Parallel-Laminated Veneer: Processing and Performance Research Review
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Parallel-laminated veneer: processing and performance research review Theodore L. Laufenberg manufacturing cost was justified by its dependable per- Abstract formance. Since that time, dwindling supplies of high- The use of parallel-laminated veneer (PLV) for quality saw logs have made PLV attractive and eco- critical structural elements has proven commercially nomically feasible in products formerly constructed of feasible for more than a decade. The uniformity of this the highest grades of solid wood. material’s mechanical properties has made it popular One attribute of PLV processing is the increased for truss components, I-beam flanges, scaffold planks, yield made possible by peeling a given log, as compared and other engineered members. to the kerf losses associated with traditional sawing Research during the last 15 years has indicated a techniques. Its highly dependable design strength number of correlations between processing param- makes it competitive with stress-graded lumber. eters and PLV product performance such as veneer Strength is achieved mainly because, when the mate- quality influence upon tensile and bending properties rial in a low-grade log is reconstituted into PLV, and the efficiency of various jointing methods. In ad- strength-reducing defects are distributed throughout dition to summarizing these findings, this paper con- the volume of the member, minimizing the amount of tains recommendations for further study. low-strength wood in any cross section. The variables that must be taken into account in predicting the mechanical properties of a PLV product include the number of veneer laminations, the size and distribution of defects, the efficiency of veneer end This paper presents an overview of the parallel- joints, the quality of the bond, the strength of the clear laminated veneer (PLV) processing and performance wood, and the depth and frequency of knife checks. technology developed and reported on in the last 15 Accurate analytical models for predicting mechanical years. The variety of PLV’s product applications (not properties have yet to be published, but stiffness tests documented herein) provides an indication of its gen- (stress-wave timing) and visual veneer-grading eral appeal. As our raw material supply diminishes in methods are used to nondestructively bracket mechan- quality and volume, PLV’s higher yield potential and ical properties. Increasing the number of laminations in adaptability to engineered end-use design will be wel- a specific configuration creates a more uniform product, comed by manufacturers and users alike. but increases production costs. Approximately 10 years have passed since PLV Processing PLV products first appeared in the marketplace, in trusses, Basic processing schemes as substitutes for select structural lumber components. Nearly all schemes presented to produce PLV for Since that time, PLV has been used for joists, box structural use somewhat resemble those of established beams, planks, and ladder rails. plywood operations. Veneer is rotary peeled, dried, PLV is processed in a manner similar to plywood, spread with adhesive, laminated in the desired con- but contains only parallel laminations. Also called laminated veneer lumber (LVL), its machinability and uniformity of mechanical properties have been appreci- ated by the furniture industry for several decades to The author is an Engineer, USDA Forest Serv., Forest produce curved furniture parts. Prod. Lab., P.O. Box 5130, Madison, WI 53705. The Laboratory is maintained in cooperation with the Univ. of Wisconsin. This Parallel-laminated Sitka spruce veneer was stud- paper was received for publication in December 1982. ied in the 1940s (30) for constructing high-strength © Forest Products Research Society 1983. wood aircraft members. At that time, its additional Forest Prod. J. 33(9):21-28. FOREST PRODUCTS JOURNAL Vol. 33, No. 9 21 figuration, pressed either in conventional plywood of joints by 16 lamination thicknesses prevented stress presses or on a continuous or step basis, then ripped to interaction of adjacent butt joints. width. Processing innovations have been developed Tensile failures at butt joints were shown to differ largely to accommodate the performance requirements significantly (21) in 1/4-inch veneer and 1/10-inch ve- of specific end products. neer PLV. A 1/10-inch veneer joint failed from its outer Continuous pressing of PLV is currently used in butt joint straight through the cross section of the mem- commercial manufacture of MICRO=LAM® (22) and ber, while the 1/4-inch failure turned 90 degrees from by Metsaliiton Teollisuus Oy, Finland, for producing the outer butt joint and followed the bondline to the next Kertowood®2 (14). Descriptions of these processes are butt joint. More study of butt joint mechanics is needed unpublished, except for the use of phenol-formaldehyde to quantify the influences of veneer thickness, joint resin and 1/8- to 1/10-inch veneers. spacing, and veneer quality on the performance of butt To produce its Press-Lam®, the Forest Products joints in PLV products. Laboratory (FPL) rotary peeled veneer, press dried it, Lap joints.—The crushed lap joint (Fig. lb) is spread the sheets with adhesive, hot laminated them currently used on the commercial product with staggered butt joints, and step pressed them in a MICRO = LAM®. No performance research data have cold end loading press (12, 13). One major drawback of been published other than code-approved stress values this process was its reliance on residual drying heat in (33) for products in which lap joints are staggered order to cure the adhesive; this necessitated short as- within a cross section. The high processing pressures sembly times, but resulted in short press cycles. To that are used to crush these overlapping veneer joints make Press-Lam’s manufacture feasible required two may result in some springback (with resultant interply pieces of new equipment: a continuous press dryer for cleavage) after the material reaches equilibrium in veneer and a continuous cold-press. moisture content (MC). These processing pressures also Continuous hot-pressing (2) was used to press increase the overall density of the PLV product. 1/4-inch cold veneer into a 1-1/2-inch-thick panel; ve- Scarf joints.—Scarf joints for individual veneers neers were laminated sequentially to form 1/2-inch-, (Fig. 1c) were (31) used as the outer plies in creating 1-inch-, and finally 1-1/2-inch-thick products. An end- 1/8-inch veneer PLV 2 by 4’s. In that application, 1:12 loading hot-press and an intermittent press sequence slopes were cut on the veneer ends, which were then were also used (2) for producing continuous PLV panels. bonded with a phenol-resorcinol adhesive. Strip-tension Methods of end jointing 3/4-inch-thick 8-foot panels tests of this joint indicated that a median strength of have been investigated (5, 6, 36), in order to make PLV 8,000 psi could be achieved. production feasible in existing plywood plants, since the In a limited test (31) of the use of scarf jointing in end jointing of 8-foot panels requires less capital in- the outer lamination of an otherwise staggered butt vestment than does that of a continuous-press system. ®3 joint C-grade veneer PLV 2 by 4, tensile strength proved Lamineer LVL (7) is currently produced in panel form more than 25 percent greater than in 2 by 4’s that had with subsequent end jointing to required lengths. butt joints throughout. The percentage of initial failure Veneer joints in outer ply joints was approximately 60 percent in the To produce PLV in construction lengths, joints must be used since the peeled veneer is usually no more than 100 inches long. A number of structural and non- structural methods have been used to end join veneers (Fig. 1). Butt joints.—Though they are inherently simple to manufacture, butt joints have numerous drawbacks (Fig. la). The joints cross the entire veneer width, sub- jecting each lamina to an artificial defect every 100 inches. Acceptability depends on the number of lami- nations and the quality of the veneer. Press-Lam®’s 1/4-inch and thicker veneers were particularly de- graded in nominal 2-inch thicknesses due to its few laminations. Stresses around butt joints, analyzed photoelastically (15), indicated that lateral separation 1MICRO=LAM is a registered trademark of the Trus Joist Corporation, Boise, Idaho. The use of trade, firm, or cor- poration names is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval of any product by the U.S. Department of Agricul- ture to the exclusion of others which may be suitable. 2KERTOWOOD is a registered trademark of Metsaliiton Teol- lisuus Oy, Finland. 3Lamineer is a registered trademark of the Weyerhaeuser Co., Figure 1. —Typical veneer joints for PLV: a) butt, b) crushed- Tacoma, Washington. lap, c) scarf, and d) reinforced butt. 22 SEPTEMBER 1983 butt jointed 2 by 4’s but only 20 percent for members Preproduction testing was carried out on the Lami- with scarf jointed outer plies. Eliminating the outer neer® PLV, which has vertical finger joints between ply’s butt joints did more to improve product per- panel-length PLV pieces. Results justified a tensile formance than could be attributed solely to the struc- design stress (Ft) of 2,200 psi for the jointed product and tural scarf joint. 2,400 psi for the unjointed (7) (Table 1). Graphite-fiber reinforced butt joints.—Butt joints Adhesive bonding were reinforced (Fig. 1d) with high-strength 0.015-inch- Basic plywood bonding technology can be used in thick unidirectional graphite-fiber composite impreg- PLV processing, so little additional development was nated with a phenol-formaldehyde resin (27). Tension required. High laminating costs have led to studies of tests were performed on specimens made of four 3/8-inch several cost-cutting methods since the rising costs of Douglas-fir veneers that contained one butt joint in a middle veneer.