa
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