Introduction

FRP (Fiber-Reinforced Plastic) water tanks are widely used in construction, fire protection, and industrial water storage due to their light weight, high strength, corrosion resistance, and a design life of up to 30 years. However, product quality varies significantly across manufacturers, with the core gap often stemming from process control. Beijing Yuanhui FRP Co., Ltd., with 15 years of production experience, maintains a constant workshop environment of 25±2°C and 40%-60% humidity to ensure stable resin curing. Below is a step-by-step breakdown of their 12 critical manufacturing processes.

1. Raw Material Inspection and Pre-treatment

1.1 Incoming Resin and Fiberglass Testing

Before production, every batch of unsaturated polyester resin must pass viscosity testing (Brookfield viscometer, 400-600 mPa·s at 25°C), gel time testing (82°C water bath, standard 8-12 minutes), and acid value titration. Reinforcement materials use a combination of E-glass chopped strand mat (450 g/m²) and surface veil (30 g/m²). Fiber moisture content must be below 0.1% to prevent white spots or delamination. Beijing Yuanhui once rejected a batch with 0.15% moisture content, avoiding downstream bubble defects.

1.2 Mold Cleaning and Release Agent Application

Mold surface finish directly affects the tank's external quality. After sanding with 400-grit paper, the mold is wiped with acetone, then three coats of semi-permanent release agent (e.g., Henkel Frekote 700NC) are sprayed evenly, with 15-minute intervals between coats. The final coat is left to dry for 2 hours before use. Mold temperature must match the workshop environment to prevent resin sagging.

2. Hand Lay-Up and Laminate Structure Design

2.1 Gel Coat Application

The gel coat serves as the first protective layer, with a thickness of 0.4-0.6 mm (2-3 spray passes). Isophthalic weather-resistant gel coat is used, mixed with 2% cobalt accelerator and 1% MEKP catalyst (by weight). The mixture must be used within 20 minutes. Spray pressure is 0.4-0.6 MPa, with the nozzle 30-40 cm from the mold. After the gel coat reaches a tack-free state (approximately 45 minutes), fiber layers can be applied.

2.2 Structural Laminate Design

Standard tank wall thickness ranges from 6 mm (for tanks under 1 ton) to 12 mm (for tanks over 100 tons). The lay-up sequence alternates surface veil, chopped strand mat, and woven roving. For a 10 mm wall: 1 layer surface veil → 2 layers CSM (450 g/m²) → 4 layers woven roving (800 g/m²) → 1 layer CSM → 1 layer surface veil. The resin-to-glass ratio is strictly controlled at 65:35 by weight, with resin weighed on electronic scales to ±5% tolerance. Beijing Yuanhui’s production logs show that a 50-ton tank requires approximately 6 hours of lay-up by a team of 4 skilled workers.

2.3 Roller De-airing and Bubble Control

After each layer, a pig-bristle roller is used at a 45° angle to expel air and ensure resin impregnation. Rolling force is maintained at 30-50 N, with at least 5 passes per layer. Empirical data shows that insufficient rolling reduces Barcol hardness by 15%-20% in affected areas. Workers use fingertip sensors to detect bubbles; any bubble larger than 2 mm must be punctured with a steel needle and filled with resin.

3. Curing, Demolding, and Post-processing

3.1 Curing Parameters

Resin curing proceeds through gel, hardening, and post-cure stages. At 25°C workshop temperature, gel time is 45-60 minutes, hardening takes 4-6 hours, and full post-cure requires over 24 hours. Beijing Yuanhui uses a step-wise heating method: 4 hours at 25°C, then a 2°C/hour ramp to 35°C, held for 6 hours, followed by natural cooling. This achieves a cure degree above 98%, preventing post-demolding deformation.

3.2 Demolding and Edge Trimming

After 24 hours of curing, compressed air (0.6-0.8 MPa) is blown from the mold edge, combined with light tapping using wooden wedges. Flash is immediately trimmed with an angle grinder, cutting 3-5 mm from the product edge. Burrs larger than 50 mm must be filled with resin putty and sanded smooth.

3.3 Drilling and Flange Installation

Openings (inlet, outlet, overflow) are drilled using diamond core bits at 800-1000 RPM with water cooling to prevent resin burning. Flanges are 304 stainless steel, with pre-embedded nuts sealed using silane-modified polyether sealant. Bolts are torqued to 20 N·m to prevent loosening and leakage.

4. Assembly, Inspection, and Final Testing

4.1 Hydrostatic Test (Full Water Test)

After assembly, the tank is filled with water and observed for 72 hours. Standard requirement: water level drop ≤3 mm per 24 hours, with no leakage at any joint. Beijing Yuanhui’s records show a pass rate consistently above 99.6%.

4.2 Barcol Hardness and Thickness Measurement

Barcol hardness (GYZJ-934-1 impressor) is measured at 5 points on each side and corners; the average must be ≥40 for isophthalic resin. Thickness is checked with an ultrasonic gauge (±0.1 mm accuracy), with a tolerance of ±0.5 mm. Areas below specification are reinforced with additional fiber layers.

4.3 Impact and Deformation Test (Sampling)

One tank per batch is sampled. A 1 kg steel ball is dropped from 1.5 m onto the tank surface; no penetrating cracks are allowed. The tank is also pressurized to 1.5× design pressure (typically 0.15 MPa), and maximum sidewall deformation must be ≤5 mm (per JC/T 658.1-2007 standard).

4.4 Packaging, Shipping, and Warranty

Finished tanks are packed in wooden crates with EPE foam; bolt holes are sealed with plastic plugs. Each tank carries a QR code tag linking to its production batch, resin lot, operator ID, and test data. Beijing Yuanhui offers a 15-year warranty (subject to proper use), covering free repair or replacement for leakage during the warranty period.

Conclusion

FRP water tank production is far from simple hand lay-up. Every variable—temperature, humidity, mix ratio, rolling pressure, and curing curve—must be precisely controlled. Beijing Yuanhui FRP Co., Ltd. has reduced its defect rate to below 0.3% through standardized SOPs and digital measurement tools. For buyers, inspecting a factory’s controlled-environment workshop, Barcol hardness tester, and hydrostatic test facility is far more meaningful than comparing prices alone.