The nozzle is the transition between the coupling bar […]
The nozzle is the transition between the coupling barrel and the mold. At the time of injection, the melt in the cylinder is injected into the mold by high pressure and rapid flow through the nozzle under the push of the screw. Therefore, the structure of the nozzle, the size of the nozzle and the manufacturing precision will affect the pressure and temperature loss of the melt, the range of the range, the advantages and disadvantages of the feeding effect, and whether or not the phenomenon of "flowing" occurs. The types of nozzles currently in use are numerous and have their applicability. Only three of them are discussed here.
(1) Straight-through nozzle This type of nozzle has a short tubular shape. When the melt flows through the nozzle, the pressure and heat loss are small, and it is not easy to cause stagnation and decomposition, so there is generally no heating device attached to the outside. However, since the length of the nozzle body is short, the length of the slit into the template hole is limited, so that the main passage of the mold used is long. To compensate for this defect, the length of the nozzle is increased to become a modified version of the straight-through nozzle, also known as an extended nozzle. This nozzle must be equipped with a heating setting. In order to filter out solid impurities in the melt, a filter can also be added to the nozzle. The above two types of nozzles are suitable for processing high-viscosity plastics, which can cause runny when processing low-viscosity plastics.
(2) Self-locking nozzle In order to prevent the flow or retraction of the melt during the injection process, it is necessary to temporarily block the nozzle passage and use a self-locking nozzle. Self-locking nozzles are the most widely used in spring and needle valves. These nozzles are self-locking according to the valve core in the spring-pressed nozzle. At the time of injection, the spool is opened by the high pressure of the melt, and the molten crucible is ejected toward the mold. When the glue is melted, the spool is reset by the spring and self-locking. The utility model has the advantages that the phenomenon of “flowing” when injecting low-viscosity plastic can be effectively eliminated, the use is convenient, and the self-locking effect is remarkable. However, the structure is relatively complicated, the injection pressure loss is large, the range is short, the feeding effect is small, and the spring is required to be high.
Lever needle valve nozzle This nozzle is the same as the self-locking nozzle. It is also a kind of temporary opening and closing of the nozzle channel during the injection process. It uses the external hydraulic system to control the linkage mechanism through the lever. Core. When used, the vertical hydraulic system can be opened accurately and in time according to the needs, and has the advantages of convenient use, self-locking controllable, small pressure loss and accurate measurement. Also, it doesn't use springs, so no more
The main disadvantage of changing the spring is that the structure is more complicated and the cost is higher.
At the time of injection, the plastic melt flows through the nozzle at a very high shear rate and enters the mold cavity under the push of the screw. Under this high shear, the melt temperature rises rapidly. Especially for PVC, PP-R, PMMA, PC, high impact ABS, etc. with high viscosity, too small nozzle hole diameter will cause high temperature decomposition of plastic. For thin-walled precision products with difficult filling, it is better to use a nozzle with a longer range, and for thick-walled products, a nozzle with a good filling effect is needed. In addition, for some plastics with very low melt viscosity (such as PA), it is necessary to use a self-locking nozzle with anti-flow function.
On many machines, in addition to general-purpose nozzles for general viscosity, special nozzles such as self-locking nozzles, PVC nozzles, PMMA nozzles are available, and if required, thin-walled walls can be provided. Special nozzle for products.
