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You may be aware of some of the many hardboard products around you. And perhaps you have already worked with hardboard, to build a birdhouse or install a "storage wall" for tools above your workbench.
But did you know, for example, that hardboard is practically a 100% wood product converted to fibers which are permanently bonded under heat and pressure into panel form?
Or that hardboard products are available in many forms other than familiar "brown board" including sophisticated woodgrains and embossed surfaces for exterior and interior uses?
Or that hardboard has superior physical properties to most materials it has replaced, and usually costs less? Look inside and learn many more interesting facts about Today's Hardboard.
Making wood even more of a resource...
"Waste no wood" makes good sense as a battle cry for all of us concerned with ecology. It is also a basic doctrine of hardboard production in the U.S. today.
No other industry makes more complete use of this great American natural resource. Hardboard is made almost wholly from wood and not just from logs or roundwood. Wood chips and board trimmings, once wastes of the lumber industry, are now routinely saved for processing into this most versatile of home building materials.
As well as efficiently utilizing the waste products of related forest product industries, American Hardboard manufacturers maintain and manage more than one million acres of forest land in soft and hardwoods.
How hardboard is used.
American hardboard manufacturers produce more than 7-billion square feet of hardboard every year, for a thousand-and-one different products: from prefinished wall panelings to furniture components; from today's widest choice of factory-primed and factory-finished exterior sidings to a variety of utility boards for home and industry-solid and perforated.
From do-it-yourselfers to professional builders, Today's Hardboard is a "how to" material. It is the happy medium for people who make almost anything.
How hardboard is made.
Hardboard is made from wood chips converted to fibers which are permanently bonded under heat and pressure into a panel. The wood fibers are combined with natural and synthetic binders and other additives that improve certain properties. Different hardboards have different fiber formulations, accounting for characteristic variations in product density, thickness and finishing properties, for almost unlimited uses.
Hardboard, made from wood fibers, should not be confused with particleboard which is made from wood chips and particles which are not refined into a distinct fiber.
Particleboard production is a "dry" process. Hardboards, on the other hand, are produced by both a "wet" or "dry" process. An abbreviated and generalized description of both processes follows.
1. Removing bark
Some hardboard manufacturers begin with roundwood or logs. Others start "one step ahead" with saw mill wood residues (which need no "debarking"). Still others begin the manufacturing process "two steps ahead" by using prechipped wood from other sources. In all cases, the sequence is the same, regardless of the locations in which these preparatory processes occur.
"Barking" of logs is usually desirable to insure quality control and uniformity of appearance in the finished product. It is achieved either (1) mechanically by cutting (2) hydraulically by high speed jets of water, or (3) by tumbling logs together in a large, rotating steel drum.
2. Reduction and preparation
Debarked roundwood or wood residues are next sent to the "chipper", where they are reduced by whirling knives into uniform chips about the size of a man's thumbnail. (A common chip size is 5/8" wide by 3/4" long.)
Chips must be screened to prepare them for defibration. Oversized chips are returned for re-chipping. Sawdust and other fine particles are sifted and saved for use as fuel in the power plant. Acceptable chips (ready for defibration) are conveyed to storage bins. Nothing is wasted.
3. Cooking
Wood chips are cooked under heat and pressure to soften them and dissolve some of the natural resin in the wood. This allows the chips to be more easily reduced to fibers in the refining process. In the "explosion" method of digestion, the pressure is built up to high levels and then released through a quick release valve. The sudden change in pressure between the inside and the outside of the chip causes it to explode into fiber bundles. This reduces the need for further refining.
4. Refining
The softened chips are fed into refiners where opposite rotating grinding disks shred them into fibers. Small amounts of chemicals may be added at this stage to improve strength and impart improved moisture resistance to the finished product. Fiber leaving the refiners is conveyed by air or water depending on the type of forming that is used.
5. Forming hardboard
The formation of felting of the fibers to form a mat can be achieved by either a wet or a dry process.
The wet process employs a continuously traveling mesh screen, onto which the soupy pulp flows rapidly and smoothly. Water is drawn off through the screen and then through a series of press rolls with wringing action similar to that of an old-fashioned washing machine.
In the dry felting process, comparatively dry fibers are laid out in much the same way but using air instead of a water medium. Air-formed mats emerge much thicker and softer than wetformed ones, and require more care in loading them into the press.
6. Pressing in hydraulic presses
From this point on, wet and dry process production procedures are virtually identical.
Pressing under heat (380°-550° F.) and pressure (500-1500 P.S.I.) puts the "hard" in hardboard. Multiple hydraulic presses are heated by hot water, hot oil or steam. The combination of applied heat and pressure welds the fibers back together and produces properties unattainable in natural wood. The amount of press time, temperatures and pressures vary widely, depending on the process and physical properties of the board being produced. "Smooth one side" board is delivered to the presses on a wire mesh screen (hence the familiar "weave" pattern found on the reverse side of the S1S hardboard). "Smooth two sides" board (S2S) is hot-pressed between two smooth plates. All boards emerge from the hot presses with an extremely low moisture content. Some are next tempered-roll coated with oil, and baked at 290°-340° F. Tempering increases hardboard's hardness, strength and water resistance.
7. Humidifying
To prevent post-press warping or buckling, the boards are conveyed through a humidifier or stored on racks in a humidity chamber. This raises the moisture content to approximate atmospheric humidity. Although hardboards are humidified, they should be allowed to adjust to local atmospheric conditions before being installed.
8. Trimming
Tungsten carbide tipped saws trim boards to standard sizes. (True to the hardboard industry's "No Waste" philosophy, even these trimmings are recycled for useful purpose.) Sheets may be cut to any size a customer wants. Also, hardboard can be fabricated and finished in a variety of ways.
After all operations and final inspection have been completed, the boards are wrapped and sent to the warehouse for shipment to customers.
Of the products shown here, how many had you already associated with hardboard?
Durability, strength, surface smoothness, superb machining and finishing qualities-all help account for hardboard's unique versatility that helps make it the preferred base material for so many and diverse products.
Hardboard is grainless, with uniform thickness, density and appearance. It resists marring, scuffing, and abrasion, as well as changes in temperature and humidity.
As a wood-base product, it follows that hardboard can be sawed, routed, shaped and drilled with standard woodworking tools. Indeed, because of its grainless uniformity of texture, working with hardboard is usually easier than working with lumber.
Hardboard can be securely glued or fastened with screws, staples or nails. Hardboard can also be laminated with plastic films, high-pressure laminates and veneers. It can even be bent, depending on the type of board, thickness and the radius of curvature desired.
Finishes can be applied in many ways: knife coat, roller coat, spray, curtain coat or dip. Clear or pigmented finishes include stains, penetrating sealer, shellac, latex base paints, oil base paints, textured or water-thinned paints, lacquer, synthetics and wax. No other material is so readily available in the range of wood-grain finishes, custom patterns and mix-or-match colors that hardboard users today take for granted.
Harbboard: A Closer Look
The uses for hardboard are diverse. It can be used wherever a dense, hard panel material in the thickness as manufactured will satisfy a performance or economical requirement better than any other material.
More and more effort is being expended by the industry to provide modifications and hardboard finishing that will permit the material to be used in more ways at less cost for application and finishing, and to permit industrial users a saving in the final product. By the selection of fiber content and advanced engineering and production techniques, the industry's objective is to produce a hardboard that will meet and perform for a specific end use.
A knowledge of the meaning of hardboard, its physical and mechanical properties, and an awareness of the influence of these properties on its performance characteristics are important to those persons specifying and working with hardboard.
(Density: weight per cubic foot.)
By formula, density is expressed as:
Density (lb/ft3) =
Weight of sample (lbs.) x 12
Area (ft2) x caliper of sample (in.)
When density is increased without any other changes, it tends to improve the following:
a. Strength-improved bending, impact, tensile, internal bond, stiffness and screw holding properties.
b. Moisture resistance-An increase in density reduces the rate at which moisture can be absorbed.
c. Hardness and resistance to abrasion.
d. Machining qualities of edges and surfaces.
A decrease in density tends to:
a. Reduce weight, resulting in lower handling and shipping costs.
b. Reduce manufacturing costs on hard-boards of equal caliper. Less fiber and resin are required.
c. Improve nailing and stapling.
Specific gravity is the ratio of the density of a material compared to the density of water.
Specific Gravity =
Density of material (lb/ft3)
Density of water (lb/ft3)
Approximate density of water = 62.5 lb/ft3
The density of hardboard is usually given as specific gravity for convenience of comparison with other materials. Because of its density hardboard is harder than most natural wood and because of its grainless character it has nearly equal properties in all directions in the plane of the board. It is not so stiff nor as strong as natural wood along the grain, but it is substantially stronger and stiffer than wood across the grain.
Modulus of rupture, the ultimate unit strength of a material in flexure or bending, has become a common measurement of hardboard strength. Even though the figure obtained is not mathematically correct, it is valuable in that it combines tensile, compressive and bending properties. It is frequently used in comparing one material with another.
MOR =
3 x Total Breaking Load (lbs.) x span (in.)
2 x Width of Sample (in.) Caliper2(in.)
(Hardness: testing for dent-resistance.)
Hardness is determined by the modified Janka ball test: The load in pounds required to imbed a steel "ball" 0.444-inch in diameter to a depth of one-half its diameter, is the hardness value. The height of drop in inches that produces visible failure on the opposite face is recorded as the index of resistance to impact.
Abrasion resistance is resistance afforded by a material to wear by friction. AR is not normally measured on unfinished hardboard, but can be determined by means of a Taber Abraser. This machine measures wear by standard abrasive filled wheels, and measures resistance to scratching under a known load. The weight of the abrased sample is subtracted from the initial weight of the sample to give the average weight loss in grams-per-number-of-cycles.
Water absorption(a test for resistance to moisture) is the amount of water absorbed by a submerged sample of hardboard in 24 hours. It is expressed by “percentage weight increase,”according to this formula:
Water Absorption (%) =
(Wet Weight-Initial Weight) x 100
Initial Weight
Thickness Swelling is the thickness increase (%) of a material after submersion in water for 24 hours. The caliper of the test sample is measured before and after submersion, after which the figures are applied to this formula:
% Swelling =
(Wet Caliper-Initial Caliper) x 100
Initial Caliper
Thickness swelling is critical where hardboard is subjected to large variations in humidity or long exposure to high humidity or water.
(Testing resistance to impact.)
Impact Strength under sudden localized load applied against the face of a panel held between supports. This value is usually determined by dropping a 2-inch diameter steel ball from increasing heights at the same point, the center, in the panel, until the panel fails. The height of drop in inches that produces visible failure on the opposite face is recorded as the index of resistance to impact.
(Pulling product apart laterally.)
Tensile strength is the resistance of a material to being pulled apart laterally. The total applied force at the time of fracture (measured on a machine) is divided by the cross-sectional area of the sample to give ultimate tensile strength in pounds per square inch.
Tensile strength =
Total Breaking Load
Width (in.) x Caliper (in.)
(Pulling product apart perpendicularly.)
Bond strength is the force that it takes to pull a material apart in a direction perpendicular to its surface. Expressed in pounds per square inch, it is the force required to fracture a 2" x 2" specimen perpendicular to the surface:
Internal bond strength is an important property of hardboard. Products with high bond strengths are manufactured for specific uses.
Bond Strength =
Breaking load, lbs.
4
(Stiffness or resistance to bending.)
Modules of elasticity is a test of hardboard's ability to resist bending. It is determined by applying a known concentrated load mid-way between two supports and measuring the deflection that occurs. ME is then determined by the following formula:
ME =
Total Load (lbs.) x Span3(in.)
4 x Width (in.) x Caliper3 (in.) x Deflection (in.)
Also See
Classification of Hardboard by Thickness and Physical Properties
Thickness Tolerances for Harboard Panels
Physical Properties of Hardboard Siding
Thickness Tolerances for Hardboard Siding
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