Nozzle external restrictions

There are several types of nozzle. The simplest is an open nozzle as shown in Fig. 4.34(a). This is used whenever possible because pressure drops can be minimised and there are no hold up points where the melt can stagnate and decompose. However, if the melt viscosity is low then leakage will occur from this type of nozzle particularly if the barrel/nozzle assembly retracts from the mould each cycle. The solution is to use a shut-off nozzle of which there are many types. Fig. 4.34(b) shows a nozzle which is shut off by external means. Fig. 4.34(c) shows a nozzle with a spring loaded needle valve which opens when the melt pressure exceeds a certain value or alternatively when the nozzle is pressed up against the mould. Most of the shut-off nozzles have the disadvantage that they restrict the flow of the material and provide undersirable stagnation sites. For this reason they should not be used with heat sensitive materials such as PVC. 

The SMD correlations in Table 24.3 for air-assist nozzles contain at least one condition that makes it difficult to compare them with each other. For example, 24.3.i derived by Sakai et al. [28] contains restrictions on the liquid, and 24.3.V by Wu et al. [29] assumes that there are no aerodynamic effects on the liquid. The most generic equation in Table 24.3 is 24.3.vii derived by Simmons [30] however, even it contains a restriction that the nozzle must be of the externally mixing type. Also, due to the constant C present, the equation has very limited applicability. The reason on these conditions was explained, in part, by Lefebvre [1]. He said that differentiating between an air-assist and an airblast nozzle is often arbitrary. And many times, a nozzle classified as one may be the other. In other words, a nozzle may be both air-assist and airblast. 

Flow balancing. Because of the non-uniform nature of the flow and the enforced nozzle positioning geometry, it is difficult to balance the flow with internal heating this restricts the potential to achieve narrow dimensional tolerances for moulded pieces. Systems with external heating may be effectively balanced, and ensure a rapid change of colour and easy start-up. All nozzle types may be used with them. 

The nozzle size is also governed by the external dimensions, and particularly by the diameter of the flange, which restricts the interval between injection points. 

Use of an HR system makes it possible to deliver the melt from the inside of the product, regardless of the core length. One restriction is the diameter of the core, since a space is needed to locate effective cooling between the nozzle and the shaping surface of the core. In these applications, nozzles with low voltage heating are slightly preferable because of the low external surface temperature of the nozzle. 

The difficulties with color change restrict the field of application of internally heated systems. The following combination can be found more often Externally heated manifold and internal heating in the nozzle area, equivalent to the design of the gate elements as a torpedo or as a thermal conductive tip. 

Equipment used in HTG systems are installed external to the tank. With existing tanks hot tapping, an installation method while the tank remains in service may be the solution when company r n lations permit. This technique is fully developed, but there are different opinions on the safety aspects. The PI transmitter must be installed as low as possible, but above maximum water and sediment level. It is important to realize that the product below the PI nozzle is not actually measured. This restriction severely limits the minimum quantity that can be measured for custody and tax purposes. 

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