Top Injection Molding Defects and How To Solve Them

Proper design of your injection molded product can prevent some common defects

Proper planning can help to ensure a successful plastic injection molded part. While many defects result from issues with the mold, operator error, and process errors, which are controlled by your injection molder, some can be prevented during the design stage. Below we discuss some defects that can be mitigated with proper design.

Flow Lines

Flow lines will appear as a wavy pattern on your part and will be slightly discolored. They will appear most often in narrow sections of the part. While they typically do not impact the part’s structural integrity, they are unsightly and must be avoided for parts where aesthetics matter.

Flow lines occur because of variations in the cooling temperature of the plastic as it flows through the mold. Thinner areas will cool faster than thicker areas, which can create flow lines as molten plastic flows over areas that have already begun to cool.

There are a couple of design element changes that can help to prevent flow lines. One is rounding the corners where wall thickness increases to keep the flow rate consistent. Another is to locate mold gates further away from the cooling apparatus to prevent the plastic from cooling too quickly.


Weldlines are slight depressions that take on the appearance of a hairline crack. Knit and meld lines are two kinds of weldlines that may occur. Weldlines occur when a feature, such as a hole, requires plastic to flow around it. The meld line is the remerging of the two flows, like cars merging on a highway. If there is enough space, the flows merge and continue through the mold. Knit lines are more severe and happen when the two flows collide, often when there isn’t much space. This can create a weakness in the part. For example, a knit line on a screw boss can cause it to crack when a screw is driven into it.

Altering your design can help to prevent kit lines. Increasing the wall thickness will help slow the plastic cooling speed and facilitate pressure transmission. However, making them too thick can create sink marks. Another way is to adjust the gate location and dimension to facilitate better flow through the mold.

Sink Marks

Sink marks are localized depressions in the part. They typically occur in areas of thick geometry and are caused when hot liquid resin cools and shrinks at different rates. The higher mass area will shrink more as it contacts the tool wall, creating a depression.

Part design plays a significant role in the creation of sink marks. Common places for sink marks to occur are areas near ribs and bosses. Parts should be designed to limit the difference in thicknesses between the wall and the thickness of the rib’s base. A gradual 7° slope at the base of a rib will help the mold fill more uniformly. Bosses should be designed so that the wall thickness equals the hole’s inner diameter. Do not attach a boss directly to an outer wall unless the wall section is cored out. Otherwise, a rib should be used to attach the boss to the wall.


Warping is a dimensional distortion that occurs on a molded part. It can cause the part to fold, twist, bend, or bow. Sometimes it is called “potato chipping” because the part can curl up like a potato chip as the part cools down.

Material shrinkage plays a part in why a material will warp. When parts of the material shrink at different rates, it can cause internal stresses. When the stresses exceed the part’s structural integrity, it can warp. Some common reasons for warpage include thickness variations and uneven cooling rates. Designing the part with symmetry, if possible, can stabilize cooling. Proper gate size and location can minimize warpage.

Let Us Optimize Your Part and Mold Design

At GreenLeaf Industries, design for manufacturing is one of our strengths. If you aren’t sure your part is designed for defect-free results, let us help. We will work with you to optimize your part and mold designs, maximizing quality and reducing costs. We can anticipate and minimize the risk upfront by rigorously applying process engineering, systems engineering, and proven molding technology to your project. We will strip out unnecessary cycle time and labor, minimize mold complexity, and ensure quality requirements are met most cost-effectively. Using these methods, we can confidently take on your most challenging jobs and consistently deliver on your expectations.

Contact us today to speak with an engineer and get started.