Extrusion basics: In thickness and in health

My monthly message up first, and in brief: No toxic plastics, no “pollution,” but people see plastics as corporate, synthetic and science-based. This negative image creates “plastiphobia,” especially the basis in science that challenges our need to believe the impossible. Mystery OK; magic no. Agree or not, please let me know.

Now to extrusion. Last week I visited Sequoia National Park in central-west California, to see the really big trees (see photo below). The largest one—the biggest living thing on Earth by volume—is the General Sherman tree: 275 feet high and 17.5 feet in diameter, even 60 feet above the ground. It is around 2200 years old, which places its “saplinghood” around the time of Archimedes, the legendary inventor of the screw pump (extruder) and the guy who said, “Eureka!” (I found it) when he realized how buoyancy worked.

Sequoia tree

When I saw such a continuous straight length, I naturally thought of extruding thick sections. The thickest extrusion I’ve personally seen is 6-inch diameter solid rod, but I’ve heard of thicker. Plastic 2 x 4s and 4 x 4s are made, and so is pipe up to 1 inch (or more?) for diameters up to 6 feet (or more?).

The big challenge is cooling. Not only is there less surface per unit mass, but plastics are generally poor conductors of heat. 

Do you really need that thickness? Get some air inside. Corrugated pipe is a good example of getting better crush resistance in a hollow product for less resin. It saves money, too, but requires special equipment, hence, there needs to be enough market to justify a 24-hour operation. Corrugated board is another example; it makes a big difference which way the corrugations run (MD or TD). My own experience is with TD corrugations in the HDPE board used for post office tote boxes throughout the USA; maybe elsewhere, too.

Foam, either chemical or mechanical, is a double-edged solution: Less mass to cool, but the bubbles are an even worse conductor of heat than plastic. Multilayers are made with foam inside, either co-extruded or (with sheet/film) laminated. The foaming agent matters, too: Some chemical agents produce heat when reacting, and others absorb. Nucleation will affect cooling, as it affects bubble size, which in turn affects both physical properties and cooling rate. Foaming needs very good control of melt temperature, as that will affect bubble size, too. Cooling the surface quickly is one way of getting more solid (stronger, more rigid) skin, while letting the interior expand to lighten weight.

Refrigerated cooling water (or for film, air) is a geographic as well as an economic issue: A line in Montana can use outside water or air for more time of the year than one in Arizona. If you do it, insulate the hoses that bring it to the product, as a lot of “cold” (money) can be lost this way.

Another problem is back pressure, which is wanted for good mixing. Big openings mean less back pressure, so thick-section tooling often has very long passages, maybe some restrictions (keep the internals streamlined), possibly some internal cooling. Other resolutions include a static mixer, more aggressive screw design (or use of a twin screw), internal screw cooling and/or judicious choice of what you are trying to mix. Concentrates, for example, should have viscosities much lower than the primary resin, so they can flow more easily between the majority particles and, thus, need less intensive mixing. If you are comparing viscosities for this purpose, try to compare at appropriate temperature and shear rate, as viscosity is dependent on both of these things. Melt index can be used—it’s usually too cool and too slow, but better than nothing.

Once out of the die, the problem continues: The product may be too heavy to move uniformly, such as sagging pipe or even too-thick upward-blown film. In the film case, the solution is to increase screw speed and pull faster. For pipe and profiles, the distance from die face to first cooling is critical—this space can be used for precooling with air or water, or it may be reduced to zero to eliminate sag. If there is water precooling of the surface, make sure there is a slight downward angle to avoid water rolling back along the product to the die face. 

Along the line, pipe and profiles use shaping plates, sleeves or rings to maintain product dimensions in the presence of shrinkage (20% for polyolefins), swell (depends on resin, temperature and die design, and is especially important for foams) and puller speed (drawdown is the primary adjustment). Some shaping devices can actually change their dimensions, and most can be moved along the cooling line horizontally, which is often useful when line speed is changed. They may be made of brass, nylon or other lubricious materials; often, anything under water is enough. 

Spray versus immersion: Immersion may look better, but unless the water is moving it may not be as good as all-around sprays. Immersion can be helped by sponges or even rags along the cooling product to break the invisible film of hot water that adheres to the product surface, allowing new cooler water to come in contact after the sponge or rag. 

Line length: Thick sections may need slower cooling; in extreme cases, fast cooling of the outside leads to vacuum voids inside—this is why we see “macaroni” pellets with holes inside. Usually this means a long linear takeoff. Keep a clear path around the back of the extruder, or build an overpass at the die end so an operator can get to the other side more quickly.   

Anything I missed? Please e-mail or call me, 301/758-7788, with questions, or leave any comments you may have below on the causes of plastiphobia or the majesty of Big Trees.

 

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 conducts live seminars across the country. The next ones are scheduled for Atlanta on Oct. 15 and Toronto on Oct. 17. Seminars in your plant are also available. If you can’t attend his live events, he offers a Virtual Seminar, which can be seen any time, any where. E-mail Griff at the address listed above for more information.

His recent webinar, What All Extruders Should Know, is now available on demand. Watch the free webinar at your convenience by clicking here.

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