AirSprayTech Academy Certificate Program

2K and 3K Coating Systems for OEM Product Finishers

Article 04 of 24

Mixing Ratios: Volume, Weight, and Allowable Tolerance

Mixing ratio is a controlled production requirement. Understanding how the ratio is stated, measured, verified, and documented is essential to producing a properly cured and repeatable coating film.

Learning Objectives

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

• Interpret common 2K and 3K mixing-ratio expressions.
• Distinguish a ratio by volume from a ratio by weight.
• Explain why component density is required when converting between volume and weight.
• Understand how ratio tolerance affects coating performance.
• Recognize equipment and material conditions that can create off-ratio delivery.
• Describe practical methods for verifying and documenting proportioning accuracy.

The Mixing Ratio Is Part of the Coating Specification

A reactive coating is designed to cure when its components are combined in a specified proportion. That proportion is not merely a convenient equipment setting. It is part of the coating manufacturer’s formulation and application requirements.

Too much or too little of one component can leave unreacted material in the finished film. The coating may appear acceptable immediately after application yet later exhibit poor hardness, brittleness, softness, loss of adhesion, reduced chemical resistance, gloss change, discoloration, or premature failure.

The approved ratio, ratio basis, allowable tolerance, and adjustment authority must be established before production begins.

Reading a Two-Component Ratio

A ratio written as 4:1 normally means four parts of the first identified component for every one part of the second identified component. If the coating documentation defines Component A first and Component B second, the ratio is:

At 4:1, Component A represents 80 percent of the combined quantity and Component B represents 20 percent.

A 10:1 ratio contains ten parts A and one part B, for a total of eleven parts. Component B is therefore approximately 9.09 percent of the total mixed quantity—not 10 percent.

Always confirm the order of the components. The notation 4:1 is incomplete unless the responsible documents identify which component is represented by each number.

Reading a Three-Component Ratio

A 3K coating can be expressed as a three-number ratio, such as 4:1:0.5. If the approved order is A:B:C, this means:

Some equipment calculates the third component in relation to Component A. Other equipment may calculate it in relation to the A-and-B mixture, or as a percentage of the final mixture.

Those calculations are not automatically equivalent. The controller configuration must match the exact method used by the coating manufacturer and facility process specification.

Ratio by Volume Is Not Ratio by Weight

A volumetric proportioning system measures the amount of space occupied by each liquid. A gravimetric system measures mass or weight. The two ratios are identical only when the components have the same density.

Coating components frequently have different densities because they contain different resins, pigments, fillers, solvents, water, or reactive materials. A gallon of Component A may therefore weigh considerably more or less than a gallon of Component B.

Never enter a ratio stated by weight into equipment that meters by volume—or the reverse—unless the ratio has been properly converted using current, approved density data.

Density Makes the Conversion Possible

Density expresses the mass of a material per unit of volume. Common coating-industry units include pounds per gallon, grams per milliliter, and kilograms per liter.

Assume a coating is specified at 4:1 by volume. Component A has a density of 10 pounds per gallon, and Component B has a density of 8 pounds per gallon.

The equivalent weight relationship is 40:8, which simplifies to 5:1 by weight. The coating is still 4:1 by volume. Both statements describe the same mixture using different measurement bases.

Density values should come from current manufacturer documentation or an approved test method. ASTM D1475 provides a recognized method for determining the density of liquid coatings, inks, and related components.

Do Not Make Unapproved Ratio Conversions

Density can vary with temperature, batch composition, pigment loading, settling, solvent adjustment, and measurement technique. Published values may be nominal rather than certified for a particular batch.

A calculated conversion should not replace the coating manufacturer’s approved ratio. When production equipment meters on a different basis than the technical data sheet, obtain written confirmation of the correct converted ratio and permissible tolerance.

Document the densities, temperatures, units, calculation, source, date, and person approving the conversion.

Understanding Ratio Tolerance

Ratio tolerance is the permitted variation from the target mixing ratio. It should be provided by the coating manufacturer, process specification, or responsible engineering authority.

A target ratio and an alarm setting are not necessarily the same. The controller may use warning and shutdown limits that are tighter than the coating’s maximum allowable variation. Tighter control can provide time to stop the process before unacceptable material reaches the product.

For example, the coating manufacturer may establish the largest chemically acceptable variation, while the facility uses a narrower production-control limit to protect quality. Those values and their purposes should be clearly documented.

Do not select a tolerance merely because it is a convenient controller default. The tolerance must reflect the coating, equipment capability, product risk, and quality requirements.

Ratio Accuracy and Ratio Repeatability

Accuracy describes how closely the delivered ratio agrees with the required ratio. Repeatability describes how consistently the equipment reproduces its result.

A system can be repeatable but inaccurate. It may deliver nearly the same mixture during every cycle while consistently delivering too much hardener. It can also average the correct ratio over a long period while producing short off-ratio portions during starts, stops, flow changes, or valve transitions.

Production controls must consider both the accumulated ratio and the quality of proportioning during actual operating conditions.

What Can Cause an Off-Ratio Condition?

Dynamic Dosing and Flow Changes

Production flow is rarely constant. A robot accelerates and decelerates, a reciprocator changes direction, guns trigger on and off, and multiple applicators may operate independently.

Electronic dosing systems may monitor Component A flow and inject measured quantities of Component B or C to maintain the programmed ratio. The controls, meters, and valves must respond rapidly enough to follow actual material demand.

A ratio test performed at one steady flow rate does not automatically prove correct operation at minimum flow, maximum flow, rapid triggering, or changing gun combinations. Commissioning should challenge the system across the intended production range.

Ratio Verification Is a Controlled Test

Ratio verification measures the individual component quantities delivered during a defined test. The equipment manufacturer’s procedure must be followed because valve sequencing, pressure, collection location, controller mode, and test duration affect the result.

A sound ratio-check procedure should identify:

• The coating and recipe being tested.
• The required ratio and whether it is based on volume or weight.
• The approved collection point and equipment test mode.
• The material pressures, temperatures, and flow conditions.
• The containers, scales, or measuring devices used.
• The actual quantities collected and calculated result.
• The allowable tolerance and acceptance decision.
• The date, technician, and corrective action when required.

Ratio-check material should not be returned to the original supply containers unless a written procedure specifically allows it. Collected components may have been exposed to contamination, moisture, or the opposite reactive material.

Ratio Monitoring Does Not Prove Complete Mixing

A proportioner can deliver the correct quantities while still producing an inadequately mixed coating. Ratio control and mixing quality are related, but they are different requirements.

Proper mixing also depends on component viscosity, temperature, pressure balance, mixer design, mixer condition, flow rate, and the location where the components enter the fluid stream.

The production process must verify both correct proportioning and adequate homogenization. Article 08 of this course will examine static and dynamic mixing in greater detail.

Responding to an Off-Ratio Alarm

An off-ratio alarm should trigger a defined response—not an improvised adjustment. The response may include:

1. Stop or inhibit coating application.
2. Identify when the condition began.
3. Quarantine potentially affected products.
4. Check material supplies, pressures, filters, valves, meters, pumps, and controller settings.
5. Correct the identified cause.
6. Purge or flush suspect mixed material as required.
7. Perform the approved ratio-verification test.
8. Document the event and authorize production restart.

Repeatedly resetting an alarm without identifying its cause defeats the quality control built into the system.

Key Takeaways

• The approved mixing ratio is a coating requirement, not merely an equipment setting.
• The order of A, B, and C must be clearly defined.
• A ratio by volume cannot be used as a ratio by weight without accounting for component density.
• ASTM D1475 is a recognized method for determining liquid-coating density.
• Ratio tolerances should come from coating and process requirements—not controller defaults.
• Accuracy and repeatability are related but different measurements of system performance.
• Ratio tests must follow the equipment manufacturer’s approved procedure.
• Correct proportioning does not by itself prove complete mixing.

Knowledge Check

1. In a 4:1 mixture, what percentage of the total is the one-part component?
2. Why can a 4:1 volume ratio produce a different numerical weight ratio?
3. Who should establish the allowable mixing-ratio tolerance?
4. Can a proportioning system be repeatable but inaccurate?
5. Does a correct ratio reading automatically prove that the components are thoroughly mixed?

Technical References and Industry Resources

• WAGNER — Mixing and Dosing Systems for Liquid Coatings
  Manufacturer overview of industrial systems that measure and control the delivery of 2K through 4K materials.
• WAGNER — 2K Smart MOD Mixing and Dosing System
  Manufacturer information describing electronic component measurement and adaptive dosing as material demand changes.
• Graco — Ratio Check Kit Instructions
  An equipment-specific example demonstrating that ratio verification must follow a defined manufacturer procedure.
• ASTM D1475 — Density of Liquid Coatings, Inks, and Related Products
  Standard test method for determining the density of liquid coatings and their components.

Manufacturer products are referenced as technical examples and not as endorsements. Use the current manual, technical data sheet, safety data sheet, approved recipe, and quality procedure for the equipment and coating installed at the facility.

Professional responsibility: Do not change a programmed ratio, ratio basis, meter factor, density value, tolerance, catalyst percentage, or reducer percentage without documented technical authorization. When coating, equipment, and facility documents disagree, stop and obtain written clarification before production continues.

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