Water Droplets in the Feed Throat
Maybe you’re tired of reading about resin drying. After all, fellow Plastics Technology columnist Mike Sepe, as well as PT contributor and drying expert Pete Stoughton, and I have written a number of articles covering this topic over the years. Yet for some reason the message isn’t sinking in.
I suppose nobody enjoys the details of drying, yet they are critical to the “Three Ps” of injection molding: Processing, part Performance, and Profits. Let me tell you a quick story. Within the last few months, I have come across two cases in which water droplets had condensed in the feed throat and nobody understood where the water had come from. In both cases, production called maintenance to find a crack in the feed throat. But there wasn’t any crack or other damage to be found. So where did the water droplets come from?
In both instances, the molder was using hygroscopic resins, materials that attract and absorb moisture from humid air. If not dried properly, they release some of the water as they are compacted and warmed in the feed section of the screw (about 50% of the flight length). Some of the water within the pellets volatilizes and then condenses in the feed throat, especially if you are running the feed throat at temperatures below 130 F (55 C). To prevent such condensation, the feed throat must be run at a temperature that will allow water and other volatiles to escape or vent.
Most processors run the feed throat too cold. Actually I like 150 F, even for polypropylene. There may be a resin that should be run below 130 F but I do not know of a thermoplastic that requires this. (Thermoset resins are a different story. There are some, but not many, that can be run at temperatures lower than 130 F.)
Bottom line: I do not know of a good reason to run below 130 F for most thermoplastic resins. And next time you get a chance, check out the feed throat (use goggles or a mirror) and let me know if you see any rust. Send me a picture.
While this may be an extreme case, if you are processing hygroscopic resins and not drying them properly, not only can you get water in the feed throat, but you also will have parts failing in service. What happens to your day when a customer calls in with part failures? Your next few days become chaotic. The extra time and effort required to find the cause—and 100% part inspections—will cost you money. By the way, you may not see splay on the parts, so visual inspection will not find the bad ones.
If you are still in any doubt about the ramifications of not paying appropriate attention to the details of drying, let’s consider a more basic question: Why do you need to dry moisture-sensitive resins—such as PC, PET, nylon, PBT, and certain elastomers—in the first place? The common answer is, “You dry these resins to get the moisture out.” While true, this is not an acceptable answer if you are practicing Scientific Molding. You need to state why these resins need to be dried.
Drying moisture-sensitive resins is important to maintain their physical properties. Processing these resins with small amounts of water causes the polymer chains to undergo a chemical reaction known as hydrolysis. Trace amounts of water react with the polymer chains in the barrel at melt temperature and literally chop the long chains into shorter chains. Water actually is consumed by the hydrolysis reaction so you will not see splay. Parts may look fine and even pass tests at room temperature, but the parts fail in use. Short chains do not provide the properties of strength, chemical resistance, etc. for long-term use or use at high or low temperatures. Often, part failure comes with liability issues that can really jack up the costs. Bottom line, the better answer to the question of “why dry” is, “to maintain part performance.” The resin needs to be dry, often below 200 parts/million (ppm) before processing.
It is critical to know before processing whether the resin is dry enough. How do you tell? My guess is 99 out of 100 molders will say they can tell if the resin is too wet. How? They claim they will see splay on the parts or minor foaming of the melt during purging. Granted, if there are significant amounts of water present, these indicators will be evident. But the moisture level required for good parts is very low, often in the range of 0.02% or 200 ppm. At a moisture level slightly above this range there will be no splay or foaming of the purge—again, because the water is consumed in the hydrolysis reaction of breaking the chains. You will have bad parts with no indication of a problem.
The only way to know if the resin is dry enough for processing is to perform a water-specific analysis (not overall weight loss) before processing. Unfortunately, most shops use weight-loss methods, which usually provide incorrect data. Such methods are not acceptable. You do not want to process a resin that has even a very low moisture content above the recommended level.
If you do process an improperly dried resin, not only will you get parts that will perform poorly, but you cannot even use them for regrind. Once the chains are broken, re-drying will not rejoin them. You’ll have to scrap the parts and use virgin material to replace them. Now you are paying double for the resin, which will erode your profit margin with no hope of recovery.
Note that not all resins require drying because they undergo hydrolysis during processing. ABS and acrylic, for example, absorb moisture just like PC or nylon, but do not undergo hydrolysis because of their particular chemistries. They are susceptible to splay or foaming if inadequately dried, however.
Also, some non-hygroscopic polymers (polyolefins, for example) are not susceptible to hydrolysis, but may need drying because water adhering to the surface of the pellets will cause visual splay, but no degradation of properties. Such resins don’t always need to be dried—often the surface moisture is just condensation when cold pellets are brought indoors to a warm, humid plant environment. If the plant air is humidity controlled, or if the pellets are allowed to warm up sufficiently before processing, non-hygroscopic resins typically need no drying at all.
Bottom line: Understand why you are drying a resin and dry it right the first time. It is worth the extra effort and/or cost. Finding time to dry it again hurts, but if you have to buy new resin, your profits will be under water.
ABOUT THE AUTHOR: John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other specialties. E-mail email@example.com or visit scientificmolding.com.
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