What are you really measuring?

Sharp-eyed readers may notice this month’s Canadian Plastics has a feature on testing and testing equipment. While researching the article, I thought about the ways we measure the various parameters that control injection and extrusion processes. When we establish a set-point and check instrumentation to verify we’re holding to that parameter, are we really measuring what we think we’re measuring?

Take barrel temperature, for example. Regular readers will recall the shearing action of the screw often supplies the vast majority of the heat for melting the resin. Therefore, heater bands work as both a supplement and a way to fine-tune the melting process.

Temperature sensors feed data to the controller, but what exactly are they measuring? Barrel temperature is only the same as resin temperature in a steady-state condition that’s independent of other factors like variation in screw speed or press cycle time. Changing to a mold with a smaller shot size and cranking up the speed will let you produce more parts per hour. But that action will mean a different residence time for the melt. With that in mind, what does the temperature registered for the Number Two heater band mean now?

Melt pressure is another example. There are ways to infer it. For my money, nothing works better than putting a transducer in the melt stream. Even though this seems fairly obvious, modern software often takes “dumb” sensor inputs and conditions the signal to the point where I’m really not sure what the real conditions are. This has an obvious benefit — mainly for ignoring spurious data or annoying transients — but I prefer seeing every glitch. And modern “smart” sensor technology massages the data stream before the controller even sees it, so it’s not possible to back probe the data cable if you have a hunch something’s wrong. Similarly, some self-diagnostic software simply sends an interrogation signal down the wire, which is dutifully fed back into the controller — like dropping a rock down a dark well. You might hear a splash, but it doesn’t mean the water’s fit to drink.

My perfect test strategy? Read the output. Before you get the wrong impression, however, don’t think that modern control software or hardware doesn’t work. On the contrary, it’s now possible to control everything, from screw speed to platen position, with unbelievable accuracy and repeatability. You can monitor a press from across the floor or around the world. But sometimes your eyes, ears and nose can sense impending trouble as reliably as the linearly-regressed graph on your engineer’s clipboard.

Technology is great, but in this business, we still need to think on our feet.