Needle shut-off nozzles

Figure 3-7 Needle shut-off nozzle (closed and open)...

A nozzle will help the material to get pumped in the mold there are different types such as shut-off nozzle, etc. Both open nozzles and needle shut-off nozzles can be used. Open... 

Needle shut-off nozzles (Figure 1.50) offers clean, smooth gates with relatively large cross-sections and low pressure losses during injection. 

FIGURE 1.50 Needle shut-off nozzle with lateral annular piston [26]... 

FIGURE 1.53 Multi- needle shut-off nozzles with grouped needle drive but individual nozzle heating [21]... 

The changeover valve is normally a part of the machinery. Also, modified hotrun-ner needle shut off nozzles are used to coaxially operate the 2nd component or the filling media (Figure 1.69). 

Finally the hydraulic GIT needle shut-off nozzle is opened and the material cushion is injected for sealing as shown in Figure 10.18c. 

This can be carried out in two ways. The gas can be introduced either through the sprue or directly into the moulded part. The hydraulic needle shut-ofF nozzle is open. The mould material is completely injected. No material cushion remains behind. This stage of the process is shown in Figure 10.19. 

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. 

Hydraulically actuated needle-type shut-off nozzle with cooling jacket... 

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