Introduction

The production process of FRP (Fiberglass Reinforced Plastic) water tanks directly determines their service life, sealing performance, and corrosion resistance. Beijing Yuanhui FRP Co., Ltd. has developed a mature set of technical parameters through over a decade of manufacturing experience. This article breaks down the entire process from raw material preparation to final inspection.

1. Raw Material Selection and Pretreatment

1.1 Resin System

The inner liner must use food-grade isophthalic unsaturated polyester resin (e.g., DSM P6-902) with styrene emission controlled at ≤35%, compliant with GB/T 8237-2005. For potable water tanks, Beijing Yuanhui FRP requires an acid value below 20 mgKOH/g to prevent harmful substance leaching during long-term immersion.

1.2 Reinforcement Configuration

A combination of E-glass chopped strand mat (450 g/m²) and 2400tex direct roving is used. Test data shows that chopped strand mat layers achieve interlaminar shear strength above 12 MPa, while unidirectional winding provides hoop tensile strength of 320 MPa. For large tanks (≥50 m³), surface veil (30 g/m²) is added to eliminate micro-cracks.

1.3 Fillers and Additives

The inner liner incorporates aluminum trihydroxide (ATH, particle size ≤5 μm) as a flame retardant at 25%-30% of resin weight, achieving an oxygen index of 32. NPG-based isophthalic gel coat is used for the outer layer, offering 40% better weatherability than orthophthalic types.

2. Mold Preparation and Release System

2.1 Mold Construction

Molds consist of steel frame skeletons with FRP laminate panels. Panel thickness is 8-12 mm with surface roughness Ra ≤0.8 μm. Beijing Yuanhui FRP molds achieve over 2,000 cycles due to a polyurethane high-gloss coating with Shore D hardness ≥75.

2.2 Mold Release Application

A semi-permanent release agent (e.g., Chemlease 2188) is applied in three coats: first a thin layer with 15-minute dwell, second a cross-hatch coat, and third after curing and polishing. Film thickness is 5-10 μm, reducing demolding force to below 0.05 MPa.

3. Core Molding Process

3.1 Inner Liner Fabrication

Gel coat (0.4-0.6 mm) is applied using airless spray at 0.6-0.8 MPa with a 2.5 mm nozzle. Beijing Yuanhui FRP uses a two-component metering system to maintain an initiator ratio accuracy of ±0.5%. Gel time is controlled at 8-12 minutes (25°C), with ambient humidity ≤75%.

3.2 Hand Lay-up for Structural Layer

Layers are placed in the order: chopped mat, woven roving, and unidirectional roving. Resin content is controlled at 45%-55%, with void content ≤2%. A four-roller debulking tool operates at 0.3 m/s. For thicknesses ≥10 mm, two-stage molding is used with intervals under 2 hours to prevent delamination.

3.3 Spray-up Molding (for Large Tanks)

For panels exceeding 6 m², spray-up is employed. Parameters: chopper speed 4000 rpm, fiber length 25 mm, spray distance 400 mm, gun speed 0.6 m/s, overlap 20 mm. Beijing Yuanhui FRP reports 60% time reduction per panel for 50 m³ tanks using this method.

4. Curing and Demolding

4.1 Curing Schedule

Room-temperature curing (8-12 hours at 25°C) is followed by post-curing at 60°C for 4 hours. Peak exotherm is limited to ≤150°C. Post-curing raises the glass transition temperature (Tg) from 55°C to 85°C, improving dimensional stability during summer heat.

4.2 Demolding and Trimming

Demolding uses pneumatic wedge tools with uniform force, maintaining 0.03-0.05 MPa. Trimming employs diamond blades at 3000 rpm, achieving cut roughness Ra ≤3.2 μm. Beijing Yuanhui FRP data shows dimensional tolerances of ±1.5 mm/m after trimming.

5. Quality Inspection and Standards

5.1 Barcol Hardness

Using a Barber-Colman GYZJ-934-1 durometer, the average of 10 readings must show inner liner hardness ≥40 and structural layer ≥45.

5.2 Hydrostatic Pressure Test

Per JC/T 658.1-2018, tanks are filled with water and pressurized to 1.5 times the working pressure for 30 minutes. Acceptable pressure drop is ≤0.02 MPa. Beijing Yuanhui FRP tests tanks under 10 m³ as complete units; for larger tanks, strain gauges monitor local deformation, with maximum allowable strain of 1500 με.

5.3 Water Quality Testing

Potable water tanks must pass GB/T 5750-2023, covering 12 parameters including turbidity (≤1 NTU), heavy metals (lead ≤0.01 mg/L), and microbiology (no total coliforms).

Conclusion

Quality control in FRP water tank production hinges on resin system compatibility, structural layer optimization, and precise curing parameters. Beijing Yuanhui FRP Co., Ltd. achieves a 99.7% first-pass yield through standardized processes and real-time inspection. For users prioritizing long-term performance, tanks with post-curing and food-grade resin liners are recommended.