T5 870 UC-NRLF U. S. Department of Agriculture, Forest Service FOREST PRODUCTS LABORATORY In cooperation with the University of Wisconsin MADISON, WISCONSIN NOTES ON THE MANUFACTURE OF PLYWOOD Revised September, 1922 Aerie . - Forestry . M ain Library NOTES ON THE MANUFACTURE OP PLYWOOD By manufacture of plywood is meant the gluing t together of plies of wood, usually an odd number, so laid that the grain of alternate layers is approximately at right angles* In three-ply panels the outside plies are referred to as faces and the center ply as a core. In a five-ply construction the outside layers are faces, the next two plies in order cross~bands and the center ply core. The details of gluing, pressing, and drying plywood are not standardized, but these notes on manufacture are "based upon observations of factory practice and upon exten- sive experiments at the Forest Products Laboratory. Gluing Panels in General The core or crossbands of the panels are coated with glue on a glue spreader consisting of two rollers, the lower one revolving in a bath of glue and the upper one sometimes being coated from another bath of glue retained > between the upper roller and a trough with its lower edge almost touching the surface of the roller. Scrapers or the pressure of the rolls regulate the thickness of the glue layer as desired. L26 519081 54-3 -i The panels are built up and placed in the press as soon as possible, The actual time consumed "between spreading glue on the first panel and the application of pressure to the stack of panels varies greatly in different factories from a few minutes to a half hour or longer. The panels are usually grouped with three to five in a set with caul "boards placed between sets and at the top and bottom of the entire lot before pressure is applied. The panels are usually left under pressure for 7 to 24 hours and then taken to the drying room. Shorter pressure periods can be used in some cases when necessary. Two different methods are used rather extensively in applying the pressure and. maintaining it on the panels The one perhaps most commonly used consists in applying the pressure by a hydraulic press and then using retaining clamps to keep the panels under pressure* The hydraulic press is usually equipped with a pressure gauge which shows the amount of pressure applied. The panels are left in the hydraulic press just long enough to apply the proper load and fix the retaining clamps in place. The bundle of panels is then re- moved on a truck to an out of the way place in the factory where they are usually left until the next day. L26 543 .2- By the other method the panels are placed in presses and left until the glue is set, !hese presses are usually of the hand screw type with no means for measuring accurately the amount of pressure applied, V/i th glues of the blood albumin, type, demanding the use of a hot press, the glue is applied as in the case of cold glue and the panels placed one or more at a time in the hot press. Pressure up to 200 pounds per square inch and temperatures of 212 degrees Fahrenheit or more are usually applied. The panels are kept in the press until the glue is set, varying from two to several minutes, depending upon the thickness of the plies and the panels and the ty^e of cauls used. Calculation of Pressure in a Hydraulic Veneer Press For plywood manufacture determination of the correct amount of pressure to use is important. Results of experiments at the Forest Products Laboratory indicate that an excess or a lack may^ produce a weak glue joint, The exact amount of pressure to apply per square inch of panel surface varies with a number of conditions. However, pressures of 75 to 1JO pounds per square inch of panel sur- face are ordinarily within the range of which good results may be obtained with most glues and under average factory conditions. When animal glue chills in a cold room before L26 54-3 pressure is applied, very he^vy pressures are then neces- sary to secure strong joints. In this case pressures of 400 pounds per square inch or more are required, The determination of the amount of pressure ap r per square inch of panel on a Iiyo.rau3.io press ecu.lpped with a pressure gauge is simply a matte:.- of calculation,. 'Vith any hydraulic press, thi3 depends upori three factors: The area of the panel, the area of the p. I:-: ton or ram of the press, and the pressure gauge reading,' 5 * The area of the piston in square inches multiplied "by the pressure gauge reading in pounds is approximately equal to the total preb. sure exerted "by the plates. The tot'al pressure ex-., erted divided by the area of the panel in square inches gives the pressure secured on the pare], in pounds per square inch. The pressure gauge reading musit. therefore, vary with the size of the panels if the same amount of pressure per square inch is to "be secured, I'cr example, if the same gauge pressure is applied on panels .10 by ']6 inches as o:.i others 36 by 40 inches the pressure -;: ] 1 >>e "approximately four times as great per square inch :ln ~,ho first case as in the second. It is assumed of course that the gauge is in good repair and indicating the pressure correctly. Pressure gauges should "be checked occasionally to see that they are correct \ L26 $43 -4- Prom the considerations above the following formulas for the calculation of pressures are derived! P = P 1 '* A' (1) * A or pr . P x A (2)* A Where P = gauge pressure in pounds per square inch. P'z pressure on panels in pounds per square in. A s area of piston or ram in square inches. A'r area of panel 'in square inches. To illustrate the use of the formulas let the fol- lowing case be assumed: On a hydraulic press with 10-inch piston what pressure gauge reading is necessary to secure 75 pounds per square inch on panels 24 by 48 inches? Use formula (l) * To determine accurately the exact prossure secured the weight of the platen to which the pressure is applied must "be taken into consideration. For practical purposes, however, it may "be omitted from the calculation-, as inaccuracies in the gauge reading, etc., may account for much larger errors. When the 'weight of the platen is taken into consideration the formulas "become: P ~ P* x A*- plus or minus W (3) A A or P f - P x A minus or plus . (4) A' A' Where W r weight of the lower platen plus weight of panels or upper platen alone, as the case may be* The sign of the last member of equation (3) is plus when the pressure is applied by the lower platen and minus when applied by the upper. In equation (4) the reverse is true, L26 543 -?- - r \ -' P = P' x A' A Here P : gauge reading required ?' ^ 75 A ? E 24 jr 43 or 1152 (area of panel) A ~ 3, 141 6 x jk or 7854, the area of the ... piston*-.. Thus P :; Z.L-2--JL4-5.2 or H30 The required gauge 78,5-- reading. A table, showing gauge readings to "be used for all sized panels manufactured and for the different pres- sures used, can "be computed and placed r.ear the press ?;here the operator may see at a glance the amount of pressure required on the gauge reading for each run of panels. Drying Panels take up a good deal of moisture in gluing and after coming from the press are usually placed on stick- ers and run into a kiln or left at room conditions for final drying* Dryingunder room conditions is blow arid, "because of the space required, is expensive, The use of panel kilns has "become more common of late years. The necessity for quick drying of panels in connection v/i t.h aircraft work has aided considerably in this development,, The area of a circle is equal to. 3.1416 times the square of the radius. L26 543 -6- Results of experiments in kiln drying aircraft panels have indicated that the essential requirements of minimum injury to the material, a sufficiently rapid dry- ing rate, and convenience and economy of operation can "best Toe met "by maintaining a constant temperature of about 120 degrees Fahrenheit and a constant maximum rela- tive humidity which will permit the stock to dry down to the final degree of dryness desired, but which will not all'ow an appreciable amount of further drying "below the required moisture content. The use of constant tempera- ture and humidity conditions which will dry the panels to a definite moisture content makes the drying simple, safe, and easy. For panels, of three and five-ply veneer or of veneer faces and cros standing and a thick core and glued at a low moisture content, drying at 120 degrees Fahrenheit and the necessary maximum humidity may "be accomplished in from a few hours to over night,, Temperatures above 120 degrees Fahrenheit have the advantage of decreasing the drying time but are more liable to lowsr the quality of the panel "by inducing checking, warping, and open joints, Panels dried from a high to an excessively low moisture content are very liable to warp and should be dried rela- tively slowly. L26 543 -7- Gluing; "by a Dry Glue Proc esjs. In connection witli aircraft work a process of laying veneer with dry blodd glue was worked out at the Tores t Products Laboratory, " It consists of making an .adhesive by coating a tissue paper with "blood albumin glue, allowing it to dry, and using the coated paper as the glue . layer for plywood* Ths sheets of glue are alternated with sheets of veneer until the required number of plies is obtained and the whole is pressed in a hot press. The use of the dry glue enables the construction of plywood with the addition of little or no moisture. Veneers as thin as 1/lpO-inch can be glued successfully into thin sheets of plywood orpanels. . ' The process can be used with thicker stock but appears to be adapted especially to fancy, cross -grained, and very thin veneer. It should also prove successful with a number of other adhesives than blood glue. The method eliminates a number of troubles result ir.g from the use of wet glues, such as checking, warping, open joints, and overlaps, and makes the handling of thin iiaterial much easier. * At least two patents on similar process have been dis- covered since that time, one a British patent ITo. 17,32? issued in 1902 and the other U.S. patent No. 1,299,74? issued in 1919. The value of these patents' is not definitely known, but it is suggested that anyone interested in producing this material should familiarize himself with them in order to avois infringement. L26 543 -8. Factors Affecting the War-rin^ of Plywood S ymni e t r i c al Construct.! on , On account of the great difference in shrinkage of wood in the direction parallel to the grain and perpen- dicular to it r a change in moisture content of plywood will inevitably either introduce or .'-..relieve internal s.tresses. Take, for example, a three-ply construction and subject it to low humidity conditions so that the moisture content of the plywood is lowered, Because the grain of the core is at right angles to the grain of the faces, the core will t:end to shrink a grea,t deal more than the faces, in the direction of the grain of the faces - This shrinkage subjects the faces to compression stresses and the core to tensile stresses. If the faces are of exactly the same thickness, of like density and otherwise "balanced the stresses are symmetrically distributed and no cupping should ensue, Now let itbe assumed that one face of a three- ply panel has "been glued with the grain in the .same, di- rection as the core, and that the mois'tare content of the panel is reduced. It is obvious that the internal stresses are now no longer symmetrically distributed, inasmuch as the compressive stresses in one face have been removed. This face now shrinks a great deal more than the other face in the direction of the grain of the latter. The result is that cupping takes place. 126 54-3 -9- The necessity for exercising care in sanding the faces of a panel is obvious, inasmuch as different thick- 4 nesses on the faces would, introduce unequal forces with changing moisture content and produce distortion. In order to obtain symmetry it is also necessary that "both faces of symmetrical plies "be of the same species, or species of approximately the same properties > To summarize! A plywood panel to retain its form with changes of moisture must "be symmetrically con- structed. Symmetry is obtained "by using an odd number of plies. The plies should be so arranged that for any ply of a particular thickness there is a parallel ply of the same thickness and of the same species o:c properties on the opposite side of the core and equally removed from the core. Direction of the Grain of the Plies b In the discussion of symmetry of construction it was understood that the successive plies were always glued with the grain either parallel or exactly at right angles to the core. In careless construction this may not always be the case. Tests have shown that deviations as small as five degrees from the standard 90-degree con- struction may introduce considerable twisting. L26 54-3 -10- Jri building up a three-ply vsieer panel the core- should, "be glued with the grain at 90 degrees with the faces, or as close to this as feasible, Mai H t ur e C c n t r ol The previous discussion brought out the fact tha a change in moisture content of a panel may intro- duce cupping and tv/i sting in the panel if it is not care- fully constructed. Hence it is highly desirable that all plies "be at about the same moisture content before glu- ing, and that the moisture content of the panel when it leaves the drying room should be about the same as it will average when in use. The limits of from 10 to 15 per cent moisture in the finished panel will usually give satisfactory results when the panel is in service in the open air v For use in heated buildings, at least a part of the year, as in furniture a somewhat lower moisture con. tent of sevenrto eight per cent will ordinarily give best results. Veneer for furniture panels should generally be low in moisture, so that when removed from the press the moisture content, increased by the moisture from the glue, will be as near as possible to that required for factory use. For fancy, cross-grained veneer the gluing at low L26 54-3 -11- moisture content is of particular importance, since dry- 4t ing of the panel frequently results in checking of the face, Non-water-res is tnnt glues also require the use of dry veneer, lout with, casein and "blood glues it is possible to use wet veneer. Good strength and water resistance can be ob- tained with these water-resistant glues with veneer at a moisture content as high as 50 per cent,, For maximum wet strength of plywood the veneer should be glued at a moisture content which would bring the panels to about 3 P er cent or more when removed from the press. The high moisture content of the panels made in this way necessitates con- siderable care in drying if a good quality panel is to be secured, and it is 1 improbable that much use can be made of this method of gluing in furniture manufacture. It offers possibilities, however, for cutting cos'cs in the manufacture of articles where glue strength, water resistance, or speed of manufacture is more important than appearance, Relation .of Densj.tv-_ of Veneer to Warping Numerous tests have shown that the warping of ply- wood panels when subjected to varying moisture contents is least for the panels made of low density veneer, such as . basswood, poplar, and cedar, and that, in general, warping increases with increasing density,, L26 54-3 -12- Effect on Warding of Increasing the Ratio of the Core to the Total Plywood Tliicknes.3. A high proportion of core to total plywood thickness helps to maintain a flat unwarped surface. In general, the core should comprise 5/10 to 7/13 of the total thickness of the panel where flatness is an impor- tant consideration. Of three-ply panels having cores of the same weight the panels having cores of low density, such as poplar and "basswood, will, in general, show less warping than the panels having high density cores, such - as maple and "birch. L26 54-3 -13- THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW 1 AN INITIAL FINE OF 25 CENTS WILL BE ASSESSED FOR FAILURE TO RETURN THIS BOOK ON THE DATE DUE. THE PENALTY WILL INCREASE TO SO CENTS ON THE FOURTH DAY AND TO $1.OO ON THE SEVENTH DAY OVERDUE. FEB 9 1935 FEB 23 1935 a 4 1 MatTSOMW ----- r- --- -MOV- DEC 1 9 1950 OEC5 G7 1 2 LOAN DEPT LD 21-100m-8,'34