How to Match Injection Molder Tonnage to Part Weight

Why Part Weight Dictates Machine Size

An injection molding machine is sized to hold a mold closed against the pressure of molten plastic. The force required depends on the amount of plastic injected, not the physical dimensions of the part alone. A small part with two cavities may require more tonnage than a large single-cavity part if the melt volume is higher.

Operators and buyers often confuse shot size with machine tonnage. Shot size is measured in cubic centimeters or grams. Tonnage is the clamping force measured in kilonewtons or metric tons. These two values are linked through the pressure the material exerts on the mold plates. If the link is weak, the mold opens. If the link is strong, the part is trapped.

This guide walks through the calculation and selection process. It assumes you have access to the mold drawings and material datasheets. If you do not have these, request them from the mold maker before discussing machine specifications with any supplier.

Prerequisites for Accurate Selection

Before calculating tonnage, gather four specific documents. First, the final mold drawings with cavity count and gate locations. Second, the material data sheet for density. Third, the part weight in grams. Fourth, the melt pressure rating from the material supplier.

Check the part weight source. If the part is in production, weigh three samples on a digital scale. Average the results. If the part is in design, use the CAD software to calculate the solid volume. Multiply that volume by the material density. Do not rely on estimated weights from previous revisions unless the geometry is identical.

Confirm the cavity count. A mold with four identical cavities requires four times the shot weight of a single-cavity mold. If the mold has two large cavities and two small ones, calculate the shot for each cavity and sum them. The machine must handle the total volume injected in one shot.

Step-by-Step Tonnage Calculation

Follow these steps to determine the required machine capacity. Each step has a specific reason. Skipping a step usually leads to a flash part or a machine that is too large for the budget.

1. Calculate the total shot volume.
   Multiply the part volume in grams by the cavity count. Add the sprue gate and runner volume. This total represents the actual amount of material the barrel must inject. The reason is that the runner and gate take up space in the mold cavity system, even if they are removed during trimming.

2. Apply the material density.
   Convert the total shot volume to grams. If you already have the part weight in grams, multiply by the cavity count and add the estimated gate weight. Use the density from the datasheet at the processing temperature. The reason is that density changes slightly with temperature, and the machine must be sized for the bulkiest state of the melt.

3. Determine the required clamping force.
   Use the formula: Required Tonnage = (Total Shot Weight in kg) x (Melt Pressure in MPa) / (Mold Area in cm squared). The mold area is the projected area of the part on the mold parting line. The reason is that pressure acts perpendicular to the mold plates. A larger projected area creates more force that tries to pull the mold open.

4. Add a safety factor.
   Multiply the calculated tonnage by a factor of 1.2 to 1.5. For rigid materials like polycarbonate, use 1.2. For soft materials like LDPE, use 1.5. The reason is that safety factors account for material density shifts, pressure fluctuations during injection, and mold wear over time. This buffer prevents flash during the first few hundred shots.

5. Select the standard machine size.
   Find the next standard tonnage rating above your calculated value. Do not select the exact calculated number. Standard machines come in increments of 50, 100, or 200 tons. The reason is that manufacturers design hydraulic systems with specific capacities. Running a machine at 100% of its rated clamping force reduces the lifespan of the cylinder and seals.

6. Check the injection stroke.
   Verify that the machine injection stroke can handle the total shot volume. A machine with high tonnage but a short stroke may not be able to inject a large part. The reason is that the ram moves a fixed distance. If the part volume exceeds that distance, the ram hits the end of the stroke before injection is complete. This causes a short shot.

7. Review the mold weight.
   Compare the mold weight to the machine tonnage. A heavy mold requires more clamping force to keep it aligned. If the mold is very heavy, you may need a higher tonnage machine even if the part weight is low. The reason is that the mold itself has a projected area that resists the hydraulic pressure.

Common Mistakes in Tonnage Selection

The most common error is ignoring the cavity count. A buyer sees a part weight of 50 grams and assumes a 50-ton machine is sufficient. If the mold has four cavities, the total shot weight is 200 grams. The required tonnage quadruples. Always sum the weights of all cavities.

Another mistake is using the wrong projected area. The projected area is the shadow the part casts on the mold plate. It is not the surface area of the part. If you use surface area, you will overbuy the machine. Measure the width and height of the part on the parting line. Multiply them to get the area in square centimeters.

A third error is assuming all materials behave the same. High-melt-flow-rate materials flow faster but may require different pressure management. Low-melt-flow-rate materials are stiffer and may generate higher internal pressures. Check the melt pressure rating in the material datasheet. If the supplier does not provide it, use a standard pressure range for the polymer family.

Final Verification Step

Once you have selected a machine, verify the setup with a test shot. Load the material into the machine. Run the mold at low pressure. Watch the part for flash. If flash appears at the parting line, increase the clamping force or reduce the melt pressure. If the part is short, increase the injection pressure or check the runner length.

Record the actual tonnage used on the machine display. Compare it to your calculated required tonnage. If the actual tonnage is significantly lower than the machine rating, you may have overbought. If the actual tonnage is near the machine limit, you are at risk of mold separation during a power fluctuation.

The final check is not just about the part. It is about the machine. An injection molding machine should operate within its design parameters for the lifespan of the mold. If the calculated tonnage is close to the machine maximum, expect higher maintenance costs and more downtime.

How to Choose Between Larger and Smaller Machines

When two machine sizes are close, consider the production volume. A larger machine is more expensive to run. It uses more electricity to heat the barrel and move the hydraulic rams. If the part is small and the cavity count is low, a smaller machine is more efficient.

If the part is large or the cavity count is high, a larger machine is necessary. The cost of the machine is spread over more parts per hour. A 1,000-ton machine producing 50 parts per hour is more economical than a 500-ton machine producing 20 parts per hour, assuming the mold fits.

Check the machine back pressure. Some machines have limited back pressure capability. If the mold has a small gate, you may need high back pressure to control the melt. A machine with a large barrel diameter can handle this. A machine with a small barrel diameter may struggle.