9 Injection Molding Defects and How to Avoid Them


Injection Molding is one of the most cost-effective manufacturing processes for mass-producing plastic products. Engineers use it in high-volume production runs to create complicated shapes and parts while maintaining a low cost-per-part.

However, as cost-effective as injection molding is, you can end up with defective parts if the manufacturing process isn’t done well. To avoid this, always make sure you use a reputable injection molding service for your projects.

With us, you can always be sure of the best quality injection molded products.

In this article, we’ll be examining nine common injection molding defects, their causes, and how to avoid them. These defects include:

Let’s take a look at them.

Sink Marks

Sink marks appear in the form of small depressions on the surface of injection molded parts. They usually occur on long, flat sections of the part.

What causes sink marks?

Sink Marks are due to insufficient cooling times for the part, which results in the part’s thicker sections cooling unevenly. Due to this, the surface cools faster than the interior of the part, causing it to collapse in on itself.

Other causes of sink marks include:

  • Low holding or injection pressure.
  • Improper part design and gate placement.
  • High melting and molding temperature.

How to avoid sink marks?

The best way to fix sink marks is to increase holding pressure and holding time while decreasing molding temperature. This gives the part enough time to cool evenly and cure adequately across its cross-section.

Also, avoid overly thick wall sections as much as possible. Ensure that the thinnest wall should be 40-60% of the thickest walls’ size.

Finally, place your gates well to ensure that the thickest sections are always filled first. Then, make sure the material is flowing from the thick to thin sections.

Burn Marks

Burn marks are black or rust-colored marks on the product’s surface and edges of injection molded parts. These surface discolorations ruin the product’s surface quality and finish but typically do not affect its structural integrity.

What causes Burn Marks?

The primary cause of burn marks is overheating. Air trapped in the mold and molten plastic can overheat to the point where the part’s surface and edges burn.

The factors behind this overheating are excessive injection speed and high mold temperatures.

How to avoid Burn Marks?

To avoid burn marks, you have to address overheating in the mold. You can do this by reducing the melting and molding temperature and adding suitable gas vents and gates to allow air to escape the mold easily.

Furthermore, reducing the mold’s cycle time (the time required to mold one shot of plastic into a part) can help combat overheating.

Finally, lower the injection speed to reduce the chances of air getting trapped in the mold.


Warping is a structural defect resulting from the twisting and bending of the finished part while it cools. As a result, the final product has a bent or twisted shape that wasn’t in the design.

What causes warping?

Warping is due to the uneven or non-uniform cooling of the parts. As different sections of the product shrink and cool at different rates, internal stresses are set up in the part.

So, as the material cools, these internal stresses are released, resulting in the bending and twisting of the part. Furthermore, rapid cooling times and bad mold design can also worsen this issue.

How to fix warping?

To fix warping, make sure the part cools slowly and gradually to avoid internal stresses being set up in it. Furthermore, lower the mold temperature to decrease the temperature variations.

It would be best to design your molds with uniform wall thicknesses and symmetry for a single plastic flow direction. This ensures stability during cooling.

Short shots

Short shots are a major structural defect that comes in the form of unfinished or incomplete parts after molding. It happens due to the molten material not filling all the molds and cavities, resulting in some regions without any plastic.

So, parts come out of the mold with missing sections.

What causes short shots?

Many factors can cause short shots inside the mold. One of the most common is a bad mold design with narrow gates. Narrow gates restrict material flow into the mold and get blocked easily.

Another reason is a viscous plastic material. If the plastic is too viscous, it will solidify before it fills up the mold. This can also occur due to low melt temperature and pressure.

Finally, trapped air pockets in the mold can also hinder material flow, resulting in short shots.

How to avoid short shots?

Avoiding short shots starts with proper mold and process design, first off all, you should always make sure your gates and channels are wide enough to allow adequate material flow throughout the mold.

Secondly, increase the melt temperature and pressure for proper flow. Alternatively, you can choose a less viscous plastic material.

Lastly, make sure the mold has enough appropriately sized air vents for proper degassing.

Flow Lines

Flow lines are surface defects found on the narrower sections of injection molded parts. They appear as a series of wavy patterns, lines, or bands that usually have a different color than the surrounding areas.

Flow lines do not compromise the part’s structural integrity. However, they can ruin its final look and finish.

What causes flow lines?

Flow lines are primarily caused by variations in the flow speed of the molten material in the mold. This variation occurs as the direction of the material change as it flows through corners and bends in the mold.

Another factor behind flow marks is a slow injection speed. If the material begins to solidify while still flowing, a wavy flow line pattern will appear.

Lastly, as the molten material flows through sections with varying wall thicknesses, flow lines can form.

How to avoid flow lines?

To avoid flow lines, make sure the injection speed, temperature, and pressure are all at optimal levels. This ensures all the cavities are correctly filled and prevents premature cooling.

Fillet the corners where the wall thickness changes to avoid sudden changes in slow velocity and direction.

Finally, add some distance between the mold and mold coolants to prevent the molten material from cooling too early.

Vacuum Voids/Air Pockets

Vacuum voids are pockets of air trapped within an injection molded part. They are found within the material’s structure and close to the surface of the part.

Vacuum voids do not significantly affect the material’s structure in small numbers. Also, if the material is colored, it will not be seen.

However, if the voids are many, it can weaken the structure.

What causes vacuum voids?

Vacuum voids occur when the holding pressure isn’t enough to force trapped air out of the mold cavity. So, the trapped air stays within the mold.

It also happens when the material closest to the mold’s walls cools too quickly. The quick cooling hardens the material and pulls it outwards, creating holes in the structure.

Vacuum voids are more prevalent in parts with a thickness of more than 6mm. The higher the thickness is, the higher the chances of voids.

How to avoid vacuum voids?

To avoid voids forming in your parts, increase the holding pressure and holding to force out all the trapped air in the mold. Also, using less viscous plastic enables the air to escape faster from the mold.

Furthermore, redesign the mold to make the mold gates closer to the thicker parts to prevent premature cooling. Finally, slow down the cooling rates to ensure the molten plastic cools evenly.


Flashes are extrusions on the edge of the plastic part formed by molten plastic escaping from the mold cavity. The molten material typically escapes through the space between the tooling plates or the injection pin.

Flashes aren’t considered significant defects unless they are pronounced in the part. They can easily be trimmed away during the post-processing and finishing of the part.

What causes flash?

Flash occurs due to poorly designed or worn-out molds that cannot seal in the molten material properly. Also, the material will leak out if the plate clamping force isn’t enough to hold the mold together.

Other causes of flashes include high mold temperatures and excessive injection pressure.

How to avoid flash?

To avoid flash, retool, or redesign the mold, especially if it’s already degraded and worn out. Make sure the plates fit together correctly to stop material from flowing out.

Also, increase the plate clamping force to seal the plates properly and avoid leakage. Lastly, adjust injection pressure and mold temperature to optimal levels.

Surface Delamination

Surface delamination refers to the separation or fracture of the surface of an injection molded part into layers. These layers then peel off the part’s surface like a coating.

Delamination is a pretty major defect that can affect both the surface quality and the structure of the part, making it unreliable.

What causes surface delamination?

Surface delamination can occur due to the presence of contaminants like water in the plastic pellets. These contaminants make it hard for plastic layers to bond together. So, they begin to separate after molding.

Another example of a contaminant that can make bonding hard is the mold release agent. If there is an excess of the release agent in the mold, delamination will occur.

Mixing two incompatible plastics can also result in surface delamination as they won’t bond well with each other.

How to avoid surface delamination?

To avoid surface delamination, dry the plastic pellets thoroughly to remove all moisture before molding. Also, make sure the pellets and the molds are stored properly to prevent any form of contamination.

Furthermore, when apply mold coatings to mold, be careful not to use excess. You can also redesign the ejection mechanism of the mold to enable easy part removal without relying on release agents.


Jetting is a part defect that occurs when an initial jet of molten material is injected into the mold cavity at high speed. Due to the high injection speed, the material fails to contact the mold properly and cools before the cavity fills up.

This results in a wavy spray line on the surface of the plastic part, as the initial material jet and the subsequent material coming after it do not fuse properly.

Jetting is both a surface and structural defect. Since the initial jet doesn’t fuse properly with the rest, it results in a weak part structure.

What causes jetting?

The primary cause of jetting is a high injection pressure coupled with a narrow gate. When the material is injected through the gate, instead of filling up the mold gradually, it sprays through it, resulting in jetting.

Another cause of jetting can be a highly viscous material or a low material and mold temperature. Both conditions result in the plastic cooling quickly within the mold.

How to avoid jetting?

You can avoid jetting by reducing the injection pressure. This makes sure the mold fills gradually instead of the molten material squirting through the gates.

Increase the melt and mold temperatures to prevent the material from cooling and solidifying prematurely inside the mold.

Finally, redesign the gate with a wider channel to ensure the material flows more evenly into the mold. Also, make sure the gate is oriented so the material flow doesn’t go directly into the mold’s center.

Want defect-free parts?

Injection Molding defects increase the cost of poor quality and eat into your revenue. To properly leverage the power and cost-effectiveness of injection molding, you have to avoid them at all costs,

So, to obtain the best product quality possible, make sure you design your molding process and tooling carefully before starting. To make the process easier, you can find a professional rapid injection molding manufacturer to help you avoid injection molding defects.


Leave A Reply