Injection Molding Defects Troubleshooting

Common injection molding defects include Blister, Air Burn Marks, Color Streaks, Delamination, Flash, Embedded Contaminates, etc. All of injection molding defects may caused by poor mold design, incorrect operation of injection molding machine or poor tooling design. This article introduced nearly all of injection molding defects and present to you the plastic molding defects pictures that you may meet in the process of injection molding. 

Weld Lines and Meld Lines

Weld Lines

During the injection molding manufacturing process it is common to see the occurrence of lines where two flow fronts meet due to the inability of those two flow fronts to knit together — these lines are called weld lines or weld mark or knit lines. Weld and meld lines can be caused by holes or inserts in the part, multiple gates, or variable wall thickness where hesitation or race tracking occurs. Weld lines are injection molding defects that cause weak regions in the molded part which could result in breakage when the part is subjected to stresses. If weld or meld lines cannot be avoided, position them at low-stress and low-visibility areas by adjusting the gate position. Improve the strength of weld and meld lines by increasing the local temperature and pressure at their locations.

Difference between Weld lines and Meld Lines:Traditionally, the meeting angle is used to differentiate weld lines and meld lines. As you can see from below picture, a meeting angle, θ, smaller than 135° produces a weld line; greater than 135°, a meld line. Normally, weld lines are considered to be of lower quality than meld lines, since relatively less molecular diffusion occurs across a weld line after it is formed. We call both weld lines and meld lines as weld lines.

Cause of weld lines defect:

Plastic Material:

Product Design:

Mold Design:

Injection Molding Process:

We will provide some plasitic molding defects pictures to you. With this plastic molding defects pictures you can learn the defects clearly.



Lacking cooling caused this molding defects. The blister raised or layered zone on surface of the plastic part, when the tool is fail to cool or heater is faulty.

Air Burn Marks


When tool lacks venting or injection speed is too high, plastic parts may have black or brown burnt areas located at furthest points from gate.

Color Streaks


The plastic part may appear localized change of color, because plastic material and colorant are not mixing completely. Anther situation is that the material has run out colorant and start come through as natural only.



Plastic part may appear thin mica like layers at its wall. This kind of defect usually caused by contamination of the material e.g. PP mixed with ABS, and it is very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond.



It also called Burrs and may caused by tool damaging, excess material injecting, too high injection speed or low clamping force. Dirt and contaminants around tooling surfaces may also cause this defect. And this defect usually demonstrates as excess material in thin layer exceeding normal part geometry.

Embedded contaminates


During the process of plastic molding, foreign particle may embed in the part. This defect is commonly caused by particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection.



This kind of defect shown as deformed parts. It is made by poor design of tool, gate position or runner. If injection speed set too high, it is easier turbulent flow of material and make this defect.

Polymer degradation


If there is excess water in the granules, or excessive temperatures in barrel, polymer may breakdown from oxidation, etc.

Sink marks


This defect happens at depression, especially in thicker zones. It is caused by low holding time or pressure, and with sprueless hot runners, it also can be caused by the gate temperature being set too high.

Short shot


It happens when plastic parts are lack of material or injection speed or pressure too low. Finally, the plastic parts can not be made completely.

Splay marks


It is made by moisture in the material, usually when resins are dried improperly. And the hot gas make circular pattern around gate.



During the holding time, if injection molding lack of holding pressure to pack out the part, it will has empty space within part. Voids also happens because the mold may be out of registration, when the two halves don’t center properly and part walls are not the same thickness.

Weld line


When plastic molds display discolored line where two flow fronts meet, we call the defect as weld line. Weld line is mainly caused by too low mold or material temperatures setting. As the material is cold when they meet, they don’t bond.



This defect may lead parts distorted. The cooling is too short, while material is too hot, it has not enough time to cooling around the tool, and has incorrect water temperatures. The parts bow inwards towards the hot side of the tool.

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