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How to Calculate the Right Press Size for Injection Molding

To 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.

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.

Blow Molding vs. Injection Molding

Plastic molding is a manufacturing process used to produce a wide range of parts and products from plastic materials. Two of the most common plastic molding methods are blow molding and plastic injection molding. While both processes rely on similar principles—e.g., heating the plastic material and applying pressure—their end products are completely different; blow molded components are hollow, while injection molded components are solid.

The following blog post provides greater detail regarding the differences between the two processes.

What Is Blow Molding?

The blow molding process is similar to glassblowing. A tube of plastic is heated and pumped with air until it turns into a hot plastic balloon (i.e., parison). The parison is then enclosed within a product mold. Air continues to flow into the parison until it expands and conforms to the shape of the mold, forming the desired part or product. Once the component has cooled, it is ejected from the mold.

This molding method has few engineering applications. It is generally used to create hollow and thin-walled components, such as bottles, cans, tanks, and other containers, suitable for use in normal operating and environmental conditions. The one-piece construction of these components eliminates the need for subsequent assembly processes, resulting in simple and fast production operations. These qualities, combined with relatively cheap machinery, result in low overall production costs.

During blow molding operations, it is important to keep in mind quality control measures. As many blow molded components feature thin walls, designers and engineers must identify the right wall thickness to ensure the component has the structural shape and integrity needed for the application. Some of the problems that may arise during operations include wall thinning, air leakage, streaks, and flash.

What is Injection Molding?

The injection molding process relies on precisely engineered molds and tooling. During operations, liquid plastic is injected into the mold (typically made from aluminum or stainless steel) at high pressures. Once the material is sufficiently cooled, it maintains the shape of the mold and, consequently, the desired part or product. At this point, it is ejected from the mold.

This molding method is used to create solid components. It is ideal for fulfilling high-volume orders of small and precise plastic pieces, such as handles, housings, and toys.

Compared to the blow molding process, the injection molding process accommodates more complex designs. Additionally, it can be used to create parts and products for demanding applications (e.g., high temperatures or stresses). For these situations, the components are generally made from engineering-grade plastics, such as ABS, glass-filled nylon, and HTPE.

Contact the Plastic Molding Experts at Plastic Design Today

Both blow molding and injection molding play a critical role in the manufacture of a wide range of plastic parts and products. If you’re looking for an injection molding partner for your next project, turn to the experts at Plastic Design today.

At Plastic Design International, Inc., we’ve provided custom plastic injection molding solutions for over 40 years. We utilize state-of-the-art technology, including automated conveyors, robotic sprue pickers, and CMM equipment, and maintain ISO certification to ensure the molded products we deliver fully meet our customers’ specifications and standards.

To learn more about our injection molding capabilities or discuss your project requirements with one of our representatives, contact us or request a quote today.