Injection molding trial process and parameter debugging
Injection molding trials are a critical step before mass production of new products. By conducting molding tests on actual molds and equipment, the rationality of mold design is verified, process parameters are optimized, and potential defects are identified and resolved, providing a reliable basis for mass production. The trial mold process must follow a scientific and standardized procedure, including pre-trial mold preparation, first injection, defect analysis, parameter optimization, and final confirmation. Detailed data (such as injection pressure, temperature, and part dimensions) must be recorded at each stage to ensure traceability of trial mold results. For example, a company trial molded a new mold for a mobile phone casing. After three rounds of parameter tuning, they reduced the part’s dimensional tolerance from ±0.15mm to ±0.05mm and reduced the appearance defect rate from 30% to below 1%, laying the foundation for subsequent mass production.
Pre-trial preparation directly impacts the efficiency and accuracy of the trial, requiring a comprehensive inspection of the mold, equipment, and materials. This inspection includes confirming that the cavity and core are free of burrs or scratches, that cooling and air channels are unobstructed (water test pressure 0.3 MPa, no leaks for 30 minutes), that the ejector mechanism is flexible (ejector stroke error ≤ 0.1 mm), and that the gate and runner dimensions conform to the design drawings (e.g., gate diameter error ± 0.05 mm). Equipment selection must match the mold specifications, ensuring that parameters such as clamping force, maximum shot volume, and tie-bar spacing meet requirements (e.g., for a mold with a projected area of 500 cm², an injection molding machine with a clamping force ≥ 1000 kN is required). All equipment systems must be verified to function properly (hydraulic system pressure fluctuation ≤ 5%, heating system temperature deviation ≤ ± 2°C). Materials must be prepared strictly according to the recipe, thoroughly dried (e.g., PA66 should be dried at 100°C for 4 hours to a moisture content ≤ 0.05%), and pre-mixed with masterbatch (mixing time ≥ 10 minutes) to ensure the stability of the raw materials. For example, a trial mold failed to check for blockage in the mold cooling water channel, resulting in uneven cooling of the plastic part. The problem was not discovered until 3 hours after the trial mold was run, delaying the project progress.
The first shot is a core step in mold trial, verifying the mold’s basic functionality and initial molding feasibility. Conservative process parameters should be used to avoid damage to the mold or equipment. Initial parameter settings should refer to the material supplier’s recommendations: Use an intermediate barrel temperature (e.g., 180-220°C for PP, set at 200°C for the initial setting), a lower mold temperature (e.g., 50°C), a slow injection speed (30-50 mm/s), a holding pressure of 50% of the injection pressure, and an appropriately extended cooling time (e.g., 15 seconds for a 2mm wall-thick part). During the first shot, closely monitor the filling process: ensuring smooth melt entry into the cavity, any leaks, and smooth ejection. If underfill is observed, gradually increase the injection pressure (increase by 10 MPa) or speed (increase by 10 mm/s) until the cavity is at least 90% filled. After removing the first molded part, inspect for obvious defects (e.g., cracks, missing material, or severe flash). Measure key dimensions (e.g., length, width, and aperture) and compare deviations with the drawing to provide a benchmark for subsequent commissioning. For example, a mold is filled to 70% during the first injection . By gradually increasing the injection pressure (from 80MPa to 120MPa) and speed (from 40mm/s to 70mm/s), it is finally fully filled. The dimensional deviation of the first plastic part is mainly concentrated in the range of ±0.2mm.
Defect analysis and parameter adjustment during mold trials are key to improving part quality. Targeted process parameter optimization is required based on the defect type. If material shortfall occurs, prioritize increasing the injection speed (e.g., from 50 mm/s to 80 mm/s) to shorten filling time, or increasing the barrel temperature (e.g., from 280°C to 300°C for PC) to reduce melt viscosity. If accompanied by flash, reduce the injection pressure or clamping force rather than blindly increasing parameters. To address warpage, adjust the cooling time (extend or shorten), the holding pressure (increase or decrease), or the mold temperature (localized heating or cooling). For example, if the edge of a PP part warps, increase the mold temperature by 10°C and extend the holding time by 2 seconds. For surface defects (e.g., weld lines and silver streaks), optimize gate location or increase venting, while also adjusting the injection speed (using pulsed injection) or increasing the melt temperature. Each parameter adjustment should change only one variable (e.g., only the injection speed, leaving all other parameters unchanged). Record the state of the part before and after the adjustment and compare them to determine the optimal value. For example, when a plastic part was tested, obvious weld marks appeared. By increasing the injection speed from 60mm/s to 90mm/s and adding venting grooves at the weld marks, the weld mark strength was increased by 25% and its visibility to the naked eye was reduced by 80%.
Dimensional accuracy tuning is a key goal of mold trial. Optimizing holding and cooling parameters ensures stable part dimensions within tolerance. Holding pressure and time are key factors influencing dimensional stability: Insufficient pressure can lead to excessive shrinkage, while excessive pressure can create internal stress in the part. Too short a holding time won’t adequately compensate for shrinkage, while too long a holding time will extend the cycle time. A “stepped holding” tuning method is employed. First, a fixed holding time (e.g., 5 seconds) is set. Part dimensions are tested at different holding pressures (from 50% to 90% of the injection pressure, in 10% increments) to identify the pressure range within which dimensions meet acceptable standards. Next, a fixed holding pressure is set and the effects of different holding times (from 3 seconds to 10 seconds, in 1-second increments) are tested to determine the optimal holding pressure combination. The cooling time must ensure that the part is fully set. Dimensional changes at different cooling times (e.g., 10, 15, and 20 seconds) are measured. The optimal cooling time is determined when the dimensional deviation is ≤0.02mm over two consecutive measurements. For example, during the mold trial of a precision gear, the gear pitch tolerance was controlled from ±0.08mm to ±0.03mm by increasing the holding pressure from 70MPa to 90MPa, extending the holding time from 4 seconds to 6 seconds, and extending the cooling time from 12 seconds to 18 seconds.
Final confirmation of the trial mold and report preparation are the finishing touches of the trial mold process, requiring verification of mass production stability and the creation of standardized documentation. Conduct a small-batch trial run (usually 30-50 molds), continuously sampling plastic parts for appearance and dimensional inspection to ensure a pass rate of ≥95% and a dimensional fluctuation of ≤0.05mm, demonstrating stable process parameters. Test the mechanical properties of the plastic parts (such as tensile strength and impact strength) and compare them with design requirements to ensure they meet operational requirements. Clean the mold, inspect components such as the cavity and ejector pins for wear or damage, and record any mold issues discovered during the trial mold (such as insufficient venting or improper ejector pin placement) and provide feedback to the mold manufacturer for modification. Prepare a trial mold report, including equipment model, material specifications, final process parameters (temperature, pressure, speed, time), defect resolution measures, dimensional inspection data, and a mechanical properties report, to serve as a guide for mass production.
