Injection molding, also known as injection molding, is a molding method that combines injection and molding. The advantages of the injection molding method are fast production speed and high efficiency, the operation can be automated, there are many designs and colors, the shapes can be from simple to complex,
and the sizes can be from large to small, and the product dimensions are accurate, the products are easy to replace, and can be made into complex shapes Parts, injection molding is suitable for mass production and complex-shaped products and other molding processing fields.
At a certain temperature, a completely molten plastic material is stirred by a screw, injected into the mold cavity with high pressure, and cooled and solidified to obtain a molded product. This method is suitable for mass production of components with complex shapes and is one of the important processing methods.
Temperature Control
Injection molding equipment and molds
Barrel temperature
The temperatures that need to be controlled in the injection molding process include barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastics, while the latter temperature mainly affects the flow and cooling of plastics.
Each plastic has a different flow temperature. The same plastic has different flow temperatures and decomposition temperatures due to different sources or grades. This is due to different average molecular weights and molecular weight distributions.
Plastics in different types of injection molding have different flow temperatures and decomposition temperatures. The plasticizing process in the machine is also different, so the temperature of the barrel is also different.
Nozzle temperature
The nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent the melt from “salivating” that may occur in the straight-through nozzle.
The nozzle temperature cannot be too low, otherwise it will cause premature solidification of the molten material and block the nozzle, or the premature solidification material will be injected into the mold cavity and affect the performance of the product.
Mold temperature
Mold temperature has a great influence on the intrinsic performance and apparent quality of products. The mold temperature 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 pressure, molding cycle, etc.).
Pressure control
The pressure during the injection molding process includes plasticizing pressure and injection pressure, and directly affects the plasticization of plastic and product quality.
Plasticizing pressure
(Back pressure) When using a screw injection machine, the pressure on the melt at the top of the screw when the screw rotates and retreats is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted through the relief valve in the hydraulic system.
In injection, the size of the plasticizing pressure needs to be changed depending on the screw design, product quality requirements and the type of plastic. If these conditions and the screw speed remain 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 counterflow and leakage, and increase driving power.
In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the color materials can be mixed evenly, and the gas in the melt can be discharged.
In the pressure curve operation of general injection molding, the plasticizing pressure should be determined as low as possible while ensuring excellent product quality. The specific value varies with the type of plastic used, but it usually rarely exceeds 20 kilograms. /square centimeter.
Injection pressure
In current production, the injection pressure of almost all injection machines is based on the pressure exerted by the plunger or the top of the screw on the plastic (converted from the oil line pressure).
The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, give the molten material a filling rate and compact the molten material.
Molding cycle
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, during the production process, the relevant times in the molding cycle should be shortened as much as possible while ensuring quality. In the entire molding cycle, injection time and cooling time are the most important, and they have a decisive impact on the quality of the product.
The filling time in the injection time is directly inversely proportional to the filling rate. The filling time in production is generally about 3-5 seconds. The holding time in the injection time is the pressure time on the plastic in the cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (up to 5-10 minutes for extra thick parts).
The holding time before the melt is frozen at the gate will have an impact on the dimensional accuracy of the product. If it is later, it will have no impact. The holding time also has a best 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 with the smallest fluctuation range of the shrinkage rate of the product is usually used.
The cooling time mainly depends on the thickness of the product, the thermal and crystallization properties of the plastic, and the mold temperature. The end point of the cooling time should be based on the principle of ensuring that no changes are caused when the product is demolded. The cooling time is generally between 30 and 120 seconds. It is not necessary for the cooling time to be too long. It will not only reduce the production efficiency, but also affect the quality of complex parts. It will cause difficulty in demoulding, and even demoulding stress will occur when forced demoulding. Other times in the molding cycle are related to whether the production process is continuous and automated and the degree of continuity and automation.
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