Injection Must Learn Knowledge Points



The melt usually flows from the nozzle into the nozzle, but with some molds, the nozzle is part of the mold because it extends to the bottom of the mold. There are two main types of nozzles: open shot nozzles and closed nozzles. In injection molding, open nozzles should be used more because they are cheaper and less likely to stay. If the injection molding machine is equipped with a depressurizing device, this nozzle can be used even with a lower viscosity melt.


Sometimes it is necessary to use a closed nozzle, which acts as a stop valve and blocks the plastic shade in the shot cylinder. Make sure the nozzle is properly inserted into the nozzle sleeve. The top hole is slightly smaller than the nozzle sleeve, which makes the nozzle easier to remove from the mold. The hole of the mouthpiece sleeve is 1mm larger than the mouth of the nozzle, that is, the radius of the nozzle is 0.5mm thinner than the radius of the nozzle sleeve.

2. Filter and combined nozzle

The impurities of the plastic can be removed by a filter of the extensible nozzle, that is, the melt and the plastic flow through a channel which is divided into narrow spaces by the insert. These narrow and gaps remove impurities and improve the mixing of the plastic.

Therefore, it is possible to use a fixed mixer to achieve a better mixing effect. These devices can be installed between the shooting tank and the nozzle to separate and remix the melt, mostly to allow the melt to flow through the stainless steel channel.

3. Exhaust


Some plastics need to be vented in the shooting tank during injection molding to allow the gas to escape. In most cases these gases are only air, but they may be melted moisture or monomolecular gases. If these gases are not released, the gas will be compressed by the melt and brought into the mold, which will expand and form bubbles in the product. To remove the gas before it reaches the nozzle or mold, reduce or reduce the diameter of the screw root to decompress the melt in the shot cylinder.


Here, the gas can be discharged from holes or holes in the shooting tank. The diameter of the screw root is then increased and the melt of the volatiles is directed toward the nozzle. The injection molding machine equipped with this facility is called a vented injection molding machine. Above the vented injection molding machine, there should be a good smoke eliminator for the flammable burner to remove potentially harmful gases.

4. Increase the role of back pressure

In order to obtain a high quality melt, the plastic is uniformly heated or melted and thoroughly mixed. Use the right screw to properly melt and mix, and have enough pressure (or back pressure) in the shot cylinder to achieve mixing and thermal consistency. Increasing the resistance to oil return creates back pressure in the firing tank. However, the screw takes longer to reset, so there is more wear and tear in the injection molding machine drive system. Keeping back pressure as much as possible, isolated from the air, also requires a consistent melt temperature and mixing level.

5. Stop valve


Regardless of the type of screw used, the tip is usually equipped with a stop valve. To prevent plastic from flowing out of the nozzle, a decompression (rewinding) device or a special shooting nozzle is also provided. If you use a stop-flow production and supply, you must check it regularly because it is an important part of the shooting tank. At present, switch-type nozzles are not commonly used because the nozzles are prone to leak plastic and break down. At present, each type of plastic has a suitable type of nozzle.

6. Screw back (rewinding)

Many injection molding machines are equipped with screw retracting or suckback devices. When the screw rotation is stopped, it is withdrawn by hydraulic pressure to suck back the plastic tip of the nozzle. This device allows the use of an open nozzle. The number of suckbacks may be reduced because entering the air can cause problems for some plastics.

7. Screw padding

In most injection molding cycles, it is necessary to adjust the amount of rotation of the screw village, so that when the screw is injected, most of the soft plastic will be left, which ensures that the screw reaches an effective advancement time and maintains a fixed firing pressure. The size of the small injection molding machine is about 3mm; the size of the large injection molding machine is 9mm. No matter how much screw padding value you use, it must remain the same. The size of the screw pad can now be controlled within 0.11 mm.

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