Plastic Troubleshooter
On-Line Solutions To Injection Molding Problems

Excessive Residence Time In Barrel

Explanation: Under the best conditions, a shot size should represent 50% of the capacity of the injection cylinder (barrel). This will result in processing the material for one cycle while preparing the material for the next cycle. Thus, a mold requiring a four-ounce shot should be run in a machine that has a barrel with an eight-ounce capacity. The more material left in the barrel between shots, the greater the likelihood of thermal degradation. This degradation is what causes the black specks.

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 because more time will be required to bring the next mass of material up to proper heat and degradation may occur.

Contamination In Injection Barrel

Explanation: Any type of contamination in the injection barrel may be the cause of streaks, spots, and specks. It may be in the form of dust particles that fell from the ceiling into an open hopper, pellets from other materials, residual resin from an improper changeover, or even pieces of food that accidentally fell into a container of material ready to be placed in the hopper.

Solution: To remove this type of contamination it may be necessary to increase the temperature of the injection barrel and, using a purging material with a wide melt range, purge the contaminate(s) from the system.


Sprue Bushing Is Nicked, Rough, Or Not Seating

Explanation: A damaged sprue may cause material to stick and be held in residence at elevated temperature until it degrades and decomposes. At that point, it will break loose and enter the melt stream as streaks or specks.

Solution: Inspect the internal surfaces of the sprue bushing. Remove any nicks or other imperfections. The tapered hole should be highly polished. Check the sprue bushing-to-nozzle seal with thin paper or bluing ink to ensure that the nozzle is centered to the bushing and that the hole and radius dimensions are compatible for the nozzle and the bushing.

Improper Venting

Explanation: Air is trapped in a closed mold and incoming molten plastic will compress this air until it auto-ignites. This burns the surrounding plastic and results in charred material in the form of spots and specks.

Solution: Vent the mold by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. Vents should take up a minimum of 30% of the perimeter of the molded part. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part.


Contaminated Raw Material

Explanation: The most common causes of black specks and streaks are molding compound contaminants. Such contamination is usually the result of dirty regrind, improperly cleaned hoppers or granulators, open or uncovered material containers, and poor quality virgin material supplied by the manufacturer.

Solution: This type of contamination can be minimized by dealing with

high quality, reputable suppliers and by using good housekeeping practices.

Properly trained material handlers will also help reduce contamination.


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 black specks or streaks.

Solution: If at all 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.
Defect - Black Specks



Black specks can be defined as small dark particles or spots on the surface of an opaque part or within a transparent part.


Some common causes and solutions are listed below.


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NOTE: For more detailed information on the causes and solutions of this defect, you can find it in our BOOK, or ONLINE SEMINAR.