Last week it got very hot over most of North America — a little less so here in the East Bay of California, where anything over 80°F is called a heat wave. Some of you in other states might consider that a summer relief. A few years ago, I visited a client in California's central valley when it was 105°F near the extruder and 105°F outside. I think it was even worse last week.
There is no such thing as cold. I mentioned this in my article on heat, “A Hot Topic, Literally, in Extrusion Technology."
Heat is the motion of molecules, so without molecules we have no temperature — but all matter is made up of molecules, including water, the air, all plastics, and us. It’s all chemistry — no organic or natural magic of any other kind.
We cool ourselves by sweating: 540 calories are lost for each gram of water that escapes us and becomes water in the air or on our clothes. The water in the air is humidity. If the air is “full” (saturated), we can't cool off by sweating. We can take showers in colder water, or drink something cold, or avoid heat-generating motion, but an extruder can’t do any of those things. We and the extruder can both use fans, but the machine won’t sweat.
In an extruder, we must put in heat to soften the plastic enough to form it, but then we have to take the heat out again to use the product. This is often done by passing the product through water. This works because water at sea-level pressure is cool enough to harden (freeze) any plastic. Some extrusions, such as sheet and cast film, are cooled by passing them over metal rolls, which are usually cooled with water. A few products, notably blown film, are cooled by blowing air on the emerging plastic. This air may be pre-cooled to accelerate production, but where humidity is high, air that is too cool may condense water onto air-ring surfaces. Also, like home or office air conditioning, there is a cost to making the air colder. Know the money numbers and keep hoses insulated to keep costs down. Remember, too, that cooling rate affects physical properties, so it’s possible to cool too fast.
All plastics shrink as they cool, some much more than others. Cooling must be managed — not too fast nor too slow, and as uniformly as possible around the surface of the product. Think of a tube floating on cooling water, bottom half submerged and top half open to air (or vacuum). This may explain thickness variations within the product.
Measuring temperature is also a part of cooling management. We have infrared thermometers for non-contact measurement, and immersion thermocouples in the extrusion head in addition to a traditional but less useful location at the end of the screw. Metal (head and die) temperature is not the melt temperature. Don’t forget recording and displaying the values so you won’t need to write them down. You can even set up alarms if they go out of desired range.
What if the melt comes out too hot or too cold?
Measuring an irregular or round product with an infrared as it emerges from its die requires a very steady hand or, better yet, a fixed measuring device. Do rounds by scanning the device up and down and watching for the high point. Laser-based gauges are common in some markets. There is still need for recording the history. Make sure you do something, as you don’t want to fly blind with such an important variable. Know answers to the question: “What happens if the melt comes out too hot or too cold?”
City water is the most common cooling medium but may be assisted with refrigeration at added cost. In some cases, water can be cooled by evaporation and recirculated. Technology for in-plant cooling systems is well known, and it should be possible to compare costs to help decide on action. In areas where outside temperature varies widely, whether season to season or day and night, some attention should be paid to how this might keep cooling costs down. I remember one small line where the temperature control device was a large drum filled with water that a man was continually feeding with snow from outside to keep it at 0°C. This was winter in the Northeast. I'm not sure what they did when the snow melted outside.
Sometimes more than one coolant or location is used, notably in blown film, where internal bubble cooling is common (more air), as well as iris strippers, second air rings, and even vacuum rings. Spray cooling is efficient and tempting but needs fine sprays and avoiding drips. A few lines have added water cooling on the outside.
For hollow sections, it’s tempting to cool from inside, but that’s not easy and is seldom done. Remember the shrinkage factor. Very thick sections like solid rod or plank have heat-transfer problems and this may limit the cooling rate.
Things to consider before increasing the production rate
Much cooling effort is aimed at increasing the production rate, but in making that effort, you should also worry about losing thickness precision and if the increased production can be profitably sold. There’s little profit in filling up a warehouse.
Last but not least, don’t waste water or heat. Cooling water and air get hot — the energy has to go somewhere — but its re-use is uncommon. It is possible to re-use barrel-cooling air to preheat feed that was stored outdoors in a cold climate, gaining the advantage of a more uniform feed temperature, as well.
I live in an area where drought affects our agriculture from artichokes to zinfandel grapes, so I’m sensitive to water loss, which may justify more extruded irrigation pipe and film, and less unlined ditches. That’s an investment that might pay off, not just in money but also as eco-help. It could even give us something responsible to advertise and show the plastics-bashers that we can do good as well as making a profit, which I learned years ago was the earned interest on investment.
And don’t waste food (or water) at home, either. Balance your nutrients, eat what you put on your plate, be a model for your children, and count. How much matters, at least as much as taste and image.
About the author
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, and now in his virtual version. He wrote Plastics Extrusion Technology, the first practical extrusion book in the United States, 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].
No live seminars planned in the near future, or maybe ever, as his virtual audiovisual seminar is even better than live, says Griff. No travel, no waiting for live dates, same PowerPoint slides but with audio explanations and a written guide. Watch at your own pace; group attendance is offered for a single price, including the right to ask questions and get thorough answers by e-mail. Call 301/758-7788 or e-mail [email protected] for more info.