Injection Molding Tonnage Calculation: How Clamping ...

Author: Daisy

May. 13, 2024

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Injection Molding Tonnage Calculation: How Clamping ...

Injection molding is a complex, dynamic system with multiple, interacting factors—all of which impact performance, cost, and quality. One of these factors is the injection molding machine tonnage calculation, also known as the clamping force.

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Let's start with the definition of tonnage. This is the measure of force that's required to keep the tool closed during the injection process, and it determines the injection molding machine size required for producing the part. This may seem simple, but a lot is riding on an accurate calculation of tonnage, which is why working with an experienced injection molder is critical to producing complex plastic components.

Impact of Improper Tonnage CALCULATION

A clamping force that is too low likely leads to inconsistent results or defects, including variable size and weight, flash, short shots, wall thickness variations, and irregular surface finish. Too much clamping force reduces the life of a tool by adding unnecessary wear and tear, which ultimately increases production costs to rebuild the tool more frequently.

Machines that deliver high clamping forces are more expensive than machines that deliver lower clamping forces, and a less experienced or commodity molder may not have the right-size machine for your project. Plastics engineers at a custom injection molder can accurately calculate tonnage, ensuring the proper machine is used and eliminating the risk for overclamping.

Cutting Costs with Tonnage Calculation

A tonnage calculation is often presented as a range (for example, from 550 tons to 750 tons for a single-cavity tool). An injection molder can certainly save money by purchasing a smaller machine that can deliver the lowest clamping force in the range and claiming it will do the job just fine. However, if the optimum tonnage calculation is actually in the mid-to-upper part of the range, this increases the risk to you, the manufacturer, that your plastic parts will have flash. Flash is when the tool opens slightly during injection because the machine lacks sufficient clamping force to produce a flash-free part.

Instead of using an injection molding machine tonnage calculator, many molders often estimate tonnage by using a “rule of thumb” equation that involves the area of the part, type of plastic, viscosity, and a “clamp factor” that ranges from 2 to 8. Depending on the part and type of material, some molders simply go with an “average” clamping factor of 5 to calculate the tonnage.

This approach can result in an inaccurate tonnage calculation that is too small (resulting in quality problems and delays) or is too large (resulting in increased mold costs and premature deterioration).

A more accurate way to determine the injection molding machine tonnage calculation is through mold filling analysis, which removes much of the guesswork from the calculation. It’s been our experience that using mold filling analysis to determine the tonnage factor, and then adding a “safety factor” of about 15 percent, results in a highly accurate clamping force and flash-free parts.

Mold Filling Analysis' Effect

Mold filling analysis is also an effective method for determining the impact of other production variables, such as number of gates (more gates will lower tonnage requirements) and flow length (longer flow lengths require higher pressures and therefore a higher tonnage). Using mold filling analysis early in the design process also helps determine the best gating scheme for the mold.

Be cautious of the flow simulation analysis software you use, as it may only provide the pressure needed to fill the part; it may not always include the pressure during the pack phase. This can be a drawback. Tonnage requirements are also a function of the pressure required to fill or pack the part. It is often easy to fill the part when using an easy-flowing material like nylon. In these cases, flow simulation analysis software will show low pressure. However, this cannot be used for the tonnage calculation because the higher packing pressure still needs to be applied to eliminate sink. If the packing pressure is higher than fill pressure, it must be accounted for in the tonnage calculation.

There are many variables that go into calculating an accurate tonnage that can heavily impact the quality and profitability of your plastic parts. Having the right injection molding partner is key to ensure these calculations are measured correctly to set your project up for long-term success. To learn more about achieving project success by working with an experienced molder, read our eBook, 5 Steps to Get New Injection Molded Products to Market Faster.

How do you calculate injection mold tonnage? -

Injection molding tonnage refers to the amount of pressure required to keep the mold in place during the injection process. This value is usually measured in tons and can be calculated using a simple formula.

The first step is to determine the projected area of the mold. This can be done by measuring the length and width of the mold and multiplying these values together.

Once you have the projected area, you will need to multiply this number by 0.69. This will give you the clamping force calculation to keep the mold from being opened during the injection molding process.

Finally, you will need to multiply this value by 2.5 to account for safety. This will give you the total tonnage required for your injection molding project.

By using this simple formula, you can ensure that your mold can withstand the stress of injection without failure.

What is the tonnage of an injection molding machine?

The tonnage of an injection molding machine capable is the amount of force that the machine can apply to the mold during the injection molding process.

This is usually measured in tons, and the tonnage of the machine will determine the size and type of product it can produce.

The higher the tonnage, the more force the machine can apply and therefore the larger and more complex the product it can produce.

Injection molding machines can range in size from small machines capable of producing relatively simple products to large machines capable of producing very complex products.

The size and complexity of the product being produced usually determine the size and tonnage of the injection molding machine required.

The tonnage of an injection molding machine is the maximum force that the machine can apply to a specific material. This is usually measured in pounds or tons.

In general, the greater the tonnage, the greater the force the machine can exert and the larger the object it can produce.

It is important to note, however, that not all materials require the same amount of force to be injection molded. For example, softer plastics may only require a few tons of pressure to mold, while harder plastics may require several hundred tons.

For this reason, injection molding machines are often classified by their tonnage capacity. The most common capacities are 100 tons, 150 tons, 200 tons, 250 tons, 300 tons, and 400 tons.

There are also larger machines that are capable of exerting forces of up to 1,000 tons. These machines are typically used in industrial applications and can produce extremely large products.

What is the formula for calculating tonnage?

Calculating required injection pressure for a particular injection molded part, you must first know the volume required for the part and the maximum safe pressure that the molding machine can apply to the molten plastic. With this information, the formula is relatively simple to calculate.

Tonnage = Volume * Maximum Safety Pressure

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For example, if the required volume of a part is 100 cubic centimeters and the maximum safe pressure that can be applied to the molten plastic is 10,000 psi, then the required tonnage is 1,000 tons.

It is important to note that this formula is only an estimate, as many other factors can affect the required tonnage, such as material properties, rotor system losses, and thermal expansion.

Therefore, it is always best to consult with an experienced injection molders to determine the exact tonnage in injection required for a specific application.

Normally, 5 tons per square inch is used for calculating the clamping tonnage for any given mould. In this example, the projected area 36 square inch multiplied by 5 Tons per square inch(36 X 5) results in 180 tons.

Always add a safety factor of 10% of the tonnage – (180 Tons +18 Tons = 198 Tons). Hence a 200-ton clamping force machine is the best choice for injection moulding this component.

How to choose the tonnage of the injection molding machine?

The maximum clamping force is expressed as the maximum value of the clamping mold. The material pressure in the injection molding space (cavity) acts in the direction of opening the mold, so it must be below the clamping force, usually, the material pressure must be less than 80% of the clamping force.

The material pressure is the product of the parallel pressure per unit area of the forming space multiplied by the projected area of the forming space.

The average pressure per unit area of forming space varies depending on the injection mechanism, about 400-500㎏/㎝ for plunger type and 250-350㎏/㎝ for screw type.

Empirical formula

Clamping force = average pressure per unit area of forming space × projected area of forming space × coefficient = 330kg/cm × length of finished product × width of finished product × 1.5

Note:

a. The unit of finished product length and width is cm;

b. The unit of calculated clamping force is kg, divided by 1000 is the tonnage of the injection machine (T);

c. The projected area of the finished product is the projected area of forming space, such as box type finished product, the length, and width of the finished product should be measured after the four sides of the finished product are expanded.

After calculating the required injection molding machine tonnage, Select the size injection molding machine according to the calculation result (the clamp tonnage of the selected injection machine must be larger than the calculation result).

The height of the injection mold must be between the maximum and minimum mold thickness of the selected injector.

How to calculate the tonnage of plastic mold?

Determine the injection volume

Assume the theoretical weight of the part is 80g. Calculate the maximum injection volume of the product screw barrel selection range between 80/0.75=107g to 80/0.25=320g. Select the range of injection moulding machine screw barrel sizes between 107~320.

Determine the clamping tonnage

Calculate the clamping tonnage = (product positive projection area (cm) * beer number + runner area) * material viscosity coefficient material viscosity coefficient: general PP 0.3~0.4 PA\PC\POM 0.65~0.75.

Determine the size of the clamping mechanism

According to the size of the mold frame, it mainly includes concave mold with variable cavity composed of concave mold combination substrate, concave mold assembly and concave mold combination card plate.

And convex mold with a variable core composed of convex mold combination substrate, convex mold assembly, convex mold combination card plate, cavity cut-off assembly, and side cut-off combination plate.

Other factors affecting the injection molding machine tonnage

Other factors affect the tonnage calculation, such as the material factor and melt factor.

The material factor takes into account the physical properties of the material being molded, such as its melting point and viscosity. Viscosity of plastic materials involves higher flow materials and lower flow materials.

The melt factor takes into account the fact that not all materials will be converted into usable products.

Ultimately, the goal is to find an injection molding machine with sufficient tonnage to fill the cavity without causing damage to the mold or requiring excessive cycle times.

Summary

Injection tonnage factor is directly related to mold size and mold product weight but may vary depending on the type of material being injected and the size and shape of the mold.

Generally, the tonnage of the injection molding machine will be noted in the price calculation of the injection molding project, if there is anything you do not understand, you can also consult ZetarMold for related questions.

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