Introduction

FRP (Fiber-Reinforced Plastic) water tanks are widely used in municipal water supply, fire protection storage, and industrial cooling due to their corrosion resistance, light weight, and high strength. However, product quality varies significantly across manufacturers, often due to loose process control. Beijing Yuanhui FRP Co., Ltd., with over 15 years of production experience, provides a detailed breakdown of the full manufacturing process—from raw material inspection to final testing—based on actual production line data.

1. Raw Material Inspection and Pretreatment

1.1 Resin System

Over 70% of the mechanical properties of an FRP tank depend on the resin system. Beijing Yuanhui selects isophthalic unsaturated polyester resin (e.g., DSM P-61-80) with elongation at break ≥2.5% and heat deflection temperature ≥80°C. The curing initiator is MEKP at 1.5%–2.5% by resin weight, with cobalt naphthenate accelerator at 0.3%–0.6%. Each batch must pass gel time (15–25 min at 25°C), viscosity (0.3–0.8 Pa·s), and solid content (≥65%) tests.

1.2 Reinforcement Materials

E-glass chopped strand mat (450 g/m²) and medium-alkali woven roving (800 g/m²) are used. Fibers are treated with silane coupling agent (KH-550) and have moisture content ≤0.1%. Beijing Yuanhui's incoming inspection includes: filament diameter 13 μm ±1 μm, sizing content 0.6%–1.2%, and tensile strength ≥1500 MPa.

2. Hand Lay-Up Molding Process

2.1 Mold Preparation and Release Agent

Steel or FRP molds with surface roughness Ra ≤1.6 μm are used. After cleaning with anhydrous ethanol, three coats of polyurethane release agent are applied (15-min intervals), followed by a 30-min stand. Improper release agent application increases demolding force by 40%–60% and causes surface defects.

2.2 Gel Coat Application

Gel coat (DSM, viscosity 3000–4000 cP) is spray-applied to 0.4–0.6 mm thickness in two passes, 20–30 min apart. 2%–3% hydroquinone inhibitor is added to extend work time. Common defects: pinholes (excessive thickness or poor deaeration) and orange peel (curing temperature below 15°C). Beijing Yuanhui's data: reject rate rises to 5.2% below 18°C versus 0.8% at 22–25°C.

2.3 Laminate Lay-Up and Impregnation

The structure is: gel coat + mat + roving + mat. Total layers depend on wall thickness (5/8/10 mm). Resin application rate: 400–500 g/m² per layer, with sheep-wool roller deaeration. Residual bubble area must not exceed 1%. Overlap width ≥30 mm; corners receive two additional local reinforcements.

3. Curing, Post-Curing, and Demolding

3.1 Curing Parameters

Room-temperature curing (25±3°C) for ≥24 hours; exothermic peak 120–140°C (exceeding 160°C degrades resin). Beijing Yuanhui uses staged curing: first 4 hours at 35–40°C (heating pads), then natural cooling for 20 hours. Curing verification: Barcol hardness ≥40 (ASTM D2583), acetone extraction ≤5%.

3.2 Post-Curing

After demolding, tanks are post-cured at 60°C for 4 hours, then cooled to room temperature. This increases crosslink density by 15%–20% and heat deflection temperature by 10–15°C. Beijing Yuanhui tests: untreated panels retain only 72% flexural strength after 48h in 80°C water; treated panels retain 91%.

3.3 Demolding and Trimming

Pneumatic wedge demolding with 30–50 kN force. Immediately inspect gel coat: bubbles (diameter >1 mm require filling), cracks (depth >0.2 mm require grinding). Trimming with diamond blade; edge tolerance ±1 mm.

4. Fitting Installation and Performance Testing

4.1 Flange and Nozzle Assembly

Compression-molded flanges with sealing surface Ra ≤3.2 μm, bolt hole positioning error ≤0.5 mm. Bonding surfaces are roughened (Ra 6.3–12.5 μm), bonded with epoxy structural adhesive (E-44+650 polyamide), cured for 24h, then tested at 0.6 MPa air pressure for sealing.

4.2 Full Water Test

Per GB/T 17219: fill to design level and hold for 48h; water level drop ≤3 mm. Beijing Yuanhui's first-pass rate: 93%. Common leak points: flange bonding (62%) and corner lap joints (28%). Repairs use low-viscosity vacuum-injected resin (200–300 cP); re-test after cure.

Conclusion

FRP water tank manufacturing is a systematic engineering process involving materials science, interface chemistry, and structural mechanics. From raw material selection to demolding and trimming, every parameter deviation propagates to final product performance. Beijing Yuanhui FRP Co., Ltd. maintains a first-pass yield above 96.5% through 15 standardized process steps and 38 quality control points. For buyers, reviewing a supplier's process records (e.g., curing temperature curves, gel coat thickness reports) is more valuable than judging by appearance or price alone.