|Before it becomes the pièce de résistance of your Thanksgiving table,
the turkey is shrink-wrapped in plastic
to facilitate transport and protect it
from harmful contaminants.
It’s Thanksgiving week and our extruded products give us a lot to be thankful for.
We are thankful for insulation of all the power lines and building wiring that light and warm our homes. When we watch the football games next weekend, remember that the synthetic turf and weatherproof domes are our stuff, too . . . as are the extrusion-blown bottles that don’t break; truck-bed liners; trash and recycle bins roto-molded from extruded micro pellets; boat hulls and ice-fishing huts; water pipes that don’t clog or rust; tubing that brings gasoline from tank to engine; and the shrink-wrap on turkeys. Also, corrugated HDPE post office tote boxes to carry things to and from you (my major project around 15 years ago). And remember to be thankful for what you don’t have: All the harmful bacteria kept away by food packaging and storage bags (it’s OK to wash and re-use them); cold winds kept out by PVC window frames; and my own favorite gratitude here in California, where I now live—no mosquitoes.
And, lest I forget, I’m thankful for the people out in the world who see the science and the truth and are not afraid to see and say that there are no toxic plastics. The broad criticism that calls us “pollution” comes from an understandable fear of big corporations and humanipulation, plus the need to resist a science that cannot coexist with magic and the impossible.
Returning to extrusion, I had a participant in my last public seminar who was new to extrusion, sent by his manager to learn as much as he could in one day. Not so easy, but I welcomed the challenge and pass on here the essence of what I told him, my 11 key principles.
- The screw unscrews itself backward out of the barrel, held in by a bearing, and the opposite and equal reaction pushes the melt out the front end.
- The material comes in solid and picks up enough heat to soften and liquefy as it travels along the barrel. The main source of this heat (energy) is the motor, helped along by barrel and die heaters. There are some exceptions: Small machines, many twins, high-temp polymers and coatings, where both barrel and motor heat share dominance.
- Screw speeds are much slower than motor speeds, so we need a reduction system, usually gears.
- Plastic is the primary coolant, as it absorbs the heat from the motor and heaters.
- Stick to the barrel, slip on the screw, in feed zone of single screws, for the most inpush per turn. Rear-barrel temperature control, thus, is very important. Particles also move better when they interlock more and slip less.
- Material is (by far) the biggest cost of manufacturing. Learn as much as possible about materials, control thickness well and reuse scrap and trim to replace new material wherever possible. Machinery factor is low (10% or less), as long as it’s running round the clock.
- Power to extrude is a minor cost, but may be amplified by auxiliary equipment like preheaters, thermoformers and annealing systems.
- Measure and understand dependent variables (“vital signs”): Melt temperature, melt pressure and motor current.
- Components of output: Volume of last-flight x screw speed, resistance of head/screens/die and overbite in rear.
- Plastic melts are shear-thinning—they get less viscous (flow easier) as they move faster. This is especially important in narrow die lips and in the clearances from screw flights to the barrel.
- The motor opposes the barrel and vice versa. Heat up the barrel and the motor sees less resistance, adds less energy. Cool the barrel, and the motor must work harder to turn, takes more energy (heat).
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].
Griff conducts live seminars across the country periodically. Seminars in your plant are also available. If you can’t attend his live events, he offers a Virtual Seminar, which can be seen at anytime, anywhere. E-mail Griff at the address listed above for more information.