Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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With baseline data, wear-monitoring procedures, and an under- standing of what is happening to the screw, maintenance and engi- neering departments can significantly reduce downtime and increase their bottom line. Replacing high-performance feedscrews can be expensive, but an experienced screw manufacturer can rebuild worn screws to like-new condition for a fraction of the cost in less time. When looking to rebuild a worn screw ask yourself these questions: 1. How severe is the overall wear? 2. What are the wear characteristics? 3. Can the current screw design be greatly improved? 4. Can the surface treatment be improved? If careful consideration of wear monitoring is ignored, screw wear can be so extreme that it is difficult or even impossible to repair reliably. Typically, the cost of rebuilding a screw depends on the severity of wear and the amount of work required. An experienced screw manufacturer can inspect worn equipment and processing performance to determine if the current design is worth salvaging. In some cases, a new, optimized screw can greatly increase overall profitability rather than relying on the original, less-efficient design. Often the worn screw can even be re-cut, improving the performance vs. scrapping the old screw. To improve abrasive and corrosive wear resistance, hardfacing and surface treatments can be improved to extend the overall wear life of a screw. Many reputable screw suppliers state that a screw can be rebuilt only three to four times. The size of a screw and a number of factors that contribute to the severity of wear can greatly impact the life of a screw, but it is not uncommon for an experienced screw manu- facturer to rebuild a screw many more times. With high-quality welding, careful consideration of the metallurgical aspects of screw manufacturing, and proper flight-profile reconditioning, the author has seen screws being rebuilt up to six to eight times. After a screw is ground to a uniform undercut diameter, the flights must be rewelded to utilize the higher-hardness hardfacing alloys that provide maximum wear resistance (Fig. 2). After hardfacing, the screw OD must be ground to original tolerances (Fig. 3). The leftover over- hanging weld must be profile ground to provide minimum flight- width loss. In many cases, poor practices reduce the effective flight width during this process, potentially affecting screw performance and complicating future rebuild potential. To help alleviate these complications, some screw manufacturers opt to reduce the width of weld. When these screws are ground to original tolerances, the welded hardface does not encompass the full flight width. This negatively impacts wear performance due to the exposed softer base material. DEVELOP A PLAN With proper baseline data and wear-monitoring procedures in place, and an understanding of what is happening to the screw, mainte- nance and engineering departments can develop a robust mainte- nance program to greatly reduce downtime and increase profitability. It is important to monitor screw wear before scrap rates and energy consumption soar. Don't be too quick to throw out worn equipment. Consulting with an experienced screw manufacturer on the potential of rebuilding screws can not only reduce downtime, but the cost of rebuilding tends to be a fraction of the cost of a new screw. Good rebuilding practices should allow for screws to be salvaged many times, passing on major savings in maintenance cost. A robust planned maintenance program rooted in baseline data, wear monitoring, and a basic understanding of screw wear is far better than reactive maintenance. ABOUT THE AUTHOR: Stephen Surley is a sales engineer at R. Dray Mfg. Inc. of Texas and was mentored by company president and famous screw designer Robert Dray. Surley has spent the majority of his time working closely with customers in both injection molding and extrusion applica- tions to design custom screw solutions to improve molding quality and efficiency. Contact: 254-386-2488; sales.rdray@gmail.com; rdray.com. Calculator Estimates Rate Loss Due to Screw Wear Results from online calculator of estimated rate loss while processing PE with a worn 3.5-in. diam. screw. An average 0.016 -in. of radial wear provides an estimated loss of 16.323 lb/hr. Results from online calculator of estimated rate loss while processing PP with a worn 3.5-in. diam. screw. An average 0.016-in. of radial wear provides an estimated loss of 21.22 lb/hr. FIG 4 FIG 5 Total Wear Clearance: 0.016 in. (Average Radial Barrel Wear + Average Radial Screw Wear) Nominal Screw OD: Flight Width: 3.5 in. 0.3125 in. Screw Lead: Head Pressure: Typically Equals Nominal Screw OD Do Not Include Commas 3.5 in. 2000 psi Material: Melt Index: Polypropylene From Material Supplier 26 g/10 min Melt Index Test Load: Melt Density of Material: From Material Supplier From Material Supplier 2.16 kg 0.7 g/cc Estimated Rate Loss: 21.22 lb/hr Total Wear Clearance: 0.016 in. (Average Radial Barrel Wear + Average Radial Screw Wear) Nominal Screw OD: Flight Width: 3.5 in. 0.3125 in. Screw Lead: Head Pressure: Typically Equals Nominal Screw OD Do Not Include Commas 3.5 in. 2000 psi Material: Melt Index: Polyethylene From Material Supplier 20 g/10 min Melt Index Test Load: Melt Density of Material: From Material Supplier From Material Supplier 2.16 kg 0.76 g/cc Estimated Rate Loss: 16.323 lb/hr 54 OCTOBER 2018 Plastics Technology PTonline.com T ips & Technique s

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