Dedrick, Dallas S.

Western Dry Kiln Clubs

Western Dry Kiln Clubs. Meeting (31st : 1980)

Undetermined

ScholarsArchive@OSU

SOME COMMENTS ON WARPINGDallas S. DedrickConsultantLongview, WashingtonDuring the past twenty five years it has been my good fortuneto have been continuously associated with the study of the funda-mentals of lumber drying. The efforts of these studies have beenrewarding since some of the findings of our drying team have re-sulted in substantial economic savings when they have beenapplied.However, from a personal standpoint, the most significantresult of the studies has been the ever-growing list of questionsconcerning drying mechanism and kinetics to which I do not knowthe answers. Two years ago before this group I recited an abbre-viated list of these questions. The paper was presented with thefrank admission that simply because I did not know the answers wasno valid reason to assume that nobody else knew the answers. De-spite considerable favorable comment concerning the presentation Ihave not been deluged with communications containing answers.One of the topics in that presentation related to the mecha-nism of shrinkage during change in moisture content below thefiber saturation point. Today, I wish to pursue this pointfurther as it relates to the warping of small log Western hemlockduring kiln drying. Unfortunately the paper will provide no newanswers---just more questions.It is not news to those of you who process Western hemlocklumber that a major problem related to the economics of kilndrying this species is the tendency for degrading warp to occur.It is also not news that the warping problem is accentuated asmore and more small log lumber is being processed. Neither is itnews to consumers of kiln dried hemlock that the product has atendency to warp further particularly under low EMC conditions andthat this tendency too is accentuated in the case of small logproduct.Warping is the change in geometrical configuration of apiece. Four types are generally recognized--crook, cup, bow andtwist. Crook is deviation from linearity of the narrow face ofthe board. Cup is deviation from linearity across the wide face.Bow is deviation from linearity of the wide face along the lengthof the board and twist is the deviation from a flat plane result-ing in one corner of the board not lying in the same plane as theother three.Anyone who has spent time around a dry kiln or a planer isfamiliar with some of the more severe examples--the ski, the chairrocker and the cup which causes planer split during later proc-essing.Warping during kiln drying is of course, due to unbalancedstresses which are set up and which react to actually deform theentire gross structure of the piece. Warping which takes placeafter drying is due to the relief of stresses which are residualwithin the piece. Much excellent work has been done in the past13to identify the mechanism of stress build-up during drying. Weare all familiar with the classical work performed by JohnMcMillen, U. S. Forest Products Laboratory, relating compressiveand tensile stress and stress reversal.Wood structure is highly complex even in the relativelysimple softwoods. Atoms are bound together chemically to formmolecules. Molecules are bound together to form chains or poly-meric structures which in turn combine to form micelles which aretied together to form the various components of the cell walls.The cells, in turn are bound to neighboring cells by some kind ofbonds. The early wood is bound to the late wood and on and on.Any volume or shape change within the gross structure must takeplace by the modification of the bonds which hold the affectedelements together. The point is that wood structure has structur-al integrity. The diagrams and models that we see frequentlysuggest that wood consists of fragmented particles and when waterenters the structure these particles simply become farther apart--like adding water to solid sand particles in the bottom of a cup.The sand particles simply get farther apart when the cup isshaken. If the water is evaporated away the sand particles simplyget closer and closer together until the water is all gone. Imaintain that this picture, although it shows that gross volumeincreases when moisture is being added and that gross volume de-creases when moisture is removed, is unrealistic and entirely toosimplistic.We have good evidence to back up the hypothesis that themajor volume change which takes place during moisture contentchange below the fiber saturation point takes place in the second-ary layers of the cell walls. In this structural element, thefibrils are aligned parallel to each other and the moisture thatis bound in the structure is positioned between these fibrils withthe result that the fibrils are pushed apart. In most softwoodsthe fibrils are aligned parallel to the bole of the tree so thatif any change in spacing between the fibrils takes place it willresult in a dimension change which is tangential or radial but notlongitudinal. However growth conditions may result in the treewhich result in laying down fibrils in the secondar y wall whichare much more perpendicular to the bole of the tree. The resultof shrinking or swelling due to moisture change is therefore muchmore longitudinal than radial or tangential. Western hemlock isa notable example. These growth characteristics in Western hem-lock are first the fact that small logs contain a high percentageof juvenile wood which is characterized by large fibril angles inthe secondary wall and secondly, there is a substantial amount ofcompression wood produced by the tree as it fights continually tostraighten up the bent-over leader which is characteristic of thespecies. The large proportion of juvenile wood and compressionwood in small log boards provides ample opportunity for atypicaldimensional changes during shrinking and swelling.The general reaction of wood substance to stress duringdrying is to relieve the stress by readjustment of bonds. Themechanism of stress relieval is postulated to be somewhat asfollows. The first tendency during shrinkage of a wood element isto change the shape of the element due to the characteristicswithin the wood that produces differential shrinkage. However,14there are crystalline elements within the wood which are not af-fected by moisture content change because no moisture contentchange takes place within these regions which tend to maintain thegeometric symmetry of the piece--that is to maintain the relativepositions of the various component elements--which direct thestress relieval principally toward bond breakage and reestablish-ment or, in the case of weak or insufficient bonds, toward inter-nal or external checking of the piece. As a gross generalizationit may be said that the greater the cross-sectional dimensions ofthe piece the greater is the tendency for stress relief to takeplace by means of bond rearrangement or by checking and that thesmaller the cross-sectional area of the piece, the greater is thetendency for stress relief to take place by means of warping. Ido not recall any timbers that I have seen which were significant-ly warped. However a piece of very thin veneer will react tostress relieval by severe geometrical change.Not only is mechanism of stress relieval important but thekinetics of the reaction appears to be of prime importance. Com-plete stress relief does not take place immediately after thestress is established. Rather in some cases the piece is filledwith residual stresses. These can be recognized by the immediatechange in shape when stress samples are prepared. There is ampleevidence that restraint during drying results in reduced warping.It is commonly noted that the incidence of severe warping isgreatest in the top courses. Ken Bassett, Research and Develop-ment at Weyerhaeuser Company, among others has reported theadvantages of mechanical restraint. Bruce Kuhnau, Research andDevelopment at Weyerhaeuser Company, has been recently granted apatent for a restraining device.However, there appear to be types of residual stresses insome pieces that are not immediately relieved by isolation suchas by making stress samples. In these cases stress relief isdefinitely time-dependent and the time-dependency is undoubtedlya function of temperature and moisture conditions within thepiece.If the crystalline regions in the specimen are not "strong"enough to direct the stress relief into bond rearrangement withoutchanging the shape of the piece aid can be provided by usingexternally applied restraints. These externally applied forcescombine with the internal "Forces" to direct the mechanism ofstress relieval.External restraint has been proven many times on many differ-ent species to have significant benefit in the reduction ofdegrade during drying. Even in kiln charges which have not beenphysically restrained the amount of warping and the severity ofwarping decreases from the top to the bottoms of the charges. Itis also significant that the greatest amount of warpage degradein restrained charges (both physically restrained top portionsand weight restrained bottom portions is due to crook, with twistand bow being much less prevalent and less severe. This can beexplained on the basis that bow and twist both represent verticalmovement in the charge whereas crook represents horizontal move-ment. Restraint of either type is applied vertically and insufficient number of places to counteract vertical movement in theboards. However the only restraint against horizontal movement is15the friction between the board surfaces and the surfaces of thecontacting stickers.Of course no restraint has value if the stock is mis-sawnsuch that there is no contact between boards and stickers.It has been further noted that restraint has an added advan-tage beyond reducing the amount of warpage during the dryingoperation. The amount and severity of warp during subsequentdrying under low EMC conditions is substantially less for boardsrestrained during kiln drying than for boards which were notrestrained. This fact suggests that the addition of externalrestraint during drying when added to the resident internal re-straint results in bond modification and realignment whichproduces less residual stress in the board when it leaves thekiln. There is therefore less stress to be relieved during sub-sequent drying.It is further noted that warp-free kiln dry lumber whensubjected to post-drying low EMC conditions develops substantiallyless warp if it is maintained in tightly banded packages beforeuse than if the material is kept in unbanded packages. This isfurther evidence of the value of restraint during drying.Thus far there have been no concrete suggestions concerningsolutions to the problem of warpage during kiln drying which arenot already well known. Nor will there be any solutions offeredby this writer because he does not know any positive answers.Some additional questions can be raised, however. There is con-siderable evidence that stress relieval is time dependent. Howlong does residual stress operate to produce warping? Mostimportantly, how can lumber be treated to reduce residual stresseswhich are normally produced during kiln drying.When considering these questions this writer is continuallyreminded of the process of bending of lumber. Here the procedureis reversed. Warp is created without residual stresses. How arethese bond modifications and realignments managed?During the past year or so I have been toying with theproblem of small log hemlock warping in an attempt to identifycauses for the very significant economic downfall which is be-coming increasingly important as the available logs become smallerin diameter.One of the approaches has been to cut longitudinal stresssamples as well as conventional stress samples from kiln dryboards and to observe the behavior of these longitudinally isolat-ed strips. Some of the results have been rather dramatic.The longitudinal stress sample technique results in theisolation of a different thin strip orientation and permits thedemonstration of stresses other than those shown by the conven-tional technique.Conventional stress samples isolate thin sections across thewidth of the board. They are invaluable for freeing up the resi-dent stresses that tend to make the board cup. Upon standing in alow EMC atmosphere the isolated segments continue to dry andshrink and thus become excellent indicators of the moisture con-tent gradient from shell to core which existed in the board whenthe stress samples were cut.The conventional technique is an excellent means for demon-strating what is commonly called case hardening and reverse case16hardening. If the surface segments deform immediately aftersawing it is evidence that the case hardening stresses arepresent.It should be pointed out that the greatest value of anystress sample procedure results when very thin sections are iso-lated. In a large section of wood the complex bonding ofstructural elements resists the full action of resident stresses.When the wood sections are very thin the stresses have greateropportunity to act. I am not aware of having observed substantialbow, crook or twist in large timbers as they dry in use. Rather,these large pieces react to shrinkage stresses by checking insteadof by changing geometric shape. A very thin strip of Douglas firveneer sliced from a green cant and allowed to dry shows theeffect of stresses which are not inhibited by three dimensionalinter-element bonding.This discussion is incomplete because it does not include anyinformation concerning moisture content distribution within thepiece when the samples were sawn. It is well known that it ischaracteristic of hemlock to contain wet streaks or pockets. Amost probable cause of these wet areas is the result of micro-organism infestation as reported by Ward of the U.S. ForestProducts Laboratory and confirmed by studies by Kozlik at theOregon Forest Products Laboratory. I am not aware of a definitiveconclusion concerning the effects of these wet streaks on productperformance in the field.The wet streaks show an unpredictable meandering through ahemlock board after kiln drying.It is my hope to have the opportunity to study the relation-ship between strip moisture content and strip reaction in variousstress samples.The obvious question in the minds of all of us is "So What?"What can the kiln operator do to reduce the costly downfall inquality in small log hemlock due to warping during drying? Per-haps someone knows the whole answer. I do not!It is obvious that the ultimate solution to the problem liesin the application of the drying procedure which results in therearrangement of inter-element bonds in the wood structure in sucha manner and at such a rate that the overall structure is atequilibrium at the lower moisture content or in use. Can this bedone economically in the kiln--and if so, how?Certainly the reverse of the situation is well known.Straight, stress-free boards are routinely bent into permanentlywarped structures which are completely stable. This can resultonly by controlled bond rearrangement. If permanent warping canbe produced in unwarped boards, perhaps the same concept of pro-grammed bond rearrangement can be applied to produce straight,stable stress-free boards from those with the tendency to warpduring drying.So where are we?Warping is a serious economic problem in kiln dry small loghemlock.More and more small log hemlock is coming to the mills forprocessing.Warping takes place as the lumber is dried.The kiln operator dries the lumber.17What can the kiln operator do to eliminate or to substantial-ly reduce the occurrence of warping economically?The challenge is great and the solution will be rewarding.18

Some comments on warping

https://ir.library.oregonstate.edu/downloads/cc08hg71p

Some comments on warping

Abstract

Similar works

Full text

Available Versions

ScholarsArchive@OSU