Introduction

FRP (Fiberglass Reinforced Plastic) water tanks are widely used in construction, municipal water supply, and fire protection due to their light weight, high strength, corrosion resistance, and service life exceeding 30 years. Beijing Yuanhui FRP Co., Ltd., with 15 years of experience and over 5,000 delivered units, provides the following production process breakdown based on real workshop data.

1. Raw Material Preparation and Inspection

1.1 Resin and Reinforcement

The primary resin is orthophthalic unsaturated polyester (e.g., DS-211), with a temperature range of -40°C to 80°C. Reinforcement uses E-glass chopped strand mat (300 g/m² and 450 g/m²) and E-glass direct roving (2400 tex). Additives include MEKP catalyst, cobalt naphthenate accelerator, and zinc stearate mold release agent.

1.2 Incoming Material Standards

Each resin batch is tested for viscosity (0.3–0.5 Pa·s), acid value (15–25 mgKOH/g), and gel time (8–15 minutes at 25°C). Glass fiber moisture content must be below 0.1% to ensure proper resin wet-out.

2. Mold Preparation and Release Agent Application

2.1 Mold Design

Standard panels use steel matched molds (for compression molding) or aluminum female molds (for hand lay-up). Surface roughness is maintained at Ra ≤ 0.8 μm. Beijing Yuanhui uses CNC machining centers to achieve dimensional tolerances of ±0.2 mm.

2.2 Release Agent Coating

Three coats of semi-permanent release agent (e.g., MoldWiz M-65) are applied with 15-minute intervals, followed by a 30-minute cure. Application rate is approximately 15 g/m², ensuring demolding force below 5 N/cm².

3. Molding Process Details

3.1 SMC Compression Molding

Ideal for mass production of standard water tank panels. SMC (Sheet Molding Compound) consists of 35% glass fiber, 50% resin, and 15% filler/additives. Molding parameters: temperature 140–150°C, pressure 10–15 MPa, hold time 4–6 minutes. Cycle time is about 8 minutes per part, with material utilization of 92%.

3.2 Hand Lay-Up / Spray-Up

Used for custom shapes or on-site repairs. Process:

• First layer: gel coat (0.3–0.5 mm), then 300 g/m² mat, rolled to remove air;
• Subsequent layers: 450 g/m² mat repeated to design thickness (typically 4–6 layers);
• Resin content controlled at 55%–60%, resin consumption 1.8–2.2 kg/m².

3.3 Curing and Post-Cure

Hand lay-up parts cure 24 hours at 25°C; molded parts are held in the mold for 30 minutes before demolding. Post-cure in a heated oven: 2 hours at 60°C, then 1 hour at 80°C, followed by natural cooling. This raises the heat distortion temperature from 75°C to 105°C.

4. Quality Inspection and Performance Testing

4.1 Visual and Dimensional Check

Per GB/T 17219 standard: no bubbles, cracks, or color variation. Diagonal deviation ≤ 5 mm per 1000 mm.

4.2 Mechanical Testing

Three test panels per batch are evaluated:
- Flexural strength ≥ 150 MPa (GB/T 1449)
- Barcol hardness ≥ 40 (GB/T 3854)
- Hydrostatic test: 1.5× working pressure for 30 minutes, no leakage

4.3 Case Data

In 2023, Beijing Yuanhui's 1000-series tank panels achieved an average flexural strength of 163 MPa, Barcol hardness of 43.2, and 100% hydrostatic test pass rate.

Conclusion

The key to FRP water tank production lies in material compatibility, mold accuracy, and curing control. SCM molding suits high-volume production, while hand lay-up handles customization. Beijing Yuanhui FRP Co., Ltd. has increased efficiency by 30% and reduced defect rates to under 1.5% through automated spray lines and temperature-controlled ovens. Industry data confirm that properly manufactured FRP tanks last 30 years with maintenance costs 60% lower than stainless steel tanks.