Troubleshooting Diesel Effect in Injection Molding

The diesel effect can occur in molds with insufficient venting, leading to burned parts in injection molding. However, several solutions can help prevent this defect.

When filling the mold cavity, air is displaced and should be directed outside the mold. This is achieved through venting channels and tiny ducts machined within the mold partition line.

You can also read: The Importance of Mold Temperature Control in Injection Molding

Air also leaves the cavity through the clearance between cores and mold cavities. Ensuring the last part filled within a mold always has a path to conduct air outside the cavity. Air will concentrate at the end of the flow path, and if there is no way for it to leave, it will impair filling or may cause the so-called “diesel effect.”

If venting is insufficient, the filling plastic traps air in the cavity. As the plastic continues to flow, it compresses these air pockets, raising cavity pressure and air temperature.

This effect, the principle under which combustion motors run, is known as the “diesel effect.” It is evident because the air turns so hot that it carbonizes the plastic, making it look and smell like charcoal.

Reasons Behind the Diesel Effect

The diesel effect in injection molding is always a signal of insufficient venting. This may happen because the conduits allowing air to exit the mold are either inadequate in number or partially obstructed.

An early fill simulation will show you where air is trapped during filling. This simulation is particularly effective when molding irregular geometries, where predicting weld lines and possible air traps is difficult.

It is always important to consider which material you are molding in order to determine the depth of the venting channel. Excessive depth may cause flash. For low-viscosity materials like PP or PE, recommended channel depths range from 0.013 to 0.03 inches, ensuring efficient airflow. Meanwhile, for more viscous materials such as ABS, standard channel depths typically range from 0.025 to 0.038 inches, optimizing mold performance.

Remember that air is a fluid, so the more resistance you offer to flow, the greater the pressure to overcome. If the channel’s landing is too large, it will oppose more resistance to air removal.

Also consider that even with properly positioned venting channels, a slow exhaust rate will slow the filling rate and extend the cycle time. It is always advisable to have as much venting as possible; excessive venting is impossible.

Excess depth within the mold channels will also cause problems, as it may produce flash. It is always advisable to machine the channels from low to large. Thicker parts will allow for deeper venting channels, as the material’s viscosity is relatively large due to the lower shear rate. Thinner parts may need thinner venting, as the shear rate lowers the viscosity and allows easier flashing.

Maintenance of The Venting Channels

Obstruction due to prior carbonization of the material is one of the most common reasons for the venting channels’ lack of performance. If carbonization of the material appears, make sure to use cotton to remove the black impurities from the venting channels. Some polymers release volatiles (such as ABS or POM, for example), and these molds may require more frequent cleaning of the venting channels.

If the mold usually performs well and the diesel effect starts suddenly, it may be a sign of a too-large clamping force. At the very high molding pressure of the clamping unit, the mold steel behaves as a spring, deforming elastically. In this case, air channels may get compressed, preventing air release. To solve this problem, make sure you are using only the clamping force required for your mold.