Large Automotive Component Molding: How to Solve Uneven Resin Impregnation and Excessive Porosity?

Under the trend of lightweighting and structural integration in new energy vehicles, the integrated molding process for large composite components has become a significant direction for industry technology breakthroughs. However, when component sizes expand beyond two meters, issues with uneven resin impregnation and excessive porosity become significantly more pronounced, directly impacting product mechanical properties and consistency. This article systematically analyzes the causes of these problems from the perspective of process mechanisms and provides implementable technical solutions.

Systematic Analysis of Problem Causes

The core challenge of large component molding stems from physical field non-uniformity caused by scale effects. When flow paths exceed 1.5 meters, the resin's flow front in reinforcement materials tends to exhibit fingering phenomena, leading to loss of control over impregnation paths. Meanwhile, a mold temperature difference exceeding 5°C can lead to localized differences in resin viscosity, causing deviations in flow front speed by more than 30%. Excessive porosity is mainly due to three compounded factors: micro-bubbles entrapped within fiber bundles during the filling process; poor venting channels for volatile small molecules during the curing phase; and secondary bubble formation due to heat accumulation at the core when thickness exceeds 5mm.

Precise Matching of Material Systems

Addressing impregnation issues starts with the synergistic design of resin-fiber systems. For large components, it's recommended to use gradient viscosity resin systems, initially controlling viscosity between 200-400 mPa·s to ensure adequate flow time. Latent promoters should be introduced to extend gel time to over 90 minutes, reserving a process window for bubble removal. It's advisable to select hybrid structures of multi-layer unidirectional fabrics and chopped strand mats for reinforcement materials, controlling single-layer weight between 300-450 g/m², ensuring permeability while avoiding interlayer slippage. Experimental data shows this configuration can reduce poorly impregnated areas by 60%.

Flow Field Optimization in Mold Design

Mold runner layout must follow the principle of "zonal flow control, sequential filling". For components longer than 2 meters, 3-5 independent injection ports should be set on the mold, combined with a sequential control system to achieve stepped injection. The cross-section of runners adopts a trapezoidal variable section design, starting from 20mm at the inlet end gradually narrowing to 8mm at the outlet end, thus balancing resistance across different zones. Venting systems should be placed in the final filling areas and geometric abrupt changes, with vent slot depth precisely controlled between 0.05-0.08mm, effectively removing bubbles without resin overflow. Mold temperature control systems must allow for zonal independent adjustment, suggesting one temperature control loop every 0.5 meters to ensure surface temperature difference ≤3°C.

Accurate Control of Process Parameters

Injection pressure curves should adopt a two-stage strategy of "low-pressure impregnation - medium-pressure compaction". In the initial stage, maintain 0.1-0.15 MPa pressure for 40-60 minutes to allow resin to fully penetrate into the internal fiber bundles under low pressure; subsequently increase to 0.25-0.3 MPa for structural compaction, requiring vacuum assistance inside the mold with vacuum degree maintained above -0.08 MPa. Curing processes are recommended to use a "stepwise temperature rise": hold at 80°C for 2 hours to achieve pre-gelation, then raise to 120°C to complete cross-linking reactions. This process can reduce porosity from the conventional 2.5% to below 0.8%.

Digital Realization of Process Monitoring

Introducing an in-mold sensor network is key to achieving quality control. It's suggested to place dielectric analysis (DEA) sensors along flow paths to monitor the position of the resin flow front and degree of cure in real-time. A pressure sensor network can capture the consolidation state in various zones, initiating compensation procedures when pressure differences exceed 0.05 MPa. Online temperature measurement using infrared thermal imaging cameras can visualize mold temperature fields, providing data support for dynamic adjustments. Based on these perception data, establish a real-time feedback system for process windows, automatically triggering corrective procedures when parameters deviate from set values by 5%.

Post-processing and Quality Validation

Post-curing treatment must be performed after molding, holding at 130°C for 4 hours to ensure core curing degree reaches over 95%. Quality validation should combine ultrasonic C-scan and X-ray CT, the former quickly locating defect areas, the latter accurately quantifying porosity distribution. For areas with porosity exceeding 1%, establish a database correlating process parameters with defect locations for subsequent iterative optimization.

Quality control in the molding of large automotive components is a systematic project requiring multidimensional coordination of materials, molds, processes, and monitoring technologies. Through implementing gradient viscosity designs, zonal runner layouts, stepwise injection strategies, and digital process monitoring, impregnation failure rates can be controlled within 3%, with porosity consistently below 1%. Enterprises should implement these solutions in stages based on their equipment conditions, prioritizing fundamental issues such as mold temperature control and injection timing, before progressively advancing towards digital upgrades, ultimately achieving stable mass production of large components.

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