Molding properties of injection molded thermosetting plastics
Injection-molded thermosetting plastics are materials that undergo a chemical cross-linking reaction during heating, forming an insoluble, infusible, three-dimensional network structure. Their molding properties differ significantly from those of thermoplastics, primarily in terms of fluidity, curing characteristics, shrinkage, and mold adaptability. Understanding the molding properties of thermosetting plastics is essential for developing a sound injection molding process and ensuring product quality. Their unique chemical curing properties dictate that the molding process involves not only physical changes but also irreversible chemical changes, necessitating targeted design of mold structure and process parameters.
The fluidity of thermosetting plastics is a key property that influences mold filling efficiency. It is typically expressed in Raschig flow (in mm), with higher values indicating better fluidity, typically ranging from 50 to 300 mm. Fluidity initially increases and then decreases with increasing temperature, with an optimal flow temperature range (e.g., 160-180°C for phenolic plastics). Beyond this range, fluidity decreases dramatically due to premature curing. Pressure significantly influences fluidity: a 10 MPa increase in injection pressure can improve fluidity by 10%-20%. However, excessive pressure (over 100 MPa) can lead to increased flash and internal stress in the part. For example, a phenolic molding compound with a Raschig flow of 150 mm can successfully fill a complex cavity with a 2 mm wall thickness at an injection pressure of 80 MPa and a barrel temperature of 170°C. However, when the pressure drops to 60 MPa, underfill occurs, requiring a 5°C increase in barrel temperature to compensate for the loss in fluidity. The fluidity of different thermosetting plastics varies greatly. Epoxy plastics have lower fluidity (50~100mm) and are suitable for simple products; unsaturated polyester plastics have higher fluidity (200~300mm) and are suitable for complex structural parts.
Curing characteristics are the core molding properties of thermoset plastics, including gel time, cure time, and cure temperature, which directly determine the molding cycle and product performance. Gel time is the time it takes for a plastic to transition from a molten state to a gel state. It decreases with increasing temperature. For example, the gel time of aminoplasts is approximately 60 seconds at 150°C, decreasing to 20 seconds at 180°C. The barrel temperature must be controlled to keep the gel time slightly longer than the mold filling time to avoid premature curing during the filling process. Cure time is the time required to achieve sufficient strength after gelation and is proportional to the part’s wall thickness. Thick-walled parts (≥5mm) require an extended cure time (60-120 seconds), while thin-walled parts (≤3mm) can be shortened to 20-40 seconds. The cure temperature must be 20-30°C higher than the gel temperature to ensure a sufficient cross-linking reaction. For example, the gel temperature of phenolic plastic is 170°C, so the cure temperature should be controlled between 190-200°C. For example, for a thermosetting plastic electrical accessory (wall thickness 3mm), the barrel temperature is set to 160℃ (gel time 40 seconds), the mold temperature is 190℃, and the curing time is 30 seconds. This ensures complete mold filling and sufficient curing, and the product’s flexural strength reaches 80MPa.
The shrinkage of thermosetting plastics is relatively low, generally ranging from 0.2% to 1.0%, much lower than that of thermoplastics. Shrinkage primarily occurs during the curing phase, with cooling shrinkage contributing a small portion. The magnitude of shrinkage varies depending on the type of plastic, filler content, and molding process. The shrinkage of thermosetting plastics containing glass fiber fillers can be as low as 0.2% to 0.5%, while the shrinkage of pure resins is higher (0.8% to 1.0%). For example, glass fiber-reinforced phenolic plastics have a shrinkage of 0.3% to 0.5%, making them suitable for products requiring high dimensional accuracy (such as gears and bearings). Pure amino plastics have a shrinkage of 0.8% to 1.0%, requiring sufficient allowance for shrinkage in mold design. Shrinkage uniformity significantly impacts product quality. Uneven mold temperatures can lead to localized shrinkage variations and warpage. Therefore, thermosetting plastic molds require higher temperature control accuracy (±1°C) and a more uniform cooling water channel layout.
Thermosetting plastics place special demands on mold compatibility. Because the curing process releases volatiles (such as moisture and formaldehyde), molds must possess excellent venting properties. Venting grooves should be 0.03-0.05mm deep (greater than for thermoplastics) and 10-20mm wide to prevent volatiles from being trapped and causing bubbles and scorch marks on the finished product. The mold cavity surface must also be wear-resistant and corrosion-resistant, as fillers (such as glass fiber and quartz powder) in thermosetting plastics can exacerbate wear. Ideally, the cavity material should be H13 or Cr12MoV, with a quenched hardness of HRC 50-55 and a surface roughness of Ra ≤ 0.8μm to minimize friction and mold sticking. For example, a glass-fiber-reinforced epoxy plastic mold with a cavity made of quenched H13 steel and a chrome-plated surface (0.01-0.02mm thick) boasts a service life exceeding 100,000 cycles. Conventional 45 steel molds experience significant wear after only 30,000 cycles.
The molding performance of thermosetting plastics is also reflected in their sensitivity to process parameters. Even slight changes in barrel temperature, mold temperature, injection pressure, and holding time can affect product quality. The barrel temperature must be strictly controlled below the gel temperature to prevent premature curing. It is typically 30-50°C lower than the mold temperature, for example, a mold temperature of 180°C and a barrel temperature of 130-150°C. The injection speed should be kept low (30-50 mm/s) to prevent localized premature curing due to shear heating of the melt. The holding time should be shorter than the gel time to prevent gate clogging, typically 5-15 seconds. For example, when molding an unsaturated polyester, an excessively fast injection speed (80 mm/s) caused premature solidification at the gate, resulting in shorting of the product. Reducing the speed to 40 mm/s restored the product to normal. High-quality thermosetting plastic products can be produced by fully understanding the molding properties of thermosetting plastics, rationally designing molds, and optimizing process parameters.
