colour changes can save significant amounts of time
There are two homemade purges which can be particularly effective: Mix #1: 50% poly, 50% diatomaceous earth: Diatom-
aceous earth is actually finely milled fossilized shells of tiny sea organisms. It is used in gardening among other things, so it is readily available in garden supply stores. The material looks like a fine powder, but on a micro- scopic scale the particles have very sharp, angular edges, which produces the scouring action. Processors who run sheet and film use this purge, as it can sometimes get rid of die lines, saving a time-consuming die tear-downs. Mix #2: Poly with kitchen cleanser and water: Mix a
slurry of water and three or four cans of Comet or Ajax kitchen cleanser in a bucket. Feed poly at a normal purging rate and temperature, and slowly pour the slurry into the main feed port. For safety reasons, warn any nearby operators that steam may be discharged from extruder openings. In addition, do not look into the die or vents unless wearing a full polycarbonate face shield and long-sleeved shirt. The water in the slurry will flash to steam, which gives a very good cleaning effect combined with the abrasive in the kitchen cleanser.
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Efficient colour/product changeovers
Figure 8: Take simple steps to avoid baking degraded polymer on to a screw
Colour or product changeovers tend to create problems. Extrusion people would ideally like to always have nice long runs. But in the real world, demands from customers mean frequent changeovers to make 40 kg of product A, followed by 100 kg of product B, and so on. So it is important to approach changeovers in a way to minimize operator effort and problems. l Understand how critical machine cleanliness is for the next product to be run. There are anecdotes around any extrusion shop of somebody purging for 16 hours, to avoid pulling the screws to brush them off. The point is, if you know the next product requires a thorough manual brush-cleaning, don’t waste time and material trying to get by with purging. l If possible, plan runs in order of decreasing cleanli- ness requirements. In terms of colours, this means
from lightest to darkest. This means you should only face one difficult changeover, when going from the dark colour back to a light colour. l Have written purging or strip-down procedures to run each product. This helps operators plan their time, and eliminates arguments later on about product being scrapped because the operator did not clean the machine properly. l Use the right size extruder for the lot size. This may seem obvious, but many processors do not plan runs for the right machine. For example, they end up running a 200 kg lot on a 75-mm extruder, producing 130 kg of scrap while getting it to run correctly. l Help operators by organizing the necessary tools, cleaning supplies, screens, die plates and other required products near to the extruder. If frequent changeovers are required, set up a workstation to have everything the operators need right by their side. l Study the flow of raw materials and end products in and out of the process area. Try to arrange it to minimize operator effort. The less operators have to hoist drums and gaylords around, the more they can concentrate on maximizing running time.
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Warm-up and cool-down procedures to avoid degradation
Warm-up: Extrusion people tend to keep the machinery hot, all the time. Operators typically turn the heat zones on the minute they come in the plant at 7:00 AM. If you’re really going to start the extruder at 8:00 AM, then there’s nothing wrong with this. But many times people get distracted with maintenance chores, and the extrud- er sits idle, heated to full operating temperature for hours at a time. The reason this is a bad practice, is because it allows
polymer to bake onto the screws and barrels (Figure 8). Once baked on, the material will continue to degrade and carbonize. Purging will not remove this material. Even worse, as the extruder is running product, black specks will periodically flake off the screws and barrels, contam- inating the product and causing customer complaints.