HR nozzles

Direct gating to a cold runner in a multi-cavity mould. In this design, the CR is only partially replaced by an HR system. A simple open HR nozzle eliminates the sprue (Figure 1.3e), thus shortening the reqnired mould opening path and making it easier to separate the sprue protrusion from the moulding on the transporter belt. This method is used when the production volume or product shape does not jnstify the use of HR nozzles for each cavity. 

Use of mixed CR and HR systems has the advantage (leaving economic considerations aside) of simplifying the problem of temperature control in the HR nozzle gate area, and additionally enables the cold plug from the injection moulding machine nozzle to be halted. 

HR nozzles operating with an externally-heated distributor may be fitted with external or internal heating. In simple applications, short nozzles without heating are still in use, i.e., nozzles conducting heat from the distributor. 

HR nozzles operating with an internally-heated distributor also have internal heating. It is difficult to install shut-off nozzles in this system. 

For amorphous plastics, where this window is very wide and also includes, for example, thermoforming, the sprue in an HR nozzle gate solidifies slowly, and at temperatures lower than Tp, it does not form a plug hard enough to block the gate. The value of AT should therefore be taken as no more than an approximate window of openness of an HR gate for amorphous plastics. 

Injection machine nozzle temperature = HR nozzle temp. ... 

Like the runner dimensions, the viscosity of the plastic influences the magnitude of pressure losses in an HR system, and thus has a bearing on the permissible runner length. When HR nozzles are being selected a permissible pressure loss of up to 25 MPa or thereabouts is often adopted. The total pressure loss in an HR system, meanwhile, should not exceed... 

The HR nozzle is the end of an HR system, and comprises a heated channel, terminating in a gate, which feeds the melt to the mould cavity or to a cold auxiliary channel. An HR nozzle may also be the sole part of an HR system, as a central nozzle. In many applications, and for various reasons, the gate is not a part of the nozzle, and is made in the mould. 

The HR manifold is the central component of an HR system in it are the flow channels feeding the melt to the HR nozzles. 

The channel must be polished. In the centre of the manifold is a sprue bushing (1), on the opposite side is a support pad (3) taking the pressure from the injection machine cylinder and a pin (4) aligning the manifold. A second pin (5) located in the end of the manifold sets its angular position. The manifold is pressed against the HR nozzles located in the mould by pressure pads (6) on the opposite side of the manifold. The transfer from the... 

When a traditional injection machine operating cycle is used for quick-setting plastics with removal of the injection unit, a cold plug forms in the standard nozzle tip. This may be the cause of a fault that is hard to identify - clogging of one of the gates of an HR nozzle. 

In this mould there are eight HR nozzles which are balanced by varying nozzle channel diameters in the 11-18 mm range, thus giving identical pressure drops at the ends of nozzles A-D. Cold runners and the moulding item zones corresponding to them are balanced in the form of separate systems. 

In a mould for radio dials (Figure 8.5), three HR nozzles have been used [3]. The cavity was fitted with a pressure meter and thermocouple, enahling the mould temperature and holding pressure to he monitored directly, thus minimising shrinkage of the plastic and making it reproducible. Too much shrinkage could cause delamination of the decorative layer. 

An example of an IML mould is shown in Figure 8.6. The HR nozzles are closed by pneumatic cylinders, and the ejector plate is also driven by a pneumatic cylinder, to prevent any oil leaks from fouling the lining. The mould has a telescopic closure to cut... 

Figure 9.18 Example of location of HR nozzle in long core...

High melt temperature Reduce cylinder temperature. Reduce screw rotation speed. Reduce temperature of manifold and HR nozzle. Check/renew thermocouples. 

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