L6. ultrasonic film and fabric welding 2 - Layers of film and fabric can be joined in continuous seams by ultrasonic welding. The ultrasonic welding horn is equipped with a smooth rounded tip and the underlying anvil has either a rotating wheel or a smooth surface. The material to be joined is pulled under the horn tip at the proper rate for seam welding. The rotating anvil can be engraved in a stitch pattern which is transferred to the fabric or sheet, simulating the appearance of a sewn seam. The process is used to join film sheet, and woven or non woven plastic fabrics. The sealing of polyester film is a common application.

L7. adhesive bonding of plastics – Information on adhesivebonding processes can be found in Chapter 7, section D. The material that follows applies only to the special steps and procedures involved when the parts to be joined are made from plastics.

L7a. solvent cementing of plastics – Some plastics can be bonded with strong joints by softening the surfaces in contact with a solvent and then pressing the two surfaces together for a short period. The plastics most suited to this approach are the amorphous type: polystyrene, acrylic, cellulosics, vinyl, polycarbonate, and copolymers of polyphenylene ether. Polyethylene and polypropylene are not bondable with this approach. The solvent used is one whose solubility is close to that of the material to be bonded. A solvent that is suitable for use with one plastic may not be suitable for another. Best results are often obtained if a small amount (up to 15%) of the base material is dissolved in the solvent beforehand. As with all bonding methods, the surfaces must be thoroughly cleaned before the bonding operation. The solvent can be applied by brushing, dipping, spraying, flooding, or capillary action. After some softening, the surfaces are pressed together under a moderate pressure - 100 to 200 psi (700 to 1400 KPa) - is sometimes recommended, but pressure must not be so high that the parts are distorted. Post heating in an oven to a temperature below the softening point of the base plastics involved may also be carried out. Objects fabricated from acrylic are commonly solvent-cemented but perhaps the major use of this method is for installation and assembly of PVC and CPVC piping.

L7b. pretreatment of plastic surfaces for bonding - See section M1 on page 197.

L7c. electromagnetic adhesivebonding – uses induction heating of an adhesive to join two plastics or other non-metallic components. The adhesive consists of a mixture of resin in which is dispersed very small ferromagnetic particles. The adhesive can be in the form of a paste, or a preform molded or cut from tape, sheet, or strands. When the induction generator is activated, radio-frequency electrical oscillations in a coil near the joint emit electromagnetic energy that causes the ferromagnetic particles to oscillate. Eddy currents generated in the ferromagnetic particles cause them to become heated. The heat from the particles and the friction from their movement quickly softens or melts the adhesive so that it wets and bonds with the surfaces to be joined. Thermosetting adhesives polymerize and harden from the heat; thermoplastic adhesives cool and solidify after the energy is shut off. The process provides good seals as well as strong joints. Joints need not be flat or regular, so long as the adhesive can fill any gaps. The process is most suited to moderate- or high-production levels, since fixtures, coils, operating settings, and possibly adhesive preforms, have to be developed for each application. Automotive parts are a major application, including such items as heating and air conditioning ducts, and seat backs. Medical devices and filters are other applications. Fig. 4L7c shows an example of electromagnetic adhesivebonding.

Fig. 4L7c Electromagnetic adhesivebonding in section view. Electromagnetic induction from the coil heats and agitates ferromagnetic particles in the adhesive, heating and softening the adhesive so that it adheres to the parts to be joined.

L8. induction welding - is similar, in many respects, to electromagnetic adhesivebonding. However, the workpieces themselves, rather than an adhesive, are melted by the induction heating. The induction effect, which requires an electrical conductor, is achieved by inserting an open-grid metallic piece between the opposing surfaces of the joint. The metallic piece may be a screen, foil, wire, or some other conductive part that will absorb the electromagnetic energy, get hot, and heat the surrounding plastics. When the power is shut off, light pressure is maintained across the joint and the melted joint materials fuse together and solidify. The metallic insert remains in the joint. The process is very fast; only a few seconds are required to weld the joint surfaces together. The equipment and tooling required are quite similar to those used to induction braze or solder joints in metal parts. See descriptions and illustration in sections 7A2h and 7B4.

L9. radio frequency sealing (dielectric sealing) - is a method for joining plastic film and sheet parts. Energy in the form of electromagnetic radiation at radio frequencies is directed at the joint, and it agitates the plastic molecules. A radio frequency of approximately 27 MHz is commonly used. Polar molecules of the plastic sheets vibrate at that frequency, and experience friction with other molecules, producing heat. The material in the joint area is thereby softened. Under pressure, the joint surfaces fuse together and, when the power is shut off and the material cools, a permanent seal is achieved. The process is used most frequently with flexible and rigid PVC but many other plastics can be processed including thermoplastic polyurethane, ABS, polyester film, EVA, acetate, and acrylic. Polystyrene, polyethylene, and polypropylene, however, are not suitable for the process. The equipment involves, in addition to a RF electrical oscillator, a press and dies for the particular application Typical products assembled by this operation include inflatable toys, swimming pool liners, shower curtains, rainwear, medical bags, looseleaf notebooks, packaging, and automobile interior components.

L10. thermal sealing (heat sealing) of sheet - In this method, the heat is applied externally and travels through the sheet sufficiently that the sheet materials in contact are soft enough to fuse together. The heat source is a bar, knife edge, metal band, wheel or roller, that is heated by electrical resistance or radio frequency energy while in contact with the top or bottom sheet. This tool is coated with PTFE (“Teflon”) to prevent the softened sheet from sticking to it. Some high-production machines use heated sealing rolls or wheels, followed by pressure wheels, followed by cooling wheels as the sheet materials pass through them. Polyethylene sheet and film are frequently bonded with this method.

L11. hot gas welding - is a method used to join plastic sheets, normally in thicknesses from 1/16 to 3/8 in (1.5 to 10 mm). The process is very similar to gas welding of metals, except for the lower temperature and the absence of direct flame. Hot gases from a hand held gun are directed to melt (soften) the edges of the parts to be joined enough that they can fuse together. A welding rod, of the same plastic material as the parts being joined, is also softened and added to the material at the joint to provide any additional material needed for a fillet. The welding rod is often of a triangular crosssection and is pressed by the operator into a vshaped space at the joint. When the heat is withdrawn, the material cools and solidifies into an integral joint. The hot gas is not supplied by a flame at the nozzle but is simply air or, in some cases, nitrogen, heated electrically to a temperature of 400 to 570 ° F (200 to 300 ° C). Edges of plates to be welded are beveled mechanically before welding. No flux is needed and there is no slag to be chipped away.

Polyvinyl chloride (PVC), polyethylene, polypropylene, acrylic, polystyrene, polycarbonate, and ABS are the plastics most commonly used for welding applications. Tanks, piping, large outdoor signs, ducting, and structural assemblies are common applications. The process is particularly useful in making components too big to mold or cast by primary methods.