Introduction

FRP (Fiber-Reinforced Plastic) water tanks are widely used in construction, municipal water supply, chemical processing, and fire protection due to their light weight, high strength, corrosion resistance, and long service life. A complete FRP water tank production process involves over a dozen steps, from raw material selection to finished product inspection. Any deviation in a single step can compromise sealing integrity, load-bearing capacity, and durability. This article uses the production line of Beijing Yuanhui FRP Co., Ltd. as a benchmark to detail technical parameters and operational essentials at each stage.

1. Raw Material Selection and Pre-Treatment

1.1 Resin System

Orthophthalic unsaturated polyester resin (UPR) is the primary choice for water tanks. It must meet a water resistance of ≥168 hours (per GB/T 8237) and a Barcol hardness of ≥45. Beijing Yuanhui FRP Co., Ltd. uses Jinling Lianlisi (formerly DSM) P61-9010 resin, which has a heat deflection temperature of 95°C, suitable for secondary curing of SMC tanks. The resin-to-MEKP catalyst ratio is strictly controlled between 100:1.2 and 100:2.0 by weight. When ambient temperature drops below 15°C, the ratio is adjusted to 1.8 to compensate for slower curing.

1.2 Reinforcement Materials

A combination of E-glass chopped strand mat (CSM, 450 g/m²) and E-glass woven roving (800 g/m²) is used. CSM provides uniform thickness while woven roving contributes longitudinal strength. Fiber moisture content must be below 0.1% to avoid interlaminar voids. Pre-treatment includes drying in an 80°C oven for 2 hours, followed by storage in sealed bags; material must be used within 24 hours of opening.

2. Mold Preparation and Release Agent Application

2.1 Mold Material and Precision

Large tanks (≥50 m³) use chrome-plated steel molds with surface roughness Ra ≤ 0.4 μm. Small tanks (≤10 m³) use FRP molds cast from a master pattern. Joint gaps must not exceed 0.2 mm to prevent resin leakage and flash formation.

2.2 Release Agent System

A semi-permanent release agent (e.g., Henkel Frekote 770NC) is applied in two coats. The first coat is left to evaporate for 30 minutes; the second coat dries for 45 minutes and is then buffed with lint-free cloth to a mirror finish. The release agent film thickness is maintained at 5–8 μm—too thick causes orange peel, too thin leads to sticking.

3. Molding Process: Hand Lay-Up and Spray-Up

3.1 Hand Lay-Up Sequence

A gel coat layer (0.3–0.5 mm thick, containing 5% fumed silica to prevent sagging) is first brushed on. Chopped strand mat is then laid and rolled with a bristle roller to eliminate air bubbles. Resin consumption per layer is 350–400 g/m², with fiber content (by weight) maintained at 35% ± 2%. For a 6 mm thick panel, Beijing Yuanhui FRP uses a 5-layer CSM + 2-layer woven roving schedule, consuming approximately 3.8 kg of resin per square meter.

3.2 Spray-Up for Complex Shapes

For curved sections (e.g., manholes, flange openings), spray-up molding is employed. Spray gun parameters: chopper speed 4000 rpm, fiber length 25 mm, resin flow rate 3.5 L/min, spray distance 400 mm. Immediate compaction with a roller eliminates interlayer voids. Ambient humidity must be below 70% to prevent moisture condensation on fibers, which would weaken the interface.

4. Curing and Demolding

4.1 Curing Schedule

Room-temperature curing (25°C) requires 8–12 hours to reach demolding strength (Barcol ≥ 35). Post-curing at 60°C for 4 hours in an oven increases hardness to above 50 and reduces water absorption from 0.5% to 0.2%. Steam-heated molds (45–50°C) shorten the curing cycle to 3 hours, suitable for batch production.

4.2 Demolding Operation

A wedge-shaped brass spatula is inserted 10–15 mm along the mold edge, and uniform force is applied. Never use a steel hammer, as it induces stress concentrations. After demolding, the part is kept at room temperature for 24 hours for stress relief, followed by trimming and edge finishing.

5. Quality Inspection and Common Defect Remedies

5.1 Key Inspection Criteria

• Barcol hardness: 5 points per panel, minimum 45 (per GB/T 3854)
• Hydrostatic test: assembled tank filled to 1.5× working pressure, held for 30 minutes, leakage ≤ 0.05%
• Fiber content: determined by ignition loss method, target 30%–40%

5.2 Typical Defects and Countermeasures

Bubbles: Caused by high resin viscosity (>2000 mPa·s) or insufficient rolling. Solutions: warm resin to 25–30°C or add 2% styrene; switch to a serrated roller.
Delamination: Often due to lay-up intervals exceeding 40 minutes or damp fiber. Maintain intervals ≤30 minutes and test fiber moisture content before use.
Warpage: Primarily from uneven curing shrinkage. Countermeasure: install cooling channels on the mold back to keep exotherm peak temperature below 80°C.

Conclusion

FRP water tank production is a systems engineering challenge—from precise resin-catalyst ratios and mold surface finish to temperature and humidity control during molding. Every detail determines whether the final product meets the JG/T 501-2020 standard. By introducing semi-automated spray lines and online Barcol hardness testers, Beijing Yuanhui FRP Co., Ltd. raised its first-pass yield from 82% to 96%. As environmental regulations tighten, low-styrene-emission (LSE) resins and vacuum infusion are gaining traction, but hand lay-up combined with spray-up will remain dominant for custom shapes and medium-to-low volumes.