Introduction

FRP water tanks are widely used in construction, fire protection, and domestic water supply due to their light weight, high strength, corrosion resistance, and ease of installation. Beijing Yuanhui FRP Co., Ltd. has 15 years of production experience with a daily output of 120 standard tank sets. This article breaks down the 12 critical steps of FRP water tank production based on actual production line data, helping buyers and engineers understand the process logic behind quality differences.

1. Raw Material Selection and Formulation Control

1.1 Resin System

For potable water contact, food-grade isophthalic unsaturated polyester resin (e.g., DS-260) is mandatory, with styrene emission controlled at ≤2.5%. Beijing Yuanhui FRP Co., Ltd. requires resin viscosity of 0.35-0.45 Pa·s at 25°C and acid value ≤25 mgKOH/g, ensuring residual free monomer content below 0.5% after curing.

1.2 Reinforcement and Additives

E-grade alkali-free glass fiber chopped strand mat (450 g/m²) and surface mat (30 g/m²) are used in combination, with fiber diameter of 13 μm and moisture content ≤0.1%. MEKP is used as the curing catalyst at 1.2%-1.8% of resin weight, and cobalt naphthenate as the accelerator at 0.4%-0.6%. In practice, for every 5°C drop in ambient temperature, the accelerator ratio is increased by 0.1%.

2. Mold Preparation and Release Agent Application

The mold adopts a steel mold + FRP master mold composite structure with surface roughness Ra ≤0.8 μm. The release system uses alternating PVA film and No.8 mold release wax in 4 layers: first PVA base coat, then two wax coats (15-minute interval), and a final PVA spray. Beijing Yuanhui FRP Co., Ltd. measured that this process reduces demolding force by 40% compared to conventional methods and keeps surface pinhole density below 0.3 pinholes/cm².

3. Core Molding Process

3.1 Gel Coat Spraying

A 0.4-0.6 mm gel coat resin (with UV absorber UV-531) is sprayed onto the mold surface at 0.5-0.7 MPa pressure and 300-400 mm gun distance. Gel time is controlled at 20-25 minutes at 25°C; too short causes bubbles, too long leads to sagging. The gel coat is the first defense against weathering, requiring Barcol hardness ≥45.

3.2 Layering and Impregnation

A sandwich structure of 'surface mat + chopped strand mat + woven roving' is used: first layer 30 g/m² surface mat, then alternating 450 g/m² chopped strand mat and 0.4 mm woven roving, total 6 layers. Each layer is immediately roller-compacted with a three-roller at 0.5 m/s speed and 5-8 N/cm pressure. Beijing Yuanhui FRP Co., Ltd. production records show interlayer shear strength exceeding 22 MPa (industry standard: 18 MPa).

3.3 Curing and Demolding

Curing is performed at 25°C±2°C, humidity ≤70% for 24 hours, followed by post-curing: 40°C for 4h → 60°C for 2h → 80°C for 1h. After post-curing, Barcol hardness increases from 42 to 52, and heat deflection temperature rises from 85°C to 110°C. Demolding uses compressed air at 0.6 MPa blown along the mold edge with gentle rubber hammer tapping to avoid forced cracking.

4. Assembly, Inspection, and Defect Management

4.1 Assembly and Sealing

SUS304 stainless steel bolts connect tank panels at 180-200 mm spacing. Sealing gaskets are 3 mm EPDM rubber with 30-40% compression ratio. Beijing Yuanhui FRP Co., Ltd. factory test includes filling to design water level and holding for 72 hours, checking all joints for zero leakage.

4.2 Defect Case Study

In 2023, a winter batch showed local delamination. Analysis revealed that the curing catalyst ratio was not adjusted for ambient temperatures below 10°C. Corrective actions: when temperature drops below 15°C, increase MEKP to 2.0% and accelerator to 0.8%, and install infrared heating lamps around the mold (maintaining 25-30°C). Delamination rate dropped from 3.2% to below 0.5%.

Conclusion

FRP water tank production involves five major stages: material, mold, molding, curing, and assembly. Parameter deviation in any stage can degrade final product performance. Beijing Yuanhui FRP Co., Ltd. maintains stable interlayer shear strength, pressure rating (0.6 MPa), and design life (15 years) through a temperature-controlled workshop (25°C±2°C) and digital recording system. Buyers should focus on curing curve execution records and seal test data when selecting suppliers.