Shut-off needles

FIGURE 1.11 Hotrunner mold dart , 32 cavities, with electro magnetically operated shut-off needles... 

FIGURE 1.51 Hotrunner shut-off needle operated by a servomotor [28]... 

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. 

A type of miniature globe valve, needle valves are used in instrument systems for throttling of small volumes. They have metal to metal seats, but due to the small size, can be used for positive shut-off (Figure 15-8). Needle valves have small passageways that may plug easily and limit their use to very small flow rates. 

A, Shut-off valve B, Micrometering valve 1, Upper stem 2, Lower stem 3, Flexible coupling 4, Cone (needle) 5, Micrometer scale. 

Most flow controller needle valves are manufactured to close tolerances. The needle valve should never be turned off too vigorously, as this can damage the O-ring shut-off seal and the fragile needle. The flow controller needle valve should therefore not be used as an on-off valve. 

Needle Valves. For control of gas 8 ow at ordinary pressures, needle valves give much better control than stopcocks. The hard-steel tapered needle, at the end of a screw-threaded shaft, seats in a cylindrical hole so that the area of open space for gas 8 ow is gradually increased or decreased on rotating the shaft. These usually do not provide a reliable shut-off and are used for 8 ow control only. Persons whose acquaintance with valves is limited to water faucets often damage needle valves by needlessly overtightening them when shutting off the 8 ow. This results in a decrease in the sensitivity of control of the gas 8 ow. It is important not to exert anymore force than necessary. 

To effect the transfer, the argon pressure in the flask containing the copper salt is reduced by inserting a needle through the septum and by shutting off the argon... 

Figure 3.24. Mettler TA 2000 C ihermoanalyzer schematic diagram. I. furnace for regular gas atmosphere 2. furnace for corrosive gas atmospheres 3. inlet valve 4. needle valve 5, gas flow meter 6. pressure gauges 7. exhaust valve 8. shut-off valve 9, bypass valve 10. plate valve 11. fore-vacuum valve 12. diffusion pump 13. rotary pump 14, balance 15. sample cup holder.

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

State-of-the-art technology allows the manufacture of such composites in 32 cavity moulds with modern cold runner technology using needle shut off valves. 

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). 

---