The sprue is a round, tapered hole which leads the plastics material from the nozzle of the heating cylinder of the injection molding machine to the distributing runners in the mold. A typical sprue is shown in Fig. below.
The heater nozzle makes contact with the sprue bushing at the small end of the taper, against some sort of accurately fitting seat which in this diagram is shown to be spherical. The diameter of the small end of the sprue should be .015″ to .025″ larger than the orifice at the end of the heater nozzle.
This provides sufficient margin for possible variation in nozzle and mold alignment, so the portion of the sprue which breaks off inside the nozzle when the mold opens will not overhang the hole in the bushing and prevent free passage of the sprue through it.
The size of the nozzle and sprue employed for a specific molding application should be in proportion to the size of the shot or charge of material required to fill the mold. Large sprues generally provide better flowing conditions than small sprues, and do no particular harm except to increase the amount of sprue scrap which must be reprocessed and used again.
A taper of 2°-30′ on a side is need. This is a practical amount which will insure easy release of the sprue but which will not increase the diameter of the large end unduly for a long sprue. The taper should be reamed smoothly, without tool marks, and it should be polished. Rough sprues may cause losses of as much as 10 seconds in the molding cycle because of the extra time the molded material requires to cool and harden sufficiently to permit it to be pulled out of the rough hole.
The use of a standard taper angle for all of the sprue bushings manufactured in a single shop makes it possible to use a single reamer for reaming a wide range of diameters and lengths. The shoulder on the sprue bushing should be rather long, because there is possibility of encountering large thrusting forces which tend to push the shank through the shoulder.
A generous radius where the diameter changes avoids stress concentration and hardening cracks. A radius at the large end of the sprue, where it meets the runner, improves the flowing conditions for the material. Mold steel should be used in making the sprue bushing, because it should be hardened to 40-45 Rockwell C. This makes the bushing resistant to crushing and brinelling, and thus it helps in maintaining good seating of the nozzle, which prevents the molding material from leaking out at this junction. Also, lodged or stuck sprues may be hammered out with a rod of brass or other soft metal without damaging the seat or taper.