What Are The Standards For Injection Molding Processes?
Injection molding process generally includes: filling, pressure protection, cooling, release and other 4 stages, these 4 injection molding process stagedirects directly determine the molding quality of the product, and these 4 injection molding process stage is a complete continuous process.
1, filling stage
(1) Filling is the first step in the entire injection cycle, the time from the mold closure to the injection molding, to the mold cavity filling to about 95%. Theoretically, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is subject to many conditions.
(2) High-speed filling. High shear rate is higher when high-speed filling, plastic due to the effect of thinning and the presence of viscosity decreased, so that the overall flow resistance is reduced; Therefore, in the flow control phase, the fill behavior often depends on the volume size to be filled. That is, in the flow control stage, due to high-speed filling, the melt's shearing effect is often very large, and the cooling effect of thin wall is not obvious, so the rate of utility occupies the upper hand.
(3) Low-speed filling. When the heat conduction controls low-speed filling, the shear rate is low, the local viscosity is higher, and the flow resistance is large. Because the heat plastic supplement rate is slow, the flow is slower, so that the heat transfer effect is more obvious, the heat is quickly taken away for the cold mold wall. Coupled with a smaller amount of sticky heating phenomenon, the thickness of the curing layer is thicker, and further increase the flow resistance at the thin part of the wall.
Due to the flow of fountains, the plastic polymer chain in front of the flow wave is ranked in front of the almost parallel flow wave. Therefore, when the two plastic melts meet, the polymer chains of the contact surface are parallel to each other, and the properties of the two fuses are different (different time of stay in the mold cavity, temperature and pressure are also different),
The structure strength of the melt junction area is poor at the microscopic level. In the light of the part placed at an appropriate angle with the naked eye to observe, you can see that there is a clear joining line, which is the formation mechanism of the fuse marks. The melting marks not only affect the appearance of the plastic parts, but also cause stress concentration due to the loose microstructure, which makes the strength of the part decrease and break.
Generally speaking, the strength of the melting trace produced in the high temperature zone is better, because in the case of high temperature, the polymer chain is more active, can penetrate the winding, in addition, the temperature of the two melts in the high temperature area is closer, the thermal properties of the melt are almost the same, increasing the strength of the fusion area;
2, the pressure-keeping stage
The role of the pressure-holding phase is to continuously apply pressure, compact the melt, increase the density of the plastic (tightening) to compensate for the shrinkage of the plastic.
During the pressure protection process, the back pressure is higher due to the fill of plastic in the mold cavity. In the process of compaction, the injection molding screw can only slowly move forward slightly, the flow of plastic is also relatively slow, at this time the flow is called pressure-holding flow. Due to the accelerated cooling and curing of the plastic by the mold wall during the pressure-keeping phase, the viscosity of the melt increases rapidly, so the resistance in the mold cavity is very large.
In the later stage of pressure protection, the density of the material continues to increase, the plastic parts are gradually formed, the pressure-holding phase will continue until the pouring seal, at this time the pressure-holding phase of the mold pressure reached the highest value.
During the pressure-keeping phase, the plastic presents partially compressible properties due to the high pressure. In areas with higher pressure, plastics are denser and denser, while in areas with lower pressure, plastics are looser and less dense, resulting in changes in density distribution with location and time.
The flow rate of plastic is very low during the pressure protection process, the flow is no longer playing a leading role, and the pressure is the main factor affecting the pressure protection process. The plastic is already filled with mold cavities during the pressure-holding process, at which point the melt that gradually cures is used as a medium for transmitting pressure. The pressure in the mold cavity is transmitted by plastic to the die wall surface, there is a tendency to open the mold, so the proper locking force is required for the locking mold.
The increase die force in normal circumstances will slightly spread the mold, for the mold exhaust has a helping role, but if the increase mold force is too large, easy to cause the mold edgy edge, spill, and even open the mold. Therefore, in the selection of injection molding machine, should choose a sufficient large lock molding force injection molding machine to prevent the phenomenon of mold rise and effective pressure protection.
3, cooling stage
In injection molds, the design of the cooling system is very important. This is because the molded plastic products only cool to cure to a certain degree of rigidity, release can avoid plastic products due to external forces and deformation. Since the cooling time accounts for about 70% to 80% of the total molding cycle, a well-designed cooling system can significantly reduce molding time, increase injection molding productivity and reduce costs. Poorly designed cooling systems can lengthy molding times and increase costs, and uneven cooling can further distort the warping of plastic products.
According to the experiment, the heat from the melt into the mold is roughly distributed in two parts, one part is 5% by radiation, convection to the atmosphere, the remaining 95% from the melt conduction to the mold. Plastic products in the mold due to the role of cooling water pipes, heat from the mold cavity of plastic through the heat transfer through the mold frame to the cooling pipe, and then through the heat convection is taken away by coolant. A small number of heat that is not taken away from the cooling water continues to be conducted in the mold and spilled into the air after contact with the outside world.
The molding period of injection molding consists of the molding time, filling time, holding time, cooling time and release time. Among them, the proportion of cooling time is the largest, about 70% to 80%. Therefore, the cooling time will directly affect the length of the plastic molding cycle and the yield size. The temperature of plastic products in the release phase should be cooled to a temperature lower than the thermal deformation temperature of the plastic products to prevent the loosening of plastic products due to residual stress or warping and deformation caused by the external forces of the release.
Design rules for cooling systems:
(1) The cooling channel designed should ensure that the cooling effect is uniform and rapid.
(2) The purpose of designing the cooling system is to maintain proper and efficient cooling of the mold. Cooling holes should be available in standard sizes for easy processing and assembly.
(3) When designing the cooling system, the mold designer must determine the following design parameters according to the wall thickness and volume of the plastic parts - the position and size of the cooling hole, the length of the hole, the type of hole, the configuration and connection of the hole, and the flow rate and heat transfer properties of the coolant.
4, release stage
Release is the last step in an injection molding cycle. Although the product has been cold solid molding, but the release of the product quality has a very important impact, improper release mode, may lead to the product in the release of uneven force, the top out caused the product deformation and other defects. There are two main ways to release: the top rod release and the release plate. When designing molds, we should choose the appropriate release mode according to the structural characteristics of the product to ensure the quality of the products.
For the selection of the top rod mold, the top rod should be as uniform as possible, and the position should be selected in the release resistance and the maximum strength and stiffness of the plastic parts, so as to avoid deformation damage of the plastic parts. The dematerialing plate is generally used in deep cavity thin-walled containers and not allowed to have pushrod marks of transparent products release, this mechanism is characterized by large and uniform release force, smooth movement, no obvious traces of the left.