Sprues of Plastic Injection Molds
The sprue is the channel along which the molten plastic first enters the mould. It delivers the melt from the nozzle to the runner system. The sprue is incoporated in a hardened steel bush which has a seat designed to provide a good seal with the nozzle.
Since it is important that the sprue is pulled out when the mould opens and there is a sprue pulling device mounted directly opposite the sprue entry. This can take many forms but typically it would be an undercut or reversed taper to provide a key for the plastic on the moving half of the mould. Since the sprue, like the runner system, is effectively waste it should not be made excessively long.
