Introduction

FRP (Fiberglass Reinforced Plastic) water tanks are widely used in residential, municipal, and industrial water supply due to their corrosion resistance, lightweight strength, and long service life. By 2025, China's annual output of SMC (Sheet Molding Compound) compression-molded tank panels has exceeded 8 million square meters. However, many small factories still rely on hand lay-up. This article draws from 12 years of production data from Beijing Yuanhui FRP Co., Ltd. (annual capacity: 120,000 m²) to walk through the complete process—from raw material intake to final inspection—highlighting the differences among three molding methods.

1. Raw Material Preparation & Formula

1.1 Resin Selection

Unsaturated polyester resin (UPR) constitutes 40%–55% of the panel weight. For potable water tanks, the resin must comply with GB/T 17219-1998. Beijing Yuanhui uses orthophthalic food-grade resin with a viscosity of 0.4–0.6 Pa·s and an acid value below 20 mgKOH/g, ensuring heavy metal leaching stays under 0.05 mg/L after curing.

1.2 Reinforcement & Fillers

E-glass chopped strand mat (300–600 g/m²) serves as the main reinforcement. A standard 1m×1m panel requires 4–6 layers. Calcium carbonate (CaCO₃) is added at 30%–45% of resin weight; exceeding this range increases brittleness. For an 8mm thick, 2m×3m SMC panel, the glass content must be stabilized at 28%±2% to avoid surface defects during molding.

2. Molding Process Comparison

2.1 Hand Lay-Up

Steps: mold cleaning → release agent → gel coat → mat layering → resin brushing → roller debulking → repeat → curing → demolding. Temperature: 20–30°C, humidity <75%, cure time 6–8 hours. Pros: low mold cost (~$700/set), suitable for complex shapes and small batches (<50 panels). Cons: thickness tolerance ±0.8mm, interlaminar shear strength only 12–15 MPa, and high styrene emissions. Beijing Yuanhui's data shows a 2.3% leakage rate for hand-laid panels vs. 0.4% for SMC panels.

2.2 Spray-Up

A spray gun simultaneously deposits chopped glass fiber (25–50mm length) and resin onto the mold. Throughput is 3–5x faster than hand lay-up, but fiber distribution depends heavily on operator skill. Recommended only for panel thickness ≤6mm. Beijing Yuanhui uses spray-up only for stiffeners in large tanks (≥100 m³).

2.3 SMC Compression Molding

The dominant method today. SMC sheet is molded at 150±5°C and 10–15 MPa pressure, with a cycle time of just 3–5 minutes per panel. Mold precision reaches 0.1mm, and thickness tolerance is ±0.2mm. Using a SY-630T hydraulic press, Beijing Yuanhui molds two 1m×1m panels per shot, achieving a daily output of 480 panels. Class-A surface molds (Ra≤0.8μm) eliminate secondary sanding.

3. Post-Processing & Quality Control

3.1 Curing & Trimming

Hand-lay and spray-up parts require 24-hour post-curing at 25°C; SMC parts can be trimmed immediately. Beijing Yuanhui uses CNC waterjet cutting for smooth edges, avoiding fiber dust from manual grinders.

3.2 Key Test Parameters

(1) Barcol hardness (GB/T 3854-2017): hand-lay ≥40 HBa, SMC ≥55 HBa. (2) Hydrostatic test: assembled tank filled to 1.5x working pressure, hold 30 min, leak rate <0.5%. (3) Accelerated aging (QUV 1000h): SMC retains 92% strength vs. 78% for hand-lay.

Conclusion

Choose the FRP tank molding method based on volume, precision, and budget. For standardized projects, SMC compression molding offers 25%–30% lower total cost (including maintenance) than hand lay-up. Hand lay-up is acceptable for irregular shapes or emergency replacements. Beijing Yuanhui FRP Co., Ltd. recommends SMC when tank volume exceeds 50 m³ or panel count exceeds 200. Before purchasing, request the resin food-grade certificate and third-party hydrostatic test footage—this separates quality tanks from leak hazards.