Downstream extrusion operations: It ain’t over till it’s over

Fenway Park scoreboardGetting the melt out of the die may look like the main objective in extrusion, but it’s just the first half. Plenty of teams go into halftime with a lead, only to lose it in the second half. Likewise, as much attention must be paid to the shaping and cooling phases of the extrusion process as to the heating and mixing stages.

Shaping is the fixing of final dimensions of the product. In some extrusions, such as compounding, precision isn’t needed, as long as all the pellets are similar—no longs or shorts—and the bulk density stays constant. As a general rule, this means a variation of no more than 1%—holding between 0.600 and 0.615 g/cc, for example. The big problem here is variation introduced by the testing process. Do you just pour the pellets into a container and eyeball it to get the level up to the mark? Do you pour to overflow and slide a ruler or big knife blade across the top to level? Do you tap the container three times on a table to get it to settle out, or use a handheld vibrator to ensure max packing? All these methods are known, as well as the common practice of not testing at all. Pellets are pellets, you may say, and we’re selling by the pound, so who cares? Well, the buyer/user should care, as more or less density may affect in-push and, thus, the screw rpm needed to get a desired rate, which, in turn, affects heat development. I see the point of the no-tester, but I still think a reliable bulk density test is better than no test.

With blown film, shaping is the interaction of die gap, output rate, blowup ratio and winder speed. The die gap is fixed by the die insert and blow-up ratio is fixed by product needs, so for a given output, the draw rate becomes the main control of average thickness. I say average, because point-to-point variation around the bubble has other causes. Some dies have automatic adjustments, but this is expensive and also doesn’t help with variation caused by short-term local changes in melt temperature, which could be due to non-uniform mixing or varying air motion around the head area. Internal air pressure also must be held steady to ensure uniform thickness.

Flat film, coating and sheet have more flexibility in adjusting gap, but this too isn’t helpful if there is still external change, such as open doors or windows, sunlight warming up part of the head or fans set up in hot weather. If you do use a fan, don’t point it at the head/die, or shield that space from moving air. Sheet is often sized and/or embossed with another roll that squeezes the melt as it passes over the main cooling roll. This may work if the thick/thin areas stay cool and aren’t too different, but remember that if the sheet is thermoformed, it won’t remember its uniformity or embossing when reheated.

Pipe, tubing and many profiles have specific tooling that forms the extrudate on leaving the die. For irregular sections, small heaters or just spot insulation (held on with magnets) can direct flow, and are more versatile than permanent machining of internal approaches. The distance between die face and first cooling is critical and should be variable, as it may need to be changed if the line is run faster or slower. Pre-cooling is used especially for PE—water spray or flow around the product just before it enters the first sizing device.

That device for hollow products is usually a closed-top water tank, covered and held under vacuum so the atmospheric pressure inside blows up the walls against the sizing sleeve. If the sleeve is too long, friction limits production; if too short, there may not be enough contact cooling. Melt temperature and both melt and solid lubricity, as well as sleeve metal, are relevant.

The die opening is usually not exactly like the final product, and must be dimensioned with experience, data or both. This must account for shrinkage from cooling; semi-crystallines like PE/PP shrink up to 20% when cooled, and some of this is after cutting/winding. Some shrinkage is countered by swell, which can vary widely depending on resin and internal die design. Don’t be too quick to say a product is oversize, or to want inline or soon-after printing if the post-cooling may distort the printing.

Water-bath cooling is usual for pipe and profiles. Consider stripper-cooling—sponges, plates, even old rags—to strip hot water from the product surface and allow cooler water in to contact the product. This may save some refrigeration cost and need for active water flow.

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