How to Control the Silver Mark in Injection Molding?

How to Control the Silver Mark in Injection Molding?

The problem of silver streaks is prevalent in injection molding. It is a silver-colored mark defect. Reasons are as follows:

  • Due to moisture in the material: In such a case, the material should be predried at the region where the silver mark has appeared. Also, If we are using ABS, then it should be predried in an oven, or if we have hopper, then it can be predried into a hopper dryer. There are various ranges of temperature at which it can be predried. For example, in the case of ABS, the temperature for pre-drying is 82 degrees Celsius. So if we leave ABS for pre-drying in a hopper at 82 degrees for one or two hours, the silver mark gets reduced. Other settings include;

  • Temperature: we need to increase or decrease the temperature of various parts to remove or reduce the silver mark. The first standard method is to reduce the temperature of the nozzle and barrel. Secondly, we should reduce the temperature of the barrel. Also, we should lower the feeding zone's temperature by lowering the temperature at the region from where the material is fed into the hopper. The feed zone's temperature should be less than the melting temperature of the material; otherwise, other opposite situations may occur like freezing, etc. Thus, we need to lower the nozzle's temperature and lower the temperature of the barrel, which will ultimately remove the silver mark.

  • The next important step is to decrease the injection speed because if the injection had been fast, there would have been intermitted flow means material would not be filling the cavity, instead of flowing discontinuously. But, if the injection speed is reduced, the material will come out swiftly and smoothly, due to which chances of occurrence of the silver mark would be negligible.

  • Injection pressure: It should be reduced. If the injection pressure is kept high, gas or air generates between material granules. Thus, it may happen that this air might also enter into the mold, and there might be chances of occurrence of the silver mark. This is why we should reduce the injection pressure.

  • The next thing is to keep the cushion minimum. The cushion should be kept of small sizes like 1 mm or 8 mm.

  • The next setting is to increase the back pressure. Increasing the back pressure might let the screw rotate fastly and move, which will allow the trapped air to let out of the machine. Besides this, the plastic granules will perfectly melt when fed through the hopper. The back pressure depends upon the type of material being used. If the material being used is ABS, the back pressure should be increased, but if the material being used is PVT, back pressure must be decreased little to avoid the silver mark.

  • The next setting to be checked is air venting in the mold is proper or not. So, the mold vent should be free from the air in order to avoid silver marks.

  • The final setting to be checked is the sealing of the nozzle if some unfinished work is left if/ any, to avoid a silver mark.

The following parameters are required to calculate the clamping force:

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