Injection Molding Ejection System: Where to Use Ejector Pins and How to Design Ejection System


The ejector pin is one of our commonly used accessories of plastic injection mold. It can be used not only to separate the workpiece but also in the mold to fix it. The ejector pin is usually made of quality steel that is durable and not easy to break. Then where should we locate the ejector pins and how to design the ejection system.

Where to Use & Locate Injection Molding Ejector Pins

1. The ejector pin should be arranged to balance the ejector force as much as possible. The demoulding force required for the parts with complex structures is large, and the number of ejector pins should be increased accordingly.

2. The ejector pins should be set at effective parts, such as bone position, column position, step, metal insert, local thick glue, and other parts with complex structures. The ejector pins on both sides of the bone position and column position should be arranged symmetrically as far as possible. The edge spacing between the ejector pin and the bone position and column position is generally d = 1.5mm. In addition, it should be ensured that the centerline of the ejector pin on both sides of the column position passes through the center of the column position as far as possible.

3. Avoid crossing steps or setting ejector pin on inclined surfaces. The top surface of ejector pins should be as gentle as possible. Ejector pins should be arranged at structural parts with good stress on rubber parts.

4. Flat ejector pin should be used when it is difficult to arrange the dome needle in the deep bone position of the rubber part (depth ≥ 20mm). When it is necessary to use a flat ejector pin, an insert should be used at the flat ejector pin as far as possible to facilitate processing.

5. Avoid sharp steel and thin steel, especially the top surface of the ejector pin. Do not touch the front die surface.

6. The side distance between the ejector pin and the water channel should be considered in the ejector pin layout to avoid affecting the processing and water leakage of the water channel.

7. Considering the exhaust function of the ejector pin, in order to exhaust during ejection, the ejector pin should be arranged at the position where vacuum pumping is easy to form. For example, at the plane with a large cavity, although the sealing force of rubber parts is small, it is easy to form a vacuum, resulting in increased demoulding force.

8. For rubber parts with appearance requirements, the ejector pin should not be arranged on the appearance surface, and other ejection methods should be adopted.

9. For transparent rubber parts, the ejector pin should not be arranged at the position requiring light transmission.

Selection of Injection Molding Ejector Pins

1. Select the ejector pin with a larger diameter. That is, when there is enough ejection position, the ejector pin with a larger diameter and preferred size should be selected.

2. The specification of the ejector pin should be as few as possible. When selecting the ejector pins, the size of the ejector pins should be adjusted to minimize the size specification, and the preferred size series should be selected as far as possible.

3. The selected ejector pins should meet the ejection strength requirements. During ejection, the ejector pins should bear large pressure. In order to avoid bending and deformation of small ejector pins, when the diameter of ejector pins is less than 2.5mm, the supporting ejector pins should be selected.


After the product completes a forming cycle, the mold will be opened, and the product will be wrapped on one side of the mold, which must be removed from the mold. This work must be completed by the ejection system, which is an important part of the whole die structure. It is generally composed of ejection, reset, and ejection guidance.

Design Principle of Injection Molding Ejection System

There are many forms of ejector systems, which are related to the shape, structure, and plastic properties of products. Generally, there are ejector rod, ejector pipes, push plates, ejector blocks, pneumatic compound ejectors, etc. The design principles are as follows:

– When selecting the parting surface, try to keep the product on the side with demoulding mechanism.

– The jacking force and position are balanced to ensure that the product is not deformed and broken.

– The ejector pins must be set at a place that does not affect the appearance and function of the product.

– Try to use standard parts as safe and reliable as possible, which is conducive to manufacturing and replacement.

– The ejection position should be set at the place with high resistance and should not be too close to the insert or core. For box type and other deep cavity molds, the side resistance is the largest, and the top surface and side should be ejected at the same time to avoid product deformation and bursting.

– When there are thin and deep stiffeners, the ejector rod is generally set at the bottom.

– Avoid setting ejector pins at the rubber inlet of the product to avoid cracking.

– For thin meat products, an ejector pin is set on the diversion channel to bring the products out.

– The fit between the ejector pin and the ejector pin hole is generally a clearance fit. If it is too loose, it is easy to produce burrs, and if it is too tight, it is easy to get stuck. In order to facilitate processing and assembly and reduce friction surface, a matching length of 10 ~ 15mm is generally reserved on the moving die, and the holes of the rest are expanded by 0.5 ~ 1.0mm to form escape holes.

– In order to prevent the ejector pin from rotating during production, it must be fixed on the ejector plate in a variety of forms, which must be determined according to the size, shape, and position of the ejector pin.