Cured Film Properties - Issues, Causes and Solutions
There is always a deep sense of satisfaction when you pull a substrate out of the curing oven and find no visual imperfections. As with most job shops, there are a number of different tests that you can perform to test the quality and consistency of the film and various other cured film properties. Now, when we say ‘cured film properties’, what we are actually talking about is which particular aspect of the film is being tested, for example, adhesion or pencil hardness.
It is always important to test the durability and quality of a job before sending it off to the customer, so we will be covering the most important cured film property issues, what causes them, and how to solve them. But first off, one piece of advice for across the board: always get the customer to specify what they expect the final product to look like and if there are any specific performance requirements from the coating. Sample panels, especially samples of the coating and substrate the customer wants, are an excellent prop for such discussions. Clarify exactly what is needed of the coating so that no one is going into a job blind.
Also, some of these issues are product and/or substrate specific. What tests are performed all depends on what equipment a job shop uses, on what kind of substrate, and what is needed of the coating. This is another important reason why it is a great idea to have a chat with your customer and determine what they need and are expecting. And of course, referring to the TDS will be a big help here!
That done, let’s get to it!
Cracked formation from impact - Image Credit: Paint Defects: Powder Coating: Failures & Analyses
Impact Resistance and/or Flexibility
The biggest factor affecting impact resistance is under or over-curing. While different product types, for example, urethanes and polyesters, have different impact resistance performance, generally it all comes down to the time and temperature in the oven. Now, the next possible tangle in this line comes from improper substrate prep. Many of you reading this already know how vital pretreatment and cleaning the substrate is in any job; you can use the best paint in the world, but if you do not pretreat properly the coating may struggle to pass the impact resistance test.
(Really, you’ll see pretreatment and cleaning crop up a lot in this guide as most issues with application and cure go back to doing neither properly.)
One other thing that can adversely affect a films impact resistance is if the film is too thick. The thicker the film (even just going over three millimeters) the less impact resistance you will get in the end. Two to three mils (max) is a good ballpark to stay in!
Refer to your TDS and whatever guidelines your suppliers/manufacturers have laid out and adjust your dwell time, cure time, and oven temperature accordingly. Every minute and degree counts! If you are following these directions and still have problems, then look at your lines pretreatment process/’s and cleaning method/’s. A good coating can still fail many tests, not just impact resistance and flexibility, because of poor prep. First off, check your equipment and concentrations, and maybe even contact the pretreatment supplier.
Top Tip: Avoid the guesswork of playing with equipment settings and concentrations by testing a coating on B1000 ACP panels. These are cleaned and pretreated already and can save you some time by essentially troubleshooting your system. If the film passes the impact resistance test, then it is your pre treat system that needs adjusting.
Largely, what causes issues with adhesion is the same as what affects impact resistance – poor pretreat and cleaning. However, one thing that we did not mention above is that if a substrate is not a uniform thickness, and a job shop does not account for this, problems can arise in the oven. It comes down to how those differing thicknesses in the substrate heat up; a one-inch area will heat up faster than a three-inch thick area. This can also apply to different types of substrate. Consult with your substrate supplier if you have issues here. Otherwise, refer to the advice above for adhesion problems!
Poor Corrosion Resistance
Guess what causes this issue? You are right! Poor pretreatment, cleaning, and/or under-curing. But also, low film build. This issue, in particular, is a biggie if the coating is going to be exposed to the elements, and if the substrate is not prepared, cured, or coated with a thick enough film, corrosion issues will pop up down the line.
Different substrates need different pretreatment systems. Cold rolled steel performs differently than aluminum, for example, and they require different formulas and systems. Using shot blast may be the right fix for one, but may irrevocably damage another. So! Sit down with the customer and clear up what they expect concerning corrosion resistance and lay down what your job shop is capable of providing. Any job shop that does not take this step is potentially setting them up for failure – not just concerning corrosion resistance.
Poor Abrasion Resistance or Pencil Hardness
Straight up, if a film is under-cured, it is not going to perform as it should. It is a shame when this happens, as visually the coating looks good, but it just fails any pencil hardness test – sometimes so badly that what you’ve got is a reject. For the most part, an under-cure is caused by improper dwell time and oven temperature.
Run an oven recorder profile to see if the oven is functioning as it should – like getting to the right temperature within a certain time. If everything is ship shape with the oven, then perhaps you need to take another look at the required temperature and dwell time for the substrate and coating. Generally, increasing the temperature and dwell time in the oven will improve this issue, but it’s easy to slip from under-cured to over-cured – cook it too much and the film will become brittle, perhaps even damaging the polymer. Refer to your supplier or TDS for accurate information.
Mandrel Bend - Image Credit: Paint Defects: Powder Coating: Failures & Analyses
The gloss of a coating can be affected by under or over-curing. Simple, right? Now, while not a common test, it is an easily measurable one – any experienced job shop will usually be able to tell immediately if the gloss coat has not come out as it should. If you have a coating that is supposed to be a 40 gloss, yet comes out of the oven as an 80 or 90, you will be able to spot it right off the bat – and so will the customer.
Fixing this issue depends a little on what kind of oven your job shop uses. Gas-fired convection oven or infrared oven? Both have their own pros and cons, but gloss levels, in particular, can be slightly trickier to fix with infrared ovens. As infrareds work by line-of-sight, a part can come out of the oven with a high gloss in some areas and low gloss in others. Unfortunately, the only way to really get around issues with gloss, concerning infrared ovens, is to have a chat with your manufacturer about the pitfalls of infrared versus gas-fired convection. Happily enough, if it is your dwell time and/or temperature that is effecting the gloss, all you have to do is adjust the time and temperature. Easy enough!
Of all the visual defects that you can get with coatings, chipping might be the most unpleasant – it looks bad for you as a job shop, opens up a substrate for corrosion, and just looks awful. What causes chipping though? Like all of the cured film issues we are covering, it comes back to the cure. But chipping can also come from film thickness and the pretreatment. How though?
Well, if the surface is not prepped properly, the powder will not be able to adhere to the surface. As for the film build, more isn’t always better. Too thick and chipping, among other things, will result.
The best way to prevent these issues is to pay close attention to your equipment. What are the settings on your gun? Check the KVs and cut back if you need to. What is the feed pressure? Maybe turn that down a notch. Plus, pay close attention to your spray time and gun to surface distance. Often, it is the little things that sneak up on you that can ruin a job.
Top Tip: Sometimes, what it really comes down to is the product itself; what it is formulated to do, how you cure it, and what type of substrate the coating is being applied to. When choosing a product, don’t just decide based on color or special effect – check out which product types suit your job shops needs best.
To wrap things up…
So many of the issues outlined above stem from poor surface preparation, film build or cure. Fortunately, these are all things that you can control reasonably easily and there are some pretty simple fixes. So armed with your customer’s expectations, the TDS, great surface prep and good line management, you can reduce rejects and create great work!
Any other questions? Drop us a line at firstname.lastname@example.org!