Best way to deal with ejector pin marks?

For injection molders, it should be very common to see white ejector mark or pin push appear on the plastic parts in contact with the ejector pin during the process of ejection. This mark doesn’t only affect cosmetic look of the A class part, but also easily gets cracked during actual use of the products Therefore it is very important to prevent ejector pin marks before they happen.

What are the reasons for ejector mark and how to eliminate them? Well, the glossy or white imprints defect is usually due to the excessive pressure at the white area of the plastic parts where the chemical resistance and mechanical properties were greatly weakened. The specific analysis could be traced to either plastic molds or molding process.

Plastic injection mold

①If the surface of cavity is too rough, release resistance is increased and ejector white mark is caused. Therefore, the surface roughness in the cavity should be reduced.
②If ejector pin is placed in the weak area of the plastic part, it will aggravate the white phenomenon. Therefore ejection system should be placed at the thick wall area.
③If the mold draft is too small and demoulding resistance of the plastic part will increase.So increase mold draft.
④ In order to improve the strength of the ejection area, the local area could be strengthened from the structure and wall thickness. Improve part design.
⑤)If the contact surface between the ejection rod and the plastic part is too small, the local stress of the plastic part is very high, causing white ejector mark. Increase the cross section area of the ejector rod
⑥For plastic parts with core, demoulding is difficult and white mark is easy to occur. Therefore, it is a good method to release vacuum between core and plastic parts, such as installing air valve in core to release vacuum.

Process
① If the temperature between molten plastic and mold differs greatly, the residual stress in plastic parts is increased, and it is easy to cause white mark when subjected to local force. Therefore, the melt temperature should be lowed to reduce the difference with mold`
②If the cooling time is too short, the stress in the plastic parts is difficult to balance fully. When the local force is too strong, the white mark will occur. So the cooling time should be prolonged appropriately.
③If the injection pressure is too high, the residual stress in the plastic parts is also high, resulting in white mark, so the injection pressure should be properly reduced.

④ If the pressure holding pressure is too high, the stress in the plastic parts is heavy, and it is easy to cause the white mark when subjected to ejector force. Thus hold pressure should be properly reduced.

⑤If the holding time is too long, it is easy to produce residual stress, and the ejectior white mark appears, so the holding time should be shortened.

If above methods won’t help, then the last way to solve ejector pin marks is mold release agent. For any technical questions, please contact our engineering team at support@hsmolds.net.

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