Mould nozzles

The process is illustrated schematically in Fig. 8.3. Polypropylene granules are fed into a continuously rotating screw of special design to permit nitrogen gas to be introduced to the melt. The screw feeds melt and gas into an accumulator where they are maintained under high pressure to keep the gas in solution . When sufhcient volume of melt has been accumulated (limit switch) a valve in the mould nozzle is opened and melt is forced into the cavity where, under reduced pressure, the nitrogen gas is free to expand. The nozzle valve closes and the accumulator continues to be charged with melt from the extruder barrel. 

For many applications, adhesive dispensing from the original package is suitable and the adhesive can be dispensed by squeezing the tube or bottle. Many of the adhesive bottles and tubes have a moulded nozzle to help control the quantity dispensed and in some applications additional control can be achieved by fitting a small nozzle onto the bottle (Figure 8.6). 

After the initial solidification of the melt, the gas pressiu e is reduced. This takes place either by allowing the gas to escape into the ambient atmosphere, or by recovering a certain proportion (up to 90%) through the machine nozzle or the mould nozzle. The process is illustrated in Figure 10.17. 

Additional costs for gas pressure generation equipment and pressure regulator modules, gas, machine nozzles or mould nozzles, and license fees Frequent jetting... 

Machine nozzles and mould nozzles are available for the introduction of the gas into the moulded part. 

Polyaryl ether ketones may be processed on conventional injection moulding and extrusion equipment, providing sufficiently high temperatures can be achieved. Melt temperatures required are typically 370°C for unreinforced PEEK, 390°C for reinforced PEEK and both unreinforced and reinforced PEK and unreinforced PEEKK, and 410°C for reinforced PEEKK. For the latter material a temperature profile from feed zone to nozzle would be... 

The moulding of Amite PETP must be carried out with dried material and because of the free-flowing nature of the melt, restricted nozzles should be used... 

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. 

Nozzles The nozzle is screwed into the end of the barrel and provides the means by which the melt can leave the barrel and enter the mould. It is also a region where the melt can be heated both by friction and conduction from a... 

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. 

The first stage of the cycle is the flow of molten polymer into the mould cavity through a standard feed system. Before this flow of polymer is complete, the injection of a predetermined quantity of gas into the melt begins through a special nozzle located within the cavity or feed system as shown in Fig. 4.45. The timing, pressure and speed of the gas injection is critical. 

One of the major innovations in recent years is the use of pulsed pressure through the gates to introduce and control the orientation of the structure (or fillers) in injection moulded products. A special manifold is attached to the machine nozzle as illustrated in Fig. 4.46. This diagram relates to the double live feed of melt although up to four pistons, capable of applying oscillating pressure may be used. 

The container shown at the top of p. 341 is injection moulded using a gate at point A. If the injection pressure at the nozzle is 140 MN/m and the pressure loss coefficient, m, is 0.5, estimate (i) the flow ratio and (ii) the clamping force needed. 

NMR microscopy is appropriate to study the flow behavior of complex materials, the flow in complex geometric structures and processes such as extrusion, injection moulding, flow in nozzles, pipes, etc., because the velocity vectors can be directly... 

Control of the temperature of rubber compound whilst being processed in extruders and injection moulding machines is vital if the product from these processes is to be uniform in quality. Failure to control temperatures in processing equipment can lead to scorching of the compound as it emerges from the extruder die or injection machine nozzle. Most modem items of processing equipment are fitted to a temperature control unit using either oil or water as the circulatory medium. 

Gas is injected into the material through a specialized nozzle. The plastic in contact with the mould is colder and therefore stronger thus the gas stays in the core of the plastic and presses it against the wall cavity. 

High-intensity vibration effect upon dissolved and molten polymers is relatively simple and can be obtained with the help of superimposition of high-frequency ultrasonic (US) vibrations. Periodic (cyclic) shear and volumetric strain of the medium is attained in this case without moving elements and due to elastic oscillations of US-vibrators of various design which function simultaneously as moulding elements (nozzles, cores) of extrusion heads. 

Developments in micro-processing have made it much easier to monitor and to control conditions in the course of manufacture, and to prepare and maintain written records. Injection moulding may be supervised with the help of melt thermocouples fitted in the nozzle, pressure transducers in the hydraulic line and in the mould, and a transducer to monitor linear movement of the screw. Information from these sensors may be displayed and depicted in relation to the control limits (upper and lower) and the operator thus made aware immediately of variations that could be significant. Conditions can be corrected more rapidly and if need be any mouldings that might not be satisfactory diverted for examination. Should the number of suspect products exceed a limit set in advance, a line can be shut down automatically. 

Injection The injection blow moulding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mould. The preform mould forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body. 

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