Injection molding nozzles

Usually the melt temperature only is taken or estimated from the inside of the barrel or the surface of the melt as it moves through the barrel. Various techniques can be used (such as IR sensors) that look at melt temperatures across the entire melt stream, as, for example, when it exits an extruder (or when it exits an injection molding nozzle into space, etc.). An automatic thermocouple system (patented by AutoProbe, Normag Corp., Hickory, NC) has a motor-driven, retractable melt thermocouple, which moves across the melt stream while simultaneously displaying temperature and temperature profile position (10). The system shows that temperature variations within the melt stream can be considerably wider than expected. 

Injection molding nozzles n. A short, thick-walled, hardened-steel tube that conducts the molten plastic emerging from an injection cylinder into the mold. It usually terminates in a hemispherical tip that closely mates a recess in the mold and is enclosed by band heaters that are controlled by a temperature sensor. 

Fig. 8. Gas-assist injection molding (nozzle mold gas entry with bubble in part). Illustration by Rick J. Bujanowski John W. Bozzelli.

Breaker Plate. A perforated plate located at the rear of the die in an extruder. It supports the screens that prevent foreign particles from entering the die. Also used without screens in injection-molding nozzles to improve distribution of color particles in the melt. 

FIGURE 3.17 Injection molding nozzle with a combination of materials... 

Using laser and/or electron beam welding, components can be produced that combine the (targeted) properties of material (e.g., a mold steel and a highly heat conductive copper base) where needed. This allows many previously structurally desirable but not technically feasible applications of copper-steel composite components. Figure 3.17 shows an injection molding nozzle on which the highly stressed tip and the thread were manufactured from a Co-base alloy. 

Injection Molding. Matched metal molds are used in the fabrication of plastic closures, specialty packages, and botde preforms. In conventional injection mol ding the plastic resin is melted in an extmder which forces a measured quantity or shot into a precision-machined chilled mold after which the nozzle of the extmder is withdrawn. 

The metal parts of the injection molder, ie, the liner, torpedo, and nozzle, that contact the hot molten resin must be of the noncatalytic type to prevent accelerated decomposition of the polymer. In addition, they must be resistant to corrosion by HCl. Iron, copper, and zinc are catalytic to the decomposition and caimot be used, even as components of alloys. Magnesium is noncatalytic but is subject to corrosive attack, as is chromium when used as plating. Nickel alloys such as Duranickel, HasteUoy B, and HasteUoy C are recommended as constmction materials for injection-molding metal parts. These and pure nickel are noncatalytic and corrosion-resistant however, pure nickel is rather soft and is not recommended. 

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