Generally, plastic parts can be taken out of the mold cavity by only one ejection, yet due to their special forms, it is still difficult to take out the parts from cavity or the parts can not fall off freely from the mold upon the completion of an ejection movement. Under such case, another ejection must be applied to make the plastic parts fall off; sometimes a secondary ejection is also adopted to avoid exceeding force on plastic parts through one ejection, such as plastic parts with thin walls and deep cavity or those of complicated shapes, which can easily get cracked or distorted upon one ejection due to the large contact area between the parts and the mold, and hence, secondary ejection is applied to such plastic parts to detract ejection force, thereby ensuring the quality of plastic parts.
Ejector pins are required to remove the green component from the tool. A sufficient quantity of these pins is required to ensure that the green component can be removed without distortion or cracking. As a natural consequence, witness marks where these ejector pins were located are evident.
The size of the pin should be selected to allow the cavity where the pin fits to be opened to accommodate larger ejector pins as the tool ages. Ejector pins are typically round, since round ejector pins are available in many standard sizes and the ejector pin housing in the cavity block is most easy to EDM.
Rectangular ejector bars are sometime used in special cases; however, the radius associated with the corner of the bar causes issues with fit-up and long-term integrity of the steel in thin sections, since these corners can act as stress concentrators for crack initiation in the tool.
a typical ejector pin blemish. These witness marks become more evident as the tool ages owing to the wear between the pin and the cavity where the pin resides.
Ejector pins are located where the greatest ejection force is required, for example near bosses, cored holes, and ribs. The aesthetics and functionality of the finished component should also be considered when selecting an ejector pin location.
Ejectors are primarily used to generate vacuum for robotic grippers, packaging and localised vacuum generation.
Tip – Ejectors should include rapid shut-off of the compressed air.
Tip – Ejector systems are often a source of compressed air leakage and must be included in the leak survey programme.
Tip – Ejectors are a substantial user of compressed air and costings for ejector use must include not only the first installation cost but also the high operational cost of compressed air used in this type of vacuum equipment.
Tip – Suction cup leakage is a contributor to the high operating cost of ejectors and suction cups must be selected carefully and kept in good condition to reduce energy use.
Ejectors provide high vacuum levels at relatively low-volume flow rates. This vacuum is not generated by a motor or blower but by the use of the ‘venturi’ effect and a feed of compressed air. A stream of compressed air flows through a drive nozzle and the velocity differential creates a lower pressure at the inlet.
Ejectors have no moving parts and therefore require no maintenance and never wear out. They generate no heat, can be very small, and permit the implementation of very short cycle times. Ejectors can be fitted with sophisticated controls to regulate the vacuum generated by modulating the compressed air supply.