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The mechanical fasteners often used on plastics are the familiar screws, rivets, nuts and bolts that join wood and metal. Many of their applications are familiar too, but the unique properties of plastics allow for some variants on joinery techniques: Screws can be set into threads heat-formed into thermoplastics; nuts can be embedded in plastic filler for a strong, invisible joint. Many plastics are virtually friction-free, a property that makes them useful for simple smooth swinging hinges in which ordinary long brads serve as hinge pins. Rivets are the fasteners of choice for connecting thin sheets of plastic to each other or to non-plastic materials. Al though they can be drilled out of their holes if necessary, rivets are best suited for permanent installations. They are also good for applications where vibrations might cause a threaded fastener to work loose. Screws or nuts and bolts provide greater strength than rivets and can be used with materials of any thickness. Most of them allow assemblies to be dismantled repeatedly. An exception is the thread-cutting screw, whose repeated cutting action in screwing and unscrewing can damage threads cut in plastic. ===Fasteners for every application. === Wood screws are best suited for connecting plastic to wood, and they can be heated to form matching threaded holes in thermoplastics. Machine screws can be used to attach plastic to any material, but they need a matched fitting, either a pre-threaded hole cut with a tap, or a standard nut or a spring clip (which is pushed rather than threaded onto the screw). Both wood screws and machine screws are avail able with flat or oval heads that lie flush with the surface in countersunk holes, or with round or pan heads that protrude above the surface and are generally used with washers to help distribute the pressure more evenly. Self-tapping screws and drive pins, both originally designed for metal, cut their own threads. Self-tapping screws carve their own path as the screwdriver turns; drive pins are forced into their holes by the pressure of hammer blows. Self-tapping screws can be used in both thermo plastics and thermosetting plastics, but drive pins should be used only in thermoplastics, as they may cause cracks in the relatively non-yielding thermosetting plastics. Rivets can be used to join plastics to plastics or to other materials. Pop rivets are installed with a special tool that expands the tip of the rivet shaft into a head; split rivets are set by having their split ends bent outward with a screwdriver. Both are inserted in predrilled holes and should be used with washers to distribute pressure. Combination post-and-screw fasteners come in a variety of lengths and can M used to join two pieces of plastic where little structural strength is required. The hole in each piece should be drilled to the diameter of the post. ---44 Adapting Traditional Fasteners to New Uses 44 With any such fasteners, the physical characteristics of plastics give rise to other problems. Most plastics are not as strong as wood or metal, so you must avoid excessive strain at any point. Tension on threads cut into plastic can strip the threads; tighten fasteners only until increased resistance is felt. Space fasteners closely to distribute strain evenly. To further distribute the pressure of each fastener, place a broad washer between the head of the fastener and the plastic. In many industrial designs a similar result is obtained by means of a thicker section, called a boss, molded into the plastic part around each fastening point. If you are molding a plastic part, you can do the same. Because most plastics are more responsive to temperature changes than other materials are, you must leave room for expansion and contraction when you at tach a piece of plastic to another material at more than one point. If plastic is rigidly attached to wood, metal, glass or ceramic—all of them relatively stable materials—the plastic may split, crack or bow between the fastening points. To avoid this problem, make the holes in the plastic larger than the fasteners, allowing the plastic to expand or contract in increments over its entire surface. The slower expansion rates of the met al fasteners themselves can cause the expanding plastic to split or crack as it presses against the heads of the fasteners. You can prevent this by simply seating the nuts and bolts or rivets less tightly against the plastic. Permanent Connections with Rivets and Washers ===45a ===Quick fastening with pop rivets. Align the holes in the two pieces to be fastened, tapping them together if necessary. Insert the long mandrel of the rivet into the nosepiece of the pop-rivet tool, then insert the shaft of the rivet into the hole in the assembly. Place a back-up washer over the protruding shaft, and squeeze the handles of the pop-rivet tool together. Continue alternately squeezing and releasing the handles until the rivet is firmly seated and you feel in creased resistance. Then squeeze the handles a final time to snap off the mandrel, leaving only the rivet head showing. If a nub of the mandrel protrudes, use a metal file to remove it. ===Installing a split rivet. Align the holes in the pieces to be joined, and insert a split rivet through them. Position the assembly, rivet head down, against a solid, padded surface, and slip a back up washer over the split end of the rivet. With a screwdriver, bend each s de of the rivet over the washer. Then use needle-nose pliers to curve the rivet ends back against the washer until the split rivet is snugly seated. 45 Hinging a Cover with a Pair of Pins ===Making a pin hinge. Clamp or tape the movable piece into its closed position, and drill holes through the stationary pieces into both ends of the movable piece. Position the holes along the same axis at each end of the assembly, and keep the drill bit aligned with that axis as you cut the two holes. Remove the movable piece end slightly enlarge its two holes. Then reassemble the pieces and drive long, smooth wire brads into the holes, using brads that fit snugly into the stationary piece. Check the assembly for free movement; file and sand away any plastic that interferes with the desired swing. Thermoplastic Threads Formed by a Heated Screw ====Shaping threads for a wood screw. Grin a bright, clean wood screw firmly in a pair of pliers and hold it over the flame of a propane torch or a kitchen stove until it’s deep blue. Then push it into a predrilled hole in the thermoplastic; the hole should be slightly smaller in diameter than the outside thread of the screw, and as deep as the length of the screw. When the plastic has melted and cooled into the shape of the screw, remove the screw with a screwdriver. In the final assembly, use new screws of the same size. 46 A Nut Sealed in Place for Extra Strength 1. Installing the nut and bolt. Drill a pilot hole for the bolt through the two pieces of plastic to be joined, making it the same size as the outside thread diameter of the bolt. Counterbore the hole in what will be the finished surface, making it large enough for a washer and a nut with 1/8 inch of clearance. Drop in a washer, insert a bolt from below and thread a nut onto the bolt. Using a socket wrench to hold the nut, tighten the bolt until the pieces are firmly connected. 2. Embedding the nut. Using an oil can with a. small spout, cover the threaded bolt end with a drop of light machine oil; insofar as possible, keep the oil off the nut. Mix a small batch of plastic patching compound to match the color of the plastic surface and pour it into the counterbored hole, filling the hole completely. Allow the patching compound to cure completely before removing the bolt from the nut, now hidden in the patching compound. Threading a Hole with a Tap Wrench ===Tapping threads in plastic. Mount a tap the size of the fastener in a tap wrench; dip the cut ting threads in soapy water and, working clockwise, slowly begin to twist the tap into a predrilled hole. Keep the tap exactly aligned with the hole, and back it out frequently to clear it of plastic chips. When you drill the pilot hole, make its length equal to that of the chosen fastener and its diameter halfway between the inside and outside diameters of the fastener’s threads (inset). 47 Parts Bonded with Cements, Glues or Solvents Bonding is to plastic as nailing is to wood and welding is to metal: It’s the method of choice for assembling or repairing. Compared to screws, bolts or nails, bonding has the advantage of distributing the load it bears over the entire length of the joint. Compared to welding, it requires far simpler equipment—often no more than a squeeze bottle for applying the adhesive and a jig to hold the bonded joint until it dries. Plastics can be bonded with a wide variety of cements and glues, some of them familiar household products, others more esoteric (chart, opposite). The type of plastic dictates the choice of adhesive, and for most plastics there is more than one choice. Two common plastics, how ever, polyethylene and polypropylene, resist cements of all kinds—because of the waxiness of their surfaces, they are best joined with heat. The preferred method for bonding most of the hard, glasslike thermoplastics such as acrylic, polycarbonate, polystyrene and the cellulosics is solvent cementing, a method peculiar to plastics. This technique takes advantage of the fact that these glassy plastics soften on contact with strong solvents. In the most common solvent-cementing process, the joint is assembled and a few drops of solvent—usually methylene dichloride, either pure or in combination with other substances—are applied to the seam with a brush or a squeeze-bottle dispenser. Capillary action draws the solvent into the joint, where it softens the abutting edges. These then intermingle and, as the solvent evaporates, create a bond that is integral with the plastic. In a variation of this technique, only one edge of the abutting plastics is softened, but it’s softened to a greater depth, producing a slightly stronger joint. The edge is soaked in a pan containing a shallow layer of solvent. When the object is assembled, solvent from the soaked edge softens the adjacent surface; the edges of the two pieces meld and harden into a strong bond. Although solvent cementing, with its crystal-clear joint, is most appropriate for joining furniture and kitchen or bath room fixtures made of glasslike plastics, it’s equally effective as a joining technique for other solvent-sensitive plastics, including polyvinyl chloride (PVC). The only precondition for its use is that the plastic parts must fit together without any gaps—since the amount of plastic softened is not enough to fill any un evenness. Square and sand the edges as described, but don’t polish them. If you are working with trans parent plastic, a handy check for the precision of the joint is the water test shown opposite. Acrylic can also be joined with a two-part acrylic cement. Useful for rough- trimmed edges and along joints that could benefit from the reinforcement of an extra fillet of material, it consists of a combination of acrylic resin and a hardener, mixed to a syrupy consistency just before use. Unlike solvent cement, acrylic cement actually requires a gapy joint rather than a close-fitting one; the opening is often produced by beveling one of the edges to create a triangular crevice. To inject cement into the crevice, use a large syringe made of polyethylene, avail able in hardware stores and pharmacies. On plastics that cannot be bonded with either of these two special cements, many familiar adhesives work well. Objects cast from thermosetting resins can be bonded with epoxy cements; contact cements will fasten polyurethane foam; ordinary white glue is suitable for joining polystyrene foam. Other common adhesives can be used on solvent-sensitive plastics when the neatness and clarity of a solvent- cemented joint are not crucial. Household cement will bond cellulosics; model glue joins polystyrene. These viscous plastic cements contain quantities of the specific plastic on which they can be used. Similar adhesives are available for PVC and for acrylic; they further extend the range of ways in which those materials can be joined. When plastics are fastened to other materials, such as wood or metal, the range of possible adhesives broadens. Many such marriages require a pliant bond, to allow for the differing rates at which the two materials expand and shrink with temperature changes. Flexible epoxies—those without fillers—and adhesives based on silicone or synthetic rubber work best for such joints. White glue and viscous plastic cements join some plastics to porous materials, such as wood, and the so-called super glues lock plastics to a number of other materials. Whatever bonding method you use, al ways observe certain preliminaries. First, read the label on the adhesive container. Not only will the label verify that you have selected the best product for the job, but it will also provide storage and use instructions and alert you to curing times—how long the joint must be immobilized before it’s ready for use. Make sure, too, that the surfaces to be bonded are clean—free of grease, polishing paste and traces of adhesive from the masking paper. To clean glasslike thermoplastics, wipe with lint-free cloth or chamois moistened in isopropyl alcohol. Objects cast from thermosetting resins, such as epoxy and polyester, must often be sanded to remove mold-release com pound and to roughen their surfaces for better adhesion. Finally, design the joint and the clamping setup before you begin to apply the cement. Most adhesives give you very little time to reconsider, and firm, steady clamping is essential in making a strong joint. Because the fumes of plastic solvents, cements, and glues are often flammable and nearly always irritating, safety guide lines similar to those for liquid plastics must be followed assiduously. Always work in a well-ventilated area. Take large jobs outdoors—but wait for warm weather; many cements cure slowly and form a weak joint at low temperatures. Don’t leave cement and solvent containers open any longer than necessary, and avoid extended skin contact with any cement or solvent. Never use these substances after drinking alcoholic beverages (the fumes may combine dangerously with the alcohol), near an open flame, or while smoking. 48 A guide to Choosing the Right Adhesive ===Fitting the adhesive to the job. In the chart above, the first column lists the plastics most commonly fastened with cements or glues. The other four columns list the preferred bonding agents for joining them to the materials at the top of the other columns. Three of these materials—reinforced plastics, wood and metal—ac count for the vast majority of surfaces to which small plastic parts are customarily joined in building or assembling larger objects. A Water Test to Verify that Edges Fit Precisely ===Locating surface irregularities. When you are joining clear plastics with solvent cement, a method requiring perfectly matched surfaces, dribble a few drops of water from a medicine dropper onto one face of the joint. Assemble the joint, then sight through the plastic to watch the behavior of the water, If it spreads in an even film throughout the joint, the joint is ready for cementing. But if the water puddles in some areas (inset), the edges require further smoothing. Continue to file, scrape and sand the unpuddled areas, repeating the water test from time to time. Be sure to dry before cementing. If the chart lists more than one bonding agent, both are qually effective. In such a case, appearance or clamping requirements may affect the choice—a fast-setting adhesive is often preferable for a complex joint, which is likely to be difficult to clamp. The bonding agents are identified by their generic names. Since they are sold in various forms and under various trade names, you will have to check the ingredients on the package label to be sure that you are getting the cement hr glue you want. Most of these bonding agents are available at hardware stores, and household cement is of course available everywhere. For model glue you may have to go to a hobby shop:. Special solvent and two-part cements are carried mainly by plastics suppliers, although solvent cement kits for joining acrylic are sometimes sold at hardware stores as well. Bonded to: Plastic Itself Reinforced plastics Wood Metal ABS Solvent Synthetic-rubber adhesive Epoxy cement Epoxy cement Acrylic Solvent, thickened acrylic cement, two-part acrylic cement Two-part acrylic cement. Two-part acrylic cement Contact cement Cellulosics Solvent, household cement Synthetic-rubber adhesive Household cement Contact cement Polycarbonate Solvent, thickened polycarbonate cement Synthetic-rubber adhesive Epoxy cement Epoxy cement Polystyrene Solvent, model glue Epoxy cement Model glue Super glue Polystyrene foam White glue Contact cement White glue, contact cement Contact cement Polyurethane foam Synthetic-rubber adhesive Epoxy cement, contact cement Contact cement Contact cement PVC Solvent, thickened PVC cement Synthetic-rubber adhesive Synthetic-rubber adhesive Contact cement Rigid thermosetting plastics (phenolics, epoxies, polyester, melamine) Epoxy cement Epoxy cement Contact cement Epoxy cement, synthetic-rubber adhesive 49 Solvent-softened Edges to Hold a Butt Joint 1--- Assembling and immobilizing the joint. Butt together the pieces of plastic being joined, and anchor them at 3- or 4-inch intervals with lengths of masking tape stretched taut. Then carefully lift the taped plastic pieces and turn them over, propping them slightly above the work surface on wooden blocks spaced to give clearance to the seam. Brace the top with additional wood blocks to immobilize the joint and prevent the pieces of plastic from bulging upward because of the tension of the tape. 2 --- Applying the solvent. Dip a No. 1 artist’s brush in the container of solvent, taking care to wet only the bristles so that the solvent does not attack the finish of the handle. Blot the brush on the lip of the container; then draw the brush along the joint, stopping to reload the brush if it becomes dry. Don’t allow excess solvent to pool on the surface of the plastic. Capillary action will draw the solvent into the narrow aperture of the joint as you apply it. If the joint shows light-colored areas, an indication that the sol vent has not penetrated the joint completely, make a second pass with the brush. Let the joint stand undisturbed for at least 15 minutes. Wait at least four hours before putting any weight on the joint and before sawing, drilling or sanding in its vicinity. 50 Using a Jig for Cementing a Corner Joint 1. Aligning and anchoring the joint. Make a right-angled jig from two pieces of ¾-inch lumber, one of which has been rabbeted to form a channel at the inside apex of the angle; nail the jig together. Position the plastic pieces n the jig so that the seam falls over the channel; this will prevent excess solvent from pooling between the wood and the plastic, blemishing the plastic. When the joint is properly aligned, secure the plastic with masking tape, as in this example, or with spring clips or large rubber bands. 2. Filling the joint with solvent. Fill the applicator bottle of a solvent-cementing kit with solvent; touch the needle-like tip of the bottle to one end of the joint and, squeezing the bottle gently, draw it along the length of the seam, dispensing an even flow of solvent. If capillary action fails to draw enough solvent into the seam to fill it, make a second pass, adding another bead of sol vent. Wait at east 15 minutes before removing the plastic from the jig, and at least four hours be fore subjecting the bonded joint to any stress. A Solvent Dip to Soften Parts for Joining 1. Softening one edge in solvent. Working in a well ventilated area, prepare the dipping bath by arranging a pattern of paper-clip supports in the bottom of an aluminum pie pan or a glass baking dish; the pattern of paper clips should match the outline of the edge being softened—in this example, a plastic cylinder. Pour just enough solvent into the pan to cover the clips, and set the plastic on top of them; only the bottom edge of the plastic should touch the solvent. Leave the plastic in the solvent for six to eight minutes, so that the edge can soften. To slow the evaporation of dangerous vapors during the dipping operation, use the smallest pan possible. For extra protection, you can fashion a cover for the pan from polyethyene film. Cut a hole in the cover just large enough to accommodate the plastic being softened, and tape the cover to the sides of the pan. 51 [missing 52] Strong Acrylic Joints with a Two-Part Cement ===Mixing the resin and hardener. Allow both the resin and hardener to reach room tempera ture, then pour the resin into a disposable glass or polyethylene container and add the pre scribed amount of hardener, following the pro portions listed on the label. Stir the mixture gently but thoroughly with a wooden tongue depressor for a minute or two, taking care not to intro duce bubbles. If air bubbles do appear, let the mixture stand for three to five minutes, until the bubbles rise to the surface and are dispersed. ===Acrylic-cementing a butt joint. Bevel the edge of one piece of plastic to an angle of 15°, as described, then butt the two pieces Of plastic together, beveled edge up in the joint. Weight the plastic between wood blocks as shown in Steps 1 and 2. Leave a 1 1/16 inch gap between the two plastic edges and secure the underside of the gap with a temporary masking-tape seal. To construct the seal, cut a strip of 1-inch masking tape slightly longer than the joint, and a strip of ½-inch masking tape, also slightly longer than the joint. Place the ½-inch strip down the center of the 1-inch strip, sticky sides together (inset), and attach the seal to the plastic. Lap the tape over the ends of the joint to form dams for the cement. The sticky margins will seal the joint, and the center strip will prevent the adhesive on the tape from interfering with the curing of the cement. Beginning at one end of the joint, fill the joint groove with cement by depressing the plunger of the syringe and moving it back and forth along the joint; overfill the groove slightly, to allow for shrinkage as the cement cures. Let the joint stand undisturbed for at least four hours. After 24 hours, sand the bead of cement level with the plastic surface. Working quickly—the syrupy mixture will hard en to uselessness in about 25 minutes—fill the disposable syringe applicator by dipping the tip in the mixture and slowly pulling up the plunger. When you have finished with the cement, allow the leftover mixture to harden before discarding both the container and the syringe. ===Acrylic-cementing a right-angled joint. Make a right-angled wooden jig using the method de scribed in Step 1, but trim the out side edge of the jig’s joint at a diagonal, forming a chamfered corner. Bevel the edge of one piece of plastic to an angle of 15°, and tape or clamp the two pieces of plastic against the outside of the jig. Position the unbeveled piece flat against the work surface and set the beveled piece against it, bevel facing out and 1/4 inch back from the edge of the unbeveled piece; this 1/ inch ledge will later be removed. Using the syringe, dispense a bead of cement into the grooved joint, overfilling it slightly to allow for shrinkage. While the cement is still viscous, shape it into a concave fillet with the tip of the syringe. Allow the joint to cure for four hours; then, for a stronger bond, make a second fillet on the inside of the joint. After 24 hours, use a table saw and a medium mill-cut met al file to trim off the ¼-inch ledge, leaving a smooth right angle. = = = = |
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