Introduction

FRP (Fiberglass Reinforced Plastic) water tanks are widely used in building water supply, fire protection storage, and agricultural irrigation due to their light weight, high strength, corrosion resistance, and easy installation. The production process is not a simple manual application but a systematic engineering project involving material science, mold design, and cure control. Based on years of production practice at Beijing Yuanhui FRP Co., Ltd., this article outlines the complete workflow from raw material proportioning to final product inspection, with a focus on key technical parameters and common quality pitfalls in SMC compression molding and hand lay-up processes.

1. Raw Material Preparation and Formulation Design

1.1 Resin System Selection

The main body of FRP water tanks uses unsaturated polyester resin (UPR), primarily orthophthalic and isophthalic types. For potable water tanks, food-grade isophthalic resin must be used, with styrene evaporation controlled below 0.5% and residual free monomer ≤0.1% after curing. Beijing Yuanhui tests each resin batch for acid value (20-28 mgKOH/g) and viscosity (0.5-1.5 Pa·s) to ensure matrix material consistency.

1.2 Reinforcement Material Matching

E-glass fiber is the standard choice, commonly 450 g/m² chopped strand mat and 800 g/m² woven roving. When used together, chopped mat provides isotropic strength, while woven roving bears directional loads. The fiber content per cubic meter of tank wall should be controlled at 30%-35% by weight—below 30% results in insufficient strength, above 35% leads to poor resin infiltration.

2. Mold Preparation and Release Treatment

The mold surface quality directly determines the inner wall smoothness and waterproof performance of the tank. Steel molds undergo a four-step treatment: sandblasting → rust removal → mold wax application → PVA release agent coating. Mold wax must be applied 3-5 times with 20-minute intervals between coats to form a complete isolation film. Beijing Yuanhui’s measured data shows that uneven release agent application can cause surface tearing during demolding, increasing the defect rate by approximately 12%.

3. Core Molding Process Steps

3.1 SMC Compression Molding (for Standardized Tank Panels)

The SMC (Sheet Molding Compound) process is suitable for mass production of standard-size tank panels. Resin paste is impregnated into glass fibers to form a prepreg, then molded at 80-150°C and 10-15 MPa. Key control parameters: mold temperature must be maintained within ±5°C; otherwise, panel warpage occurs. The pressing cycle depends on thickness—6 mm panels require 5.5-6 minutes, while 12 mm panels need 8-9 minutes. Beijing Yuanhui uses a PLC temperature control system to keep zone temperature fluctuations within ±2°C.

3.2 Hand Lay-Up Lamination (for Custom Shapes and On-Site Repair)

Large tanks or complex corner sections still rely on hand lay-up. The process involves: applying resin → placing fiber layers → roller degassing → repeating to design thickness. Each resin layer should be controlled at 350-400 g/m²—too little results in insufficient fiber wet-out, too much causes localized exothermic concentration. Overlap width between layers must be ≥50 mm, with staggered joints to eliminate stress concentration points. Curing conditions: initial set time 35-45 minutes at 25°C, full cure requires 24 hours. Winter construction must use low-temperature initiators (BPO content 0.8%-1.2%), otherwise insufficient cure will cause later leakage.

4. Curing, Demolding, and Post-Processing

Cure degree is the core indicator of tank longevity. Barcol hardness ≥45 is the demolding standard; below this value, deformation may occur. After demolding, products undergo trimming → grinding → reinforcement. Beijing Yuanhui’s quality inspection requires ultrasonic thickness measurement for each panel, with wall thickness deviation not exceeding ±0.5 mm from design value. For depressions or bulges exceeding 0.5 mm, putty repair using the same resin system followed by re-curing is mandatory.

5. Assembly and Leakage Testing

Tank assembly uses stainless steel bolts to connect panel joints, with EPDM rubber strips as sealing material, compressed at 30%-35%. After installation, a full water test is conducted: fill to design water level and hold for 72 hours; a drop ≤3 mm/24h is acceptable. Beijing Yuanhui performs a 0.05 MPa pneumatic pre-check on every tank before shipment, detecting over 90% of potential micro-cracks.

Conclusion

The production process of FRP water tanks involves the coordination of materials science, mold design, and environmental control. Among the 12 critical steps from raw material inspection to final leakage testing, any deviation at a single step can propagate to the final product quality. Experience at Beijing Yuanhui FRP Co., Ltd. shows that SMC compression molding is ideal for standardized products (40% efficiency improvement), while hand lay-up remains irreplaceable for customized projects. It is recommended that industry practitioners establish a complete batch traceability system, binding process parameters, inspection data, and operators to each panel—this is the most effective means of controlling quality variation.