Plastics Technology

FEB 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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FOLLOW DIRECTIONS A common mistake with CPCs is not following sup- plier instructions. While experimentation is encour- aged, it should be conducted only after establishing a baseline by following the step-by-step purging instruc- tions. Try purging with or without the die, or purge with the screen packs in or out. Maybe consider other grades of purge compound, and increase temperatures by about 50⁰ F where extra cleaning is necessary (like areas of heavy contamina- tion). Optimization is always the end goal of every process, and your purging program is no exception. Consult with a purging expert before you decide to experiment on your own. ANALYZE COST FACTORS Smart companies will thoroughly and continuously examine processes to evaluate where cost can be reduced. If you're not already keeping track of changeover time, scrap rate, production reject rate, and lost production due to downtime, start now; you can't know if the changes you make are working unless you know where you started. Being consistent is also critical to accu- rately understand whether your efficiencies are improving. One of the most common objections processors have about purging compounds is that they are priced more expensively than regrind or virgin resin. But you should also consider the costs associated above, and see that scrap rate, production reject rate, and changeover time will all increase by not using a CPC. If you think about these costs in terms of cost per purge, rather than the cost per pound of CPC, you'll see that the purge compound will actually save you a substantial amount of time and money. Refer again to Fig. 2. In a trial using 5 lb of Asaclean's U-grade purging compound, the changeover took 20 min., reducing the machine downtime cost from $200 to $33 and lowering the changeover material cost from $18.75 to $16.70. This results in a $49.70 total changeover cost using CPC as opposed to the $218.75. Assuming the same five changeovers a week for 48 weeks a year, this totals a cost of $11,928 annually. The customer would save $40,572, or 77% annual savings in changeover cost simply by using CPC instead of virgin resin. What about home-made purges? Some think in-house purges are better because they are less expensive. However, ask yourself these questions if you are using an in-house purge: Is my in-house purge resistant to oxidation? If I shut down and seal with my in-house purge, will I have defect-free product when I start up again? Am I positive my in-house purge is not adding another layer to the contamination? If the answer to any of these questions is no, and you are looking for a better way, maybe changing to a CPC is the way to go. DISCO PURGING This tip is specific for extrusion and is even effective for twin- screw setups. As mentioned earlier, not purging during every changeover can cause long-term contamination issues. However, using a purging compound and a technique known as a "disco purge" lets the machine do the work and remove those set-in layers. In a disco purge, the operator runs the machine at the maximum safe speed, then stops the screw abruptly, repeating that process several times. This allows the purge material to settle into those layers when the machine is not running, but when it starts up again, it creates more turbulence and starts to remove the layers with the CPC. Repeating this process removes more and more of the layers, and eventually the remaining contamination should shake loose and come out before running the next product. If you're not already keeping track of change- over time, scrap rate, production reject rate, and lost production due to downtime, start now. ABOUT THE AUTHOR: Jarred Packard is the project engineer for Sun Plastech Inc., Parsippany, N.J., the manufacturer and distributor of Asaclean Purging Compounds. He is a purging compound expert specializing in new product development for Asaclean. Packard joined Sun Plastech in 2017 after working in the packaging industry. Contact: 973-257-1999 x248; jpackard@asaclean.com; asaclean.com. Virgin Resin CPC Changeover Time 2 Hr 0.33 Hr Machine Running Cost $100/hr $100/hr Machine Downtime Cost $200 $33 Amount of Material Used 25 lb 5 lb Price of Material 75 ¢/lb $3.34/lb Changeover Material Cost $18.75 $16.70 Total Changeover Cost $218.75 $49.70 FIG 2 A Profile Processor's Purge Cost (Virgin Resin vs. CPC) Profile processor slashed 83% in changeover time and cut changeover cost by 77% by using CPC to purge as opposed to virgin HDPE. 64 FEBRUARY 2018 Plastics Technology PTonline.com P U R G I N G T ips & Technique s

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