Diesel Effect (Burn)
Black burn spots formed by trapped air igniting spontaneously. A pure venting problem.
The diesel effect is air trapped in the mold cavity being rapidly compressed by the melt and igniting. It takes its name from the compression-ignition principle of the diesel engine. It leaves a charred, dark brown or black mark on the part, and this mark is **always at the same point** — where the air cannot escape.
How to Recognize It
- Intense black/brown burns at the end of the flow path, in blind holes or at the point where the fronts meet.
- A short shot may accompany it in the same area.
- If it started SUDDENLY during production, the vents have become blocked.
Physical Causes
- Air that cannot escape is compressed at the end of injection; its temperature rises to the ignition point and it burns the plastic (the diesel principle).
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- Reduce the injection speed (give the air time to escape) — slow down especially in the last stage.
- Reduce the clamping force (with a 20% safety margin) — breathing through the parting surface improves.
Mold, Design and Material Solutions
- Add venting at the burn area; clean the existing channels of CONTAMINATION (periodic maintenance).
- Correct the vent positions according to a flow front analysis.
- Avoid air traps by changing the flow design; if necessary, vent the mold actively (vacuum/evacuation).
Important Notes
- ⚠️ The combustion products are aggressive; they can cause damage to the mold surface and plate-out — burns must be solved before they become persistent.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Burn Marks | A burn mark appears scattered along the flow and comes from thermal damage to the melt. The diesel effect is at a single point, where the air is trapped. |
| Short Shot | With a short shot the material never reaches that area. With the diesel effect it reaches it but the trapped air burns; the two are often seen together in the same area. |
Frequently asked questions
What does it mean that the mark is always in the same place?
A fixed position is the clearest sign of insufficient venting at that point. Adjusting temperature or speed can ease the problem, but the permanent solution is to add venting to that area.
Does the clamping force have an effect?
Yes, and it is often overlooked. A clamping force higher than necessary crushes the vents closed; the air has no way out. Reducing the tonnage alone solves the problem in some cases.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice