The Pursuit of Reality in Extrusion and Elsewhere
What am I thankful for?
Life, that I have lived to super-senior status — for us, the future is now —with a conscious and open mind, clear voice, and plenty to say.
Liberty, which allows me to cooperate with others, and be free from self-indulgence.
And the pursuit of reality, especially in science, where the need to believe the impossible has affected popular beliefs about plastics (and food, health, and a lot more).
Apply reality to extrusion? Yes. For example, energy matters — it is countable and doesn’t magically appear or disappear. It’s a small part of current costs but its excess, as in overheating, can damage the product or slow down production.
Understand the greenhouse gas problem and help others to do the same. This is very hard to do, as climate change has been politicized and misunderstood. It’s certainly real, but all gases are not equal — numbers matter, but cow belch doesn’t — and the role of the oceans as temperature stabilizers and carbon dioxide absorbers — all our bubbly drinks — is either unclear or ignored. Who’s affected? The coastal populations, for sure, but everyone else, too, because weather uncertainty affects food production.
Our extruded products for agriculture and packaging are critical here, as well as water-resistant building products and water-moving pipe.
Thinner products save money but so does less thickness variation, if it allows a lower aim. That may mean real attention to die temperature, or cooling uniformity, or equipment like gear pumps and moving screens for pressure control.
Material selection and purchase are often problems, as they may be less than ideal or less consistent because of the need to buy cheaply and sometimes quickly. Know your “primary pair” — melt index and density — in advance and measure them if no data or unreliable data are available. Remember, too, that all parts of a big shipping container are not necessarily the same. Many polymers are made in a continuous process, so if melt temperature goes up in the reactor, the polymerization rate may be higher, and longer molecules (lower melt index) may be made.
Suppliers may say they test it all, but that’s not enough: How do they test and how often? If they have a continuous viscosity test and guarantee a melt index range, that’s good if the range is OK. But one test in a railcar or even a truckload may not be enough, unless there are data showing consistency in that size container. Not all purchases are direct from the resin maker, and smaller distributors and compounders may or may not provide tighter control. And you still have to know the range that the tests define. It’s not a simple problem, so there’s motivation to avoid it.
Melt index is pushing a melt through a standardized hole, usually 0.083-in. diameter. It is properly criticized as being too slow to duplicate extrusion and especially injection molding, but is nevertheless common in the sale of polyethylene and polypropylene. It works for other plastics, too, but conditions, such as load and temperature, are different for each. The test is ASTM D1238 (ISO 1133). Polyethylenes are run at 190°C and 2.16 kg, but there is a high-load variation (21.6 kg load, still 190°C), used for some extrusion grades. It’s a quick and well-known test, but be sure of test conditions before comparing.
Extruding PVC or PET? It’s complicated
If you’re extruding PVC or PET or their close relatives, there’s more to learn. PET officially is classified by intrinsic viscosity (IV), the viscosity of a solution in an uncommon solvent, and PVC is similar with a different solvent. With PET, it’s easy to run melt index if you don’t overheat and you pre-dry the samples. If an IV is quoted, it may be a real IV tested with solvent, or an IV from a plot of melt index vs IV. Melt index conditions may vary, too. The original Reilly & Limbach work used 285°C and 2.16 kg. This is important today because of the importance of recycled PET. It is possible to change the IV up or down, so recyclers and processors of recycled PET should understand the history and technology.
With PVC, although resin viscosity varies and matters, so much depends on the formulation and its additives that a viscosity test of the compound (such as a torque rheometer) means more. If you want to know about the base resin(s), too, get solvent IV or K-value, ISO 1628.
Density is easy to test via ASTM D792 and a good indicator of properties, especially stiffness, for polyethylenes, hence low-, medium-, and high-density PE. It is an absolute value, usually grams/cc, while specific gravity is a comparison with water; the two are usually the same in lab testing. Temperature matters, as materials expand when heated (water-ice is different and very important in world biology; ask a fish). Remember that there is added energy needed to separate crystals in semicrystalline polymers. Required energies are available on the net, but extrusion is imperfect, so if I need a number to understand real costs, I double the theoretical to get a practical, thus allowing for losses to the surroundings.
There are three important densities:
- the solid material, used to identify in sales;
- the density of melt inside the extruder and die;
- the bulk density of feed, which depends on particle size and shape as well as solid density.
Back to thankfulness, I am thankful for the ability to accept and understand scientific realities, even if it costs me the magic of miracles, and for the liberty that allows me to believe what I see, not just see what I believe. I won’t convince the plastiphobes that plastics are good for us, but I will understand why they think they are bad.
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.