Biopolymers have been part of the plastics industry now for almost 20 years, with polylactic acid (PLA) being the most well-known of these materials. When PLA first came on the scene, processing challenges came along with it, but today tsuppliers have done an excellent job at perfecting their formulations to make this resin easier to run.
Most of my experience has been in processing the NatureWorks Ingeo PLA resin; actually I was the first person to ever extrude this material into sheet. It has actually been said that, “Tim extruded PLA before it wanted to be extruded”. Yes there were growing pains, but that is not uncommon when new resins are being developed. And as usual, they were resolved.
One thing we learned pretty quickly is that PLA can be corrosive to the carbon/iron-based materials of construction of which most plasticating components are made.
PLA is not as aggressive to corrosion attack as is rigid PVC or fluoropolymers, but in order to extend the life of the plasticating components, they should all be built of materials that will not be effected by the acidic nature of PLA, or at least coated to protect the carbon steel that might be used.
For screws of any plasticating process, the best material of construction is 17-4 ph stainless steel. This grade of stainless is readily available and can easily be machined. It can also have a nickel-based flight hard facing welded on to it, and is a superior product compared to a 4140 HT based screw that has been chrome plated. The 17-4 ph stainless has approximately 16% chromium in its chemical matrix, whereas 4140 steel only is 0.8 to 1.1% chromium. It is that extra chromium that provides the excellent corrosion protection.
The cost of the 17-4 ph stainless screw will be at a premium. But consider that on chrome-plated screws, once the chrome has worn away, the PLA will corrode the 4140 base material. Also, the 17-4 ph stainless screw can be cleaned over and over without the worry of polishing through the 0.001-in. thick chrome that normally is applied to the screw surface.
The other big advantage for using 17-4 ph stainless steel for the base material of the PLA screw, especially for small screws—2-in (50mm) and smaller—is that you will be less likely to overtorque the screw and break it. Again, in comparing 17-4 ph stainless to 4140, 4140 HT Steel has about 95,000 psi yield strength, whereas 17-4 has a yield strength of approximately 175,000 psi, almost twice as much. Again, a great reason to use 17-4 ph stainless for any small screw application.
THE BARREL
With the barrel, things aren’t quite as complicated. Basically there are two kinds of barrel liners: iron-based or nickel-based matrixes. The barrels that most OEMs supply with new machines for injection molding, blow molding and extrusion, have iron-based lined barrels. These liners are industrially known as the Xaloy 102, Wexco 666 or Reiloy 112.
For corrosion protection of the barrel is it best to use the nickel-based barrel bimetallic liners, which have the industrial nomenclature Xaloy 800, Wexco 777 and Reiloy 121.
When molding PLA, not only do the screw and barrel need to be protected, but the components in front of the screw and barrel also need to be made of corrosion resistance materials. The endcap, nozzle adapter and tip should be made of 17-4 ph stainless steel or chrome plated internals. The 17-4 ph stainless is the preferred way to go for the best overall corrosion protection.
For extrusion and blow molding applications, all of the adapters, screen changers, melt pumps, and flow tubes should also be made of 17-4 ph stainless steel or some other corrosion-resistant type of stainless, or all of the polymer flow paths should be chrome plated a minimum of 0.002-in. thick, just is done for processing rigid vinyl.
Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com.
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