HUIWEN Precision Injection Molding Manufacturer states that with the increasingly widespread application of plastic injection molding products and the rapid development of plastic molding technology, people's requirements for plastic products are also getting higher and higher. In recent years, scientific and technological workers in the plastic molding field have conducted in-depth discussions, research, and practice on how to expand the application scope of injection molding, shorten the injection molding cycle, reduce molding defects, improve the quality of injection
Molded Plastic Parts, and reduce production costs, achieving gratifying results. New mold technologies and new injection molding processes are emerging one after another. Here, we will only introduce the increasingly widely used thermosetting plastic injection molding, gas-assisted injection molding, precision injection molding, low-foaming injection molding, co-injection molding, venting injection molding, and reaction injection molding.
I. Overview of Thermosetting Plastic Injection Molding Process
While the injection molding principles and processes of thermosetting and thermoplastic plastics have many similarities, there are also significant differences between them due to their different chemical properties. The principle of thermosetting plastic injection molding is to feed the molding material from the hopper of the injection molding machine into the barrel, heat it, and melt and plasticize it under the action of the screw rotation, making it a uniform viscous melt. Driven by the high pressure of the screw, this melt is injected at a high flow rate through the nozzle at the front of the barrel into the high-temperature cavity. After a period of pressure holding, shrinkage compensation, and cross-linking reaction, it solidifies into the shape of a plastic part, and then the mold is opened and the part is removed. Clearly, in principle, the main difference between thermosetting and thermoplastic injection molding lies in the solidification stage after the melt is injected into the mold. The solidification of thermoplastic injection molded parts is basically a physical process of transformation from a high-temperature liquid phase to a low-temperature solid phase, while the solidification of thermosetting injection molded parts must rely on a cross-linking chemical reaction under high temperature and high pressure. It is precisely because of this difference that the injection molding process conditions for the two are different.
(I) Temperature
(1) Material Temperature: Similar to the thermoplastic injection molding process, the material temperature includes the plasticizing temperature and the injection temperature, which depend on the temperatures of the barrel and the nozzle, respectively. However, due to the different properties of thermosetting and thermoplastic injection molding processes, the temperature requirements for the barrel and nozzle differ. For thermosetting plastics, Dongguan Machike Injection Molding Factory tends to use a lower barrel temperature to prevent premature hardening of the melt within the barrel, while also considering that the influence of barrel temperature on plasticization is less than the influence of shear friction within the material. However, when the barrel temperature is too low, the material melts more slowly, and a large amount of frictional heat is generated between the screw and the raw material. This heat can actually cause premature hardening of the melt more easily than when the barrel temperature is higher. Therefore, the barrel temperature should be strictly controlled during production. Typically, the barrel temperature is set in two or three stages. When setting the temperature in two stages, for different materials, the temperature of the latter stage can be selected within the range of 20~70℃, while the temperature of the former stage should be selected within the range of 70~95℃. For the latter stage (e.g., in the case of feeding), the frictional heat between the melt and the nozzle should also be considered, as this heat generally results in a very high temperature rise. In principle, it is generally required that the temperature of the melt after passing through the nozzle has both good fluidity and is close to the critical value of the hardening temperature. This ensures both filling and good hardening. Therefore, the nozzle temperature is generally set higher than the material temperature. For different materials, the nozzle temperature can be selected and controlled within the range of 75~100℃. At this temperature, the temperature of the melt after passing through the nozzle can reach... 100~130℃, which may satisfy the above two requirements.
(2) Mold temperature: Mold temperature is a key factor affecting the hardening and setting of thermosetting plastic parts, directly related to the molding quality and production efficiency. If the mold temperature is too low, the hardening time is long; if the mold temperature is too high, the hardening speed is too fast, making it difficult to expel low-molecular-weight volatile gases, leading to defects such as loose structure, bubbling, and dark color in the plastic parts. Generally, for different materials, the selection and control range of mold temperature is 150~220℃. In addition, the moving mold temperature sometimes needs to be 10~15℃ higher than the fixed mold temperature, which is more advantageous. (ii) Molding Cycle
With the application of new injection molding technologies, the time components of the thermosetting plastic injection molding cycle are basically the same as those for thermoplastics. However, the cooling and setting time for thermosetting plastic parts should be replaced by the hardening and setting time. The most important time components in the thermosetting plastic molding cycle are the injection time and the hardening and setting time, while the holding pressure time can be considered as either the injection time or the hardening time, but it is often considered separately. Generally, the injection time for domestically produced thermosetting injection materials is 2-10 seconds, and the holding pressure time is 5-20 seconds. The curing and setting time is selected within 15~100s, and the total injection molding cycle requires 45~120s. However, it should be noted that when determining the curing and setting time of the same type of part, not only the structural integrity and quality of the part need to be considered, but also the curing speed of the part itself. Generally, the curing time of domestic injection molding materials after mixing can be calculated based on the size of the part. However, with the continuous development of plastic production technology, the curing speed of some thermosetting injection molding materials has basically reached the curing speed of foreign fast injection molding materials.
(III) Pressure
(1) Injection pressure and injection speed: Similar to the thermoplastic injection molding process, the injection pressure and injection speed of the thermosetting injection molding process are also closely related. Due to the large amount of filler in the melt, the high viscosity, and the requirement for temperature rise of the melt during injection, the injection pressure is generally selected to be higher. Depending on the material, the commonly used range of injection pressure is 100~ 170MPa, but for a few materials, values lower or higher than this range can be used. In principle, the injection speed related to the injection pressure should also be higher, which helps to shorten the flow filling and hardening time, and at the same time avoids early hardening of the melt in the runner, reducing weld lines and flow marks on the surface of the plastic part. However, if the injection speed is too high, it is easy to draw air into the cavity and melt, resulting in defects such as bubbles on the surface of the plastic part. According to current production experience, the injection speed of thermosetting plastics can be 3~4.5m/min.
(2) Holding pressure and holding time: Holding pressure and holding time directly affect the cavity pressure and the shrinkage and density of the plastic part. At present, because the hardening speed of thermosetting injection melt is much higher than before, and most molds use point gates, the gate freezes relatively quickly, so the commonly used holding pressure can be slightly lower than the injection pressure. The holding time is slightly less than that of injection thermoplastics, but it should be determined according to different materials, the thickness of the plastic part and the freezing speed of the gate, and is usually taken as... 5~20s. The cavity pressure for thermosetting injection molding is approximately 30~70MPa.
(3) Back pressure and screw speed: When injecting thermosetting plastics, the back pressure of the screw should not be too high. Otherwise, the material will be subjected to long-distance compression in the screw, making injection difficult or causing premature hardening of the melt. Therefore, the back pressure for injecting thermosetting plastics is generally lower than that for injecting thermoplastic plastics, ranging from 3.4 to 5.2MPa, and can be close to zero when the screw starts. In some cases, the back pressure valve can even be relaxed, using only the frictional resistance of the injection screw during retraction as the back pressure. However, it should be noted that if the back pressure is too low, air is easily incorporated into the material, resulting in unstable metering and uneven plasticization. When injecting thermosetting plastics, the screw speed, which is related to back pressure, should not be too high. Otherwise, the material is prone to uneven heating in the barrel, resulting in poor plasticization. Generally, the screw speed is selected in the range of 30~70 r/min.
(IV) Other Process Conditions
(1) Residence Time and Injection Volume of Material in the Barrel After each injection, a portion of the plasticized melt remains in the screw groove. Although this melt is gradually pushed out of the barrel during subsequent injections, it is prone to cross-linking and hardening due to prolonged residence time in the barrel. This can affect the molding quality of the plastic part or even cause the injection molding machine to malfunction. To continue operation, the residence time of thermosetting plastics in the barrel must be controlled. The residence time of the material in the barrel is related to m/m and the molding cycle t, but t must never exceed the allowable plasticizing time of the material; otherwise, the material will harden in the barrel. During production, empty injection is often required to prevent premature hardening of the material in the barrel. Obviously, this results in significant waste of raw materials.
(2) Venting: Because a large amount of reactive gases evaporate during the hardening and shaping process of thermosetting injection molded parts, venting is crucial for thermosetting material injection. In addition to designing a proper venting system in the mold, the injection molding process itself must also be considered. Does the molding process require pressure relief and mold opening for venting? Generally, this measure is necessary for thick-walled plastic parts, and the pressure relief and mold opening time can be controlled within 0.2 seconds.
(3) Typical process conditions for thermosetting injection molding materials: The process conditions for thermosetting injection molding have been generally described above. Dongguan Machike Injection Molding Factory lists nine typical injection molding process conditions for thermosetting plastics for reference in production. However, it should be noted that thermosetting injection molding technology is still under development, and injection molding processes will continue to improve. Furthermore, the injection molding process for the same plastic can vary depending on the grade, the type of plastic part, or the manufacturer.
