How to Prevent Diesel Effect in Injection Molding

How to Prevent Diesel Effect in Injection Molding

Diesel effect, also known as dieseling, is an undesirable phenomenon that occurs when trapped air or gas inside a mold cavity ignites due to high temperature and pressure during injection molding. This can cause thermal degradation of the plastic material, resulting in burn marks, short shots, poor surface finish, or reduced mechanical properties of the molded part.

In this blog post, we will explain the causes and consequences of diesel effect in injection molding, and provide some practical tips on how to prevent it or minimize its impact.

Table of Content


In the world of injection molding, achieving high-quality and consistent results is crucial for meeting the demands of various industries. One significant challenge that manufacturers often face is the “Diesel Effect.” This phenomenon, also known as “Dieseling” or “Diesel Streaking,” occurs when tiny black or dark specks appear on the surface of molded plastic parts, resulting in an undesirable appearance and compromised product integrity. In this comprehensive guide, we will delve into the intricacies of the Diesel Effect, its root causes, and most importantly, explore a range of effective strategies to prevent this issue from arising during the injection molding process.

Understanding the Diesel Effect

What is the Diesel Effect?

The Diesel Effect is a common defect observed in injection-molded plastic parts, particularly those with light-colored or transparent resins. These unsightly specks are akin to the fine soot produced by a diesel engine, hence the name “Diesel Effect.” When present, it degrades the aesthetic appeal and overall quality of the final product, leading to potential rejections and increased manufacturing costs.

The diesel effect is a phenomenon that can occur in hydraulic systems, where air bubbles mixed with oil particles are compressed and ignited, causing combustion and damage to the system. The diesel effect is named after the combustion process in a diesel engine, where fuel is injected into compressed air and ignites spontaneously. The diesel effect can result from cavitation, which is the formation of voids or bubbles in the hydraulic oil due to pressure changes or shearing forces. Cavitation can cause material damage, noise, pressure surges, oil aging, and seal destruction. To prevent the diesel effect and cavitation, hydraulic systems should be designed to avoid low pressure zones, high flow velocities, sharp edges, and pulsating pressures.

The Root Causes of Diesel Effect

To address the Diesel Effect effectively, we must first identify its root causes. This phenomenon can be attributed to a combination of factors that come into play during the injection molding process:

1. Resin Contamination

One of the primary culprits behind the Diesel Effect is resin contamination. The presence of foreign particles or impurities, such as dust, moisture, or leftover material from previous runs, can introduce dark specks into the plastic material. These contaminants can arise from improper storage, handling, or transportation of the raw resin, emphasizing the significance of maintaining a clean and controlled production environment.

2. Poor Mold Venting

Inadequate mold venting is another crucial factor contributing to the Diesel Effect. When the mold lacks sufficient venting channels, air and gases trapped inside during the injection process cannot escape properly. As a result, the trapped air gets compressed, leading to overheating and the degradation of plastic material, which manifests as dark streaks on the molded parts.

3. High Processing Temperatures

Excessive processing temperatures can lead to the degradation of the plastic resin and the formation of carbonized particles, which ultimately cause the Diesel Effect. This issue can occur if the temperature settings are too high for the specific resin being used, or if the injection molding machine experiences temperature fluctuations during production.

Strategies to Prevent Diesel Effect

1. Material Selection and Quality Control

The foundation of preventing the Diesel Effect lies in selecting high-quality plastic resins and establishing rigorous quality control measures. Choosing resins from reputable suppliers and conducting thorough material testing to ensure they meet the required specifications can significantly reduce the risk of contamination-related defects.

2. Proper Resin Handling and Storage

To avoid resin contamination, it is essential to adhere to proper handling and storage practices. The production facility should maintain a clean and dust-free environment, with designated storage areas for different resin types. Additionally, employing moisture-resistant packaging and ensuring a first-in, first-out (FIFO) inventory system will further safeguard the quality of the resins.

3. Mold Design and Venting

Investing in well-designed molds with adequate venting is crucial to preventing the Diesel Effect. Collaborating with experienced mold designers and manufacturers can help optimize the mold’s structure, ensuring that air and gases can escape efficiently during the injection process. Moreover, regular mold maintenance and cleaning are essential to prevent any obstructions that could hinder venting.

4. Optimal Processing Parameters

Maintaining proper processing temperatures is vital to preventing the formation of carbonized particles and subsequent Diesel Effect. Conducting comprehensive tests to determine the optimal processing parameters for each resin type and adjusting the machine settings accordingly can significantly improve the quality of the molded parts.

5. Real-time Monitoring and Inspection

Implementing real-time monitoring and inspection systems can aid in early detection of any deviations or irregularities during the injection molding process. Integrating advanced sensors and automated inspection technologies can provide valuable data, allowing operators to take prompt corrective actions before the Diesel Effect becomes a significant issue.


The Diesel Effect in injection molding is a persistent challenge that demands a comprehensive approach to prevent and address effectively. By understanding the root causes of this defect and implementing the strategies outlined in this guide, manufacturers can elevate the quality of their injection-molded products, reduce waste, and ultimately enhance customer satisfaction. Remember, successful prevention of the Diesel Effect is not only a testament to the expertise and authority of the production team but also a testament to the trustworthiness of the products that bear the mark of excellence.

What causes diesel effect in injection molding?

Diesel effect in injection molding is similar to the combustion process in a diesel engine, where air is compressed and heated to a point where it can ignite fuel without a spark. In injection molding, the fuel is the plastic material and the air or gas is trapped inside the mold cavity.

The main factors that contribute to diesel effect in injection molding are:

  • High injection speed and pressure: These increase the temperature and pressure of the melt and the trapped air or gas, creating favorable conditions for ignition.
  • Poor mold design and venting system: These prevent the escape of air or gas from the mold cavity, leading to compression and accumulation of gas pockets.
  • Unsuitable material and additives: Some materials and additives are more prone to thermal degradation or oxidation than others, making them more flammable or combustible.

What are the consequences of diesel effect in injection molding?

Diesel effect in injection molding can have negative impacts on both the quality and performance of the molded part. Some of the common consequences are:

  • Burn marks: These are dark or black spots on the surface of the part, caused by charring or carbonization of the plastic material due to high temperature.
  • Short shots: These are incomplete parts that fail to fill the entire mold cavity, caused by gas pressure that opposes or blocks the flow of melt.
  • Poor surface finish: This is a rough or uneven appearance of the part surface, caused by bubbles, blisters, cracks, or pits formed by gas expansion or explosion.
  • Reduced mechanical properties: This is a loss of strength, stiffness, toughness, or durability of the part, caused by molecular degradation or chain scission of the plastic material due to high temperature.

How to prevent diesel effect in injection molding?

The best way to prevent diesel effect in injection molding is to eliminate or reduce the sources of trapped air or gas inside the mold cavity. This can be achieved by optimizing process parameters, improving mold design and venting system, and choosing suitable material and additives. Here are some practical tips on how to do so:

  • Optimize process parameters: Adjusting parameters such as injection speed, pressure, temperature, time, and holding pressure can help control the flow and cooling of melt and minimize gas generation and compression. Generally speaking, lower speed and pressure, higher temperature and time, and moderate holding pressure can reduce diesel effect.
  • Improve mold design and venting system: Designing a mold with proper gate location, size, shape, and number can help direct the flow of melt and avoid air entrapment. Adding vents or vacuum systems to strategic locations on the mold can help release air or gas from the mold cavity. The vents should be sized and positioned according to the material properties and flow pattern.
  • Choose suitable material and additives: Selecting a material with high thermal stability and low viscosity can help prevent thermal degradation and improve flowability. Adding additives such as antioxidants, stabilizers, lubricants, or flame retardants can help enhance resistance to oxidation, decomposition, friction, or combustion.


Diesel effect in injection molding is a common but avoidable problem that can compromise the quality and performance of molded parts. By understanding its causes and consequences, and applying some preventive measures such as optimizing process parameters, improving mold design and venting system, and choosing suitable material and additives, you can reduce or eliminate diesel effect in your injection molding process and produce high-quality parts.