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How to Calculate Injection Molding Machine Tonnage?

Apr 24.2023

injection molding machine tonnage

For your ongoing injection molding business, determining the proper tonnage for your injection molding machine is critical to producing high-quality parts. But figuring out the correct tonnage can be a challenge with so many factors in mind. In this article, we will guide you through the process of calculating the tonnage of your injection molding machine to ensure your production runs smoothly and efficiently.

1.What is the Tonnage of an Injection Molding Machine?

Injection molding machines are used in the manufacturing process of plastic products. They are designed to melt plastic pellets and inject them into a mold cavity to create a specific shape. One of the most important aspects of an injection molding machine is its tonnage, which refers to the clamping force of the machine.


2.The Importance of Calculating the Tonnage of the Injection Molding Machine

  • Determining the Optimal Production Capacity

Determining the appropriate tonnage of an injection molding machine is a pivotal step in ensuring the production capacity is optimized. The tonnage must be carefully calculated since if the tonnage is too low, the machine will be unable to produce the desired output, but if it's too high, it will lead to oversizing of the machine and an unnecessary rise in cost.

  • Matching the Machines to Specific Applications

The calculation of tonnage is also essential for matching the injection molding machine to specific applications. Each product necessitates a specific tonnage, and the usage of an improper tonnage can result in defective products and squandered resources.

  • Efficiency and Cost Savings

Last but not least, computing the tonnage of an injection molding machine can lead to cost savings and efficiencies. Manufacturers can curtail waste, optimize production time, and minimize downtime by utilizing the correct tonnage.


3.Key Parameters for Calculating the Tonnage of the Injection Molding Machine

  • Die Size

The size of the die is one of the key parameters for calculating the tonnage of an injection molding machine. The die is the part of the mold that shapes the plastic material into the desired shape. The larger the die, the higher the tonnage required to produce the desired product.

  • Part Size and Weight

The size and weight of the part being produced is another important parameter for calculating the tonnage of an injection molding machine. Larger and heavier parts require higher tonnages to produce.

  • Material Type

The type of material being used is also important for calculating the tonnage of an injection molding machine. Different materials have different characteristics, and some may require higher tonnages than others.

  • Clamping Force

The clamping force of the injection molding machine is another critical parameter for tonnage calculation. Clamping force refers to the force required to keep the mold closed during injection. Higher clamping forces require higher tonnages.

  • The Melt Flow Rate

The melt flow rate is the rate at which the plastic material flows through the injection molding machine. It is a critical parameter for tonnage calculation, as higher melt flow rates require higher tonnages.

  • Injection Pressure

Finally, the injection pressure is the pressure at which the plastic material is injected into the mold cavity. It is another critical parameter for tonnage calculation, as higher injection pressures require higher tonnages.


4.How to Calculate Injection Molding Machine Tonnage

  • Experience Formula 1:

    Clamping force (T) = Clamping force constant Kp * Product projected area S (CM * CM) * Safety factor (1+10%)

    To correctly calculate the necessary clamping force, you must first know the projected area of the molded part plus any runners. The area can be calculated by multiplying length by width, A = L x W, or A = πr2. The above clamping force experience formula is often used for quick calculations, which can have a rough size. Most engineers even select a larger size from the result. There is still a risk for products with complex structures or large thickness changes in different positions or many vertical runners/corners.

    Kp empirical values in square centimeters:

    • PS/PE/PP: 0.32;
    • ABS: 0.30-0.48;
    • PA: 0.64-0.72;
    • POM: 0.64-0.72;
    • Adding glass fiber: 0.64-0.72;
    • Other engineering plastics: 0.64-0.8;

    For example, if the projected area of a product is 410 cm2 and the material is PE, how to calculate the clamping force during injection molding?

    Calculate according to the formula: P=KpS=0.324101.1=141(T), it is recommended to choose an injection molding machine of 150-170 tons.

    Kp empirical values in square inches:

    Most injection molding companies set the surface square inches of the part to 2.5 times (for higher fluidity materials such as PS, PE and PP) or 4 times (for lower fluidity materials such as PA, PC and PSU), and add an additional 10% as a safety factor.

    If you have a part with a surface area of 100 square inches, you need an injection molding machine with a clamping force of 250 tons for injection molding.

    After adding a 10% safety factor, the required press size is 330 tons clamping force. Then your project will require an injection molding machine of 330-350 tons.

  • Experience Formula 2:

    Clamping force (T) = Material molding pressure * Product projected area S (CM * CM) * Safety factor (1+10%) 350bar

    S (cm^2) / 1000 * (1+10%)

    Similarly, 350410/10001.1=160T, choose a 160T injection molding machine.


  • Precision Calculation Formula

    There are two important factors for calculating clamping force:

    1. Projected area
    2. Cavity pressure.

    The projected area (S) is the maximum area seen in the line of sight of the opening and closing mold.

    Cavity pressure measurement (P) is influenced by the following factors:

    Number and position of gates

    • Gate size
    • Product thickness
    • Viscosity characteristics of the material used
    • Injection speed
    1. Flow Characteristics of Thermoplastic Plastics

      Group 1: PS, PE, PP

      Group 2: PA6, PBT, PET

      Group 3: PU, Soft PVC

      Group 4: ABS, POM

      Group 5: PMMA, PC/ABS, PC/PBT

      Group 6: PC, PEEK, Rigid UPVC

    2. Viscosity Grade

      Each group of plastics has a viscosity (flow) grade. The relative viscosity grades of each group of plastics are as follows:

      Group Multiplier Constant (K)

      Group 1 x1.0

      Group 2 x1.3-1.35

      Group 3 x1.35-1.45

      Group 4 x1.45-1.55

      Group 5 x1.55-1.70

      Group 6 x1.70-1.90

    3. Cavity pressure is determined by wall thickness and the ratio of flow rate to wall thickness

      P = P0 x K (multiplier constant)

    4. Determination of Clamping Force (F)

      All of the above locking force calculation formulas require knowledge of how to calculate the injection molded projected area. Therefore, calculating the projected area of injection molding is the key to calculating the locking force.

      • Calculation of Process/Wall Ratio

        Process/Wall Ratio = Longest Melt Process/Thinnest Part Wall Thickness = 200mm/1.9mm = 105:1

      • Application of Cavity Pressure/Wall Thickness Curve

        Provides the relationship between cavity pressure and wall thickness and process/wall ratio. From the graph, it can be seen that for a 1.9mm cavity thickness and a flow/wall ratio of 105:1, the cavity pressure is 160Bar for Group 1 plastics. For other groups of plastics, the corresponding multiplication constant K should be used.

      • Determination of PC Cavity Pressure

        The flowability of PC belongs to the sixth group of viscosity grades. The viscosity of PC is 1.7-1.9 times that of Group 1, and the different viscosity is reflected in the mold cavity pressure. Therefore, the mold cavity pressure of the PC lamp holder should be 160Bar x K (viscosity grade of PC). P = 160 x 1.9 Bar = 304 Bar, and for safety, we take it as 1.9 times.

      • PC Lamp Holder

        The projected area value of S = π x (outer diameter of the lamp holder)^2/4 = 3.14 x 22 x 22/4 (cm^2) = 380cm^2

      • Clamping force of PC lamp holder

        F = P x S = 304 Bar x 380 cm^2 = 304 kg/cm^2 x 380 cm^2 = 115,520 kg or 115.5 tons, so a 120T~130T injection molding machine can be used.

        F = P x S = P0 x K x S

      All of the above locking force calculation formulas require knowledge of how to calculate the injection molded projected area. Therefore, calculating the projected area of injection molding is the key to calculating the locking force.

  • Use CAE software (MOLDFLOW, etc.) for precise calculation.

    The main criterion for classifying injection molding machines is the clamping force. Indeed, large machines have high clamping force while small machines have low clamping force. When you try to select a suitable injection molding machine for your project, the clamping force during injection molding is one of the most important factors.

    This is a traditional concept. It seems that the clamping force is the main indicator to measure the size of the machine. However, as the market becomes more segmented, the clamping force of the machine cannot completely evaluate the size of the machine. More and more specially optimized machines are emerging.

    For example, when I produce thick-walled products that require a large amount of melt, the clamping force needs to be small. This is different from the design concept of traditional machines. The rigidity of the equipment plate can be lower, the clamping force can be smaller, the screw can be thicker and deeper, the opening distance can be larger, and the opening and closing speed can be slower.

    However, when producing thin-walled products, a large clamping force, a small amount of melt, a small opening distance, and a short cycle time are required. At this time, the design is different. The rigidity of the equipment plate needs to be high, the clamping force needs to be large, the screw needs to be small, the opening distance needs to be small, and the opening and closing distance needs to be fast.

    For example, when I make PVC, PA, and PS products, the screw design has its own characteristics. The production of precision products requires high repeatability of equipment.

    Therefore, clamping force is the main parameter to measure the machine, but when evaluating the machine, other parameters need to be comprehensively evaluated.


5.Practical Application of Calculating the Tonnage of Injection Molding Machines

Calculating the weight of an injection molding machine constitutes a crucial stage in the injection molding process. The tonnage required for a particular project hinges upon a multitude of factors, including but not limited to the dimensions and mass of the part, the type of material employed, and the intended production output.

One pragmatic application of determining the weight of an injection molding machine lies in identifying the optimal production capacity. By evaluating the necessary tonnage, manufacturers can select a machine that accords with their particular production requisites. This ensures that the machine can bear the necessary workload, thereby sidestepping the pitfalls of underutilization or overloading.

Another pragmatic application entails aligning machines with specific applications. Distinctive projects may necessitate disparate tonnages, and aligning the machine with the project can heighten the quality and uniformity of the final product. For instance, smaller parts may necessitate machines with a lesser tonnage, while larger parts may call for those with a greater tonnage.

Calculating the tonnage of an injection molding machine can also result in cost savings and efficiencies. By choosing the right machine for a specific project, manufacturers can avoid unnecessary expenses associated with underutilization or overloading. Additionally, using the optimal tonnage can result in faster cycle times and reduced scrap rates, leading to improved efficiencies and profitability.


6.The Benefits of Correct Calculation of Tonnage

Correctly calculating the tonnage of an injection molding machine can result in numerous benefits for manufacturers. These benefits include:

  • Improved quality and consistency of the final product

  • Enhanced production efficiency and cost savings

  • Reduced scrap rates and waste

  • Extended machine lifespan

  • Improved safety and reduced machine downtime

By understanding and calculating the required tonnage for each project, manufacturers can optimize their production processes, resulting in improved product quality, increased profitability, and a competitive advantage in the market.


7.Disadvantages of Incorrect Calculation of Tonnage

On the other hand, incorrect calculation of tonnage can result in various disadvantages, such as:

  • Poor product quality and consistency

  • Increased scrap rates and waste

  • Reduced machine lifespan

  • Increased risk of machine failure and downtime

  • Safety hazards for operators and other personnel

Using a machine with incorrect tonnage can also result in increased operating costs and reduced profitability, as well as a negative impact on the manufacturer's reputation in the market.


8.Avoiding Misunderstandings in Tonnage Calculation for Injection Molding Machines

When calculating the tonnage required for an injection molding machine, there are several common misunderstandings that manufacturers should be aware of. These misunderstandings can lead to incorrect tonnage calculations, which can result in suboptimal production capacity, increased costs, and even part defects.

One common misunderstanding is the belief that the tonnage required for a given project can be determined simply by multiplying the projected part weight by a specific multiplier. While this method can provide a rough estimate of tonnage requirements, it does not take into account several key factors, such as material type, part size and shape, and other parameters that can affect the performance of the injection molding machine.

Another common misunderstanding is the assumption that a higher tonnage machine is always better for the job. While it is true that higher tonnage machines can handle larger and more complex parts, they also require more energy and can result in higher operating costs. Additionally, if the machine is significantly larger than what is required for the specific application, it may not operate efficiently, leading to reduced production capacity and increased part defects.

Finally, some manufacturers may assume that once a tonnage calculation has been made, it is set in stone and cannot be adjusted. In reality, the tonnage required for a given project can vary depending on several factors, such as material type, part geometry, and processing parameters. It is important for manufacturers to regularly review and adjust their tonnage calculations as needed, in order to ensure that their injection molding machines are operating at peak efficiency and delivering the best possible results.

By avoiding these common misunderstandings and taking a comprehensive approach to tonnage calculation, manufacturers can ensure that their injection molding machines are matched to the specific application, and that they are operating at optimal performance levels. This can result in greater cost savings, improved part quality, and a more competitive edge in the market.

injection molding machine

9.Injection Molding Machine Tonnage Optimization Technology

To ameliorate the tonnage of injection molding machines, manufacturers have at their disposal several strategies. These strategies comprise modifying both the mold and part designs, selecting the most suitable machine, refining processing parameters, and leveraging simulation software.

One efficacious approach to optimize tonnage involves meticulously crafting the mold and part designs with appropriate dimensions and wall thicknesses. This endeavor minimizes the required tonnage for a given injection molding undertaking, ultimately leading to considerable cost savings over time.

Another crucial factor in tonnage optimization is the selection of the most appropriate machine for the task at hand. The machine's tonnage rating should be commensurate with the specific application, and other parameters like injection speed and clamping force must be taken into account as well. The judicious selection of the machine can guarantee optimal production capacity while diminishing the probability of part defects and machine downtime.

Fine-tuning processing parameters also plays a pivotal role in tonnage optimization. Manufacturers can optimize their injection molding machines' performance and mitigate the risk of part defects and other issues by modifying parameters like injection pressure and melt flow rate.

Lastly, the use of simulation software represents a potent instrument for tonnage optimization. The software empowers manufacturers to model and test various injection molding scenarios, facilitating the optimization of their processes and the mitigation of errors and defects. In turn, this culminates in greater efficiency, cost savings, and better overall performance.



Manufacturers must consider the tonnage of their injection molding machines to maximize production capacity and minimize costs. Accurately calculating the required tonnage ensures that the machine is tailored to the specific application and operating at peak efficiency. Employing tonnage optimization strategies, such as modifying mold and part designs, selecting the appropriate machine, fine-tuning processing parameters, and utilizing simulation software, further enhances efficiency and cost savings. By adhering to best practices for tonnage calculation and optimization, manufacturers can sidestep common misconceptions, enhance their injection molding operations, and provide greater value to their customers.


Following the steps outlined in this article, you should now have a better understanding of how to calculate the right tonnage for your injection molding machine. Remember to consider all factors that can affect tonnage, including materials used, part design and production volume. With careful consideration and calculation, you can achieve consistent, high-quality results in your injection molding process.

If you are looking for injection molding machine suitable for your business, then you can contact us, we have extremely rich experience in injection molding machine production and injection molding solutions, we can help you.

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