D&M Plastics utilizes the latest plastic injection molding technology to create high quality parts for a diverse range of industries. Typically used for mass production, plastic injection molding is a process which allows manufacturers to create immensely high volumes of the same part within tight tolerances. While this process has numerous advantages, errors in the plastic injection molding process or tooling setup may cause defects that impact the appearance or functionality of the final product. Understanding these potential issues makes it easier to avoid them in future projects.

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Avoiding Plastic Injection Molding Defects

Avoiding Plastic Injection Molding Defects

Here are some of the most common defects to watch for during the plastic injection molding process:

Burn marks

These discolorations appear on the outer surface of the component and may be black or rust-colored. They are often caused by trapped air or resin overheating within the mold. Shorten the mold cycle time or lower the mold temperature to avoid burn marks. Adding venting can help with trapped gas.

Flow lines

Although they typically do not impact the functionality of a product, flow lines—off-color streaks or patterns across a product’s surface—may look unprofessional when visible to consumers. This defect is caused when plastic cools too rapidly during the injection molding process. Increasing the nozzle diameter and injection speed will help prevent flow lines.

Short shots

Short shots weaken the part and are commonly caused by a lack of molding pressure, which results in trapped air within the product.

Sink marks/voids

When the plastic does not have enough time to cool within the mold, the surface material may shrink inward. This results in surface depressions known as sink marks. Increasing pressure and cooling time can help avoid this defect. Proper part design is necessary to avoid sink marks, i.e. uniform wall stock rather than uneven/thick walls. Uneven cooling speeds may also create voids (internal sink marks) within the part. Limit the risk of this defect by increasing the injection pressure or choosing low-viscosity materials.

Weld lines

Weld lines are caused by multiple molten materials colliding at too low a temperature, resulting in a thin line where the materials did not properly bond. This defect can weaken a component. To help plastic injection molding materials bond properly, increase material temperature and injection speed or eliminate partitions within the mold. Proper gate location can help to avoid weld lines.


Often caused when residual resin is left in a mold from a previous project, discoloration impacts the aesthetics of a part. Occasionally, discoloration may also occur when the coloring agent is unstable. Utilizing a high-quality coloring agent and thoroughly cleaning the production equipment between production runs helps reduce the risk of discoloration.


One of the more serious defects is delamination, which presents as a flaking surface layer that separates from the surfaces beneath it. Generally caused when the resin is contaminated or exposed to excess moisture, delamination will negatively impact the strength of the part. It can be avoided by properly storing the resin pellets and increasing the temperature of the mold.


Jetting appears as a distinctive snaking spray line on the product’s surface. It typically occurs when the molten material is injected too quickly without contacting the mold’s wall. Jetting ultimately weakens the part but may be avoided by increasing the temperature of the material and lowering the injection pressure. Proper gate location can help to avoid jetting.

Eject marks

When a part is ejected too early or with too much force, it can be left with unattractive markings. Waiting until the part is fully cooled and reducing the force of the ejector rods can reduce the likelihood of eject marks.

About D&M Plastics

At D&M plastics, our experienced team crafts high quality molded products at competitive prices from our ISO 9001:2015 certified quality management system and ISO 13485:2016 certified quality management system certified facility. We can mold parts in both vertical and horizontal molding machines at tolerances as low as ±0.001 inches and offer insert molding, overmolding, and cleanroom molding to meet specialized needs. Visit our injection molding services page to learn more, or request a quote to see how we can help with your next project.