Tag Archive for: venting defect

What is Burn marks troubleshooting

Burning marks, or dieseling defects, This defect will often show up as black, gray, or brown discoloration on the part surface. This is the result of gasses and volatiles getting trapped, compressed, and heated in the mold during injection. This gas will burn the front of the polymer flow front by the high injection pressures and speed. Burning marks normally occur at the end of fill, or the flow ends, such as the end of the ribs or deep ribs area., we call gas traps, venting defects as well.

Burnt marks defects

Burn marks defects

Cause and solutions

The mean reason to cause the Burn marks issues are the air, which the air did not escape from the mold cavity.

The two main things when troubleshooting burn marks issues are:

  1. Where is the gas coming from? In the mold cavity that any empty place is the air before fill.
  2. Why is the gas not escaping the mold?

Burning defect caused by excessive injection speed or pressure.

If injection pressure is too high, the plastics resin will flow into the mold cavity so fast that any air trapped in mold cavities is not having enough time to be pushed out ahead of the material flow. Then this stuck air has been compressed and its temperature rises sharply. The hot air ignites the surrounding melt plastic materials, which is burning until the air is consumed, leaving a black, brown or white color marks at the air trapped area.

Reducing the injection speed & pressure will give enough time for the trapped air or bas to escape to mold cavity, in the means time increase the venting grooves, or add a venting insert at the air trapped area will improve this burn marks defects.

Burn marks caused by Lack of venting.

Venting systems is important in the mold, it is placed in mold cavity to exhaust any gases or trapped air in the mold. If the there is no vent grooves in the mold, or vents is not deep enough or wide enough, the air will be compressed before it is all exhausted and then it ignites and get burning to cause the burnt marks. All over the parting line will be venting grooves and there will be place to lead the air out to the mold cavity and the end of the fill area, if the are deep bottoms  of ribs in mold which is blind areas, should add venting insert to lead the air out.

Burn marks caused by the property of material

Excessive regrind use. If use regrind material may cause this burn marks issues as well,

especially use PVC regrind material. This Regrind PVC material may tend to absorb heat in the injection barrel at a slower rate than virgin, so sometimes the PVC material get burning in the barrel,

Finally to solve the burn marks is not hard task as long as you finding the good way to lead the air out of the mold cavity, but sometimes the gate location is important to improve the burn marks as well, because the right gate location will improve lead the venting easy to escape out the mold cavity.