Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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basis. Plastic materials on the surface of the screw and barrel can be removed with a wire brush and copper gauze. Precision wear measurement instru- ments, accurate and easy to use, are available in a variety of sizes and capa- bilities. Once the barrel is purged of all plastic, an electronic bore gauge is inserted into the barrel, and readings are taken at every inch mark to generate data on the condi- tion of the ID. The screw's flight diam- eters and depths are measured using a flight micrometer. Digital versions provide LCD readouts and enable the data to be logged into a computer. Taking those measurements on a regular basis to determine rate of wear will enable the operator to predict when the screw or barrel will need to be re- placed or repaired and to schedule the downtime during a favorable time. The recommended clearance for a new 2-in. screw and barrel is 0.004 to 0.006 in. A rule of thumb is one thousandth per inch per side. Screws should be re- built when they exceed the maximum clearance. Repair services can give re- newed life to worn components at lower cost than complete replacement. Barrels for injection molding can be resleeved in the worn sections. Extruder barrels typically have to be resleeved along the whole length, which is more expensive and not always cost-effective. Once a screw is worn past a certain point, the melt starts coming back over the flights, causing both screw and barrel wear to accelerate further. A serious wear condition wastes a significant amount of resin and causes a drop-off in productivity. But wear can be caught before it gets to that point through peri- odic PM inspections. Keeping the screw/ barrel clearance between the recom- mended minimum and maximum read- ings is the best policy. ABOUT THE AUTHOR Ron Anderson is president and founder of Flite Technology Inc., Cocoa, Fla., a supplier of screws, barrels and wear instrumentation. Contact: (800) 327-9310; email: ronander-; website: Severe scouring of this screw surface was also a result of binding due to a mismatched barrel and screw combination. Your Primary Source For Knowledge On Drying and Conveying t 4ZTUFN JOWFTUNFOU WT QBZCBDL t "DIJFWJOH FOFSHZ TBWJOHT t $PNQBSJTPO PG ESZFS BOE DPOWFZJOH TZTUFN UZQFT t 4PMWJOH ESZJOH BOE DPOWFZJOH TZTUFN JTTVFT t 4ZTUFN PQFSBUJPO BOE NBJOUFOBODF t '"2 o "OTXFST UP DPNNPO RVFTUJPOT 8FCJOBST BOE 7JEFPT HOSTED BY PLASTICS TECHNOLOGY MAY 2012 53

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