Machinery's Handbook, 31st Edition
596 DESIGNING PLASTIC PARTS FOR ASSEMBLY With some materials the calculated bending stress can exceed the yield point stress con siderably if the movement is done rapidly. In other words, the flexing finger passes through its maximum deflection or strain and the material does not respond as it should if the yield stress has been greatly exceeded. It is common to evaluate snap-ins by calculating the strain instead of stress. Dynamic strain ε for the straight beam, is calculated from L Yh 2 3 o 2 ε = and for the tapered beam, from L K Yh 2 3 2 0 ε = The derived values should be compared with the permissible dynamic strain limits for the material in question, if known. A tapered finger provides more uniform stress distribution and is recommended where possible. Sharp corners or structural discontinuities that will cause stress concentrations can be avoided through the use of judicious fillets. Snap-in arrangements usually require undercuts produced by a sliding core in the mold as shown in Fig. 21 . Sometimes the snap finger can simply be popped off when the mold is opened. An alternative to the sliding core is shown in Fig. 21, which requires an opening in the molding at the base of the flexing finger. Other snap-in assembly techniques that take advantage of the flexibility of plastics are shown in Fig. 22. Molded-in threads in holes usually are formed by cores that require some type of unscrewing or collapsing mechanism, leading to tooling complications. External threads can often be molded by positioning them across the parting plane of the mold. Molding of threads finer than 28 threads per inch (0.9 mm pitch) is sometimes not practical. Chemical Bonding. Chemical bonding is suited to applications that must be leak-tight or where stresses due to assembly must be minimized. However, adhesives and solvents can be hazardous, and their preparation and cure times can be prolonged. Properly applied cyanoacrylate adhesives achieve moderate strength in seconds or minutes and work well with assemblies that can be brought together very quickly after applying the bonding agent. Chemical bonding may use solvents, solvents bodied with resin, or so-called “100% solids” adhesives, such as epoxies and cyanoacrylates. Use of solvents is limited to com patible materials that can be dissolved by the same solvent. Because most of the solvent must gradually diffuse out of the joint, solvent cementing is used mostly where a very thin film of the solvent can be applied to the accurately mating surface(s) to be joined, followed by rapid assembly, usually with clamping. Bodied solvent cements permit more flexibility in such operations. Safety precautions must be observed in handling the solvents to protect workers and in solvent recovery. Adhesives that set up or cure with little or no loss of volatiles include epoxies, acrylics, polyurethanes, phenolics, rubbers, polyesters, and vinyls. Cyanoacrylates are often used because of their rapid setting with many materials, both similar and dissimilar, and because they are naturally catalyzed by atmospheric humidity. Their downside is the very short working time they allow for assembly. Recommendations of both plastics suppliers and adhesives suppliers should be sought because many adhesives contain active components that can partly dissolve the plastics’ surfaces, giving improved adhesion. However, some adhesives attack certain plastics, causing crazing and even joint failure. The main disadvantages of adhesives are that their bonds require time to reach full strength, may need long clamp times, may require fixtures and may involve special ovens or curing conditions. Also, surface preparation may be difficult because of the presence of grease or mold-release compounds; even a fingerprint can spoil a bond. Some adhesion-resistant materials such as polyolefins, nylons, and fluo rocarbons usually need pretreatment with surface-activating primers or flame or plasma treatment prior to applying an adhesive. Some materials may need mechanical roughen - ing to improve joint strength.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online