Torpedo nozzle

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

FIGURE 2.57 Cross section through a torpedo nozzle , shows torpedo and heater position... 

The metal parts of the injection molder, ie, the liner, torpedo, and nozzle, that contact the hot molten resin must be of the noncatalytic type to prevent accelerated decomposition of the polymer. In addition, they must be resistant to corrosion by HCl. Iron, copper, and zinc are catalytic to the decomposition and caimot be used, even as components of alloys. Magnesium is noncatalytic but is subject to corrosive attack, as is chromium when used as plating. Nickel alloys such as Duranickel, HasteUoy B, and HasteUoy C are recommended as constmction materials for injection-molding metal parts. These and pure nickel are noncatalytic and corrosion-resistant however, pure nickel is rather soft and is not recommended. 

The earliest injection moulding machines were of the plunger type as illustrated in Fig. 4.30 and there are still many of these machines in use today. A predetermined quantity of moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by conduction from the external heaters. The material is thus plasticised under pressure so that it may be forced through the nozzle into the mould cavity. In order to split up the mass of material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel as shown. 

If the ultimate in protection is required, a membrane of metal less than 0.1 mm thick is left across the nozzle face and provides visible tamper-evidence. Sometimes a special double-ended cap is used, incorporating an integral piercing device. The tube is pierced by removing the cap, reversing it and screwing it back onto the tube until the inverted cone pierces the membrane. If the product is of the unit dose type a membrane tube is used without a cap or threaded portion, e.g. taper or torpedo ended tubes. In this case piercing of the nozzle must be carried out with a pin or similar device. Alternatively a break-off style tip can be used. 

In turbulent flow conditions, both rigid and flexible macromolecules diluted solutions exhibit drag reduction, which was described for the first time by Toms (25) and often reviewed since [Hoyt (26), Berman (27), Virk (28)]. Many practical applications have been proposed for this effect increase of the range of water-hose nozzles, better efficiency of fluidized beds and central heating, increase of the flow rate in pipe lines, increase of the speed of torpedos etc... But the fundamental origin(s) of drag reduction is still debated. 

Figure 12-2. Processing via the elasto-viscous state (schematic). (I) Injection molding (with torpedo), (II) extrusion, (III) rolling or milling. In injection molding, the granulate is plasticized by heat and then injected into the mold by a torpedo (or with a screw). When the molded article is cold, the mold is removed and the article ejected. In extrusion, the heated material passes through a metal screen (filter) placed in front of the nozzle.

A nozzle without a gate has a partial body, and ends in a channel leading to the gate, or ends with a tip introduced into the gate. There is also a simplified version of the bodyless nozzle known as a torpedo. 

For melts which cause stringing, there are open nozzles with a mini-torpedo (Husky) located at the entrance to the taper channel, i.e., at the point where the sprue is broken away. 

The next type of nozzle is one with a heat-conducting tip (Figure 4.20b), also called a mini-torpedo, located inside the central channel. The tip is heated by the heat of the nozzle casing and the molten material. The insulating chamber is basically the termination of the channel and the melt does not get left behind in it. Colour can be changed rapidly. 

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. 

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...

Figure 4.22 External heating of nozzle with torpedo, facilitating colour change Reproduced with permission from EWIKON Heiflkanalsystem GmbH Co. KG)...

In design 4.24(b), a short torpedo is part of a screw-in insulating bushing. A copper core inside the torpedo helps to feed heat to the gate. At the same time the steel jacket of the torpedo protects it from abrasion. This nozzle is suitable for all plastics. 

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. 

The telescopic connector is rendered leakproof by a fit with a very small clearance (0.005 mm, lapped aperture) and by differentiation of thermal expansion. The bushings are heated by band heaters, whereas the connectors are heated by cartridge heaters. The basic nozzle in this system is one with a heated torpedo. The nozzle cartridge heater may easily be replaced through a hole in the mould clamping plate. 

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. 

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