Extrusion basics: If you know what good is, you’ll know what fishy is

Man holding a fresh fish

Years ago, we had clipboards. We would put a piece of paper in the clipboard, look at the displays once in a while and write down some numbers. When everything was running well, we would sometimes just put in ditto marks. But when something went wrong, we didn’t have much data to rely on beyond our experience. That was OK if an experienced operator happened to be around who had seen the problem many times before.

Now, we don’t have as many experienced operators, especially at 4 AM, but we do have better data collection and acquisition, which means we can see what has happened as well as what is happening. What I’m recommending is the self-discipline to look at such records and get familiar with the numbers and how they vary—the range, envelope, standard deviation . . . any way you want to express variation. Average values aren’t enough. I’m talking about the vital signs—the melt temperature, melt pressure and motor load in percentages or amps. Screw speed is vital, too, but we set that. It usually holds to set value very closely, and when it does vary it’s rare and serious, except if there is a control loop, such as with a gear pump, where screw speed may be deliberately changed to keep the pump full and at constant output.

While we’re recording the data, we can also worry about how they are measured (yes, data is a plural). Dual temp-pressure gauges at the screw tip are OK for pressure but not temperature. It’s better than nothing, as it will show some material changes, but don’t trust the absolute values.

And it’s melt temperatures we’re measuring, not just the usual barrel and die settings, which all lines have. It’s best to measure melt in the adapter or die, after the screens.

Motor power is easy to measure, but units may be confusing, as they may read either in true amperes (amps) or in motor load. Sometimes the limit is shown above the display, or it may be alarmed, so when the limit is reached a light flashes or a buzzer sounds. Lights are better, as they can be seen at a distance. Also, it’s best to set the alarm at 80 to 90% of the limit so you have a chance to do something, rather than just stop the line and run away.

Maintenance schedules should include calibration of all the sensors/displays so you can believe what you see. And put a phone number on the screen to be called in case a reading looks fishy.

How easy is it to see the recorded data later? If it isn’t easy, it won’t be done, so print out condensed pages every week and keep them in a notebook at the extruder or production office. And maybe hold a meeting every so often to present and discuss the averages and ranges. If you know what good is. . . .

Last week, I was in the northwestern United States and Canada, where salmon is king, and also sockeye, coho, pink and chum. This is the spawning season when the ones that we and the bears don’t eat swim upstream to lay more eggs and hatch more salmon. That explains this column’s headline.

On this trip I also learned of yet another misunderstanding—a proposed ban of all plastics, even biodegradable ones, in single-use applications, unless they compost in six months. The proposers don’t care that there are no toxic plastics, nor that the anti-plastic image comes from fear of the changeable and anything synthetic. “Plastic” is bad, and people are scrambling to show that their products are not plastic, even if they are. Paper is OK, but the officials don’t understand or don’t want to understand that paper cups need plastic coatings to resist water. “Paper is wood fiber, and wood is waterproof, or we wouldn’t make boats out of it.”

PS: Chinook is the same as king salmon. Chum has the least taste, doesn’t swim very far upstream, but its eggs are best for sushi. All salmon is high in fish oil, which is good for most of us (for the chem-curious, learn the difference between omega-3, -6 and -9). Numbers matter.

Image: Africa Studio/Adobe Stock

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years as a consultant, expert witness in law cases and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or e-mail him at [email protected].

Griff conducts live seminars across the country. The next ones are scheduled for Houston on June 25 and Chicago on June 27. Seminars in your plant are also available. If you can’t attend his live events, he offers a Virtual Seminar, which can be seen any time, any where. E-mail Griff at the address listed above for more information.

His recent webinar, What All Extruders Should Know, is now available on demand. Watch the free webinar at your convenience by clicking here.


Comments (0)

Please log in or to post comments.
  • Oldest First
  • Newest First
Loading Comments...