Shut-off nozzles

Shut-off nozzles were developed with large-diameter gate designs in mind, and also with the idea of eliminating the undesirable vestige left by the sprue (see Chapter 4.1.1). In the 

In contrast to other nozzles, a shut-off nozzle leaves no vestige, just a round mark from the pin. This is why it is used to manufacture products with high surface quality requirements. 

A shut-off nozzle is the only type of nozzle that allows HR to be used for injection moulding of structural foam, and for sequential and cascade moulding. 

Despite mechanical gate shut-off, there is a problem with the melt freezing in the gate, just as with other types of nozzle. 

In addition, specific requirements regarding cooling are made of shut-off nozzles  

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The sharp melting poiat and the low melt viscosity also mean that nylon can give problems with nozzle drool and/or premature freeze-off For this reason, it is normally necessary to use either a reverse-taper nozzle (fitted with a heater to avoid freeze-off), a mechanical shut-off nozzle, or melt decompression. Melt decompression, or suck-back, iavolves the screw retractiag slightly after the screw retractioa time, pulling the material back and... 

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. 

With backpressure the process is performed in conventional injection molding machines (IMMs) (Chapter 4). The cover stock is inserted and located in an open mold. A shear edge mold permits draw-in of the cover stock during the closing cycle to avoid wrinkles and damage by stretching of the fabric. Molds require special attention. They generally use a hot runner system with its shut-off nozzle(s). All mold elements such as ejector, core pulls, and slides have to be on the injection side mold half. 

The compression is low-level with duroplastic screws. There are practically no shut-off nozzles or nonreturn valves for blind comers and crosslinking deposits. 

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

Shut-off nozzles cannot be used with certain highly sensitive molding compounds because material could accumulate and decompose at the shut-off point. An open nozzle is used to process molding compounds of this type (PVC, POM) or to minimize the cost of machinery in general. It is impossible to completely prevent the escape of material through an open nozzle. This must be taken into account in the feed operation (by decompressing at the end of the feed phase). 

When back-flow valves are used they must be inspected, and serviced, regularly as they play an important role within the injection unit. These comments apply particularly to high speed machines where screw cushion may not be used and extreme accuracy is demanded. Shut off nozzles are not widely used nowadays, due to material leakage and degradation taking place within the nozzle assembly. Guidance will be given on the types of nozzles used for each material. No matter what nozzle is used, the correct nozzle contact force should be used as, if it is too much, then the fixed mold half may be pushed off the platen. 

Tlie use of a shut-off nozzle is not recommended due to high pressure losses and the risk of degradation the use of a shut-off nozzle also results in an increase in injection cylinder pressure and, an increase in water absorption. An open nozzle, together with decompression, is suggested cellulosics will not drool if properly dried and then processed at the correct temperature. 

The heart of the process lies in the special nozzle block which is, in effect, a composite shut-off nozzle consisting of two nozzles, one inside the other, and actuated by separate hydraulic cylinders. 

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... 

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

A type (b) mould has been converted by replacing the cold sprue bushing with an HR shut-off nozzle. The cycle time has been reduced by 8%, and the production cost has fallen by 18%. 

Number of injection points. Besides single nozzles, there are also multi-point nozzles, with two to six or more injection points. These are mainly tip nozzles and side gating nozzles, but there are also open and shut-off nozzles. The use of multi-point nozzles enables the cost of a HR system to be reduced by bringing down the number of nozzles and possibly doing away with the manifold. 

Figure 4.36 Comparison of design of partial-body (a) and full-body (b) and shut-off nozzles used depending on the plastic structure Reproduced with permission from Husky Injection Moulding Systems.

The full-body shut-off nozzle (a) is designed for the processing of quick-setting crystalline plastics, e.g., PA, PAEK, PEEK, POM, PPA, PPS and others. Proper heat supply to the gate prevents gate freezing before the holding time is completed, and facilitates the essential final compaction of the material. 

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