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43.3 Sprue Sticking Troubleshooting  435



          using sprue break stay very aware of any nozzle drool as it will lead to damage of
          the tip or sprue bushing over time (and may well cause sticking).



          43.3.2„ Sprue Sticking Troubleshooting Mold Issues

          Below are some of the potential mold concerns for sticking sprues:
             ƒ Undercuts in sprue bushing
             ƒ Incorrect finish in sprue bushing
             ƒ Sprue puller
             ƒ Cooling of sprue bushing
             ƒ Sharp corners
             ƒ Oversized sprue or runner

          43.3.2.1„  Mold: Undercuts in Sprue Bushing
          Any undercuts in the sprue bushing may cause  a sprue to stick.  If the sprue
            bushing has been damaged from steel tools the undercuts will lead to problems.
          Sprue bushings are used reamed with a tapered reamer that will leave circular
          rings with the sprue bushing that can cause sticking. There will be cases where a
          sprue needs to be draw polished to allow proper release of the sprue. See Figure
          43.2 for a magnified view of circular polish lines on a sprue that could lead to stick-
          ing; draw polishing the sprue bushing would remove these undercuts.




























          Figure 43.2  Magnified view of sprue: notice the circular cutter lines
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