Most common injection molding problems

Most Common Injection Molding Problems

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Injection Molding Problems

Injection molding is the most versatile method for mass-producing identical components, and this is done by injecting molten plastic resin into a mold. It can be performed using various thermoplastics.

Despite its many benefits, such as fast production speed, high efficiency, operation automation, and shape and size flexibility, some issues and defects may arise during this process. This article will explain typical injection-molding problems. You’ll understand where problems usually come from and how Abtec Inc. can prevent these issues early via your part and tooling design and via our efficient manufacturing process.

Short Shots

Incorrect calibration or plasticizing capacity causes insufficient molten plastic to fill voids. A short shot is the incomplete filling of a mold cavity, resulting in an unfinished part. It occurs when molten plastic freezes off before filling the flow paths, and incorrect degassing may prevent molten plastic from filling the mold chamber.

Air Traps

Formed bubbles in the molded component are caused by trapped air in the parting surface, the ram, or the vent when the two melt fronts meet. Additionally, misaligned mold halves and insufficient holding pressure allow molten plastic to condense, creating these unwanted component space bulk.


Numerous factors, including insufficient dry conditions, excessive use of recycled materials, incorrect injection temperature, an inappropriate gate and runner system, and a low melting point, contribute to plastic’s brittleness and fragility.

Burn Marks

Burn marks are rusty discolorations generated by extreme heating or high injection speeds. Likewise, trapped air may overheat and etch molded parts.


Excess molten plastic escapes through the parting lines and ejector pins from the mold cavity.


Surface delamination refers to thin layers of material that can be peeled off from a part and are caused by foreign material in the molten plastic and excessive releasing agents.


Sometimes, molten plastic fails to adhere to the mold, creating wavy folds on the portion. These are often caused by low injection speed, a low melting temperature, or viscous molten plastic. The plastic cools quickly and thickens when it contacts mold walls, and molten plastic scrapes against cooling plastic, leaving marks.

Flow Lines

Flow lines, like jetting, are undesirable “off-tone” wavy lines or patterns. Injection speeds that are too slow cause the plastic to cool down excessively during the injection. As a rule, injection speed must match the material’s cooling rate. Injection pressure, mold temperature, and pressure holding time are other factors to be adjusted.


Typically, melt fracture produces a cloud-like discoloration around the gate. It occurs when the entrance is too tiny, or the cavity at the glue is too thin. Fog also occurs when the melt flow and shear rate are high or frequently increased or when the break area is small. These factors cause the melted rubber to break and the fog to exist.


Water, air, or char are spread in the direction of flow along the part’s surface to form streaks. This output defect is caused by the raw material’s high moisture and trapped air content, as well as polymer degradation.

Sink Marks

When the cooling time or mechanism is improper, and the plastic does not cool sufficiently while in the mold, sink marks form.

Weld Lines

Weld lines result from poor bonding between two or more flow types.


Work With Abtec Inc. For Your Plastic Molding Injection Needs

Are you having trouble with your injection molding process and need expert service? How about having Abtec fix your common problems for maximum product quality? Let Abtec Inc. help solve your injection molding needs!

We have over a century of experience and proven skills in recognizing and resolving plastic injection molding issues. We work using reputable technology and innovative mold-making techniques. Our experts at Abtec Inc. promptly respond to your tooling and manufacturing demands in the shortest lead time.

Contact us now to learn more about our services and capabilities.