Extrusion Basics: When Conversions Get Lost in Translation

Greek up, Latin down. Some of you will know right away what I am referring to — the rule for multiples of standard units. Some are already familiar to American-system users: We pay our power bills in kilowatt-hours, and we see medicine and supplement content in milligrams (mg). Chilia is 1000 in Greek and mille is one thousand in Latin. We don’t have the Greek χ sound in English, so we use k, even when spelled with ch, as anyone familiar with college fraternities, sororities, or statistics may know. Therefore, if it’s “Greek up,” a kilogram is 1000 grams, and if it’s “Latin down,” a milligram is one-thousandth of a gram.

man showing confusion
Image: Olly/Adobe Stock

Not everyone knows this, and it gets important with the thickness of extruded products, which is why I mention it here. There is a website where people from all over the world can get technical help — SPE’s The Chain — and recently someone from far away asked about extruding a 3-inch-thick part! We asked if he really meant 3 inches, maybe 3 mils (0.003 inches) or maybe 3 millimeters? Turned out he wanted 3 mm (0.12 inches), which is quite reasonable, but the poster was trying to use American units (inches), as he was posting on an American site, and got it wrong.

Lesson to us all: Just because we know something, we can’t assume everyone else does, and there is always a need to get everyone familiar with important conversions. The millimeter-to-mil conversion is especially confusing (40 mils to a mm). Film people sometimes have to convert a mil into 25 microns (a micron is one-thousandth of a millimeter). Some other useful ones are 2.2 lb to a kilo or 454 grams to a pound, 3 kilowatts = 4 horsepower, and 30 cm to a foot and 3.3 feet to a meter. These conversions are deliberately inexact; I know that 39.37 inches is a meter, but I want to make them easy to remember, as more accuracy isn’t usually needed right at the extruder.

There is a natural reluctance among humans to count accurately, and sometimes to count at all, as it may speak against what we want to do or not do. A good example is food, where if something is deemed good, more must be better, or for some things any amount is bad. Remember 9-7-4-0: 9 Calories/gram for all fats/oils; 7 Cal/gm for ethyl alcohol; 4 Cal/gm for protein, sugar and starches (dry weight); and close to zero for everything else. Lessons here are:

  1. Fats aren’t inherently evil: We need some to help digest proteins, and they don’t deserve the obsessive demonization they often get;
  2. Ethyl alcohol has energy value, but also has other effects which limit its nutritional role;
  3. Proteins, sugars, and starches all are around the same, despite (in)famous diets that restrict one or the other. Many of us eat too much of all of these, so counting matters;
  4. The zero isn’t truly zero, but veggies and fruits are so bulky that they can’t really affect our energy intake very much, and have other micronutrient and roughage value, too.

Back to extrusion. If the product is too “fat,” you may be wasting money, as most resins are bought by the pound (or kilogram) but sold by length or area. That’s a reason why we try to reuse as much as possible of our scrap/trim to replace new resin, the measure of true recycling. For a few products like pipe/tubing, precise thickness control is needed to ensure proper performance. If a pipe doesn’t fit in its fitting, it can’t be used. This is the justification for accurate thickness-measurement equipment, sometimes fed back to the extruder die or screw. If there is a gear pump between the screw and die, it does some of this job but not all, as the die comes after the pump and can introduce thickness variations, too.

Another conversion, important to resin buyers, is pounds of crude oil per barrel = 375 to 400. Next time you read about changes in crude price, figure out how it relates to resin pricing — probably less than you think. Throw in the fact that most resin makers don’t buy their feedstock at open-market prices, and you are flying in a fog, until you see that resin prices are controlled as much by demand, volume, contract terms, and specialized performance as by oil supply.

And if you’re looking for my usual declaration of polymer nontoxicity, you can see this and much more in my just-published “Open Letter to Plastiphobes,” including the reasons why so many people all over the world love to hate us.  

As for live seminars, nothing planned in the near future — you know why — but you can still “attend” online, even better than live, via my video seminar: No travel, no waiting for live dates, same PowerPoint slides but with my audio explanations and a written guide. Watch it at your own pace, group attendance is offered for single price, and the right to ask questions and get thorough answers by e-mail. Call 301-758-7788 or email me at [email protected] for more info.

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].

Comments (0)

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