Machinery's Handbook, 31st Edition
598 DESIGNING PLASTIC PARTS FOR ASSEMBLY Welding Plastics: Plastics welding techniques include ultrasonic, hot-plate, spin, induc tion, and microwave energy, melt-bead sealing, and hot-gas welding. Thermal methods are also used for staking, swaging, and other post-thermoforming procedures. Materials must be melt-compatible and have similar melting temperatures. Plastics welding encompasses a variety of processes in which a film melt is created at the surfaces to be joined. Ultrasonic welding is frequently used for joining small- and medium- sized parts of a single material or melt-compatible materials that share the same melting- temperature range. High-frequency (20 to 40 kHz) acoustic vibrational energy is briefly applied to the surfaces to be joined, creating localized molecular excitation that causes a thin layer to melt. The surfaces are pressed and held together while the melt freezes. With proper joint designs, welds can be made in two seconds or less and on cooling can be as strong as the base resins. The lower part of the assembly is supported in a rigid nest fixture and the upper part is aligned, usually by the joint design. This upper part has the freedom to couple acoustically when it is in contact with the horn through which the ultrasonic energy is transmitted. Fig. 23 shows two typical joint designs for this process. The example on the left shows a simple butt-type, energy-director design that works well with amorphous materials. The inverted-cone projection, known as an energy director , concentrates the energy in a small area on both faces of the joint. This area melts quickly and the material flows as the parts are pressed together. Shown at the right in Fig. 23 is a shear-interference joint. Melting of both components starts in the small initial contact area, and flow continues along the near- vertical wall as the parts are pressed together, creating a continuous, leakproof joint with a strength that often equals that of the parts joined. With hygroscopic plastics, welding should be done as soon as possible after molding because even small amounts of moisture can weaken bonds. Drying immediately before welding may be advisable. Drawbacks of ultrasonic welding are the following: design, quality control, equipment maintenance, and settings are of critical importance for consistent, high-strength welds; the equipment is costly; the process uses large amounts of electric power especially with large parts; and parts to be joined must be of the same or compatible plastics. Filled and fiber-reinforced materials are more difficult to weld. Although horn frequencies in ultrasonic welding are not audible to the ear, sounds that cause discomfort may be generated when plastics parts vibrate at lower frequencies, necessitating sound-proofing.
Interference 0.005–0.012 in.
45°
Before Welding
Before Welding
After Welding
After Welding
Fig. 23. (left) Energy-Director Type of Ultrasonic Weld Joint for Molded-Part Assembly and (right) Typical Shear-Interference Joint
Vibration Welding: Vibration welding resembles ultrasonic welding except that the parts to be joined are rubbed together to produce frictional heat that melts the faces. The energy is transferred in the form of high-amplitude, low-frequency, reciprocating motion. After a short period of vibration, the weld area cools and solidifies while the parts are held in the
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online