DISCOLORATION

 

 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.

Machine

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.

Excessive Residence Time

Explanation: The longer a material resides in the barrel before being injected, the more heat the material absorbs. The barrel temperatures are set based on the cycle time of the machine. If that cycle time increases or is interrupted, the material residence time increases. That increase may degrade the material and cause discoloration due to the molecular breakdown. In addition, if the mold is in a press that is too large, the residence time for the material will be too great and degradation will occur.

Solution: 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

Explanation: When barrel temperatures are too high, the material will overheat and undergo thermal degradation. Due to the tendency for the material to carbonize, the more it degrades, the darker it becomes. This results in discoloration in varying degrees from slight to extreme.

Solution: 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

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.

Excessive Cycle Time

Explanation: A cycle time that is too long will increase the residence time for the material in the heating cylinder. The longer residence time may cause the material to begin to degrade and discoloration will occur as the material begins to carbonize.

Solution: 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

Explanation: Injection molding machines are furnished originally with a "general purpose'' screw. This screw is designed to perform adequately with most materials. However, each material has a specific optimized screw design available. If the screw is not properly designed for a heat sensitive material, the resin may be degraded by the screw action. Degraded material causes discoloration in the molded part.

Solution: 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.

MOLD

Improper Mold Temperature

Explanation: A hot mold will cause a molded part to be darker than if it had been molded in a cold mold. This is because the hotter mold allows the material molecules to pack tighter before they solidify and the part is more dense. The colder mold causes the material to solidify before the molecules are packed tightly so the part is less dense.

Solution: 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

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.

MATERIAL

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.

OPERATOR

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.

HOME  DEFECTS  RESOURCES  BOOKSTORE  SHOWCASE  SEMINARS  CONTACT US
Copyright Texas Plastic Technologies
Worldwide rights reserved