UV dose calculation is the step between reading an adhesive datasheet and setting up a production process that actually meets the material’s requirements. Done correctly, it eliminates guesswork from exposure time selection and gives process engineers a quantitative basis for qualification, troubleshooting, and process change management. Done poorly — or skipped entirely — it leaves cure quality dependent on trial and error.
Start with the Adhesive Specification
Every UV dose calculation begins at the adhesive product datasheet. The datasheet will specify, at minimum:
- Recommended wavelength (the curing lamp’s emission peak must match this)
- Minimum irradiance (mW/cm²) — the minimum UV intensity at the cure surface
- Recommended dose range (mJ/cm²) — the target range of total UV energy delivered
These are the target values. The calculation’s goal is to determine what lamp settings and exposure time are required to simultaneously meet both the minimum irradiance requirement and the dose range.
Measure the Actual Irradiance at the Cure Surface
Dose cannot be reliably calculated from lamp power ratings alone. Optical losses in the light guide, reflections at lens surfaces, and the working distance all reduce the UV intensity from the lamp’s rated output to what actually reaches the adhesive. The starting point for any dose calculation is a measured irradiance value.
Use a calibrated UV radiometer with a detector peak response matched to the lamp’s emission wavelength. Position the detector at the actual working distance used in production — not at the distance used in the lamp specification, which may differ. Record the irradiance in mW/cm².
This measurement is the process irradiance: the real value that will drive photopolymerization. It is the quantity to use in all subsequent calculations.
The Core Calculation
With measured irradiance in hand, the exposure time required to deliver a target dose is:
Exposure Time (seconds) = Target Dose (mJ/cm²) ÷ Irradiance (mW/cm²)
For example: an adhesive requires a minimum dose of 2,000 mJ/cm² and a minimum irradiance of 300 mW/cm². The lamp delivers 800 mW/cm² at the production working distance. The minimum exposure time to reach 2,000 mJ/cm² of dose at 800 mW/cm² is:
2,000 mJ/cm² ÷ 800 mW/cm² = 2.5 seconds
Because the irradiance (800 mW/cm²) exceeds the adhesive’s minimum (300 mW/cm²), both requirements are met at 2.5 seconds. A shorter exposure would deliver adequate irradiance but insufficient dose. A longer exposure would deliver additional dose, which may be acceptable if within the upper dose limit specified by the adhesive manufacturer.
Accounting for the Minimum Irradiance Requirement
The irradiance check is separate from the dose calculation and must be performed first. If the measured irradiance at the production working distance is below the adhesive’s minimum irradiance requirement, increasing exposure time cannot compensate.
For instance: if the adhesive requires 300 mW/cm² minimum and the lamp delivers 200 mW/cm² at the production working distance, no exposure time will produce a correct cure. The process must be changed — by reducing working distance, using a higher-power lamp, or upgrading the optical coupling — to bring irradiance above the threshold before dose calculation becomes relevant.
Calculating for Conveyor (Moving Substrate) Processes
In conveyor-based UV curing systems, the part moves continuously beneath a flood lamp. The exposure time is determined by the cure zone length and the conveyor speed:
Exposure Time (seconds) = Cure Zone Length (cm) ÷ Conveyor Speed (cm/s)
And the dose received by the moving part is:
Dose (mJ/cm²) = Average Irradiance in Cure Zone (mW/cm²) × Exposure Time (seconds)
Measuring average irradiance in a conveyor cure zone requires a data-logging radiometer or a scanning measurement across the cure zone, because irradiance is not uniform along the conveyor path — it peaks under the lamp center and falls off at the edges of the cure zone. A single-point measurement under the lamp center overestimates the average dose received by a moving part.
If you need support setting up dose calculations for a conveyor UV curing process, Email Us and an Incure engineer will assist with the measurement approach and process specification.
Building in Process Margin
The calculated minimum exposure time for target dose is not the recommended production setting — it is the minimum acceptable value. Production processes should operate with margin above the minimum to account for:
- Lamp aging: UV LED output declines over time. A lamp delivering 800 mW/cm² today may deliver 700 mW/cm² after several thousand hours of operation. If the process is set at exactly the minimum exposure time for current irradiance, aging will push it below the minimum dose requirement before the next maintenance interval.
- Part-to-part variation: Working distance variation in manual operations, substrate surface absorption differences, and adhesive dispensing volume variation all affect the effective dose.
- Measurement uncertainty: Radiometer calibration tolerance introduces a measurement error band around the reported irradiance value.
A practical approach is to target a dose at 125–150% of the adhesive’s minimum, verify that this dose does not exceed the upper dose limit, and confirm that the irradiance at the production working distance exceeds the minimum by at least 20–30%.
Dose and Over-Cure
UV adhesive datasheets sometimes specify a maximum dose in addition to a minimum. Exceeding the maximum dose can cause over-cross-linking, brittleness, color change, or elevated residual stress in the cured adhesive. For critical structural or optical applications, the target dose should be within the recommended range — not simply as high as possible.
Documenting the Process
A complete UV curing process specification documents:
– Adhesive designation and lot qualification
– Lamp wavelength and model
– Measured irradiance at production working distance (mW/cm²)
– Programmed exposure time (seconds)
– Calculated UV dose (mJ/cm²)
– Irradiance measurement date and re-measurement interval
– Minimum acceptable irradiance before corrective action
This documentation provides the basis for process audits, equipment qualification, and failure investigation if bond performance deviates from specification.
Contact Our Team to review your UV dose calculation methodology and ensure your process parameters are correctly specified and documented.
Visit www.incurelab.com for more information.