Detailed Guide: How to Calculate the Update Cycle for Electrocoating

1. Update Cycle

The update cycle (also known as the replacement or turnover cycle) for electrocoat refers to the time interval during which the bath liquid, due to consumption, carry-out losses, and aging, requires the addition of fresh paint to maintain stability. During the operation of an electrocoating production line, the paint is consumed as the coated product surface area increases, reducing the solid content of the bath. Fresh paint should be added regularly—per shift, hourly, or continuously—to keep the solid content within ±0.5%. When the cumulative amount of consumed (or added) paint equals the initial bath preparation amount, this is defined as one update cycle. A reasonable update cycle ensures coating quality while controlling per-unit consumption costs.

Key Factors Affecting the Update Cycle:

1. Coating Volume (Production Output): Higher production leads to faster consumption and a shorter update cycle.

2. Paint Consumption Rate: Higher per-unit consumption accelerates replenishment, shortening the cycle.

3. Carry-Out Losses: Workpiece shape and hanger design affect carry-out; UF/RO recovery systems can reduce losses.

4. Bath Liquid Aging: Solvent evaporation, resin decomposition, and contamination from pretreatment accelerate bath aging, shortening its lifespan.

2. Calculation Formula

The per-unit consumption of electrocoat (kg/m²) is closely related to the bath liquid's update cycle. A reasonable update cycle optimizes consumption and ensures coating quality. The formula for calculating the update cycle under normal production conditions is as follows:

$T_0=\frac{M_0}{M_1}$

Where:

• T₀​: Initial paint amount required for bath preparation (kg).

• M₁​: Monthly paint consumption (kg).

Per-Unit Consumption Formula

$\text{Consumption (kg/m²)}=\frac{\text{Dry Film Density}\times \text{Film Thickness}}{\text{Average Solid Content}\times \text{Coating Utilization Rate}\times (1-\text{Heating Loss})}$

Per-unit consumption is a critical parameter in electrocoating, directly affecting production costs, resource efficiency, and coating quality. Choosing a low-consumption electrocoat product helps reduce coating costs.

3. Update Cycle Calculation Example

Example: A car body electrocoating line uses cathodic electrocoat with a total bath capacity of 200 tons and a solid content of 20%. The line produces 200 vehicles daily, each with a coating area of approximately 100 m², operating 25 days per month. Assume the emulsion has a solid content of 35%, the color paste has a solid content of 58%, and the mixing ratio is emulsion:color paste = 6:1. The heating loss is 8%, dry film density is 1.25 g/cm³, average film thickness is 20 μm, and coating utilization rate is 95%. Calculate the update cycle for this electrophoretic line's bath liquid.

Step 1: Calculate Per-Unit Consumption for Car Body Coating

Average solid content of the paint:

$\text{Average Solid Content}=\frac{(35\mathrm{\%}\times 6+58\mathrm{\%}\times 1)}{7}=38.28\mathrm{\%}$

Per-unit consumption:

$\text{Consumption (kg/m²)}=\frac{1.25\text{g/cm³}\times 1000\times 20\text{μm}\times 10^{-6}}{38.28\mathrm{\%}\times 95\mathrm{\%}\times (1-8\mathrm{\%})}\approx 0.072\text{kg/m²}=72\text{g/m²}$

Paint consumption per vehicle:

$72\text{g/m²}\times 100\text{m²}=7200\text{g}=7.2\text{kg}$

Step 2: Calculate Initial Bath Paint and Monthly Consumption

Initial bath solid content:

$200\text{tons}\times 1000\times 20\mathrm{\%}=40,000\text{kg}$

Initial paint amount:

$M_0=\frac{40,000}{38.28\mathrm{\%}}\approx 104,493\text{kg}\approx 104.5\text{tons}$

Monthly paint consumption(M1):

$72\text{g/m²}\times 100\text{m²}\times 200\text{vehicles}\times 25\text{days}=36,000\text{kg}=36\text{tons}$

Step 3: Calculate Update Cycle

$T_0=\frac{104.5}{36}\approx 2.9\text{months}$

Daily paint consumption:

$\frac{36,000\text{kg}}{25\text{days}}=1,440\text{kg}$

• Color paste: ~205 kg

• Emulsion: ~1,234 kg

Thus, approximately 205 kg of color paste and 1,234 kg of emulsion should be added daily, with adjustments based on actual pigment-to-binder ratio test results.

4. Summary

In actual electrocoating production, a long update cycle is detrimental to bath liquid stability. Electrocoating is suited for high-volume, continuous production, and a recommended update cycle for most lines is 2–3 months. Cycles exceeding 6 months are not suitable for electrocoating. Therefore, when designing an electrocoating line, the update cycle should be carefully considered. For scenarios with longer cycles, the bath size should be designed as small as possible while ensuring coating conditions are met.