Guide to Working with Plastics -- Man-made Materials: Many Uses (part 2)

Where and How to Buy Stock

Despite the exotic names of the plastics in the chart, you can buy many of them without a trip to a specialized plastics-supply house. Glass-supply and hardware stores stock sheets of clear acrylic and polycarbonate for glazing, usually in thicknesses of 1/8 to 3/8 inch. Lumber yards carry PVC pipe, corrugated sheets of glass-reinforced polyester used for covering patios, decorative laminates for countertops and cabinet facings, and polyethylene film. Large lumberyards, hardware stores and paint stores are the best sources for many plastic-base coatings, adhesives and fillers.

Even if the type of plastic you need should call for a trip to a specialized plastics dealer, there are likely to be several listed in Google or the classified telephone directory of any major city under “Plastics Supply Centers.” Although many of them technically are wholesalers, you will usually not be required to buy in wholesale quantities.

Most plastic suppliers will cut rod, sheet and tubing, all sized in inches, to your order. But film—plastic sheets 0.01 inch or less in thickness and more commonly sized in mils (a mil equals 0.001 inch)—is available in rolls of standard widths and lengths.

In choosing a plastic, compare your requirements with the characteristics of the plastics listed. Take into ac count the stresses to be placed on the finished object, including the weight it will bear, its exposure to impact, abrasion and corrosive liquids, and whether it will have to endure high temperatures and direct sunlight. Also consider the desired appearance of the plastic object.

If the characteristics you seek don’t match any plastic on the chart, chances are you are looking for a laminate or for a reinforced—or filled—plastic. The laminates, which are far tougher than most pure plastics, consist of layers of non-plastic materials, such as paper or cloth, bonded together with intervening layers of plastic. Filled or reinforced plastics contain materials other than plastic— most often, powdered wood or cot ton fibers—that are randomly dispersed through the plastic before it sets. Such loose, fibrous fillers impart a degree of strength and resistance to chipping that is difficult to attain in pure plastic.

A third type of hybrid, structural foam, consists of a core of foamed plastic sheathed in a solid, unfoamed skin of the same plastic. The structural foams offer a tough, attractive exterior and exceptional stiffness and may be made from many sorts of plastics.

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== Basic plastics offspring. == Layers of coarse brown kraft paper soaked in phenolic resin form the base of a tough, moisture-proof decorative laminate, ideal for countertops and cabinet facings; the penultimate layer, just beneath a surface film of clear, tough melamine, is a sheet of colored or patterned paper. To make fiberglass, alternating layers of glass-fiber mat and woven glass-fiber fabric are impregnated with polyester resin. A layer of clear resin on top of the glass-reinforced layers hardens into a smooth, tough surface.

Wood or cotton fibers, scattered randomly throughout a phenolic resin, form a reinforced plastic with sufficient strength and chip-resistance to be used for coffeepot knobs and electric- switch toggles. Unlike the filler in many other rein forced plastics, this filler material does not form a distinct layer in the finished plastic.

In a typical structural foam, a smooth skin of plastic covers the multi-celled core. The skin is formed during manufacture when the foaming resin, injected into a closed mold under high pressure, compacts against the mold walls, yielding a light, stiff product. Structural foam is increasingly used for prefabricated housing components, such as window frames and door frames, shutters and shingles.

Storage System for the Raw Materials

A home warehouse in a closet. Store plastic s and long rods and tubes on end in a closet subdivided with particle board into narrow compartments. Fill the compartments completely, so that all the plastic is held upright; without bowing or sagging. If you cannot fill a compartment, wedge small cardboard boxes between the sheets or rods and the nearest divider to hold the stock vertical. Short rods and tubes may be laid flat on a shelf, but don’t let them over hang the edge, lest the unsupported ends droop or sag. Wrap any unmasked rods in tissue paper to protect them against scratches, and leave the protective masking paper on plastic sheets. Never leave the closet open to sun light; when masked stock is exposed to sunlight, the adhesive paper can bake on and become difficult to remove.

Liquid resins and hardeners may be stored in the same closet. Solvents, glues and paints should be stored elsewhere to prevent vapors, leaking from the containers, from damaging the surfaces of the sheets and rods.

The Unmasking of a Paper-covered Sheet

=== Stripping off protective paper. === Peel back the protective paper from one edge of the plastic sheet and hold it against a wooden rod, such as a broom handle, about 1 to 1½ inches in diameter. Holding the paper securely against the rod, roll the rod across the plastic, wrapping the paper tightly around the wood. Fasten the paper for storage and later use with a strip of masking tape. Don’t uncover a greater area of plastic than you need to in order to bend, weld or cement the plastic.

To reuse the protective paper, release the tape securing the paper against the rod, and press the free end of the paper against the unmasked plastic. Gradually, unroll the paper across the plastic, smoothing it as you go and pressing it against the surface. At the opposite edge, trim the paper with scissors and then re-tape the unused end of the roll.

Readying the Stock for Cutting or Drilling

Like wood and sheet metal, plastics can be sawed, drilled and sheared. Also like wood and metal, plastic stock, whether it’s flat or tubular, must be marked with lines for layout before the cutting tools are applied, As they come from the factory or the dealer, plastic sheets are reliably square.

So are the ends of rods and rigid tubing.

However, if you are using stock that you have cut in the course of previous projects, you should check the squareness of corners, ends and edges. If necessary, true them by clamping the stock in a wooden jig and filing away any uneven plastic. Having squared the edges, you can then calculate cuts that will minimize waste, since straight edges will re quire no further trimming.

Marking the plastic surface without marring it’s the crux of plastics layout:

Hard plastics are scratch-prone. Fortunately, most plastic sheets provide a ready-made solution to the problem. At the factory, the sheets are wrapped with protective masking paper; this allows you to indicate cutting lines and locations for drill holes without touching the plastic surface. You can then cut and drill through the marked paper. It you have saved masking-paper wrappings from previous plastics projects, you can also wrap unmasked rods and tubes for marking.

Only a few tools are required for marking plastics. For straight lines on flat surfaces-sheets or the wider faces of rectangular rods-use a square and either a lead pencil or a china-marking pencil, depending on whether the plastic surface is covered with masking paper. Keep in mind that traces of china marker left on the plastic will bake on if the plastic is heated for bending or welding, so re move the layout lines with a damp cloth after they have served their purpose.

Remember, too, that even a carefully sharpened china marker leaves a line that is fairly broad, although adequate for most work. When your work calls for an especially precise cutting line, or if the plastic surface is so dark that a china marker won’t leave a visible line, score the plastic lightly with a scriber. But never scribe marks other than the cutting lines; a scribed fold line or a scribed X marking a drill hole will remain visible in the finished project and may open the way for a crack. For foam plastics, both stiff and flexible, the standard marking tool is a felt-tipped or ball-point pen; test both to determine which one is best suited to the surface.

When you are drawing or scribing lines on round plastic rods or tubing, a simple V-shaped jig provides both support and control. Two boards, nailed together at a right angle to form a V and propped between two rows of bricks, will serve well, So will an angle iron or a wood beam channeled with a 90-degree V groove. As shown, the jig can be used to mark lines around a rod or a tube and also along its length.

Use a compass, with a pencil on one leg, to lay out arcs and circles on paper masked plastic; use dividers, with two metal points, to scribe an unmasked surface. Before you draw a circle or an arc with a compass or a divider, build up a foundation of several layers of masking tape at the center of the circle to provide a firm footing for the pivot point, thus protecting the plastic surface from mars.

A more complex shape can be drawn on a separate sheet of paper, then transferred in one of several ways. You can lay carbon paper between the pattern and the masked or unmasked plastic, then trace all the layout lines. Or you can cut out the pattern and fasten it directly to the sheet, rod or tube with water-soluble glue. You can also simply scribe or draw around the cutout pattern.

To avoid scratching unmasked plastic, cover your work surface with a soft pad of newspaper. And to reduce the chance that the completed object will crack or chip, make sure your design includes as few corners sharper than 90° as possible, and no interior angles-they are common starting points for cracks. Curves and broad angles -- 90-degree and up -- are the safest to use with plastic sheets.

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Testing a Cylinder or a Sheet for Squareness

Checking rods or tubes. Fit a combination square or a try square over the end to be tested, Sight between the tool's blade and the stock; if they touch completely while the arm is held firmly against the side of the stock, the end is square. To be sure, move the tool and check the end from an angle perpendicular to the first, If the end is more than Y inch out of square use the method shown on page 18 to scribe a true cut line around the plastic, making it as close as possible to the end of the workpiece; then saw off the uneven portion. If the discrepancy is less than i inch, simply file the end of the piece until it’s square.

===Checking a sheet for square corners.=== Use a carpenter’s steel square to check all four corners of a large sheet of plastic; use a combination square or a try square on smaller rectangles. If you find a square corner, with no space showing between the plastic’s edges and the arms of the square, use the adjacent edges to determine corrections needed to square the other corners. If the sheet is more than 1/8-inch thick, make sure the edges are square; swing the arm of the square perpendicular to the sheet, and slide if along the edge, checking several times.

If an edge is less than ½ inch out of true, either crosswise or along its length, tile it square. Otherwise, draw or scribe a new cut line as close as possible to the uneven edge and trim the sheet with a saw. If none of the corners are square, correct the one that needs the least work, then proceed as above. To ready an edge for filing, peel the masking paper back and lay a steel square across the plastic. Work an arm of the square toward the edge of the plastic until only the material that must be filed away to produce a square edge protrudes; then scribe a guideline to mark off the waste plastic.

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Leveling a Cylinder End

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1 --- Aligning the jig. Select two boards with square ends and with widths greater than the diameter of the plastic rod or tube to be trued. Sandwich the plastic between them. With this assembly resting on a worktable, fit one end in a woodworking vise so that two or three inches extend beyond the front of the vise. Use a square to align the board ends, then position the rod or tube so that only the part to be filed away projects beyond the boards. Tighten the vise just enough to hold the assembly; if you are working with tubing, be careful not to crush it.

2 --- Filing the end. Use the boards as a guide while you file away excess plastic with a medium-coarse mill-cut metal file until you have a perfectly square end. On soft plastics such as polyethylene, the file may clog; clean it as needed with the short wire bristles of a file card.

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A Wooden Sandwich for Squaring Edges and Corners

=== Filing away an uneven edge. === Sandwich the plastic sheet between a jig made of two boards with straight edges, each a few inches longer than the edge to be trimmed. Align the boards with the scored guideline. To make sure that the boards are evenly placed, fit a combination square over the jig, once at each end; both boards should touch the blade. Secure the assembly to the edge of the workbench with two C clamps; if the edge to be filed is more than 2 feet long, use three clamps. With a medium- coarse metal file, remove the excess plastic, scraping away the material with strokes running nearly horizontal along the length of the plastic edge, moving the C clamps as needed.

Marking Straight Cut Lines on Sheets and Cylinders

=== Marking sheets and square rods. === Measure and mark reference points for cutting to size with a rule and, depending on the surface, a lead pencil, a china marker, or the point of a scriber. Then align a square or a straightedge with the reference points, and use it to guide the marking tool as you mark the cutting lines.

To indicate the position of holes to be drilled, use a pencil or a marker and the rule to mark two short intersecting lines to form an X with its center at the center of the planned hole. Don’t scribe these lines; in most cases they would be longer than the hole’s diameter and would be visible as scratches on the completed work.

==Girdling tubes and round rods.== Lay the tube or rod in a V-shaped jig with a stop at one end; the V groove should be no deeper than one third the diameter of the plastic. Butt one end of the plastic against the stop, and use a ruler or a square to mark the position of the cutting line. Brace a pencil, a china marker, or a scriber against the edge of the jig, pressing the point firmly against the cutting-line mark; then rotate the plastic, extending the cutting-line mark completely around its circumference. Make sure that the end of the plastic stock rests securely against the stop throughout the operation.

==Marking a longitudinal line.== Butt the rod or tube against the stop of a V-shaped jig no deeper than one third the diameter of the plastic. Hold the plastic securely with one hand and, using the edge of the jig as a guide, draw a line down the length of the plastic with a pencil, a china-marking pencil or a scriber.

A Padded Center Point for Swinging Curved Lines

==A compass or dividers for sheet layout. == Set a compass or dividers to the radius of the curve or circle you plan. Without actually touching the plastic, use this radius to locate the center point of the circle for minimum waste, position the center so that the circle or curve will touch one or more edges of the plastic sheet. Mark the center with a pencil or a china marker, then pad the center, with several layers of masking tape. Position the point of the compass or the dividers against the center, forcing the point into, but not through, the masking tape. Then draw or scribe the circle or curve.

Graph-Paper Patterns for Complex Shapes

A carbon copy for a shelf bracket. Plot the pattern for the bracket on stiff graph paper, using a steel square for the angles and straight lines and drawing the curves either freehand or with a trench curve (inset). Use the pattern to choose the portion of the plastic sheet that is to be marked and cut, and cover this area with sheets of carbon paper, inked side down. Lay the pattern over the carbon, and secure both to the plastic sheet with masking tape. Trace the pattern onto the plastic, using a ruler for the straight portions of the pattern and drawing the curves freehand. Check to make sure all the lines have been transferred to the bare or masked plastic surface. Then remove the pattern and carbon, and cut the stock.

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A paper pattern for a plastic scoop. Cut a pat tern for the scoop from graph paper and glue it, with water-soluble glue, to a plastic tube of the desired size. In plotting the pattern, locate a base line equal to the circumference of the plastic tube, and a perpendicular center line (inset). Then construct the contours of the scoop, using a ruler for the straight Sections and drawing the curved sections freehand or with a french curve. The two sides of the scoop should be sym metrical so that their curves can be cut from the plastic simultaneously.

Make a second bracket in the same way. Glue the two brackets to the underside of a plastic shelf and to a vertical plastic support, which in turn is screwed to the wall. For instructions on gluing plastic to plastic.

In mounting the pattern on the plastic, line up the base line with the end of the tube and be sure the center line lies along the long axis of the tube. Check to see that the edges of the pattern are well secured. When the glue is dry, cut out the scoop with a coping saw or a band saw, and file the cut edges smooth, Remove the pattern and complete the scoop by gluing a disk of plastic to the end of the tube, then shaping and gluing a handle of plastic rod to the out side face of the disk. For instructions on how to glue plastics, see upcoming discussion.