April 10, 1922 



Hardwood Record — Veneer & Panel Section 



39 



Drying and Dry Storage of Plywood Materials 



By C J. M. 



There is probably no other branch in the woodworking industry 

 v?^here correct moisture content of the raw^ wood materials as well 

 as of the built up product, is of so great an importance to success, 

 as in plywood manufacture. 



Much unnecessary disappointment and monetary loss will be 

 avoided once the moisture conditions are fully understood and 

 given the same painstaking attention which must be paid to adjust- 

 ment of machinery or other exacting operations in the manufacture 

 of these same plywood products. 



Since it is the purpose of this article to deal ■with the moisture 

 and drying problems only, to the exclusion of construction or glue 

 problems, these latter are eliminated from the discussion. 



Veneer Easily Changes Moisture Content 



Wood is a hygroscopic material, and thin veneers present a 

 maximum of surface with a minimum of substance, and therefore, 

 if exposed to humid air either in transit or in storage, will, regard- 

 less of any previous drying, quickly adjust their moisture content 

 to a balance with that of the surrounding air; and the intensity of 

 this moisture absorption will depend upon the length of exposure 

 and the relative humidity of the air. 



A manufacturer of veneers may sell his product as of a certain 

 dryness or moisture per cent, and the buyer may find the actual 

 moisture content far in excess of the seller's statement when he is 

 ready to use the material, after transit and storage. Yet the seller's 

 statement may have been strictly based on fact. 



The only certain and safe method is to ascertain the exact mois- 

 ture content of veneer shipments, immediately on arrival, and if 

 not sufficiently low, to correct this moisture content before these 

 veneers are placed into production. In many cases a reasonably 

 short time in dry storage may prove sufficient. Thin veneers, pro- 

 perly spread, will give up moisture quickly, and for the same reason 

 w^hich causes its ready absorption, to-wit, extreme surface w^ith 

 minimum substance. 



Uniform Moisture Content Des!red 



In the case of thin veneers a very low moisture content may not 

 be essential, but the moisture content of the materials used for the 

 different plies of the same piece should be as nearly identical as it 

 is possible to make it. If the moisture content of the different plies 

 is identical, then the glue moisture will diffuse more uniformly into 

 the veneer, and stresses due to unequal swelling and subsequent 

 shrinkage w^ill be avoided, particularly if re-drying after glueing 

 becomes necessary. 



Of greatest importance, however, is the correct dryness before 

 gluing, of the core stock in the construction of the heavier panels. 

 It is a sad mistake to depend upon the re-drying operation to rectify 

 what the kiln should have done in the first place. 



As a rule cores are made of coarser grained species, which 

 absorb the atmospheric moisture more readily and likewise the 

 glue moisture. If the corewood is of sufficient thickness and dried 

 to about 5 per cent before gluing, it may safely absorb all of the 

 glue moisture without physical change, and it may retain this glue 

 moisture without necessity for further drying. There w^ill be no 

 sw^elling, and consequently no subsequent shrinkage to affect the 

 face veneer. 



Something Must Let Go 



If, on the other hand, the core is fairly heavy but insufficiently 

 dry (8 to lU per cent) before gluing, its moisture content will be 

 augmented by the glue moisture, which will increase the expanded 

 condition of the fibres, and something must let go when dryine and 

 shrinkage finally takes place, either in the re-drier or in dry storage. 



Moreover, if the corewood is insufficiently dry before manu- 

 facture of the panels, its subsequent drying out will be found a 



more difficult and slow problem, because the glue and the denser 

 face veneers will delay the transfusion of the core moisture to the 

 surface of the panel. The thicker the core the more uraent be- 

 comes the necessity for its low moisture content, before it enters 

 into the construction of the panel, and the more important becomes 

 pre-drying rather than re-drying. The former is preventative, the 

 latter merely corrective. 



The foregoing only emphasizes the necessity for systematic and 

 correct testing of all materials for moisture content before manu- 

 facture, as a preventative measure; and, to go further still in a 

 preventative sense, all materials should be tested for moisture con- 

 tent immediately on arrival and their moisture condition recorded 

 in systematic manner, in order that materials which are insuffi- 

 ciently dry may be so disposed that the same shall become of 

 sufficient dryness before use, and other materials which may have 

 the correct moisture content, shall be so preserved, without further 

 absorption of moisture. 



Ample Dry Storage Facilities Profitable 



The most practical and profitable assistance in the solution of 

 the drying problems of a plywood plant or plant using plywood, 

 will be found in an ample dry storage capacity, with air, condi- 

 tioned to correct temperature and humidity, mildly but thoroughly 

 and uniformly circulated, in a good building of airtight construc- 

 tion, where w^indows are always closed, and doors are opened only 

 when required to pass men and materials. Such a dry storage 

 space can be held to uniformly counterbalance a moisture content 

 of 5 or 6 per cent in the materials stored therein, and when fresh 

 materials of a higher moisture content are properly disposed in 

 such a space, the same will gradually give up their moisture dow^n 

 to the point for which the temperature and humidity of the air is 

 conditioned. 



Such dry storage, ahead of the production, will save much han- 

 dling labor, and probably considerable re-drying in special dry 

 rooms, if of ample capacity and properly operated. 



The temperature of such a dry storage space can be held com- 

 paratively low, approximately 90 degrees dry bulb, with about 25 

 to 30 per cent relative humidity, which would correspond to 65 

 to 67 degrees wet bulb temperature. A mild but positive and 

 uniformly thorough circulation of the air would be essential, to 

 be maintained by a fan blower, which continuously re-circulates 

 the w^arm air w^ith a small addition of fresh air. The warming coils 

 can be massed at the intake or the discharge of the fan, outside 

 of the storage space, and the effect of direct radiation upon the 

 stored materials w^ill be thus prevented. 



Dry Storage for Temporary Uses 



A similar dry storage space would prove equally as valuable for 

 temporary storage of the glued products and would in a large 

 measure eliminate the necessity for passing the same through spe- 

 cial re-drying rooms, especially if particular attention is paid to 

 the proper condition of dryness of the materials before manu- 

 facture, and if the glue moisture only is to be removed. 



In special re-drying rooms for glued plywood a positive and 

 uniform air circulation is as necessary as heat and low humidity, 

 only the more so. If glue moisture only is to be removed, the 

 amount of heat to vaporize this will be small, and the temperature 

 need not be very high, but to obtain reasonably speedy and uni- 

 form results the heat must be carried continuously and uniformly 

 to all wood surfaces by the air in circulation over same. This 

 result can be obtained only by proper sticking and an ample 

 volume of air in circulation at the right velocity. The heated air 

 can be re-circulated continuously v/ith but a small amount of fresh 

 air added to replace an equal amount thrown away, in order to 

 hold down the humidity. 



