We live by rules — some biological imperatives, some traditions, some well-rooted, and some frivolous, but even the frivolous bonds us with other frivolers. Their values change with time and technology, and the longest-lasting are those that can manage such changes, responsibly responding to changes, balancing past and future and avoiding blind commitment to either.
This reflects the tension that exists between old and new, stability and uncertainty. Usually parents and school stress stability (security) — the anchor of the boat — while our desire to try new things is the counter force — the sail or the propeller. Rejecting stability is wasting the lessons from the old. Avoiding uncertainty means we’re not trying hard enough, as standing still means you’re falling behind. This may be fear of rocking the boat, and relates to risk management, but I want to apply it to extrusion. So, here are a few angelized practices — not always wrong, not always right.
When do you turn on the heat?
On starting a line that has cooled down, someone has to turn the heat back on again. Too early with a full head/die may degrade the plastic in contact with the hot metal surfaces; if all is empty, it may just waste time, as much less time might be needed. Things to consider:
- Automatic startup switches, more than one to allow starting heat at different times if masses are different;
- heat to temperature enough to allow proper flow but below that expected in full operation — this will discourage degradation and allow longer “soak” time;
- at shutdown, use a purge or any more thermally stable formulation, to discourage degradation during reheat;
- insulate the head and die, even temporarily during reheat at start — these sections are usually exposed bare metal, and insulation will reduce radiation loss and, thus, heat up faster;
- watch amps at startup to judge if preheat is long enough and, if safe, soak a little less each time, until amps rise and get closer to max on the motor.
Change the formulation?
Sometimes we can't make changes because of purchase requirements or other regulations. But if there is any wiggle room, you can improve almost any property of a material with additives, such as antioxidants, process aids, clarifiers, and impact modifiers. The key here is getting good dispersion to ensure uniform distribution, which, in turn, depends on the use of concentrates (masterbatches) and their carriers, dispersion aids, particle sizes, and relative viscosities.
Viscosities should be in proportion to the proportions, at appropriate temperatures and shear rates. Melt index is not enough. Example: I was asked recently about mixing “an eight-melt PP with an eight-melt PE.” I couldn’t answer because I didn’t know the proportions. Also, they have different viscosities despite the common number eight, as PP is measured at 230°C and PE at 190°C. PP and PE are similar but not fully compatible — that can be helped by functional additives, which add cost, but saving material saves money, too.
Screw design is everything
To be honest, nothing is everything, even winning (sorry, Vince). Screw design matters, surely, but you have to know it to criticize it, and it’s too easy to say “just put in a different screw” to solve a problem. Sometimes it makes sense, but to have an opinion I’d want to know the old and new designs — numbers, not just types or names; why the new design would help; who pays, and who is responsible if it doesn’t work; and the materials to be used, both the resins and the screw metals. It is my own nature to avoid changing equipment, which means first to consider change of conditions, including screw cooling, if possible; use of additives; more or less scrap percentage; or another material grade or even supplier. Before any of this, define the problem and what success looks like. It’s not as simple as it sounds, and some problem-solving actions never get past this point.
The profile refers to the shape of the graph of barrel settings vs. distance:
- If all settings are the same, it’s a level profile.
- If they rise from rear to front, it’s rising or normal, as the temperature of the plastic is rising.
- If it’s hottest in the rear, it’s a reverse profile.
- If it’s hottest in the middle, it’s hump or camel.
- If lowest, it’s valley.
These descriptions are OK provided they are not considered sacred and provided they are not expected to work regardless of the actual settings. A reverse profile of 450-435-400-375 is not the same as 390-380-375-360. The settings may not even match temperatures of the inner wall, because of cool feed at the rear and excess frictional heat at the output end. And the head and die sometimes are included, sometimes not, but are always important. My answer: Forget about the shape, and focus on the actual numbers used for specific resins at specific throughput rates (screw speeds). Settings that work at 40 rpm may cause trouble at 80 rpm.
Proverb of the month. My new feature, a reward for reading this far. Never ask a question three times. First time, maybe they’re busy, or didn’t hear you, or are unsure of the answer. OK. Ask again, as the person may want to answer, and may then do so quickly and even apologetically. But if you ask a third time, the person doesn’t want to answer for whatever reason (think about it), or if they do, I wouldn’t trust the answer without further checking.
And, unchanged each month: Plastics are not toxic, but people want to believe they are, maybe need to. For more on this, see my article, "An Open Letter to ‘Plastiphobes’ about the Material You Love to Hate."
No live seminars planned in the near future, or maybe ever, as my virtual audiovisual seminar is even better than live. No travel, no waiting for live dates, same PowerPoint slides but with my 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.
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 new audiovisual 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].