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Injection mold flow lines

The flow lines are related to the one side of the injection mold, not necessarily the geometry of the part or the composition of the plastic. Follow the instructions below to find the problem.

If the injection mold is in the press, check the following:
There is usually an offset between the injection mold steel that comes in contact with the plastic and the mold base. This is usually between .004" and .010", (.10mm to .25mm). With the injection mold closed and locked up, insert a feeler gage into the gap between the A & B sides. The gap should be very close from the left side of the mold to the right side of the mold. The right side of the injection mold will have a smaller gap. This makes the shut off surfaces of the tool not parallel.

The flow lines problem may be a Ding or Burr on the injection mold base tool steel or on one of the platens. This can be found easily by using a large flat stone on the surfaces of the mold base, and the platen used to clamp up the injection mold. The ding or burr would be on the right side of the injection mold or the mold base. This will show up as a shiny spot when you stone the mold or platen.

If your injection molding machine is a toggle type, the problem may relate to worn out toggle bushings on the left side of the machine. After a mold maker has confirmed the squareness of the mold, check the toggle bushings and pins for wear with a strain gage. Your maintenance department or machine manufacturer could help you with this.

The last most probable cause of flow line is that the injection mold has been running non-parallel for a period of time. Have a mold maker check the vent depth on the left side of the injection mold and the right side of the mold. If you find this to be an issue, remember this is very likely a symptom of one of the problems listed above. The vents will need to be changed to the depth on the left side, and the root cause of the problem must be determined and corrected.

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