Thermoplastic resins: just heat ‘n serve

Last month, I compared the molecular structure of polyolefins to popcorn on a string. That model is extremely simplified, but it’s illustrative of the effect of heat on thermoplastic resins. Resin can be thought of as a tangled collection of popcorn strings of many different lengths, i.e. molecular weights. Add heat, and you excite the kernels (molecular repeat units) into movement, both within and between strings. That excitement is what makes the resin flow. In a typical extruder or IMM barrel, the heat comes from friction, the shearing action of the screw, and from heater bands or cartridges. How orderly the strings pack in the solid resin determines its degree of crystallinity, an important parameter. Highly crystalline materials (an ice cube is a good example) have sharp, well-defined temperatures at which the material melts into the liquid state. On the other hand, amorphous polymers are much less ordered and soften progressively with increasing temperature, so that in theory, the more heat, the easier the melt flows. An example of a highly crystalline resin is neat polystyrene; most as-molded polyolefins are amorphous.

Unfortunately, all that heat does more than make resin molecules dance; it also breaks down the molecular chains, causing, among other things, a reduction in average molecular weight. The resin coming out of the nozzle will be different from the resin going into the hopper. Additionally, oxygen, moisture, and excessive barrel residence time will make a processor more of a chemist than he or she realizes. Striking the balance between good melt flow properties (rheology) and melt degradation is the key. More on this next month.

THE NUTS AND BOLTS OF NUTS AND BOLTS

Compared to dancing molecules, I know that nuts and bolts lack sex appeal, but a fresh look at the way we hold things together can also yield a revelation or two. Honest. What is a bolt? Nothing but a spiral doorstop that wedges itself into a threaded hole (or nut). Without getting into the intricacies of thread designs and classifications, the object is to use that shaft to apply a clamping force between two objects through the underside of the bolt head or nut face. Before you say, “that’s obvious”, think for a moment about what happens when you torque that fastener down: it stretches.

Bolts stretch? Absolutely, and in more than one direction. Twisting, or torsional stretch, occurs when the assemblies pull up, or where a bolt or stud bottoms in a blind hole. Twist isn’t necessarily a bad thing, since it forms the essential tactile feedback that tells the technician about the tightness of the fastener. Torsion IS a problem, however, when the bolt or stud doesn’t spring back when the wrench or socket is released. That’s “plastic deformation” to metallurgists, and a seriously weakened fastener to you and me. The trouble is, repeated over-torquing of that fastener, for example, during mold changes, will result in a time consuming and potentially expensive failure. Hardened components resist the bending, but will yield less before their elastic limit is reached. The result is bolts that take more torque, but break suddenly, with little or no tactile warning. The same phenomena are seen along the length of the bolt, too, if the excessive axial loading stretches the bolt or stud beyond its elastic limit.

How do you minimize or eliminate fastener failures? Next month, I’ll offer a few tips on several techniques that give good results at low cost. CPL