Clamp Tonnage: More Is Better...Right?

Many factors must be considered to determine if an injection molding machine has the capability to mold parts on a consistent basis. The injection unit must have the correct shot volume to avoid degradation and unmelt. It must also have pressure in abundance and a flow rate high enough to allow for proper part filling.

Plastic variables also must be considered, focusing on how important it is to process from “the plastic’s point of view.” It’s important to dry the material to appropriate moisture content, process within the recommended melt temperature, select the correct flow rate for injection, apply adequate hold time and pressure, and cool long enough to maintain dimensional stability. However, these variables only focus on the injection aspect of the process. The clamp end of the machine is responsible for providing force to counter the pressure that is applied from the injection unit. How much tonnage is applied, location of that force, type of clamping mechanism, and size of the mold base all affect the success of the molding process. Determining the correct tonnage is not a simple task. It provides a foundation that must be rock- solid to avoid flash and mold damage. Is it possible to apply too much clamp force, narrowing the process window before molten material is ever injected? 

Part quality is not the only item affected when excessive clamp tonnage is applied. There are long-term effects from over- clamping that will damage the mold and machine. First let’s review several of the side effects on the mold:

• Crushed vents;
• Rolled parting line;
• Broken inserts;
• Cracked core or cavity block.

Figure 3 is an example of a mold that was clamped at four times the required tonnage. Based on surface area, material, and gating, the mold required roughly 100 tons (at 3 tons/in.2). The molder was inexperienced and set the clamp tonnage to the machine maximum of 400 tons (12 tons/in.2). Despite a robust mold design, quality steel, proper machining, and correct heat treating, the result of excessive tonnage was that the cavity block separated in two locations. Each split ran the entire depth of the cavity block (roughly 10 in.) from parting line to clamp plate. This was a catastrophic failure that caused delays in production and upwards of $100,000 to expedite the manufacturing of a new cavity block.

Here are some potential failures on the molding machine when excessive clamp force is applied:

• Cracked hydraulic-cylinder mounting plates;
• Deformed platens;
• Fractured machine frame.

In Fig. 4, there were several contributing factors that caused the frame rails to break on this vertical C-clamp molding machine. First, the mold required about 10 tons to be held shut against the forces of injection, but the clamp tonnage was set to 100 tons. Second the mold-base size covered less than two-thirds of the platen in both direc- tions. Combining these two poor practices over years not only fractured the machine rails, but caused molding defects and mold failure. No one was sure when the failure occurred on the machine, but they did identify when it was first discovered and documented its progression over time. Replacing both machine rails cost thousands of dollars for materials alone. If we factored in the labor hours to disassemble and reassemble nearly the entire machine, and the scrap produced over the years before the rails failed, the number could easily exceed $200,000.