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.

Now, 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.

Higher pressure in the extruder, no matter what the cause, will improve mixing and probably raise melt temperature, unless there is a gear pump in place to push through the head/die and leave conditions in the extruder unaffected or even improved (lower pressure = lower melt temperature). More or finer screens are often the first-aid remedy for mixing trouble, but the better they work, the more often a screen change is needed. Mixing will get better as contamination builds up on the screens (more pressure in extruder) and worse after a screen change, but the difference may be too small to matter.

Twin screws, usually intermeshing and either co- or counter-rotating, are traditionally associated with good mixing, but are not necessary. Their basic advantage is the ability to mix with less frictional heat, thus reducing melt temperature and allowing less expensive formulations for PVC and other resins, where stabilizer cost is higher than resin cost.

Static mixers are common and often work well, but present problems in location, selection of basic type/supplier and managing details such as the number of elements and the diameter enclosure. Get the expected pressure drop if you are thinking of using one; the supplier should be able to estimate this if the material and grade are well-known.

Water in the screw is rarer than it used to be but is still effective, as it makes the melt nearer the screw root more viscous and slows its flow, pushing more melt out toward the barrel wall and effectively reducing the channel depth. Shallower channels mean better mixing, but also may cut output per rpm. If there is enough rpm and HP in reserve, this may not matter. Water on the screw has the advantages of being adjustable (more or less) and removable (just stop the flow).

If you are using water (or oil, for UPVC for different reasons), be sure the hardware is safely fixed in place to avoid fatigue failure of flexible hoses, and keep a clear plastic shield over the fittings just in case something does blow out.

Internal leakage from one channel over the flight into another is an aid to mixing. Its effect on output can be either good or bad, depending on which way the leakage is going (forward or backward, depending on pressure gradient).

Particle fusion (in PVC, for example) is not the same as mixing, though they sometimes go together.

I’m not covering premixing in separate vessels, high- or low-speed, as well as starve-mixing, where components are continually dropped downward from several hoppers through a chute over the screw(s). These are well-known techniques that require dedicated equipment and sometimes do an adequate job of stuff-mixing by themselves. Remember, however, that even a perfectly-stuff-mixed mass can be de-mixed by passing it through a head and die that differentially heats/cools different parts.

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 will present live seminars in Chicago on Jan. 23 and Costa Mesa, CA, on March 6. If you can’t attend these live events, he offers a Virtual Seminar, which can be seen at any time, anywhere. E-mail Griff at the address listed above for more information.