Tip nozzles

Use of tip nozzles is not recommended for shear-sensitive plastics (which include plastics with flame retardants or organic dyes), since the small annular gap in the gate may cause large shear stresses to arise, with a rise in temperature and degradation of the plastic or the additives in it. 

Because of insufficient heat transfer, use of steel torpedoes and tips is not recommended for quick-setting crystalline plastics (apart from special products). 

Tip nozzles are equipped with a full body, a partial body, or - in the case of torpedoes - no body at all. The nozzle body facilitates the use of insulating gaps, and thus assists the processing of crystalline plastics and colour changes. Nozzles designed for injection of reinforced and filled plastics are supplied as a special product. 

3 - insulation bushing 4, 5 - torpedoes with various tips 6 - body with gate. (RK - Radius) 

This kind of nozzle may be used for most plastics, with the exception of plastics with flame retardants, PUR and elastomers. 

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Melt too cold at the nozzle or hot tip] nozzle too cold/temperature sensor error/ too few heater bands/heater bands to far from nozzle tip/hot tip heat source too far from orifice or faulty/sharp corners near the gate. 

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. 

As a result of internal heating, annular gates of tip nozzles are far less susceptible to clogging. When the melt is injected, a layer arises adjacent to the wall, but because of the hot tip it does not entirely stop up the gate. When amorphous plastics are injected, this layer forms a ductile plug in the vicinity of the tip (Figure 4.8a) which is easy to eject. 

The gate size sometimes requires optimisation following trial injections. The adjustment to the gate should be carried out in a way that does not alter its configuration. Special recommendations concerning adjustments to the gate of a tip nozzle are to be found in Chapter 4.1.3. 

Tip nozzles may be divided into three basic types (Figure 4.20) ... 

Nozzles with heated torpedo. The simplest kind of tip nozzle with torpedo (Figure 4.21a) is internally heated by a cartridge heater and attached directly to the manifold (mould plate) with internal heating. It thus serves at the same time to heat the feed channel in the manifold. The end of the nozzle is supported by three pins or short ribs in the bottom of the chamber. Sealing of the channel between the plates is provided by an external frozen layer of melt. 

Figure 4.21 Examples of tip nozzles with heated torpedo...

A similar tip nozzle with mini-torpedo with helical channel 4.24(c) allows faster colour changes by forcing the melt stream in the insulating chamber to rotate. 

Figure 4.25 Examples of tip nozzles with tip-terminated channel...

Multiple tip nozzles have a singie temperature controi zone for aii gates, which may restrict their use. Advantages are ... 

In a multi-tip nozzle (Figure 4.26a) the melt is delivered to each insulating chamber from a radial channel, and flows past small tips which may be made of various materials, depending on the material being processed. The tips are installed in a single heated copper/ beryllium block. 

Multiple tip nozzles may also be supplied as intermediate products with a termination in the form of a replaceable copper alloy head (intermediate product). This gives the user broad freedom of application in terms of the location and angling of the screw-in tips (Figure 4.27). The nozzle head is heated by conduction, and this means the nozzle should... 

An example of an HR system with multiple tip nozzles is shown in Figure 4.28. 

Figure 4.28 HR system with six-point tip nozzles for 36-cavity mould...

Figure 4.29 Set of four-point tip nozzles in comb layout with 8 mm pitch...

Use of multi-tip nozzles is, however, subject to numerous restrictions. The common insulating chamber gives rise to considerable forces associated with the effect of the melt pressure (Figure 4.30a). 

Figure 4.30 Analysis of multi-tip nozzle selection for moulding of small rollers...

Figure 4.31 shows the use of a tip nozzle with several points to mould a roller wheel of PA. To avoid the roller turning oval, the melt had to be delivered through multiple points. 

Figure 4.31 Application of 4-point tip nozzle for moulding of an idler roller from PA 66 Reproduced with permission from EOC Normalien)...

Gate adjustment. Adjnstments to the diameter of a tip nozzle are always made on the insulating chamber side by increasing the entire taper (Figure 4.33). 

Pressure losses in the gate in tip nozzles are greater than for other nozzles because of the annular nature of the flow and the tendency towards minimisation of the gate size. It may be said, roughly speaking, that the pressure loss for PA 6 in a tip nozzle is around 40-50% greater than in an open nozzle of the same gate diameter [13]. 

Edge nozzles are used both as central nozzles with one or more injection points, and as manifold nozzles, thus making it possible to design multi-cavity moulds with a reduced number of nozzles. Edge nozzles have enabled the economic problem underlying moulding of small products to be resolved, in much the same way as multi-tip nozzles did. 

Tip nozzles. In recent years, tip nozzles for edge injection have also been designed. Because the tip is introduced into the gate, the length of the gate ceases to be a critical value, the cold plug in the nozzle is eliminated, and the mould wall has been thickened to 2-3 mm. 

Use of a tip nozzle (see Figure 4.52a), however, necessitates the use of a split insulating chamber to make installation possible (introduction of tips into gates). 

A system with internal heating (EWIKON) makes it possible to employ side gating with the aid of a manifold and standard tip nozzles located parallel to the parting line (see Figure 4.53). 

Figure 4.53 Edge gating effected using a manifold and tip nozzle located horizontally...

Figure 4.75 Three-level manifold with tip nozzles, with 5 V heating in 32-cavity mould...

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