Introduction

FRP (Fiberglass Reinforced Plastic) water tanks are widely used in industrial and commercial applications for corrosion resistance and high strength-to-weight ratio. Based on production data from Beijing Yuanhui FRP Co., Ltd., a mature production line can deliver 3,000 standard tanks annually, but yield heavily depends on process control. This article dissects the entire manufacturing workflow.

1. Material Selection & Pretreatment

1.1 Resin and Reinforcement Matching

Unsaturated polyester resin (UPR) with a viscosity of 0.3–0.5 Pa·s is preferred for optimal fiber wet-out. E-glass chopped strand mat (450 g/m²) is used for hand lay-up, while Sheet Molding Compound (SMC) is adopted for compression molding. At Beijing Yuanhui, a resin-to-fiber weight ratio of 60:40 yields a flexural strength above 180 MPa.

1.2 Fillers and Additives

To reduce shrinkage and improve fire resistance, 15–20% aluminum trihydrate (ATH) by resin weight is added, along with 0.5% MEKP as a curing agent. The mixed compound must be degassed at 25°C for 20 minutes to avoid pinholes on the final surface.

2. Mold Preparation & Release Agent Application

Mirror-finished stainless steel molds (RA ≤ 0.4 μm) are used. Three layers of release agent are sprayed: first a wax-based sealer, then a PVA film, and finally a semi-permanent fluorocarbon coating. Each layer is allowed to dry for 10 minutes, followed by a 30-minute oven bake at 60°C to ensure adhesion.

3. Molding Processes: Hand Lay-Up vs. Compression Molding

3.1 Hand Lay-Up (for Custom/Small Batches)

Resin is brushed onto the mold, followed by a glass mat layer. A roller is used to remove air bubbles. Layers are repeated to the design thickness (typically 4–6 layers). Beijing Yuanhui’s hand lay-up team achieves 2.5 m² per man-hour with a thickness tolerance of ±0.3 mm. Each layer requires a 45-minute gelation wait before the next.

3.2 Compression Molding (for Mass Production)

SMC sheets are cut and stacked inside the mold cavity. The mold is closed at 140–150°C under 8–12 MPa pressure. Hold time equals 1 minute per mm of thickness. Beijing Yuanhui’s compression line produces one part every 6 minutes (for 5 mm thick), with dimensional accuracy of ±0.1 mm and superior surface finish.

4. Curing, Demolding & Post-Processing

Hand-laid parts cure at room temperature (≥20°C) for 24 hours before demolding. Compression-molded parts are demolded hot. After demolding, flash is trimmed, edges are sanded, and holes for inlet/outlet/manhole are cut with a water jet. Beijing Yuanhui’s QC standard requires Barcol hardness ≥35 and no blistering or leakage after 48-hour immersion in 60°C water.

5. Assembly & Hydrostatic Test

Panels are joined with stainless steel bolts, and seams are sealed with silicone structural sealant (movement capacity ±25%). After assembly, the tank is filled to design water level and left for 72 hours. Water level drop must not exceed 2 mm. Beijing Yuanhui’s 2024 statistics show a first-pass rate of 97.3%; failures are mainly due to incomplete sealant curing.

Conclusion

The choice between hand lay-up and compression molding depends on order volume and precision requirements. Hand lay-up offers flexibility for custom shapes; compression molding delivers consistency for standardized production. Companies should prioritize resin/fiber ratio, curing temperature profile, and mold maintenance — these three factors directly determine yield and service life.