Introduction

FRP (Fiberglass Reinforced Plastic) water tanks are widely used in building water supply, fire protection storage, and industrial cooling due to their corrosion resistance, light weight, and long service life. Beijing Yuanhui FRP Co., Ltd., with 20 years of specialized manufacturing experience, has developed a mature production system. This article breaks down the entire process from material selection to final inspection, highlighting key control parameters for practitioners.

1. Raw Material Selection and Formulation

1.1 Resin System

Orthophthalic unsaturated polyester resin (e.g., 196# resin) is preferred for water tank applications due to superior water resistance. The typical resin-to-curing agent ratio is 100:1.5-2.5 (MEKP) with 0.5-1.0 parts cobalt naphthenate accelerator. Beijing Yuanhui adjusts the ratio seasonally—1.8 parts in summer and 2.2 parts in winter—to compensate for temperature effects on curing kinetics.

1.2 Reinforcement Materials

Surface layers use 0.4mm and 0.6mm E-glass chopped strand mats, while structural layers employ 0.8mm medium-alkali woven roving. Fiber content is controlled at 30-35% by weight. Fiber areal weight per square meter ranges from 450g to 600g for surface plies. Exceeding 35% risks poor resin wet-out, while below 30% compromises mechanical strength.

2. Hand Lay-Up Molding

2.1 Mold Preparation

Steel or FRP master molds with surface roughness Ra ≤ 3.2 μm are used. A three-layer release agent system (PVA + paraffin emulsion) is spray-applied with 15-minute intervals between coats. Beijing Yuanhui's data shows that uneven release agent application increases demolding force by over 30%, potentially causing surface damage.

2.2 Layering and Rolling

The sequence follows "surface layer first, structural layer second." Surface plies use 0.4mm mat with 70% resin content for densification. Each structural ply is consolidated with a bristle roller to maintain void content below 2%. Lay-up speed is limited to 0.5-0.8 m²/person-hour; faster rates entrap air. Corner sections receive two additional woven roving layers for reinforcement.

3. Curing and Demolding

3.1 Curing Temperature Profile

Curing proceeds through gelation (30-45 min at 25°C), hardening (2-3 hours), and post-curing (24+ hours). Beijing Yuanhui uses a segmented heating schedule: 4 hours at 25-30°C followed by 2 hours at 40-45°C. This achieves 75% of design strength before demolding, reducing cycle time by 40% compared to ambient curing.

3.2 Demolding and Trimming

Demolding force must not exceed 15% of part weight; forces above 20% indicate under-curing or release agent failure. Pneumatic tools apply uniform force along mold edges to avoid stress concentration cracks. Immediately after demolding, flash is removed and edges are smoothed with 400-grit sandpaper.

4. Sealing and Testing

4.1 Seal Design

Panel joints employ dual EPDM rubber gaskets with polysulfide sealant. Gasket compression is set at 25-35%; below 25% risks leakage, above 35% causes extrusion. Polysulfide sealant is applied at 6-8mm thickness, achieving cured tensile strength ≥ 2.0 MPa.

4.2 Water Fill Testing

Per CJ/T 205-2000 standard, assembled tanks undergo 24-hour water fill testing with allowable leakage ≤ 0.05% of tank volume. Beijing Yuanhui applies a stricter internal limit of ≤ 0.03%, plus individual joint pneumatic testing at 0.1 MPa. From 2019 to 2023, process optimization improved first-pass yield from 92% to 97.5%.

Conclusion

FRP water tank production is a multidisciplinary process combining materials science, chemical formulation, and precision workmanship. Beijing Yuanhui's experience demonstrates that quantitative control of process details—seasonal curing agent adjustment, lay-up speed vs. void content, and precise seal compression—directly determines product quality. While automation in spray-up and compression molding will boost efficiency, hand lay-up remains indispensable for complex geometries and custom designs.