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Materials Manufacture



In addition to knowing how a house is constructed and how its mechanical systems operate, it also is valuable to know the methods of manufacture of the raw materials and components that are used in house construction. Every component part of a house has a complex manufacturing process. In addition, most building materials and supplies are graded for quality according to industry regulations. These standards are outlined in this section, along with brief descriptions of manufacturing processes.

WOOD

A tree grows by adding a new layer of cells directly under its bark each year. Each year’s growth is distinguishable from the previous year’s, which is what gives wood its grain. When a branch develops in the tree and grows through the grain, a knot composed of rings of grain running in small circles is formed. Sometimes a separation, called a shake, develops between the layers of grain. When the grain separation becomes filled with pitch (liquid or solid resin), it's called a pitch pocket. Knots, shakes and pitch pockets are considered imperfections in the wood and when wood is graded, the presence of these imperfections results in a lower grading for the wood.

The chemical composition of wood is about 70 % cellulose. The material that binds the cellulose together is called lignin and makes up most of the balance of the wood except for the small remainder of materials distinctive to each species that give the species its individual color, odor and decay resistance.

An important property of wood is that it's hydroscopic (absorbs moisture). It expands when it absorbs moisture and shrinks when it dries out, a characteristic that must be considered whenever wood is used. In the tree, the wood is in a wet (green) condition, but when wood is used as a building material, it must be dry. When the tree is felled and cut into lumber, its moisture content begins to drop. Eventually, the lumber will reach a point of equilibrium at which the moisture in the wood will equal the moisture in the air. This ensures that the wood will shrink or expand only a small, tolerable amount after it's fabricated or installed.

Lumber

The manufacturing process of lumber begins with felling the tree in the forest and transporting it to one of more than 30,000 sawmills. There the logs first are debarked by equipment that peels, scrapes or blasts the bark off the log with high-pressure water jets. Next, the log is fed into the head saw, which saws it into rough pieces called cants, timbers, planks or boards. Then these pieces are fed into a series of smaller saws called edgers that remove the rounded edge and rip wide pieces into narrower widths. The pieces go into the trimmer saws that trim off the round ends and end defects and cut the wood into the desired lengths. The next step is the green chain where the pieces are sorted (usually manually) according to grade, species and size as they move along a conveyor belt. Once the lumber is sorted, some of it may be dried and the rest may be sent without drying directly to manufacturing plants for resawing, dressing, ripping, planing or other treatment.

The best way to reduce the moisture content of lumber in the shortest time is to dry it in a kiln, a large building in which the heat, humidity and air circulation are controlled. Kiln drying can reduce the moisture to any desired percentage, usually 6 to 12 % for softwoods and 15 to 19 per cent for hardwoods.

Lumber also may be dried by standing it outdoors or indoors in an unheated building or it can be shipped undried to the lumberyard where it's air-dried. The use of unseasoned lumber or improperly dried lumber results in poor construction caused by the shrinking and bending of the wood after it has been installed.

After the lumber is seasoned, it's sent to a planing mill where the surface is planed to make it smooth and the lumber is further sawed. The amount of planing and sawing varies. Little is needed to produce rough boards, dimension stock and timbers, but more planing is necessary for the finished grades of these items.

Some of the lumber is further processed or “worked.” It may be matched for use where tight joints are needed between boards as in sheathing and subflooring. A tongue is cut into one side of the board and a groove into the other. When the board is installed the tongues are fitted into the grooves.

Shiplapping is another cut that will produce a tight joint but not so good a joint as the one produced by matched lumber. Shiplapping also is used for subflooring, wall and roof sheathing or siding. Each edge is rabbeted and , when installed, the edges overlap.

Patterned lumber is a type of lumber that's cut into various shapes and is used for siding and moldings.

Lumber is graded and sized according to American Lumber Association (ALA) standards. The standards vary from species to species. Each individual board is stamped with its size, grade and manufacturer. There fore, a builder may easily obtain the proper grade of lumber for each part of the house.

Plywood

Plywood is manufactured by bonding together several thin sheets of hardwood or softwood (veneer) or by bonding the sheets to a core of lumber or particle board.

Manufacturing starts with the selection of large peeler logs, 8 feet long and 1 to 4 feet or more in diameter, cut by a giant lathe into continuous sheets from 1/so to /16 of an inch thick. An alternate process is to slice the sheets from large blocks of wood. The sheets are cut to the de sired widths by a clipping machine and then are oven-dried to reduce their moisture content so that the glue will bond properly. Next, the sheets are sorted into various grades depending on the number of knots and other imperfections.

The final panel is made by spreading glue on both sides of the veneer and core, laying them on top of each other and putting them into a hydraulic press where, under pressure, they are bonded into plywood panels.

Shingles and Shakes

The manufacture of shakes starts with the selection of cedar logs cut to the length of the finished product, 18, 24 or 32 inches. The logs then are quartered into blocks that will fit the splitting machines or, if they are to be hand-split, cut into convenient, workable sizes. Most shakes are split by machines, although the trade of splitting shakes by hand with a hard wood mallet and a steel froe still thrives.

The three basic types of shakes made are taper-split, straight-split and hand-split and re-sawn. Taper-split shakes are produced by turning the block over after each shake is split off, while straight-split shakes are cut from the block without turning it. Hand-split and re-sawn shakes are produced by splitting the blocks into boards of desired thickness and then passing them through a thin band saw to form two shakes, each with a hand-split face and a sawn back. When they are guided through the saw diagonally, thin tips and thick butts are formed. Packers then bundle the finished shakes into standard-size frames, compressing the bundles slightly and binding them with wooden “bandsticks” and steel strap pings.

The manufacture of a shingle begins like that of a shake, with the se lection of logs, cutting them to shingle length and then quartering them into blocks that will fit into the sawing machine. The sawing machine has two blades, one that cuts the shingle from the block and the other that trims the edges. After being sawed, the shingles are graded and bundled like shakes. They are held flat in the bundles by two bandsticks and metal straps while they are stored and seasoned.

Standard shingles may range from 3 to 14 inches wide. Dimension shingles are available in one equal width. There also are complex standards for thickness that vary with the length. Shingles with extreme cross graining aren't acceptable in any grade. There are four grades of shingles. The best is no. 1 (blue label), most often used for roofs and side walls. No.-1-grade shingles must be 100-percent heartwood, 100-percent clear and 100-percent edge-grained. No. 2 (red label) is a good grade that contains some imperfections but is usable in most places that the no.-1 grade can be used. No. 3 (black label) is a utility grade for economy applications. No. 4 is suitable only for undercoursing.

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CONCRETE

Concrete contains portland cement, water, fine and coarse aggregates and admixtures.

Cement Manufacture

Portland cement is a compound of lime, silica, alumina and iron. The materials are carefully selected and the manufacturing process is closely controlled. The materials these compounds are obtained from are lime stone dug from a quarry, oyster shells, clay, marl (a rock that contains limestone), iron ore, silica sand and blast-furnace slag.

These ingredients are mixed in proper proportions. Then they are ground up by either a wet-grinding or a dry-grinding process. The ground-up mixture is fed into a rotary kiln that burns it at a temperature of approximately 2,700 degrees. At this heat, the ingredients combine into clinkers. The clinkers are cooled, combined with a small amount of gypsum and pulverized into a very fine powder that then is packed in pa per bags or shipped in bulk in special railcars and trucks.

Cement Quality Standards and Grades

Manufacturers of portland cement market it under their own trade and brand names. Specifications have been established by the American Society for Testing and Materials (ASTM). Practically all cement is made to these specifications.

Normal portland cement types I and IA are suitable for almost all residential construction purposes. In areas where a large amount of sulfates is present, types II and IIA should be used because they are more resistant to sulfate attack. Types III and IIIA are used when fast setting is required, such as in cold-weather construction. Type IV isn't suitable for residential construction. Type V is used mostly in the West where the soil and water subject the cement to very high sulfate attacks. The “A” after the type number indicates that it's resistant to frost action and to the effect of salt application, an advantage in the North where salt is used for snow and ice removal.

Aggregates

Fine and coarse aggregates make up about 75 % of the final concrete mixture. Fine aggregate consists principally of sand. The type of sand best suited for concrete manufacture ranges from pieces that are very fine to pieces up to 1/4 of an inch in size. The range of sizes must be even. Coarse aggregate consists principally of crushed stone and gravel larger than 1/4 of an inch to 1 1/2 inches. The maximum size should depend on the final use of the concrete.

Aggregates should be sound, hard and durable. If soft and flaky aggregates are used, the concrete is weakened. The aggregates also must be free of loam, clay and other vegetable matter for these, too, will weaken the concrete.

Water

The water used to make concrete must be free from oil, alkali or acid. In general, water that's suitable for drinking is suitable for use in concrete. Water that contains high quantities of sulfates shouldn't be used because sulfates attack cement.

Admixtures

Admixtures are materials that are added to the concrete mixture to pro duce special qualities such as improved workability, reduction of the separation of coarse and fine aggregates, entrainment of air or acceleration or retardation of setting or hardening. They must be used carefully for they may, in addition to producing the desired effect, also produce undesirable effects. The three frequently used admixtures are accelerators, retarders and air-entraining agents.

Accelerators, usually calcium chloride, increase the rate of early strength development. These may be needed to reduce the waiting time for the finishing operations to start, to allow earlier removal of the forms, to reduce curing time, to lengthen the time that the structure can be used, to offset effects of cold weather in slowing up setting time and to perform emergency repairs when delay of use causes problems.

Retarders, a wide variety of chemicals, delay the early stiffening action. They often are used in hot weather and in difficult installations where more than normal installation time is required.

Air-entraining agents are mixtures that improve the workability and durability of concrete and increase the concrete’s resistance to damage from frost action and from the corrosive effects of salt. Their use is so common and their cost so low that they are being added during the manufacturing process. There is little excuse not to use them for all residential installations.

Mixtures

The goal of a proper concrete mixture is to surround completely each particle of aggregate with the cement water paste. This will produce the best workability, strength, durability, water-tightness and wear-resistance.

The proportion of water to cement, stated in gallons of water per 94-pound sack of cement, is the water-cement ratio. The more water that's added, the less the strength, durability and watertightness of the cement. Therefore, just enough water should be used to produce the required workability. The longer the mixture remains moist after it's poured, the stronger it becomes. It may be kept moist by covering it and spraying it with water.

It is beyond the scope of this guide to cover the details of how a proper mix is made for each job. The type of job must be considered, along with the size and moisture content of the aggregate, the temperature and the humidity. Usually, a small trial batch is made to verify that the proposed mixture is correct.

A slump test may be used as a rough measure of the consistency of concrete. A specially designed cone is filled with the mixture in stages and “puddled” with a metal rod a prescribed number of times between each stage. The top is struck off (smoothed) with a trowel and the cone is gently removed immediately after being filled. The amount that the top of the pile drops from its original height then is measured.

Another test is used to measure the compression strength of the concrete. At regular intervals throughout the discharge of the concrete mixture, samples are taken, poured into a cone, puddled and left to set. The strength of the mixture then is measured.

All concrete ingredients are mixed until they are uniformly distributed. These ingredients are mixed by hand or in mechanical mixers on the site. The use of ready-mixed concrete, however, is increasing. The concrete is mixed in a special plant and hauled to the site in agitator trucks or transit mixer trucks operated at agitator speed. In any case, the cement must be discharged from the truck within 1 1/2 hours after the water has been added to the mixture.

CLAY BRICK and TILES

Manufacture

Winning (mining) is the first step in the brick-making and tile-making process. Clays are dug from the open pits or mines with power shovels, crushed and placed in storage bins. To minimize the variation in the properties of the clays, clays from different areas of the pit or mine and from different mines and pits are blended together.

After the stones are removed, the mixture is pulverized on huge grinding wheels that weigh up to 8 tons each. Usually, the mixture then is passed through vibrating screens to further control the particle sizes. In a pug mill, water is added to the clay and huge blades on a revolving shaft mix the clay and water into a homogeneous mixture.

The mixture then is formed into bricks or tiles by one of three processes. The stiff-mud process is the most common. During this process, just enough water is added in the pug mill to make a mixture of plastic consistency. The mixture is placed in a de-airing machine that has a vacuum inside it. This vacuum removes many of the air holes and bubbles in the clay. The mixture then is forced through an extrusion die that forms it into bars, which are the final width and shape of the brick or tile. Automatic cutters cut the bars into bricks or tiles that are larger than the final product to allow for shrinking during drying and burning. Textures, if any, also are stamped or scratched on at this point.

The soft-mud process is used mostly when the clay is too moist for the stiff-mud process. The clay is poured into molds, the sides of which have been lubricated with sand (bricks called sandstruck) or water (bricks called waterstruck). This process isn't used for tiles.

The dry-press process is used for dry clays. The bricks or tiles are formed by placing the clay into steel pressure molds that press the clay into the desired size and shape.

After the bricks or tiles are shaped, they are dried in kilns at temperatures from 100 to 300 degrees for one to two days. An optional step either at this point or after the next step is glazing, which produces a glasslike colored coating on the exterior surface. High-fired glazes are sprayed on the units before and after the preceding drying process. The bricks then are completed in the normal way: low-fired glazes are applied and then the bricks are re-fired at lower temperatures.

Burning, the next process, hardens the clay-and-water mixture. The units are placed into a tunnel kiln or a periodic kiln. In a tunnel kiln, the bricks move along from one stage of the process to the next in special cars; in a periodic kiln, they remain in place.

Burning takes place in stages. In the first water-smoking stage, any free water is evaporated at a temperature of about 400 degrees. The temperature then is raised to 1,800 degrees during the dehydration and oxidation stages and finally to about 2,400 degrees in the vitrification stage. Near the end of the burn, the units may be flashed to produce different colors and shades by reducing the oxygen in the kiln.

After the temperature has reached its final height, it's slowly reduced and the cooling process begins and lasts from two to four days. The kiln then is unloaded (which is called drawing) and the bricks or tiles are sorted, graded and either stored or shipped by rail or truck.

CONCRETE BRICK and BLOCK

Manufacture

Raw materials are received and elevated to storage bins as the first step in the manufacture of concrete bricks and blocks. Then they are released into a weight batcher under the storage bins that regulates the amount of each ingredient by weight. The weight batcher is moved over the mixer and the ingredients are released into the mixer together with the correct amount of water. After mixing, the ingredients are discharged into a hop per above the block machine and controlled quantities are fed into the machine. The ingredients are compacted into the mold with pressure and vibration.

From the block machine emerge green concrete bricks or blocks molded onto steel pallets that are placed on a steel curing rack. The curing rack is moved into an autoclave or steam-curing kiln.

Kiln curing begins with a holding period of one to three hours inside the kiln at normal temperatures. The heating starts when saturated steam or moist air is injected into the kiln and the temperature is gradually raised over several hours to about 180 degrees. When the desired heat is reached, the steam is turned off and the units soak for 12 to 18 hours while their strength develops. Next, they are dried as the temperature continues to drop. The drying process may be speeded up by again raising the temperature, this time with dry heat. The total time for curing and drying is about 24 hours.

High-pressure steam curing is performed in autoclaves that are steel cylinders from 6 to 10 feet in diameter and 50 to 100 feet long. The green molded units are put into the autoclave and held at normal temperatures for two to five hours. Saturated steam is forced into the autoclave at about 150 pounds per square inch of pressure. The only purpose the pres sure serves is to hold the steam at the high temperature. The temperature is raised gradually over about a three-hour period until 350 degrees is reached. Temperature and pressure are maintained from five to ten hours while the units soak. The pressure then is released over a half-hour period, which facilitates the rapid loss of moisture from the block without setting up shrinking stresses.

It also is possible to cure the green units by keeping the moisture at normal temperatures for about a month without using an autoclave or kiln.

After the curing is completed in the kiln or autoclave, the units are taken to the cubing station where they are assembled into cubes and then stored until they are shipped. Blocks usually are made locally and , there fore, almost always are shipped by truck.

MORTAR

Straight-lime mortar, made of lime, sand and water, hardens at a slow, variable rate. It develops low compressive strength and has poor durability to the freeze-thaw cycle. It does, however, have high workability and high water retention. It tends to self-mend small cracks that appear, which helps keep water penetration to a minimum.

Portland-cement mortar, made of portland cement, sand and water, has the opposite characteristics of straight-lime mortar. It hardens at a quick, constant rate, develops high compressive strength, has good resistance to the freeze-thaw cycle but has poor workability and low water retention and does not self-mend small cracks.

Portland cement—lime mortar, made of portland cement, lime, sand and water, combines the characteristics of the straight-lime and portland- cement mortars and makes a much more satisfactory mortar for general use.

Masonry-cement mortar is made of masonry cement, sand and water. Masonry cement consists of portland cement, natural cement, finely ground limestone, air-entraining agents and gypsum. This is a convenience item, for these ingredients are premixed by the manufacturer rather than at the site.

GLASS

Most house glass is a mixture of silicone, usually in the form of silica obtained from beds of fine sand or from pulverized sandstone, soda used as an alkali to lower the melting point, lime as a stabilizer and cullet (waste glass) to assist in melting the mixture. These are the same materials that have been used from ancient times. Small amounts of other materials are added to produce special types of glass.

The ingredients are mixed together and then melted in tank furnaces with capacities as high as 1,500 tons or in pot furnaces that hold up to 2 tons. The glass is released from the furnaces and rolled into sheets, cooled and cut into the desired sizes. Plate glass is polished on one or both sides.

Glass formerly was made by blowing large bubbles, cutting them and allowing the half spheres to flatten out on a hot bed. Making large panes (called “lights” in the building trades) was almost impossible, which explains why most windows in colonial houses consisted of many small panes of glass.

RESILIENT FLOORING PRODUCTS

Linoleum

True linoleum is made of oxidized linseed oil, resin binders, wood floor pigments, mineral fillers and ground cork. The ingredients are mixed and then bonded to an organic backing of burlap or asphalt-saturated rag felt. Hot linseed oil is placed in closed cylinders where it solidifies into a jellylike mass. Next, the oxidized oil is fused with the resins into a tough cement that's blended with the other ingredients and onto the backing material. It is seasoned in ovens for several weeks to cure it and then waxed or lacquered to improve its stain resistance.

Asphalt Tile

Asphalt tile is made of asbestos fibers, finely ground limestone fillers, mineral pigments and asphaltic or resinous binders. Asphaltic binders are made from a blackish, high-melt asphalt, mined principally in Utah and Colorado. Resinous binders are made from asphalt that's a product of petroleum cracking or from coal tars. The ingredients are mixed at a high temperature and rolled into sheets that are sprinkled with colored chips. The sheets are cooled and cut or stamped into tiles. Sometimes they are pre-waxed.

Rubber Tile

Rubber tile is made of natural or synthetic rubber, clay and fibrous talc or asbestos fillers, oils or resins and non-fading organic or chemical color pigments. The ingredients are mixed, rolled into sheets and then vulcanized in hydraulic presses under heat and pressure. The sheets are sanded on the back to uniform thickness and cut into tile.

Cork Tile

Cork tile is composed of the bark of cork tree, found in Spain, Portugal and North Africa, and synthetic resins. The cork is granulated, mixed with the resin and pressed into sheets or blocks and baked. Then it's cut into tiles. Often, wax, lacquer or resin is applied under heat to the surface to provide a protective coating. (more info on cork)

Vinyl Tile

Vinyl tile is made of polyvinyl chloride resin, mineral fillers, pigments, plasticizers and stabilizers. The ingredients are mixed at high temperatures, hydraulically pressed or pressure-rolled into sheets of the required thickness and cut into tiles. (more info on vinyl and laminate tiles)

Rag-Backed Vinyl Tile and Sheets

These products are made with a vinyl wear surface bonded to a backing of vinyl, polymer-impregnated asbestos fibers or asphalt-saturated or resin-saturated felt. New vinyl products have a layer of vinyl foam bonded between the backing and the wear surface. The process substantially expands the decorative possibilities, making possible tiles that look like brick and many other materials. These tiles are very attractive but tend to be quite expensive.

Carpeting

Woven carpet is made on a loom by one of four weaving processes known as velvet, Wilton, Axminster and loomed. Velvet is the most formal weave and also the simplest. The Wilton loom is a complex form of the velvet loom fitted with a special Jacquard mechanism that uses punched cards to select various colors of yarn and to make the complex pattern. The Axminster loom simulates hand weaving because each tuft of yarn is individually inserted and theoretically each tuft could be a different color. This flexibility offers unlimited design possibilities. Loomed carpet recently has been developed for use with bonded rubber cushioning.

Tufted carpet is made by inserting tufts of pile into a pre-made backing material that then is coated with a layer of latex to hold the tufts permanently in place. The tufting machine has thousands of sewing- machinelike needles that operate simultaneously.

ASPHALT ROOFING and SIDING PRODUCTS

Manufacture

Usually, asphalt roofing and siding products are made by a continuous process. The felt rolls are placed on a dry looper that provides enough slack so that one roll can be attached to the next without stopping the ma chine. The felt goes through a saturator tank filled with asphalt that soaks into the felt. Next, a wet looper holds the saturated asphalt and allows it to shrink. For products that require coating, the next step is a coating machine that applies a thin coating of asphalt over both sides of saturated felt. The thickness of the coat is controlled by rollers and automatic scales.

When smooth roll roofing or siding is being made, the surface is coated with talc or mica and rolled into the surface by pressure rollers. When mineral-surfaced products are made, the mineral-surfacing materials, which have been stored in hoppers, are spread over the hot asphalt- coated surface and run through a series of pressure and cooling rollers. To make shingles, the coated sheets are cut, stacked and packaged. When rolled materials are made, the coated sheet is rolled on rollers, cut and packaged.

GYPSUM PRODUCTS

Gypsum rock is mined and crushed in a hammer mill to a 1/2-inch particle size. It is calcinated in a rotary steel, 150-foot cylinder lined with fire- brick or a kettle calciner by heating it to 350 degrees. When aggregates are used, they are washed, crushed and graded. Vermiculite and perlite aggregates are heated to 2,000 degrees to expand them. When lime is used, it requires special processing. It is calcinated and slaked (combined with water).

Plaster is manufactured by taking the gypsum from the kettle calciner and treating it in a tube mill, a rotary cylinder containing thousands of steel balls that grind up the gypsum. Retarders are added to control the rate of set, which should be up to four hours. For fibered plasters, organic fibers are added, and for ready-mixed plasters, lightweight aggregates are added. The plaster then is packaged and shipped.

Plasterboard manufacture is started by taking the gypsum from the calciner and mixing it with water to form a slurry, which is fed onto a conveyor belt between two layers of paper. It sets in a few minutes to a hardness that permits it to be cut into pieces and conveyed to kilns for drying. Special backings and finishes, such as aluminum foil, decorative vinyl or paper finishes, are affixed and the boards are stacked and shipped.

ASBESTOS CEMENT PRODUCTS

Manufacture

The first steps of the three common manufacturing processes (wet-mechanical, dry and wet-formed) are the same. In most plants, it's a continuous process starting with mixing asbestos fiber, portland cement and water and forming the mixture into a plastic sheet. The sheet is hardened, trimmed to its final size, punched with nail holes and cured. Colors are either mixed in during the mixing stage or coated onto the hardened sheet. Textures are embossed on the sheet while it still is plastic.

The most commonly used wet-mechanical process is similar to paper- making. The ingredients are dry-mixed, after which water is added to form a thin slurry. The slurry is picked up by wire-mesh rollers, deposited on a continuous carrier blanket and then transferred into an accumulator roll where the thickness is built up to the required amount. The sheets, now in a plastic form, are removed and processed as described previously. This process produces a built-up, laminated sheet with asbestos fibers aligned in one direction of the fiber alignment.

The dry process is used by some manufacturers to produce roofing and siding sheets that are homogenous in nature rather than laminated. These sheets have uniform strength in all directions because the asbestos fibers aren't aligned in any one direction. The ingredients are dry-mixed and distributed in an even layer on a continuous moving belt. The layer of material is sprayed with water and compressed with steel rollers. The rest of the process is the same as described previously.

Large flat sheets of asbestos cement are made by a molding process called the wet-form process. The ingredients, including water, are mixed into a slurry and poured into the mold of a hydraulic press that forces out most of the water and compresses the mixture to the required density. The fibers are nonaligned and the sheet produced has uniform strength in all directions.

After the product has been formed by any of the previously described three processes, it must be cured. Like any product made with portland cement, it will become harder and stronger with age and with proper moisture conditions.

Normal curing is accomplished by stacking the finished products when they leave the production line in a curing room for approximately a month. The temperature and humidity in the curing room are carefully controlled. Steam not under pressure sometimes is used to provide the required warm, moist air.

Curing time may be reduced substantially by the autoclave curing (high-pressure steam) method; after a few days of normal curing, the material is placed in an autoclave and subjected to high-pressure steam.

CABINETS

Manufacture

A standard cabinet is made of various components: front frames, end panels, door, backs, bottoms, shelves, drawers and hardware. The front frame usually is made of 1/2-inch to 3/4-inch thick hardwood. The pieces, known as rails, stiles and mullions, are doweled or mortise-and-tenoned and glued together.

The end panels usually are made of 3/16 to 1/4-inch plywood. When thin pieces are used, they are glued to a frame to give them stiff ness, but when thick pieces are used, no frame is necessary.

Doors are made in a variety of ways. Some are solid wood or plywood, others are hollow or filled with particleboard, wood, high-density fiber or plastic. Some doors are faced with plastic; others are solid plastic. Backs are made of hardboard or plywood. The bottoms and tops are 3/4-inch to 1/2-inch hardwood plywood. Shelves usually are boards, ply wood or particleboard from 1/2 to 3/4 of an inch thick. Drawers often are made of hardwood lumber sides and backs and plywood bottoms.

The large cabinetmaking factories are almost completely integrated operations. The raw material comes in and is completely processed into a finished cabinet. Only a few minor components are manufactured else where. These plants include complete milling operations as well as assembly facilities. Other smaller cabinetmaking factories only assemble parts that have been milled elsewhere. In the large plants, cabinets are made on a production line like automobiles. Each part is made in its own department and then fed simultaneously into the production line for final assembly.

The front frame is made from stiles, rails and mullions that have been cut previously and shaped in the mill section. They are assembled in an air press that holds them in place until they are glued and stapled. After being left to dry for a day, the frame is sanded and holes are drilled in it for hardware.

DOORS

Manufacture of Wood Doors

Most doors used in houses are factory-made in standard sizes and with standard detailing. In very expensive houses, however, doors may be specially made to an architect’s specifications. Builders today rarely make doors themselves, although in the past this was more common.

Wood doors may be made out of many hardwoods and softwoods. Most flush doors are made with hardwood-veneered faces. Most stile and rail doors are made from ponderosa pine and other softwoods. Accordion-folding doors are made from hardwoods and softwoods.

The stiles, rails and lock blocks for flush doors are cut from kiln- dried, surfaced lumber. The stiles and rails are joined by metal fasteners or dovetailed joints. The frame then is glued to the face panel and core. Hollow flush doors are pressed in a cold press to assure a good bond. Solid core doors are pressed in a hot press to assure proper setting of the glue. Additional optional operations on some doors include routing for louvers and glass openings and installing muntins and bars prior to glazing.

The kiln-dried lumber for stile-and-rail doors is surfaced and cut to exact width and length. Holes are bored in the stiles and rails to accept the dowels. A molding machine is used to form the sticking slot (the slot for the glass or panel) on the edge. The dowels are inserted into place and glued. In a separate operation, the panels are cut, sanded and shaped to the desired size. Next, the panels are set into the sticking slot, glued with a water-resistant adhesive and clamped in a jig until the glue has set. Then the door is sanded to a smooth finish on both sides.

If the door contains glass, the glass will have been previously cut to the correct size. It is set in place with a glazing component and the stops that hold the glass are nailed in place.

Additional processes that may take place at the factory to reduce on- site preparation are coating the door with a seal coat or complete surface finishing. Usually, prefinished doors are pre-fit to exact opening measurements so that they will not have to be trimmed and their finishes marred. Sometimes the hardware is factory-installed.

Pre-hung doors, where the door is attached to the frame, also are factory-made. The complete frame then is set into the wall frame or partition, thus eliminating the difficult step of hanging the door at the site.

Manufacture of Aluminum Doors

The use of aluminum for sliding doors and combination storm doors and screens continues to increase. The principal materials used are wrought aluminum and aluminum screening. Cast pieces are used for the locks, hinges, handles and corners.

Doors are produced with natural mill finish, protective finish and decorative finish. The natural mill finish is a silver sheen when new that turns to a soft gray with age. In sea coast and industrial areas, a protective finish known as anodizing often is applied. This electrolytic process provides a heavy oxide film on the surface that can be clear or contain permanent color tones. Decorative finishes of colored or opaque synthetic resin enamel also are used. They last up to 20 years but aren't so permanent as anodizing.

Aluminum doors are fabricated by taking the extruded and cast stock and sawing, drilling and punching it to obtain the desired size and shape and then assembling it into the finished product including the glass, screens and hardware.

(more info on doors)

WINDOWS

Manufacture of Wood Windows

In the past, the vast majority of windows installed in houses were wood stock windows, which still are very popular. Wood has good insulating qualities and takes either natural or painted finishes. In addition, wood windows are easy to repair with simple tools.

Techniques of kiln-drying the wood have reduced the shrinking and warping problem. Water-repellent preservatives and chemical treatments further reduce swelling and warping, improve paint retention and in crease the wood’s resistance to decay and insect attack under all climate conditions.

The best wood for windows seems to be ponderosa (western) pine be cause of its excellent workability, gluability, nail-holding capacity and uniform light color suitable for natural or painted finishes. The wood must be kiln-dried, sound and free from defects such as loose knots and checking.

The lumber is cut into the proper lengths and widths and then milled to the desired profile. A cross-milling machine cuts the grooves and channels and routing machines cut notches for the hardware. The pieces then are treated to make them water-repellent and decay-resistant. Sometimes, they also are factory paint-primed or even paint-finished. If they are to be weather-stripped, this is done at this stage of fabrication. The pieces then are assembled into the desired shape and nailed together on an automatic nailing machine. Next, the completed sash is sanded to a smooth finish by a three-step (coarse, medium, fine) sanding machine. The final steps consist of glazing, fitting the sash to the frames and installing the hardware.

Manufacture of Aluminum Windows

The use of aluminum windows continues to increase and they now ac count for about half of the stock house windows being produced today. Their popularity can be attributed to their ease of production, attractive appearance and long maintenance-free life.

The principal materials used in manufacturing these windows are wrought-aluminum alloys that are formed by the extrusion process and then artificially aged by heat treatment (see the section on aluminum products in Section 7). The piece is extruded into the complex shape required for glazing, weather-stripping, condensation control and assembly. Cast pieces are used for the locks, hinges, handles and corners.

Most windows are produced with a mill finish that's a natural, silvery sheen. As the window ages, an aluminum-oxide coating forms that dulls the surface to a soft gray and protects it from further corrosion.

Protective finishes are necessary only in industrial and sea-coast areas where the air carries excess corrosive materials. The most common protective finish used is an electrolytic process that provides a heavy oxide film on the surface known as anodizing. This process can leave a clear coating or it can contain permanent color tones. Other protective coatings occasionally used are paint and methacrylate-type lacquer.

Decorative finishes of colored or opaque synthetic resin enamel aren't so permanent as anodizing, but they will last up to 20 years when baked on at the factory.

Aluminum windows are fabricated by taking the extruded and cast stock and sawing, drilling and punching it to obtain the desired size and shape and then assembling it into the finished window, including the glass and hardware.

(more info on windows)

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