Oven Curing 101

Q. We manufacture miscellaneous casted ductile grey iron parts. After we chip and grind the casting, we shot-blast the parts to remove the casting sand. The parts are then cleaned, degreased and dried before the painting process. 

 Lately we have noticed that many of the parts have paint blistering after spraying and going through our IR oven. Our oven has been designed with three separate zones, and lately our maintenance department has been adjusting various setting in an effort to fix this problem. 

We are not really sure what is happening with our enamel coating, because to my knowledge even with maintenance’s changes, we have not deviated too far with the settings. Can you offer some suggestions as to what we should be evaluating? What are some possible solutions we can implement?

A. Thank you for providing such detailed information about your operation. The low-hanging fruit would be to ask if you have made any changes with your current coating supplier or have modified the existing enamel? Often when suppliers are changed or existing products are modified with various solvents, it can have a dramatic impact on the curing properties of coatings when subjected to wide ranges of temperature outputs. 

Regardless if you have switched suppliers or are adjusting the current product, it is always a good idea to first consult them for proper solvent adjustments. For example, if you are experiencing dry spray on your parts, your paint operator may adjust the product with a retarder or leveling solvent. When this is done, it slows the dry time down, which can have a major impact on your blistering issues. 

Before even tackling the chemical properties, I would verify the temperatures in the various zones of your oven. You mentioned that your maintenance department has been working on the oven. Are they adjusting heat outputs in the different zones, air velocity, line speeds? These all have an impact on drying parts, especially if the coating has been changed or modified. Do you have a meter or digital temperature gauge for each zone? Do you know for sure the if the thermal sensors that measure the temperature output are correct, or do you have an I.R. gun or heat tape applied to test parts to validate these temperature outputs? 

One crucial note is to make sure your oven zones are set up to slowly dwell the temperature exposures to the painted surface. For example, Zone 1 should be the lowest temperature, Zone 2 a little higher, and finally Zone 3, the highest. If the temperature in Zone 1 has been changed to be the same a Zone 2 and 3, this may be too much front-end heat exposure to the coating surface, which can contribute to your issue. So, why are slowly dwelling temperatures by zone so important? Enamels are typically designed with front, middle and tail-end solvents to help control the flash-off and drying property of the coating. If you expose the paint surface to too much heat on the front end, you force-evacuate key solvents too rapidly, thus altering proper cure/dry properties, (potentially your blistering issue). 

Lastly, in regards to the coating application, have you made any changes with the spray operators or spray equipment? If you have, did the new operators go through the required spray training needed to ensure proper mil application of material in accordance with you operation? If you have not changed who applies the coating, but have made changes to your spray equipment, this can impact how much coating is being applied, due in part to bigger tip sizes, more fluid pressure, etc. 

It is always a good idea to check with your coating supplier for recommendations for ideal coating application. Once you have validated the oven’s working temperature parameters, validate proper curing properties with your current or new supplier. What works with one company may not work with another. Most coating suppliers have an application specialist who can help you establish the proper standard operations and application processes. I hope this will take some heat off your blistering issues!