What is Back Pressure and Its Role in Injection Molding?

What is Back Pressure and Its Role in Injection Molding


Back pressure plays a vital role in the injection molding process, influencing the quality and characteristics of the final plastic product. It refers to the pressure exerted on the back of the screw in an injection molding machine. In this article, we will explore the definition, function, effects of too little or too much back pressure, and the importance of controlling it in injection molding.


Definition of Back Pressure

Back pressure is the pressure applied to the screw during the injection molding process. It is controlled by adjusting the return throttle of the injection cylinder. In electric injection molding machines, the speed of the back pressure is regulated by an AC servo valve. By manipulating the back pressure, manufacturers can achieve optimal results in terms of resin mixing, density, size stability, and overall product quality.


Function of Back Pressure

The primary function of back pressure is to control the speed at which the screw retreats, enabling the molten resin to be uniformly mixed and compacted in the barrel. This ensures consistent density, improves the weight of the plastic part, and enhances the stability of the molding size. Additionally, back pressure aids in eliminating gas from the molten resin and barrel, preventing issues such as plastic bloom, bubbles, and internal shrinkage defects.


Moreover, back pressure promotes uniform mixing of color powder or masterbatch with the resin, reducing or avoiding the formation of color patterns. It also helps increase the temperature of the molten resin, enhancing its fluidity and improving the shrinkage properties of the plastic parts.


Effects of Too Little Back Pressure

Insufficient back pressure can lead to several undesirable outcomes in the injection molding process:


  1. Less dense molten resin and gas presence: If the screw retreats too quickly due to inadequate back pressure, the molten resin may be less dense, resulting in increased gas content. This can make the resin more prone to producing bloom, negatively affecting the final product’s quality.


  1. Large size and light weight of molded part: Insufficient back pressure can impact the molding size, causing the part to be larger than intended. Additionally, the weight of the plastic part may be lighter than desired, compromising its functionality and strength.


  1. Shrinkage and bubbles in the plastic part: When back pressure is too low, the plastic part’s surface is susceptible to shrinkage issues. Transparent parts may develop bubbles, while non-transparent parts can experience internal shrinkage defects, affecting the overall appearance and integrity of the product.


Effects of Too Much Back Pressure

Excessive back pressure can also lead to various challenges in the injection molding process:


  1. Salivation (glue overflow) from the nozzle: High back pressure can cause salivation or glue overflow from the nozzle, resulting in excessive material wastage and production inefficiencies.


  1. Increased metering time and reduced efficiency: Excessive back pressure can prolong the metering time, reducing the overall efficiency of the molding process. This can lead to slower production rates and increased costs.


  1. Discoloration of colorant or resin degradation: When the screw retains molten resin for an extended period due to excessive back pressure, it can result in discoloration of the colorant or degradation of the resin. This occurs due to overheating, negatively impacting the visual appearance and performance of the final product.


  1. Increased wear and tear between screw and barrel: Excessive back pressure can cause increased friction and wear between the screw and barrel of the metering equipment. This can lead to higher maintenance costs, reduced equipment lifespan, and potential production delays.


Importance of Controlling Back Pressure

Controlling back pressure is crucial for achieving optimal results in injection molding. It ensures uniform resin mixing, consistent density, and size stability of the plastic parts. By carefully adjusting the back pressure, manufacturers can prevent defects such as shrinkage, bubbles, and bloom, resulting in high-quality products that meet customer expectations.


Factors Influencing Back Pressure

Several factors influence the ideal back pressure setting in injection molding:


  1. Material viscosity: Different materials have varying viscosities, requiring specific back pressure settings to achieve optimal flow and mixing.


  1. Screw design: The design of the screw, including its length, flight depth, and compression ratio, affects the required back pressure for effective resin mixing.


  1. Molding machine type: Different types of injection molding machines may have varying back pressure control mechanisms and optimal pressure ranges.


  1. Process parameters: Process parameters such as temperature, injection speed, and cooling time can impact the ideal back pressure settings.


Adjusting Back Pressure in Injection Molding

To control back pressure effectively, manufacturers can follow these best practices:


  1. Start with the manufacturer’s recommendations: Manufacturers often provide recommended back pressure ranges for specific materials. Begin with these guidelines and make adjustments as needed based on your specific application.


  1. Gradually increase or decrease back pressure: Make incremental adjustments to the back pressure settings to find the optimal level. Monitor the results closely, considering factors such as resin quality, part appearance, and production efficiency.


  1. Consider material properties: Take into account the viscosity, melt temperature, and flow characteristics of the material being processed. Adjust the back pressure accordingly to achieve the desired flow, mixing, and part quality.


  1. Regularly monitor and fine-tune: Continuously monitor the injection molding process and observe the molded parts for any signs of defects or inconsistencies. Make necessary adjustments to the back pressure settings to optimize performance.


Best Practices for Back Pressure Control

To ensure effective back pressure control, consider implementing the following best practices:


  1. Conduct material testing: Before full-scale production, perform material testing to determine the ideal back pressure for each specific material. This helps identify any material-specific requirements or limitations.


  1. Optimize screw design: Work closely with screw designers to develop screw profiles that facilitate efficient resin mixing and reduce the need for excessive back pressure.


  1. Maintain machine cleanliness: Regularly clean and maintain the injection molding machine to prevent contamination that could affect the back pressure control and overall process stability.


  1. Train operators and technicians: Provide comprehensive training to operators and technicians on the importance of back pressure control and the correct adjustment procedures. This ensures consistent and accurate settings during production runs.


Common Myths and Misconceptions

Several myths and misconceptions exist surrounding back pressure in injection molding. Let’s debunk some of them:


  1. Higher back pressure always improves part quality: While back pressure is essential, excessive pressure can lead to negative outcomes such as degradation, wear, and increased cycle times. It is crucial to find the right balance for optimal results


  1. Back pressure affects cycle time significantly: Back pressure does influence cycle time, but other factors such as injection speed, cooling time, and mold design also play significant roles. A holistic approach to process optimization is necessary.


  1. Back pressure should always be set to the maximum: Setting back pressure to the maximum does not guarantee better results. Each material and application has specific requirements, and excessive pressure can lead to issues.


Back Pressure in Different Injection Molding Machines

Different types of injection molding machines may have variations in back pressure control mechanisms. It’s essential to understand how back pressure is adjusted in your specific machine to optimize its performance. Consult the machine manufacturer’s guidelines and documentation for detailed instructions.


Back Pressure and Material Selection

The selection of the appropriate material for injection molding is crucial to achieve the desired properties and product quality. Different materials have different viscosity, flow characteristics, and processing requirements, which directly impact the ideal back pressure setting. Consider the material’s melt temperature, viscosity range, and processing guidelines when determining the optimal back pressure.


Troubleshooting Back Pressure Issues

If you encounter back pressure-related issues during injection molding, consider the following troubleshooting steps:


  1. Insufficient back pressure:

   – Increase the back pressure gradually and monitor the resin mixing and part quality.

   – Check the screw design and ensure it is appropriate for the material being processed.

   – Verify that the return throttle and AC servo valve (in electric machines) are functioning correctly.


  1. Excessive back pressure:

   – Decrease the back pressure gradually and observe the impact on part quality.

   – Check for any obstructions or wear between the screw and barrel that could increase resistance and require higher back pressure.

   – Ensure the return throttle or AC servo valve is not causing unnecessary pressure buildup.


Industry Applications and Examples

Back pressure control is critical in various industries that utilize injection molding, including automotive, electronics, consumer goods, and medical devices. In the automotive sector, for instance, maintaining precise back pressure ensures consistent part quality, minimizing defects and optimizing performance. Similarly, in medical device manufacturing, accurate back pressure control is vital to ensure product safety and reliability.



Back pressure is a crucial parameter in injection molding that affects the quality, dimensional stability and appearance of plastic parts. By understanding its definition, function, and effects, manufacturers can optimize back pressure settings to achieve high-quality products consistently. Implementing best practices, monitoring process parameters, and troubleshooting any issues will result in improved efficiency, reduced defects, and increased customer satisfaction.


FAQs (Frequently Asked Questions)


Q1. Can back pressure be adjusted during the injection molding process?

Yes, back pressure can be adjusted during the injection molding process. Manufacturers can monitor the resin mixing, part quality, and other parameters to determine the optimal back pressure level.


Q2. Is it necessary to adjust back pressure for every material used in injection molding?

Yes, different materials have varying viscosities and processing requirements. Adjusting back pressure for each specific material ensures proper resin mixing, density, and part quality.


Q3. Can excessive back pressure cause damage to injection molding equipment?

Yes, excessive back pressure can increase wear and tear between the screw and barrel, potentially causing damage to the metering equipment. It is essential to find the optimal back pressure level to maintain equipment integrity.


Q4. How often should back pressure be monitored and adjusted?

Back pressure should be regularly monitored during the injection molding process. Adjustments may be required based on factors such as material changes, part design variations, and evolving process requirements.


Q5. Is back pressure the only factor that influences part quality in injection molding?

No, back pressure is just one of several factors that impact part quality in injection molding. Other factors include temperature, injection speed, cooling time, mold Design, and material properties. A comprehensive approach is necessary to achieve optimal results.

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