8 Common Injection Molding Defects and How to Avoid Them

Introduction

When it comes to efficient mass production of identical parts, injection molding is the most preferred method. You can produce hundreds of thousands of pieces rapidly at a cost-effective rate. But here you must note that there’s a lot more room for expensive errors in this process. Seemingly minor errors in the early stages of injection mold manufacturing can lead to significant defects.

These not only degrade the final product quality but can also disrupt the entire injection molding process, ultimately reducing the system’s speed and efficiency. The matter does not end here; these defects can increase scrap and rework. This raises production time and total cost. But do you want to know what causes defects? There can be several reasons for this.

This article serves as an essential guide to understanding the common defects, their causes, and solutions. It will surely help engineers and operators to take a proactive approach to risk mitigation. And also will assist them in reducing such defects.

Common Injection Molding Defects

Basically, Injection molding requires significant technical expertise. There are several types of defects that may arise for various reasons. When there are mistakes in the injection molding manufacturing process, such as wrong process parameters, issues with tooling, or poor design, you see defects in the molds. Additionally, some defects arise from problems with material flow and cooling.

Here are some common injection molding defects that are as follows:

Flow Lines

Flow lines are visible streaks or wavy lines on the surface of the part. They appear as regions with slightly different color or gloss than the nearby area.

Flow lines form when molten plastic flows at different speeds or cools at different rates inside the cavity. This often happens near gates, sudden changes in wall thickness, sharp corners, or flow obstructions.

Burn Marks

Burn marks mostly appear near the end of flow paths, vents, or thin sections of moulded plastic parts. They seem blackish or rust-colored. But they do not compromise the part’s integrity unless it is burnt to the point of degradation.

When compressed air and other gases are trapped or overheated in the resin during the injection molding process, these burn marks appear. Other causes include excessive melt temperature or injection moulding process speed, resulting in overheating.

Sink Marks

In most cases, thick sections of injection-molded parts take longer to cool. This can cause that section of the part to compress or shrink from the inner side, and the outer skin cannot resist the pull. That’s why you see slight depressions, small recesses, or dents on the mold’s outer surface. These are called sink marks. These defects are usually seen on thick sections, ribs, bosses, and around screw posts. They affect the part’s strength.

Sink marks may look the same as vacuum voids, but their effect and impact are the reverse of sink marks. Instead of rapidly cooling the mold part’s outer surfaces, these sections cool more slowly, resulting in depressions.

Weld Lines

Weld lines often appear as thin, hair-like lines or faint seams on the mold surface. They often appear where two flows of molten resin meet as they move through the geometry. This defect is usually found in a part with a hole.

When the temperature setting is incorrect, the molten material fills or wraps the mold around the hole, and the area where the two flows meet appears as a weld line. This is because the material does not fuse properly due to a temperature difference between the two moulded fronts, resulting in partial solidification. These defects are also referred to as knit lines. They affect the part’s overall strength and geometry, making it weaker and less durable.

Jetting

The squiggly lines that appear on the surface of the finished part are usually known as jetting defects. These defects also result from partial solidification and often start just downstream of the gate region.
This happens when the initial “jet” or narrow stream of mould material shoots into open space and quickly solidifies, leaving the entire cavity improperly filled. A snake-like, wavy motion pattern appears, which reduces the part’s strength. 
The main cause of this defect is the excessive injection pressure. As the molten material is injected at high pressure through the tiny gate, it often squirts rather than fills the entire cavity. This is when it starts cooling rapidly, causing jetting defects. The frozen “string” is then covered by later melt.

Short Shot

Short shorts are types of defects that arise due to incomplete filling of the cavity. The main cause of these defects is an inaccurate or poorly maintained mold design.

When narrow or blocked gates restrict flow, the molten material cannot reach certain areas, leading to short shots. Other reasons include insufficient injection pressure, a trapped air pocket, low injection speed, or not enough shot volume. The final parts are incomplete and unusable, with missing sections or rounded, unformed edges.

Warping

Warping is the distortion or twisting of the part after ejection. The final part does not match the intended shape and may not fit or assemble correctly. Warping is usually caused by uneven shrinkage during cooling. Different regions cool and shrink at different rates, which introduces internal stresses that bend the part.

Bubbles/Voids

Bubbles and voids are gas pockets inside the part or just under the surface. They may appear as clear blisters, white spots, or internal holes visible after cutting the part. These are minor defects that do not significantly affect the injection-molded part. While larger voids and air pockets have a significant impact, they weaken the part’s structure.

They can form when air or volatiles are trapped and cannot escape during filling. They also occur when the surface freezes early and the inner material shrinks, leaving internal voids instead of surface sinks.

How to Avoid Injection Molding Defects?

Flow Lines

To reduce flow lines, here are some solutions:

• Keep wall thickness uniform and avoid abrupt thickness changes.
• Use smoother flow paths and rounded corners, and place gates to keep the flow direct and balanced.
• Process fixes include higher melt temperature, higher mold temperature, and higher injection speed and pressure. These changes help the melt stay fluid longer and fill the cavity with more uniform flow.

Burn Marks

Here are some solutions to prevent causing burn marks:

• Venting should be improved at the end of the thin section or cores.
• For the gases and air to escape easily, lower the injection speed and pressure.
• To prevent overheating during injection molding, maintain a lower temperature.
• Shortening of the cycle periods to avoid overheating of gases. 

Sink Marks

These are some implementations operators can follow to avoid sink marks:

• Good part design is the first defense against sink marks. Design engineers can reduce the wall thickness and the injection mould rib thickness relative to the main wall. In the overall design, try reducing the thickness of massive sections.
• Sink marks indicate that the part needs more time for cooling. To ensure the part has cooled properly, operators must increase the cooling.
• Lowering the temperature while increasing the holding pressure can allow the part to cool effectively.

Weld lines

Common ways to prevent weld lines are:

• From a design perspective, avoid unnecessary obstacles in the flow path, or redesign ribs and holes to ease flow around them.
• Different polymers have different melting points, so choose resins with lower viscosity and lower melting points to ensure they fuse properly and prevent weld lines.
• Increasing the temperature of the molten material can help prevent partial solidification.
• By increasing injection speed and pressure, the cooling process may be shortened.

Jetting

To avoid jetting defects, here are some common implementations:

• Excessive pressure is the leading cause, calling for reducing the injection pressure of molten plastic through the narrow gates. This will avoid gradual fills.
• Also, increase the gate size to slow the jet’s speed.
• Regarding the gating design, use a gate that directs flow along a wall, such as an edge gate, rather than a straight-through gate into an open volume.
• Use low viscosity plastics to avoid jetting.
• Increase the temperature of the molten material and the mold to avoid partial solidification. 

Short shots

To fix short shots, here are some ways:

• Increase injection pressure and injection speed within safe limits.
• Check that the shot size is adequate and that the screw fully recovers before injection.
• Raising the melt temperature reduces viscosity and improves flow into thin or distant regions.
• Better venting and larger or better‑placed gates also help material reach all parts of the cavity.

Wrapping

Here are some ways to reduce warpage:

• On the design side, use uniform wall thickness and balanced geometry.
• Avoid large flat areas without ribs or supports, and avoid sudden changes in thickness.
• Process actions include optimizing cooling time, using uniform mold temperature, and balancing cooling channel layout.
• Lower packing pressure and changing gate location can also reduce residual stresses that lead to warping.

Bubbles/voids

To control bubbles/voids, apply these techniques:

When the defect is due to the trapped air:

• Improve venting and reduce injection speed to allow air to exit.
• Dry hygroscopic materials properly to remove moisture that turns into steam.
• To avoid defects like internal voids from shrinkage:
• Increase packing pressure and packing time so the core of the section is fully packed.
• Use proper wall thickness and rib design, and ensure adequate cooling to achieve more uniform shrinkage.

Conclusion

Now that you know the injection molding defects, their causes, and solutions, manufacturers and operators can take a proactive approach to avoid them by applying preventive measures. Understanding each defect’s appearance, root cause, and basic remedies allows better mold design, parameter setting, and material selection. It is always better to get the right injection mold design for the molding process in the first place, rather than spending on costly measures. For that, a reliable and experienced injection molding partner can do it all for you. From stable injection molds to the right material selection, to new product development and prototyping, MoldPartner can provide a customized solution for your business. So, contact us now!