KNIT LINES (WELDS)
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 part.
Low Barrel Temperature
Explanation: When barrel temperatures are too low, the material will not be heated to the proper temperature for adequate flow. The material will push slowly into the mold and the flow fronts that form around obstructions will not be hot enough to reunite after they travel around those obstructions.
Solution: 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
Explanation: The back pressure control is used to impart a resistance to the molten material being prepared in the barrel for the upcoming cycle. This resistance is used to help preheat the material and also control the density of the melt before it is injected into the mold. If the back pressure setting is too low, the material may not be brought to the proper temperature and the knit line areas will be more evident due to their inability to reunite.
Solution: 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
Explanation: If the injection speed or pressure is too low, the molten plastic will not be pushed into the mold cavity fast enough to fill and knit at converging flow fronts because it cools down and solidifies too soon.
Solution: 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
Explanation: Generally, a hot mold will allow a material to stay molten longer than a cold mold and cause the molecules to flow farther before they solidify. This results in a dense part with maximum welding at knit line locations. If the mold is too cold, the molecules solidify before they are packed and the knit lines will be more evident and much weaker.
Solution: 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
Explanation: Gates and/or runners that are too small will cause excessive restriction to the flow of the molten plastic. Many plastics will then begin to solidify before they fill the cavity. The result is an unpacked condition and knit lines will be weaker and more evident.
Solution: 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
Explanation: If certain materials are injected directly across a flat cavity surface they tend to slow down quickly because of frictional drag and cool off before the cavity is properly filled. The flow front breaks into many streams and they have difficulty welding back together before they solidify.
Solution: 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
Explanation: The area that surrounds the gate itself is called its land. It determines the distance a material must travel in a restricted state immediately before it enters the cavity. The length of this travel (land) should be no more than 1/8''. The land acts like a tunnel when the mold is closed and if the tunnel is too long the material begins to cool off before it can get to the cavity. This causes the material to split into streams that can create knit line conditions.
Solution: 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
Explanation: Resin manufacturers supply specific formulations in a range of standard flow rates. Thin-walled products may require an easy flow material while thick-walled products can use a material that is stiffer. It is better to use as stiff a flow as possible because that improves physical properties of the molded part. But the stiff material will be more difficult to push and this may result in a breakup of the flow front as the material enters the gate. This may cause streams of flow that cannot weld back together properly and they will form weak knit lines.
Solution: 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
Explanation: The machine operator may be the cause of delayed or inconsistent cycles. This will result in erratic heating of the material in the injection barrel. If such a condition exists, the colder particles may not fill the mold before they fully solidify. The flow front may break up into streams that cannot weld back together properly and weak knit lines may form.
Solution: 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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