OK, let’s get my ritual intro over with now. There are no toxic plastics. None of them. Have you done your pushback to the contrary public image today? If your friends and family have heard enough, try a stranger—airplane seatmate, Starbucks' barista, your friendly Uber driver. And if you want help, or you don’t really believe it yourself (plastophobia is contagious), call or email me.
|The 20% shrinkage rate of polyolefins is a great help in manual melt removal and is the basis of the “pliers-and-air-hose” process: Grab the melt with the end of long-nose pliers, direct an air hose at the metal-melt joining lines, and pull the melt away at just the right rate.|
Now, on to the topic of the day—cleaning small, and sometimes not so small, components. When a system is taken apart, there is a lot of sticky stuff, and it seems logical to remove it. With most plastics, the rule is to get it off hot, but some of that is a result of the “speed syndrome,” the common assumption that everyone is in a hurry. That’s why cooperative phone-helpers are trained to use phrases like “quick minute.” I sometimes tell them, “take a slow minute and be more thorough and careful.”
When cleaning an extruder, time matters if it controls how fast you get back into production and if that speed really means more product sold as well as made. If there is enough time, there are solvents for some resins—PVC, PS—so items can soak overnight and cleaning is much easier. Precooling the items (in a bucket of water, for example) will avoid the dangers of hot solvents (vapors, fire). Counter-argument: Solvents have a direct cost, while mechanical removal costs are driven more by labor than material. Counter-counter: Scrape off most of the melt mechanically, dissolve the rest and keep using the same solvent as long as it works.
The “devil’s tools” are a hammer and (steel) screwdriver. Don’t use them on critical areas, such as sealing surfaces, where dents and scratches may lead to leaks, or resin-flow surfaces such as die lips, where scratches and lip-edge nicks lead to die lines in the product that don’t go away. These devil’s tools are common and the first resort in the minds of many. Fight the “quick and now” impulse and have brass or hardwood strips or bars available for safe cleaning.
Polyolefins—PE, PP—don’t dissolve easily, and the solvents are expensive—they do better hot and have dangerous vapors. Therefore, it’s customary to remove melt manually with non-scratch brass or wood pieces, as needed. The 20% shrinkage from melt to solid is a great help here and the basis of the “pliers-and-air-hose” process: Grab the melt with the end of long-nose pliers, direct an air hose at the metal-melt joining lines, and pull the melt away at just the right rate. The rate will vary with the resin and its temperature. Too fast will tear the melt; too slow will take forever to finish. See the photo, and use a pair of brass-tipped pliers if you’re working on a sensitive surface. There may be a flush of pride when you get it to work well and peel off a lot with one pull, much like peeling an orange in one strip, but after a few jobs it becomes work like so much else, and the pride may remain but is less obvious. Safety warning: Any time you use an air hose, wear long sleeves and heavy gloves to avoid blowing air at an open wound, and wear all-around eye protection, as the air may blow things around at eye level.
There are “die soaps,” such as solid, weak organic acids, that may help in removing polyolefins and other resins. Like the solvents, they add a cost, but used carefully they may save time and justify their cost that way.
Ovens have been around for a long time and many types are used in component cleaning, ranging from ordinary heated spaces and vacuum ovens to ovens that get hot enough to vaporize some or all of the organic (plastic) material and fluidized-bed ovens, which swirl small particles of very hot sand (or alumina) around the part to help melt/dislodge/vaporize what’s stuck on the surfaces.
Salt baths are a specialized oven-like treatment, known in the synthetic fiber industry, where dies have very small holes. These use a mixture of sodium and potassium chloride, which melts at a temperature lower than either component.
Blasting the surfaces with a torch is considered a no-no, as it may warp the component, ignite the plastic or both, but if done carefully it can work as an example of the proverb, “if you do something wrong, do it right.” With breaker plates that means doing it horizontally—no vise marks on sealing surfaces and letting melt fall through the holes (watch for foot/shoe protection). Also, with all components, it means doing it slowly, as the job is to melt and let flow, not search and destroy. This is no place for the “speed syndromers.”
Do you really need to get it all off? All is one of the “extreme” words, like now, and when used strongly and in the right place, it means no halfway, no compromise, no balance. Even the word compromise has taken on a negative value these days, implying a half-surrender, with the compromiser in a tough spot and giving up more than he gets in return. Call it balance, cooperation, synergy or even a win-win deal, and you’ll get a more positive feel.
Lastly, consider whether we need cleaning at all. If the next resin coming through is the same, maybe we don’t, except perhaps at sealing surfaces to avoid later leaks. Even if the change is to a different color or formulation, or there is suspicion of degradation/contamination in what’s still in the extruder, on startup the first production is scrap anyway, so it might as well be low-value discolored or contaminated scrap.
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 firstname.lastname@example.org.
Griff will present live seminars in Houston on Sept. 27, Chicago on Nov. 8 and Seattle on Nov. 27. Seminars in your plant are also available. If you can’t attend his live events, he offers a Virtual Seminar, which can be seen at anytime, anywhere. E-mail Griff at the address listed above for more information.
|PLASTEC Minneapolis 2018 held October 31-November 1 is part of the Midwest’s largest advanced design and manufacturing event that also includes MinnPack brings you the latest in materials and additives, injection molding, rapid prototyping, coatings, automation, packaging and more. For details, visit PLASTEC Minneapolis.|