Plastic Troubleshooter
On-Line Solutions To Injection Molding Problems

Inadequate Injection Pressure

Explanation: If too little injection pressure is used, the molten plastic material will tend to cool off and solidify before the mold cavity is packed. If no packing is achieved, the material will not be forced against the mold cavity hard enough to replicate the finish. The less dense material will appear dull and have very little gloss.

Solution: Increasing the injection pressure will force the plastic against the steel of the mold cavity and duplicate the gloss of the finish on that steel.

Low Nozzle Temperature

Explanation: As material is transported through the heating barrel, it is gradually brought up to the ideal processing temperature by absorbing heat from the heating bands and frictional heat, which is created by the shearing action of the rotating screw within the barrel. In the last heating zone, the material is exposed to is the nozzle. By the time the material gets to the nozzle, it should already be at ideal molding temperature and only a small amount of heat needs to be applied at this point to keep the resin flowing. If the nozzle is not hot enough, however, the material will begin to cool off too quickly as it leaves the barrel and the flow front will not be forced against the cavity steel to squeeze out the flow lines.

Solution: Increase the nozzle temperature. As a rule-of-thumb the nozzle temperature should be set at 10 degrees F higher than the setting for the front zone of the barrel. This helps compensate for heat loss due to metal-to-metal contact between the nozzle and the sprue bushing, and keeps the material hot enough to pack the mold, eliminating low gloss.


Low Mold Temperature

Explanation: Generally, a hot mold will allow a material to stay molten longer than a cold mold and cause the molecules to pack against the mold steel before they solidify. This results in a dense part with high gloss. If the mold is too cold, the molecules solidify before they are packed out and low gloss may result.

Solution: Increase the mold temperature to the point at which the material has the proper flow and packs out the mold. Start with the material suppliers recommendations and adjust accordingly. Allow 10 cycles for every 10-degree change for the process to re-stabilize.

Poor Polishing Of Cavity Surfaces

Explanation: A properly molded product will duplicate the finish that is present on the molding surfaces of the mold in which it was formed. If that finish was not properly prepared (normally by hand polishing), the molded part will not have the high gloss that is normally desired.

Solution: Prepare the molding surface finish to the requirements of the molded product. There are industry standards available that describe the degree of gloss required for specific finishes. These should be utilized to ensure consistency of finish on all molded products, and the finish should be specified on the product drawing as well as on the mold design.


Improper Flow Rate

Explanation: Resin manufacturers supply specific formulations in a range of standard flow rates. Thin-walled products may require an easy flow material while thick-walled products can use a material that has a stiffer consistency. It is better to use the stiffest flow possible because it improves physical properties of the molded part. However, the stiff material will require higher injection pressures, which may blow the mold open and cause loss of pressure, which results in low surface gloss. If an easy flow material is used, the physical properties will not be as great. In addition, the material will maintain pressure and create proper gloss.

Solution: Utilize a material that has the stiffest flow possible without causing non-fill. Contact the material supplier for help in deciding which flow rate should be used for a specific application.


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 of the material may not absorb enough heat to travel far enough to pack the cavity and low gloss will occur because the material cannot replicate the steel surface.

Solution: If possible, operate 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.



Low gloss can be defined as a dulling of the product finish, usually caused by insufficient force to push the molten plastic against the steel surface of the cavity.


Some common causes and solutions are listed below.

Defect - Low Gloss
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