Welds and/or Knit lines can be defined as the inability of two or more flow fronts to “knit” together, or "weld", during the molding process. This normally occurs around holes or obstructions and causes locally weak areas in the molded parts.
Possible Causes & Remedies:
◇ LOW BARREL TEMPERATURE
Remedies: Increasing the barrel temperatures will allow the flow fronts to stay hotter longer and knit better when they reunite. It is practically impossible to eliminate knit lines once they are formed but they can be minimized.
◇ INADEQUATE BACK PRESSURE
Remedies: Increase the back pressure to raise the melt temperature and improve the ability for the fronts to unite. This is best accomplished by starting at the minimum of 50 psi and increasing in 10-psi increments until the knit lines improve. Do not exceed 300 psi. The higher the back pressure, the hotter the plastic, and excessive back pressure will thermally degrade the plastic.
◇ INJECTION PRESSURE 0R SPEED TOO LOW
Remedies: Increase the injection pressure or speed. While these two parameters are related, it is not proper to adjust them both at the same time. Adjust them each independently and monitor the results closely to determine whether or not the other needs adjustment. As a rule-of-thumb, it is best to make adjustments in increments of no more than 10% of the original setting.
◇ LOW MOLD TEMPERATURE
Remedies: Increase the mold temperature to the point that the material has the proper flow and packs out the mold with maximum knit line strength. Start with the material suppliers recommendations and adjust accordingly. Allow 10 cycles for every 10-degree change for the process to re-stabilize.
◇ SMALL GATES AND/OR RUNNERS
Remedies: Examine the gates and runners to determine if any burrs or other obstructions exist. If possible, perform a computer analysis to determine the proper sizing and location of gates and runners. Ask the material supplier for data concerning gate and runner dimensioning for a specific material and flow rate.
◇ IMPROPER GATE LOCATION
Remedies: Relocate, or redesign, the gate so that the molten plastic is directed against an obstruction such as a core pin. This will cause the material to disperse and continue to flow instead of slowing down.
◇ EXCESSIVE GATE LAND LENGTH
Remedies: Decrease the gate land length. It is best to construct the mold so that the gates are located in replaceable inserts. That way they can be replaced easily at times when adjustments are needed. The insert should include the land area. This land length should be no less than 0.030'' and no greater than 0.125''.
◇ IMPROPER FLOW RATE
Remedies: Utilize a material that has the stiffest flow possible without causing knit lines. Contact the material supplier for help in deciding which flow rate should be used for a specific application.
◇ INCONSISTENT PROCESS CYCLE
Remedies: If possible, run the machine on the automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is necessary. In addition, instruct all employees on the importance of maintaining consistent cycles.
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