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EXTRUSION A number of die-flow issues can affect surface finish, but one that's often overlooked is the "sharpness" of the die exit geom- etry. Polymers will stick to the die surfaces unless you use additives such as fluoropolymers, which can lend a degree of stick/slip flow. Moreover, all polymers have some degree of die swell due to molecular orientation that occurs in the die exit. This is due to uniaxial flow, which untangles the long polymer chains in the direction of flow. Once relieved of the flow velocity, the polymer returns quickly to its "relaxed" state and reorients to a random configu- ration. This results in expansion, primarily in the transverse direction, both horizontally and vertically, and is referred to as swelling of the extrudate. Die swell increases with velocity, reduced land length, decreasing temperature, and the molecular properties of the polymer. Adhesion to the die surface, combined with die swell, causes the polymer to flow along the face of the die after the exit if there is not a sharp break-off point, rather than moving directly downstream away from the die face. This situa- tion is exacerbated by any radius on the die exit, which causes the polymer to continue following the die surface because of the combined effects of adhesion and swelling. Such exit radii are not usually there by design but are caused by periodic maintenance and/or gauging. Contacting the exit edges of the die with hard objects will eventually wear the sharp edge, creating a radius. Sharper Dies Make for Smoother Extrusions Although it's wise to use soft tools to clean the die, it should be noted that all metals that have a "gold" color are not necessarily soft. Typical low-carbon steel has a Rockwell Hardness (Rb) of less than 71. Pure copper has a typical Rb of <10, but some copper alloys are as hard as 98 Rb while brass alloys can be 63 Rb. It's not uncommon to observe operators using razor knives, steel spatulas, hardened drills, steel-wire brushes, stainless feeler gauges, and even screw drivers to work on the die. These types of tools are often harder than the die-exit edges unless the edges are made of heat-treated, high- carbon steel, and as a result can wear them quickly. The "breakaway" effect for a sharp edge is contrasted by one with a radius in the accompanying figure. The polymer tends to follow the radius because of its adhesion to the wall, and then swell when it needs to break away cleanly from the die face and move downstream. This can cause micro-tearing of the surface Get more insights on Extrusion from our expert authors. short.ptonline.com/extrudeKH Learn more at PTonline.com KNOW HOW EXTRUSION Even protective materials such as brass are hard enough to abrade the sharp edge of the die exit, creating a radius or chamfer. Dies that have a radius at the exit will result in poorer surface finish. By Jim Frankland Die Exit Radii Influence Extrudate Smoothness On die exits with a large radius (bottom), polymer tends to follow the radius because of its adhesion to the wall, and then swells, when it should break away cleanly from the die face and move downstream. This can cause micro-tearing of the surface and leave polymer on the radius or above it on the die face as die buildup. Sharp edges (top) result in a much smoother extrudate. Sharper Lip Radius—4X Smooth Surface Tears Lip Buildup Large Lip Radius—4X Die Lip Die Lip 40 JUNE 2017 Plastics Technology PTonline.com K now How