Announcement posted by Rockingham Montessori School 28 Nov 2024
How Ejector Pins Are Used in Injection Molding
Ejector pins are a crucial part of the injection molding process, responsible for pushing molded parts out of the mold cavity once they have cooled and solidified. Along with other ejector systems like ejector blades and blocks, these pins help ensure efficient part removal, minimizing defects and improving productivity.

What Are Ejector Pins in Injection Molding?
In injection molding, ejector pins are used to push the cooled, molded parts out of the mold after the plastic has solidified. The mold typically consists of two parts: Side A, the movable part, and Side B, which contains the ejector system. After the plastic part cools, the mold opens, and the ejector pins push the finished part out. This mechanism is critical to speeding up the process and ensuring that parts are efficiently removed from the mold without damaging them.
Types of Ejector Pins
There are several types of ejector pins, each suitable for different molding requirements:
- Through-Hard Ejector Pins: These pins are heat-treated to handle high temperatures, making them suitable for plastics that do not exceed 200°C.
- Nitride H13 Ejector Pins: Known for their ability to withstand temperatures up to 600°C, these pins are commonly used in harsher molding environments. However, they are softer than other pins and can chip easily.
- Black Ejector Pins: These pins can endure temperatures up to 1000°C and are ideal for automotive parts. They are coated with a self-lubricating layer, although they tend to be more expensive.
- Ejector Sleeves: These hollow pins protect and guide the ejector pin, helping prevent damage to the molded parts.
- Ejector Plates and Blocks: Ejector plates help hold the pin in place, while ejector blocks include lubricating grooves that reduce ejector pin marks, ideal for high-surface-finish products.
How Do Ejector Pins Work?
The injection molding process is divided into three phases: injection, cooling, and part release.
- Injection: Molten plastic is injected into the mold cavity.
- Cooling: The plastic solidifies within the mold, taking the shape of the cavity.
- Part Release: Once the part has cooled, the ejector pins push it out of the mold. This can be done manually or automatically, with the latter being much faster and more efficient.
The ejector pins exert force on the molded part, pushing it out of the mold cavity along the axial direction. The use of automatic ejector pins ensures a smooth and efficient part release.
Ejector Pin Marks: Causes and Solutions
During the ejection process, ejector pins can leave marks on the molded part. These are called ejector pin marks and are generally inevitable, but they can be minimized. Common causes of ejector pin marks include:
- Insufficient Cooling Time: If the molded part is ejected before it is fully cooled, the pin may penetrate the part, leaving marks. Solution: Ensure the part has adequate cooling time to solidify completely before ejection.
- Thin Wall Thickness: Parts with very thin walls are more likely to show ejector pin marks. Solution: Use a minimum wall thickness of 2.5mm to reduce the risk of marks.
- High Dwell Time or Temperature: Excessive dwell time or high temperature can increase the likelihood of ejector pin marks. Solution: Reduce the dwell time or adjust the temperature to ensure the part is solidified before ejection.
Choosing the Right Ejector Pins
Selecting the right ejector pin is crucial for minimizing defects and ensuring smooth production. Key factors to consider include:
- Pin Size: Larger diameter pins are better for larger parts as they distribute pressure more evenly, reducing penetration and marking. However, the size should be optimized based on the part's size.
- Material Strength: The ejector pin must be strong enough to withstand the high pressures of injection molding. A pin diameter of at least 2.5mm is recommended to avoid bending.
- Mold Design: The design of the mold and the complexity of the part will determine how many ejector pins are needed and where they should be placed. Tips: Place pins on flat surfaces and in areas that experience higher stress, such as ribs or thick sections.
- Mold Type: Ensure the right injection molding machine is used for the specific part size and volume. Large machines may not be suitable for small molds as they can cause internal stress.
Reducing Ejector Pin Marks
To minimize ejector pin marks, you can:
- Use abrasion-resistant plastics like nylon or polyethylene.
- Apply a mold release agent to ensure smoother ejection.
- Incorporate a draft angle in the mold design to facilitate easier part release.
Conclusion
Ejector pins are essential components in injection molding, facilitating the efficient removal of molded parts. By understanding their types, functions, and how to select the right ejector pin, manufacturers can optimize their injection molding process. Proper management of ejector pin marks, including thoughtful mold design and material selection, can improve both the functionality and appearance of the final product.
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