Introduction

FRP (Fiber Reinforced Plastic) water tanks are widely used in building water supply, fire storage, and industrial cooling due to their light weight, high strength, and corrosion resistance (design life up to 20-30 years). However, product quality varies significantly across manufacturers, and the difference lies in process details. Beijing Yuanhui FRP Co., Ltd., with 15 years of experience, presents a step-by-step breakdown of the manufacturing process with real production data.

1. Raw Material Preparation and Ratio Control

Key materials include unsaturated polyester resin (UPR), glass fiber reinforcements (chopped strand mat, surface mat, woven roving), and additives (catalyst MEKP, accelerator cobalt naphthenate, pigment). The resin-to-catalyst ratio determines curing speed and final mechanical properties. Standard ratio: 100 parts resin : 1-2 parts MEKP : 0.5-1 parts accelerator. Adjustments are made based on ambient temperature: reduce MEKP to 0.8 parts in summer, increase to 1.8 parts in winter to prevent cracking or deformation.

Beijing Yuanhui uses electronic scales for precise weighing and keeps batch records. Case: In 2023, a fire project in Hebei suffered surface cracks due to a ratio error (2.5 parts MEKP), resulting in a 80,000 RMB rework loss. Ratio control is the first lifeline of the process.

2. Mold Making and Surface Treatment

Tank flatness and sealing depend on mold precision. Mainstream molds are steel or FRP with surface roughness Ra ≤ 1.6 μm. Process: design drawing → wood/steel framework → apply mold release wax (3-5 coats, 10-min intervals) → spray gel coat (0.3-0.5 mm thickness, viscosity 3000-4000 mPa·s).

The gel coat provides weather resistance and anti-leakage. It must be left to tack-free (about 40 min at 25°C) before lay-up. Air bubbles in the gel coat or brushing back-and-forth cause pinholes or wrinkles. Beijing Yuanhui uses high-pressure airless spraying to keep bubble rate ≤ 2%.

3. Forming Process: Hand Lay-up vs. Spray-up

Two main processes are used:

3.1 Hand Lay-up

Suitable for small batches and customized shapes. Steps: apply resin, lay chopped strand mat (300-450 g/m²), roll with a ribbed roller to remove air, repeat to target thickness (tank wall typically 4-8 mm). Hand lay-up requires skilled workers. Beijing Yuanhui data: experienced workers achieve bubble rate below 3%, while trainees may reach 8%.

3.2 Spray-up

Ideal for standardized mass production. A spray gun simultaneously deposits resin and chopped glass fiber (25-50 mm length) in a single pass (2-3 mm thickness), boosting efficiency 4-5 times over hand lay-up. Fiber-to-resin ratio must be controlled (typically 35:65). Beijing Yuanhui's automated spray line achieves ±2% ratio accuracy, with tensile strength exceeding 120 MPa.

4. Curing, Demolding, and Post-processing

After lay-up, cure at room temperature (20-30°C) for 24-48 hours, or heat-cure at 50-60°C for 4-6 hours. Cure degree is checked by Barcol hardness (≥35). Demold using wedge tools or compressed air; never hammer to avoid delamination.

Post-processing includes: trimming flash, grinding burrs, drilling holes (diamond bit at ≤800 rpm), and assembling metal flanges and gaskets. Finally, perform a full water test (fill to design level, hold 72 hours, leakage ≤ 1% volume). Beijing Yuanhui conducts 100% full-water tests before shipment; defective units are scrapped, not repaired.

5. Common Defects and Prevention

Conclusion

FRP water tank manufacturing is not simply "painting and brushing" — it is a systematic engineering process involving material science, mold engineering, and process control. From the precision of resin ratio to the force of every rolling pass, each step determines final performance. Beijing Yuanhui FRP Co., Ltd. retains samples from each batch for tensile, flexural, and Barcol tests, ensuring compliance with GB/T 21238-2007. For buyers, choosing a manufacturer with transparent processes and traceable data is far more reliable than focusing on price alone.