When it comes to decorating plastic parts, particularly when using specialty processes, it is crucial to account for particles that could otherwise mar the finished product. As a result, particle monitoring technology is becoming a vital part of cleanroom environments. Plastics Decorating sat down with PMT Director of Sales Mark Berdovich to discuss some of the basic aspects of particle monitoring technology and how it can be applied to decorating applications.
How does particle monitoring technology work?
In a nutshell, the laws of physics dictate that technology that is ideally suited for detecting very small particles won’t work well for larger particles and vice versa. That said, most cleanroom particle counting systems are optical: An air sample is directed through a region of uniform light intensity, usually provided by a laser, and a detector captures light scattered from any particles present in the sample. Generally, the stronger the light signal received at the detector, the larger the particle.
Why is particle monitoring important as it relates to coating and decorating?
There are two primary problems caused by particles in coatings applications. The first is appearance and the second is durability. In order for the finished product – in this case the coated surface – to look smooth and pristine, there had better not be any large particles present on the surface before it’s coated.
This is particularly important in thicker or multi-layer coating applications. A 15 to 20 µm particle can “grow” into a 100 to 150 µm mushroom defect by the time all the coating material is deposited. And, perhaps more importantly for certain applications, having a particle present when the surface is coated sometimes means the coating process itself is incomplete. Where the coating in question is designed to protect the surface, this incomplete coating is an obvious weak point where a coating failure could begin.
What types of products and processes are vulnerable to large particle issues?
Manufacturing everything from IV antibiotics to catheters, computer chips to automobile hoods and trunk lids – all are vulnerable to large particle issues. But in decorating applications, as an example, anyone painting piano black should be especially concerned about particles larger than about 15 or 20 µm, even though a commonly accepted limit of detection for the naked eye is a particle around 50 µm in diameter. As already noted, a 15 to 20 µm particle will grow into a much larger defect that is easily visible to the naked eye on a shiny black surface that shows everything. So on a high-contrast surface such as piano black, multiple depositions of particles as small as 15 µm can turn a perfectly smooth-looking surface into a gritty, sandy-looking mess relatively easily.
Why is large particle monitoring in cleanrooms important?
In coating applications, it’s because of a combination of three things: (1) larger particles are present in very small population numbers, (2) larger particles settle out on surfaces before they can be removed by the HEPA air filtration system, and yet (3) they tend to be responsible for what’s commonly referred to as a killer-defect – i.e. that one particle of sufficient size that’s known to cause a product failure.
It should be noted that large particles in cleanroom environments are almost always human-generated. Each day, the workers in the cleanroom replenish the supply of these large particles that are not removed from the environment, except by manual cleaning. In a cleanroom that is mopped once a week, for example, not monitoring specifically for these larger particles translates to not knowing when the population of these particles increases enough to pose a threat to product quality or coating appearance due to normal personnel movements stirring up previously settled-out particles.
What are some challenges associated with this technology?
The biggest hurdle is understanding that a multi-tool approach is needed to capture all risks to coating performance and quality that are potentially caused by particulate contamination. As noted previously, no one technology can cover all particle size ranges that must be monitored to be confident that critical surfaces are clean and particle-free.
The cleanroom HEPA air filtration system must be running properly for a cleanroom to function, and aerosol particle counters are the best tools for monitoring these systems. But it is a common misconception that just because a process is housed in a cleanroom and the HEPAs are running that there will automatically be no particle contamination issues.
To the contrary, and this is especially true in cleanrooms with less than 100% HEPA filter coverage of the ceiling system – such as in ISO 6 – ISO 8 spaces common in the coatings industry: The lower the average velocity of the air moving through the cleanroom, the more readily large particles divert from the airflow streamlines and settle to the floor or onto a critical surface. Aerosol particle counters are not well suited to monitoring for these big particles. So, again, a complementary approach is needed where the rate of particle deposition onto surfaces is monitored in hopes of preventing a killer-defect particle from doing its killer-defect thing.
Mark Berdovich, director of sales for PMT (USA) LLC, has 25 years of experience, all in customer-facing roles, solving problems related to particles and other unwanted sources of contamination. His first role was as an applications engineer with Micro Measurement Labs developing products and test methods for detecting particulate contamination in injectable pharmaceuticals and implantable and terminally sterilized medical devices. He joined PMT in 2017. He is a subject matter expert in both optical particle counting and manual visual inspection for parenteral products. For more information, visit www.pmt-us.com.