Know Factors to Consider When Choosing Straight Ejector Pin

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Know Factors to Consider When Choosing Straight Ejector Pin

Posted By Jin hong     June 10, 2021    

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Jinhong uses Straight ejector pin, and their placement depends on a number of factors. Obviously the shape of the part is one (see example in illustration). Factors like draft and texture of sidewalls and depth of walls and ribs can increase the likelihood that areas of the part will cling to the mold. Resin choice can also affect pin placement or size. Some resins are "stickier," requiring more force for release from the mold. Softer resins may also require the use of more or wider pins to spread force and prevent puncturing or marring of the cooled plastic.

In our process, the ends of ejector pins are flat and perpendicular to the direction in which the pin moves. To be effective, the pins need a flat "pad" to push against, and the surface of the pad must be perpendicular to the direction of pin movement. If the part surface at that location is textured, the smooth surface of the pad will be apparent. And if the surface of the part is not parallel to the flat end of the ejector pin, the cosmetic impact will be even more obvious.

In a traditional steel production tool it may be possible to machine the end of the pin to match the contour of a part surface that is not perpendicular to the direction in which the pin moves, producing a contoured pin.


The above illustration shows three basic types of pin assignments on a non-flat surface. From top to bottom, these include a center cut pin, which is indented on a curved part surface; shortest cut pin, which adds a pad, shown in yellow, above the curved part surface; and longest cut pin, which fully indents the pin into the part.

Our process, however, usually does not support the production of contoured pins unless a customer requests it. It is done on a case-by-case basis.

If a pin needs to act on a part surface that is not parallel to the pin-end, there will have to be a pad provided that is in the same plane as the pin-end rather than that of the part surface. Because it is in a different plane than the part surface, the pad may be raised slightly above the part surface at one edge or recessed slightly below the part surface at one edge. Configuring a pad that is slightly recessed into the part surface is the default configuration for pins on contoured surfaces.

The default configuration is a center-cut pin, which on an angled or curved face means the pin hits the tangent of the surface. The pin would hit with half indenting the part and half raised pad on the part. See the illustration that shows center cut and shortest and longest pins on the surface. Jinhong does have two other options besides just center cut: shortest, which leaves standing pad under pin; or longest, which fully indents the pin into the part. Keep in mind, with a shortest pin, you will be making a thicker section of plastic, which, if too thick, could potentially lead to a risk of sink on the back side of the part. Additionally, a longest pin, which is fully indenting, makes the plastic area thin, so make sure it is not too thin so that you do not end up with a hole in the part because of a short shot or the pin punching through the surface entirely. You can work with Protolabs' applications engineers to discuss pin locations and pin type on critical areas to ensure molding and part design concerns are solved.

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