Several Factors Affecting Plastic Injection Molding Processes (Part two)

Influence of temperature during injection molding
Temperature is one of the key factors affecting plastic injection molding. Under normal conditions, if the temperature is too high, the Rockwell hardness, flexural modulus, and elongation stress at the break of the product will decrease. Mold temperature, barrel temperature and nozzle temperature are the key research objects for plastic injection molding.
 
1) Analyzing the influence of mold temperature
The mold temperature has a great impact on the product quality of plastic injection molding. At present, the methods of controlling the mold temperature in the production process mainly include heating, air cooling, natural heat dissipation, water cooling and other methods. The mold temperature plays a decisive role in the crystallization conditions and crystallinity of plastics. The higher the mold temperature is, the lower the cooling rate becomes. The higher mold temperature is conducive to the relaxation of PET molecular bonds and the reduction of molecular orientation. Generally speaking, when the mold temperature is low, the elongation stress at break and the cantilever impact strength will increase while the flexural modulus and tensile yield strength will be reduced.
 
2) Analyzing the influence of barrel temperature
The temperature of the barrel has a great influence on the fluidity and plasticity of the plastic part. The raw material is heated in the barrel, and the raw material is very prone to oxidation reactions under high temperatures, which will eventually affect the performance of the plastic product. For example, the preparation of heat-sensitive plastic materials such as POM and PVC requires strict control of the heating temperature. Generally speaking, as the temperature of the barrel increases, the surface gloss of the plastic product, the molding fluidity of the product, and the impact strength will increase, while the shrinkage, warpage, orientation, and internal stress will decrease.
 
3) Analyzing the influence of nozzle temperature
The nozzle temperature directly affects the fluidity and plasticity of plastic products. The nozzle temperature has a great influence on the mechanical properties and molding conditions of plastic parts. When the nozzle temperature is too high, the surface gloss of the plastic product increases and the viscosity of the melt decreases. However, as the temperature increases, the problem that the molten material is easy to decompose will occur, thereby affecting the performance of the plastic product.
 
Influence of time during injection molding
In the plastic injection molding production process, the time influence mainly includes injection time, pressure maintaining time and cooling time. Generally speaking, the longer the holding time, the better the quality of the injection-molded product becomes. A shorter cooling time allows the molten material to stay in the container for less time and reduces the possibility of degradation.
 
Influence of stress during injection molding
The properties of each plastic are also very different due to the wide variety of plastic products. Some plastics will crack under stress. To solve the problem of plastic cracking, additives are generally added during plastic injection molding to increase the crack resistance of plastic parts. For example, some experimental data have studied the microscopic cracking of polyetherimide injection molding products by using SEM technology. The research results show that the stress of plastic parts has a great relationship with the selection of barrel temperature, mold temperature and material drying temperature. From the SEM image, we can see the filling process of the molten plastic in the injection molding process. According to the flow ripple, we can judge whether the mold temperature and barrel temperature meet the production process conditions and whether the dry condition meets the processing conditions.
 
In the plastic drying process, if the drying is too much, the internal stress of the plastic will be reduced. Increase the temperature of the barrel to reduce the viscosity of the melt, which is beneficial to reduce the internal stress of the plastic. However, if the temperature of the barrel is too high, the melt will easily decompose, which will increase the stress of the plastic part. Therefore, the temperature of the barrel should be selected according to the actual production process. Increased injection pressure and extended time will increase the orientation stress and reduce the shrinkage stress.
 
The effect of crystallinity
Generally, according to whether the thermoplastic material crystallizes in the condensation process, the thermoplastic material can be divided into two categories, that is, crystalline plastic and non-crystalline plastic. When the polymer is in the molten state, the molecules are free to move, satisfying the conditions of Brownian motion, and there is no order. Once the temperature drops, the moving molecules will stop. Such molecules have a fixed position. This phenomenon is called crystallization. Crystalline and non-crystalline materials can be distinguished by their appearance, with non-crystalline materials being transparent and crystalline materials being translucent or opaque. In the injection molding process, crystalline plastics have poor ductility and great brittleness, while permeability and insoluble properties will increase. Factors affecting the formation of crystals in injection molding are mainly time, temperature and pressure. Among them, temperature is the most important influencing factor. Raising the temperature can speed up the movement of molecules, and the effect of external force also increases the probability of crystallization during the process of lowering the temperature.
 
The effect of shrinkage
The shrinkage rate of the melt in the flow direction of polymer molecules is greater than that in the vertical direction in plastic injection molding, resulting in warping deformation. Severe deformation may turn plastic products into waste products, so it is necessary to find out the cause of the deformation to ensure the quality of plastic products. Equipment, molds and processes are all factors that affect the shrinkage rate of injection products. Among them, the plastic injection molding process parameters are the most critical factors affecting the shrinkage rate of the finished product, which are mainly temperature, time and pressure. The rise of the melt temperature will cause a change in the shrinkage rate. The main reason is that the high melt temperature will bring a lot of heat into the mold cavity, and the shrinkage rate will increase due to the increase in mold temperature. The type of plastic also affects the shrinkage rate. Thermoplastics will change in volume due to crystallization in the pouring process, resulting in changes in the shape of plastic products.
 
2. Conclusion
In the process of preparing plastic products, the injection molding process has a great influence on the performance of plastic products. To prepare products with good performance, it is necessary to analyze and optimize the plastic injection molding process conditions and production process. The main factors affecting the surface quality of plastic products are temperature, injection speed, and injection pressure. In the injection molding process, the melt needs to be filled into the mold. It should be noted that high pressure cannot be generated. Otherwise, the plastic product will crack. There are many factors that affect the injection molding process. To ensure the good performance of plastic parts in the production process, it is necessary to analyze the factors to improve the quality of plastic products.