Measures for improving quality and shape stability of sawn softwoof timber during drying and under service conditions:Best Practice Manual to improve straightness of sawn timber

In the EU funded STRAIGHT project, different processing methods (drying, sorting, conditioning and re-engineering) were investigated in order to minimise the distortion of sawn softwood timber. This Best Practice Manual summarises the main outcome from the study and was primarily written for industry. This manual describes the methods studied, and the advantages and disadvantages they bring to the sawmilling process and end uses of sawn timber. The methods for improving straightness are ranked according to a distortion acceptance percentage measured over the central 2000 mm portion of structural batten, when straightness is the main criteria (twist 4 mm / 100 mm, bow 4 mm, spring 3 mm & cup 2 mm / 100 mm). Other important criteria included in each assessment is extra drying and handling costs, the impact of using different methods on other factors of quality and the improvement in the saleability of material from normal sawmill production. The methods studied were a) pre-sorting of logs according to the angle of spiral grain, b) twisting small diameter logs during sawing to counteract natural direction of twist, c) re-engineering boxed-pith battens using green gluing (splitting battens along their length and re-engineering whilst "green"), d) twisting the drying load in the opposite direction to natural twist (the support sections on the kiln wagon were angled to counteract the normal direction of twist), e) top-loading of the kiln, f) oscillating drying schedules to introduce mechano-sorptive creep to reduce twist, g) high-temperature drying and finally, h) new conditioning techniques where dried, twisted timber was re-stacked and stickered on angled supports to promote opposite twisting during special conditioning. It was found that the best straightening results were achieved by the re-engineering and green gluing of boxed-pith battens before drying. The most cost-effective method of improving straightness of timber was top-loading. How cost-effective the opposite twisting will be depends on the practical solutions to be developed. The main disadvantage of top-loading and counter-wise twisting during drying was the amount of "spring back" which occurs when timber is subjected to gradual changes in environmental conditions. These methods of drying are deemed suitable if the timber is kept under pressure or used soon after drying or re-engineered so that the spring-back effect is reduced