In the compression molding process of carbon fiber sheet molding compound (SMC), "bubbles" are one of the typical defects that limit product quality. These bubbles often manifest as surface bulges, internal pores, or interlayer voids, which not only reduce the mechanical properties of the product (such as a 10%-30% decrease in tensile strength and flexural modulus), but may also lead to the penetration of corrosive media and a shortened fatigue life. This severely impacts the application of carbon fiber SMC in high-end fields such as automotive structural components and aerospace auxiliary parts. The formation of bubbles in carbon fiber SMC compression molding is not caused by a single factor, but rather the result of the synergistic effect of multiple factors, including material characteristics, process parameters, mold design, and operating procedures.

I. Core Reasons for Bubble Formation in Carbon Fiber SMC Molding

(I) Intrinsic Factors of the Material System ItselfThe material composition of carbon fiber SMC (resin matrix, carbon fiber, fillers, and additives) and its pretreatment state are the fundamental causes of bubble formation. The resin matrix, as the core binder phase, directly affects the formation and escape of bubbles through its moisture content, viscosity characteristics, and reactivity. If the resin absorbs moisture from the air during storage (moisture content > 0.2%), the high temperature during molding will cause the moisture to vaporize rapidly, forming water vapor bubbles. Due to the high fiber volume fraction of carbon fiber SMC (usually 40%-60%), the dense network formed by the interwoven fibers hinders the upward movement of bubbles, ultimately trapping them inside or on the surface of the product. Simultaneously, poor compatibility between the resin and the curing agent/accelerator, or an excessively fast curing reaction rate, can lead to the accumulation of small molecular volatile substances (such as amines and esters) produced during the reaction, forming chemical bubbles.The quality of the carbon fiber reinforcement pretreatment is also crucial. If oil stains, mold release agents, or incompletely removed oxide layers remain on the carbon fiber surface, it will affect the interfacial bonding between the resin and the fibers, leading to the presence of tiny air gaps between layers. Uneven dispersion of chopped carbon fibers can lead to the formation of fiber agglomerates in some areas. The air inside these agglomerates is difficult to displace by the resin during molding, ultimately forming porous bubbles.Furthermore, defects in the SMC sheet preparation process can also create potential problems—insufficient mixing of fibers and resin during sheet formation, air entrapment during winding, or moisture absorption and aging during sheet storage will significantly increase the probability of bubble formation during molding.

(II) Mismatch of Molding Process ParametersThe coordination of parameters such as temperature, pressure, holding time, and molding cycle in the molding process directly determines whether air bubbles can be effectively removed. Unreasonable temperature parameters are the most common cause: if the mold temperature is too low (5-10℃ below the resin curing temperature), the resin viscosity will be too high (>5000 mPa·s), resulting in poor fluidity and inability to quickly fill the mold cavity and displace air; if the mold temperature is too high, the resin curing reaction rate will far exceed the bubble escape rate, forming "locked-in bubbles." Especially in carbon fiber SMC molding, the thermal conductivity of carbon fiber (approximately 10-15 W/(m·K)) is much higher than that of glass fiber, leading to faster local heat accumulation and premature gelation of the resin, preventing the escape of air bubbles.Improper control of pressure parameters is equally critical. When the molding pressure is insufficient (<15 MPa), the wettability and fluidity of the resin decrease, making it difficult to penetrate the fiber network, and the air between the fiber and resin interface cannot be effectively squeezed out; while excessively rapid pressure increase will cause the SMC sheet to flow rapidly and seal the mold's venting channels, trapping internal air and forming bubbles. Insufficient holding time will lead to incomplete resin curing, incomplete escape of small molecular volatile substances produced by the reaction, and insufficient filling of pores by the resin, ultimately resulting in residual bubbles. In addition, unreasonable design of the molding cycle (such as excessively fast heating rate and uneven cooling during the cooling stage) will lead to thermal stress inside the product, indirectly exacerbating the appearance of bubbles.

(III) Inherent Defects in Mold Design and Structure

The mold's venting system, cavity structure, and surface condition are the "hardware foundation" for the removal of air bubbles. Insufficient venting system design is the most significant mold factor: the mold may lack vent grooves, the vent groove positions may be unreasonable (not avoiding dead corners of the cavity or the end of fiber flow), or the vent grooves may be too shallow (<0.1mm) or too narrow (<5mm), preventing the timely release of air and volatile substances generated during molding, causing them to accumulate inside the mold cavity. For complex carbon fiber SMC products (such as automotive brackets with ribs and holes), sharp corners and abrupt changes in wall thickness of the mold cavity easily form flow dead zones, where air is trapped, and the directional arrangement of fibers further hinders bubble migration, ultimately forming fixed air bubbles.The surface condition and positioning accuracy of the mold also affect bubble formation. When the surface roughness Ra of the mold cavity is >0.8μm, it will generate greater friction with the SMC sheet, leading to poor material flow and air entrapment; insufficient accuracy of the mold's guiding mechanism (positioning error >0.05mm) can lead to uneven cavity closure during mold clamping, creating local gaps where air can easily enter and become trapped. In addition, uneven temperature distribution in the mold (temperature difference >5℃) can lead to inconsistent resin curing rates, causing the resin to gel prematurely in some areas, forming "closed areas" where air bubbles are trapped.

(IV) Irregularities in Operating Procedures

The standardization of operating procedures in industrial production directly affects the effectiveness of bubble control. Improper cutting and placement of SMC sheets are common problems: cutting the sheets to too small a size can lead to excessive material flow distance during molding, making it easy for air to be trapped; failure to arrange the sheets in an orderly manner according to the fiber flow direction during placement, or overlapping and wrinkling of the sheets, will prevent internal air from being smoothly expelled. Too fast or uneven mold closing operations can cause the air inside the mold cavity to be rapidly compressed but unable to escape through the venting system, forming high-pressure bubbles; and premature demolding of the product during the molding process can lead to incomplete resin curing, causing rapid release of internal pressure and the formation of "secondary bubbles."In addition, inadequate cleaning and maintenance of the mold can also indirectly induce bubbles. Residual release agent, resin carbon deposits, or fiber debris on the mold cavity surface can affect the wetting and flow of the resin, leading to the formation of local air gaps; and excessive use or improper selection of release agents (such as those with poor compatibility with the resin) can release volatile substances during molding, forming bubbles.