Ejector pins are the “bouncers” of the injection molding world. They let technicians apply a force to eject a part from the mold, and, in some cases, can leave marks. The goal for engineers and molders is to design and position pins to minimize their effect on parts; although molders typically determines pin placement, customers get to sign off on pin locations before an order is finalized. In some cases, manufacturers provide contour pins, which are ejector pins manually ground at an angle to closely match the part’s contoured surface.
This is an example of the illustration some molders provide early in the process of designing the mold so that the location and size of both the gate(s) and ejector pins can be approved.
Pins are in the B-side half of the mold, the side in which the part remains when the mold is opened. Once the mold is opened, the pins extend into the mold cavity, push the part out, and then retract, letting the mold close and be refilled.
Factors to Consider
The number of ejector pins and their placement depends on several factors. Obviously, the shape of the part is one. Other factors, such as draft and texture of sidewalls, and depth of walls and ribs, increase the likelihood that areas of parts will cling to the mold. Resin choice also affects pin placement or size. Some resins are “stickier,” requiring more force for release from the mold. Softer resins may also require more or wider pins to spread the removal force, as well as to prevent puncturing or marring of the cooled plastic.
Some molders use round ejector pins with flattened ends perpendicular to the direction in which the pin moves. To be effective, the pins need a flat “pad” on the part to push against, and the pad’s surface 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 pin, the cosmetic marring will be even more obvious.
In traditional steel production tools, it may be possible to machine the end of a pin to match the contour of a part’s surface that is not perpendicular to the direction in which the pin moves. These are called contoured pins.
Many molders usually do not support production of contoured pins unless customers request it. It is done on a case-by-case basis.
The above illustration shows three basic types of pins 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.
If a pin needs to act on a part surface that is not parallel to the pin-end, there should be a pad, provided it 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 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 tangent to the surface. There are two other options besides just center cut: Shortest leaves the standing pad under pin, while longest fully indents the pin into the part.
Keep in mind that with the shortest pin, you will be making a thicker section of plastic—which, if too thick, could lead to sinking on the back side of the part. Additionally, a longest pin (which is fully indenting) makes the plastic area thin, so designers should make sure it is not so thin that you end up with a hole in the part on account of a short shot or the pin punching through the surface entirely. You can work with molders’ applications engineers to discuss pin locations and type on critical areas to ensure molding and part design concerns are resolved.
A post gate lets resin be injected through an ejector-pin hole.
A post gate is an extreme example of a raised ejector pad (see illustration above). In cases in which an edge gate cannot be used, resin gets injected through an extension of an ejector-pin channel. When the part cools, the ejector pin pushes against the resulting post and, in the process, clips off the runner. The post is typically removed from the finished part in a secondary operation.
In most cases, ejector pads (or the vestiges left by their removal) are on the non-cosmetic sides of parts. In some cases, however, this may not be possible. Take for example the case of a clip formed using a pass-through core (see clip hook illustration). In this case, because the clip increases the surface area of that side of the part, the “clip-side” part surface will adhere more strongly to its mold half. This will make that mold half the B-side. The clip would normally be on the cosmetic side of the part, but its presence requires that ejector pads also be on that side of the part.
In addition, ejector pins are also sometimes used to help vent deep features in a mold to prevent gas trap at the end of fill.
The bottom of the clip hook and blue face of the clip shaft will be formed by a pass-through core of the A-side mold half, which protrudes through a hole in the base of the part. The rest of the clip is formed by the B-side mold half.
If Surface Area is Limited
All the examples assume there are surfaces that pins can push against to eject parts from a mold. There are, however, designs lacking such surfaces. Take, for example, a grate, in which all that faces into the B-side mold half are the tops of ribs.
If the rib edges do not provide enough surface area for the pins to push against, the designer would need to add some bosses to act as ejector pads.
Another example would be parts made of liquid silicone rubber (LSR). In those cases, ejector pins are not used. Instead, parts are manually pulled from the molds.
In most cases, ejector pin placement is a relatively minor concern in the early phases of part design.
Mike Adams is senior applications engineer at Protolabs. If you have any questions regarding injection molding or gates and ejector pins, please feel free to call a Protolabs application engineer at (877) 479-3680 or e-mail [email protected].