Melt usually flows from the nozzle to the injection port, but in some moulds, the nozzle is part of the mould because it extends to the bottom of the mould. There are two main types of nozzles: open nozzles and closed nozzles. Open nozzles should be used more in injection moulding because they are cheaper and less likely to remain. If the injection moulding machine is equipped with a pressure relief device, the nozzle can be used even if the viscosity of the melt is low.
Sometimes it is necessary to use a closed nozzle, which acts as a stop valve to block the plastic shade in the injection cylinder. To ensure that the nozzle is properly connected to the nozzle sleeve, the top hole is slightly smaller than the nozzle sleeve, which makes it easier for the nozzle to withdraw from the die. The hole of the nozzle sleeve is 1 mm larger than that of the nozzle, that is, the radius of the nozzle is 0.5 mm smaller than that of the nozzle sleeve.
02. Filter and combined nozzle
Plastic impurities can be removed by filters with extensible nozzles, i.e. melting and plastics flow through a channel separated by inserts into narrow spaces. These narrows and gaps remove impurities and improve the mixing of plastics.
Therefore, when extended, a fixed mixer can be used to achieve a better mixing effect. These devices can be installed between the ejector cylinder and the nozzle to separate and re-mix the melt, mostly through the stainless steel channel.
Some plastics need to exhaust in the injection cylinder to let the gas out. In most cases, these gases are only air, but they may be water or single-molecule gases released from melting. If these gases cannot be released, the gas will be compressed by the melt and carried to the die, which will expand and form bubbles in the product. To remove the gas before it reaches the nozzle or die, reducing or reducing the screw root diameter can decompress the melt in the injection cylinder.
Here, gas can be discharged from holes or holes in the ejector cylinder. Then the diameter of screw root is increased, and the melt glue for the devolatilization is directed to the nozzle. The injection machine equipped with this facility is called exhaust injection machine. The top of the exhaust injection moulding machine should have a good exhaust fume exhauster with catalytic burner, which can remove potentially harmful gases.
04. The Role of Increasing Back Pressure
In order to obtain a high quality melt, plastics should be heated or melted uniformly and mixed sufficiently. The correct screw is used to melt and mix properly, and sufficient pressure (or back pressure) is provided in the injection cylinder to achieve consistency of mixing and heat. Increasing the resistance of return oil can produce back pressure in the cylinder of shooting material. But the screw needs to be reset for a longer time, so there is more wear and tear in the drive system of injection moulding machine. Maintain backpressure as much as possible, isolate from air, and also need the same melt temperature and mixing degree.