How to Calculate the Right Press Size for Injection Molding

Custom Plastic Injection Molding ServicesTo accomplish the highly technical process of injection molding plastic components, injection molding machines exert an extreme amount of pressure. The mold is subjected to two strong opposing forces, holding the mold open and forcing it closed. The pressure that pushes the mold together is called the clamping force, which is measured by tonnage. The strong opposing pressures are needed to keep the mold together during and after the molding material is inserted, while the opening pressure needs to be strong enough to hold the mold open while injecting the molding material. Each of these pressure types must be exerted to a specific factor, which needs to be carefully calculated before the injection process begins.

What Is Tonnage?

The tonnage of an injection molding machine refers to the clamping force rate of the injection mold machine. For example, a 50-ton machine can produce a clamping force that is equivalent to 50 tons. Size is also important in injection-molded machines, as the larger the tonnage rate, the larger the machine. Tonnage is also determined in ton per square inch, and usually ranges from a 2 to 8-ton factor. It is extremely important to understand the required tonnage needed for a mold, which can be determined by a series of formulas.

Calculating Tonnage for Injection Molding

To calculate the correct tonnage for injection molding, there are three critical steps: surface area, melt flow, and part depth dimension. Finding the correct tonnage is critical in the molding manufacturing process, as using the wrong machine and clamp rate can damage the final part or product.

  • Surface area: The first step to calculate the tonnage is to determine the surface area. This is done by measuring the length and width of the mold’s cavity, and multiplying the two together. If there are multiple cavities of the same size in the mold, multiply the number of cavities by the number of cavities. After the surface area is found, the tonnage factor is multiplied to the area. For precaution, a 10% safety factor should be included, to ensure that the pressure is not too strong.
  • Melt flow: After finding the surface area, the next step is to calculate the melt flow, by using the Melt Flow Index (MFI). The MFI rates material’s viscosity rate, which has an inverse relationship with the MFI. The higher the viscosity, the lower the flow rate, and the lower the viscosity, the higher the flow rate.
  • Part depth dimension: The final step to determine the correct tonnage is the depth of the part. If the dimension of a part is one inch, a factor of 10% clamping force must be added. An additional 10% is added for every inch about the initial one inch.

Dangers of Excess Tonnage

Using an injection molding machine that exceeds the pressure rate that is required can be dangerous to the product, machine, and mold. Excess tonnage can result in the problems of viscosity and flash. Injection molds need to have a very specific viscosity, as too thick or thin flow can result in an inoperable part. Flashing can also occur when the wrong tonnage pressure is applied, which results in an unwanted excess of material on the edges of the part. Using too little clamping force can also cause production problems, but an excess tonnage has the potential to be more destructive.

<Learn more in our Plastic Injection Molding 101 guide.>

Contact Plastic Design International Inc. for Your Injection Molding Needs

At Plastic Design International Inc., we specialize in the production of a wide range of products, many of which are produced by our accurate and reliable injection molding machines. Our dedicated and knowledgeable staff has the expertise to produce our clients required part requirements, manufactured with the correct clamp force to ensure the highest quality possible. Request a quote to begin the process of receiving your expertly crafted injection mold product. Contact us with any further questions about the injection molding process, and how it can benefit your company today.

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