On-Line Solutions To Injection Molding Problems
Excessive Shot Size Ratio
Explanation: Ideally, a shot size should equal 50% of the capacity of the barrel. That results in processing one cycle while preparing for the next cycle. However, this is a general statement because, depending on the material, the ratio can be as small as 20% for non-heat-sensitive materials such as polypropylene, and up to 80% for heat-sensitive material such as PVC. As the ratio drops, the residence time of the material in the barrel increases so does the risk of thermal degradation. Degraded material will discolor due to the molecular breakdown. The material will start showing dark yellowing that progresses towards black as the degradation worsens.
Solution: 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.
Nozzle Temperature Tool Hot
Explanation: As material is transported through the heating barrel, it is gradually brought up to the ideal processing temperature. The material absorbs heat from the heating bands and frictional heat, which is created by the shearing action of the rotating screw within the barrel. The last heating zone that the material is exposed to is the nozzle. By the time the material reaches the nozzle, it should already be at ideal molding temperature and any further heat introduced may cause the material to begin to degrade. The degraded material darkens as it becomes more degraded and discolors the molded part.
Solution: 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.
Explanation: Anything that impairs the cooling process within a mold will have a direct effect on the color of the molded parts. A blocked or kinked waterline will cause sections of the mold to run too hot and this will cause darker areas to be molded on the part. If there are not enough waterlines the mold will be too hot in general and the whole part will be molded darker because it is denser. If there is too much cooling in one area, that will produce a lighter area of color on the molded part.
Solution: 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
Explanation: If the raw material contains any contamination, such as dirty regrind, dust from storage areas, etc., the molded part will show varying degrees of discoloration based on the location and type of contaminant. Even mixing different grades of the same material or different flow values of the same material can cause differences in color.
Solution: 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
Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in excessive residence time of the material in the injection barrel. If such a condition exists, heat sensitive materials will degrade, resulting in discoloration.
Solution: 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.
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.
Some common causes and solutions are listed below.
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