The Sprue Bush

The diameter of the sprue bushing should be at least 1.6 mm larger than the diameter of the main runner and slightly larger than the nozzle orifice. A standard taper of 4-6 mm/m is a good rule-of-thumb choice. 

The molding compound flowing from the nozzle enters the mold and is distributed into the cavities. This is the task of the sprue- and numer system (see Figure 4-1). It consists of several sections, that may differ in design, subject to requirement. As the melt leaves the nozzle, it passes through the sprue bush and into the runner, which connects to the gates of the cavities. 

That part of the injection molding tool, against which the nozzle pushes during the injection process to inject the melt. The bore of the sprue bush is cone-shaped and with many single impression molds connects directly into the cavity... 

The sprue is the metal attached to the nmner that solidifies in the sprue bush. The overflow is a recess with entry from the die cavity, to assist in the production of sound castings and ensure complete filling of the cavity. 

Nozzle opening at least 4.75 mm and usually 7.9 mm and 0.8 mm smaller than the orifice dimension of the sprue bushing land length < 4.75 mm and nozzle... 

A nozzle insert of internal diameter equal to that of the sprue bush is fitted. Use of too small a nozzle is a common cause of light components due to a decrease in pressure in the runner, or in extreme cases scorch with attendant problems. 

Once the interlocks are satisfied, the mold closes. Then the injection unit occasionally moves slightly forward to coimter a motion called sprue breakaway. With cold runner systems, the injection rmit is moved slightly away from the sprue bushing to facilitate sprue removal and/or prevent nozzle freeze-off. Thus, injection unit must be moved forward to provide intimate contact between the nozzle and sprue bushing during injection. Sprue breakaway is not used with hot runner systems. 

The support plate backs up the B plate in the mold, and melt is delivered from the nozzle to the sprue bushing. The locating ring aligns the mold with the nozzle, whereas the leader pins and bushing align the two sides of the mold. In the ejection system, the sprue puller and ejector pins, are attached to the ejector plate, which is, in turn, connected to the machine s ejection plates by the ejector rod. When the mold is opened, the sprue puller draws the part to the B side of the mold, whereas the... 

The front end of the sprue bushing contains a spherical depression or recess to fit the cylinder nozzle in order to permit easy release of the plastic sprue. Standard taper should be 1/2 in./ft. The orifice at the nozzle end of the sprue will separate readily from the nozzle when the mold opens. The sprue orifices should be at least 1/4 in. in diameter. The length of the sprue channel should be as short as possible and never longer than 4 in. 

To prevent drool and nozzle freeze-off, good nozzle temperature should be maintained. The heater band is provided and should be as far forward as possible on the nozzle and should not touch the platen. Nozzle can also be heated by heat pipes. Nozzle designs have been successfully used, with the radius of the nozzle tip being less than the radius of the sprue bushing. 

The locator ring helps fit the injection mold into a press clamp. The sprue bushing is a channel by which the plastic flows from the injection... 

Shutoff nozzles are used to prevent nozzles from leaking into empty molds or when the nozzle is retracted from the sprue bushings. Valves of all kinds —i.e., spring-loaded ball check valves, hydraulically and mechanically operated needle valves, and rotary valves —have been incorporated into nozzles. 

The seating ring centers the sprue bushing on the injection platen of the machine directly in line with the nozzle of the injection cylinder. The sprue bushing should be as short as possible from the nozzle to the parting line of the mold and have a taper of at least 3 deg in order to maintain the heat of the melt. Cold slug wells should be used wherever possible to permit the leading cold resin to be trapped and to permit the hot resin to... 

It is suggested that the radius where the sprue bushing meets the nozzle be 1/8-1/4 in. greater than the radius of the nozzle. This provides a contact line rather than a broad surface, which is helpful when stray glass fibers get between the surfaces. ... 

The figure shows examples of solutions to this problem with the rheological balance maintained. Because of the flow disruptions that occur, it has been necessary to eliminate the central channel located opposite the sprue bushing channel. In Figure 4.71b a two-level manifold has been used, with a central channel with a diameter that is smaller than the diameter of the outside channels. In Figure 4.71c, a manifold with two small-diameter inclined channels has been used. 

The channel of an open nozzle in an injection machine should have a diameter that matches the channel of the sprue bushing (see Figure 4.84). 

In view of the low temperature of the manifold plate, the sprue bushings must be heated. Both external and internal heating is used (for the use of a torpedo, see Figure 4.92). 

The numbers 1-8 have been given to the nozzles/cavities, starting with the nozzle which is nearest to the guide pillar with the smallest diameter, and proceeding clockwise. The numbers 9-11 have been allocated to the manifold zones in a similar sequence (9-10 - side distributor channels, 11 - main channel), and 12 has been allocated to the sprue bushing as the final component of the system. 

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