Plastic Troubleshooter
On-Line Solutions To Injection Molding Problems

Low Barrel Temperature

Explanation: Low injection barrel temperature results in an improper melting and blending of the plastic resin. Some of the unplasticized pellets enter the  melt stream and appear in the molded part as small chunks of clear material. The clearer the base resin, the more obvious the particles are, but even dark opaque materials can display these unmelted particles. And, improperly blended additives can have the same general appearance.

Solution: Increasing the barrel temperature will reduce the amount of unplasticized material. It will also help to mix additives with the base resin. Increase the barrel temperature by adding 10 degrees F to each zone until the clear spots disappear. Wait 10 cycles between adjustments for the barrel heats to stabilize. And, watch for signs of degradation as the material temperature rises.

Slow Screw Rotation

Explanation: Screw rotation helps to impart shear heat into a material and aids in blending the resin and additives. A slow speed will keep the material from achieving proper heat to fully plasticize and some of the unmelted particles will enter the melt stream.

Solution: The screw rotation speed should be set at the material supplier's recommendation. As a rule-of-thumb you can start at 100 rpm and adjust upwards or downwards in 10-rpm increments until the proper speed is achieved. However, excessive screw speed will degrade the material.


Cracked Mold (Water Leak)

Explanation: A mold that is cracked due to weak waterlines may cause water droplets to enter the cavity. These will be trapped by incoming molten resin and will show up as clear spots in the molded part.

Solution: Waterlines that are leaking may be welded if the cracking condition is not severe. Tubing can be threaded through the cracked waterline and the water can flow through the tubing much like air within a tire inner tube. Or, a reverse water system can be utilized that sucks water through the mold instead of pressurizing it through the mold.


Contaminated Raw Material

Explanation: A common cause of clear spots is contaminated raw material. Often, the raw material containers are left uncovered and pellets from other incompatible resins may get deposited in the container. Because they melt at different temperatures the incompatible pellets may never get hot enough to melt and they will be dragged into the cavity appearing as clear spots.


Inconsistent Process Cycle

Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in erratic heating of the material in the injection barrel. If such a condition exists, some pockets of material may not melt all the way and unmelted particles will cause the clear spots.

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.



Clear spots can be defined as spotty, translucent imperfections on the molded part surface and usually are caused by unplasticized particles of resin. They might also be caused by improperly dispersed additives, such as colorant.


Some common causes and solutions are listed below.


Defect - Clear Spots

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


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