Tonnage factor in injection molding is a material-specific coefficient used to calculate the clamping force (tonnage) required by the mold.
Its value depends on the resin properties, component design and process conditions.
The right choice can ensure the integrity of the mold, the quality of components and cost-effective production.
How to calculate the tonnage of an injection molding machine
The calculation method of the tonnage factor for injection molding machines involves several factors.
Including product size, material type, structural complexity, and safety factor.
Here are the detailed calculation steps:
- Determine Base Ton
- Material Type Coefficients
- Structure Complexity Factor
- Empirical Formula
- Clamping force calculation
- Safety factor
- Final calculation
Determine Base Ton
Determine a base tonnage based on the size and weight of the product.
This step is achieved by estimating the clamping force required for the product during the molding process.
Material Type Coefficients
Different material types have different coefficients.
These coefficients are usually provided by the injection molding machine supplier or can be found in the relevant information.
For example, some materials may require higher clamping forces to ensure molding quality.
Structure Complexity Factor
Determine an appropriate factor based on the structure and complexity of the product.
In general, more complex products require higher coefficients.
Empirical Formula
Use an empirical formula to calculate the required tonnage.
The formula is: Tonnage = Tonnage/Clamping Force Constant x Surface Area x Safety Factor.
The projected area is the two-dimensional area of the part, which is obtained by multiplying the length and width of the part.
Clamping force calculation
Clamping force = average pressure per unit area of the forming space ×coefficient of the projected area of the forming space.
The average pressure per unit area of the forming space varies depending on the injection mechanism.
And is about 400-500 kg/cm for the plunger type and 250-350 kg/cm for the screw type.
Safety factor
The safety factor takes into account potential variations or uncertainties in the molding process.
Ensuring that the manufacturing process is reliable and resilient.
Generally, the clamping force of the injection molding machine is 1.17 times larger than the actual clamping force of the product.
To ensure the stability of production and the safety of the load of the injection molding machine.
Final calculation
All of the above factors are substituted into the formula to calculate the required tonnage of the injection molding machine.
For example, if the projection area is 25 square inches and the injection pressure is 15000 PSI, then tonnage = (25 x 15000) / 2000 = 187.5 tons.
Factors affecting tonnage of an injection molding machine
- Material viscosity and fluidity
- Injection pressure/rate
- Part geometry
Material viscosity and fluidity
High-viscosity materials (such as PC and ABS) require a higher tonnage (3-8 tons per square inch) due to greater resistance during the filling process.
Low-viscosity materials (such as PP and PE) require a lower tonnage (2-4 tons per square inch).
Crystalline resins (such as nylon) typically require 3 to 5 tons per square inch.
Amorphous resins (such as PS) require 1 to 3 tons per square inch.
Injection pressure/rate
Higher injection pressure or faster filling rate will increase the demand for tonnage.
Part geometry
Complex parts with thin walls, ribs or complex features require a higher tonnage to prevent deflection.
Why is injection molding tonnage important?
- Defects of injection-molded products
- Pressure loss
- Wear of the mould
Defects of injection-molded products
When the mold is clamped, the cavity will be filled with air that needs to be released.
For this reason, mold manufacturers will install a small passage on the parting line, which is called an exhaust port.
These exhaust ports are not very deep, averaging only about 0.001 to 0.0015 inches.
Therefore, be careful not to press them tightly with excessive force.
If the applied force exceeds the required amount, the air will be trapped, resulting in a defect known as charring.
This charring is caused by the compression of air at the flow front.
As the air compresses, it will heat up to the point where the material begins to burn.
Pressure loss
If an overly large tonnage is used to close the vent hole, it will increase the additional pressure required to fill the mold.
In short, every time the pressure required to fill the mold and push the plastic to the end of the cavity is increased, the volatility of the process is enhanced.
By maintaining an appropriate tonnage, we can minimize pressure loss in the mold to the greatest extent.
Wear of the mould
Another long-term problem with using excessive tonnage is the wear of molds.
Frankly speaking, the mold will wear out over time.
The tonnage of the mold will continuously squeeze the parting line, causing the parting line to wear out prematurely.
We have seen some molds that are so severely worn that when the tonnage drops to the normal level, flash will appear on the molds.
How to choose the right tonnage of injection molding machine
- Determine the product and plastic type
- Mould size and structure
- Calculation of clamping force
- Injection volume and screw diameter
- Injection speed and pressure
- The need for special equipment
Determine the product and plastic type
First, determine the type or series of injection molding machines based on the weight and size of the product, as well as the type of plastic used.
Different plastic types (e.g. PS, PE, PP, ABS, AS, PC) have different material pressure coefficients.
Which will affect the calculation of the clamping force.
Mould size and structure
Make sure that the size of the mould allows for production
And that the mould opening and ejection stroke is sufficient to remove the finished product.
The complexity of the mould and the material (e.g., metal) may require a larger tonnage.
Clamping force is one of the key factors in determining the tonnage of an injection molding machine.
The empirical formula is: clamping force (tons) = average pressure per unit area of forming space × projected area of forming space × coefficient × finished product length × finished product width.
In addition, the pressure coefficient of the material is also important.
Injection volume and screw diameter
Choose the right injection molding machine according to the injection volume of the product and the required screw diameter.
The injection amount is usually related to the weight of the product or the amount of injection molding.
while the screw diameter affects the injection pressure and speed.
Injection speed and pressure
Confirm whether the injection speed and pressure of the injection molding machine meet the product requirements.
High-speed injection and high-volume injection products often require larger tonnage injection molding machines.
The need for special equipment
For some special products, additional equipment such as pressure accumulators, closed circuits, etc. may be required to improve production efficiency.
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.
Haichen‘s advice on choosing tonnage for injection molding machine
In some cases, customers may need the manufacturer’s assistance in deciding on the specifications of the injection molding machine.
Or even providing samples or ideas to ensure the most suitable model is selected.
Haichen has dozens of different tonnage injection molding machines for customers to choose from and is a trusted business partner.
