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Process Qualification

Mon, 08/08/2011 - 11:22am
Kyle Rhodes
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A common question posed to adhesive companies supplying into the medical device industry revolves around the need to qualify a process to ensure a hermetical seal or bond line in the device itself. It all boils down to “How do I design and qualify a process to ensure that I have 100% yield of good parts, with 0% failures?” When working with light-curable medical device adhesives, a solid process can go a long way to achieve this goal, as long as proper care is taken to understand the variables which could affect the process. A few of these are listed below:

Viscosity: All adhesives are manufactured to a viscosity range due to raw material variations in the chemical process. Qualifying a process that will work with both the high and the low end of the viscosity specification will allow you to dial in the various dispenser settings, with the knowledge that you have covered the lot-to-lot variation.

Intensity: Light-curable adhesives cure with a specific amount of energy. The total energy is the intensity of the lamp multiplied by the time of exposure, effectively giving you a dosage of UV exposure. Measuring this value is critical to most applications, so a radiometer is a required tool for setting up a process. Whatever style of lamp that you might choose, from a flood lamp to a conveyor system, or from a bulb-style spot lamp to a new LED-style spot lamp, all require that you track and know the minimum amount of intensity for a set period of time that is needed to cure the adhesive. This helps with the process set-up phase as well as regular line maintenance. Bulb-style lamps degrade as they age, so tracking the intensity is important in order to not fall below the minimum intensity for your process, as an undercured adhesive can ruin your whole day. LED lamps do degrade as well, but take a longer time to degrade, which is one reason they are becoming so popular. It is still a good idea to identify the intensity at the bond line on a periodic basis to make sure that there are no films or deposits on the end of the lightguide and that the lightguide did not shift accidently.

Time of Exposure: Just like intensity, these values work together hand in hand. You can vary the intensity, or you can vary the time of exposure, to get to the same minimum energy required for your adhesive and process. By setting an intensity at a constant level, you can adjust the time of exposure to determine your optimum manufacturing window. By holding either time or intensity constant, and varying the other parameter, a curve can be generated which usually shows a plateau for a given value, such as tensile strength or durometer. Additional time and exposure do not give any higher tensile strength or durometer, so complete cure has been achieved. There is a back side of the plateau, so we don’t want to think 30 seconds is good, so therefore 30 minutes will be better.

By understanding some of these variables, and running them through your process qualification, a solid and robust process can be build so that you can achieve 0% failures.

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