Extrusion basics: Troubleshooting? It’s OK to monkey with the resin

First, my monthly reminder: There are no toxic plastics, none of them. Our materials are safe and do far more good for the environment than harm. When you meet anyone who thinks otherwise, try to find out why they want to believe the contrary so fiercely. You may be surprised and maybe even sympathetic. If you ask for statistics, toxicity data and their sources, you probably won’t get any, or they will be selective or come from questionable sources. Anything you think is really significant, please let me know.

MonkeyNow, I can talk about extrusion. I’ve done a good deal of extrusion troubleshooting over the years, and found that many people want to believe that the equipment is the problem. If we can just change the screw or the screen pack or maybe do something to improve cooling, the problem(s) will go away. That sometimes happens, of course, but I’m a chemical engineer and my “grad school” was my job in tech service for a resin supplier, so I also look to materials for solutions, especially because that’s where most of the money goes, too.

One thing we can do is blend resins with each other. We often do this, anyway, if we work our trim and scrap into the product, but sometimes it makes sense to blend two or more different resins. For example, we might blend some regular LDPE with LLDPE to raise the speed we can run without melt fracture. Proportions will depend on needed properties and relative resin costs. 

Dissimilar resins sometimes blend, too. ABS may be used to toughen PVC, and SEBS resins are typically sold as additives to other resins.

With all additives, mixing is important, and even if the resins are compatible, their viscosity differences have to be considered. My rough rule is to use viscosities in proportion to the proportions—that is, a minor component should be less viscous than the major component, so that the minor component can flow more freely and, thus, be dispersed more evenly. We can use melt index to represent viscosity, but melt index is tested much more slowly and usually at different temperatures than in actual production. The ideal is to have viscosity data at mixing temperatures and shear rates—the data are easy to get but rarely used. 

With polyethylene (PE), blending different densities can be done. Density is important, as even the third decimal place (0.922 versus 0.925, for example) may have a major effect on rigidity (modulus) and lesser effects on barrier and physical strength. This assumes a similar type of PE, as linear lows and regulars at the same densities will still be different. The proportion-to-the-proportions rule applies here, too.  Remember that density also depends on cooling rate, and the same resin(s) may give different densities if cooled differently. 

Whatever the polymer, it pays to specify density and melt index: Make sure that what you are buying meets that specification (it’s best to specify a range rather than a single value). And be prepared to test both incoming materials and finished product. Test often, with multiples on each

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