Injection Molding Basics: Common Surface Defects

Warpage occurs with injection molded parts when there is uneven shrinkage during the cooling process, causing stress that results in bending and twisting.

Possible Sources of Warpage:

• Mold design, such as gate location resulting in uneven cooling
• Material selection
• Part Design, such as wall thickness variations or even the shape of the part
• Short cooling times

Ejector pin marks are imprints caused when the ejector pins push the part out the mold and the mold will occasionally stick, resulting in the marks.  These cosmetic defects are highly common on injection molded parts, which is why you'll almost always see them.

Possible Sources of Ejector Pin Marks:

• High injection pressure
• Insufficient use of mold release agent
• Ejector pins design and speed of which they eject the parts
• Improper part design

Flashing issues are one of the most common defects you'll run into with injection molding. It is caused when molten plastic escapes out of the mold cavity. 

Possible Sources of Flash:

• Poorly designed mold, resulting in insufficient venting
• Worn down mold
• Excessive high-temp mold
• High injection pressure
• High-moisture absorption of the resin

In order to reduce/prevent flashing, you'll need to ensure there is proper venting to remove any trapped gasses, and you'll also want to ensure you're using the optimal melt temperature, injection speed, and mold temperature.

Sink marks are the small depressions around the thicker areas of injection molded components that typically form when shrinkage occurs.

Possible Sources of Sink Marks:

• Wall thickness variations
• Insufficient gate and runner sizes
• Insufficient vents not allowing enough air to escape
• Gate location (Needs to be relocated to the thicker sections to pack before thinner sections freeze off)

While not always visible, weld lines (aka knit lines) are where two resin flows meet.  This defect is caused when the polymer resin is injected into a mold and flows around a solid piece and forms when the resin collides back together without completely fusing back together.

The weld line can either be a minor cosmetic issue or potentially bad enough to negatively impact the part's strength.

Possible Sources of Weld/Knit Lines:

• Temperature from the injection molding machine, runners, and/or the mold itself in order to ensure the plastic resin flows properly
• Insufficient pressure resulting in the flow fronts not being merged back together with strong enough
• Mold design, especially location of gates and runners impacting the resin flow

Flow lines are going to typically show as off-color, wavy patterns on the narrow sections of molded parts, or as the ring-shaped near the entry point of the mold.

These defects are typically caused when molten plastic cools quickly and hot plastic flows past sections that have already cooled down.

Possible Sources of Flow Lines:

• Low injection pressure/speed
• Low mold temperature/material temperature
• Insufficient gate channel
• Gate location

Jetting is a defect where molten plastic ends up not sticking to the mold's surface as a result of the speed of the injection.  This results in the plastic solidifying as wavy folds of the jet stream on the surface of the part.

Possible Sources of Jetting:

• Melt temperature is too low, resulting in a higher viscosity of the molten plastic
• Injection speed
• Gate size/design