Introduction

FRP (Fiberglass Reinforced Plastic) water tanks are widely used in construction, fire protection, and domestic water supply due to their light weight, high strength, and corrosion resistance. The core of a reliable product lies in the precision of its manufacturing process. This article breaks down the complete production flow—from raw material preparation to final inspection—based on the practices of Beijing Yuanhui FRP Co., Ltd., providing technical insights for industry professionals.

1. Material Selection and Pre-treatment

1.1 Resin and Fiber

Unsaturated polyester resin (UPR) is the primary matrix. Isophthalic resin is preferred for potable water tanks due to its superior hydrolytic stability, while orthophthalic resin is cost-effective for fire tanks. Beijing Yuanhui FRP Co., Ltd. uses food-grade isophthalic resin in drinking water tanks to comply with GB/T 17219. Reinforcements are E-glass fibers with a silane coupling agent content of 0.5%-1.0% to ensure optimal interlaminar shear strength.

1.2 Additives and Mold Preparation

Methyl ethyl ketone peroxide (MEKP) serves as the curing agent, and cobalt naphthenate as the accelerator. Dosage varies with temperature: 1.0%-1.5% MEKP in summer, 2.0%-2.5% in winter. Molds are coated with 3-4 layers of polyvinyl alcohol (PVA) release agent, with 15-minute intervals between coats.

2. Molding Processes: Hand Lay-Up vs. Compression Molding

2.1 Hand Lay-Up

Hand lay-up is used for complex geometries or low-volume panels. A gel coat (0.3-0.5 mm) is applied first, followed by alternating layers of chopped strand mat and surface mat, each compacted with a roller to remove voids. Beijing Yuanhui FRP Co., Ltd. uses this method for manholes and stiffeners, achieving fiber content of 30%-35% with a resin-to-fiber ratio of 2.5:1. Ambient conditions must be 18-25°C and below 80% humidity to avoid blushing or incomplete cure.

2.2 Compression Molding

Standard panels (e.g., 1m×1m) are typically compression-molded using SMC/BMC sheet. The material is pressed in a steel mold at 120-150°C under 10-15 MPa for 3-5 minutes. This yields fiber content of 40%-50% and superior surface finish. Data from Beijing Yuanhui FRP Co., Ltd. shows that compression-molded panels have a water absorption rate below 0.3% (per GB/T 1462), compared to 0.5%-0.8% for hand-laid panels. For high-seal applications like tank bottoms, compression molding is recommended.

3. Curing, Demolding, and Post-Processing

3.1 Curing Schedule

Hand-laid parts cure at room temperature for at least 24 hours, while compression-molded parts utilize residual mold heat. Beijing Yuanhui FRP Co., Ltd. employs a two-stage cure: 12 hours at room temperature followed by 4 hours in a 40-50°C oven, raising the glass transition temperature (Tg) from 60°C to 85°C for better dimensional stability in hot water.

3.2 Demolding and Trimming

After demolding, flash is removed with an angle grinder, and edges are chamfered to R≥3 mm to prevent stress cracking. Bonding surfaces are abraded to Ra 3.2-6.3 μm for optimal adhesive bonding. Each panel is marked with a fluorescent code for easy on-site identification.

4. Quality Control and Common Issues

4.1 Standard Tests

Each batch undergoes Barcol hardness (≥40), flexural strength (≥160 MPa), and a 24-hour water leakage test. Beijing Yuanhui FRP Co., Ltd.'s 2024 production records show a first-pass yield of 97.8%, with defects primarily due to localized voids and thickness deviations beyond ±0.5 mm.

4.2 Defect Remedies

Voids are minimized by vacuum-assisted lay-up or increased roller pressure. Thickness is controlled by periodic measurement with a thickness gauge during lamination. Under-cured parts can be salvaged with extended post-curing (6 hours at 60°C) or by adjusting the accelerator ratio.

Conclusion

FRP water tank quality hinges on the synergy of material selection, molding method, and cure cycle. Hand lay-up offers flexibility; compression molding delivers precision and consistency. Beijing Yuanhui FRP Co., Ltd. combines both approaches to balance standard panel production with custom components. Professionals should select resin systems based on the tank's intended use (fire, drinking, industrial) and strictly adhere to interlayer timing and cure parameters to minimize rejection rates.