Blush can be defined as a clouded discoloration normally found at gate locations, but they can be found anywhere on the part surface. These visually defective areas have very weak physical properties due to loose molecular structure.
Possible Causes & Remedies:
◇ EXCESSIVE INJECTION FILL SPEED
Remedies: One solution is to adjust the fill speed rate until the optimum has been achieved. This will help eliminate blushing.
◇ MELT TEMPERATURE TOO HIGH OR TOO LOW
Remedies: Melt temperature must be adjusted to the optimum for a specific material and specific product design.
◇ LOW INJECTION PRESSURE
Remedies: Increasing the injection pressure forces the material against the mold surface, producing a truer finish that replicates the steel finish.
◇ NOZZLE DIAMETER TOO SMALL
Remedies: Enlarging the nozzle diameter will minimize the condition. The nozzle tip is interchangeable and a tip with the opening the same as, or slightly smaller than, the sprue bushing opening is recommended.
◇ LOW NOZZLE TEMPERATURE
Remedies: Increasing the mold temperature will allow the material to flow easier, and for a longer time, thereby packing the mold and replicating the steel finish. Normally, the nozzle temperature should be set the same as, or 10 degrees hotter than, the front zone of the barrel.
◇ LOW MOLD TEMPERATURE
Remedies: Increasing the mold temperature allows the material to flow farther and pack properly. The material temperature could also be raised to accomplish the same effect.
◇ IMPROPER VENTING
Remedies: Vent the mold by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. Vents should take up approximately 30% of the perimeter of the molded part. Add vents in local areas that show blush. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part.
◇ SMALL SPRUE BUSHING DIAMETER
Remedies: Size the sprue bushing major diameter so its cross-sectional area is equal to (or greater than) the sum of the cross-sectional area of all the runners leading from it. Then, taper the sprue diameter to match the nozzle. That will ensure proper pressure drop adjustments to pack the mold.
◇ IMPROPER GATE LOCATION
Remedies: Make sure the gate is located so the thicker sections fill first. The material should flow from thick section to thin section. That ensures equal packing of all areas.
◇ SHARP CORNERS
Remedies: Radius all sharp corners, especially in the gate area, as that is the last place to pack. Sharp corners should not be allowed on any molded part.
◇ EXCESSIVE MOISTURE
Remedies: Dry the material to the supplier's recommendations and make sure it is used within two hours of that drying activity.
◇ INCONSISTENT PROCESS CYCLE
Remedies: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining consistent cycles.
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