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a purging compound is that starting up is much easier, because the production resin can be run directly after the purging compound is purged. The barrel will be clean and free of contaminants, so a start-up step is eliminated by using this method. SCREW PULLS For difficult changeovers (black to natural or white, for example), a screw pull can be an effective option. Pulling a screw is time con- suming and labor intensive, but using a CPC can significantly lighten that burden. By running the barrel full of certain CPCs, then pulling the screw, the technician could have to exert signifi- cantly less force to pull the screw and also have much less manual cleaning to do, because the purging compound does a good portion of the cleaning. This decreases the overall changeover time. There is even a grade on the market available that will not adhere to the barrel/screw and peels off the screw similar to peeling a banana (see photo at left). PREVENTIVE PURGING If you're worried about increasing production numbers and decreasing scrap rates, preventative purging can be a life saver. The first step in establishing a preventive purging program is to start tracking production rates, changeover time, and even the amount of purging material used during changeovers. When production rates start to fall, it might be time for a preventive purge, even if it is not time to change over. This method effectively removes deposits in their beginning state and prevents them from settling in and becoming more difficult to remove. clean the barrel. Chemical CPCs are used in lower-pressure applica- tions, such as extrusion, blow molding, etc. This type of purging compound uses an endothermic chemical reaction that expands to create the pressure and agitation necessary to clean the barrel. Depending on the grade and severity of the problem, this reaction generally requires 5 to 30 min of soak time in the barrel. If you don't make sure you're using the right grade, there's a good chance that your contamination and changeover issues won't improve. CHANGEOVERS Changeovers are unavoidable for almost all processors. In cases of changing resins or colors, the previous resin/color can some- times linger in the barrel, creating scrap. The material will travel through the barrel following the path of least resistance, which causes layers of resin to accumulate over time. Resin alone cannot remove these layers as effectively as a CPC can. Changeovers introduce a way for the layers to "shake loose" because the machine is not at steady state. Figure 1 shows how contamination and layering can occur. By not purging during the last changeover, an operator might see some of those layers come out in the next product. In other words, there's a ripple effect of not purging during every changeover. It is critical to understand your total machine downtime cost in order to truly understand how well the purging compound is working. This requires gathering a bit of data, but it is necessary to quantify a purging trial. So rather than subjectively determining "it was OK" or "good," you can make decisions based on concrete data. Figure 2 details a profile extruder's real-world cost-savings analysis for its changeovers. This particular processor was using virgin HDPE to purge. The changeover took it 2 hr, and it calculated that since the machine runs at $100/hr, the downtime cost was $200 per changeover. They used 25 lb of purge material, priced at 75ยข/lb, to give a changeover material cost of $18.75. Adding in the machine downtime cost equates to a total purge cost of $218.75. Assuming this profile processor performs five changeovers a week for 48 weeks in a year, this computes to $52,500 annually. If you bring additional staff onto the line to help speed up the changeover, how much more is that purge costing you in terms of labor? SHUTDOWN & SEALING One of the most often overlooked functions of a CPC is to use it to shut down and seal. Unless your operation runs 24/7/365, you will have periods of extended downtime for holidays and weekends. Even if you do run 24/7/365, you'll eventually have shutdowns for preventive maintenance and repairs. In any of these circumstances, sealing the barrel with a CPC will help prevent degradation-causing oxygen from entering the barrel, thus reducing carbon from settling into the system. Another benefit to shutting down and sealing with QUESTIONS ABOUT PURGING? Visit the Purging Zone. If you're worried about increasing production numbers and decreasing scrap rates, preventive purging can be a life saver. As material flows through the barrel, it will travel along the path of least resistance. Deposits are left at the edges of the flow channel, creating layers if left over time. FIG 1 How Lingering Layers Create Scrap Material Flow @plastechmag 63 Plastics Technology P U R G I N G