Determining the Press Tonnage on a New Custom Injection Molding Project

Determining the Press Tonnage on a New Custom Injection Molding Project

HTI Plastics Injection Molding Press

When HTI gets ready to create a new plastic product, we begin by determining injection molding machine press size. It's nowhere near as simple as "bigger is better" or any other easily memorable pattern - our products require very specific setups to be made to the highest standards. Here's a look at what goes on behind the scenes to bring plastic products to life:

Measuring Press Size

Press size, when talking about plastic injection molding machines, means more than width, height, and depth. It's measured in tons, and specifies the amount of clamping force the machine can apply to keep the mold closed during the injection. HTI has presses ranging from 50 to 950 tons.

Why Is There Different Press Tonnage?

We take the size, material, and number of cavities in each plastic part into consideration before determining press size. In the molding process, we'll inject plastic into the mold at extremely high pressure, which naturally attempts to force the mold open. The press is designed to keep it shut. Larger parts require heavier presses, and smaller parts require less tonnage and force.

To ensure a successful product, a simple calculation helps us determine proper press size. If you project the part shadow onto the parting line plane, the area (in square inches) multiplied by 3 and then multiplied by the number of mold cavities gives a conservative estimate of press tonnage required. This calculation works for most cases, but material selection and part shape must also be taken into consideration.

Determining Press Size

Why wouldn't we just use the heaviest press all the time? Many larger presses can't accommodate smaller molds because they can't close far enough. If the minimum distance between machine platens is larger than the stack height of the mold, it won't close and the injection process won't work. We call this distance the Minimum Daylight of the press, and always ensure that it's smaller than the stack height of a particular mold.

Alternately, smaller presses tend to have tie bar spacing that's too narrow for some of our larger products. If the mold won't fit between them horizontally or vertically, we move up to a larger press. Maximum Daylight may be too small to allow the mold to open far enough for the parts to be removed.