Introduction

FRP (Fiberglass Reinforced Plastic) water tanks are widely used in construction, chemical, food, and fire protection industries due to their corrosion resistance, light weight, and long service life. The production process directly determines sealing performance, pressure capacity, and durability. Beijing Yuanhui FRP Co., Ltd., with 15 years of experience and over 5,000 delivered tanks, provides a practical reference for the industry. This article breaks down the complete manufacturing workflow, from raw materials to finished product, with real data and case studies.

1. Raw Material Selection and Pre-treatment

Resin and Fiber

The matrix material is typically 196-type unsaturated polyester resin or vinyl ester resin. 196 resin is cost-effective for ambient water storage; for hot water (>60°C) or corrosive media, vinyl ester is required. Reinforcement uses E-glass chopped strand mat (450 g/m²) and spray-up roving (2400 tex), with alkali content below 0.5% to prevent hydrolysis strength loss. Beijing Yuanhui FRP Co., Ltd. adopts a double-mat plus three-layer cloth lay-up in pressure-rated tanks, achieving an initial tensile strength of 280 MPa.

Fillers and Additives

To reduce shrinkage and improve flame retardancy, aluminum trihydrate (ATH) filler is added at 15–20% of resin weight. The curing system uses MEKP as initiator and cobalt naphthenate as accelerator, at 1.5–2.0% of resin weight. Precise metering is critical to avoid cracks from exothermic heat. Beijing Yuanhui's batching room uses an automatic dosing system with ±0.5% accuracy.

2. Mold Preparation and Molding Techniques

Mold Design and Treatment

Steel or FRP master molds are used. Steel surfaces are polished to Ra ≤0.8 μm, then coated with mold release wax (3–4 layers) and PVA release agent. Mold temperature is controlled at 25–30°C, humidity below 70%. Beijing Yuanhui employs zone heating, maintaining mold temperature uniformity within ±2°C.

Hand Lay-up

For custom tanks (5–50 tons), hand lay-up is used: a resin gel coat (0.3–0.5 mm with UV absorber) is brushed first, followed by chopped strand mat, compacted with a roller to remove air. Two to three layers of mat then glass fabric are applied, total thickness 4–6 mm. Resin content per layer is kept at 55–65%. Case: a 20-ton hydrochloric acid tank for a Beijing chemical plant, hand-laid with 5 mm wall thickness, showed no leakage after 7 years.

Compression Molding

For mass-produced standard tanks (100–5000 L), compression molding improves efficiency by over 40%. SMC sheet is cut to mold size, placed, and closed under 150–160°C and 15–20 MPa for 3–5 minutes. Low-profile additive (LPA) reduces shrinkage to below 0.05%. Beijing Yuanhui's molding line uses a 2500-ton press with an 8-minute cycle time, achieving mirror-finish surfaces.

3. Curing, Demolding, and Finishing

Curing Control

Hand-laid parts cure at room temperature (25±2°C) for 24 hours to demolding strength (Barcol hardness ≥35), then post-cure at 40°C for 6 hours, achieving a glass transition temperature (Tg) above 80°C. Molded parts cool naturally below 50°C before further processing. Beijing Yuanhui's curing workshop has a temperature/humidity monitoring system recording 48-hour data, with automatic alerts.

Demolding and Trimming

After demolding, flash is cut with a water jet, dimensional tolerance ±1 mm. Openings (pipe, manhole) are milled with diamond tools and sealed with resin to prevent capillary absorption. Sealing surfaces are ground to Ra ≤3.2 μm using a surface grinder.

Assembly and Leak Testing

Panels are bolted with 304 stainless steel fasteners, using EPDM gaskets (Shore A 60±5). After assembly, the tank is filled to design full water level and pressurized for 24 hours. Beijing Yuanhui's standard exceeds GB/T 17219-1998: allowable leakage ≤0.03 L/(m²·h); actual average is 0.01 L/(m²·h).

4. Quality Inspection and Shipment

Each batch undergoes Barcol hardness test (≥35), thickness measurement (ultrasonic, ±0.1 mm), and resin content test (ignition method, 0.001 g precision). Final hydrostatic test at 1.5 times working pressure for 15 minutes. In 2023, Beijing Yuanhui achieved a 99.7% pass rate and a field failure rate below 0.5%.

Conclusion

FRP water tank production integrates material science, mold design, molding techniques, and quality control. Every step, from raw material selection to final testing, ensures product reliability. Beijing Yuanhui FRP Co., Ltd. maintains a yield rate above 99% through automated batching, zone temperature control, and rigorous testing. Understanding these process details helps buyers evaluate suppliers and avoid risks such as leakage or deformation caused by process defects.