Manifold torpedo

The straight-through gate is a simple inexpensive way of gating that can accommodate probe or torpedo shifts resulting from heat expansion of the manifold. Gate temperature control is poor... 

Figure 7-15. Suggested Design for Side or Edge Gating of a Hot Manifold Mold (Essentially a straight-through gate perpendicular to torpedo) "...

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. 

In the classic D-M-E version, the manifold has large-diameter (24-50 mm) drilled flow channels in which cartridge heaters in a tubular body are located. The melt thus flows through an annular channel between 4 and 9 mm wide (see Chapter 1). Because of the heating pipe in the centre of the channel, the channels cannot cross over as in externally-heated manifolds, but must be connected adjacently. Torpedoes must also be connected with a butt joint to the flow channels. This creates certain restrictions as regards the mould cavity arrangement, and it is not easy to achieve a natural flow balance (see Figure 4.91). 

To simplify cleaning, and also colour changes, a split manifold design has been developed, where the manifold may be rapidly dismantled when the mould is removed from the injection machine. The flow channels may be located in one or both halves of the manifold, and they may intersect as a result of the use of heaters set in one end of the channel. The ends of the heaters must be supported to prevent their being skewed by the flow of melt. To clean the manifold, the heating should be switched on, the manifold opened up and the heaters and torpedoes pulled out of the frozen sprue. 

Use of an internal channel heating system, coupled with the low manifold plate temperature, have enabled torpedoes to be set within the manifold and for them to be used to heat the exit channels. A torpedo is normally fitted in a fixed manner (see Figure 4.93), but there is also the potential to fasten it in such a way that it can move (Figure 4.93b). 

Figure 4.93 Methods of setting torpedoes in manifold with internal heater...

As has already been mentioned, the problem of thermal expansion does not arise in this type of manifold, but compensation for thermal expansion AL of torpedoes and heaters is required. 

Nozzles with the power lead running in from the side (see Figure 4.94) are set between the manifold and the cavity plate in the axis of the channel (not adjacent). These nozzles may be in the form of a torpedo, a mnlti-tip nozzle or an edge nozzle. 

Figure 4.94 Setting of torpedo (nozzle) under manifold...

In view of the low temperature of the manifold plate, the sprue bushings must be heated. Both external and internal heating is used (for the use of a torpedo, see Figure 4.92). 

Using a two-level system of channels in the manifold, partial natural balance was achieved. Rheological adjustment was performed through differentiation of the insulating torpedo chamber diameters. This differentiated the thickness of the flowing melt layer around the torpedoes. 

In view of the advantages of HR systems with internal heating (described in Chapter 4.2.2), they are also used in stack moulds. Of particular importance here is the potential to reduce the mould height. Figure 9.14 shows a fragment of a stack mould with a long sprue bushing, internally-heated manifold and two-way torpedo. A characteristic feature of this system is that the manifold is the mould plate. 

Figure 9.14 Example of use of an HR system with internal heating in a stack mould 1 - sprue bushing 2, 3 - cartridge heaters 4 - manifold 5 - two-way torpedo Reproduced with permission from D-M-E Belgium)...

A system with external heating enables the channels to be purged relatively quickly. A system with internal heating does not allow the channels to be purged completely, and in some cases it is necessary to take the manifold apart and clean the channels, torpedoes and insulation chambers manually. Some systems are designed in such a way as to allow the channels to be opened up (see Figure 4.91) sometimes this can even be done on the machine without removing the mould. 

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. 

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