A Harmonic Three Plate Mold with Double Acting Runner Level Ejection
One of the most difficult problems in mold design utilizing floating plates is to consistently control the movement and position of the plates. This is especially true when building a mold with runners that must be ejected that are on a different level from that of the molded parts.
When this is done, the runner is usually retained against the sprue side with 'sucker' pins. These are pins with a negative draft that project into the runner. When the mold opens. The runner is retained by the 'sucker' pins and the gate between the runner level and the molded part level is broken, usually at the part, leaving the gate and runner intact. Then the problem becomes one of stripping the runner from the 'sucker' pins and allowing it to fall out of the mold in a reliable manner.
One design method uses shoulder bolts and springs to control which plates open first and how far each plate moves. This provides good plate control but does not guarantee that the runner will drop. Also, there tends to be a good bit of clanging and banging as the mold cycles. Among the reasons the runner may not drop is the possibility of a vacuum between the runner and the stripper plate, or more likely, a tiny amount of flash around the 'sucker' pin.
The linkage in this figure solves all the problems. It is designed so that the cavity plate moves away from the sprue side of the mold faster than the runner stripper. This causes the gate to break before the runner is stripped from the 'sucker' pins. Continued opening motion strips the runner and allows the ejector side of the mold to open far enough to provide plenty of room to eject the parts and allow them to drop free.
Another action that takes place is that the runner stripper plate pulls the runner completely off of the 'sucker' pins. At this point, the runner is free to shrink freely which causes it to misalign slightly with the 'sucker' pins. As the mold continues to open, the runner stripper plate moves back toward the sprue which causes the 'sucker' pins to project through the stripper plate. Since the runner 'sucker' holes are slightly misaligned with the 'sucker' pins, the pins dislodge the runner causing it to drop.
As the mold closes the runner stripper action reverses, causing the 'sucker' pins to retract into the stripper plate again before they project into the runner space again to capture the next cycle's runner.
There has been only one complaint with this design. The molders tell us that they cannot tell when the mold is operating because it cycles so quietly.