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: molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, we should try to shorten the relevant time in the molding cycle on the premise of ensuring the quality. In the whole molding cycle, injection time and cooling time are the most important, which have a decisive impact on the quality of products.
The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in the production is generally about 3-5 seconds. The pressure holding time in the injection time is the pressure time of the plastic in the mold cavity, which accounts for a large proportion in the whole injection time, generally about 20-120 seconds (the extra thick parts can be as high as 5-10 minutes).
The amount of holding time before the melt is frozen at the gate has an effect on the dimensional accuracy of the product, but in the future, it has 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 sprue and gate. If the size of sprue and gate as well as the process conditions are normal, the pressure value with the minimum fluctuation range of shrinkage rate of products is usually taken as 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 point of cooling time shall be based on the principle of ensuring no change when the product is demoulded. Generally, the cooling time is about 30-120 seconds. It is unnecessary to have too long cooling time, which will not only reduce the production efficiency, but also cause difficulty in demoulding for complex parts, and even produce demoulding stress when forced demoulding. The other time in the forming cycle is related to the continuity and automation of the production process and the degree of continuity and automation.
The 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. Driven by the pressure, the plastic melt enters the vertical channel (which is also the main channel for some molds), the main channel and the shunt channel of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate. This process is called the injection molding process, or the filling process. The existence of pressure is 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 the smooth filling process.
In the process of injection molding, the pressure at the nozzle of injection molding machine is the highest to overcome the flow resistance in the whole process of melt. After that, the pressure decreases gradually along the flow length to the front of the melt. If the cavity is well vented, the last pressure at the front of the melt is atmospheric pressure.
There are many factors affecting the filling pressure of melt, which can be summarized into three categories:
Material factors, such as type and viscosity of plastics;
Structural factors, such as the type, number and location of the gating system, the shape of the mold cavity and the thickness of the product, etc;
Forming process elements.
The injection time here refers to the time required for the plastic melt to fill the cavity, excluding the auxiliary time such as mold opening and closing. Although the injection time is very short and has little influence on the molding cycle, the adjustment of injection time has a great effect on the pressure control of gate, runner and cavity. Reasonable injection time is helpful to the ideal filling of melt, and it is very important to improve the surface quality of products and reduce the dimensional tolerance.
The injection time is much lower than the cooling time, about 1 / 10 ~ 1 / 15 of the cooling time, which can be used as the basis for predicting the whole molding time of plastic parts. In the mold flow analysis, only when the melt is completely driven by the screw rotation to fill the cavity, the injection time in the analysis results is equal to the injection time set in the process conditions. If the pressure maintaining switch of screw occurs before the cavity is filled, the analysis result will be greater than the setting of process conditions.
Injection temperature is an important factor affecting injection pressure. There are 5-6 heating sections in the barrel of injection molding machine, and each raw material has its proper processing temperature (for detailed processing temperature, please refer to the data provided by the material supplier). The injection temperature must be controlled within a certain range.
Too low temperature and poor plasticization of melted material affect the quality of formed parts and increase the process difficulty; too high temperature makes raw materials easy to decompose. In the actual injection molding process, the injection temperature is often higher than the barrel temperature, the higher value is related to the injection rate and material performance, up to 30 ℃. This is due to the high heat generated by the shearing of the melt through the injection port. There are two ways to compensate for this difference in mold flow analysis: one is to measure the temperature of the melt when it is injected into the air, the other is to include the nozzle when modeling.
Pressure maintaining pressure and time
At the end of the injection molding process, the screw stops rotating and only pushes forward. At this time, the injection molding enters the pressure maintaining stage. In the process of pressure maintaining, the nozzle of the injection molding machine continuously feeds the cavity to fill the volume that is vacated due to the shrinkage of the part. If the cavity is filled without pressure maintaining, the part will shrink about 25%, especially the shrinkage trace at the rib due to excessive shrinkage. Generally, the pressure maintaining pressure is about 85% of the maximum filling pressure, which should be determined according to the actual situation.
Back pressure refers to the pressure to be overcome when the screw reverses and retreats to store materials. High back pressure is conducive to the dispersion of pigment and the melting of plastic, but at the same time, it extends the screw retraction time, reduces the length of plastic fiber, and increases the pressure of injection molding machine. Therefore, the back pressure should be lower, generally no more than 20% of the injection pressure