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Joining Plastic

Nov. 15, 2002
There are several techniques for joining plastic parts.

There are several techniques for joining plastic parts. Equipment cost and labor for each method vary considerably. Most techniques have limits on the sizes and types of plastic that can be joined.

Mechanical fastening: The simplest way to join plastic parts is to design a fastening element (hinge, latch, detent) into the parts. Only stronger plastics are suitable for this method since the joint must survive the strain of assembly, service load, and possible repeated use. This form of fastening is suitable only for lightly loaded, nonrigid assemblies where precision is not critical.

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Mechanical fasteners (screws, rivets, pins, sheet-metal nuts) are the most common joining method. They require a plastic that can withstand the strain of fastener insertion and subsequent high stress around the fastener. Conventional machine screws are rarely used except with extremely strong plastic.

There are a number of fasteners designed for use with plastics. Threaded fasteners work best on thick sections. Thread-forming screws are preferred for softer materials, while thread-cutting screws work best on harder plastics. Push-on locknuts and clips may be better for thinner sections. If a fastener has to be removed a number of times, metal inserts are recommended. They may be molded in place, forced, glued or expanded into holes, or inserted ultrasonically.

Fusion bonding: Plastic parts too complex or large to be fabricated on available molding equipment are sometimes made as subcomponents and welded together by fusion bonding. Holding fixtures ensure accurate mating and alignment of the parts to be joined.

To plasticize the part edges, the fixtures press the parts against a heating platen. As the platen melts the part surface, the plastic material is displaced while the fixture maintains part pressure against the platen. This initial fusing produces a smooth surface by removing surface imperfections, warps, or small sinks. Melted material continues to be displaced until "melt stops" on the platen contact the holding fixtures. Material is then no longer displaced and the parts are held against the platen until each part edge is plasticized to a predetermined depth. Melt depth is regulated by contact time, which is usually from 3 to 6 sec.

After part edges are plasticized, the fixtures open and the platen is withdrawn. The fixtures then reclose, forcing the parts together until "seal stops" on the fixtures come into contact. The parts are held together under pressure as the melted material cools, bonding them together.

Usually, fusion bonding joins thermoplastics such as polyethylene, polypropylene, and ABS. However, with slight changes, the process can be used to join both filled and unfilled nylon.

To fuse unfilled nylon, a thin, nickel-chrome blade, rather than a platen, is heated to about 1,100°F. The heated blade is brought to within 0.0005 in. of the material in a brief heating cycle. The blade is then removed and the part conventionally fixtured. Glass-filled nylon can be softened using direct heat, if care is taken to clean the blade or platen with a steel brush between each cycle.

Cooling time normally is the same as melt time -- from 3 to 6 sec. When cooling is complete, the gripping mechanism in one of the holding fixtures releases the part, the fixtures open and the finished part is removed manually or automatically. Total machine time from start to finish is generally about 15 sec, well within the range of the injection-molding systems often used in conjunction with fusion bonding.

Hot-gas welding: This is a low-speed process for fabricating large structural parts from sheet stock. A thermoplastic rod is heated with the parts to be joined until they soften and can be pushed together. The heat source is usually an inert gas. Top speed on long, straight welds is about 40 in./min. Intricate parts require more time. Operator skill is critical for both weld strength and appearance.

Vibration welding: Vibration welding produces pressure-tight joints in circular, rectangular, or irregularly shaped parts made from almost any thermoplastic material -- even in dissimilar materials having a melt-temperature spread as great as 100°F. The process is particularly suited for hollow, container-type components having the weld joint in a single plane.

The parts are frictionally heated by pressing them together and vibrating one of the parts at 120 to 240 Hz, in the plane of the joint.

After 2 to 3 sec, vibration is stopped at the exact required relative position of the two pieces. Pressure is maintained briefly while the softened plastic cools. Joint strength is very near that of the parent material. Cycle time, including manual loading and unloading, ranges between 5 and 8 sec for most parts. The process is adaptable to fully automated systems.

Vibration welding accommodates large parts that are impossible or impractical to weld by other methods. Parts can be rectangular or irregular, as long as the weld joint is in a single plane and a small amount of motion (at least 0.12 in.) in that plane is possible. Components with weld surfaces as long as 20 in. have been successfully joined.

Solvent bonding: Plastics are softened by coating them with a solvent, then clamped or pressed together. The plastic molecules mix together, and the parts bond when the solvent evaporates. This process is limited to thermoplastics. Fusion time is a function of the solvent's evaporation rate and may be shortened by heating.

Pure solvents provide the simplest, lowest-cost bond. Doped solvents, which cost more, contain solutions of the plastic being bonded to fill gaps in imperfectly fitting parts. Next in complexity and cost come monomer and polymerizing solvents. These materials contain catalysts and promoters added to doped solvents to produce polymerization at room temperature or a temperature below the softening point of the thermoplastic.

Solvent-bonded parts must be pressed together for 10 to 30 sec before the joined parts can be handled. Pressure is critical, as too much pressure causes parts to distort. A day or more at room temperature or several hours at elevated temperature may be needed to cure the bond.

Ultrasonic welding: Pulses are transmitted to the part by a resonant vibrating tool called a horn, causing two plastic materials to vibrate against each other. Vibration heats and fuses the parts together. Plastic products including blends or alloys of different resin families can be joined by ultrasonic welding. Such dissimilar parts should be designed carefully, and both the resin and equipment suppliers should become involved early to ensure that ultrasonic techniques can produce a suitable bond.

Ultrasonic assembly is often done at 20 kHz to achieve the vibrational amplitude and power needed to melt thermoplastics. However, higher frequencies that produce less vibration can also join thermoplastics, especially engineering thermoplastics such as reinforced polymers. For some applications, use of 40 kHz means less material degradation. Tooling used for 40-kHz welding is smaller than that used for 20 kHz; therefore, the welds produced at 40 kHz are generally smaller.

Pressure is critical. Too much causes the parts to vibrate as an integral structure with no heating. Too little does not provide enough contact friction or heating.

Ultrasonic welding is fast. Assembly rates of more than 25 parts/min are possible with a single station. There are no secondary operations, such as coating, inserting, or cleaning. The process requires fairly rigid materials. Dissimilar-material sonic welds can be made, but the melting temperatures of both materials must be quite close, otherwise only the lower-melting material will soften and a bond will not form.

Ultrasonics can also be used to insert metal components into plastic, stake metal and plastic parts together, or spot-weld plastic sections.

Induction welding: Induction welding can be done by pressing two pieces of plastic material together around a metal insert. When passed through a magnetic field, the encased metal is heated, and the compression produces a fusion weld. The metal remains sealed inside the part.

Thermoplastic bonding agents filled with either electromagnetic or ferrite materials may also be used in induction welding. The material, in the form of a preformed ring or strip, or as a hot melt, for instance, is inserted between the mating parts before induction heating. If metallic particles are used, the alternating magnetic field induces current flow within the particles, generating heat. When ferrite is used, no current is produced. Instead, heat is produced by molecular friction as the particles try to retain their magnetic charge when the fields are reversed.

To eliminate the need to add metal to the joint, metal powder can be added to the original plastic molding, but a much higher frequency is needed.

Induction welding is a high-cost technique and is suitable for difficult-to-weld plastics such as polypropylene, and for shapes that cannot be fitted into an ultrasonic welding machine. The process is best suited for bonding most polypropylene, polyethylene, styrene, ABS, polyester and nylon in high-volume, highly automated joining operations.

Heat-resistant polypropylenes that cannot be joined with adhesives or other welding techniques may be successfully joined using induction welding. Bonding agents heated inductively reach temperatures of 300°F in 0.1 sec to fuse with the heat-resistant substances.

Dielectric welding: Dielectric welding is used on films and thin sheets up to about 60 mil, primarily in packaging. The technique uses the breakdown of plastic under high voltages and frequencies (13 to 120 MHz) to produce dielectric heating and fuse the plastic. Welding speed is a function of dielectric-loss factor, material thickness, and the area subjected to the voltage. Dielectric welding is ideal for PVC materials.

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