How to Calculate Tonnage in Injection Molding

There are mainly three methods to calculate tonnage in injection molding:

  • Standard method

  • Wall thickness x Maximum flow

  • Thumb rule method

First, we need to understand what tonnage is and why we must calculate tonnage for an injection mold.

Tonnage is the size or weight of the plastic product calculated in tons. Now, is it required because of the presence of the clamping force. Clamping force is the force on the mold towards the opening direction received from injected resin. If the clamping force is less, we get a burr on the parting line. If the clamping force is more, then machine life will be more. Both machine life and mold life are directly proportional to the clamping force.

For a better clamping force, few methods are considered for a molding machine. They include:

  • Shot Capacity

  • Mold Size Feasibility

  • Mould Clamping.

The following parameters are required to calculate the clamping force:

  • The projected area of the part (multiplied by the number of cavities if present more than one) + projector area of the runner (in square centimeters.)
    Projected Area means the area of the shadow of the projected part. Therefore, if we have following inputs, like, length= 8 cm, width= 5 cm, number of cavities = 2 and projector area of runner is 10 percent of the actual projector area, then, we have total projected area= total number of cavities x (Length x Width) + projector are of runner= 2x {(5x8) + 10/100x (5x8)} = 44x2=88 square centimeters.

  • Opening pressure of resin (Can be referred from the chart, explained further): The standard pressure for any raw material is the amount of pressure applied by a specific material on the given projected area. The specific pressure which has been fixed as standard is called effective pressure and is 15000 PSI. When we convert this quantity into kg /centimeter square, we get 1054.604 kg/ centimeter square. We can select the opening pressure as per the following chart based on the type of material used.

  • For the calculation of clamping force, we have the formula:
    Clamping force= effective pressure x projected area= 15000x88 kg/square centimeters converted into tons.
    The amount of pressure exerted by a material due to its projection area is called tonnage, and this pressure is the clamping force in tons.

  • Set limit FOS: Finally, the factor of safety is applied to the clamping force for better mold life. We should always consider 20 percent more clamping force or clamping tonnage than calculated mold tonnage. Also, clamping tonnage should never be five times greater than mold tonnage. This is important to maintain the safety factor as it is suitable for mold and the machine.


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