Page 422 - Injection Molding Advanced Troubleshooting Guide
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42.3 Splay Troubleshooting  421



             ƒ Lubricants
             ƒ Cracked mold
             ƒ Leaking air
             ƒ Venturi effect

          42.3.2.1„ Mold: Venting
          One of the biggest concerns with tooling issues creating splay is the impact of
          venting. Often times by allowing the gas to escape the mold through vents splay
          will be eliminated. If wiping the parting lines of the mold seems to improve the
          problem for a short period of time look at optimizing venting. All gas that is ge-
          nerated in the plastic melt stream as well as the air in the cavity must be allowed
          to exit the mold through vents. Many molders underestimate the importance of
          proper venting (see Chapter 7 on venting methods and design).


                Case Study: Splay Caused by Lack of Venting
                The splay was near the start of fill approximately 2–3 in away from the gate
                and to one side. The splay did not appear to be coming directly from the
                gate. The tool went back and forth to the tool room multiple times to review
                the venting; it was eventually vented well beyond the original venting, to the
                point where the whole perimeter of the parting line was now being vented.
                Feedback from processing was that the issue went away when aluminum
                tape was added on the parting line near the gate leading them to believe
                there was still an issue with lack of venting. Closer observation of where the
                aluminum tape was being placed, which was near the runner, showed this
                area was not vented. The mold was pulled from the press and vents were
                added to the runner. When the tool was set in the press and restarted the
                splay was completely gone! Venting on runners is as important as it is on
                the part (see Chapter 7 on venting methods and design).
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                Case Study: Splay from Screw Boss

                In this case the screw boss was 1 in deep and with a shutoff hole in the
                  center. The splay was occurring downstream from the boss. The mold was
                pulled from the press so the tool room could investigate venting options.
                The boss had an ejector pin and a sleeve that were not vented. The tool
                room added a ring vent to both the pin and the sleeve. When the mold was
                run again the splay was gone. If the boss has core detail cut in the solid a
                small vent hole can be added in the core detail and vented to the shutoff
                (see Chapter 7 on venting).
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