Preparation of injection molding test mold
Injection mold trials are a critical step in verifying mold design rationality and determining the optimal molding process. Adequate preparatory work directly impacts trial efficiency and the reliability of the results. Thorough preparation reduces unexpected downtime during trial runs, ensures accurate data on mold performance and product quality, and provides a foundation for mold improvement and mass production. Trial preparation encompasses mold inspection, raw material preparation, equipment commissioning, parameter planning, and personnel arrangements. It requires coordination among various departments and adherence to standardized processes to avoid trial failures or data distortion due to oversights.
A comprehensive mold inspection is the core of mold trial preparation. It is crucial to ensure the mold’s structural integrity, proper function, and compliance with the mold trial requirements. First, verify the mold’s basic information, including the mold number, number of cavities, and the product name, to confirm consistency with the mold trial plan. Check the mold’s appearance and key components: Ensure the parting surfaces are smooth and free of scratches; ensure the guide pins and bushings are well lubricated and free of wear; and ensure the ejector mechanism (ejector pins, ejector plate, bevel ejector, etc.) moves smoothly without sticking or unusual noises. Ensure the cooling water channels are unobstructed, using a high-pressure air gun or water jet test to ensure that all branches are unobstructed and that the inlet and outlet connections are clearly marked. Ensure the gates and runners are smooth and free of burrs or foreign matter. Ensure the venting grooves are machined as designed and meet the required dimensions (depth, width, and length). For hot runner molds, check the hot runner system’s wiring, the integrity of the heater coils and thermocouples, and the tight connection between the hot runner nozzle and the runner plate. Clean the cavity interior to remove oil, iron filings, and other impurities. Wipe with alcohol if necessary to ensure the molding surfaces are clean. Before the mold trial, the mold must be pre-installed and the mold must be opened and closed manually 3-5 times. The coordination of the moving parts must be checked to confirm that there is no interference or abnormality. Any problems found (such as bent ejector pins and loose guide pins) must be repaired in a timely manner to avoid damage to the mold during the mold trial.
Raw material preparation and processing must strictly adhere to product requirements to ensure stable raw material properties and meet molding conditions. Confirm the required plastic type and brand based on the design drawings. Inspect the raw material’s appearance for discoloration, lumps, and impurities. Verify the raw material’s melt flow rate (MFR), density, moisture content, and other indicators to ensure they are consistent with design requirements. Hygroscopic plastics (such as PA, PC, and PBT) require rigorous drying: PA6 should be dried at 80-100°C for 4-6 hours, and PC at 120-140°C for 6-8 hours to ensure the moisture content is reduced to below 0.05%. After drying, the material must be sealed and stored to prevent reabsorption of moisture. If the product requires color matching, prepare the matching pigment or masterbatch in advance, mix them in the correct proportions, and conduct small batch color tests to ensure the color meets the requirements. Prepare a sufficient amount of raw material, typically 3-5 times the total weight of the trial molded product, to avoid interruptions caused by insufficient raw material during the trial molding process. For reinforced plastics containing glass fiber or other fillers, verify the filler’s dispersion to prevent agglomeration that may affect melt flow. After the raw materials are prepared, the batch, brand, processing method and other information of the raw materials must be recorded on the trial mold sheet for easy traceability.
Commissioning the injection molding machine and auxiliary equipment is a crucial step in mold trial preparation. Ensure the equipment is in good condition and compatible with the mold. Select an appropriate injection molding machine based on the mold size and product requirements. Verify parameters such as the machine’s maximum mold capacity, tie-bar spacing, maximum shot volume, and clamping force. For example, if the mold thickness is 400mm, the machine’s maximum mold capacity must be at least 400mm, and the clamping force must meet the projected area requirements of the product (usually calculated as 40-60 MPa). Check the machine’s barrel and screw for cleanliness. If it has previously produced other plastics, clean them to avoid raw material contamination. For example, when switching from PE to PC, use PP for transition cleaning. Debug the machine’s temperature control system, setting barrel section temperatures based on the plastic’s characteristics. For example, when processing ABS, set the barrel temperature to 220-230°C for the front section, 230-240°C for the middle section, 210-220°C for the rear section, and 210-220°C for the nozzle. Preheat the barrel to the set temperature and maintain it for at least 30 minutes to ensure stable temperatures. Check the injection system, including parameters such as injection pressure, speed, holding pressure, and back pressure, to ensure they are functioning properly and that the hydraulic system pressure is stable (typically 12-15 MPa). Adjust the cooling system to ensure the cooling water pressure (0.3-0.5 MPa) and flow rate meet mold requirements and that the water is clean and free of impurities. For mold trials using auxiliary equipment such as manipulators and conveyor belts, these devices must be adjusted in advance to ensure coordination with the injection molding machine.
The planning and setting of trial mold process parameters must be based on the plastic’s properties and mold structure, resulting in a scientifically sound parameter plan. Preliminary injection parameters are set based on the plastic’s melt flow characteristics and the product structure: The injection speed is controlled in stages, with a low speed (30-50 mm/s) in the initial stage to prevent melt jetting and air entrainment; the speed is adjusted in the middle stage (50-100 mm/s) based on the complexity of the cavity; and the final stage is a low speed (30-50 mm/s) to prevent flash. The injection pressure is initially set to standard plastic molding pressure (e.g., 80-100 bar for PP and 100-130 bar for PC) to ensure that the melt fills the cavity. The holding pressure is typically 50%-80% of the injection pressure, and the holding time is set based on the product thickness (usually 1-2 seconds per millimeter of thickness). The cooling time must ensure that the product is fully formed and is initially set at 10-15 times the product thickness (e.g., 30-45 seconds for a 3mm thick product). The back pressure is set at 5-15 bar to ensure uniform plasticization of the raw material. For hot runner molds, the hot runner temperature must be set, typically 5-10°C higher than the barrel temperature. Develop a parameter adjustment plan, clearly defining the adjustment range and step size for each parameter. For example, injection pressure should be adjusted no more than 10 bar at a time, and temperature should be adjusted no more than 5°C at a time. Record the initial parameters in a mold trial parameter table as a baseline for the mold trial process.
