Extrusion basics: If it ain’t broke, mix it

My column leads off, as usual, with the reminder that plastics are not toxic—none of them. The public image is the opposite, as shown last week by the McDonald’s announcement that it will stop using all foamed polystyrene worldwide by the end of the year. Scientifically wrong but commercially right. McDonald’s did what competitive businesses do all the time—announce policies that look good to customers. And these customers, the general public, need to believe in magic to stay sane. As science challenges magic (the impossible), the public needs to push back, all the more because plastics are seen as synthetic (unnatural), corporate and petroleum-based.

Problem solvedNow, to mixing in extrusion. This involves not only the mixing of stuff—colorants, stabilizers, fillers, resin blends—but also thermal mixing, the production of a mass all at the same temperature without hot or cold spots or streaks. Fortunately, what’s good for one is also good for the other. Screw design is important but it’s not the only factor, as mixing can also be promoted by static mixers, water-cooled screws and wise selection of ingredients to be mixed. The latter especially relates to the particle size of non-plastic additives and viscosity management—the less the amount to be mixed, the less viscous it should be at mixing conditions. This allows minor ingredients to more freely penetrate between the major particles and become more uniformly distributed. (Distribution refers to uniformity throughout a mass, and dispersion refers to particle size.)

The rule that emerges is that the viscosities should be in proportion to the proportions. It’s usually impractical and unnecessary to follow this rule very closely, but knowing it at least will keep you from trying to match viscosities of concentrates and primary resins in the belief that equal viscosities mix better than differing ones. Yes, if the proportions are 50/50; otherwise, no.

For some hard-to-mix ingredients, liquid concentrates are used, but they may add cost as well as raise the problem of selecting a carrier that either dissolves in the resin (plasticizes) or otherwise disappears (vents out).

In single-screw design, channel depth, especially in the metering zone, is a measure of mixing; a shallower depth results in more mixing but less output. There are dozens of designs that split and redirect the melt to improve mixing, but all of them increase energy input from the motor and may create excessive heat. What’s excessive varies for different compounds; sometimes an improvement in thermal stability will allow more vigorous mixing without the degradation that might otherwise limit rate.

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