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Cutting Rigid or Flexible Foams Plastic foams can be rigid or flexible, and they require different cutting methods. Rigid foams such as polystyrene and polyurethane can be cut with most of the same tools and techniques used for acrylic glass. A jig saw, saber saw or band saw, for example, will make quick work of cutting curved or straight lines in rigid foam. The band saw should be fitted with a wood-cutting or metal- cutting skip-tooth blade having at least 10 teeth per inch. A table saw will also cut rigid foam quickly, but it’s limited by its blade height to foams less than 3 inches thick. Thin sheets of rigid foam, up to ½-inch thick, can be cut with a razor knife, and very thin sheets can be cut with a paper cutter or with tin snips. Flexible foams require more ingenuity because they cannot be cut with the power tools associated with woodworking. For thin flexible foam, scissors or a razor blade is usually satisfactory, and an electric carving knife is an excellent tool for cutting straight lines or contours in thicker flexible foam. Another possibility is a commercial foam cutter. Like the carving knife, it consists of two blades moving in opposite directions, but the blades are up to 12 inches long and are attached to a flat base that makes it easier to hold a perpendicular edge. Rough or uneven edges on rigid foam can be smoothed with a forming tool. Uneven edges on flexible foam are more difficult to deal with, so it’s best to make the initial cut as carefully as possible. However, most flexible-foam edges will eventually be concealed—by the upholstery fabric that usually covers them. For an interior cut, puncture the foam with a sharp knife, and push the electric-knife blade through the slit. Proceed as above. Using an Electric Carving Knife to Shape Foam 29--- ===Making a straight cut. Set the foam on a worktable, positioning it so that the cutting line overhangs the table edge at least an inch. To begin the cut, hold the electric knife at a 45° angle to the top plane of the foam, and draw it into the foam about two inches. Then move the knife to a perpendicular position and continue cutting. Be sure to keep your fingers clear of the blades at all times. To use a foam cutter, start with the blades upright. Grip the handle, pull the trigger and guide the machine forward—a wheel on the underside rolls the machine along the table. ===Cutting curved shapes. Stand the foam on edge, cutting outline facing you, and start the cut at the top edge. Follow the outline, keeping the knife blade perpendicular to the foam surface. To make cutting less awkward, you can stop the knife from time to time and change the position of the foam so that you are always cutting with a downward stroke. For an interior cut, puncture the foam with a sharp knife, and push the electric-knife blade through the slit. Proceed as above. To cut curved shapes with an electric foam cutter, lay the foam flat on the work surface and guide the machine along the curve shifting the position of the foam as necessary. Drilling and Counterboring Drilling holes in plastics, whether for fastener’ or for decoration, is done with the same tools used for drilling holes in wood. But added procedures and precautions are needed, to accommodate plastics incompressibility, brittleness and low softening temperatures. The first of these, incompressibility, calls for greater accuracy in drilling holes that must align: Wood fibers will com press to a certain extent when a bolt, screw or plug penetrates two holes that don’t precisely match, but some plastics will crack. Where alignment is important, therefore, it’s best to use a drill press or to mount a portable drill in a drill stand. In less critical situations, holes can be drilled freehand with a manual drill or a power drill. When you are using a variable-speed power drill, the speed set ting should be in the slow range (500 to 1,000 rpm’s) with a high-speed twist bit, and in the high range (up to 5,000 rpm’s) with a spade bit. The drill should always be fed into the work slowly and steadily but without excessive pressure, then slowed even more as the tip nears the breakthrough point on the opposite side. For the actual piercing of the plastic, a high-speed twist bit is preferable, especially when you need to penetrate all the way through stock. But the bit needs modification to keep it from splitting or cracking the plastic as it cuts. The tip of the bit must be blunted very slightly to keep it from causing small split lines on the other side of the plastic. Except on very thin plastics, the blunted tip permits the full width of the bit to enter the stock before the tip br through on the opposite side, stabilizing the drill’s action so that it cuts a straight, clean hole. The two sharp leading edges of the bit, called the cutting lips, should also be blunted, so that they plow rather than slice their way along. Some twist bits made for plastics include these blunted features, but it’s easy enough to convert an ordinary twist bit with a file. The configuration of the bit will depend on the size and type of hole need ed. A brad-point bit, a spade bit or the drill fitting called a hole saw will work well for drilling plastics. And for counter sinking fasteners, you can use the counterbore, countersink and combination drill-countersink bits made for wood. As further precautions against chipping the plastic, leave the protective paper in place until the drilling is complete. In anchoring the plastic to a work surface or in a vise, use wood scraps to protect the plastic surface from the jaws of the clamp or vise. Working with the plastic at room temperature, rather than when the material is cold, will also discourage chipping. The low softening temperature of some plastics is both a bane and a blessing in drilling. Bits cutting deep holes should be backed out often to clear the material. Bits should not be stopped in the hole, lest they be seized by plastic cooling around them. On the other hand, the softened plastic lends itself to a technique for producing a clean, smooth hole. Drill a pilot hole slightly smaller than the ultimate hole, and put beeswax or paraffin into it. Then drill again with the proper size bit—the wax lubricates the bit as it spins, helping to expel more chips, and together with the softened plastic, it smooths the walls of the hole. Finally, when you are working with thin plastics such as acetates and laminates, holes can be punched as well as drilled. Any of the hand punches used for leatherwork will penetrate these plastics. 30--- ====Drill bits for plastics work. A high-speed twist bit, with its tip slightly blunted, is the best choice for drilling plastics. One designed for wood cutting will drill holes in acrylic glass; a metal- cutting type is best for tough thermoplastics such as nylon, polycarbonate and cellulosics. A brad-point bit, available in diameters of up to 1 inch, is good for drilling holes that go just partway through the plastic. A spade bit can be used to counterbore holes up to 1 inch wide; its broad paddle provides a convenient surface on which to mark the desired - hole depth, and its tip leaves a centering mark for a second bit when you are making a small hole at the bottom of the wide one, for a bolt or screw. It should be very sharp, not modified. A hole saw, available in diameters up to 4 1/2 inches, is useful for large holes in any thickness of plastic. A pilot bit in the center positions the saw on the work, and knockout holes in the sides make it easy to push out the cut disk. Modified Wood Bits, Special Supports 31a--- 1 --- Modifying the bit. Round the tip of the bit with a metal file or a bench grinder. Then file or grind the two sharp edges of the cutting lips, making both uniformly blunt. The drawing below shows a twist bit before (left) and after modification. The cutting angle has been changed so that the drill will emerge without fracturing the plastic. Check the bit by drilling a practice hole in a scrap of acrylic glass. The drill should produce continuous spirals of waste plastic, all the same width. Before drilling, use a punch to make a small dent to hold the tip in place. 2 --- Drilling the hole. Clamp the plastic in a vise between two pieces of scrap wood; the scrap be hind the work should be large enough to back up the hole. Turn on the drill and apply even pres sure to bore the hole. On a drill press, back the plastic where the hole is to be drilled with a piece of wood. Center the hole mark under the bit; clamp the wood and plastic to the table, using additional pieces of scrap wood to protect the plastic from the clamp jaws. Drill as above. For large holes in thin plastic, place scrap wood over as well as under the plastic, and drill through the sandwich. For holes in round stock, use a V block to hold the rod or tubing, and clamp the V block to the work surface. A Two-Part Hole 31b---- === Counterboring a hole for a bolthead. === Fit a spade bit or a hole saw into the drill press and center the hole mark under the tip of the bit. Clamp the plastic to the drill-press table, protecting the plastic from the clamp jaws with wood scraps. No backing is needed when the hole is being drilled only partway through the plastic. Set the depth stop on the drill press, turn on the drill and lower the bit to make the first cut. Then replace the spade bit with a twist bit 1 6 inch larger than the diameter of the bolt. Check to make sure the tip of the twist bit lines up with the small hole left by the tip of the spade bit. Put a piece of wood behind the plastic. Reset the depth stop, then drill the second hole. To drill a matching bolt-hole in the second piece of plastic, align the pieces, and push a pencil or an awl through the top piece to mark the second one. Set the depth stop and, using the same twist bit, drill the matching hole. Rigid Sheets and Tubes Made Pliant with Heat Several plastics have the useful property of becoming pliable when they are heated to temperatures ranging from 250 to 300 F. Once heated, they can be bent into new shapes. The plastics best suited for heating and bending at home are acrylic glass (commonly sold as Lucite or Plexiglas) and polyvinyl chloride (PVC) pipe. The acrylic, fabricated in sheets of various thick nesses that may be transparent, translucent, tinted, or even mirrored, is often used for making room dividers, break- resistant windows and windscreens. PVC pipe, available in many colors and diameters, is most commonly seen in plumbing disposal systems and drainage fields. After it has been heated and bent, PVC pipe can no longer be used in plumbing, since its walls are usually weakened at the bends. But it’s still strong enough to be used for shelf supports or the frames of outdoor furniture. Sheets of acrylic glass that are 1/8 inch thick become pliable when exposed to 300° heat for about 4 minutes; sheets that are ¼ inch thick need to be exposed to 300° for about 10 minutes. The 1 acrylic is the thickest that is practical to bend at home. PVC pipe softens even faster, in about a minute, at even lower temperatures—around 250°. Neither plastic should be exposed to heat over 300°; both may bubble, scorch and undergo irreversible molecular and structural change if overheated. Both acrylic glass and PVC pipe cool rapidly and should be shaped quickly while they are at their maximum forming temperature. Moreover, in most cases acrylic should be bent about 5 degrees beyond the desired finished angle, then allowed to retract to that angle, at which point it should be held or clamped for a few minutes of final cooling. Over-bending is necessary because the polymers in the plastic (page 97) tend to resist the bend and to return to their former con figuration as they cool—a phenomenon known as “memory.” When the acrylic is completely cool, this tendency is thwarted, and the polymers are in effect frozen in place. The memory phenomenon has one useful result: It permits novices to correct mistakes and try again simply by re-warming the plastic. An oven is the most obvious source of heat for bending acrylic glass at home, but the best way to warm PVC pipe to forming temperature is to pour heated sand inside it; this not only softens the plastic but also prevents the pipe walls from collapsing during bending. Infrared heat lamps may be used to heat large sheets of acrylic glass or isolated sections of a single sheet. In this procedure, the acrylic should be placed on a light-reflective surface, such as sheet metal; shields of the same material placed on top of the plastic can further localize the area to be heated. A special heating iron, sold at paint stores to soften paint for removal, will also do an adequate, though slower, job of heating acrylic to forming temperature, provided the area to be bent is small enough for the iron to heat it all at one time. One of the most useful heaters for localized areas of acrylic glass is a strip heater; its key feature is a ½-inch-wide tape interlaced with electrical wire to form a flexible heating element. Al though commercial models are available, a homemade strip heater () is less costly and satisfies most home bending needs; the tape is available at plastics-supply stores in 36-inch lengths, complete with the wiring needed to connect it to a house outlet. Because a strip heater turns a plastic sheet pliable in a very limited area, bending can often be done with only the eye to judge the final configuration. For greater precision, however, the plastic can be bent over a form and locked in place with blocks or clamps. If the bend line is warmed over a strip heater, the form can be no more than two sides of an angle (bottom). But if a sheet of plastic is heated in an oven, you can con figure it with more complex forms—a coffee can, perhaps, or the matching halves of a wooden mold cut on a jig saw, or blocks and clamps that hold the plastic in a multi-angled shape until it’s cool. Although acrylic can be bent into an acute angle, the actual apex of the angle will be slightly rounded—unless you adopt a special procedure. A sharp interior corner can be achieved if a V groove is cut along the bend line before the acrylic is heated; the groove should be half as deep as the thickness of the plastic. When the plastic is bent, the V will form a sharp inside corner; the outside of the corner, however, will be rounded. Despite the ease with which acrylic glass and PVC pipe can be bent, there are pitfalls. One, to which acrylic is vulnerable, is scratching. To protect the acrylic surface, it’s best to leave its masking paper in place except in areas that are to be heated. All paper must be removed, of course, from plastic that will be heated in an oven. But when you use a strip heater, all of the paper can be left in place except for a narrow band. Another pitfall is under-heating or over heating the plastic, When the plastic is bent before it has reached the proper temperature, its internal structure will be strained and small cracks, called crazing, may appear. At the other extreme, over heating may cause the plastic to bulge at the edges of the bend—although these protrusions can be ground down in the finishing process. A third pitfall is plastic’s tendency to pick up the texture of the material against which it’s formed. To prevent this kind of unwanted mimicry, the form can be covered with kraft paper, felt or finely woven cloth. Of course, no plastic should contact a flame or heated electrical wire, or be left unattended while it’s being heated. In addition, when an oven is used for heating, its door should always be left slightly ajar so that potentially toxic fumes re leased from the plastic won’t build up inside, and the kitchen should be adequately ventilated. A Simple Heating Tool for Bending a Narrow Strip 33a--- ===Anatomy of a strip heater. A homemade strip heater, shown assembled in the inset at right, is a layering of wood, foil and insulation paper into which a band of electric heating tape is placed. The base is a length of ½- inch-thick plywood 6 inches w de and about 6 inches longer than the heating tape. Nailed on top of the base are two 1 plywood strips set ¾-inch apart to form a shallow channel as long as the heating element. Covering the strips and the space between them are two layers of heavy-duty aluminum foil. At one end, the tolls pierced by a screw for a ground wire, which should be long enough to reach a ground connection such as the cover-plate screw on an electrical outlet. Placed over the foil is a double layer of high-temperature insulation paper, normally used as an oven liner and available in hardware stores. The paper is scored or dampened to follow the contours of the channel and stapled in place, along with the foil, against the plywood. The heating tape is centered within the channel, pulled taut and tied with its strings to pairs of nails tacked near the ends of the plywood base. Forming an Angle with a Homemade Strip Heater 1 Fashioning a cardboard guide. Score and fold a piece of heavy cardboard into the desired angle, anchoring it with masking tape against a second piece of cardboard. Check the angle with a protractor, and adjust the form if necessary. 2. Preparing the acrylic. Measure and mark the location of the bend line on the two edges of the plastic -- using a grease pencil or a china-marking pencil. Measure out 1½ inches on each side of these marks, then draw two pencils across the face of the acrylic, still covered with its protective masking paper. Score the masking paper lightly along the lines, using a dulled glass cutter so that you don’t mar the surface of the acrylic. (To dull the blade of the cutter, run it back and forth several times over a piece of metal.) Exerting just enough pressure to score on y the paper takes practice; try it on acrylic scraps first. Finally, peel off the 3-inch strip of masking paper between the lines, a long with its adhesive, and remove any traces of the adhesive with the recommended petroleum base solvent. For a precise 90-degree angle, groove the bend line with a router’s V bit. You may polish the faces of the V by sanding them, then wiping with a cloth soaked in methylene dichloride, but observe the procedures and cautions noted below. 3. Heating the acrylic. Position the acrylic on the strip heater so that the two marks for the bend line are centered over the heating tape. Weight the acrylic with metal blocks to keep it from shifting position and, with a soft cloth, rub off the bend-line marks. Plug in the strip heater and warm the plastic until its edges droop slightly. 34 4. Bending the acrylic. Remove the weights from the acrylic on the far side of the heater. Hold down the near side of the acrylic with one hand while you grasp and gently pull the far edge toward you, bending it to the desired angle and then about another 5° beyond. Remove the acrylic from the heater, set it on the workbench and allow the bend to retract. Then, while the acrylic is still warm, hold it over the form—the one shown here is in the shape of a tent—until it has cooled, about a minute. To bend an acrylic sheet that has already been grooved, set it in position over the heater so that its grooved side is facing up. Bend the warmed acrylic sheet only until the two faces of the V meet; over-bending is not necessary in this case. The Softening Influence of a Heated Oven 1. Heating in an oven. To soften a sheet of acrylic to a rubbery consistency, remove its masking paper and place it on a clean, flat cookie sheet in a preheated 300° oven. Leave the oven door slightly open so that any fumes released by the plastic can escape, and ventilate the kitchen. Acrylic that is 1/8 inch thick will soften in about 4 minutes, ¼-inch thick acrylic in 10 minutes. To guard against overheating, check the oven controls first with an accurate oven thermometer. 35 2. Forming the sheet. Curve or twist the softened sheet into the desired shape, working by eye or using a prepared form. For a cylindrical shape or a half cylinder, hold the sheet over a coffee can or an oatmeal box, padding the can or box with felt to prevent the acrylic from mimicking irregularities. For compound angles, such as those needed to form a lipped shelf, clamp the plastic between blocks and wedges, keeping n mind that the corners of the blocks must be slightly beveled to accommodate the plastic’s rounded corners. For an undulating shape, clamp the plastic between the two matching halves of a wooden block that has been cut into a male female mold (inset). Bending PVC Pipe into Shape with Hot Sand 1---Filling the pipe with sand. To warm the sand, place it in an old pot over low heat on the kitchen range. Stir it occasionally to distribute the heat, and check its temperature every few minutes by putting the end of a scrap of PVC pipe into the sand. When the sand is warm enough to make the PVC pliable, temporarily cap one end of the pipe to be formed and pour sand into the other end, through a funnel. Cap the pipe when it has been filled. Depending on the weight of the sand within the pipe, you can use regular PVC caps or several layers of aluminum foil se cured with rubber bands. 2---Bending the pipe. Test the sand tested for flexibility, and as soon as it’s easy to bend, shape it by hand or around a form. To hasten the cooling process, have a helper remove the caps; then you can empty the sand out or flush it out with a hose. Spraying cool water on the pipe will also help shorten the setting time. 36 +++++++++++ +++++++++ +++++++ ++++++++++ Giving Luster and Polish to Edges and Surfaces
Most decorative plastics come fully polished, protected by paper masking. But unmasked edges must be smoothed, as must any cut or scratched surface. This smoothing process consists of scraping, filing and sanding to remove rough edges and tool marks, followed by buffing and polishing to bring a high gloss to the surface. Not all surfaces need to receive the full treatment. The edges of a tabletop or a shelf should be buffed and polished until they are crystal clear, but edges that are to be glued or welded can simply be smoothed and left opaque. The first step, scraping, applies especially to hard plastics, such as acrylics and polycarbonates. A refinishing scraper is the most efficient tool, but you can improvise by grinding down an edge of a triangular file. Some softer plastics, such as polyvinyl chloride (PVC), can be smoothed with a block plane. Along inside edges and interior cutouts where a scraper cannot reach, filing is the principal finishing technique. Wood working tools, such as smooth-cut rasps and bastard-cut mill files, are best for finishing edges; half-round, triangular and curved riffler files are useful for tight curves and fine details. A piece of chalk rubbed over the blade will keep the file from sticking. Sanding, the next step, is best done wet because the friction-caused heat of dry sanding can soften most thermoplastics. Abrasive papers for plastics are avail able in hardware stores. Steel wool can be substituted for abrasive paper. Mechanical sanding with an orbital or a belt sander is faster and easier than hand sanding. To avoid heat build-up, keep the sander or the plastic in constant motion, avoid extreme pressure and frequently wet the surface being sanded. Like sanding, buffing can be done either by hand or mechanically. To buff by hand, use either a shoeshine buffer or a piece of soft flannel wrapped around a wood block. When buffing irregular forms or curved surfaces, use a strip of flannel in the same way a shoeshine cloth is used. Buffing compounds of the type commonly used on metal are also suitable for plastic. They come in several grades, colored according to coarseness. The finest of these compounds, usually white, are the coarsest you will usually need for plastics. Mechanical buffing can be done on any kind of buffer. You can adapt a double-shafted bench grinder for buffing by substituting loose cotton buffing heads for the grinding wheels. For large sheets, use either a portable electric shoe buffer or an electric drill fitted with a buffing disk. Coat, or charge, the buffing head by holding a stick of buffing com pound against the rotating wheel or disk until it’s well covered with compound. Any good-quality floor wax or car wax can be used to polish plastics, but plastics manufacturers often recommend specific compounds. These polishing pastes are applied in exactly the same manner as the buffing compounds. In thermoplastics, such as acrylic and polycarbonate, drilled holes can be polished with methylene dichloride solvent, available in solvent-cementing kits. The solvent, poured into the hole, works on the plastic by dissolving the thin frosty layer. This solvent can also be applied with a cloth to smooth and polish the sides of a V groove. Shallow scratches on the face of plastics can be polished out with white toothpaste. If the scratch is deep, sanding may be necessary, but because optical distortion may result, clear acrylic windows or skylights should not be sanded. = = = Abrasives Useful for Sanding and Buffing - - - Abrasive | Recommended coarseness [Rough, Medium, Fine] | Comments - - - - Sandpapers: Garnet Silicon carbide Tungsten carbide -- 100 grit 80—100 grit 36 grit -- 180 grit 180—220 grit 80 grit -- 280 grit 330-600 grit 150—200 grit -- Durable Fast-cutting, durable Very durable, can be cleaned with a wire brush or methylene dichloride solvent -- -- -- Steel wool | No. 0 | No. 00-000 | No. 000-0000 | Can be used instead of sandpaper Buffing compounds Rouge Tripoli Available in stick or cake form Cuts slightly faster than rouge
==== A full range of abrasives. Abrasive materials commonly used on plastics are listed in the column on the left. Listed to the right of each abrasive is the suitable range of grit or grain for rough sanding, semi-finishing and fine finishing. All of the sandpapers listed can be used wet, and for plastics they should be. Always start with the finest grade, or highest grit, abrasive pa per so that the surface of the plastic won’t be roughened more than necessary. Step-by-Step Smoothing of Tool-Roughened Edges 1--Scraping to remove saw marks. Clamp the damage in a vise, protecting the masked faces from damage by inserting scrap wood between the plastic and the vise jaws. Grasp the scraper with both hands and position it across the stock, tilting it back toward you at an angle of 60°. Starting at the far end, drag the edge of the scraper over the length of the stock repeatedly, from back to front. Never push the blade forward. Use moderate pressure and long strokes to avoid creating depressions in the piece. 2--Wet sanding. Fit a slab of plate glass or wood into one end of a shallow metal pan, and add water to a level just below the surface of the slab. Lay a full sheet of 100-grit wet-or-dry sandpaper on the slab, grit side up, and wet the paper. Dip the plastic in the water and move the edge over the paper in an oval pattern. Keep the edge flat on the paper and use even pressure. At frequent intervals, wet the plastic and rinse off the abrasive paper. When the surface is even and smooth, repeat the process with progressively finer grades of paper until the sanding marks are almost undetectable. Remove the fine dust by rinsing the plastic under running water. 3--Buffing and polishing. To buff an edge, lift one end of the plastic against the bottom quarter of a buffing wheel coated with white com pound. Keep the stock in constant motion, moving it from side to side and up and down. Buff the edge up to its center point, then reverse the work to buff the opposite end. Wash the stock with soap and water to remove the abrasive buffing compound. Then polish the edge by coating the second buffing wheel with polishing paste or paste wax, and move the edge over the wheel, as above. Continue polishing until the edge is shiny, and then wipe the edge with a clean, soft cloth. To buff and polish an edge by hand, clamp the plastic in a vise between protective wood scraps (insert), and use a sheepskin shoeshine buffer coated with the same compound and wax as above; use separate buffing heads for the two operations. Hold the buffer flat against the edge and move it back and forth, parallel to the edge. Shift the position of the plastic often, to smooth the edge evenly. Smoothing and Buffing an Inside Curved Surface 1 --- Filing with a half-round file. Clamp the plastic in a vise between protective wood scraps and, gripping a single-cut, half-round file with two hands, tile across the plastic at right angles to the edge. Use light, even strokes and a forward motion, twisting the file a half turn on each stroke. Rotate the plastic in the vise as needed to keep the surface that is being filed just above the jaws. Use a file card to clean the teeth of the file after every few strokes. 2--Sanding and buffing. Wrap a piece of 100-grit wet-or-dry sandpaper around a dowel smaller in diameter than the curve to be sanded. Wet the sandpaper and the plastic with water, then sand with a combination back-and-forth and rolling motion, keeping the dowel level with the edge and working slowly around the circle. When the edge is fairly smooth, change to 240-grit, then to 400-grit paper. Rinse the plastic well with water each time you change the paper. Replace the abrasive paper with felt. Coat the felt liberally with white compound and buff the edge, using the same technique as in sanding. Buff until smooth, then wash off the buffing compound with soap and water. 3--Polishing to a shine. Saturate a strip of lint- free cloth with polishing paste or paste wax. Holding the ends of the cloth firmly, draw if briskly back and forth over the surface. Polish until the surface shines, then repeat the procedure with a clean, dry piece of cloth. Removing a Scratch from an Acrylic Sheet 40a--- 1. Sanding the sheet. Lay the plastic on a piece a at work surface, and wet the plastic with water using a sanding block and a fine-grade sandpaper (approximately 280-grit or shallow scratches), sand very lightly in a star pattern. Keep the sheet and the sandpaper wet, and work over a wide area to avoid creating a noticeable depression. As the scratch disappears, use progressively finer grades of wet-or-dry paper in the same star pattern until the surface is uniformly smooth. Wash off the dust with soap and water, and dry the plastic with a damp chamois. 2. Buffing and polishing. Use a portable electric drill fitted with a wool buffing disk, and coat the disk with buffing compound. Keeping the disk in constant motion, buff the sheet in small, over lapping loops until the sanding marks have disappeared and the stock is slightly shiny. Wash away the buffing compound, using your bare hand and soap and water. Polish the sheet in the same manner as above, using a clean wool buffing disk, and either paste wax or polishing paste. When the sheet has the desired luster, finish the job by hand-polishing it with a clean, dry cloth. Two Ways to Polish a Small-Bore Hole 40b--- ===Using a drill stand. Cut a piece of wooden dowel, slightly longer than the depth of the hole to be polished and 1/8-inch smaller in diameter, and cut a ½-to 1-inch slit at one end. Slip a small piece of 280-grit wet-or-dry sandpaper into the slit (inset) and tighten the dowel in the drill chuck. Clamp the plastic, supported by a piece of scrap wood, against the drill-stand table, centering the hole under the dowel. Squeeze a few drops of lubricating oil into the hole, then raise and lower the rotating dowel into the hole until the inner surface is smooth. Clean the hole, then repeat the process, substituting a strip of felt, coated with white com pound, for the sandpaper. Clean the hole again and repeat the process, using a new piece of felt coated with paste wax or polishing paste. The same results can be accomplished by hand if you simply twist the dowel between your thumb and your forefinger, at the same time moving it slowly in and out of the hole. 41--- ===Solvent polishing. After knocking any loose particles or dust from the hole, hold the plastic up right and use a medicine dropper to fill the hole to the brim with methylene d chloride. Let the solvent work for 30 seconds, then pour it into a disposable container. Flush the hole immediately with soapy water; rinse with clear water. = = = = The Light Touch That Cleans Plastics For all their toughness, plastics require a surprisingly gentle hand when it comes to maintenance. Thermoplastics—especially acrylic, polycarbonate and polystyrene—can be easily scratched, and because these plastics are usually clear, scratches caused by rough cleaning will be highly visible. To remove superficial dirt from a thermoplastic, rinse the surface first with running water to rid the surface of any loose particles, which can scratch. Then wash with soap or a mild, non- abrasive detergent and water, using your bare hands or a soft cloth. Don’t wipe with a completely dry cloth, since doing so can generate an electrostatic charge that attracts more dust and dirt. Antistatic solutions called demagnetizers, available from plastics suppliers, ne gate this charge and also serve as a protective polish to fill in shallow scratches and prevent future ones. Avoid abrasive scouring compounds, boiling water and harsh solvents such as acetone, benzene, toluene, tetrachloride, and lacquer thinners—all of which will attack thermoplastics. So will many window cleaners. isopropyl alcohol will remove the adhesive residue sometimes left by the paper masking on thermo plastic sheets. And if a solvent is needed to cut grease on these surfaces, use a good-quality kerosene or naphtha; re move the solvent immediately with soap and water, and dry the surface with a damp chamois. To remove yellowing from polycarbonates, buy butyl cellosolve from a plastics supplier and re-polish the surface with a polishing paste or paste wax, buffing it back to its original luster with a soft cloth. Thermosetting plastics, such as the laminates used on kitchen or bathroom countertops, are not as sensitive as the thermoplastics are to solvents or abrasives, but they can be scratched with sharp objects and will discolor if subjected to high heat. Don’t try to re move spots with metal scrapers or scouring pads; wipe with a wet sponge and soap or a mild scouring compound. Polish with a clean cloth. Fiberglass-reinforced plastics have a tendency to craze, or develop fine surface cracks, when attacked by harsh sol vents. Washing with detergent and water will remove most dirt; use isopropyl alcohol or kerosene when a solvent is needed to clean tough stains. = = = = |
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