Understanding Injection Molding Tonnage, Key Factors and Importance

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Injection molding tonnage is one crucial aspect of the injection molding process, which refers to the clamping force applied to the mold during the molding process. Tonnage plays a vital role in ensuring the successful production of quality parts. Adequate tonnage is achieve proper mold closure and maintains uniform pressure during the injection process.


Several factors influence the tonnage requirements in injection molding. These factors include the part size and geometry, material type and viscosity, mold design, and the desired part quality. Understanding these factors is crucial for determining the appropriate tonnage needed to achieve optimal mold performance and part quality.


In the following sections, we will delve deeper into these factors and explore how they influence tonnage requirements in injection molding. By understanding the importance of tonnage and its influencing factors, manufacturers can make informed decisions to ensure successful and efficient production processes.

What is Tonnage in Injection Molding?

Define of Injection Molding Tonnage

Tonnage in injection molding refers to the clamping force applied to the mold during the molding process. It is a measure of the force required to keep the mold closed and withstand the pressure exerted by the molten plastic material during injection.

Measure of Injection Molding Tonnage

Tonnage is typically measured in tons, where one ton is equivalent to 2,000 pounds or 907 kilograms. The tonnage rating of an injection molding machine indicates the maximum clamping force it can generate.

Tonnage vs Clamping Force

The clamping force is the force applied to the mold halves by the machine’s clamping unit to keep the mold closed during injection and cooling. It is directly related to the tonnage and is determined by factors such as the part size, material properties, and mold design.


Higher tonnage machines are capable of exerting greater clamping forces, allowing for the production of larger and more complex parts. On the other hand, smaller parts may require lower tonnage machines.


The relationship between tonnage and clamping force is not linear. It depends on various factors, including mold design, part geometry, material viscosity, and required part quality. It is crucial to determine the appropriate tonnage for a specific injection molding application to ensure proper mold closure, part quality, and manufacturing efficiency.


In the next sections, we will explore the factors that influence tonnage requirements in injection molding and the significance of selecting the right tonnage for successful production processes.

Factors Affecting the Injection Molding Tonnage Requirements

Several factors influence the tonnage requirements in injection molding. Understanding these factors is essential for determining the appropriate tonnage for a specific molding project. Here are some key factors to consider:

Part Size and Complexity:

Larger and more complex parts typically require higher tonnage to withstand the forces exerted during injection. The size and shape of the part, including features such as ribs, bosses, and thin walls, can impact the tonnage needed for proper mold closure.

Material Properties:

Different materials have varying flow characteristics and viscosity, which can affect the required tonnage. High-viscosity materials or those with high melt flow rates may require higher tonnage to ensure complete filling of the mold cavity.

Mold Design and Layout:

The mold design, including the number and configuration of cavities, affects tonnage requirements. Multi-cavity molds may require higher tonnage to accommodate the increased material flow and pressure. Multi-cavity mold has multiple cavities, allowing for the production of multiple identical parts in each injection cycle.

Wall Thickness and Flow Characteristics:

Thicker walls or areas with restricted flow paths may require higher tonnage to overcome resistance and achieve proper filling. Parts with intricate geometries, such as thin ribs or complex curves, may also require higher tonnage for effective molding.

Injection Pressure and Injection Rate:

The injection pressure and rate at which the molten material is injected into the mold influence tonnage requirements. Higher injection pressures or faster injection rates may necessitate higher tonnage to ensure proper mold filling and part quality. Click here to understand what is injection pressure.


It is important to evaluate these factors during the design and planning stages of an injection molding project to determine the appropriate tonnage. Working closely with mold designers and process engineers can help optimize the tonnage requirements for efficient and successful production processes.

Injection Molding Tonnage Calculating

Determining the required tonnage for an injection molding project involves considering various parameters and using a specific formula. Here is an overview of the calculation process:

Injection Molding Tonnage Calculation Formula

The commonly used formula for calculating tonnage requirements is:


Tonnage = (Projected Area of the Part) × (Injection Pressure) / (Safety Factor)


Key parameters for calculating the tonnage of the injection molding

  • Projected Area of the Part: This refers to the total area of the part that is projected onto the mold parting line. It is typically measured in square inches or square centimeters.


  • Injection Pressure: The injection pressure is the force required to inject the molten material into the mold cavity. It is measured in pounds per square inch (psi) or megapascals (MPa).


  • Safety Factor: The safety factor is a value used to account for uncertainties and variations in the process. It ensures that the calculated tonnage is sufficient to avoid issues such as flashes or short shots. The safety factor typically ranges from 2 to 4, depending on the specific application and material.

Injection Molding Tonnage Calculation Examples

To illustrate the calculation process, let’s consider two examples:


Example 1: A simple rectangular part with a projected area of 20 square inches and an injection pressure of 20,000 psi. Assuming a safety factor of 3, the tonnage calculation would be:


Tonnage = (20 square inches) × (20,000 psi) / (3) = 133,333.33 pounds or approximately 67 tons


Example 2: A more complex part with irregular geometry and a projected area of 35 square inches. The injection pressure is 15,000 psi, and a safety factor of 2 is used. The tonnage calculation would be:


Tonnage = (35 square inches) × (15,000 psi) / (2) = 262,500 pounds or approximately 131 tons


These examples demonstrate how the projected area, injection pressure, and safety factor are used to calculate the required tonnage for injection molding.


By considering the projected area, injection pressure, and safety factor in the tonnage calculation, manufacturers can determine the appropriate tonnage requirements for successful injection molding operations.

Effects of Injection Molding Tonnage Selection

Selecting the proper tonnage in injection molding is crucial for achieving optimal part quality, avoiding process issues, and maximizing cost-effectiveness. Here are the key reasons why proper tonnage selection is important:

Consequences of inadequate Tonnage:

Inadequate tonnage can result in various quality issues such as sink marks, warpage, or flash. Insufficient pressure may not fully fill the mold cavity, leading to incomplete or defective parts. By choosing the right tonnage, the material can be properly compressed, ensuring consistent and high-quality parts.

Consequences of Excessive Tonnage:

Excessive tonnage can lead to unnecessary wear and tear on the machine, molds, and components. It can also result in overpacking the material, causing stress and potential part damage. Moreover, excessive tonnage increases energy consumption, prolongs cycle times, and raises production costs.

Optimization for Cost-Effectiveness:

Selecting the appropriate tonnage helps optimize the injection molding process for cost-effectiveness. Using excessive tonnage not only incurs higher production costs but also requires larger, more expensive machinery. On the other hand, inadequate tonnage may lead to more frequent mold maintenance, rework, and increased scrap rates. Optimizing tonnage ensures efficient material usage, shorter cycle times, and lower overall production costs.


By carefully considering the tonnage requirements for each injection molding project, manufacturers can achieve consistent part quality, avoid unnecessary expenses, and optimize the overall production process. Proper tonnage selection is a critical factor in ensuring successful and cost-effective injection molding operations.


In conclusion, tonnage plays a crucial role in injection molding, impacting part quality and production efficiency. Factors such as part size, material properties, and mold design influence tonnage requirements. Proper tonnage selection is essential to avoid defects and optimize cost-effectiveness.


Zhongde is significant in calculating injection molding tonnage. With our expertise and advanced technology, we can accurately determine the required tonnage for specific projects, ensuring optimal part quality and efficiency. Choose Zhongde for your injection molding needs.

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