A Helping of Plastics Extrusion Insights with a Side of Shakespeare

The shortest day this year is Dec. 21, the winter solstice. I will have some nine and one-half hours between sunrise and sunset here at around the 38th parallel north. Further north, it’s even less. Before electricity, long nights meant more need for candles and early rising. But the axis of the earth is pointing up to Polaris, the north star, as it always does. "It is the star to every wandering bark, whose worth's unknown although his height be taken" — Shakespeare.

If you want to understand this more, talk to a smart 12-year-old, ask about Galileo, and listen openly. Put a toothpick through a grape to represent the Earth on its axis, and use a round electric light bulb for the sun. Or stab a chopstick through an orange.

As for Shakespeare, ask me. That poem — Sonnet 116 — has been rewritten, as Shakespeare was not a sailor.

These observations won't help your extrusion directly, but it might remind you that time has units and we can't change them, even if we can change their worth. How much matters. In formulations, in probability, and in test results — the big ones. And even if you have numbers, know what they are for. Mysteries yes, magic no.

Safe storage

What will help is understanding the safe storage of products, such as temperature between manufacture and shipment, which can relieve stresses and change dimensions and properties.

For film to serve as a barrier to oxygen and carbon dioxide, it will start absorbing those gases from the air as soon as they are made, which may affect test results.

Effects are similar in humid areas, but the resistance to moisture is different for different polymers, so I squirm when I see the word plastic used too loosely. They are not all the same.

Winding tension can affect properties

If you are winding on a roll or reel, the tension in and across the line will matter and can affect properties. Talk to winder makers if you don't have an in-house expert.

Nips need to avoid anything falling in and possibly damaging very expensive surfaces. If it’s removable, you can wire it to not start unless all protections are on.  

Compounders and other pellet makers worry about floating contamination in water baths. Easy to manage with screens, but someone has to do it.

Screening melt is also easy but needs decisions as to which mesh, angle of pairing if doubles are used, and when changed (on a time or pressure basis). Continuous changers may pay for themselves if used to recycle dirtier feed (like ag film).

Wire moves so fast that online testing is common. The displays should be in an air-conditioned room and fitted to self-correct or at least make a big noise if they are off.

Know what limits line speed and go there, if an increase can raise production speed and earn its cost. There is a cost to all change, but not always in monetary terms.

Extrusion blow molders may be able to test stresses at different temperatures and also may be able to apply closures when made. For a sterile interior, in-mold sealing is needed. The plastic should be good enough. If sterilized with ethylene oxide, some are not resistant. Blowing air may not be sterile, either.

Bug-based problems

Bugs — some factories have them all over, and some keep them out. Not funny if you have them at a problem level. They can be in resin silos or storage boxes. Use liners in gaylords for scrap. Apply insecticide at loading docks and other openings. In one infamous case, the bugs in a silo in New Jersey were a species only found in that state and not in Texas, where the resin was made.

Yes, it's Christmas time again. Time for temporary magic and a lot of fire-resistant plastic greenery and high-class music. Remember to collect and recycle your waste wrapping and unwanted junk.

I don't know if Santa Claus lives at the South Pole for the Australians, but they have their (summer) solstice, too!