Causes of Poor Surface Gloss in Injection Molded Parts and How to Fix It

Poor surface gloss in injection molded parts is usually related to either mold condition or molding process settings. If the surface of the plastic part appears dull, dark, or uneven, the root cause should be checked from both the mold and the injection molding process.

1. Mold-related causes

(1) Poor cavity surface finish
If the mold cavity is not processed properly and has scratches, micropores, wear marks, or excessive roughness, these defects will transfer directly to the plastic part surface and reduce gloss. In this case, the mold cavity should be carefully reworked to achieve a finer surface finish. If necessary, polishing or chrome plating can be used to improve surface quality.

(2) Oil, moisture, or excessive release agent on the cavity surface
If there is oil, water, or too much mold release agent on the cavity surface, the molded part may appear dark and dull. To prevent this, the mold surface should be kept clean, and release agent should be used only in a controlled amount.

(3) Insufficient draft angle
If the draft angle is too small, demolding becomes difficult and excessive force may be required during ejection, which can damage the surface finish of the plastic part and reduce gloss. In this case, the draft angle should be increased appropriately.

(4) Poor mold venting
If the mold does not vent properly, trapped gas may remain in the cavity during filling, which can affect the surface quality and reduce gloss. The mold venting system should therefore be checked and improved where necessary.

(5) Gate or runner size is too small
If the cross-sectional area of the gate or runner is too small, or if the flow path changes too abruptly, the melt may experience excessive shear and unstable flow, which can lead to poor surface gloss. In this case, the gate and runner dimensions should be optimized to allow smoother material flow.

2. Injection molding process-related causes

(1) Injection speed is too low
If the injection speed is too low, the melt may not pack tightly against the mold surface, which can result in poor surface density and reduced gloss. Increasing injection speed appropriately may help improve the part finish.

(2) Insufficient cooling for thick-wall parts
For thick-wall plastic parts, insufficient cooling can cause poor surface quality, dull appearance, or surface haze. In such cases, the cooling system should be improved to ensure proper and stable cooling.

(3) Insufficient holding pressure or short holding time
If holding pressure is too low or holding time is too short, the molded part may not be packed densely enough, which can reduce surface gloss. Increasing holding pressure and extending holding time may improve the result.

(4) Melt temperature is too low
If the melt temperature is too low, material flowability becomes poor, and the plastic may not replicate the mold surface properly, resulting in a dull finish. Raising the melt temperature appropriately can help improve gloss.

(5) Uneven cooling of crystalline resins
For crystalline materials such as PE, PP, and POM, uneven cooling can lead to poor gloss and inconsistent surface appearance. In this case, the cooling system should be improved to provide more uniform cooling throughout the mold.

(6) Injection speed is too high with a small gate
If the injection speed is too high while the gate cross-section is too small, the area near the gate may become dark or show poor gloss due to excessive shear and localized overheating. In this case, injection speed may need to be reduced and the gate size may need to be increased appropriately.

In summary, poor gloss in injection molded parts is usually caused by mold surface condition, venting, gate and runner design, or improper molding parameters such as injection speed, melt temperature, cooling, and holding pressure. A systematic review of both mold and process conditions is the best way to identify the cause and improve the final surface finish.