When you start on a new project, and more specifically are looking for a new injection molding partner, your design team will need to understand how large of a machine will be required to make your new product. The calculation to determine the right press tonnage size isn’t complicated, but it does require a few known variables. If you haven’t done it before, we’ll walk you through the quick process before you start sourcing a new injection molding partner.
What is Press Tonnage?
Before we start to get into the process of calculation press tonnage, you have to know a little of the basics of injection molding and why tonnage is important. Injection molding is completed by forcing molten resin through a nozzle and into two halves of an injection mold. The halves are held together with an extreme amount if pressure, and that pressure is called press tonnage’. Too little of pressure to hold the halves together will result in malformed parts, missing material, and extra material squeezing outside of the mold. Too high of pressure can also lead to damage to the mold halves, excess material outside of the mold (also called flashing), and overall can be just a waste of machine power that isn’t required to make a correct part. Think of it like Goldilocks and the 3 Bears, you’re looking for the machine tonnage that is ‘just right’.
How to Calculate Correct Tonnage for Injection Molding
The calculation for press size, aka tonnage, requires three main considerations: surface area, melt flow, and part depth dimension.
Surface area – To calculate tonnage required, the first step is to determine the surface area. Surface area is determined by measuring the length and width of the mold’s cavity side and multiplying the two together. If the mold has multiple cavities of the same size, multiply the number of cavities by the number of cavities. Once the surface area is calculated, the tonnage factor is multiplied by the area. Most designers will use a 10% safety factor to ensure the calculated pressure is not excessive.
Melt flow – The next main consideration is calculating the melt flow of the resin to be used. This can be done by using the Melt Flow Index (MFI). The Melt Flow Index follows the ISO standard 1133-1 where a small amount of the polymer (typically 4 to 5 grams) is taken to a special MFI apparatus. The polymer is preheated to a specific temperature for an amount of time until it can be forced through a small die inside the apparatus. A sample of the melt is taken after a determined amount of time and then weighed. This indicates the MFI, which is expressed in grams per 10 minutes of duration.
Part depth dimension – The final consideration for tonnage is the depth of the part to be made. If the part is approximately one inch, a factor of 10% must be added to the clamping force. If the part is larger than one inch, an additional 10% should be added for each additional inch of depth.
Injection Molding Isn’t One Size Fits All
The mantra “bigger is always better” can be a potentially dangerous business plan in injection molding. Using a machine larger than required may not work as the larger machines can’t always accommodate a smaller injection mold. The smaller machines typically can’t fit a larger mold inside the operating area of the machine, nor can they open wide enough to accommodate the larger mold. The best option once you have the approximate tonnage required is to find a molding partner that has machines in that range.
SEA-LECT Plastics has 16 MILACRON injection molding machines that range from 33-ton with a 2.27-ounce shot size to 600-ton with a 105-ounce shot size. The part weight x number of cavities + runner weight should be ~ 60% of the machine shot size, so offering multiple machines in various sizes allows for more opportunity with molding services.
Calculating the tonnage requirements for your next project isn’t a complicated process, but it does require certain considerations before you start the process. If you’re unsure of the results, it may be easier to find an injection molding partner that is fluent with the calculations and how they affect daily molding operations. If you need that partner, look no further than SEA-LECT Plastics. At SEA-LECT Plastics we pride ourselves in tool and die manufacturing that allows us to deliver high quality products free from defects. We have multiple options for manufacturing to help your business succeed. Our expertise is in a wide range of molding disciplines. SEA-LECT Plastics has an elite team that can discuss the available options for your new project, we produce world-class prototypes and products, and we have decades of experience with plastic injection molding operations. Our tooling experts can create molds from 3D designs and make fine adjustments to ensure smooth molding operations on a daily basis. We can offer support to determine what type of tooling you need, what materials to choose, and can support the generation of documentation to win new business in your industry. Give us a call at (425) 339-0288 or email us at firstname.lastname@example.org
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.