Sensitive resin meets old equipment

Let’s face it, to make money in thermoplastics, it’s all about “pounds on the ground”. Optimizing the throughput of an injection molding or extrusion process, especially for older equipment, is a lot easier if you understand a little about how temperature affects resin flow through molds and dies.

We perceive thermoplastic melts as having complex and difficult to understand properties, because they’re different from fluids we normally encounter in daily life. Consider another common material that flows: water. Water has a sharp, well defined melting point. Ice cubes don’t turn to putty as they melt in a glass of Canadian Club. The flow properties of water in the liquid state are relatively easy to understand, mainly because of water’s consistent viscosity over the 100 degree range of its liquid state. Viscosity is a measure of a liquid’s resistance to flow.

Molding or extruding TPs is all about flow. The volumetric flow rate (which ultimately limits machine productivity) isn’t quite as simple to define. In the 1830s, Poiseuille created an equation to describe a simple fluid moving through a tube. The bottom line is this: the flow rate of a simple fluid increases both with pressure and tube radius (such as mold runners) and decreases with viscosity. Unfortunately, Poiseuille was a medical doctor, so the fluids he worked with were more like blood than polypropylene.

Unlike water, the viscosity of thermoplastic melts is not linearly proportional to the force pushing the melt through the runner or die. You can’t simply double the pressure to halve the viscosity. However, if you increase pressure, these polymer melts can get dramatically thinner and flow more readily. From a practical point of view, this resulting flow, or “shear rate” is the very thing a processor is attempting to maximize: machine operating speed.

If the goal is maximum shear rate for the best throughput, the primary way to control viscosity is temperature. Different polymers, however, have different temperature sensitivities. Acrylics, for example, are very temperature sensitive; an increase of just a few degrees can create a drastic viscosity drop. Polyethylene, on the other hand, would need an increase of tens of degrees to produce an appreciable melt thinning.

This has a couple of consequences that go beyond the need for speed. One is that equipment, particularly older machines whose temperature controllers oscillate significantly around the preset temperature settings, may work smoothly with less temperature sensitive resins, such as polystyrene, but then give poor results when switching to something temperature-sensitive like acrylic.

For custom shops running lower volume jobs on older equipment, temperature controllers that show the mean barrel temperatures may still produce inconsistent parts with a highly temperature-sensitive resin. Switch materials, and the problem can disappear.

A simple solution may involve a good set of temperature viscosity charts from the resin supplier. The absolute values on the charts are less important than the slopes on the graph. Steep lines translate into high temperature sensitivity, requiring tighter machine control.

Another productivity consequence of polymer temperature-sensitivity is related to the shear thinning effect that I mentioned earlier. One way to compensate for erratic temperature control with a sensitive polymer is to bring the melt into a working range where it behaves more like water (that is, has a direct relationship between pressure and viscosity). It turns out that at very low and very high flow rates, polymer melts have that water-like behaviour. The faster you flow, the thinner your melt. On older equipment, then, one way to cheat poor temperature control when running temperature-sensitive resin is to slow down the machine.

The real solutions, then, are temperature control appropriate to the resin and the throughput required for job profitability. Having a handle on the temperature sensitivity of your resin can protect your margins, especially on older equipment.