5 Factors to Consider When Buying Plastic Injection Molding ...

14 Jul.,2025

 

5 Factors to Consider When Buying Plastic Injection Molding ...

As many of us in the machine industry are aware, Plastic Injection Molding is just as relevant and important as any other machining process. There are many things to consider when considering purchasing plastic injection molding equipment, such as: Shot Size, Tie Bar Spacing, Ejector Stroke, Platen Size, and Tonnage.

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Shot Size

Shot size is best defined as the maximum amount of plastic that the injection molding machinery is capable of injecting into the molding cavity during one molding cycle. The amount is rated in ounces of “general purpose polystyrene” (GPPS) for U.S. machines, and cm3 for European and Asian machines.

When searching for a machine, it's always a best practice to look for a machine that is capable of producing 30-40% greater shot sizes than what your parts require.

Tonnage

In this case, I mean “clamp tonnage or clamping pressure.” This could be considered the same concept as when you're examining press brakes. With press brakes, tonnage capabilities are measured by how many tons of downforce the machine is capable of directing towards a workpiece. However, with plastic injection molding machines, tonnage is measured by how many tons with which the machine is capable of pressing together the platens, which hold the mold cavity and form the plastic that is injected into said cavity to produce the desired part.

Platen Size

The platen is the "table" or tool that holds the mold cavity. Spaced apart, the mold is inserted and clamped securely to the platens. Once the two platens come together and are held under the pressure of tonnage, the plastic is heated, injected under pressure, and then allowed to cool through a molding cooling cycle. Here, the plastic takes on its hardened, final shape while continuing to be kept in place with many tons of pressure to retain the desired shape.

Tie Bar Spacing

Tie Bar Spacing is defined as the space between the horizontal tie-bars on an injection molding machine. Basically, this measurement, along with the platen max spacing, determines the maximum size of molds that can be placed in the molding machine.

Ejector Stroke

Simply put, Ejector Stroke is the action of the machine pushing out (ejecting) the final workpiece from the platens using ejector pins. The ejector in the molding machine pushes against an ejector plate on the mold, Ejector "Pins" or rods are attached to this plate and perform a pushing operation on the molded part after the plastic has hardened and the mold has opened.

Once the action of molding the workpiece is completed, the mold opens and then the ejector pins slowly extend outwards, pushing (or ejecting) the finished part out of the mold cavity. Pin movement is timed with the mold opening—because pins that move too quickly could impact the plastic workpiece and damage the final product and pins that move too slowly may not fully eject the molded part.

The 8 Key Parameters in Injection Molding Process Optimization to ...

We talk a lot about injection molding process optimization, but what does that entail? What parameters need to be perfected in order to reach maximum optimization? Here are the 8 key parameters in injection molding that you need to align in order to produce consistent, high quality plastic parts.

1. Temperature Control:

Plastic or Melt Temperature: Proper control of the plastic temperature is essential for melting the plastic uniformly. Different materials have specific temperature requirements, and maintaining the correct temperature range prevents issues like incomplete melting or thermal degradation.

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Mold Temperature: Controlling the mold temperature affects the part’s crystallinity, shrinkage, and cycle time. Keeping the mold at the recommended temperature helps to ensure that the resin has the correct mechanical characteristics.

2. Injection Speed:

The rate at which molten plastic is injected into the mold affects the filling pattern, part density, and overall quality. Fill speed also has an effect on the packing phase of the molding process. Adjusting the injection speed helps prevent issues like flow lines or air traps.

3. Cooling Time:

The cooling time is the duration the part spends in the mold after the material has been filled and packed. Proper cooling time is crucial for preventing warpage and ensuring the part solidifies uniformly. It also influences the overall cycle time of the injection molding process.

4. Plastic Material Selection:

The choice of plastic material has a significant impact on the injection molding process. Different materials have varying melting points, shrinkage rates, and flow characteristics. Selecting the appropriate material for the intended application is essential for achieving the desired part properties.

5. Screw Speed and Back Pressure:

Screw Speed: The rotational speed of the screw in the injection molding machine affects the material’s overall shear. Screw speed should be selected based on the type of material and the additive content. The correct screw speed will ensure that the correct amount of shear is provided by the rotation of the screw and the screw geometry.

Back Pressure: Maintaining the right back pressure on the screw ensures proper mixing and homogenization of the molten plastic. It also helps in preventing voids and improving part quality.

6. Injection Molding Machine Calibration:

Regular calibration of the injection molding machine is crucial for maintaining accuracy in the process. This includes calibration of the machine’s pressure, temperature, and timing settings.

7. Mold Design and Maintenance:

Well-designed molds with proper venting, gating, and cooling channels contribute significantly to the optimization process. Regular maintenance of molds is essential to prevent issues like wear, corrosion, or damage that can affect part quality.

8. Monitoring and Control Systems:

Implementing advanced monitoring and control systems (such as CoPilot®) allows real-time tracking of various parameters. This enables quick identification of deviations and facilitates prompt corrective actions, contributing to continuous improvement in the injection molding process.

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