Ejector System Design: Principles for Clean Part Release

The ejector system is the part of the mold that gets the least attention during design, yet it is responsible for one of the most critical functions in the molding cycle. If the part does not eject cleanly, the entire cycle stops. Getting ejection right requires understanding the forces involved, the part geometry, and the material behavior. ## Understanding Ejection Forces The force required to eject a part depends on three factors: the shrinkage of the plastic onto the core, the coefficient of friction between the plastic and the steel, and the draft angle of the part. Shrinkage creates a compressive force that clamps the part onto the core. For semi-crystalline materials like nylon or polypropylene, this shrinkage force can be significant. The coefficient of friction varies by material. Polycarbonate has a relatively high coefficient of friction against tool steel, while polypropylene is lower. Adding mold release to the material reduces the friction but can affect subsequent finishing operations like painting or bonding. Draft angle is the most controllable factor. A draft angle of 1 degree per side is the minimum for most materials. For textured surfaces, the draft must be increased by 1 degree for every 0.001 inch of texture depth. Parts with deep ribs or bosses may need 2 to 3 degrees of draft to release cleanly. ## Ejector Pin Layout The layout of ejector pins determines how evenly the ejection force is distributed across the part. Uneven ejection causes the part to tilt during ejection, which can result in stuck parts or damage to the mold. The general rule is to place ejector pins at the deepest features of the part, such as ribs and bosses. These areas have the greatest shrinkage force and need the most assistance to release. The pins should be distributed so that the part is pushed straight off the core without cocking. For large flat surfaces, a combination of ejector pins and ejector blades works better than pins alone. Blades distribute the force over a wider area and are less likely to mark the part. The trade-off is that blades are more expensive to machine and more prone to galling. Ejector pin diameter should be as large as the available space allows. Small-diameter pins are more likely to buckle under the ejection force, especially when ejecting deep parts. A 6-millimeter diameter pin is a good starting point for most applications. For pins over 200 millimeters in length, the diameter should be increased to prevent buckling. ## Ejector Stroke and Speed The ejector stroke must be long enough to clear the part from the core, but no longer than necessary. A stroke that is too long adds unnecessary cycle time and increases the risk of the part falling and getting damaged. The required stroke depends on the depth of the core and the draft angle. For a part with a 50-millimeter deep core and 2 degrees of draft, the part will release after about 10 to 15 millimeters of travel. The remaining stroke should be enough to clear the part from any undercuts or core features. Ejector speed should be fast enough to minimize cycle time but slow enough to prevent part damage. A speed of 10 to 20 millimeters per second is typical for most applications. Thin-wall parts can be ejected faster, while parts with deep ribs or fragile features should be ejected slower. ## Stripper Plate Ejection For cylindrical parts like bottle caps or syringe barrels, stripper plate ejection is often preferred over pin ejection. The stripper plate contacts the entire circumference of the part, distributing the ejection force evenly and eliminating ejector pin marks. Stripper plates require more space in the mold than pin ejection systems. The plate must be guided by guide pins that are separate from the ejector housing. The clearance between the stripper plate and the core must be tight enough to prevent flash but loose enough to allow free movement. The return of the stripper plate is typically done by springs or hydraulic cylinders. Springs are simpler but can fatigue over time. Hydraulic return gives more positive control but adds complexity to the mold. ## Air-Assisted Ejection For deep-draw parts or parts with a high shrinkage rate, air-assisted ejection can be used in combination with mechanical ejectors. Compressed air is introduced through the core to break the vacuum that forms between the part and the core surface. The air inlet should be located at the deepest point of the core, where the vacuum is strongest. A small-diameter hole, typically 0.5 to 1.0 millimeter, is drilled through the core and connected to an air supply. The air is pulsed on during the ejection stroke and turned off before the next injection cycle. Air-assisted ejection is particularly useful for polypropylene parts, which tend to stick to the core due to their high shrinkage. It can also help with parts that have a large projected surface area, where the vacuum force is significant. ## Troubleshooting Ejection Problems Sticking parts are the most common ejection problem. The first step in troubleshooting is to check the draft angle. If the draft is less than 1 degree, increasing it is the most reliable fix. If the draft cannot be changed, adding mold release or polishing the core surface can help. Part deformation during ejection indicates that the ejection force is too concentrated. Adding more ejector pins or switching to a stripper plate distributes the force more evenly. Reducing the ejection speed can also help. Ejector pin galling is caused by inadequate lubrication or misalignment. The pins should be lubricated with a high-temperature grease during mold assembly. If galling persists, check the alignment of the pin bores and the clearance between the pin and the bore.