Extrusion Basics: Material Matters, and So Do Quantity and Measurement
The Romans needed a holiday in midwinter as they didn’t do Christmas or New Year and followed the agricultural year, as did the Jews. The new moon told them when another month had passed. So they started the feast of Februa, which led to this month’s name.
They knew about many elements and even made glass and bronze but didn’t understand chemistry and biology. We get it in school but most of us forget soon thereafter. Worse yet, many reject the rules of science and are proud of it.
They are then “free” to follow the lawless pulls of convenience and popular images. They may even agree to the existence of plastic pollution even though plastics are not toxic to us, can’t be digested, and are not even a major contributor to global warming.
Chemistry is not magic
In extrusion, chemistry tells us why melt temp control is critical. Energy is stored in connected atoms, is countable, and doesn’t suddenly appear or disappear. Chemistry is not magic or even a danger, but is just a language that describes substances and their interaction. A lot of people fear chemical-sounding names; to them water isn’t a chemical with two hydrogens and one oxygen (H2O) but is “natural.” And all additives “leach.”
Another “natural” chemical is chlorophyll, the green stuff in plants that takes carbon dioxide from the air, water from the ground, and energy from the sun to make fruits and leaves. People are afraid of chlorine, but chlorophyll has no chlorine. The root chloro just means green in Greek. Chlorine gas is, in fact, green, but PVC (polyvinyl chloride) isn’t green because it’s a chloride, like common salt (sodium chloride). PVC has no more chlorine gas than salt, but chemicals that make our tap water drinkable may. That’s the language of chemistry in action.
Biology is important if you make food packages, but also in our daily lives, from eating to food waste to masking (no, it’s not gone yet). Additives in food and plastics do a lot of good. And natural can be harmful — consider bacteria, earthquakes, malaria-bearing mosquitoes, sunburn, and the fugu fish.
Here are links to my two-page reviews of simple science, as published in PlasticsToday:
Finding value in rejects
And now, on to materials — resins and additives, including color. Particle shapes and sizes may affect cost and production rate. Specifying numbered products is an alternate to specifying performance by tests, but you can do both.
If you reject a lot, it still may have value. Blend with more resin or additive to “rescue” the value. It’s better than food, as it usually doesn’t spoil.
Additives need their own specification. How they are added matters, as mixing time/temp/geometry all will affect how much additive is needed. Putting several additives in one compound may save money and get better dispersion. It also keeps the proportions constant, unless you add more of one component separately.
If you are buying formulated compounds, who knows what’s in them and how much? The compounder may not want to disclose details to prevent the buyer from taking the business elsewhere, but the buyer will want to know as much as possible, too. You may have to pay more if you get such information, either for the compound or by testing. Every case is unique.
Think beyond price per pound
One supplier or several? It’s a business decision, but don’t forget about it. Prices always matter but may not be a simple $X/lb equation. Get promises in writing, as memory is notoriously unreliable where money is concerned. Packaging vs. bulk vs. big bags? What to do with empty packages? Particle shape and size, as noted above. If it’s recycle or scrap, who grinds it?
Amounts matter. We may not be as important as we think, although salesmen are telling us we are. Another related area is delivery: Can you go and get it if there is a problem? Easy if nearby and you have trucks or direct rail; not so easy if it’s far away. Risk of delivery delay may matter, so ask what you do if it’s delayed or even failed.
A few extruders and molders reserve in-plant scrap and even rejected product for emergency. They need space (at what temperature?) and grinding capacity. Don’t put paper-based containers like Gaylords outdoors, where rain could destroy them; even with film liners the contents become difficult to salvage.
Some lines can feed only one resin at a time, with or without places for additive entry. Others have multiple hoppers (How big? Drying needed?) Can it blend resins (Premix or let extruder mix)? Can you change proportions?
A word about melt temperature
Measuring has changed with modern technology, and especially number processing. I’m always concerned with melt temperature, which can be different at different places in the system. Time matters, so I want to know how long the same molecules were at high temperatures. Other measurements of note: Melt pressure at the screw tip and in the die; motor current and energy input; lubricant temperature at the gears; and feed temperature at entry especially if there is drying. Does available motor HP depend on screw speed?
Product thickness is easily measured at appropriate places in the product, sometimes fed back to control screw or gear-pump speeds. But don’t forget shrinkage when “freezing” to a solid and during all the cooling.
And for all measurements consider alarms and recording the data so you can learn from them.
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.