Powder coating provides tough, durable finishes to a broad range of surfaces, but to maximise this effectiveness requires proper application.
A critical part of the powder coating process is the curing of the coating, and here there is the potential for error.
To avoid either under-curing or overcuring, you must follow a cure schedule for the specific part you are coating.
The curing stage of powder coating involves baking the part or surface you have coated in an oven.
This stage can vary, depending on the type of powder coating material you are using and the size, shape and thickness of the powder-coated part.
The length of the curing process comes from the curing schedule (see below).
Once the part is in the cure oven, the temperature starts to increase. When this reaches the melt temperature of the powder, the powder then melts and flows, forming a continuous liquid film.
This part of the curing process is known as flow out, and it ensures that the powder forms into an even coating.
Flow out differs depending on the broad type of powder coating you are using.
These two broad types of powder coating are:
Themoplastic powder coatings include synthetic materials such as polyvinyl chloride (PVC), polyolefin, polyester and nylon.
Thermoset powder coatings include epoxy resins, epoxy-polyester hybrids, acrylic and silicon.
Whereas flow out occurs in both types of powder coating, with thermoset coatings a process known as cross-linking also occurs.
Cross-linking is a chemical reaction, which alters the physical properties of the powder coating during flow out. Here, the latent heat-hardeners become active, causing the curing reaction to begin.
Cross-linking is an irreversible process, and thermoset coatings generally dry harder than thermoplastics. This makes them highly heat-resistant, sometimes up to temperatures of 500°C.
To cure a powder coating effectively requires a combination of temperature and timing, so that a coated part reaches its optimum curing temperature. This is its cure schedule.
The part metal temperature (PMT) of the specific part you are coating forms the basis of its cure schedule.
Finding the PMT of the part requires taking its temperature with an infrared gun or laser. The part itself must reach its required PMT before you can then begin to time the curing process.
It is also important to note that powder will begin to melt at a lower temperature than the optimum cure temperature, so checking the part temperature is essential. This should always be at the thickest area of the part, to avoid starting your timing of the cure process too early.
If your cure schedule is correct, then you should end up with a coating that is 100 percent cured, making it tough and impact-resistant.
Where the timing is off, this can result in either under-curing or overcuring.
Under-curing is a common curing mistake, usually through misinterpretation of the cure schedule and how it should work.
Without meeting the combined requirements of timing and temperature, you will have a finish that is under-cured.
The coated part may appear to look finished, but the side-effects of under-curing include:
Where the under-cured part looks incorrect too, this can appear as cracking. You can test for under-curing by applying a cotton pad to the surface and rubbing it continually. Under-curing may leave some colour on the pad.
Overcuring is the opposite problem to under-curing. It occurs when you bake the coating for too long in the curing oven, or bake it at too high a temperature.
Accurate curing requires that you follow the cure schedule closely. This is especially true when coating a part that has varying thicknesses.
Here, thinner areas will, naturally, cure before the thicker areas. Therefore, your cure schedule must allow for this discrepancy.
It is true that most powders will have a degree of bake stability built into them, so they can handle being baked a bit longer than specified. But this stability has its limits. These limits will vary from powder to powder.
If you overcure your coating, then it can become brittle. Sometimes this overcuring will show in the appearance of the finish, giving a gold or yellow hue to it.
Most of the time, issues with overcuring arise from leaving parts in the curing oven for too long, rather than baking them at temperatures that are too high.
Where this is the case, one precaution can be to lower the baking temperature and extend the curing time.
Under-curing and overcuring also highlight the importance of controlling oven conditions, and it is always vital to measure the temperature of the parts themselves, and to be aware of how differing metal mass will affect the heat-rate of a part.
The most effective means of dealing with either under-cured or overcured parts is to strip the powder coat completely and re-apply it.
A professional powder coating service will provide the optimum conditions for powder coating, taking accurate part metal temperatures and devising and following clear cure schedules.
This ensures you get powder-coated parts that are strong, durable and have an excellent, uniform finish.
For more information about our powder coating service, or to get a quote, please contact us.