Plastics Technology

NOV 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Which ElboW ProblEm Do You NEED to SolvE? ProblEm cAuSE SolutioN Formation of streamers, angel hair and snake skins when conveying plastic pellets Plastic pellets skidding and/or bouncing against the outside radius of sweep elbows create friction and heat, melting pellet surfaces. Smart Elbow ® replacements from hammertek eliminate these problems by eliminating impact Unlike conventional "impact" elbows and "plugged-tee" elbows that rely on material impact to change direction, HammerTek's Smart Elbow ® design features a spherical chamber that protrudes partially beyond the desired 90º or 45º pathway, which causes a ball of pellets suspended in air to rotate, gently deflecting incoming pellets around the bend. No impact means no wear and no fines and no streamers, angel hair or snake skins. Wear and failure when conveying glass-filled, mineral-filled or other abrasive pellets Abrasive pellets hit the outside radius of conventional impact elbows at high speed, continually wearing through the elbow wall. FF-0975 Free, No-risk trial offer See how Smart Elbow® deflection elbows eliminate conventional and plugged-tee impact elbow problems—at no cost or obligation. 1-610-814-2273 45° and 90° elbows available in diameters ranging from 1.25 to 18 inch shape that is close to that of the molded part, while the ash from the bad part is a pile of disconnected fibers. The reason for the difference in appearance is the fiber length in the two parts. Longer fibers stay more entangled and will maintain the overall envelope that defines the part shape; shorter fibers will not. Fiber-length retention actually starts with mold-design considerations. Flow paths that are too restrictive produce higher shear rates and shear stresses. This is true for the entire melt-delivery system and extends to the cavity itself. Material composition also needs to be considered, specifically any colorant being added to the material. Titanium dioxide, the most common pigment used to produce white and other light colors, is very abrasive and can promote a much greater level of fiber attri- tion. But in this case the mold had not changed between the two production runs and no color was being added to the raw material. (The slight yellow color of the good part arose from a somewhat longer time in the dryer). This points to the process. The biggest culprit in decreasing fiber length is the conditions associated with screw rotation; a combination of revolutions per minute, recovery time, and backpressure. Some backpressure is needed to provide a stable volume of material in front of the screw at the start of injection. But if it is excessive it will increase the mechanical work on the material to a point where it does more harm than good. In addition, running too low a temperature in the rear zone of the screw will create additional mechanical work that will cause more fiber breakage and also cause the screw and barrel to wear much faster. I once saw a processor who was running a 65% filled PPS go through three screws in two months because they insisted on running the rear zone at a temperature lower than the melting point of the polymer. Worn screws and barrels also cause excessive fiber breakage. And if the screw employs a mixing element that is of the high-intensity variety, this will also ensure a high degree of fiber-length reduction. Vented barrels are an example of this, but mixing elements that produce high levels of shear to achieve a homogeneous melt will also negatively impact the integrity of fiber-reinforced materials. The biggest culprit in decreasing fiber length is the conditions associ- ated with screw rotation: a combi- nation of revolu- tions per minute, recovery time, and backpressure. @plastechmag 21 Plastics Technology M AT E R I A L S

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