Since their initial introduction into manufacturing processes over 25 years ago, UV curing adhesives and coatings have continued to gain recognition as significant drivers for improved productivity and overall process cost reduction. In fact, they have become the preferred assembly method in many manufacturing industries.

The basic components of a UV curing process consist of a UV light-activated adhesive and UV-curing system. Often, the best consultants for the adhesives and equipment are the manufacturers of these products. The key is to ensure a compatible match between the two. Once the appropriate selection of adhesive and curing equipment is made, a manufacturing process that incorporates these components must be qualified prior to production start-up, and then steadfastly maintained during actual production. Validating a UV curing process is essential to its success.

Once a manufacturer has identified the adhesive and UV curing spot lamp system they need, the exposure time and an acceptable intensity range must be specified. The following process is suggested to determine the exposure time and intensity range required:

1. Define full cure—Identify a parameter (or group of parameters) that can be practically measured to indicate full cure. Physical properties of the cured adhesive or coating are most often used for measurement and correlation to full cure. Full cure is defined as the point at which additional cure time or additional intensity no longer improves these physical properties. Commonly used criteria include bond strength, hardness, and surface tack. Measurements are typically made on parts that have returned to room temperature after UV cure.

2. Determine minimum intensity and exposure time—Determine the minimum exposure time and intensity required to achieve full cure. Users can determine the minimum intensity and exposure time in one of two ways:

Set exposure time and vary the intensity to determine the minimum intensity. In many cases, exposure time is selected so as to avoid creating a bottleneck in the assembly process. In most manufacturing processes, there is a rate-limiting step that dictates throughput. As long as exposure time is not slower than the rate-limiting step, it will not be the bottleneck. If the minimum intensity associated with the chosen exposure time results in unacceptable bulb life, either a higher intensity UV curing system or multiple curing systems may be required.

Set intensity and determine the minimum exposure time. In many cases, the processing intensity is selected so as to provide acceptable bulb life. Your spot lamp manufacturer can likely recommend this intensity. If the minimum exposure time associated with the chosen intensity is considered too long, a higher intensity or multiple curing systems may be required.

 

Determining the minimum intensity required for full cure in a specific application requires empirical testing. This testing typically involves measuring some physical property indicative of cure (adhesion or hardness, for example) while varying either exposure time or intensity. Chart 1 (above left) shows how this testing might be accomplished by setting exposure time and varying intensity. Some of today’s UV light curing spot lamps allow users to adjust intensity manually.

3. Determine Your Safety Factor–Apply a safety factor to the minimum intensity determined in step 2 above to determine the lower intensity limit. For example, if the minimum intensity required to cure an assembly within 5 seconds is 4.0 W/cm2, the lower intensity limit would be 6.0 W/cm2 with a 50% safety factor. A safety factor helps to insure that the UV curing process can withstand unavoidable variations in the parts and process. As applications and manufacturing environments can vary significantly, it remains the responsibility of the user to assess and establish the minimum intensity limits and safety factors.

4. Define the upper intensity limit—Determine the highest intensity that still produces acceptable parts without causing damage to the bonded substrates or resins (typically caused by overheating). This intensity may or may not exceed the maximum intensity of the UV curing system employed.

Your UV light curing process now has both a lower and upper intensity specification and employs a safety factor as shown in Chart 2 below. By utilizing these four guidelines, a UV spot lamp user can be very confident that the appropriate cycle time and curing intensity range has been established for the specific application under consideration.

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