Principle of Thermosetting Plastic Injection Molding

The main component of thermosetting plastics is linear or branched low-molecular-weight polymers, with reactive functional groups on the molecular chains. Therefore, during the heat-induced molding process, thermosetting plastics undergo not only physical state changes but also irreversible chemical reactions. When thermosetting plastics are fed into the barrel, they are heated to become a viscous flow state, forming a melt with certain fluidity. However, chemical reactions may increase viscosity or even crosslink and harden into a solid. To ensure smooth injection molding, the molding material must first be pre-plasticized to a semi-molten state in a relatively low-temperature barrel. During the subsequent injection and mold filling process, it is further plasticized, and must reach the optimal viscosity state when passing through the nozzle. After being injected into the high-temperature mold cavity and continuously heated, the material undergoes crosslinking and curing reactions through its own reactive groups or active sites with added curing agents over a certain period, gradually transforming linear resins into a three-dimensional structure. Low-molecular substances (such as ammonia and water) released during the reaction must be discharged in a timely manner to ensure smooth progress of the reaction and achieve the best physical and mechanical properties of the material in the mold, after which the product can be demolded.

From the basic process and requirements of thermosetting plastic injection molding described above, it can be seen that thermosetting plastic injection differs significantly from thermoplastic injection.  

Plasticization of Thermosetting Plastics in the Barrel  

The temperature of the barrel must be strictly controlled. Low temperatures result in poor material fluidity, while slightly higher temperatures can cause material curing and reduced fluidity. Therefore, the barrel temperature should be as uniform as possible, with minimal curing products and sufficient fluidity to ensure smooth injection of the material from the barrel. For example, phenolic resin generally melts at around 90°C, and exothermic heat from crosslinking reactions can be observed above 100°C. Therefore, the temperature of the high-temperature heating section of the barrel is preferably set at 85-95°C. Screw speed and back pressure during pre-plasticization of thermosetting plastics should not be too high to avoid uneven heating of the material and premature curing of part of the material due to temperature rise caused by strong shearing. Minimizing the residence time of the melt in the barrel is also an important measure to ensure the quality of the plasticized melt.  

Flow of Thermosetting Plastic Melt During Mold Filling  

The mold filling flow process of thermosetting plastics is also a process of further plasticization of the melt. Since both the nozzle and the mold are in a heated high-temperature state, the melt does not form an immobile solid plastic insulation layer on the wall of the channel like thermoplastics when flowing through the nozzle and runner. Moreover, due to the large velocity gradient near the wall, the melt near the wall flows in a turbulent form, which improves the heat transfer effect from the hot wall to the melt. In addition, the flow rate during mold filling is very high, and the melt generates a large amount of shear friction heat when passing through the nozzle and runner, causing the temperature to rise rapidly, further plasticizing the melt and significantly reducing its viscosity, so that it has good mold filling ability after entering the mold cavity.  

For non-Newtonian pseudoplastic fluids, increasing shear stress can reduce their viscosity, but for thermosetting plastics, shear stress activates crosslinking reactions, instead accelerating reactions and increasing viscosity. Therefore, correct process control should be carried out during the mold filling flow stage, with the key being how to fill the mold cavity with melt before significant crosslinking reactions occur. Measures such as high pressure and high speed and minimizing the length of the runner system are conducive to completing the mold filling process in the shortest time.  

Curing of Thermosetting Plastics in the Mold Cavity  

The shaping of the melt after taking the shape of the mold cavity is completed by curing reactions at high temperatures. The crosslinking reaction rate of the resin increases with the rise in temperature, so only by controlling the mold temperature at a higher level can the plastic be fully cured and molded in a shorter time.  

The crosslinking of thermosetting resins is an exothermic reaction, and this heat can raise the temperature and expand the material in the mold cavity, compensating for the volume shrinkage caused by crosslinking reactions. Therefore, there is no need for pressure-holding and material-supplementing after mold filling is completed. Moreover, the material in the gate usually cures earlier than that in the mold cavity, so thermosetting plastics cannot be supplemented into the mold after mold filling, and backflow does not occur.  

The crosslinking and curing reaction of thermosetting plastics is essentially a polycondensation reaction, and low-molecular substances are precipitated during the curing process. Therefore, the clamping part of the injection molding machine should meet the requirement of being able to discharge these reaction by-products from the mold cavity in a timely manner to ensure the full progress of the polycondensation crosslinking reaction.