The injection molding process of plastic parts mainly includes four stages, such as filling – pressure holding – cooling – demolding, etc., which directly determine the molding quality of the product, and these four stages are a complete continuous process.
1.Filling stage filling is the first step in the whole injection cycle process, the time is calculated from the mold closure to the mold cavity filling to about 95%. In theory, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is limited by many conditions. The shear rate is high during high-speed filling and high-speed filling, and the viscosity of the plastic decreases due to the effect of shear thinning, which reduces the overall flow resistance; Local viscous heating effects can also thin the thickness of the cured layer. Therefore, during the flow control phase, the filling behavior often depends on the size of the volume to be filled. That is, in the flow control stage, due to high-speed filling, the shear thinning effect of the melt is often large, while the cooling effect of the thin wall is not obvious, so the utility of the rate prevails. Low-speed filling heat conduction control When low-speed filling is controlled, the shear rate is low, the local viscosity is high, and the flow resistance is large. Due to the slow replenishment rate and slow flow of thermoplastics, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. Coupled with a smaller amount of viscous heating, the thickness of the cured layer is thicker, which further increases the flow resistance at thinner walls. Due to the flow of the fountain, the plastic polymer chain in front of the flow wave is arranged in front of the almost parallel flow wave. Therefore, when the two strands of plastic melt intersect, the polymer chains on the contact surface are parallel to each other; In addition, the two strands of melt have different properties (different residence time in the mold cavity, different temperature and pressure), resulting in poor microscopic structural strength in the melt intersection area. When the parts are placed at an appropriate angle under the light and observed with the naked eye, it can be found that there are obvious joint lines, which is the formation mechanism of the welding line. The welding line not only affects the appearance of the plastic part, but also easily causes stress concentration due to the loose microstructure, which reduces the strength of the part and fractures.
Generally speaking, the strength of the welding line produced in the high temperature area is better, because under the high temperature situation, the polymer chain activity is better and can penetrate and wind each other, in addition, the temperature of the two melts in the high temperature area is relatively close, and the thermal properties of the melt are almost the same, which increases the strength of the welding area; Conversely, in the low temperature area, the welding strength is poor.
2.The function of the holding stage is to continuously apply pressure, compact the melt, and increase the density (densification) of the plastic to compensate for the shrinkage behavior of the plastic. During the holding process, the back pressure is higher because the mold cavity is already filled with plastic. In the process of holding compaction, the screw of the injection molding machine can only slowly move forward slightly, and the flow speed of the plastic is also relatively slow, and the flow at this time is called the holding flow. Since the plastic is cooled and cured faster by the mold wall during the holding stage, and the melt viscosity increases rapidly, the resistance in the mold cavity is very large. In the later stage of packing, the material density continues to increase, the plastic parts are gradually formed, and the holding stage continues until the gate is solidified and sealed, at which time the mold cavity pressure in the holding stage reaches the highest value.
In the packing phase, the plastic exhibits partially compressible properties due to the rather high pressure. In areas with higher pressures, plastics are denser and denser; In areas with lower pressures, plastics are looser and dense, causing the density distribution to change with location and time. The plastic flow rate during the holding process is extremely low, and the flow no longer plays a dominant role; Pressure is the main factor affecting the holding process. During the holding process, the plastic has filled the mold cavity, and the gradually solidified melt acts as the medium for transmitting pressure. The pressure in the mold cavity is transmitted to the surface of the mold wall with the help of plastic, which tends to open the mold, so the appropriate clamping force is required for clamping. Under normal circumstances, the mold expansion force will slightly stretch the mold, which is helpful for the exhaust of the mold; However, if the mold expansion force is too large, it is easy to cause the burr of the molded product, overflow, and even open the mold.
Therefore, when choosing an injection molding machine, an injection molding machine with a large enough clamping force should be selected to prevent mold expansion and effectively maintain pressure.
3.Cooling stage In the injection molding mold, the design of the cooling system is very important. This is because molded plastic products can only be cooled and cured to a certain rigidity, and after demolding, the plastic products can be avoided from deformation due to external forces. Since the cooling time accounts for about 70%~80% of the entire molding cycle, a well-designed cooling system can greatly shorten the molding time, improve injection molding productivity, and reduce costs. An improperly designed cooling system will lengthen the molding time and increase the cost; Uneven cooling will further cause warping and deformation of plastic products. According to the experiment, the heat entering the mold from the melt is roughly dissipated in two parts, one part has 5% transmitted to the atmosphere by radiation and convection, and the remaining 95% is conducted from the melt to the mold. Due to the role of the cooling water pipe in the mold, the heat is transferred from the plastic in the mold cavity to the cooling water pipe through the mold base through heat conduction, and then taken away by the coolant through heat convection. A small amount of heat that is not carried away by the cooling water continues to be conducted in the mold until it comes into contact with the outside world and is dispersed into the air.
The molding cycle of injection molding consists of mold clamping time, filling time, holding time, cooling time and release time. Among them, the proportion of cooling time is the largest, about 70%~80%. Therefore, the cooling time will directly affect the length of the molding cycle and the output of plastic products. The temperature of plastic products in the demolding stage should be cooled to a temperature lower than the heat deflection temperature of plastic products to prevent the slack phenomenon caused by residual stress or warping and deformation caused by external force of demolding of plastic products.
The factors that affect the cooling rate of products are: plastic product design.
Mainly plastic products wall thickness. The greater the thickness of the product, the longer the cooling time. In general, the cooling time is approximately proportional to the square of the thickness of the plastic product, or to the 1.6th power of the maximum runner diameter. That is, the thickness of plastic products is doubled, and the cooling time is increased by 4 times.
Mold material and its cooling method.Mold materials, including mold core, cavity material, and mold base material, have a great influence on the cooling rate. The higher the thermal conductivity of the mold material, the better the heat transfer from the plastic per unit time and the shorter the cooling time. Cooling water pipe configuration.The closer the cooling water pipe is to the mold cavity, the larger the pipe diameter and the greater the number, the better the cooling effect and the shorter the cooling time. Coolant flow. The larger the cooling water flow rate (generally it is better to achieve turbulence), the better the cooling water takes away heat by heat convection. The nature of the coolant. The viscosity and thermal conductivity of the coolant also affect the heat transfer effect of the mold. The lower the coolant viscosity, the higher the thermal conductivity, the lower the temperature, and the better the cooling effect. Plastic selection. Plastic refers to a measure of the speed at which plastic conducts heat from a hot place to a cold place. The higher the thermal conductivity of plastics, the better the heat conduction effect, or the specific heat of plastics is low, and the temperature is easy to change, so the heat is easy to escape, the heat conduction effect is better, and the cooling time required is shorter. Processing parameter setting. The higher the feed temperature, the higher the mold temperature, the lower the ejection temperature, and the longer the cooling time required. Design rules for cooling systems: The cooling channel should be designed to ensure that the cooling effect is uniform and rapid. The cooling system is designed to maintain proper and efficient cooling of the mold. Cooling holes should be of standard size to facilitate processing and assembly. When designing a cooling system, the mold designer must determine the following design parameters according to the wall thickness and volume of the plastic part – 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.Demolding stageDemolding is the last link in the injection molding cycle. Although the product has been cold-set, but the demolding still has a very important impact on the quality of the product, improper demolding method may lead to uneven force of the product during demolding, and cause product deformation and other defects when ejecting. There are two main ways to demold: ejector bar demoulding and stripping plate demolding. When designing the mold, it is necessary to choose the appropriate demolding method according to the structural characteristics of the product to ensure product quality.
Post time: Jan-30-2023