Views: 0 Author: Site Editor Publish Time: 2025-09-08 Origin: Site
First things first: "Tonnage" is a measure of an injection molding machine's clamping force. A 100-ton machine can generate a clamping force equivalent to 100 US tons (or roughly 1,000 kN) to keep the mold closed.
This isn't just a random number; it's the machine's primary defense against the immense forces generated during injection.
Imagine injecting high-pressure, molten plastic into a closed mold. This plastic pushes against the walls of the mold cavity with tremendous force, creating what's known as the "molding separation force" or "bursting force."
The machine's clamping force (tonnage) must be greater than this separation force. If it isn't, the injection pressure will literally force the two halves of the mold to separate—even just a tiny bit.
The result? Flash.
Flash is the thin, unwanted layer of plastic that seeps out into the gap between the mold plates. It creates defective parts that require costly secondary trimming, wastes material, and can even damage the mold itself over time. Insufficient tonnage is the primary cause of flash.
So, how do you calculate the clamping force you need? It comes down to two main factors dictated by your mold and part design:
Projected Area (A): This is the largest surface area of the part (and the feed system, like runners) as seen from the direction of the clamp. Imagine shining a light through the part onto a wall—the shadow it casts is the projected area.
Bigger Area = Larger separating force = More tonnage required.
Cavity Pressure (P): This is the pressure inside the mold cavity during injection. It's influenced by the material viscosity (e.g., PP flows easier than PC), wall thickness, gate design, and injection speed.
Thinner walls, longer flow paths, and stiffer materials require higher cavity pressure.
The Simple Calculation:
You can estimate the required clamping force using this formula:
Clamping Force (tons) = Projected Area (in²) × Cavity Pressure (psi) / 2000
Cavity Pressure is typically an empirical value between 3,000 and 5,000 psi (20-35 MPa) for most materials. For easy-flow materials like PS or PE, use the lower end. For engineering-grade, viscous materials like PC or PC/ABS, use the higher end. For very thin-walled parts, pressure can exceed 8,000 psi.
Example:
Let's say you have a part with a projected area of 20 in² (including the runner) and you're using ABS material. You estimate the cavity pressure to be 4,000 psi.
Clamping Force = 20 in² × 4,000 psi / 2000 = 40 tons
It's always wise to add a safety factor of 10-20%. In this case, a 50-ton or 60-ton machine would be a safe and appropriate choice.
While tonnage is the star of the show, a successful marriage between mold and machine depends on other crucial factors:
Mold Size: Will the mold physically fit between the machine's tie bars? Check the mold's length, width, and height.
Shut Height: Does the mold's thickness fit within the machine's minimum and maximum mold height adjustment range? A mold that's too thick won't close; one that's too thin won't be clamped properly.
Ejection Stroke: Does the machine's ejector stroke provide enough travel to push the finished part completely out of the mold?
Shot Capacity: Is the machine's maximum shot volume (in ounces or grams) larger than the total weight of your part(s) and the runner system? A machine that can't inject enough plastic will produce short shots.
The mold design dictates the machine tonnage required, not the other way around.
Insufficient tonnage causes flash; excessive tonnage is wasteful and can potentially damage the mold.
Always calculate the required force based on projected area and estimated cavity pressure, and add a safety margin.
Never forget to verify the other compatibility factors like mold size, shut height, and shot capacity.
Understanding this relationship is the mark of a skilled molder. By carefully matching your tool to the right machine, you ensure a smooth, efficient, and profitable production process, shot after shot after shot.
