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How to Choose the Right Injection Molding Machine Size: Guide to Sizes, Parameters, and Selection
Created on 01.07
When it comes to manufacturing, the injection molding machine is a key piece of equipment. But stepping onto a factory floor or looking through a catalog can be confusing. Machines range from 50 tons to 3,000 tons, each having a detailed list of specs. Picking the wrong size can cause mold damage, wasted stuff, or parts that just can't be made.
This guide cuts through the technical talk and gives you a simple way to figure out the right injection molding machine size for your project.
What Determines an Injection Molding Machine Size?
People often use size when discussing injection molding machines, but they usually mean clamping force, which we measure in tons. Really, size includes a few things, like physical and mechanical limits.
The Tonnage (Clamping Force)
The tonnage is the amount of pressure the machine can apply to keep the mold halves pressed together during the injection process. When molten plastic is forced into a mold at high pressure, it tries to push the mold open. If the machine's tonnage is too low, the mold will breathe or part slightly, causing "flash"—excess plastic leaking out of the seams of the part.
Platen Dimensions and Tie-Bar Spacing
Think of the platens as the "walls" that the mold is bolted to. The tie-bars are the large steel rods that guide these walls.
- Platen Size: The total surface area available to mount the mold.
- Tie-Bar Spacing: The horizontal and vertical distance between the bars. Your mold must be small enough to fit between these bars to be installed in the machine.
Shot Size (Injection Capacity)
This refers to the maximum volume of plastic the machine can inject in one cycle. It is determined by the diameter of the screw and the distance it travels. If your part is too large for the machine’s shot capacity, the mold won't fill. If the part is too small, the plastic sits in the heated barrel for too long and begins to degrade (thermal degradation of polymers) or "cook."
Detailed Parameters and Sizing Factors
Beyond the basic tonnage, several other parameters dictate whether a machine is a good match for your production needs.
1. Injection Pressure
Different materials require different levels of "push" to fill the mold. For example, a thin-walled electronic housing requires much higher injection pressure than a thick, simple bucket. The machine must be able to generate enough pressure to overcome the resistance of the mold’s internal pathways.
2. Injection Molding Machine Nozzle Size
The nozzle is the interface between the machine barrel and the mold. The injection molding machine nozzle size must match the mold’s sprue bushing. If the nozzle tip is too large, plastic will leak; if it is too small, it creates a "pressure drop," making it harder to fill the mold and potentially causing the plastic to overheat due to friction.
3. Opening Stroke and Daylight
"Daylight" is the maximum distance the platens can move apart. If you are manufacturing a deep bucket, you need a machine with a very large opening stroke. Even if the part is small in diameter, you need enough space to move the mold halves apart and have room for the finished part to fall out or be grabbed by a robot.
How to Calculate Injection Mold Size and Requirements
To choose the right machine, you must work backward from the part you want to make. Here is how you can perform the basic calculations.
Step 1: Calculating the Projected Area
The projected area is the 2D surface area of the part when viewed from the direction of the injection. Imagine a flashlight shining on the part; the shadow it casts on a flat wall is the projected area.
Calculation: Projected Area = Length x Width
Example: If you are molding a rectangular tray that is 10 inches long and 5 inches wide, the calculation is: 10 inches x 5 inches = 50 square inches
Step 2: Determining Required Tonnage
Most materials require between 2 to 5 tons of clamping force for every square inch of projected area.
- Easy-flow materials (like Polypropylene): Use 2 to 3 tons per square inch.
- High-viscosity materials (like Polycarbonate): Use 4 to 5 tons per square inch.
Example Calculation: Using our 50-square-inch tray made of a standard plastic (using 3 tons per square inch): 50 square inches x 3 tons = 150 tons In this case, you would look for a machine with at least 150 tons of clamping force.
Step 3: Calculating Shot Capacity
You must calculate the total weight of the plastic used in one cycle. This includes the part itself plus the "runner system" (the plastic channels that lead to the part).
The 20/80 Rule: For the best results, your total shot weight should be between 20% and 80% of the machine’s maximum capacity.
- If you use less than 20%, the plastic stays in the barrel too long and degrades.
- If you use more than 80%, the machine may struggle to maintain consistent pressure, leading to part defects.
How Do I Know What Size Injection Molding Machine I Need?
Once you have your calculations, you can follow this practical decision-making framework to select the specific machine.
1. Check the Physical Fit
First, look at the mold's external dimensions. If your mold is 20 inches wide and the machine's tie-bar spacing is 18 inches, that machine is immediately disqualified, regardless of its tonnage. Ensure there is enough "daylight" to eject the part comfortably.
2. Compare Average Injection Molding Rates by Machine Size
Machine size affects how fast you can produce parts.
- Small Machines (Under 500 Tons): These often have very fast cycle times and are great for high-volume, precision parts like micro-gears, electrical connectors, or bottle caps. Because the components are light, the machine can open and close in just a few seconds.
- Medium Machines (500 to 1000 Tons): This is the "sweet spot" for many manufacturers. These machines are versatile enough for automotive interior components, power tool housings, and larger household goods. They offer a balance of moderate speed and enough physical space to accommodate more complex molds with side actions or sliders.
- Large Machines (Over 1000 Tons): These move much more slowly due to the massive weight of the platens and the time required to build up high clamping pressure. These are reserved for large-scale items like car bumpers, plastic pallets, or large appliance panels.
If your project requires millions of parts, you might choose a larger machine that can hold a "multi-cavity" mold (e.g., making 16 parts at once) rather than a small machine making one part at a time.
3. Factor in Material Sensitivity
Certain materials are sensitive to heat. If you are using a clear plastic like Acrylic or Polycarbonate, you must be very precise with the shot size calculation. Using a machine that is too large for the part will cause the material to yellow or become brittle because it sits in the heated barrel for too many cycles.
Understanding the Relationship Between Tonnage and Other Specs
A common question is: “If all 200-ton machines have the same clamping force, are they the same size?”
The answer is no. While the tonnage might be the same, manufacturers offer different "frames." You might find a "200-ton wide-platen" machine designed for large, flat parts that don't need much plastic, or a "200-ton large-barrel" machine for thick, heavy parts that need a lot of material but don't have a huge surface area.
Always look at the machine specification sheet rather than just the tonnage label. The spec sheet will list:
- Maximum mold thickness: The largest mold the machine can close on.
- Minimum mold thickness: If your mold is too thin, the machine cannot close tightly enough to build pressure.
Common Mistakes in Machine Selection
1. Selecting Based Only on Tonnage
This is the most frequent error. A machine might have the 500 tons of force you need, but if the tie-bars are too close together to fit your mold, the tonnage doesn't matter. Always verify the mold dimensions first.
2. Ignoring the Ejection System
Different machines have different ejection patterns. Ensure your mold's ejection plate is compatible with the machine's ejection bars. If they don't align, the part will not be pushed out of the mold.
3. Forgetting the "Safety Factors."
Never run a machine at 100% of its rated capacity for long periods. If your calculation says you need 195 tons, do not buy a 200-ton machine. Aim for a machine where you only need to use about 80% of its maximum capability. This extends the life of the machine and provides a buffer if the material behavior changes slightly during production.
Summary of the Selection Process
- Choosing the right injection molding machine size is a logical process of elimination:
- Calculate the project area to find the minimum tonnage.
- Calculate the shot weight to ensure it falls within 20% to 80% of the machine’s capacity.
- Check mold dimensions against the tie-bar spacing and platen size.
- Verify the opening stroke to ensure the part can be removed safely.
By following these steps, you ensure that your production run is efficient, your parts are high quality, and your machinery stays in good working condition for years to come.
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