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1. Hand Lay-up Molding  

Hand lay-up molding is the most traditional method for forming fiberglass-reinforced plastic (FRP) flanges. This technique involves manually placing resin-impregnated fiberglass cloth or mats into a mold and allowing them to cure. The specific process is as follows: First, a resin-rich inner liner layer is created using resin and fiberglass cloth. After the liner layer cures, it is removed from the mold, and the structural layer is constructed. Resin is then brushed onto both the mold surface and the inner liner. Pre-cut fiberglass cloth layers are laid according to a predetermined stacking plan, with each layer compacted using a roller to ensure thorough impregnation. Once the desired thickness is achieved, the assembly is cured and demolded.  

The matrix resin for hand lay-up molding typically uses epoxy or unsaturated polyester, while the reinforcement material is medium-alkali or alkali-free fiberglass cloth.  

Advantages: Low equipment requirements, ability to produce non-standard flanges, and no restrictions on flange geometry.  

Disadvantages: Air bubbles formed during resin curing can lead to porosity, reducing mechanical strength; low production efficiency; and uneven, unrefined surface finish.  

2. Compression Molding  

Compression molding involves placing a measured amount of molding material into a flange mold and curing it under pressure using a press. Molding materials vary and may include pre-mixed or pre-impregnated short-cut fiber compounds, recycled fiberglass cloth scraps, resin-impregnated multi-layer fiberglass cloth rings/strips, stacked SMC (sheet molding compound) sheets, or prewoven fiberglass fabric preforms. In this method, the flange disk and neck are molded simultaneously, enhancing joint strength and overall structural integrity.  

Advantages: High dimensional accuracy, repeatability, suitability for automated mass production, ability to form complex tapered-neck flanges in one step, and aesthetically smooth surfaces requiring no post-processing.  

Disadvantages: High mold costs and limitations on flange size due to press bed constraints.  

3. Resin Transfer Molding (RTM)  

RTM involves placing fiberglass reinforcement into a closed mold, injecting resin to impregnate the fibers, and curing. The process includes:  

• Positioning a fiberglass preform matching the flange geometry in the mold cavity.  
• Injecting low-viscosity resin under controlled temperature and pressure to saturate the preform and displace air.  
• Heating to cure and demolding the finished flange.  

Resins are typically unsaturated polyester or epoxy, while reinforcements include fiberglass continuous mats or woven fabrics. Fillers like calcium carbonate, mica, or aluminum hydroxide may be added to enhance properties or reduce costs.  

Advantages: Smooth surfaces, high productivity, closed-mold operation (minimizing emissions and health risks), directional fiber alignment for optimized strength, low capital investment, and reduced material/energy consumption.  

4. Vacuum-Assisted Resin Transfer Molding (VARTM)

VARTM modifies RTM by injecting resin under vacuum. The process involves sealing a fiberglass preform on a male mold with a vacuum bag, evacuating air from the mold cavity, and drawing resin into the preform via vacuum pressure.  

Compared to RTM, VARTM produces flanges with lower porosity, higher fiber content, and superior mechanical strength.  

5. Airbag-assisted resin transfer molding

Airbag-assisted RTM molding is also a kind of molding technology developed on the basis of RTM. The process of preparing flanges by this molding method is as follows: a flange-shaped glass fiber preform is placed on the surface of an airbag, which is filled with air and then expands outward and is confined to the space of the cathode mold, and the flange preform between the cathode mold and the airbag is compacted and cured.

Advantages: the expansion of the airbag can make the resin flow to the part of the preform that is not impregnated, ensuring that the preform is well impregnated by the resin; the resin content can be adjusted by the pressure of the airbag; the pressure exerted by the airbag is applied to the inner surface of the flange, and the flange after curing has low porosity and good mechanical properties. Generally speaking, after preparing FRP flange with the above molding method, the outer surface of the flange should also be processed according to the requirements of the use of turning and drilling through holes around the circumference of the flange.