See the factors that affect UV curing in this article, they include:
The above factors are interdependent to some extent. Increasing the intensity of UV light will decrease the time needed for the adhesive to cure. But both ends have limits. Using very high intensity short wave UV light will support a tack free surface cure with most products but can reduce the depth of cure. Lower intensity and longer wavelength light can increase the depth of cure but may produce a tacky surface.
Being in control of the cure process provides advantages such as the ability to reposition or align parts, allow the adhesive to wick or flow into areas, etc… But it also presents questions of how much intensity and what wavelength of UV light is needed for adequate adhesion.
Permabond manufactures adhesives that can be cured by a variety of UV light, including both mercury arc lamps and LEDs and even the sun.
Ideally, the UV light source is measured with a radiometer Eit to ensure a consistent curing process. Automated dispensing equipment ensures proper amount and placement of adhesive and a conveyor at a set speed passes the assembly at the same distance from the UV light for the same amount of time. In many cases the process controls are enough to satisfy the manufacturer’s quality control requirements, but some manufacturers do destructive testing as well for added assurance.
Other manufacturers rely solely on UV lamp hours as an indication of when the bulb should be changed. This method is not as controlled as a radiometer but can work effectively when additional quality control techniques, such as destructive testing, are in place.
There are even large manufacturers in sunny climates – that use old fashioned sunshine very effectively.
The methods and controls for each UV curing process are as varied as each adhesive application. Contact either the adhesive manufacturers, UV lamp manufacturers, or radiometer manufacturer for suggested UV curing schedules for your specific application.
Try UV curing at half that time and test for adhesion. If the adhesion is still the same, half the cure time again until you see a drop in strength. The time prior to the drop in strength is the lower limit of your cure process range. I also like to test adhesion at twice the suggested time. If adhesion has increased, the original time was inadequate. When further curing does not increase strength, you have complete cure.
It is tempting to manufacturers to increase cure times or intensity just to be sure. If a little is good, more is better, right? In discussing and reading others discussions of bond strength, temperature resistance, and a variety of other adhesive properties on a daily basis, I see the phrases “ensure the adhesive is fully cured” or “after thoroughly curing the adhesive…” or “upon complete cure, the adhesive…”. So it is no wonder that we have some overly zealous “curers” out there!
Clearly, it is critical to ensure complete cure, so manufacturers with less controlled processes tend to err on the side of over curing. Generally, extending cure time 2 to 3 times the required amount of light is not detrimental to the adhesive. However, take care to ensure the additional exposure to heat is not degrading components. Often, a fan is sufficient. UV light aging and thermal aging processes produce many of the same effects on plastics, including color changes, shape-size changes, hardness and elongation changes, and embrittlement. To test for thermal or UV aging of the adhesive, consider the cured adhesive as you would a plastic. Extreme over curing can degrade both plastic and adhesive.