15 Injection Molding Defects [Know The Causes & How To Prevent Them]

Injection Molding Defects

At times, getting perfect injection molded plastic parts and components can be an overwhelming task.

It is due to a range number of injection molding defects.

This guide will help you identify the most common 15 injection molding defects, their causes, and solutions.

So, if you want to get perfect injection molded plastic parts – read this guide.

injection molding unit

injection molding unit

1. Flow Lines

Normally, the defect appears as either ring-shaped bands or a wavy pattern. Flow lines usually have a different color from that of the final component.

Although they do not affect the overall integrity of the components, the aesthetic value of the injection molded part is compromised.

flow lines

flow lines

Some of the main causes of flow lines include:

  • Varying cooling speed of the injection molded part
  • Variations in the wall thickness of the components may result in inconsistent cooling

In case you wish to reduce flow lines in injection molded parts, you should consider the following:

  • You should ensure plastic material fills the mold before the cooling process begins. You can easily achieve this by increasing material temperature and injection speed. Of course, increasing pressure will also help.
  • When you increase the diameter of the nozzle, the material will flow at high speed, thereby preventing early cooling
  • Consider relocating mold gates
  • When the wall thickness increases, it is vital to round the corner of your mold.

2. Brittleness

It is a situation where parts easily break or crack.



Some of the main reasons for this injection molding defect include the following:

  • Unsuitable drying condition
  • Use too much-recycled material
  • Setting the wrong injection temperature
  • Setting the wrong runner and gate setting

There are many ways of fixing these defects, such as:

  • Try to minimize the use of recycled material; in case you must use recycled material, mix it in the right proportion with virgin raw material
  • Before the injection molding process, you must set appropriate drying conditions
  • Try increasing the size of the gate, main runner, and branch runner
  • Using a high-strength plastic may minimize brittleness
  • During plasticization, ensure the temperature distribution is even. You can achieve this using a well-designed screw.

3. Burn Marks

Burn marks are discoloration on the molded plastic part which may appear rusty or black. You may see burn marks on the surface or edges.

Burn marks in injection molding

Burn marks in injection molding

Some of the main causes of burn marks include:

  • Trapped air
  • Overheating the plastic resin in the mold cavity
  • Excessive injection speed of heating

In case you wish to prevent burn marks on injection molded plastic parts, you should consider the following:

  • Try to regulate mold and melt temperature since this will prevent overheating
  • Reduce mold cycle time – in case there is trapped air, there will be no possibility of overheating
  • Allow trapped air to escape easily by enlarging gates and vents
  • By reducing injection speed, the risk of trapping air inside the mold will reduce significantly

4. Injection Molded Parts Sticking In The Mold

It is a situation where molded parts do not separate from the mold. If you are not careful, there is a high possibility of breaking the ejector pins.

sticking parts in injection molding

sticking parts in injection molding

So, what are some of the main causes here?

  • High residual pressure in the mold
  • Molded parts shrink significantly
  • Irregular finishing is not good
  • Small mold draft, i.e., it is not large enough

Here is how you can fix this problem:

  • Decrease pressure and temperature settings
  • Redesign the mold or increase draft
  • Consider using mold parting agents

5. Warping

In this injection molding defect, there is always uneven shrinking of parts. As a result, this causes warping during the cooling process.

Of course, this causes undue stress on various parts, thereby causing bending or twisting.

warping in injection molding

warping in injection molding

Some of the main causes of warping include:

  • Rapid cooling
  • Low thermal conductivity or excessive temperature may make warping worse
  • Design of the mold, especially if the wall thickness is not uniform

Here are some ways of reducing the warping phenomenon:

  • Give the molded components enough time to cool – ensure it is gradual to prevent the possibility of uneven stress on the injection molded part
  • Ensure the mold has a uniform thickness
  • Try lowering the temperature of the mold and material
  • You can use material with a low shrinking rate during cooling, such as particle-filled thermoplastics

6. Parting Lines

These injection molding defects appear as seams or lips. Usually, parting lines are recessed.

In most cases, they may go around the injection molded part.

parting lines in injection molding

parting lines in injection molding

Some of the main causes of these defects in injection molding include the following:

  • The section where two halves of the mold meet forms a parting line.
    After removing the mold, the injection molded part may have a characteristic line around its surface.

Now, how can you eliminate the parting line defects?

  • Incorporating linear features may hide the parting line
  • A matte surface finish or rough texture may hide the parting line
  • Repainting or sanding can remove the parting line
  • When you locate a parting line under some features, such as a cap or rim, it may hide it.

The parting line will always be in virtually all injection molded plastic parts. You can only find a way to hide or reduce it.

7. Air Pockets Or Vacuum Voids

These are air bubbles trapped in the final injection molded product.

Although vacuum voids are minor injection molding effects, if they appear on many surfaces, they will compromise the integrity of the part.

vaccum void and sinks

vacuum void and sinks

Some of the main causes of air pockets include:

  • When the molding pressure is significantly low, it will not remove trapped air in the mold cavity.
  • Whenever the material closest to the mold wall cools faster, it may cause air pockets to form
  • A significant change in material density from a molten state to a solid state may contribute to forming air pockets.

Here are some of the main ways of eliminating this molding defect:

  • Increase the injection molding pressure since this will help remove trapped air in the mold
  • Low-viscosity material reduces the possibility of bubble formation
  • To prevent premature cooling, especially around thick parts, consider having gates

8. Sink Marks

These are small depressions of recess on the plastic injection molded components.

It is mainly due to the shrinking of a molded component which pulls material toward the inner section of the component.

The main causes of sink marks include:

  • When the material cools too slowly, especially near the exterior section
  • Thicker parts are susceptible to sink marks

You can prevent sink marks by adopting the following:

  • Try to increase the cooling timeframe to prevent the possibility of shrinkage occurring
  • Mold may have thin component walls to enhance cooling, especially near the surface
  • You can increase holding time and pressure

9. Gate Witness Marks

gate used in injection molding

gate used in injection molding

Gate witness marks appear in many forms, such as ridges, dimples, or burrs.

You may find these defects in either one or more parts on the injection molded surface.

Some of the main causes of gate witness marks include:

  • The runner system design determines where you will find the gate witness mark. Remember, a witness mark refers to a section where the waste spruces break free from the finished component.

Here is how you can eliminate the gate witness mark:

  • You can use some features to disguise the marks
  • Try relocating the witness location to an unobtrusive area
  • Post-machining or sanding can help eliminate gate witness marks. So, the location of the witness should be easily accessible
  • By using smaller and multiple runners, you can easily distribute both resin and pressure
  • Using a fan gate can help in resin distribution

10. Weld Lines

They appear on the surface of injection molded parts like lines.

It is normally at the meeting points of two flow fonts.

Mostly, this happens when two or more flow fronts do not “knit” during the injection molding process.

In weak material bonding, they will appear as hair-like weld lines. Generally, this will affect the strength of injection molded components.

weld lines

weld lines

Some of the main causes of weld lines are:

  • It occurs different polymers fail to bond effectively. It happens when they fail to maintain a certain temperature during the collision.

You can eliminate weld lines in the injection molding process by:

  • Prevent partial solidification of the injection molding material by increasing temperature.
  • Ensure the material does not cool before filling the mold by increasing injection molding pressure and speed
  • You can eliminate partitions in the mold by redesigning
  • Materials with low viscosity and melting temperature will help reduce weld lines

11. Flash

It is a phenomenon where excess material appears like a thin protrusion along the edge of the injection molded component.

These injection molding defects occur when material flows beyond the intended channels. In such cases, you will trim the excess material.

flash lines

flash lines

The main causes of flash in injection molding include:

  • A worn-out or degraded mold is a major cause of flash in injection molding
  • Whenever the injection molding pressure and temperature are excessively high, you may experience flash
  • When the clamping force is not adequate, the material pass through cavities, thereby forming flash

Here are some of the effective ways to eliminate flash:

  • You should redesign the injection molding tooling system to ensure the plates fit properly. It will prevent excessive material from flowing to form a flash.
  • Ensure the clamping plate is sufficient enough to confine material during the injection molding process
  • For the best material flow, you should make adjustments to the injection pressure, mold ventilation systems, and mold temperature

12. Short Shot

A short shot is a major injection molding defect that will inhibit the final component’s overall functioning.

It occurs whenever the molten material fails to fill the entire mold cavity. As a result, you will have incomplete parts and components after the cooling process.

short shots

short shots

The main causes of short-shot injection molding defects are:

  • It is mainly due to the flow restrictions of the molten plastic material. It can be due to narrow or constricted gates in the mold.
  • The flow may be restricted when the mold is too cold, and the material is also dense. As a result, it will cause the short shot defect.
  • Trapped air pockets in the mold system may prevent the flow of material in the mold
  • When there is no inadequate pressure, the flow of material may also be restricted hence causing a short shot

Some of the most reliable ways of eliminating short-shot injection molding defects include:

  • You may consider redesigning the mold to enhance flow.
    The main design considerations may include wider gates and channels to improve material flow.
  • Use thinner baser materials for better flow
  • Increase injection pressure and speed for efficient material flow
  • Try to increase the mold temperature since this will prevent the material from cooling before filling the mold
  • Enlarging existing vents and adding more vents will help remove trapped air. As a result, this will improve material flow in the mold.

13. Jetting

Jetting is another plastic injection molding defect that can lead to part weakness. It is common when the mold has an initial “jet” of molten plastic material.

In such situations, it will solidify before filling the entire mold cavity. You will see the jetting injection molding defect as a squiggly line.

 jet effect

jet effect

The main causes of jetting defects include:

  • A small gate injecting molten plastic at very high pressure may cause squirts.

Of course, this happens as opposed to the gradual filling of the mold. Consequently, the material begins to cool on the mold walls.

To eliminate jetting, you should consider any of the following strategies:

  • It would be helpful if you prevented the possibility of squirting material into the mold cavity. You can achieve this by reducing the injection molding pressure
  • At any given time, you should prevent initial material that enters the mold from early solidification.
    You can achieve this by increasing the temperature of both mold and material
  • Consider redesigning the mold – the gate should be such that it directs material across the mold instead of lengthwise.

14. Delamination

Delamination occurs when some layers of plastic material tend to peel off from the main injection molded plastic part.

It is a plastic injection molding characterized by a flaking surface layer.

The defect will compromise the structural integrity of the component.



Additionally, it will affect the aesthetic value of the injection molded component.

Some of the main causes of delamination include:

  • Contamination of plastic resin – this may cause improper bonding of materials
  • The coating on the mold – when the mold release some of these agents during injection molding, a reaction may occur, causing delamination
  • Excess moisture may be another cause of delamination

You can eliminate delamination by considering the following key factors:

  • Pre-dry the mold and increase mold temperature – this will remove any moisture in the mold
  • Handle resin or base material carefully to prevent any possible contamination.
  • You can redesign the mold – adjust the injection nozzle so that you don’t use any release agents

15. Discoloration

It is a phenomenon where the final product does not have the intended color. Normally, discoloration happens in a localized area.

Although the injection molding defects do not affect the components’ strength, aesthetic value is highly compromised.

Possible causes of discoloration include:

  • Residual resin or leftover plastic pellets in the hopper or nozzle
  • Poor thermal stability may lead to discoloration
  • Improper mixing of material

You can eliminate discoloration by considering the following;

  • Hopper and nozzle must be clean – always clean the injection molding machine after every production
  • Using a purging compound can help remove the discoloration
  • Use a color agent which is thermally stable

Once you consider these, you will definably get an injection molded component with uniform color.


As you can see, plastic injection molding defects affect the final product’s structural strength and aesthetic appeal.

The good news, there is always a way to eliminate or prevent these plastic injection molding defects.

So, before you begin mass production, start with prototypes to ensure the end product has no defects.

Have you experienced any plastic injection molding defect we did not discuss here?

You can share the defect with us.

More Resources

Plastic Injection Molding

Polycarbonate Injection Molding

Acrylic Molding

Injection Molding Process

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