Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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rapid (with fillers) in the solids-conveying section of the screw, it is common for fillers to be added downstream where the polymer is already molten to allow for better lubrication. Mixing sections and barrier flights also tend to wear, as their function requires resin to flow over their flight geometry. Dispersive-mixing designs incorporate some form of undercut geometry that imparts high shear forces on the resin to promote a more isothermal melt and uniform color. As the mixing design wears, there is a loss of adequate dispersion. Much like dispersive- mixing designs, barrier flights are undercut to help separate the melt pool from the solid bed. As the undercut flight wears, barrier designs can lose melting capacity and efficiency. PERFORMANCE ISSUES Screw wear is generally a slow process that can go unnoticed until performance is greatly reduced. Minor wear will have little effect on overall performance, as machine parameters can be adjusted to main- tain productivity. When the radial clearance between the screw flights and barrel wall increases, leakage flow is inevitable. The operator will typically see a reduction in throughput and an increase in melt tem- perature, forcing higher rpm and energy consumption to maintain the desired rate. With increased radial clearance, the screw is unable to develop necessary pumping pressure to maintain output. Obviously, the amount of radial clearance is a key variable in leakage-flow calculations. A common "rule of thumb" considers a flight clearance four times the original tolerance to be the cutoff point for screw replacement, but it is important to consider the viscosity of the resin processed. A resin with higher viscosity will provide less loss of output than will lower-viscosity resins with the same radial clearance. These rules of thumb should be viewed only as guidelines. To help better predict the effect of diametrical wear, you can calculate leakage flow by considering overall clearance, flight width, lead, head pressure, viscosity, and melt density. These calculations can become quite cumbersome when trying to accurately predict throughput; but fairly accurate estimations can be generated that predict potential rate loss. To help processors and maintenance departments predict this loss in productivity, the author has devel- oped a rate-loss calculator that estimates the potential loss of rate due to screw wear. This tool helps conceptualize the severity of wear and helps teams develop a plan for screw replacement or repair. This tool was used to generate the results shown in Tables 1 and 2, which reveal the estimated rate loss for PP and PE under the same conditions. Notice the more viscous PE suffers less rate loss than does PP. When processing other shear-sensitive resins such as PVC and PC, minimal wear can adversely affect quality due to the increase in melt temperature from excessive shear. As resin leaks over the screw flights, the material experiences excessive shear rates. This raises melt temperature, which is only compounded by the higher rpm needed to maintain output, as noted previously. Higher melt temperatures can lead to major quality concerns, as molded part physical properties are greatly reduced at these excessive temperatures. When processing amor- phous resins, these high shear forces can easily burn and degrade the extrudate, leading to high scrap rates. Root wear generally creates pockets and areas for material to hang up and degrade. This is observed as black specs in molded parts, and surging. Material degradation can become a serious concern when processing resins like rigid PVC. PVC tends to generate hydrochloric acid at elevated temperatures, leading to extreme corrosive wear on equipment. In extreme cases of root wear, the sides of the flights can become washed out, leaving little substrate to support the welded hardface inlay, as seen in Fig. 1. With no support, the brittle hardfacing material easily chips away, leaving rough edges, reduced flight width, contaminated extrudate, and potential damage to the downstream components. REPLACE OR REBUILD? As many processors will agree, downtime is the enemy of suc- cessful production. Many extrusion and injection molding plants rarely, if ever, pull screws to record wear. Tight production sched- ules and maintenance complications can force companies to squeeze every ounce of life from existing equipment. The cliché, "If it ain't broke, don't fix it," comes to mind. Trouble is, these types of practices can leave a processor in limbo when screw wear can no longer be ignored. Remember: Screw wear is a gradual process that can go unnoticed until scrap rates spike, energy consumption soars, or in extreme cases, catastrophic failure results. To save downtime and costs associated with buying and installing new equipment, many processors spend a fortune on the hidden costs of increased energy consumption and material waste. When a screw is finally pulled for inspection it may not be salvage- able, forcing longer downtime waiting for new equipment. It is important to monitor screw wear before scrap rates and energy consumption soar. Screw OD being final ground to original OEM tolerances after hardfacing procedure. FIG 5 @plastechmag 53 Plastics Technology S C R E W W E A R

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