Introduction

FRP (Fiberglass Reinforced Plastic) water tanks are widely used in construction, municipal, and industrial water storage due to their lightweight, high strength, corrosion resistance, and long service life. According to 2023 data from the China FRP Industry Association, domestic annual production exceeds 1.2 million cubic meters, with SMC compression-molded tanks accounting for 45% and hand lay-up tanks for 35%. Beijing Yuanhui FRP Co., Ltd. (hereafter 'Yuanhui FRP') has 15 years of production experience, manufacturing over 100,000 tank panels annually and serving projects like Beijing Daxing International Airport and Xiong'an New Area Citizen Service Center. This article breaks down the complete production chain based on Yuanhui FRP's real-world processes.

1. Raw Material Selection and Pretreatment

1.1 Resin System Selection

Resins for water tanks must comply with drinking water hygiene standards (GB/T 17219). Yuanhui FRP primarily uses isophthalic unsaturated polyester resin (grades 191#, 196#), whose water resistance outperforms orthophthalic types, with water absorption below 0.3%. For high-temperature applications (above 60°C), vinyl ester resins (e.g., Derakane 470 series) are recommended, increasing cost by about 40% but doubling heat resistance.

1.2 Reinforcement Configuration

A hybrid structure of E-glass chopped strand mat (300-450 g/m²) and woven roving (0.4-0.8 mm thick) is employed. Yuanhui FRP's test data show that when fiber content is controlled at 28%-35% (by weight), tensile strength reaches 180-220 MPa, and flexural strength ≥250 MPa. Below 25% fiber content, panels are prone to deformation; above 40%, poor resin wet-out causes delamination.

1.3 Fillers and Additives

Calcium carbonate filler is added at 8%-12% of resin weight to reduce shrinkage from 4% to below 1.5%. MEKP hardener is used at 1.5%-2.5% of resin, with cobalt naphthenate accelerator at 0.5%-1.0%. When ambient temperature falls below 15°C, accelerator dosage should be increased by 30%.

2. Mold Preparation and Molding Processes

2.1 Mold Design and Treatment

Molds for tank panels are made of steel or FRP master molds, with surface roughness ≤ Ra 0.8 μm. Yuanhui FRP controls mold temperature differentials within ±0.5°C to prevent panel warping due to thermal expansion. Semi-permanent wax mold release (e.g., Marbocote 227C) is used, allowing 8-12 releases per application—50% more efficient than traditional PVA films.

2.2 Hand Lay-Up Process

For a standard 1 m × 1 m panel, process parameters include:
- Base layer: 0.3 kg/m² resin, 300 g/m² chopped strand mat, rolled with a bristle roller to keep bubble residue ≤0.5%.
- Middle layers: alternating 0.6 mm woven roving and resin at 0.5 kg/m² per layer, with layers offset by 45° for torsional strength.
- Surface layer: 30 g/m² surface veil, resin mixed with 5% paraffin wax to suppress styrene evaporation, ensuring a smooth finish.
Yuanhui FRP's measurements show 3 mm thick panels produced with this method achieve a Barcol hardness of 45-50, meeting national standards.

2.3 Spray-Up Process (for Large Tanks)

For panels exceeding 5 m², Yuanhui FRP uses spray-up forming. A dual-gun system (resin + chopped fiber) operates at 8-12 kg/min, with fiber cut length of 25-40 mm. Fiber content deviation is controlled within ±2%, tripling efficiency over hand lay-up, though equipment investment is around 350,000 RMB per unit.

3. Curing, Demolding, and Post-Processing

3.1 Curing Schedule

Under ambient curing (25°C), gel time is controlled at 20-30 minutes, and demolding time at 4-6 hours. Yuanhui FRP's workshop uses staged temperature control: 30°C for the first 2 hours to promote gelation, followed by natural exotherm to 50-60°C for full cure. If curing temperature exceeds 80°C, exothermic cracking may occur. In winter, infrared heating panels maintain mold surface at 35-40°C.

3.2 Demolding and Trimming

Pneumatic ejectors apply 0.3-0.5 MPa for demolding. Panels are then post-cured at 25°C for 24 hours to reduce residual styrene below 0.5%. Yuanhui FRP's quality standards require edge burrs ≤0.5 mm and flatness tolerance ≤2 mm/m.

4. Assembly and Quality Inspection

4.1 Modular Assembly

Standard tanks use bolted connections with EPDM gaskets (compression ratio 25%-30%, service life up to 15 years). Yuanhui FRP's assembly sequence: base panel placement → side panel joining → tie rod installation (1.5 m spacing) → top panel closure. A 100 m³ tank takes 2-3 days to assemble.

4.2 Inspection Items and Standards

Per GB/T 17219-2001, water immersion tests (24 hours) must show oxygen consumption ≤2 mg/L. Yuanhui FRP's factory inspection includes:
- Hardness: Barcol ≥40 (ASTM D2583)
- Leak test: 0.3 MPa water pressure for 30 minutes, no leakage
- Dimensional tolerance: diagonal difference ≤5 mm, thickness tolerance ±0.5 mm

Conclusion

The core of FRP water tank production lies in precise resin-to-fiber ratio control, mold accuracy, and strict adherence to curing schedules. By optimizing fiber content (28%-35%), implementing staged temperature curing, and using EPDM sealing systems, Yuanhui FRP has raised product pass rates from the industry average of 92% to 97.5%. For buyers, it is recommended to verify the manufacturer's raw material certifications (e.g., NSF/ANSI 61) and third-party test reports to ensure long-term tank reliability.