Properly Curing Powder Coat with an Infrared Thermometer



To achieve a high quality and durable powder coated finish, knowing the surface temperature of parts while they are curing in the oven is essential. To fully realize all of the benefits of powder coating, the powder must be cured at the specified temperature for the specified amount of time. The most universal method of checking the part surface temperature while it is in the oven is with a non-contact infrared thermometer, AKA an IR thermometer or IR temp gun. This article will explain why you need to measure part temperatures, how to use an infrared thermometer correctly in your powder coating process, and the best IR thermometers for the job.

powder coating part metal temp ir thermometer

An infrared thermometer can measure the temperature of an objects surface without actually making any contact with it. Very affordable models exist for hobbyist powder coaters and their are more feature-rich options for professionals. They are equipped with a laser pointer to help you aim at the intended area, and they are able to display the temperature in both Fahrenheit and Celsius. These tools make it very easy to quickly check the temperature of powder coated parts while they are curing in the oven. 








Correct Part Temperature is Vital for Durable  Powder Coating!


Every powder coating powder will specify a certain curing schedule. The curing schedule specifies the temperature that the entire part surface must be maintained at and how long it must be maintained. These curing schedules are specified by the manufacture of the powder to provide the best properties once fully cured. Without a way to accurately measure the part temperature, it is impossible to satisfy the specified cure schedule, resulting in an undercured or overcured powder coat finish.



Undercuring Powder Coat

Undercuring, or underbaking, occurs when a powder coated part is cured for too little time, at too low of a temperature, or some combination of the two. This is a very common occurrence among new powder coaters that do not realize that the temperature on the oven display is not an accurate representation of the temperature of the part, and they tend to start the cure timer as soon as the part goes in the oven. This is not the proper curing procedure almost always result in an undercured part. An undercured part will likely display some or all of the following deficiencies:

  • Brittleness: Non-flexible powder coat that chips or cracks easily
  • Poor Chemical Resistance: Mild chemicals cause discoloration
  • Poor UV Light Resistance: Color will fade or chalk outdoors
  • Poor Corrosion Resistance: Powder does not sufficiently protect part from the elements
  • Incorrect Appearance: Color, gloss, or texture not accurate
Following the procedures in the remainder of this article will help to make sure that you do not undercure your parts. However, a simple test to ensure that a part is not undercured after it has come out of the oven, is a M.E.K. swab test.

The M.E.K. (Methyl Ethyl Ketone) swab test, sometimes called "Polymerisation Test", involves swabbing a cotton swab soaked in M.E.K. solvent on a powder coated part after it has been removed from the oven and allowed to reach room temperature. The swab should be lightly rubbed back and forth over one spot of the powder coat 20 times in each direction. If the powder is undercured, you will begin to see significant color removal, sometimes even to the point that bare metal is showing once the test is completed. Other symptoms of undercured powder is if the powder seems very soft or dull after the test, especially if it can then be easily chipped off with a fingernail. It is normal for some powder chemistries such as Hybrids to slightly lose some gloss during the M.E.K. test so it should be done in an inconspicuous area or on a test part. The availability of M.E.K. is becoming more and more limited but it can still be found at Ace Hardware, marine stores, and is available online. M.E.K. is an industrial solvent so be sure to follow all safety precautions listed for the product you purchase.


Overcuring Powder Coat

Overcuring, or overbaking, occurs when a powder coated part is cured for too much time, at too high of a temperature, or some combination of the two. Forgetting to set the oven timer is the most common cause of this issue. Most powders are specified as "Overbake Stable", which means that even if the part is left in the oven a little too long, its properties will not be affected. Powder coaters tend to lean more on the side of overbaking for a couple of minutes to ensure that they are not undercuring, and as long as the powder is overbake stable, this is not an issue. An overcured part will some or all of the following deficiencies:

  • Incorrect Appearance: Colors with yellow or brown tints, reduced gloss, incorrect texture
  • Adhesion Failure: Powder lifts off of part in sheets
An overcured part can not be fixed as the powder coat is chemically damaged. It must be stripped back to bare metal and re-coated.




How to read the Temperature of a Part in the Oven


Lets learn how to use this tool. The simplest way is to simply set the temp on your oven, and when you see that your oven is up to temp, that is when you can start checking with the IR thermometer. I simply open the door every couple minutes to take the temp, and adjust the oven temperature accordingly. When the part is at my desired temperature, that is when I start the timer. I check the temp about 4 times in a 10 minute long cure.  However, opening the oven door does allow some heat to escape from your oven, which changes the readings that you just took. This is why its important to not leave the oven door open for a minute, just open the door quickly, get your IR thermometer in there, check the temperature, and shut the door. Just make sure not to burn yourself, it is a hot oven after all.  Also make sure not to accidentally hit the part with the IR thermometer.


The most common curing schedules for powders are:
 400°F for 10 minutes (PMT)
 375°F for 15 minutes (PMT)


When you first start using the IR thermometer, you will probably be checking the temperature every 30 seconds, don't worry, that is normal.  You will learn as you go along how the temperature behaves when dealing with different sizes and different types of metal.  The bigger the part, the longer it will take to get up to temp. Iron and steel will take longer to heat up than aluminum. A small part such as a bolt can be up to temp in 30 seconds, a larger part like a wheel can take 30 minutes to get up to the 400 degree temperature. Your ovens efficiency will also have an effect on this, so if you ever change ovens, be sure to check the temperature more often until you learn your new oven.

Sometimes larger parts will heat up inconsistently, you will want to take your temp in a couple different spots to make sure the entire part is up to temp. It is not uncommon to have one side of a part 50 degrees off from the other side of the part.  A convection oven helps reduce this affect a lot.  If the side of your part closest to the heating element is getting up to temp way faster than the rest, you can put a metal shield over your heating element which will help your oven heat more evenly. If your heating element is on the bottom of your oven like mine, you can just sit a cookie sheet on an oven rack at the lowest position.




Part emissivity can reduce the accuracy of an Infrared Thermometer


While infrared thermometers are very well-suited for powder coating, you should be aware that the accuracy of the temperature reading can be affected by the type of material, shape, angle of measurement, surface temperature, thickness, and surface quality. This is because of a factor called emissivity. All things emit some amount infrared energy and emissivity is used to describe this amount on a scale from 0 to 1. An emissivity of 0 would emit no infrared energy, and an emissivity of 1 would emit the highest amount of infrared energy. Infrared thermometers factor emissivity into their calculation of a surface temperature, so in order to obtain an accurate temperature reading, the emissivity of the object being measured should be accounted for.

Infrared thermometers by default, have the emissivity set at 0.95 as this works for a wide variety of materials, and many powder coats are in this range. Cheaper infrared thermometers do not allow you to adjust the emissivity so some of the temperature readings will be affected, especially metals, which generally all emit at much less than 0.95. For example, with an infrared thermometer set at the default emissivity of 0.95, reading the temperature of a 400°F polished aluminum sheet with an actual emissivity of 0.05, can result in a temperature error...

There are many tables available that list the emissivity of different materials and coatings. You can easily reference these charts to get a ballpark figure of the emissivity of your specific material. Keep in mind that because IR thermometers read the surface temperature, that you would reference the emissivity value specific to the outer surface of the material. Meaning, if you are measuring the temperature of an aluminum wheel that is powder coated, you would not adjust your IR thermometer to the emissivity of aluminum, but instead you would adjust it to the emissivity of the powder coat.

Unfortunately, there are not many tables that reference the emissivity of powder coat, but luckily, regardless of color, gloss level, or texture, they are mostly in the emissivity range of 0.9 to 0.95. However, some powder coats like chrome and other metallics have metal flakes in them which can reduce their emissivity down to the 0.4 - 0.5 range.


Distance reduces the accuracy of an Infrared Thermometer


The further away you get from the part, the less accurate they will be. This is called the "Distance-to-Spot ratio" or "Optical Resolution". The farther away your IR thermometer is reading, the larger the area it is reading. If are trying to take the temperature of a small bolt in an oven 10 feet away, even though your laser is pointed right at the bolt, you are actually taking the temperature of the entire interior of your oven. For the most accurate temperature readings of your part, and only your part, you should try to get as close as possible to the part with the infrared thermometer.

Optical Resolution is a ratio of the IR thermometer-to-part distance, and the size of the area being read. If an infrared thermometer says it has an Optical Resolution of 12:1, this means that if your infrared thermometer is 12 inches away from a part, it will read the temperature of an area that is 1 inch in diameter. At 24 inches, it would read a 2 inch diameter area, and at 36 inches, it would read a 3 inch diameter area. Think of the IR thermometer giving off a cone shaped beam that spreads out the further away it gets as shown in the picture below.

ir thermometer optical resolution
Understanding the optical resolution of an Infrared Thermometer.


The optical resolution of an IR temp gun is often correlated with the price. Meaning, the cheaper the IR temp gun you get, the lower the optical resolution is likely to be, so the closer you need to get to take your temperatures. For convenience while using these tools, try to find an infrared thermometer with the highest optical resolution that fits your budget. The higher the resolution, the further away you can be and still be accurate. An optical resolution of 12:1 is about average for the $100 range temp guns.

Important: When measuring very small parts and you have the IR thermometer close to the part, it is better to ignore where the laser is pointing and instead, focus on aiming the lens of the thermometer at the part.


If you powder coat for a living or you are a hobbyist that takes it seriously, then a Fluke Infrared Thermometer is the standard. Fluke makes professional tools that are highly accurate and trusted by workers in many industries. I would recommend the Fluke 62 Max+.  It has a 12:1 Distance-to-Spot Ratio, adjustable emissivity, and it has dual lasers so you know exactly what area you are reading.

powder coating part metal temp ir thermometer






If you are on a budget, here is an infrared thermometer that still has great reviews, but won't break the bank and still reads at 12:1, it also has adjustable emissivity:  Kizen Infared Thermometer





While infrared thermometers are very useful for powder coating, they are also handy in many other situations. These IR temp guns are useful in the kitchen, around the house, and when working on a car.


12 comments:

  1. Wow! That's really great and this is such a helpful post. Infrared thermometers are the best thing to have on hand. They are just so amazing because they give an accurate and quick readings without even touching the object.


    http://thermee.org

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  2. Hows it goin! What happens if you just cant get the part to the required temp? I am having issues getting a harley head to the required 400 degrees. those damn fin's disperse heat pretty well i guess. I am just trying to outgas it first.

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    1. Larger masses of of metal like a head will take longer to heat up, you may even need to set the temperature higher than usual. If you are using a infrared thermometer to check the temperature, just set it at 400 and check it in 30 minutes to see if its up to 400. If not, try bumping up the temp to 425 and check again in another 10 minutes or so. Some wheels that I do take an hour to heat up to 400.

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  3. I was doing a chain guard for a motor cycle, I had my Eastwood oven set at 400 degrees but my part was around 330 degrees. It's a rather small part and it is made of aluminum, I'm not sure if all the holes in the part were displacing the heat somehow but will it be okay to crank the oven up to 450 degrees to get my part to 400? Also is it common for parts to be at a lower temp than the set temp on the oven? (I was also using a dormant powder if that matters at all)

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  4. Loving your site, I am building an oven like the one in your howto. Can you tell me what happens if you bake your part to long?

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    Replies
    1. That's great to hear and good luck on the oven build. There are many people that have followed this guide and are very happy with their ovens.

      As far as baking the part too long, many powders are overbake stable, meaning if you keep them in for an extra 5 minutes, it is not going to hurt them. I will often bake my parts an extra 2-3 minutes past the cure schedule to ensure they are completely cured. However, if you are using a powder that does not specify that it is overbake stable or if you leave the part in the oven for double the curing time, the powder is likely to be more brittle and less than durable. It can also affect the color of the powder, often showing a brown tint. If this happens, the only way to fix it is to strip the part and start over.

      Hope that helps!

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    2. Thanx Sean, yes it does help. Finished the loven last weekend, works great first parts came out perfect mostly thanx to your site.

      thanx again.

      Bob

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    3. Sean, would you face the same danger with heating the part too much? Say the part calls for 400 till flow out and 375 to cure. Would it be alright to heat to 425 and cure @ 400? I'd rather use a little extra electricity running the oven than end up with an under-cured part...

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    4. Usually powders do not require two different curing temperatures. Usually they will flow out and cure all at the same recommended temp. If you want to ensure that you are not under-curing a part, I feel that it is safer to cure the part at the recommended temp but for a longer time frame instead of raising the temperature. When I am using my household oven to cure, I leave parts in for an extra 3-5 minutes to ensure the part is not under-cured. Too high of a temperature can lead to yellow or brown tints in the color.

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  5. I'm having a problem with powder coating chipping with just one color can paint the same part a different color and it not chip can you please give me a idea on what to do

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    Replies
    1. Some powders are more durable than others. However, if it is chipping easily then its probably one these factors:
      -too much powder
      -undercured
      -part has no texture for powder to grab onto (media blast or treat part with phosphate wash)
      -rare but its possible that the powder is defective.

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