(I) Pretreatment and Optimization of the Material System

To address the inherent defects of the material, control measures must be implemented at the source. The core of resin matrix pretreatment is to reduce moisture content and optimize reaction characteristics: vacuum drying (temperature 80-100℃, time 2-4h) is used to remove moisture from the resin, ensuring a water content of ≤0.1%; compatible curing agents and accelerators are selected through experiments to control the curing reaction rate (gel time 100-150s), preventing excessively fast reactions that lead to volatile substance retention. Simultaneously, a suitable amount of defoaming agent (such as organosilicon defoaming agent, added at 0.1%-0.3%) can be added to the resin to disrupt the surface tension of bubbles and promote bubble fusion and escape.

The pretreatment of carbon fibers requires focusing on interface optimization and uniform dispersion: plasma treatment or coupling agent modification (such as KH-550 silane coupling agent) is used to remove impurities from the carbon fiber surface and improve the interfacial bonding strength with the resin; the length of chopped carbon fibers (3-6mm) and the dispersion process are optimized, using a combination of mechanical stirring and ultrasonic dispersion to prevent fiber agglomeration and ensure uniform distribution of fibers in the resin. The preparation and storage of SMC sheets also require strict control: vacuum degassing is used during sheet molding to remove air trapped during the mixing process; the storage environment should be kept dry (relative humidity <60%) and cool to prevent moisture absorption and aging. If moisture absorption is detected before use, secondary drying is required (temperature 60-80℃, time 1-2 hours).

(II) Precise Control of Molding Process Parameters

The optimization of process parameters needs to be based on the rheological characteristics of the material and the mold structure, achieving a synergistic match of "temperature-pressure-time." The core of temperature parameter control is "gradient heating and uniform temperature control": a three-stage heating system is adopted, with a preheating stage (60-80℃) to reduce resin viscosity and facilitate air removal; a molding stage (120-140℃) to control the curing reaction rate; and a holding stage (130-150℃) to ensure complete curing. Simultaneously, thermocouples embedded in the mold are used to monitor the temperature in real time, ensuring that the temperature difference in each area of the mold cavity is ≤3℃, thus avoiding premature gelation of the resin due to local overheating.

The optimization of pressure parameters should follow the principle of "low-pressure exhaust and high-pressure molding":  During the initial stage of mold closing, a low pressure (5-8 MPa) is maintained for 30-60 seconds to allow the resin to flow slowly and expel most of the air; then, the pressure is gradually increased to a high pressure (20-30 MPa) to ensure that the resin fully wets the fibers and fills the mold cavity; the holding time is adjusted according to the product thickness (for every 1 mm increase in thickness, the holding time is extended by 1-2 minutes) to ensure that the small molecule volatile substances produced by the reaction are completely released. In addition, the heating rate (2-3℃/min) and cooling rate (1-2℃/min) should be optimized to reduce the impact of thermal stress on bubble formation. Demolding should only occur when the mold temperature is below 60℃ to avoid secondary bubble formation caused by premature demolding.

(III) Optimization and Upgrading of Mold Design and Structure

Mold optimization should focus on exhaust efficiency and molding adaptability. The design of the exhaust system is crucial: exhaust grooves with a depth of 0.1-0.2 mm and a width of 5-10 mm should be set in areas prone to air accumulation, such as dead corners of the cavity, the end of fiber flow, and the root of ribs. The total length of the exhaust grooves should be no less than 60% of the cavity perimeter. For complex structures, exhaust pins or vents (1-2 mm in diameter) can be added to improve local exhaust efficiency. At the same time, the mold cavity structure should be optimized by designing sharp corners as rounded corners (radius R≥3 mm) to avoid sudden changes in wall thickness, ensuring smooth resin flow and reducing dead zones. A combination of parting surface venting and internal venting should be used to ensure a clear path for air expulsion.

The surface treatment and precision control of the mold also need to be strengthened: the cavity surface should be polished (Ra≤0.4μm) to reduce material flow resistance; the mold should be cleaned regularly to remove residual release agents and carbon deposits, preventing impurities from affecting interfacial bonding; and the precision of the mold guiding mechanism should be improved (positioning error ≤0.03mm) to ensure uniform cavity closure during mold clamping and prevent air ingress through gaps. In addition, a mold heating system with uniform heating (such as electric heating tubes + heat transfer oil circulation) should be used to ensure consistent temperature distribution in the cavity, providing a stable environment for resin curing and bubble removal.

(IV) Standardized Control of Operating Procedures

Establish standardized operating procedures to minimize the impact of human factors on bubble formation. The cutting and laying of SMC sheets must be standardized: cut precisely according to the product dimensions, controlling the gap between the sheet edge and the mold cavity edge to 5-10mm to avoid excessively long flow distances due to undersized sheets; when laying, stack the sheets in an orderly manner according to the fiber flow direction, avoiding overlapping and wrinkling; for large products, multiple sheets can be spliced together, leaving a 5-10mm overlap at the splice to ensure sufficient resin filling. Mold closing operations should be smooth and slow, with a closing speed controlled at 5-10mm/s to prevent air from being trapped by rapid closing; monitor mold pressure and temperature in real time during the molding process, and if abnormal pressure fluctuations are detected, promptly investigate whether there are problems with inadequate venting.The use of mold release agents must be scientific: select a mold release agent that is highly compatible with the carbon fiber SMC resin system (such as polytetrafluoroethylene-based mold release agents), strictly control the dosage (≤5g per square meter of mold cavity), and apply it evenly by spraying to avoid localized accumulation. In addition, establish a product quality inspection mechanism, using ultrasonic testing or X-ray inspection technology to promptly detect internal bubble defects, trace and optimize problems in materials, processes, or molds, forming a closed-loop control system.

III. Industrial Application Case Verification

A certain automotive parts company encountered problems during the production of carbon fiber SMC car door rings, including a surface bubble rate exceeding 8% and an internal porosity of up to 5%, resulting in products that did not meet the required mechanical performance standards. Through systematic investigation, the main causes of bubble formation were identified as: excessive moisture content in the SMC sheet (0.35%), insufficient molding pressure (12 MPa), and an unreasonable mold venting groove design. To address these issues, the company implemented the following measures: vacuum drying the SMC sheet (90°C, 3 hours) to reduce the water content to below 0.1%; optimizing the molding process by using a parameter combination of "low-pressure venting (6 MPa, 40 seconds) - high-pressure molding (25 MPa) - pressure holding for 15 minutes"; and adding eight venting grooves (0.15 mm deep, 8 mm wide) at the root of the ribs and in the dead corners of the mold cavity. After these improvements, the product's surface bubble rate was reduced to below 1%, the internal porosity was ≤1.5%, the tensile strength increased by 22%, and the bending modulus increased by 18%, fully meeting the requirements for automotive structural components.

Controlling bubbles in carbon fiber SMC molding is a systemic engineering challenge that requires coordinated management across four dimensions: materials, process, mold, and operation. The core logic lies in: reducing intrinsic bubble formation through material pretreatment, creating conditions for bubble removal through process parameter optimization, facilitating exhaust pathways through mold design upgrades, and avoiding human-induced defects through standardized operations.