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2K and 3K Coating Systems for OEM Product Finishers

Article 10 of 24

Flushing, Color Change, and Material-Waste Control

A controlled color change must remove the previous coating, clean every required passage, introduce the next material without contamination, and return the system to production with the least practical loss of coating, solvent, time, and labor.

Learning Objectives

After completing this article, the reader should be able to:

• Distinguish flushing, purging, filling, and color changing.
• Identify which portions of a 2K or 3K system require cleaning during a color change.
• Explain why color-change and mixed-material paths require different procedures.
• Understand purge-volume, purge-time, and flow-verification controls.
• Recognize the causes of color carryover and incomplete flushing.
• Identify opportunities to reduce coating, solvent, waste, and production downtime.

A Color Change Is a Controlled Process

A color change is not complete merely because the material leaving the gun appears to match the next color. Residue can remain inside valve passages, fittings, filters, regulators, mixers, hoses, and applicators.

In a reactive system, incomplete cleaning creates two risks: contamination of the next coating and curing of mixed material inside the equipment.

The change sequence must control material recovery, component isolation, purging, flushing, new-material filling, remixing, verification, and safe waste collection.

Working Definitions

Separate Component Paths and the Mixed Path

Upstream of the mix point, Components A, B, and C remain separate. Downstream of the mix point, the material is reactive and subject to pot-life limits.

A color change may require cleaning only the selected color circuit and mixed path while an unchanged hardener circuit remains filled. A chemistry change may require cleaning the resin, hardener, third component, color stack, meters, valves, manifold, mixer, hose, and applicator.

The required scope depends on component compatibility and equipment design. A procedure written for a same-family color change should not be used automatically for a change between incompatible chemistries.

Typical Automatic Color-Change Sequence

The actual sequence must follow the installed equipment manual and approved facility procedure. A representative sequence may include:

1. Complete or stop application of the current recipe.
2. Close and isolate the current color valve.
3. Recover usable coating where the approved system permits recovery.
4. Discharge remaining mixed coating to the designated waste receiver.
5. Flush the mixed-material path using the approved sequence.
6. Clean the required color valve and common color passage.
7. Select and introduce the next Component A color.
8. Verify that the new color has displaced solvent and the previous coating.
9. Begin proportioning with the approved B and C components.
10. Perform the required mixed fill through the mixer, hose, and applicator.
11. Verify recipe, ratio, flow, spray pattern, and acceptable color before coating production parts.

Color-Change Valve Stacks

A color-change valve stack connects several color supplies to a common outlet. The controller opens the selected color valve while keeping the other color circuits isolated.

Internal geometry is important. Dead spaces, long common passages, threaded cavities, oversized fittings, or leaking valve seats can retain the previous color and increase cleaning demand.

An effective arrangement should provide:

• Short common material passages.
• Minimal dead volume.
• Positive valve shutoff.
• Compatible seals and wetted materials.
• Accessible maintenance and leak testing.
• Controlled solvent, air, and dump connections.

A valve can leak internally without producing an external drip. Internal leakage may contaminate another color circuit or the common outlet.

The Cleaning Material Must Be Approved

A solvent that dissolves one coating may react with another component, damage seals, precipitate resin, form gel, leave residue, or create an unacceptable transition into the next material.

Waterborne systems may require water, detergent, solvent, intermediate rinses, or a controlled combination. Water must not be introduced into a moisture-sensitive isocyanate circuit without an specifically approved changeover procedure.

Use only cleaning materials and sequences authorized by the coating supplier, equipment manufacturer, and facility process documentation.

Solvent-Only and Air-Solvent Sequences

Some approved systems use a continuous solvent flush. Others alternate controlled quantities of solvent and compressed air to create movement and scrubbing action within the fluid path.

Air-solvent sequencing must be specifically designed and approved. Introducing compressed air into a flammable-liquid system can affect static control, vapor concentration, discharge behavior, and pressure.

Do not manually improvise an air-solvent flush. Use the equipment’s approved valves, timing, grounding, waste receiver, ventilation, and control logic.

Purge Time Versus Purge Volume

A timed flush assumes that a predictable flow occurs throughout the programmed period. If pressure, restriction, viscosity, valve opening, or solvent supply changes, the actual delivered quantity may be insufficient.

A volume-controlled flush verifies that the required quantity moved through the system. Flow switches, meters, pressure monitoring, and purge-volume alarms can provide additional confirmation.

Time and volume may both be used. The system must achieve the established cleaning result rather than merely complete a software countdown.

Determining the Required Purge Quantity

The required purge quantity should be established during commissioning and process validation. It depends on the internal volume and geometry of the path being cleaned.

Include the volume of:

• Color valve and common color passage.
• Component meter and dosing valve where included in the flush.
• Mix manifold and injection passages.
• Static or dynamic mixer.
• Mixed-material hose.
• Regulator, flow-control valve, splitter, and fittings.
• Every gun, applicator, bell, and branch line.

One calculated internal volume may not produce complete cleaning. Displacement patterns, coating color, viscosity, geometry, and surface wetting can require a greater validated multiple.

Dark-to-Light and Light-to-Dark Changes

A dark or strongly pigmented color can contaminate a lighter color at a concentration too low to be obvious in the waste stream but high enough to change the product’s appearance.

Metallics, pearls, effect pigments, transparent colors, high-gloss finishes, and critical color matches may require longer or different cleaning sequences than ordinary solid colors.

Facilities may sequence production from lighter to darker colors or group compatible colors to reduce cleaning demand. Production scheduling can therefore be an important waste-reduction tool.

Mixed Fill and Return to Production

After flushing, cleaning material remains in the mixed-material path. Fresh coating must displace it before acceptable production material reaches the applicator.

The controller may use a programmed mixed-fill quantity. This quantity should account for the manifold, mixer, hose, regulator, applicator, and any branch lines.

Before coating production parts, verify:

• The correct recipe is active.
• The new color and correct hardener or catalyst are selected.
• The system is proportioning within its approved ratio tolerance.
• Required fill volume has passed through every active applicator.
• The discharged material is free from unacceptable solvent and previous-color contamination.
• The spray pattern, flow, atomization, and color are acceptable.

Never Spray Purge Material Into the Booth as a Disposal Method

Purge and color-change waste should be directed into the approved grounded and ventilated collection system. Spraying cleaning material into the booth wastes solvent, increases vapor and emissions, loads booth filters, and can create unnecessary exposure.

Waste collection procedures must account for chemical incompatibility, reactive coating, closed-container requirements, labeling, fire protection, spill control, and applicable environmental regulations.

Material Recovery and Solvent Push

Some properly designed systems can use a controlled solvent push or another approved method to move usable coating from a fluid line before the cleaning portion of the cycle begins.

Recovery reduces coating sent to waste, but the interface between coating and cleaning material must be controlled. Contaminated coating must not be returned to the original supply.

Recovery procedures require verified line volume, pressure control, collection routing, interface detection, material compatibility, and written authorization.

Common Causes of Incomplete Color Change

Reducing Material and Solvent Waste

Waste reduction opportunities include:

• Shortening material paths where technically appropriate.
• Selecting valves and manifolds with low internal volume.
• Locating the mix point close to the applicator when the complete design permits.
• Using verified volume-based purge and fill controls.
• Maintaining valves so they close completely.
• Scheduling compatible colors together.
• Recovering usable coating through an approved method.
• Separating waste streams where recovery or recycling is available.
• Tracking actual coating and solvent used during each change.

The goal is not to use the smallest possible quantity regardless of outcome. The goal is to use the least quantity that repeatedly achieves the validated cleaning and fill requirements.

Useful Color-Change Performance Measures

• Time from the last acceptable part in one color to the first acceptable part in the next.
• Coating sent to waste.
• Cleaning material consumed.
• New coating used for mixed fill.
• Number of rejected startup parts.
• Frequency of purge alarms and incomplete changes.
• Waste-disposal or recovery cost.
• Downtime and labor required.

These measurements allow the facility to improve efficiency without weakening the validated cleaning process.

Color-Change Verification Checklist

• The correct outgoing and incoming recipes are identified.
• Component compatibility has been confirmed.
• The approved purge and flushing sequence was completed.
• Required purge volume passed through every affected path.
• Color valves closed correctly and show no internal leakage.
• The next color filled the component path completely.
• Fresh mixed material displaced solvent from the mixed path.
• Ratio and pot-life controls are active.
• Spray pattern, flow, color, and appearance are acceptable.
• Waste was collected, identified, and handled correctly.

Key Takeaways

• A color change includes purging, cleaning, filling, remixing, and verification.
• Separate component circuits and the mixed-material circuit may require different procedures.
• The cleaning material must be compatible with the outgoing coating, equipment, and incoming coating.
• A completed timer does not prove that sufficient purge volume flowed.
• Dead spaces and internally leaking valves are common sources of color carryover.
• Mixed fill must remove cleaning material before production resumes.
• Purge material should be directed into an approved collection system.
• Waste reduction should be based on measured and validated process improvements.

Knowledge Check

1. Why is a color change not complete when the discharge first appears to match the new color?
2. What is the difference between a same-family color change and a chemistry change?
3. Why can purge volume provide stronger control than purge time alone?
4. What is the purpose of mixed fill?
5. Why should purge material not be sprayed into the booth for disposal?

Technical References and Industry Resources

• WAGNER — Activeflush for Liquid-Coating Systems
  Manufacturer information concerning structured flushing, material-loss reduction, cleaning-fluid control, and color-change time.
• WAGNER — Color-Change Block
  Manufacturer information concerning short material paths, reduced dead space, fast changes, and low flushing-agent consumption.
• Graco — ProMix 2KS Operation Manual
  Manufacturer instructions addressing purging, mixed fill, solvent push, color changes, component totals, and alarms.
• Graco — Purge-Volume Alarm Guidance
  Manufacturer guidance concerning verification that solvent flowed during a programmed purge.
• U.S. EPA — Solvent Use and Surface-Coating Regulations
  EPA resources for surface-coating and industrial-cleaning solvent emission requirements.
• U.S. EPA — Hazardous-Waste Resources
  Federal information concerning hazardous-waste identification, generator responsibilities, handling, and disposal.

Manufacturer products are referenced as technical examples and not as endorsements. Use the current coating technical data sheet, safety data sheet, equipment manual, validated color-change program, environmental permit, and facility waste procedures.

Professional responsibility: Cleaning materials, purge pressures, valve timing, purge volumes, air-solvent sequences, mixed-fill quantities, waste routing, and color-change recipes are controlled process parameters. Do not shorten a validated cycle, substitute cleaning materials, discharge waste into the booth, or bypass purge alarms without documented technical and environmental authorization.

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