Get the mix ratio off by even a few percentage points on a two-part adhesive, and the secondary cure mechanism can fail entirely — producing a bond that looks identical to a correctly mixed one until it’s put under load.

Dual-component adhesives, such as UV/epoxy or UV/acrylic systems, require precise mixing of two parts — resin and hardener or activator — to ensure the secondary cure mechanism functions correctly.

The Problem

• Incorrect ratio: If the two parts aren’t measured accurately, or if dispensing equipment is miscalibrated, the chemical reaction of the secondary cure will be incomplete. The result is a soft, non-curing, or low-strength final product that may not reveal itself until the assembly sees a real load.
• Poor homogenization: Even when the overall ratio is correct, poor mixing creates localized pockets with too much or too little hardener. This produces a bond line with inconsistent hardness, internal stress points, and areas that are more vulnerable to chemical attack than a properly mixed section of the same joint.

The Solutions

• Use static mixers: Always dispense through a properly sized static mixing nozzle — a spiral element — designed for the specific mix ratio in use. This ensures Parts A and B are homogenized immediately before the material reaches the substrate.
• Confirm equipment calibration: Regularly verify the dispensing equipment’s metering pistons or pumps to ensure the specified A:B ratio, by volume or weight, is maintained consistently throughout an entire production batch rather than drifting as seals wear.
• Purge and discard the lead material: Always purge the initial amount of mixed adhesive until the flow is visibly uniform and consistent before applying it to parts. This clears any unmixed material left behind in the tip or manifold from the last shutdown.

Pigmented and Filled Adhesives

Many single-component UV adhesives contain pigments for color or light-blocking, or inorganic fillers to reduce shrinkage or increase strength and thermal conductivity. These components are typically denser than the liquid resin around them, which introduces a separate handling problem from mix-ratio accuracy.

The problem. Over time — especially during storage — dense pigments and fillers settle to the bottom of the container. Material drawn from the top will be thinner, less pigmented, and have different curing and strength properties than material drawn from the bottom of the same container. Left unaddressed, this produces exactly the kind of inconsistent-cure and inconsistent-strength defects that a well-controlled mix ratio is supposed to prevent. If the container is used without stirring, the first parts bonded are under-pigmented or under-filled and may over-cure or lack strength, while the last material drawn is over-pigmented and may not cure properly due to excessive light blocking.

The solutions. Gently stir or roll containers of pigmented or filled UV adhesives immediately before use — avoid vigorous shaking, which can introduce bubbles and voids into a material that otherwise had none. For prolonged production runs, keep adhesive in constant, slow suspension using a low-speed agitator or roller rack rather than relying on a single stir before the shift starts. Regularly inspect the adhesive in the dispensing reservoir to confirm it remains uniform in appearance and viscosity, and stop and re-agitate immediately if any visible separation appears.

Building a Mix-Verification Step Into the Process

Because incorrect mixing and settled fillers both produce defects that are invisible until the part is tested or put into service, the most reliable safeguard is a scheduled verification step rather than relying on operator vigilance alone. A simple shore-hardness spot check on a cured witness sample, pulled at set intervals through a production shift, can catch a drifting mix ratio well before enough defective parts accumulate to matter. For filled or pigmented systems, periodically drawing material from both the top and bottom of a reservoir and comparing viscosity or color is a fast way to confirm suspension is holding through the shift rather than degrading. Documenting these checks by lot number turns a vague “the adhesive seemed off” complaint into a traceable process record. If you’re setting up a new two-part dispensing line and want help specifying a verification schedule, Email Us with your production volume and mix ratio.

Incure’s two-part systems are supplied with fixed-ratio cartridges and static mixers specifically to reduce this class of field-mixing error. Two-part adhesive systems deliver real performance advantages — faster secondary cure, better gap-filling, tunable mechanical properties — but only when mixing accuracy and material homogeneity are actively maintained rather than assumed. A static mixer that hasn’t been checked for wear can silently under-mix a batch for weeks before a bond failure finally traces the problem back to the dispensing equipment rather than the adhesive itself. For a broader comparison of when a two-part chemistry outperforms a single-component UV system, see UV glue versus epoxy for heavy-duty repairs, and for conveyor-based dispensing setups handling high daily volumes, matching a UV conveyor’s lamp head to line speed covers equipment-side considerations that affect mix consistency too. Contact Our Team to review your current two-part dispensing setup.

Visit www.incurelab.com for more information.