Introduction

FRP (Fiberglass Reinforced Plastic) water tanks are widely used in construction, chemical, and municipal sectors due to their light weight, high strength, and corrosion resistance. However, production defects such as leakage and insufficient strength often arise from improper process control. This article, based on the experience of Beijing Yuanhui FRP Co., Ltd., details the full production workflow—from raw material preparation to final inspection—to help manufacturers avoid common pitfalls and improve yield.

1. Raw Material Selection and Pretreatment

1.1 Resin and Curing System

Unsaturated polyester resin (e.g., grade 196# or 197#) is commonly used, with viscosity controlled at 0.3–0.5 Pa·s and acid value ≤30 mgKOH/g. The curing agent is methyl ethyl ketone peroxide (MEKP) at 1–2% by weight of resin, and the accelerator is cobalt naphthenate at 0.5–1%. Beijing Yuanhui FRP Co., Ltd. heats the workshop to 18–25°C in winter and adjusts accelerator ratio to 1.2% to prevent incomplete curing.

1.2 Reinforcement Materials

A combination of E-glass chopped strand mat (450 g/m²) and woven roving (800 g/m²) is used. Fiber moisture content must be ≤0.1%; exceeding 0.3% reduces interlaminar shear strength by 40% in lab tests.

2. Mold Preparation and Release Agent Application

Steel or FRP molds with surface roughness Ra ≤3.2 μm are used. Polyvinyl alcohol (PVA) or semi-permanent release agent is applied in 3–4 coats with 15-minute intervals. A case study: failure to fully dry the release agent caused three tank rejections. Beijing Yuanhui FRP Co., Ltd. mandates infrared temperature checks after each coat.

3. Hand Lay-Up Core Process

3.1 Laminate Design

Wall thickness is typically 4–8 mm, built with alternating surface mat (30 g/m²), chopped mat, and woven roving. The first layer creates a resin-rich surface (0.3–0.5 mm) to prevent fiber exposure. Load-bearing layers use 0°/90° cross-ply. Bubbles are removed with a grooved roller at 0.5 m/s and 0.2–0.4 MPa pressure.

3.2 Curing Control

Gel time is set at 30–50 minutes, and demolding occurs only when the Barcol hardness reaches ≥45. Beijing Yuanhui FRP Co., Ltd. uses infrared thermography; if local temperature exceeds 80°C, the process is halted to avoid exothermic runaway.

4. Post-Processing and Quality Testing

4.1 Demolding and Trimming

Compressed air and pry bars assist demolding. Flash and burrs are ground with an angle grinder; internal surfaces are cleaned to ensure hygiene for potable water tanks.

4.2 Leak and Pressure Tests

Air pressure test at 0.05 MPa for 30 minutes (pressure drop ≤0.005 MPa) and hydrostatic test at 1.5 times working pressure. In one project, micro-leaks at joints were traced to resin/fiber ratio imbalance; corrective adjustment reduced leak rate from 12% to 1.5%.

Conclusion

Controlling dozens of parameters—resin viscosity, fiber moisture, lay-up sequence, and curing temperature—is critical for FRP tank quality. Beijing Yuanhui FRP Co., Ltd. boosted first-pass yield from 85% to 96% by implementing standardized operating procedures (SOP). We recommend regular worker training and investment in viscosity meters, Barcol hardness testers, and infrared cameras to minimize process variation.