The process of powder coating can be tricky as there are several unusual variables at play like the grounding of the metal part being coated and whether the plastic powder has been reclaimed and reused properly, etc.
But there is one aspect of the process that is absolutely vital and often overlooked or at least uncontrolled: before a coating can even conceivably adhere to a surface, that surface needs to be thoroughly cleaned and prepared.
There is no powder coating process that does not include some manner of cleaning and surface preparation; however, these steps are usually only monitored visually, if at all, due to a lack of effective cleanliness measurement tools available for use on the production line.
Whether the powder coating process is done manually on a few parts every hour or is done through automation that is able to coat tens of thousands of parts every day, if there is no way to measure how clean the surfaces are getting before applying the coating, then failure is inevitable.
Powder Coating Processes and When to Measure Cleanliness
A typical powder coating process begins with an area to load the parts to be coated onto a conveyor that might be suspended from the ceiling or on the floor. This is the first Critical Control Point (CCP). Any place where a surface has the opportunity to change chemically (become contaminated or have contamination removed) is a CCP.
The introduction of parts to a process is always the first CCP, and manufacturers need to take care to measure the surface cleanliness of all incoming parts to get a base-level understanding of how soiled or contaminated materials are at the very beginning. This step allows manufacturers to catch critical cleanliness issues before the part gets processed and costs the company rework and scrap.
The next step is to clean the parts using any combination of methods ranging from spray washing, chemical cleaning, etching and then rinsing and drying. This is the process gap that, if left unchecked, can result in orange peel texturing in the coating, pinholes, inconsistent coverage and a whole host of other very common failures.
It has to be understood that every time something comes into contact with the part, even if that something is soap, a technician’s hands moving it from one step to the next or even just water, molecular level contaminants are able to be transferred to the surface and cause the powder coating to adhere poorly or not at all.
High Surface Free Energy
Clean surfaces are very delicate and can change rapidly. This is because clean surfaces have what surface scientists call “high surface free energy.” When a surface is clean it wants to react with anything it comes into contact with, which is great for adhesion, but also means that if a surface is exposed to air, contaminated fluids or unclean machinery, substances that are adverse to adhesion can transfer to the material surface.
The best way to combat this transfer is to know the cleanliness level of your surfaces at each CCP. This is done by measuring the cleanliness or what we like to call surface quality using a simple contact angle measurement. This is done with an in-line inspection tool that deposits a tiny drop of purified water on the surface and the way that droplet interacts with the surface is very predictive of how coatings will interact with it.
After coating a surface there is usually a curing phase where the polymeric particles that were sprayed onto the surface will be subjected to radiant energy, convection heat and other forms of thermal energy to melt the plastic into a film that will flow evenly over the surface and ultimately crosslink with the material underneath creating a strong bond.
Surface quality has a major impact on how well the film can flow over the surface. In the contact angle measurement test, the droplet will either bead up or wet out (or flow) on the surface. The wettability of the surface (i.e. the surface energy or cleanliness) will predict the coating and curing outcomes. By measuring how the droplet reacts with the surface, manufacturers can put a number to their cleanliness and surface quality, so they know that each part is ready to be coated, cured, assembled and sold.
Issues That Arise With Measuring Surface Energy
Like was said earlier, the main reason this measurement of surface quality isn’t often utilized is because of the lack of available, effective means to do so.
Devices used on real parts need to be able to measure surface quality on edges, in corners, over welds, in holes and in all of the places powder coating experts call Faraday areas, which are difficult to coat places due to a phenomenon called the Faraday Cage Effect.
The measurements used to assess cleanliness also need to be very sensitive to minute changes in surface energy. Because if a coating flows too much it might run off an edge and lead to inconsistent coverage. Sometimes this is due to supplier formulations but can it also be managed through surface control.
There are also possibilities of trapped gasses and migratory substances in materials with porosity that can cause defects on the surface of coatings. These issues typically manifest in three different ways:
- Enclosed porosity - a gas or oil is fully trapped within the material being coated causing bumps in the coating
- Blind porosity - a gas or substance is mostly trapped within the material but has a small break at the surface causing bubbles to appear in the coating
- Through hole porosity - a gas or oil has created a fissure all the way through the material (even invisibly) resulting in pinholes and voids in the coating
All of these can be mitigated by using powder coatings that are formulated to overcome these defects and measuring the surface quality at every CCP. This is where root cause analysis can come into play. When substances are heated, it is not only the coating on the surface that will flow. Substances within the bulk material can also become migratory and move to the surface during heating. A contact angle measurement will be able to detect these substances and help investigate root causes of failures.
Often multiple coats need to be applied or primers and phosphate coatings are required and therefore any surface quality measurement device used needs to be able to take several measurements quickly at many points in the process.
These coatings need to withstand prolonged (think years or decades) exposure to the harshest environments and continue to protect the materials underneath from corrosion and maintain their fetching appearance. Leaving a major gap in the production process leaves these products vulnerable and prone to failure.
To learn more about measuring surface quality at each CCP and how to discover what all of your CCP’s are, download BTG Lab's free eBook: The Manufacturer’s Roadmap to Eliminate Adhesion Issues in Production.
BTG Labs is a cutting-edge materials science based company that delivers effective adhesion quality control for manufacturers who are concerned with adhesive bonding, painting, coating, printing, and cleaning.
