Fasteners

Fasteners have been used to assemble thermoplastic components almost as long as there have been thermoplastic components. Of course, some of the earliest plastic parts were things like combs that did had no assembly requirement, but it wasn’t long before someone drilled a hole in a part, put a wire loop in it, and hung it on a chain around his/her neck. Thus the realm of fastener use on plastic parts was born. Thermoplastics parts have been bolted, screwed, riveted, clipped, stapled, and held together with just about every fastener imaginable, but there are a few key points to consider when using fasteners on plastic parts. First, few plastics exhibit a really high degree of crystallinity, so are therefore amorphous to some degree or another when “solidified.” Fully amorpous materials, which includes a great many thermoplastic materials, can never be said to be really “solid.” They just flow so slowly at normal temperatures that we humans experience them as solids. What this means is that almost all thermoplastic materials exhibit more or less creep; creep being the tendency of a thermoplastic part to change dimension over time. Since most thermoplastic parts have some degree of internal tension, the parts generally get smaller over time. This is why the vinyl trim in an older car will pull away from openings and open up gaps. Creep is exacerbated by stress on the material in the form of temperature extremes, vibration, and such. Creep can be a major problem for any thermoplastic assembly but is particularly troublesome where screws may loosen as material shrinks, or where a lessening wall thickness will cause a rivet to become loose and start “working” or moving around in a hole. In some cases, where a fastener needs to be set to a certain torque to remain tight, like in an under-hood automotive application, it becomes necessary to insert metal sleeves into bolt holes to prevent crushing of the plastic material and looseneing of the bolt as the plastic shrinks. Where bolts need to be threaded into a thermoplastic component, especially when disassembly is a possible need, threaded inserts are strongly recommended. There are several great books on fastener use in thermoplastic components, so again we’ll not belabor a point that is near the edge of our expertise here. Just be sure to have done your homework before incorporating fasteners in a new design to avoid unpleasant surprises later in product life.