A Developmental Study of Drying System Applying RF-Heating/Vacuum Method to The Boxed-Heart Heavy Timbers Like Logs.

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In Japan, sugi is very often used as a boxed- heart timber of log or square column. It is difficult to dry these timbers in a short time, as having a big cross section and large aria of heart wood. It connects with the increase of the drying cost. And the severe checks occur easily for the boxed heart timbers of sugi during drying , because of anisotropic shrinkage. RE/vacuum drying method is wellknown for an effective means of rapid and checkless drying. But When the moisture distribution became approximately uniform after drying, surface check often occurred.In this research, measurment of internal stress and drying set was performed on the sugi log and boxed-heart square timber dried with RE/vacuum method and it's combination with heat treatment or transpirational drying. As a result of these investigations, it became clear as follows. (1) These stresses of Sugi logs dried with RE/Vacuum method were relatively large tension , when the moisture distribution in the cross section became approximately uniform after drying. By the heat treatment of 80ﾟC for 1 day, these large tensile stresses at the surface layer disappeared and surface checks were improved. (2) The conbination method of kiln- drying and RE/Vacuum drying improved these drying time and surface check for the boxed-heart square timbers. It seems that one of the reason for surface check decrease is drying set made at the former stage of kiln- drying. (3) By the transpirational drying, moisture content of only sap wood was decreased. So the moisture contents of outer part in the cross section of the boxed-heart square timbers became approximately lower than inner part during RE/Vacuum drying, and surface checks were decreased comparing with untreated.