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Purity and danger is the title of a book by British anthropologist Mary Douglas (Mary Douglas). He shows the basic instinct of people to believe in definable, unmixed, and pure things because they think that everything else is right. The limited predictability of life poses a threat. This may explain the habit of wearing white clothes and repeating certain words and even food as comfort in religious ceremonies, which distinguishes us from them. Douglas focuses mainly on human culture, but I extend it to explain the negative image of plastics worldwide due to its synthetic, man-made (and therefore unknown and unpredictable) nature.

Last week, the plastic recycling industry held a two-day presentation and exhibition in Washington, DC. The program, the list of exhibitors and some presentations are available from the sponsor Plastic Recycling Department. Since there is an extruder at the end of most recycling lines to make pellets or sometimes finished products, extrusion plays a major role, but it is actually a link in a complex operation chain: collection, packaging, separation, washing, Grinding and shredding (not the same thing), sometimes mixed with other ingredients, and finally fed to the extruder.

There are several integrated recycling line manufacturers that send the items to be recycled into one end, and the particles are discharged from the other end. As with all online systems, the production speed is limited by the slowest process. The design of the components must take this into consideration, which requires an understanding of how much material (pounds or kilograms per hour) to be processed and its form and state (particle size, cleanliness).

This is where purity comes in, because recycling always has pollution problems, especially post-consumer recycling (PCR), which is the main focus of this conference and trade show. Even if the resin does not degrade due to heat, for PET, PC and PA, moisture will reduce their performance, embedded particles will reduce their performance, these particles can act as stress concentrators (for example, in pressure pipes), or just Decorative high-end packaging. These particles can be removed by filtration (sieving), and thanks to the textile industry, this technology is far ahead of most of our needs. The difficult part is to estimate the commercial significance of the pollution to justify the cost of finer screening.  

Such technologies include laser drilling panels instead of screens (some with tapered holes instead of through holes); sintered metal fibers; corrugated or 3-D screens, which greatly increase the screen surface (less screen replacement); A cleaning device that continuously scrapes or blows contaminants back to the outside; and other continuous equipment, replaces contaminated screens with new, clean screens.

The screening decision is also important because it is related to melt temperature and resin degradation, which may affect color, physical properties, or both. The finer the filter, the higher the back pressure in the extruder, the greater the power required to push the motor, and the energy consumed by the motor makes the melt hotter. The barrel can be cooled, but this is not very effective, because the cooling will make the melt at the barrel wall more viscous, which means that the motor needs to do more work and therefore add more heat.

The gear pump between the head and the mold takes over the work of pushing through the mold, so the melt temperature can be reduced because the pressure in the last screw area is only the pressure required to push through the screen and trap the contaminants on the screen. A screen must be set before the pump to protect the pump. This is a more precise processing equipment than the extruder screw.

More screen and panel knowledge:

Image courtesy of Pavel/Adobe Stock.

Allan Griff is a senior extrusion engineer. He initially provided technical services for a major resin supplier. Now he has been working independently for many years as a consultant and expert witness in legal cases, especially public and internal through webinars and seminars. . As early as the 1960s, he wrote the first practical extrusion book and the plastic extrusion operation manual updated almost every year, and it is available in Spanish, French and English. Learn more on his website www.griffex.com or send an email to [email protected].

Griff conducts live seminars nationwide. The next one is scheduled for June 25th in Houston and June 27th in Chicago. You can also hold seminars in your factory. If you can't participate in his live events, he provides a virtual seminar that you can watch anytime, anywhere. Send an email to Griff at the address listed above for more information.

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