With the use of injection molding presses and high-performance resins, plastic injection molders are able to craft a broad range of quality custom parts. Just as all resins are unique, these sophisticated machines offer varied capabilities and performance options depending on press size and type.

To ensure optimal quality and value, it’s critical to partner with a manufacturer who can provide the right size press for your specific needs.

How Are Plastic Injection Molding Presses Rated?

An experienced plastic injection molder can help you select the correctly sized machine for your project, and will be able to provide you with an accurate size estimate right from the start based on a few key pieces of information.

Presses are rated, or classified, based on tonnage, which indicates how much clamping pressure a particular machine can offer. Press tonnage, or force, can range from less than 5 tons to over 4,000 tons. The higher a machines tonnage is rated, the larger it is.

Many plastic injection molders provide a list of molding equipment used in their facilities on their websites, along with each machines’ tonnage capabilities.

How Much Clamping Force Do I Need?

Pressure keeps a mold closed during the injection process; too much or too little can compromise quality and result in flashing — the appearance of excess material on the part edge. To determine the appropriate size press for your application, consider the following key variables:

• Press rating — A machine with a 68-ton rating, for instance, will be able to deliver 68 tons of clamping pressure, or force. 

• Material choice and MFI — The melt flow index (MFI) of plastic, or melt flow rate (MFR), indicates the ease of flow of a molten plastic material. A high MFI will require more pressure than a low MFI.

• Size — The size of the part will naturally affect the size of the machine needed. Many calculations include platen size in addition to mold and part size.
• Safety factor — A numerical percentage incorporated into size calculations to help avoid defects in the final part, the safety factor acts as a buffer; some experts recommend adding 10% to the overall press size estimate.
  

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By Aileen