Screw-holding, internal bond, and related properties of composite board products for furniture and cabinet manufacture: a survey of the literature

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1 Screw-holding, internal bond, and related properties of composite board products for furniture and cabinet manufacture: a survey of the literature by J. Dobbin McNatt Technologist, USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin Abstract This paper discusses selected properties of particleboard, hardboard, medium-density fiberboard (MDF), and plywood related to their use in furniture and cabinet manufacture. Properties covered are screw-holding, internal bond, density profile, and edge appearance. Studies of direct screw withdrawal of panel products indicate that resistance to withdrawal from the edge is 75 to 80 percent of resistance to withdrawal from the face. In all cases, screw-holding powers of these products are considerably less than that of solid wood of the same density. Internal bond strength, density profile, and edge appearance are all related to each other to some degree and are largely governed by press conditions, particle or fiber characteristics, and adhesive content. A particular panel product can be accepted or rejected for use in furniture and cabinet manufacture for a variety of reasons. In 1968, Suchsland (36) published the results of a survey on the use of panel materials by furniture and cabinet manufacturers. Most frequently mentioned reasons for using particleboard,for example,were: economics, dimensional stability, no telegraphing, no warping, sizes available, and uniform thickness. Reasons given for not using particleboard were: difficult edge treatment, fastening problems, customer objection, and low strength/high weight. Concerning customer objection, one manufacturer commented, "We are using particleboard whereverit is superior to any other material,but we would not like our customers to know about it." It is no surprise then that particleboard used in furniture is disguised. To a great extent, the results of this survey are still valid. Component joints (especially where screws are used)and treatment of panel edges are still major concerns of furniture and cabinet manufacturers. This paper discussesscrew-holding, internal bond (IB),density profile, and edge appearanceof particleboard, hardboard, MDF, and plywood as they relate to performance of these products in furniture and cabinets. Additional important properties are covered in other papers in this proceedings. Screw-holding: resistance to direct withdrawal The resistance of particleboard and MDF to direct screw withdrawal is considerably lower than solid wood of the same density (14,15,43,44). This is usually attributed to the reconstituted nature of the products. A limited amount of data available indicates that withdrawal resistance of plywood is also somewhat lower than that of solid wood of the same species. The standard ASTM method for determining the screw withdrawal resistance of wood-based panels (6) specifies a 1-inchlong, No. 10 wood screw inserted into a 7/64-inch-diameter lead hole to a depth of 2/3 inch. Lead hole diameter is about 90 percent of the screw root diameter. The ANSI standards for particleboard (1)and MDF (2)call for a Type A or AB self-tapping screw instead of a wood screw. Whittington and Walters (43)and Johnson (18)indicate that there is little difference between the withdrawal resistance of self-tapping and wood screws in particleboard and plywood for the same depth of thread penetration. 30

2 Superfesky (40) reported that the Type A screw averaged slightly greater withdrawal resistance from particleboard (+ 4%) and MDF (+ 10%) than did Type AB. Both types had a shank diameter of inch. Threads in the Type A screw were cut deeper than those in the Type AB screw, which offset the fact that the Type AB had 16 threads per inch compared to 12 for the Type A. It is interesting that a special particleboard screw showed greater withdrawal resistance than the wood or self-tapping screw from fiberboard, but not from particleboard (13). ANSI standards for particleboard and MDF (1,2) list minimum screw-holding (No. 10, self-tapping) requirements which vary by panel grade and type for particleboard and by panel thickness for MDF. Face withdrawal resistance requirements for particleboard vary from 125 pounds for Grade 1-L-2 (low-density door core) to 450 pounds for Grade 1-H-3, a highdensity product. Grades 1-M-2 and 1-M-3 are most often used in furniture and cabinet manufacture. Face withdrawal resistance requirements for these are 225 pounds and 250 pounds, respectively. Face withdrawal resistance requirements for MDF are 325 pounds for panels 3/16 inch-thick or less, and 300 pounds for panels thicker than 13/16 inch. Screw withdrawal resistance requirements for screws inserted into the edges of MDF or particleboard panels average about 80 percent of face withdrawal requirements. This difference in face and edge screw-holding requirements reflects the results of test data (13,20,33,37,40,44).Coefficientsofvariation for screw withdrawal resistance are about 10 to 15 percent for commercial particleboard and MDF. Eckelman (14,15) used some of these published data to develop equations to provide furniture and cabinet designers with reasonable estimates of the screw-holding power of wood-base panels. Average ultimate withdrawal resistance from the face of particleboards is predicted by: F = 2655D 1/2 (L-D/3) 5/4 G 2 [1] and from the edge of a panel by: F = 2055D 1/2 (L-D/3) 5/4 G 2 [2] A comparable expression for MDF face withdrawal is: F = 3700D 1/2 (L-D/3) 5/4 G 2 [3] A comparable expression for MDF edge withdrawal is: F = 2860D 1/2 (L-D/3) 5/4 G 2 [4] where: F = ultimate withdrawal resistance (lb., at 65%RH) D = shank diameter of the screw (in.) (D = N, where N is the screw gauge or number) L = depth of embedment of the threaded portion of the screw G = specific gravityofthe material based on ovendry weight and volume at test These equations yield an edge withdrawal resistance that is 77 percent of the face withdrawal resistance for both particleboard and MDF. Also for both edge and face withdrawal, predicted resistance values for MDF are 39 percent greater than those for particleboard. Plywood standards do not include screwholding requirements. However, the American Plywood Association (APA) has published average ultimate withdrawal loads (5). Comparison of these APA values and those reported by Johnson (18) and Carroll (11) indicates that face withdrawal resistance of Douglas-fir plywood is about 85 percent of the value for sidegrain Douglas-fir solid wood. Wilkinson and Laatsch (44) reported a withdrawal resistance of 420 pounds for a No. 10, Type A self-tapping screw inserted 3/4 inch into Douglas-fir side grain. Withdrawal resistance perpendicular to the face of Douglas-fir plywood, adjusted for the same depth of penetration, averaged about 385 pounds. In a National Particleboard Association (NPA) study (27), the face withdrawal resistance of A-D plywood was essentially the same as that of core stock particleboards (about 45 pcf, average). Inserting screws into plywood edges is not normally recommended but may be necessary at times. The APA data (5) and that of Johnson (18) indicate that the ratio of edge-to-face withdrawal resistance of plywood is about 0.85, almost the same as that for MDF and particleboard. Screw-holding data from the ASTM standard test give comparative values for different board products, but actual joint strength depends greatly on the amount of torque used to set the screw. Too little torque leaves a loose joint; too much torque will strip the threads cut by the screw in the panel. The NPA (27) reported that the maximum torque required to set a No. 8 screw (wood or self-tapping) ranged from about 20 to 50 in./lb. for 30 to 57 pcf particleboards and 30 to 40 in./lb. for plywood. Carroll (11,12) related driving torque to screw withdrawal resistance of particleboard and plywood. He found that maximum withdrawal resistance of Douglas-fir plywood is reached when the screw is inserted only one-half turn past flush against the face of the panel. In the panel edge, any turning past flush will reduce withdrawal resistance. In particleboards, maximum withdrawal was developed at about one turn past flush in the face and onequarter turn past flush in the edge. This small difference between torque required to set the screw and that needed to strip the threads formed in the panel often causes problems when screws are power driven. Torque requirements are affected by screw size, panel density, and depth of penetration. A lead hole diameter range between 50 and 90 percent of screw root diameter does not seem to have much effect on torque requirement or withdrawal resistance in particleboard (11,18). Lead hole diameters between 40 and 70 percent of root diameter also did not affect face withdrawal resistance of Douglas-fir plywood. When lead holes were 100 percent of screw root diameter, face withdrawal resistance was reduced, but edge withdrawal resistance was increased. This reflects the tendency of panels to split when screws are inserted into the edge of a panel. Didriksson et al. (13) used the IB test to Evaluate edge splitting tendency of fiberboard (hardboard and MDF), particleboard, and spruce plywood (Fig. 1). They found that the edge-splitting tendency decreased as the lead hole diameter increased from 60 to 85 percent of the outer diameter of the screw threads. However, the edge withdrawal resistance decreased. The edge splitting tendency of fiberboard was considerably greater than that of particleboard or 31

3 Figure 1. - IB test used to measure splitting tendency of wood-base panels due to edge screwing (13). Figure 2. - V-fold method of particleboard edge treatment (29). plywood. Lateral loading of screws in furniture or cabinet joints could increase the edge-splitting tendency. Increasingresin content has been shown to increase screw withdrawal resistance of MDF and particleboard. Markstrom et al. (23)reported an average increase of about 50 percent for MDF and 25 percent for particleboard when the resin content was increased from 6 to 10 percent. Putting glue in the lead hole before inserting the screw reinforces wood fibers and increases withdrawal resistance. Kelly and Pearson (21) reported a 5 to 10 percent increase in face withdrawal resistanceand 35 to 70 percent increase in edge withdrawal resistance for MDF when polyvinyl acetate was put in the lead hole. All the screw withdrawal resistance data given above are maximum values from short-term tests used for comparisons between materials. They are not meant to be used in design for long-term loading. Design values for particleboard and MDF have not been published. Design values for solid wood (26,42) are based on wood specific gravity and amount to about 18percent of average ultimate withdrawal resistance. The same percent has traditionally been used for plywood for normal load duration (5). Internal bond strength Tensile strengthperpendicular to surface (IB)is an important indicator for quality control in wood-based panel manufacture. Internal bond is quite sensitive to a number of raw material and processing variables: particle or fiber characteristics, resin content, panel density, etc. If IB changes very much, something has happened in the manufacturing operation. Although IB is generally not considered to be directly related to performance, insufficient bonding between fibers or particles will quite often show up later as poor performance of the panel in use. For example, low IBs with failures occurring at or near the panel surface indicates resin precure. If not corrected, it can result in delamination if a high pressure plastic laminate is bonded to the core panel with water-based adhesives. IB requirements in the ANSI particleboard standard (1) vary from a low of 20 psi for a lowdensity door core grade (1-L-1) to 300 psi for a high-density phenolic-bonded grade (2-H-1). IB requirements for furnituregrade particleboards, 1-M-2 and 1-M-3, are 60 psi and 80 psi, respectively. The ANSI MDF standard (2) requires 90 psi for thinner panels (13/16-in. and below) and 80 psi for thicker panels. The ANSI hardboard standard (3) requirements range from 25 psi for industrialite (Class 5) to 130 psi for tempered (Class 1). There are no equivalent strength requirements for plywood. However, River et al. (31) reported an average value of 127 psi in IB tests for samples of 112-inch-thick Douglas-fir plywood. Some unpublished data indicate that plywood IB strength increases with the number of plies for a given panel thickness. Reported IB values for commercial particleboards range from 45 psi to 450 psi for a specific gravity range of 0.62 to 0.92 (13,24). In his review of relationships between particleboard processing variables and properties, Kelly (19) noted that most researchers have found higher IB with increasing resin content and increasing press time and temperature. IB also increases as the core particle configuration changes from long, wide flakes (wafers) to planer shavings or slivers (8,16,22,23,33). Shuler and Kelly (33) and Brumbaugh (8) found that increasing flake thickness increased IB. One explanation for this is that for a given resin content, thicker flakes result in a lower total surface area per panel, and thus more resin 32

4 per unit surface area (30). Markstrom et a (23) and Burrows (9) have shown that increasing the resin content of panels mad from wafers from 2 or 3 percent to 5 or percent (increasing resin per unit surface area) will nearly double the IB. Superfesky and Lewis (41) reported I strengths of 133,140, and 186 psi for three commercial MDF products with densities of 0.66, 0.68, and 0.78 g/cm 3, respectively Suchsland et al. (37) reported a range of 10 to 282 psi for eight commercial MDF products. IB strength values and failure plan location can be significantly affected b overall board density and differences in density between panel face and core. IB coefficients of variation ranged from 10 to 2 percent for both particleboard and MDF. No published IB values were found for commercial hardboards. Unlike particleboard, IB of the commercial MDF did not correlate with overall par el density or even core density. The tendency for linear expansion to increase, an thickness swelling to decrease as IB increased suggested a possible vertical component of fiber orientation. Work by Myers (25) indicated that vertical orientation is more likely with shorter fibers. Edge appearance and density profile Because of the nature of mat-formed particleboard, finishing the edges is considerably more involved than finishing the faces. Porosity of the edges precludes the conventional procedures used on particleboard flat surfaces. In most applications, exposed edges must be banded with veneer or lumber, V-grooved and folded (Fig. 2), or filled with a paint-type material (28,29). Edgefilling levels out the voids and provides a smooth, sandable surface which can then be finished, Proper edge finishing is particularly important for kitchen and bathroom cabinet doors in order to meet the ANSI performance requirements (4). The amount of filling needed is dictated by the porosity of the panel edge. This in turn depends on such factors as particle size and geometry, panel density, and density gradient through the thickness. Finer wood elements produce tighter edges and reduce the amount of filling needed. Panels produced from fibers have superior edge quality. This is one reason MDF is readily accepted for furniture and cabinet parts (37). Density gradient produces a nonuniform edge surface which complicates edge treatment. Suchsland and Woodson (38,39) found that the face density of commercial MDF was as much as 60 percent greater than core density (Fig. 3). Kelly and Pearson (20) found that density gradient in MDF increased as overall density increased from 25 to 42 pcf, and as panel thickness increased from 318 to 314 inch. The time to reach a core temperature of 225 F can be 8 minutes for 1-1/8-inch-thick MDF as compared to 3 minutes for 1/2-inch-thick MDF (7). Shen and Carroll (32) found that density gradient in commercial particleboards also increased as panel thickness increased. Manipulation of press cycle variables can change or essentially eliminate the gradient (17,38) (Fig. 3). Stevens and Woodson (35) reported that for medium- (44 pcf) and low- (38 pcf) density boards, the density gradient of highfrequency-cured boards was less than that of hot-presscured boards. At a higher board density (50 pcf) there was practically no difference in density gradient between the two curing methods. Carll (10) found that lower platen temperature and high frequency heating produced 1-inch-thick particleboards with a more uniform vertical density profile. While edge finishing of hardboard is usually not a consideration in furniture and cabinet manufacture, differences in density profile can affect IB strength levels and failure location. Spalt (34) discussed the density profile of wet- and dry-process hardboard. Based on differences in pressure cycle and heat and moisture transfer in the mat during pressing, he theorized the following differences in density profiles (Fig. 4): 1. Wet-formed/wet-pressed: High density at the smooth surface, decreasing gradually through the panel to a minimum at the screen surface. 2. Wet-formed/dry pressed: Two dense surfaces with lower core density. 3. Dry-formed/dry pressed: Lower face Figure 3. - Density profiles of commercial MDF; compared with lab-made board; press temperature 335 F, initial pressure 820 psi; pressed to thickness in unheated press, then heated at 285 F, initial pressure 820 psi (38). Figure 4. - Conceptualizations of density profiles through the panel thickness for wetand dry-formed hardboards (34). (A) Wet/ wet (wet-formed/wet-pressed) (B) wet/dry, (C) dry/dry. Figure 5. - Density gradient measured on samples of 1/4-inch-thick hardboard. 33

density and higher core density. However, Spalt did not report any actual hardboard density gradient measurements.

Conclusions Screw-holding, IB strength, and edge appearance are properties of wood-based panels of considerable importance for their use in furniture and cabinet manufacture.

The amount of torque used to set screws when joining wood-base panels is more important than screw type.

5 density and higher core density. However, Spalt did not report any actual hardboard density gradient measurements. Measurements made at FPL on a few samples of 1/4-inch-thick commercial hardboards do not support the above statements (Fig. 5). Conclusions Screw-holding, IB strength, and edge appearance are properties of wood-based panels of considerable importance for their use in furniture and cabinet manufacture. An evaluation of published information on these and related characteristics leads to the following conclusions: 1. The amount of torque used to set screws when joining wood-base panels is more important than screw type. There is little difference between torque required to bring the screw flush with the surface and that required to strip the threads formed in the panel. 2. Because of splitting tendency, inserting screws into the edges of wood-based panels should be avoided whenever possible. Resistance to edge withdrawal averages about 70 to 80 percent of face withdrawal resistance. 3. IB strength of particleboard generally increases as pressing time and temperature and resin content increase. IB also increases as particle type changes from long, wide flakes to planer shavings or slivers. Presence of short fibers in MDF panels with an increased tendency toward vertical orientation leads to increased IB. 4. Edge appearance is influenced by particle type (flakes, shavings, sawdust, fiber), panel density, and density gradient through the thickness. Density gradient can be controlled to a degree by manipulating press cycle variables. Literature cited 34

6 In: Hamel, Margaret P., ed. Composite board products for furniture and cabinets-innovations in manufacture and utilization: Proceedings 47357; 1986 November 11-13; Greensboro, NC. Madison, WI: Forest Products Research Society; 1989:

SCREW WITHDRAWAL RESISTANCE SHEET METAL SCREWS IN PARTICLEBOARD AND MEDIUM-DENSITY HARDBOARD

SCREW WITHDRAWAL RESISTANCE SHEET METAL SCREWS IN PARTICLEBOARD AND MEDIUM-DENSITY HARDBOARD SCREW WITHDRAWAL RESISTANCE OF TYPES A AND AB SHEET METAL SCREWS IN PARTICLEBOARD AND MEDIUM-DENSITY HARDBOARD U.S.D.A. FOREST SERVICE RESEARCH PAPER FPL 239 1974 U.S. DEPARTMENT OF AGRICULTURE FOREST