What’s happening in the barrel?

In a recent issue of Canadian Plastics, I took a very quick trip down the barrel of a typical injection molding machine and talked about basic screw design, and the difficulty of plasticizing resin without degrading it along the way.

I mentioned that different screw profiles, lengths and length/diameter (L/D) ratios are needed for optimal performance with a given polymer. There is no one-size-fits-all screw, or machine. In this column, I’ll explain one of the reasons why.

Few manufacturing processes are as materials-dependent as plastics, especially in the case of injection molding. The number of ways to generate lousy parts is enormous, as are the variables that can be tweaked to rectify a bad situation. However, before we start changing settings, let’s start with some more material considerations.

Basically, thermoplastics don’t flow the same way water does. Most substances we regularly encounter have viscosities that are affected by temperature in a fairly linear way: increase the temperature and the fluid gets thinner — a phenomenon motor oil manufacturers love to remind us of. Heat a thermoplastic like polyethylene (PE) and it will also get thinner, and become easier to propel down the barrel. But “stressing” a non-Newtonian fluid such as PE will result in a strange phenomenon: it will flow more easily.

The relationship is exponential, where further pressure produces a more linear response. Put simply, resin in the barrel responds to both temperature and the shearing force applied by the screw. If money didn’t matter, this would not be important; but in the real world, it’s crucial. Screw speed and profile need to be matched to the material you’re molding.

Now add another factor: residence time. Many thermoplastics are very temperature sensitive, so the torture they endure in the barrel needs to be brief. Cranking up the temperature will decrease melt viscosity, but it can be a crude and unreliable tool if the screw design and speed can’t properly shear the material. And playing with screw speed has its own drawbacks, not the least of which is the need to run it as fast as possible. Why? Because in commodity molding, it’s pounds on the ground that make the difference between survival and losing your shirt, so running it “flat out” makes good financial sense.

Now add another variable: molecular orientation. I once described polymers as spaghetti-like masses of entangled carbon chains. Stress the resin by shoving it down the barrel and the molecules lose some of their random orientation and line up along the axis of the barrel. Not perfectly, but enough to have consequences in filling the mold and freezing the resin.

There are more variables, but the point is this: even well-understood “neat” (no additives or other resin species) polymers can be surprisingly difficult to consistently mold well, especially if you’re running at the machine’s capacity.

Maybe an example from my experience will illustrate it better. The job was a simple polypropylene (PP) pad — a flat rectangle about 1×3 inches. It ran well, but the press could have handled a bigger mold, and could have delivered a larger shot size. A new mold with 50 per cent more cavities promised serious throughput improvement. Filling that mold, however, required a greater proportion of the machine’s cycle time, and when combined with the larger shot size, meant spinning the screw faster.

Needles to say warpage reared its ugly head, even though the gating remained the same and pressure/temperatures were well within the resin supplier’s recommendations.

What did we do wrong? By increasing the shear rate, the subsequent but slight increase in molecular alignment was frozen in the wall region of the mold cavities, while the relatively warmer centre of each cavity allowed the resin to relax a little, from a molecular order point of view. The result was internal stress that guaranteed warpage. We ended up running at a little less than full output to get quality parts, but the additional cavities paid off regardless.

There are so many variables that no one will have trouble finding books on melt flow and control. For most of us, however, we can forego the heavy reading and survive by sticking to the variables we understand, and more importantly, control. Just cross your fingers they’re the same ones!