Sprue bushing heated

Shear heating of melt] injection rate too fast/injection pressure too high/gates too small/nozzle orifice too small < 0.8 of sprue bushing/nozzle dirty/sharp corners/injection rate too fast/shutoff nozzle used instead of a general purpose nozzle/improperly designed or defective non-return valve. 

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 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... 

Since an HR system essentially comprises two functional parts, the distributor with sprue bushing and the nozzles mixed systems can also be found, i.e., a distributor with external heating or nozzles with internal heating. 

A type (a) mould of three-plate design with heated sprue bushing is converted using an HR manifold and nozzles with side gating. The cycle time has been reduced by 35%, and the production cost has fallen by 43%. 

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). 

Figure 4.95 Heated sprue bushing with extra temperature balancing...

Figure 5.16 Example of marking of heating zones 1-8 - nozzles 9-11 - manifold 12 - sprue bushing (Reproduced with permission from D-M-E Belgium)...

Heat conduction from the manifold takes place chiefly via the spacer pads and fastening bolts, and perhaps also via the non-heated sprue bushing. A diminution in heat losses caused by conduction may be attained by ... 

The manifolds most frequently used have a very long heated sprue bushing located on the mould axis and operating on the principle illustrated in Figure 9.10. As can be seen, the melt flowing out of the sprue bushing may, when the mould is taken away from the injection machine nozzle, gets into the ejector plate space. 

In view of the advantages of HR systems with internal heating (described in Chapter 4.2.2), they are also used in stack moulds. Of particular importance here is the potential to reduce the mould height. Figure 9.14 shows a fragment of a stack mould with a long sprue bushing, internally-heated manifold and two-way torpedo. A characteristic feature of this system is that the manifold is the mould plate. 

Fig. 19.35. Schematic of injection molding operation. A. When the mold starts to close, the screw has just finished charging the front end of the cylinder. B. With the mold closed, the heating cylinder then moves toward the sprue bushing and is ready to commence the injection cycle. C. The hydraulic cylinder then forces the screw forward under the injection head and fills the cavity. D. Cooling and charging cycle the mold is filled and, after the holding time, the screw starts to rotate and charge the front end of the cylinder. E. The screw is still charging the front end of the cylinder, and the cylinder starts to move away from the sprue bushing. F. The finished cycle the mold opens and the molded part is ejected with the sprue bushing. Du Bois, J. H., and John, F. W., Plastics, Reinhold, 1967 by permission of Van Nostrand Reinhoid Co.)...

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