How can manufacturers achieve a reliable UV curing process?
Peter Swanson, managing director, Intertronics, explains how medical device manufacturers can achieve a reliable and repeatable UV curing adhesive process.
Process speed and control are two of the many benefits that UV curing adhesives can bring to manufacturers bonding medical plastics. To build an accurate and repeatable new process, all process variables need to be understood, so they can be eliminated or kept within acceptable tolerances.
In a UV curing adhesive process, the two key areas of focus are making sure the adhesive has fully cured, and that it has been applied in the correct quantity and location. For both areas, medical device manufacturers can identify the process variables, and either eliminate them or keep them within acceptable tolerances. This must be done using actual production parts, including the specified substrates, adhesive, and manufacturing tolerances.
Ensuring an optimal cure
The adhesive data sheet will guide the manufacturer on an appropriate curing lamp, which could be a mercury arc or LED-based technology. Once a lamp type and wavelength has been specified, it should not be changed without requalification.
For the adhesive to cure completely, it is important to ensure that it receives the correct “dose” of UV curing energy. The “dose” means the total energy arriving at the surface per unit area — the intensity of the light combined with the time duration of exposure.
Every application is different. Factors like the substrate, bondline topography, and differing light transmission characteristics impact the amount of UV energy getting into the bondline, which makes it essential to establish the dose for an optimum cure using practical tests on production parts.
Full cure is the point at which additional cure time/intensity no longer improves the physical performance of the cured material. Medical device manufacturers can choose a measurable parameter to determine the amount of cure. Additionally, some materials offer features that visibly demonstrate when cure is complete, such as Dymax See-Cure colour change technology.
Know your limits
A safety factor helps build a robust process that is capable of withstanding unavoidable variation. It is added to the minimum intensity, creating a minimum intensity limit. For example, if the minimum intensity for full cure in five seconds is determined to be 75 mW/cm2, a safety factor of 50% would make the minimum intensity limit 113 mW/cm2. While we recommend a 25% safety factor, it is highly application dependent, so we advise consulting an expert.
Checking the intensity
Once the limits are defined, it is important to develop a process that stays within them. To assess the health of the system and ensure it is within the process testing limits, manufacturers can check and document the output intensity of their curing lamps. The intensity of mercury arc broad spectrum lamps decreases over time, and should be checked regularly.
Mercury arc lamps have a warm-up and cool down period, typically taking up to five minutes to reach full intensity. In many cases, we recommend that mercury arc lamps are left on all day to avoid variation and save time. LED UV lamps, however, are instant on/off and have no warm-up or cool down time. LED UV lamps have minimal intensity degradation over time, and introduce less process variability, so are a good choice if they are a viable option to cure the specified adhesive.
Ensuring the correct deposition of adhesive
Dispensing UV curing adhesives is relatively straightforward, especially as they are single part materials. Process qualification requires the manufacturer to establish the minimum and maximum quantities of adhesive needed for the desired bond strength. Based on this, the choice of application technology can vary from simple time/pressure dispensing controllers with lower accuracy, up to volumetric dispensing equipment that can dispense small quantities with an accuracy of ±1%, >99% of the time.
The more variation that can be eliminated, the better. Adhesives applications are complex, and so the best approach is to work with an adhesives partner who understands all the details of the process.