Ejector Stroke Injection Molding: A Guide for Mold Designers and Manufacturers

Ejector stroke injection molding is a technique that uses a mechanical device called an ejector to push the molded part out of the mold cavity after it has cooled and solidified. Ejector stroke injection molding is widely used in the plastic industry for producing complex and high-quality parts with minimal waste and defects.

In this guide, we will explain the basic principles of ejector stroke injection molding, the advantages and disadvantages of this technique, and the factors that affect the ejector stroke design and performance. We will also provide some tips and best practices for mold designers and manufacturers who want to optimize their ejector stroke injection molding process and achieve better results.

This article will provide an overview of ejector stroke injection molding, a process that uses a mechanical device to eject the molded part from the mold cavity. Ejector stroke injection molding is widely used for producing plastic parts with complex shapes and high dimensional accuracy. The article will cover the following topics:

• What is ejector stroke injection molding and how does it work?
• What are the advantages and disadvantages of ejector stroke injection molding?
• What are the factors that affect the ejector stroke and how to calculate it?
• What are the best practices for designing and manufacturing molds with ejector stroke injection molding?

What is Ejector Stroke Injection Molding and How Does It Work?

Ejector stroke injection molding is a method of obtaining molded products by injecting plastic materials molten by heat into a mold, and then cooling and solidifying them. The mold consists of two halves: a fixed half and a movable half. The fixed half contains the cavity where the molten plastic is injected, while the movable half contains the core that forms the inner shape of the part. The movable half also has a set of ejector pins that are connected to a knockout rod. The knockout rod is attached to an ejector plate that is driven by a hydraulic or pneumatic cylinder.

The ejector stroke injection molding process can be divided into four stages:

1. Clamping: The mold halves are closed and clamped together by a clamping unit. The clamping force depends on the size and shape of the part, as well as the injection pressure.
2. Injection: The molten plastic is injected into the mold cavity through a nozzle at high pressure and speed. The injection time and pressure are controlled by an injection unit. The molten plastic fills the cavity and conforms to the shape of the core and cavity.
3. Cooling: The molten plastic cools down and solidifies in the mold cavity. The cooling time depends on the material properties, wall thickness, and mold temperature.
4. Ejection: The mold halves are opened and the movable half moves away from the fixed half. The ejector plate pushes the knockout rod, which in turn pushes the ejector pins. The ejector pins contact the molded part and eject it from the mold cavity.

What are the Advantages and Disadvantages of Ejector Stroke Injection Molding?

Ejector stroke injection molding has several advantages over other molding methods, such as:

• It can produce parts with complex shapes, fine details, and high dimensional accuracy.
• It can use a wide range of plastic materials with different properties and characteristics.
• It can achieve high production efficiency and low scrap rate.
• It can reduce post-processing operations, such as trimming, drilling, or machining.

However, ejector stroke injection molding also has some disadvantages, such as:

• It requires high initial investment for designing and manufacturing molds.
• It requires high maintenance and repair costs for molds and machines.
• It may cause defects in molded parts, such as warping, shrinkage, flash, or sink marks.

What are the Factors that Affect the Ejector Stroke and How to Calculate it?

The ejector stroke is the distance that the ejector pins travel to eject the molded part from the mold cavity. The ejector stroke affects the quality of the molded part, as well as the cycle time and energy consumption of the molding process. Therefore, it is important to optimize the ejector stroke for each mold design.

The factors that affect the ejector stroke are:

• The shape and size of the molded part: The larger and more complex the part, the longer the ejector stroke required to release it from the mold cavity.
• The material properties of the molded part: The higher the shrinkage rate and elasticity of the material, the longer the ejector stroke required to overcome its resistance to deformation.
• The design of the mold: The number, size, location, and angle of the ejector pins affect the distribution of force on the molded part and its ease of ejection.
• The operating conditions of the molding machine: The temperature, pressure, speed, and timing of the injection, cooling, and ejection stages affect the behavior of the material and its interaction with the mold.

To calculate the ejector stroke for a given mold design, one can use this formula:

Ejector Stroke = Part Thickness + Ejector Pin Length + Clearance + Overtravel

Where:

• Part Thickness: The maximum thickness of the molded part measured perpendicular to the direction of ejection.
• Ejector Pin Length: The length of the ejector pin that contacts the molded part.
• Clearance: The gap between the ejector pin and the mold cavity wall to allow for thermal expansion and contraction of the material and the mold.
• Overtravel: The extra distance that the ejector pin travels beyond the part thickness to ensure complete ejection of the part.

For example, if the part thickness is 10 mm, the ejector pin length is 15 mm, the clearance is 0.5 mm, and the overtravel is 2 mm, then the ejector stroke is:

Ejector Stroke = 10 + 15 + 0.5 + 2 = 27.5 mm

What are the Best Practices for Designing and Manufacturing Molds with Ejector Stroke Injection Molding?

To achieve optimal results with ejector stroke injection molding, one should follow these best practices for designing and manufacturing molds:

• Design molds to use minimal ejector stroke and relief (providing more bearing surface) to reduce cycle time, friction, and wear on the components.
• Design molds with sufficient ejector pins to distribute the ejection force evenly and avoid part deformation or damage.
• Design molds with proper ejector pin location and angle to align with the direction of ejection and avoid interference with other mold components.
• Design molds with adequate cooling channels to control the mold temperature and prevent premature ejection or sticking of the part.
• Manufacture molds with high-quality materials and precision machining to ensure dimensional accuracy and durability.
• Maintain and repair molds regularly to prevent corrosion, wear, or damage that may affect the performance and quality of the molding process.

Conclusion

Ejector stroke injection molding is a widely used method for producing plastic parts with complex shapes and high dimensional accuracy. It involves injecting molten plastic into a mold cavity and then ejecting it with a mechanical device. The ejector stroke is an important parameter that affects the quality, efficiency, and energy consumption of the molding process. Therefore, it is essential to optimize the ejector stroke for each mold design by considering the factors that influence it and following the best practices for designing and manufacturing molds.