In this presentation we will look at the various temperatures in the painting process and their relative importance with respect to improving finish quality outcomes. [click]
So who is Saint Clair Systems anyway?
For nearly 25 years, we have specialized in advanced point-of-application temperature and viscosity control systems for industrial fluid dispensing processes and, with more than 3500 active installations, we have worked with just about every type of dispensing process out there.
While this IS a shameless plug for SCS, it is also why are we qualified to be here discussing this topic… [click]
Each successful project with demanding customers such as these has increased our understanding of the ramifications of temperature and viscosity variations in a host of fluid dispensing applications.
Here’s just a bit of what we’ve learned… [click]
Virtually all manufacturers understand that If the parameters of a proven “painting recipe” are held constant, the resulting surface finish will be consistent and repeatable. One of the key parameters to control is temperature. But it turns out that there are a host of temperatures in the process that can affect the outcome. These include…[click]
[Read List] [click]
It is widely believed that it is important to carefully control booth temperature because it directly affects the temperature of the paint as it is being applied. On first blush, it seems a logical assumption. After all, the atomized droplets are extremely small, and there’s a huge number of them, which presents a large surface area to the ambient air when compared to bulk fluid. The result is that companies spend millions of dollars in capital and operating expenses each year to control this variable.
The reality, however, is much different. In a paper presented at the 2018 Waterborne Symposium, SCS’ Michael Bonner revealed a thermal model that debunked this long prevailing myth. [click]
While it is virtually impossible to measure the temperature of individual droplets in the cloud, it is fairly straightforward to calculate their average change in temperature. Saint Clair Systems has developed tools such as this to perform these calculations quickly and easily in order to assist coaters in better evaluating and planning their process control strategies.
Here we can see a typical scenario played out to show the impact of air temperature on droplet temperature. [Click]
In this specific example, we look at the situation where the [Click] booth temperature is 77°F (25°C) and the paint temperature is at 90°F (32°C) coming into the booth from a circulation system that is run in the truss level from the mix room on a summer day — a fairly common scenario for many painters. [Click] [Click]
So let’s look at the impact [click] that a 13°F (7.2°C) temperature differential between the paint and the booth air has on droplet temperature. [click]
We can see that, with the high particle velocities created by a gun and resulting shorter time in the air, [Click] the paint loses between [Click] 0.25°F – 0.75°F (0.1°C – 0.4°C) — thus reaching the part still above 89°F (~32°C). Even with the relatively longer time in the air caused by the lower velocities of a bell, the paint only changes by 1.1°F – 2.3°F (0.6°C – 1.3°C) — again in the worst case, still reaching the part at nearly 88°F (31°C). If you are assuming that your paint is being applied at 77°F (25°C) and it is actually at (or above) 88°F (31°C), you may find it very difficult to make the right decisions to keep your finish quality in spec. [Click]
This is why modern progressive coaters consider controlling paint temperature at the point of application to be more important to finish quality than controlling both temperature. [Click]
We know that [click] paint temperature inversely affects viscosity. As the temperature increases, the viscosity falls. Conversely, as the temperature falls, the viscosity increases. [click]
This affects things like flow out, flash off, and run & sag, just to name a few. With this kind of impact, [click] the paint temperature becomes more important than the ambient air temperature. [click]
It’s all about viscosity…
All liquids show some change in viscosity as a function of temperature. Modern coatings are no different. Here we see the viscosity-temperature curve for a common paint. It’s the typical curve we’re all familiar with - the non-linear relationship – the viscosity falling as the temperature increases…
Valspar recommends [click] that this coating be applied at a viscosity of 26 seconds. If we extend this over to the curve and down, we can see that this correlates to a temperature of 28°C. As long as it is held at exactly 28°C, it will be at the optimal coating viscosity – but how often does that happen? Right, almost never. Valspar knows this, so they give us a tolerance of ±2 seconds. Again, [click] extending this over to the curve and down we can see that this corresponds to a 3°C temperature range from 26.5°C – 29.5°C. So as long as we stay within this narrow temperature range, which is about 80°F - 85°F, we should be OK.
In a system without temperature control, little can be done when the paint is [click] above 29.5°C (85°F) and the resulting viscosity is below the 24s lower limit. Other parameters (pressure, speed, flow, etc.) must be adjusted to compensate. More often, however, the coating temperature is [click] below 26.5°C (80°F) and the resulting viscosity is above the 28s upper limit. The most common practice in this instance is to add solvent to reduce its viscosity. But adding solvent changes the carefully crafted formulation, so controlling the paint temperature at the point-of-application is a much better solution. [click]
The impact of substrate temperature is often acknowledged and seldom addressed. This can be complicated because [click] substrate varies. The part may be made from metal, plastic, composites, or even wood, just to name a few. But the real reason substrate is important is mass. [click] The substrate will usually have a mass that is orders of magnitude greater than the mass of the paint film. This means that the paint will very quickly assume the temperature of the substrate. As a result, [click] the substrate has a greater influence on the paint than does the ambient. [click] This makes the temperature of the substrate the most important on the list. [click]
Teams that understand this go to great lengths to control their substrate as it enters the paint booth! [click]
Turbine Air Temperature is often neglected – in fact, it is rarely considered unless it is creating issues. But the reality is that [click] compressed air gets cold when it is released, which causes condensation when it falls below the dew point, which is high in a booth due to high humidity caused by the waterfalls used to carry away paint waste. [click] Condensation can cause paint defects and even equipment failure when it gets into moving parts. [click] This makes turbine air more important than booth air. [click]
Shaping Air is also often neglected. And yet, [click] this comes from the same compressed air source as the Turbine air, so it gets cold, too. As a result, it also causes condensation when it falls below the dew point in the booth. [click] Because this air is in direct contact with the paint it is even more prone to creating paint defects. Moreover, [click] it can have more influence on paint than booth air, primarily because it can be much colder due to the refrigeration effect caused by the change in pressure. [click] This makes the shaping air more important than booth air. [click]
Purge solvent is regularly run through the atomizer to keep it clean. [click] It is often at temperatures well below optimal paint temperature – even in systems with paint temperature control installed. [click] As a result, it cools all of the paint passages in the process [click] which affects the next parts being painted. This creates a systematic temperature variation as shown here…[click]
Here we see more than 8°F total variation during painting over a 5 day period. Note that the [click] painting temperatures are shown in red, and the [click] solvent purge temperatures are in blue. This shows that the point-of-application temperature varies by [click] more than 8°F over the course of a week – well outside the tolerances set forth by the paint supplier.
The impact on finish quality is easier to see when we zoom in so we can see individual painting cycles… [click] [click]
Here we can see that, during the course of the process, there are four distinct actions happening: [click] Painting [click] Idle Time [click] Solvent Purge [click] Painting restart after solvent purge
Moreover, we can see the impact that [click] cold solvent has on the delivery system, and the subsequent cycles. [click]
So, based on this comparison, we find the new hierarchy of importance to be:
[click] Substrate [click] Paint [click] Purge solvent [click] Shaping Air [click] Turbine Air, and finally… [click] Booth Air [click]
I’d like to thank you for taking the time to join me for this presentation.
Though we have only scratched the surface of the subject of temperature control in fluid dispensing processes, and the impact that it has on quality and performance, we hope that this cursory introduction has given you a different window through which to view your operation.
We encourage you to contact us to gain a more in-depth perspective of how you can reduce costs, improve quality and shorten your production cycle by controlling the temperature variable in your application system.