Plastic parts design fastening

A projection on a plastic part designed to add strength, facilitate aligmnent during assembly, and provide for fastening. 

With plastic materials, the designer also has a greater choice of bonding techniques than with many other materials. Thermosets must be adhesively bonded or mechanically joined, but most thermoplastics can also be joined by solvent or heat welding. Additionally, plastic parts can be designed for assembly by means of molded-in, snap-fit, press-fit, pop-on, and threaded fasteners so that no additional adhesives, solvents, or special equipment is required. 

There are basically two methods of mechanical assembly for plastic parts. The first uses fasteners, such as screws or bolts the second uses interference fiL such as press-fit or snap-fit and is primarily used in thermoplastic applications. This latter method of fastening is also called design for assembly or self-fastening. 

It is often possible and desirable to incorporate fastening mechanisms into the design of the molded part itself. The two most common methods of doing this are by interference fit (including press-fit or shrink- fit) and by snap-fit. Whether these methods can be used will depend heavily on the nature of the plastic material and the freedom one has in part design. 

Self-fastening joint designs generally produce very high stresses in the plastic part during the assembly operation. With brittle plastics, such as thermosets, press-fit assembly may cause the plastic to crack if conditions are not carefully controlled. In addition, certain plastics, especially thermoplastics, are subject to cold flow under stress. Under continued stress, which is the fundamental adhesive that holds self-fastened parts together, the plastic may relax, causing the joint to fad. 

Metal fasteners of high strength can overstress plastic parts, so torque-controlled tightening or special design provisions are required. Where torque cannot be controlled, various types of washers can be used to spread the compression force over larger areas. 

Metal fasteners of high strength can overstress plastic parts, so torque-controlled tightening or special design provisions are required. 

Once parts are optimally consolidated within an assembly, there are additional DFMA guidelines that should be considered to reduce assembly times and costs. Perhaps most importantly is the use of top down assembly in which parts are assembled from above in straight-down motions this strategy also supports fully automated assembly in applications with high production volumes. The motivation for the top-down approach is that it allows for the fixturing of a base part on an assembly line without need for subsequent reorientation for access to the reverse side of the base. An operator can then rapidly insert other parts in the assembly given visual access to the assembly. With both hands free, the operator can use one hand to insert the part and their second hand to use a tool if necessary. Plastic parts should be designed (and fasteners specified) to be identical or clearly differentiated by size, color, or obvious asymmetry. In this manner, the number of components in the assembly s bill of materials is further reduced... 

Nuts, bolts, and washers are also commonly used in assembly of plastic parts. In general, the strength of metal fasteners is much higher than the attached plastic material, so nuts and bolts with small diameters are typically used with washers having a larger outer diameter to reduce the compressive and shear stresses in the plastic component and avoid failure. Fig. 27.17 provides some common designs as well... 

Design of a handle determines to a great extent its service life. Quite often handles are found fastened directly to the hot object without regard for any temperature limitation at the junction the plastic becomes brittle because of high temperature, and failure occurs. If the handle can be separated from the heated part and some cooling arrangement is included as part of the design, improved performance is to be expected. 

---