Ejector Pin Design Principles

The ejection of a plastic product is the last step in the plastic injection molding process. The quality of the ejection determines the quality of the plastic product. When the mold opens, the plastic product must be retained on the mold half (usually the core) with the ejector mechanism that is used to release the plastic product.

(1) In order to prevent the plastic product from deformation during ejection, the ejection force has to be distributed as evenly as possible, and as close as possible to the core wrapped by the shrinking plastic or to the part that is difficult to eject, e.g., the elongated pillar of a plastic product is ejected with an ejector sleeve.

(2) The ejection force should be applied to the part with the strongest rigidity and strength of a plastic product (avoiding action on the thin section), and the active surface should also be as large as possible, such as the outstanding flange, the rib and the wall edge of the shell, etc. Most barrel-shaped products are released by using the ejector plates.

(3) To avoid ejector marks affecting the appearance of the plastic product, the ejection position should be set on the concealed surface (internal) or non-appearance surface of a plastic product; for transparent plastic products, the selection of the ejection position and the ejection form should be taken special care of.

(4) To avoid ejector marks and deformation of a plastic product caused by vacuum adsorption, the combined ejection approach or ventilation with porous steel may be applied. For example, ejector pin combined with ejector plate, or ejector pin combined with ejector block. The clearance of ejector pin should be appropriately enlarged for ventilation and use an intake valve when necessary.

(5) The ejector mechanism should be reliable and flexible, with sufficient strength and wear resistance. For example, with regard to ejections with swing rod and wedge, the strength and wear resistance of the sliding surface should be improved, with lubricating grooves designed on the sliding surface; Nitriding treatment may be applied to improve surface hardness and resistance to wear.

(6) The length of the mold return pin should ensure contact with the cavity or a distance of less than 0.1 mm after the mold closes, as shown in the figure above.

(7) A return spring is usually used for the return of the ejector plate; since the return spring is not reliable, it cannot be used as the reliable early return mechanism.