burn marks in injection molding

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Injection molding can’t be exempt without defects and errors. There are many rooms for expensive defects. Some defects are unrepairable, but some can fix. Many defects, as burn mark, can rectify with the help of troubleshooting tips and methods. Burn marks are common molding defects and appear as black and brown discoloration. They’re also known as gas trapped and dieseling effects in injection molding. Burn marks appear when air traps and they occur on the end of the flow path. During the mold filling stage, the gas trap causes ignition that becomes a reason for burn marks. It’s essential to know the reason for their appearance. So you can solve this issue by reducing product defects and quality concerns.

How to Fix Burn Marks In Injection Molding

To fix these burn marks, you must be aware of where trap gas comes from and the reason for its trapping in the mold? You can check for this problem: velocity, back pressure, melting temperature, and decomposition. Burn marks occur due to many reasons. You need to know to fix these burn marks. They can be due to many factors.

Like injection molding machines, material and mold are the main contributors. Burn marks may appear if your resin or material is wet or your content is volatile. Let the material dry when you have to start the molding application. Expertise injection molder chooses the resin that is having a small melt index. Reduction in the amount of lubricant is also proving useful for reducing burn marks. Hopper dryer and pre-drying methods can cut the burn marks in injection molding. It improves your product integrity and quality.

What Are Main Causes For Burn Marks, And How To Control Them In Injection Molding

If you don’t control molding conditions, it may become a cause of a burn mark. When melt injects into the cavity and your melt flow state is laminar. The speed of injection molding increases to some extent, and the flow state disturbs. The surface of a plastic part that forms with turbulent conditions causes burn marks. If you want to control the flow state, injection speed should be less to prevent burn marks. At high melt temperatures, it’s very easy to create burn marks on the plastic molded parts. So back pressure should be less than 2mpa and screw speed less than 90rmp. The injection molder experts recommend fixing the burning issues in molded parts.

In injection molding, excess fractional heat produces due to a lengthy rotational period. You can increase the screw speed and the molding cycle for this problem. Raw material having poor lubricant and lowering back pressure proves helpful.

When the resin is changing into charring, it happens due to the high resin temperature. For fixing this, you can make sure of the proper functioning of the heater bands. Check temperature and if you find too high a melt temperature. You can reduce it for proper processing.

With the improper function of barrel and screw, you may face black color and specs striation on the parts. You can replace the barrel and screw to solve it when we use the anti-reverse ring in the molding machines. It may become a cause of retention and make discolor and discompose melt. Black or brown burn marks create when molders inject this discolor melt into the mold cavity. You should clean the nozzle screw system and avoid this issue. You should choose high-velocity resin. It’s better to choose a long diameter machine with low pressure for the whole process.

The reason for burn marks may be the degradation of the material. That occurs due to overheating and fast injection speed. It can be due to high melt temperature and a restrictive flow path. Screw high speed also affects the material and becomes a reason for burn marks.

Overheating of trap air is also one of the major reasons for burn marks in injection molding. You may find burn marks at the end of the filling process. The reason for this may be too high speed for injection molding. You can solve this by lowering pressure and controlling the filling rate.

Improper venting also is a cause of air trapping and burn marks at the end of the filling. Slowing down the injection rate may prove effective for fixing this type of problem. Due to high injection speed and pressure, air can trap in the runner system. Compress air will generate polymer to degrade on the surface near the blind area. One remedy is to place an adequate venting system that can help vent out the tarp air. They’re very important in the blind area and near the end of the flow path. For crystalline polymer, the venting size is 0.025mm, and for the amorphous polymer, it should be 0.038 mm.

Nozzle size and gate size also produce defects in your finished parts. Too small nozzle and gate size can create black streaks in the molded parts. You can open the nozzle mouth or opening and can lower the resin flow. This may remove this issue. Mold failure also becomes a cause of many problems. It should give serious consideration. Mold gate type and location are critical in this regard. You should use a proper amount of release agent, and the surface of the mold cavity should be glossy.

Mold failure is also one of the major reasons for burn marks. So it needs to improve the mold functionality for fixing this issue.

Some factors that become a cause of mold failure are:

  • Mold vent blocks because of release agents
  • Unsuitable mold venting capacity
  • Poor design location & high filling speed
  • Over grease and breakdown of grease is also a cause of burning.
  • Generating high temperature as a gas trap and resin burn because of compression
  • For fixing all these issues, make sure the clamping force must be low. There should be no hindrance, and the mold venting should improve.

What Is Melt Fracture And Why It Occurs and Cause Burn Marks In The Molded Parts?

Melt Fracture

In plastic injection molding, we find fractures of the polymer stream on the surface of the mold. We can see fractures at the early stage of mold filling, and they start at the gate to the cavity. Melt fracture in the most moderate shape can lessen the surface gloss. We can’t see the roughness of the surface. In some critical cases, we can see zigzag lines, short waves, and long waves. Short waves are like melt fractures, and they can appear inside a film. Haze ( a defect ) also generates when melt fracture occurs on a film’s surface.

It’s very tough to know what problem you are facing? Is it a melt fracture, or is it an interfacial instability? You may find interfacial instability between the layers of film. It may appear as short waves or long waves patterns. You can distinguish both defects with a simple test by putting a film sample in a glass of water. The melt fracture that you find on the surface will disappear in the water. But interfacial instability can’t disappear in the water, and water does not affect it.

Causes Of Melt Fracture

With an in-depth analysis of melt fracture and its mechanism. We found that we inject melt into a large cavity. In this procedure, melt fracture creates due to high speed and pressure. Fractured areas surface can mix with the melt surface. For making burning marks on the surface of the plastic part. A small amount of molten melt causes more burn marks on the surface of the products. We found some causes for its creation.

  • Due to high shearing stress in the melt during the filling process of mold.
  • Elastic behavior of the melted polymer is common for burn marks and melt fractures.
  • Lower polymer surface cohesive force is also a cause of melt fracture.
  • Pulsations in the melt pressure become a cause by a sticking phenomenon. Pulsations pass through the melt, and it leaves the die.
  • Skin raptures are also causing melt fracture. It happens when the surface of the film stretches when separating the die. It occurs on the film’s outer surface.
  • It appears when micro-tears create due to fast stretching and cooling.
  • We can see skin rapture inside of the surface. It happens when using cold, IBC air.
  • Poor polymer or mold adhesive is also one reason for melt fracture.

How To Solve/Fix Melt Fractures In Molded Parts

It’s essential to solve these melt fractures to eliminate burn marks. There are some steps you can follow to fix these problems.

  • You can raise the material temperature and can reduce the melt relaxation time.
  • You should control the screw and injection speed. It can fix by changing equipment and processing conditions.
  • You can enlarge the pinpoint gate and tunnel gate for resolving melt fracture. You can choose the appropriate gate type and location.
  • Make sure that you first inject molten material into a transition cavity. And afterward, it will inject into the large size cavity. Your melt should not flow into the mold cavity.
  • For raw material, you can use low molecular polymers as it’s better for wider distribution. It also proves helpful for minimizing the elastic effect.
  • One solution for this problem is to add a processing aid to the resin for coating the metal’s surface. It can reduce the coefficient friction.
  • You can blend high melt index resin with low viscosity. It may reduce the shear stress in the die. For eliminating the issues of skin rapture, blending also proves effective.
  • Reducing output is also beneficial for fixing melt fracture. You can use fine mesh filters for increasing recirculation flow. It provides more time for friction in the area barrel and between the screws.
  • Molders use one of the most common methods for reducing melt fracture. They use a wide die gap for the reduction of shear stress. This method has some limitations for drawing down a thin film.
  • By applying all these remedies and methods. You can reduce melt fracture, which can also reduce the chances of creating burn marks.


Burn marks are the main injection molding defects. They cause material degradation and mold properties. You can control burn marks with the help of the methods mentioned above. It’s essential to fix these burn marks. To reduce machine downtime and quality issues. They can appear at any time during the injection molding procedure. You need to choose one of the best companies and in-house master injection molders. An experienced molder can fix and solve burn marks and all other defects of molding. It may increase the product’s rate and quality of the finished parts.


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We will contact you within 1 working day, please pay attention to the email with the suffix “@cavitymold.com”. 

or email direct:jerry@cavitymold.com

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We will contact you within 1 working day, please pay attention to the email with the suffix “@cavitymold.com”. 

or email direct:jerry@cavitymold.com