Introduction

90% of an FRP water tank's lifespan and sealing performance depends on the manufacturing process, not the raw materials. Beijing Yuanhui FRP Co., Ltd. has produced over 3,800 tanks in 12 years with a rework rate below 0.7%, largely due to standardized control across 16 process steps. This article walks through the complete production chain, from fiberglass roving to finished tank, using Yuanhui's workshop as the reference model.

1. Raw Material Preparation & Resin Formulation

1.1 Fiber Reinforcement Selection

E-glass chopped strand mat and surface mat are used in combination, with an areal weight of 450 g/m² ± 5%. Every batch of mat must pass a single-filament tensile test with breaking tenacity ≥ 0.35 N/tex at Yuanhui's facility.

1.2 Resin System Mixing

Isophthalic unsaturated polyester resin (UPR) is blended with 1.5%-2.0% MEKP catalyst and 0.3% cobalt naphthenate accelerator. Workshop temperature is held at 22-26°C, and gel time is set to 18 minutes ± 2 minutes—any deviation triggers batch rejection.

2. Molding Methods: Hand Lay-Up & Compression Molding

2.1 Hand Lay-Up (with SMC supplement)

Bottom and side panels are fabricated using a hand lay-up + spray-up composite process: mold release agent is applied 8 times per mold, surface mat is laid, resin slurry is sprayed, then chopped strand mat is added and compacted with an aluminum de-airing roller. Resin content per layer is controlled at 65%-70%, thickness deviation ≤ ±0.2 mm.

2.2 Compression Molding (for stiffeners and flanges)

Stiffeners are molded from sheet molding compound (SMC) in a 500-ton hydraulic press at 145-155°C with a dwell time of 90 s/mm thickness. Molded parts achieve density ≥ 1.85 g/cm³ and Barcol hardness ≥ 45.

3. Curing, Demolding & Post-Treatment

Hand-laid parts are cured in a 25°C constant-temperature room for 12 hours (initial cure) plus 48 hours (full cure). After demolding, a secondary heat treatment at 80°C for 4 hours is applied to eliminate residual stress. Yuanhui's test data shows that heat treatment increases flexural strength from 280 MPa to 340 MPa.

4. Panel Assembly, Welding & Sealing

4.1 Panel Jointing

A socket-type lap joint with 40 mm overlap is used. Two-part epoxy putty (tensile lap shear strength ≥ 12 MPa) is applied, and M8 stainless steel bolts are fixed at 150 mm intervals.

4.2 Sealing Procedure

Internal seams are first covered with 300 g/m² surface mat, then coated with a resin gel coat containing 5% fumed silica. Gel coat thickness is controlled to 0.8-1.2 mm with Barcol hardness ≥ 38.

5. Hydrostatic Testing & Final Inspection

Every tank must pass a 72-hour water-fill seal test with water level drop ≤ 3 mm/24h. Yuanhui also performs destructive sampling on 3‰ of production panels to verify Barcol hardness ≥ 40 and water absorption ≤ 0.5% per ASTM D570.

Conclusion

Quality differences in FRP water tanks typically stem from three critical control points: formulation accuracy, curing temperature consistency, and seal layer thickness. Beijing Yuanhui FRP Co., Ltd.'s 16-step process standard minimizes human error through parametric control. For procurement professionals, requesting gel time records and hydrostatic test curves for each batch is the most direct way to verify process stability.