Discoloration can be defined as a change in the original color of a plastic material usually caused by overheating, mechanical shear, contamination, or chemical attack.
Possible Causes & Remedies:
◇ EXCESSIVE SHOT SIZE RATIO
Remedies: Strive for a 50% shot-to-barrel ratio. This is ideal but can go as low as 20% if the material is not too heat sensitive (like polypropylene) and up to 80% if the material is extremely heat sensitive (like PVC). It is not a good idea to empty the barrel for every shot as more time will be required to bring the next mass of material up to proper heat and degradation may occur.
◇ EXCESSIVE RESIDENCE TIME
Remedies: Size the mold to run in a press that supplies an injection shot size of 50% of the barrel if possible. Optimize the overall cycle of the machine, and eliminate any interruptions to that cycle to minimize the amount of time the material is in the heated barrel.
◇ HIGH BARREL TEMPERATURE
Remedies: Reduce the barrel temperature to the range recommended by the material supplier. Make sure the profile is such that the material heats progressively from the rear to the front of the barrel.
◇ NOZZLE TEMPERATURE TOOL HOT
Remedies: Reduce the nozzle temperature to be the same as, or 10 degrees F hotter than, the front zone of the barrel. The extra 10 degrees is used to make up for any heat loss occurring between the nozzle and the sprue bushing against which it seats.
◇ EXCESSIVE CYCLE TIME
Remedies: Optimize the cycle time to accommodate the material being used and the wall thickness of the part being molded. If longer cycles are actually needed, reduce the barrel temperature to minimize degradation.
◇ IMPROPER SCREW DESIGN
Remedies: The compression ratio of the screw should be right for the material being molded. The material supplier can provide the ideal compression ratio and the screw manufacturer can provide the compression ratio of the screw being used. If the ratio is not the same, you may have to purchase a screw with the right compression ratio, especially for heat sensitive materials.
◇ IMPROPER MOLD TEMPERATURE
Remedies: Adjust the mold temperature to that recommended by the material supplier. If the parts are too light, heat the mold more. If the parts are too dark, reduce the mold temperature. Make adjustments in 10 degree increments and allow 10 cycles between adjustments for the machine to stabilize.
◇ INEFFICIENT COOLING
Remedies: The cooling lines should be designed from the beginning to be efficient and properly located. This is the job of the mold designer. If the mold was not designed right it can be a major problem trying to accommodate for that in the molding process. One thing that can be done is to ensure each mold half has its own temperature control system and that a single unit does not control both halves.
◇ CONTAMINATED RAW MATERIAL
Remedies: The solution is to keep all materials clean, covered, and stored in proper containers with the contents clearly identified, and make sure grade and melt flow are included.
◇ 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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