The mold temperature has a great influence on the intrinsic performance and apparent quality of the product. The temperature of the mold depends on the crystallinity of the plastic, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).
The pressure in the injection molding process includes two kinds of plasticizing pressure and injection pressure, and directly affects the plasticization of plastics and product quality.
(Back pressure) When a screw type injection machine is used, the pressure at the top of the screw when the screw rotates backwards is called the plasticizing pressure, also known as the back pressure. The magnitude of this pressure can be adjusted by the relief valve in the hydraulic system. In injection, the plasticizing pressure needs to be changed according to the screw design, product quality requirements, and the type of plastic. If these conditions and the screw speed are unchanged, increasing the plasticizing pressure will strengthen the shear. The effect is to increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase the counterflow and leakage, and increase the driving power. In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the mixing of the color material uniform, and the discharge of the gas in the melt. general
Pressure curve in injection molding
In operation, the decision of plasticizing pressure should be as low as possible on the premise of ensuring good product quality. The specific value varies with the type of plastic used, but usually rarely exceeds 20 kg/cm2.
In the current production, the injection pressure of almost all injection machines is based on the pressure exerted by the plunger or screw top on the plastic (converted by the oil line pressure). The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, give the molten material a filling rate and compact the molten material.
The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It actually includes the following parts:
Injection molding cycle
Molding cycle: The molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, on the premise of ensuring the quality, try to shorten the relevant time in the molding cycle. In the entire molding cycle, the injection time and cooling time are the most important, and they have a decisive influence on the quality of the product.
The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in production is generally about 3-5 seconds. The pressure holding time in the injection time is the pressure time for the plastic in the cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (extra-thick parts can be as high as 5~10 minutes). Before the melt is frozen at the gate, the holding time will affect the dimensional accuracy of the product. If it is later, it will have no effect. The holding time also has the most favorable value, which is known to depend on the material temperature, mold temperature and the size of the main channel and gate. If the dimensions of the main channel and gate and the process conditions are normal, the pressure value that results in the smallest fluctuation range of the shrinkage rate of the product is generally the standard.
The cooling time mainly depends on the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature. The end of the cooling time should be based on the principle of ensuring that the product does not cause changes during demoulding. The cooling time is generally between 30 and 120 seconds. If the cooling time is too long, it is not necessary. Not only does it reduce production efficiency, but also reduces the efficiency of complex parts. It causes difficulty in demolding, and even demoulding stress may occur during forced demoulding. The other time in the molding cycle is related to whether the production process is continuous and automated, and the degree of continuous and automated.
Injection pressure is provided by the hydraulic system of the injection system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Under the pressure of the plastic melt, the plastic melt enters the vertical flow path of the mold (the main flow path for some molds), the main flow path, and the split flow through the nozzle of the injection molding machine. Channel, and enter the mold cavity through the gate, this process is the injection molding process, or called the filling process. The pressure exists to overcome the resistance in the melt flow process, or conversely, the resistance in the flow process needs to be offset by the pressure of the injection molding machine to ensure that the filling process proceeds smoothly.
In the injection molding process, the pressure at the nozzle of the injection molding machine is the highest to overcome the flow resistance of the melt throughout. After that, the pressure gradually decreases along the flow length toward the front end of the melt. If the exhaust inside the mold cavity is good, the final pressure at the front end of the melt is atmospheric pressure.
There are many factors that affect the filling pressure of the melt, which can be summarized in three categories:
Material factors, such as plastic type, viscosity, etc.;
Structural factors, such as the type, number and location of the casting system, the shape of the cavity of the mold and the thickness of the product;
Forming process elements.
The injection time mentioned here refers to the time required for the plastic melt to fill the cavity, excluding auxiliary time such as mold opening and closing. Although the injection time is very short and has little effect on the molding cycle, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. Reasonable injection time helps to fill the melt ideally, and it is very important to improve the surface quality of the product and reduce the dimensional tolerance.
The injection time is much lower than the cooling time, which is about 1/10 to 1/15 of the cooling time. This rule can be used as the basis for predicting the total molding time of plastic parts. In the mold flow analysis, the injection time in the analysis result is equal to the injection time set in the process conditions only when the melt is completely driven by the rotation of the screw to fill the cavity. If the pressure retention switch of the screw occurs before the cavity is filled, the analysis result will be greater than the setting of the process conditions.
Injection temperature is an important factor affecting injection pressure. The barrel of the injection molding machine has 5 to 6 heating sections, and each raw material has its own suitable processing temperature (detailed processing temperature can refer to the data provided by the material supplier). The injection temperature must be controlled within a certain range.
If the temperature is too low, the melt plasticization will be poor, affecting the quality of the molded parts and increasing the difficulty of the process; if the temperature is too high, the raw materials will be easily decomposed. In the actual injection molding process, the injection temperature is often higher than the barrel temperature, and the higher value is related to the injection rate and the performance of the material, up to 30 ℃. This is due to the fact that the molten material is sheared as it passes through the injection port and generates high heat. There are two ways to compensate for this difference when doing mold flow analysis. One is to try to measure the temperature of the molten metal injection molding, and the other is to include the nozzle during modeling.
Holding pressure and time
Near the end of the injection molding process, the screw stops rotating and only advances, at which time injection molding enters the pressure holding stage. During the pressure maintaining process, the nozzle of the injection molding machine continuously replenishes the cavity to fill the volume vacated by the shrinkage of the parts. If the pressure is not maintained after the cavity is filled, the part will shrink by about 25%, especially the ribs will shrink due to excessive shrinkage. The holding pressure is generally about 85% of the maximum filling pressure, which of course should be determined according to the actual situation.
The back pressure refers to the pressure that the screw needs to overcome when reversing the storage material after reversing. The use of high back pressure is conducive to the dispersion of the colorant and the melting of the plastic, but at the same time it extends the screw retraction time, reduces the length of the plastic fiber, and increases the pressure of the injection molding machine, so the back pressure should be lower, generally not more than injection molding 20% of stress. When injection molding foam, the back pressure should be higher than the pressure created by the gas, otherwise the screw will be pushed out of the barrel. Some injection molding machines can program the back pressure to compensate for the reduction in screw length during melting, which reduces the heat input and lowers the temperature. However, because the result of this change is difficult to estimate, it is not easy to adjust the machine accordingly.