What Happens if Tonnage Is Too Low?
For manufacturers injection molding large parts, getting the tonnage right is essential for cost-effective, high-quality production. Insufficient clamping force is one of the most common causes of large-part injection molding defects. Without enough tonnage, the mold may not stay tightly shut during the injection cycle, and molten plastic can escape along the parting lines. This is called flash, and it not only mars the surface of the part but also indicates improper sealing.
You may also see short shots where the mold cavities are not fully filled. This leads to incomplete or structurally weak parts unsuitable for end-use applications. Surface imperfections and warping can also occur when there isn’t consistent pressure throughout the mold, especially in large or complex geometries.
Beyond visible flaws, low tonnage wears out molds faster. Uneven clamping can damage leader pins and bushings due to misalignment or the mold opening mid-cycle. Over time, this increases downtime, tool rebuilds, and maintenance costs. In short, under-clamping leads to waste, shortens the mold’s life, and increases total production costs.
What Happens If Tonnage Is Too High?
Excessive tonnage can be just as damaging as too little, even if it doesn’t cause visible effects. It often increases long-term costs by degrading the mold and machine. Too much force compresses mold vents, preventing trapped air from escaping. This trapped air can ignite under heat and pressure, resulting in burn marks on the part.
Excessive clamping also increases internal mold pressure, making the injection process less consistent and harder to control. The added force may even lead to tool damage, including deformed platens, cracked inserts, or worn parting lines.
Plus, over-clamping consumes more energy per cycle and places strain on the mechanical components of the press. The machine works harder than necessary, leading to premature wear and lower overall efficiency.
Finding the Right Tonnage for Your Mold
So how do you determine the right tonnage? The standard rule of thumb is 2 to 8 tons per square inch of projected area, but this is just a starting point. Real-world applications require more nuanced analysis of:
- Projected area. This includes all surfaces where pressure is applied, including runners and gates.
- Material viscosity. High-flow materials tend to need less pressure to fill the mold, while more viscous or filled materials require more force.
- Safety factor. Most molders add 10–15% extra force to ensure the mold stays sealed under various production conditions.
- Part geometry. Thin-walled or intricately designed parts often need higher packing pressure to fill correctly.
Using mold flow simulation software or conducting pressure sensor studies can help determine more precise requirements, significantly accelerating the trial-and-error stage in production.
Optimizing Tonnage for Cost and Quality
Accurate tonnage settings can dramatically improve efficiency in plastic molding large parts. You’ll see fewer rejects, faster cycle times, and reduced wear on tools and machines.
Best practices include:
- Sensor-based validation. Use cavity pressure sensors to determine the minimum force required to keep the mold sealed.
- Incremental adjustments. During trial runs, gradually increase or decrease tonnage and watch for defect indicators like flash, burns, or short shots.
- Avoiding over-clamping. More tonnage isn’t always better. Instead of defaulting to full machine force, adjust based on mold needs.
- Right-sized machines. Using a 3,000-ton press for a 500-ton part wastes energy and reduces process control. Always match your machine size to the projected area and resin type.
With large-part molding machines—often in the 1,500–3,500 ton range—getting tonnage wrong can be costly. Proper optimization ensures you get the most from your equipment investment while protecting part quality and mold longevity.
Proper Clamping Force Prevents Defects and Downtime
Whether you’re molding automotive bumpers, pallets, or custom housings, it all starts with setting the right tonnage. Too little pressure risks costly defects and tool damage, while too much increases energy use and wears out machines and molds prematurely.
At Lerner Molded Plastics, we specialize in injection molding large parts and have a deep understanding of the engineering that drives quality production. Our team can help evaluate your mold, resin, and part requirements to determine the optimal clamp force for your project.
Ready to produce high-quality large plastic parts with fewer defects and greater efficiency? Request a quote today to learn more about our capabilities or start your project.