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404   41 Sink




               41.3.2„ Sink Troubleshooting Mold Issues

               The root cause for sink can be the mold itself. Some of these factors include:
                  ƒ Plugged cooling lines
                  ƒ Poor cooling design
                  ƒ Hot runner temperatures
                  ƒ Valve gates not actuating
                  ƒ Wall stock issues
                  ƒ Cavity balance
                  ƒ Gate size, location, and quantity
                  ƒ Pull sink

               41.3.2.1„ Mold: Plugged Cooling Lines
               To have effective cooling all water lines in the mold must be flowing well. If a water
               line becomes plugged there will be either restricted flow or no flow. This restriction
               in flow will lead to increased mold temperatures, which in turn could lead to sink.
               Water  flow through the cooling circuits should be measured and documented
               during process development. Once the baseline of cooling capability is established
               during process development it can always be referenced to when troubleshooting.
               If water flow through a circuit has decreased the cooling line may need to be
               cleaned out.
               There are many things that can plug a cooling line including:
                  ƒ Teflon thread tape
                  ƒ Scale buildup
                  ƒ Baffle inserted too far
                  ƒ Metal shavings from machining process
               Scale buildup in cooling lines can be another major cause of hot spots in molds. If
               the plant has water that is not treated properly mineral deposits will form inside
               the cooling lines. Scale buildup in a cooling line is an insulator and will restrict the
               cooling ability of the water traveling through the mold.
               A molding plant must have a quality water treatment plan in place. If the plant
               runs without water treatment, scale buildup will be the result (see Figure 41.4). At
               its worst, scale buildup will choke off water circuits and severely limit cooling. Also
               understand that if the molds are experiencing scale buildup like this so is the heat
               exchanger and feed throat of the molding machine. Overheating oil may be the
               next issue that scale leads to.

               If a mold has scale buildup, the mineral deposits must be removed either mechan-
               ically by drilling or chemically with an acid flush.
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