How to Build a Powder Coating Oven Part II

     Welcome to "How to Build a Powder Coating Oven - Part II."  See How to Build a Powder Coating Oven Part 1.  In part 1, we build the oven frame, we started skinning it in sheet metal, we added lights and the convection blower, and also insulation and heating elements.  I am going to pick up right were we left off.  This powder coating oven build was done by KNJ Custom Coating and I want to thank them for contributing all of the information and pictures.

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Oven Door

The door for this powder coating oven is constructed the same way as the rest of the oven using steel studs and sheet metal, using rivets to hold everything together.  This door also features a window so you can your parts inside of the oven.  A cool thing about the window is that it is built into a smaller access door so there is actually a door within a door.  The small access door allows you to open it up and check the part temperature using an Infrared Thermometer, if you are not using an Infrared Thermometer to check part temperature, learn why you should be, read this article: Properly Curing Powder Coat with an Infrared Thermometer.  The small door is very small and will not let out much heat in the oven.  If you opened the big door every time you wanted to check your part temperature, it would cause a drastic temperature drop in the oven because you allowed the heat to escape.  This will cause fluctuations in the temperature which is the last thing you want when you are curing powder coated parts.

Here is how the frame of the door is constructed:

After the basic frame is constructed, the framing for the window is started, the access door was an afterthought and the framing was redone after the following 2 pictures.  However these pictures give you an idea of exactly how it was framed.

Here is the finished window frame.  In this shot, you can see exactly where the rivets where used to secure the steel studs together:

Here is the frame for the access door.  As you can see, it is too large to fit into the recently built frame in the above pictures.  The upper square boxed section will be used as the window and all of the other open sections will be filled with insulation.

Here is the door frame re-designed to fit the access door:

One side of the door was skinned in this picture and also corner braces were added to the door for extra support.  Since the door will not be a part of the main oven frame and will just hang when the door is opened, it needs all the support it can get to remain square. 

A finished shot of the door framing including the access door:

Here you can see the dimensions of the door.  A very important aspect of building this door is the angled cuts used on the sides of the door.  If the access door and door frame were perfectly square, the door would not be able to open.   These next 2 pictures explain how the angled cuts were used to allow the door to open.

After the access door frame is completed, it is then skinned on one side.  A handle and hinges were also added and riveted in place:

Then the hole for the window is cut out leaving a lip of sheet metal that extends past the stud about 1/2" on all 4 sides.

Now the window installation is started.  Installing the oven window takes more steps than you would think.  When heated, glass and metal will expand but they will not expand exactly the same at the same temperatures.  If you sealed the window directly to the metal using high-temp sealant, this would not allow for any flex and the window would likely crack due to the different expansion rates of the metal and glass. To solve this, first high-temp sealant is added to the inside lip of sheet metal, and then an oven window seal is added to all 4 sides in strips.

High-temp sealant applied on one of 4 sides:

And the oven window seal is laid down over the sealant. 

Then the window is laid in place and secured with thin strips of angle iron. Each of the angle iron pieces also has a piece of oven window seal applied using the high-temp sealant.  The window is now secured in place and sealed, but it still has a little room for expansion and movement due to the soft seals. The window used appears to be about 8" x 8" and for safety, I recommend tempered glass.  Your local hardware store should have pieces of glass that you can buy or you can easily find it online.

 

This process is repeated on the inside of the door.  This way, you will have 2 sheets of glass and an air gap to keep the heat from escaping.  This window is installed in reverse order of the first window.  First the thin angle iron with the oven window seal applied.  Then the glass is laid into place.

Then you will start work on the inside sheet metal door skin.  Cut out the hole for the window leaving a lip that extends past the stud, apply the high-temp sealant and oven window seal on all 4 sides.  

Fiberglass insulation is then added inside of the door and the inside door skin is riveted into place. Insulation can be cut by a utility knife, just use a straight edge like a 2x4 laid down on the insulation as a guide. When working with fiberglass, make sure to cover all of your skin up, wear a dust mask, gloves, and safety goggles.  As for the sheet metal, you can either have it cut by your sheet metal supplier to your specifications or you can make the cuts yourselves.  All cuts in this oven build (except for the large sheets) were cut using a Dremel with a reinforced cut-off wheel. 

The access door is now completed and can be set aside.  The main door-frame is then insulated and the inside sheet metal skin is installed and riveted into place.

Before the oven door is installed, the front facing of the ovens studs is finished off with strips of sheet metal so there is a smooth surface.  This could have been avoided if steel framing tracks were around the front of the oven instead of conventional steel studs as the framing tracks have a smooth surface..  Then a tadpole gasket is installed around the perimeter of the door opening and riveted into place.  A tadpole gasket is used specifically for high heat environments, and can withstand more than 1000 °F (538 °C). 

Here are the steel sheet metal strips being installed:

Then, the tadpole gasket is installed in one continuous loop all the way around the front of the oven:

The access door is then installed into the main door. You can see that the access door uses two slide latches to keep it latched in place. The main door is installed using four door hinges. While it was not done on this oven build, it is beneficial to reinforce the area that the door hinges mount to with steel angle to prevent the door from sagging when it is opened.

 

The finishing touches on the door are two of these Powertec Latch-action Toggle Clamps.  These will keep the door compressed against the door gasket to prevent heat from eascaping. 


That wraps up the build of the powder coating oven door.  As you can see, a lot of work went into this particular door to have all the features that the builder wanted.  Another method for installing an access door with a window, into the powder coating oven door, is a bit of a shortcut compared to this. It does not look as quite as nice, but it does save some money and construction. You could use an ordinary household oven door in place of building your own. Most oven doors already have a window installed and it is easy to add hinges to it. Here is an example: (credit for this idea goes to the user Duke46 of Powder365.com).

Control Box

 

The heating element installation was shown in Part 1 of this oven build.  This oven uses 4 2000-watt heating elements.  You will need a way to control the heating elements.  This is done by building (or buying) a control box.  You can always salvage the control panel from an existing household oven and figure out how to wire it up to control multiple heating elements but if you are building your own powder coating oven and have already come this far, then you should build your own control box.  The best thing about building your own is the use of a PID oven controller.  Your standard household oven controls the heating element using an On/Off controller.  Meaning that if you set your oven to 400 °F (204.4 °C), ideally it would keep the elements on until it reaches that temperature and then turn them on and off to maintain it.  Notice I said "ideally". Your standard household oven will not cycle the elements on and off quickly enough to maintain an accurate temperature. If you set your temperature to 400 °F, it may turn the elements on until 425 °F is reached and then turn them off. Once the oven drops to 375 °F, it will turn them on again. You can see there is a temperature swing of 50 °F using this method.  If you want properly cured powder coated parts, this is not ideal at all. A PID controller supplies a 0-100% signal to the heating elements.  Using the PID controller, you will tune your powder coating oven so that a certain percentage will equal 400 °F, and it will maintain that signal throughout the curing process so you have a constant 400 °F.  Another thing I want to point out is that not all PID controllers are created equal. There are lots of PID controllers on eBay and Amazon that come with poor instructions and die very quickly. It is important to get a high quality PID controller.  Trusted brands for PID controllers are Auber or Watlow, Watlow being the higher quality unit.

Solid State Relay or Contactor?

A PID controller cannot handle the amperage needed to control the heating elements directly so instead, a Solid State Relay (SSR), or a Contactor is used in combination with the PID controller.  Whether you decide to use a contractor or an SSR is a personal decision but for whichever one you choose, people generally swear by one or the other.  When purchasing your PID controller, you must select one that is either compatible with a SSR or contactor.  Multiple SSR's or contactors can be used depending on how many total watts are required for your heating elements.  This oven build uses 2 contactors, each one operating 2 heating elements, totaling 4 heating elements.  If using a solid state relay, it is very important to order a high quality unit or read reviews about the specific item you are ordering as there are many knock-off's out there that burn up quickly.  Fotek, Watlow & Omega are all good brands however there are many counterfeit Fotek SSR's on eBay and Amazon. Another step that must be taken for the SSR is cooling. A SSR specific heatsink should be installed on the SSR with thermal paste and you can also go a step further and use a heatsink with a fan mounted on it, like a CPU cooler. If you use a heatsink/fan combination, the fan will operate with DC power, so to wire it up in your control box you will also need to incorporate an AC to DC converter board. A CPU cooler was not made to mount to a SSR but you can adapt them by drilling holes in the heatsink and mounting the SSR to it. If you decide to use a contactor, the cycle time must be increased in the PID controllers settings, otherwise the life of the contactor will be significantly shortened.

This is the finished control box for this particular oven build: 

As you can see, it is very well done and has controls for everything installed in the oven.  The exact equipment used to build this control box were not mentioned by the builder but I will put together a list of similar equipment below:

Metal Electrical Box
Auber PID Controller - SYL-2342 if using a Contactor, or SYL-2362 if using a Solid State Relay
Inkbird IDT-E2RH or Sestos B2E 110-220v Timer (upper left hand corner)
4 Green Lights
2 Red Led Buzzer Alarms
4 Toggle Switches

You don't have to make your box identical to this one, but using the links above will give you an idea of what type of parts you will need and where to purchase them.  The builder did not go into wiring schematics inside of the box so unfortunately, that will not be discussed in this article.  There is tons of information about how to wire the control box and it is just a Google search away. A great place to help you select parts and wire your control box is the Caswell Plating Oven Building Forums, specifically this thread. Something to keep in mind is that you are dealing with AC electrical wiring which can kill you or burn your garage down, so please take this part of the oven build very seriously and do adequate research.

Control Box Kit with Included Wiring Diagram

These options are more expensive than piecing together your own control box but the benefit is that when you purchase one of these kits, a wiring diagram that takes the guess work out of it. Auber instruments has put together several control box kits that you assemble yourself. There are two wattage classes available: up to 12,000 watts and up to 7,200 watts. The 12,000 watt kit would be plenty for this oven build featured in this article. The 7200 watt kit would not be able to handle the four 2000 watt heating elements mentioned in this specific oven build, but if you were building a slightly smaller oven or an oven with fewer or smaller heating elements, this would be a great option. The kits include just about everything you need for the oven control box and it includes a large heat sink to keep the solid state relay cool.


Two newer kits are available that have a fan and light controller, however, your fan and light must have a 3 prong plug so that they can be plugged into the top of the box.  If you would prefer to hard-wire your fan and lights into the box, it would be cheaper and cleaner to get one of the basic oven controller kits and add your own wiring and switches for fan and light control.  Assembly is required. Click the links below for more info.

12,000 watt Oven Controller Kit with Light & Fan Control
12,000 watt Oven Controller Kit 
7,200 watt Oven Controller Kit with Light & Fan Control
7,200 watt Oven Controller Kit

12,000 watt Oven Controller with Fan and Light Control

When installing the control box on the oven, it is a good idea to use standoffs to space it away from the oven a little bit.  If you install it directly on the oven, it can cause the electronics in the control box to heat up and kill them prematurely.  A 1/2" air gap between the control box and powder coating oven is enough to keep the box cool.

Thermocouple

 

The powder coating oven uses a thermocouple which reads the temperature of the oven and connects to the PID controller.  The PID controller has to know what the temperature is inside of the oven in order to regulate it.  Considering every component of this oven is recessed, of course the thermocouple is too.

A box will be constructed to house the thermocouple recessed inside of the wall.  Here you will see how a piece of sheet metal is transformed into a professional looking product:

And here is the thermocouple installed:

Wiring, Conduit, and Finishing Touches 

 

Now that we have the control box and the heating elements installed, we need to wire them together.  14 gauge high-temp wire to connect to the individual components and pass through the wall.  Once the wire is passed through the wall and is on the outside of the oven, you can switch to normal wiring which is much cheaper. 

The wiring in this oven build has to travel from the control box to the heating elements, lights, fan, and thermocouple.  The wiring passes through the oven wall to and is run on the outside through metal conduit and fittings.  When wiring the powder coating oven, you must use high-temp wire anytime you are wiring inside of the oven walls.  This oven uses

Here you can see the layout of the 4 recessed heating elements along with the insulated panels mounted behind them.

Some elements will come with the wiring terminals needed to connect your wires to them.  In the case that your heating elements do not come with them or if you are salvaging a heating element from a household oven, you can use a non-insulated ring terminal crimped on the wire and screw it into the hole on the heating element connector. Very similar to this picture:

After the high-temp wire is connected to the elements, the wire passes through the insulation panel and out of the side, drilling holes were necessary.  Whenever a wire passes through the sheet metal, you will need to use a bulkhead fitting around the hole to prevent the wire insulation from being cut by the sharp sheet metal.  At the wire exit point from the insulation panel, a metal conduit fitting is used and then it travels through rigid metal conduit using 90 degree fittings until it reaches the control box.

Here you can see the metal conduit fittings and the wire passing through them:

The conduit travels from the left oven wall heating element to the back heating element, then to the right oven wall heating element and then up to the control box. Also you can see now that the wiring is done, that the exterior of the oven has been skinned in 20 gauge sheet metal, the insulation has been installed into each of the insulation panels on the outside of the oven and the insulation panels have also been skinned in sheet metal.

This is the wiring on the right side of the oven, it passes around from the rear heating element
insulation panel to the left wall, through conduit, then through the convection duct insulation panel, the last bit of wiring from the right heating element is added to the bundle, then up to the control panel.

Then the panels are insulated:

 These are the metal conduit runs for the lights and the convection blower:

Everything is now skinned in sheet metal except for the top, it has come a long way since that metal stud frame in Part 1.  

This is the exterior sheet metal for the top of the oven with cutouts for the lighting and convection blower:

Insulation panels are made for the top of the oven using steel studs, you can also see the sheet metal skin for the insulation boxes with electrical boxes used to connect the wiring to the lights:

The top insulation panels are all insulated and lighting electrical boxes and are installed and riveted into place:

From here the last insulation panel on the top is skinned in sheet metal.

The Finished Product

Let there be light!

A view through the access door:

 Heating elements work:

Control panel all lit up:

The complete powder coating oven:

So the powder coating oven build is now done.  The builder of this oven at KNJ Custom Coating can now powder coat anything that will fit inside of this beautiful 6.5' tall x 3.4' deep x 3.4' wide powder coating oven.  After the PID is tuned using the provided instructions, it is ready to go.  The oven reaches 400 degrees is just under 20 minutes and the exterior sheet metal remains cool to the touch.

I want to thank you for reading through this powder coating oven build.  I hope you see that you can also build your own large powder coating oven and I really hope that this article and How to Build a Powder Coating Oven Part 1 help you achieve it.

If you enjoyed this article, let me know,  You can leave a comment below, or like Powder Coating: The Complete Guide on Facebook, Twitter, or Goolge+ or subscribe for email updates using the links in the upper right hand corner of the site.  Stay tuned for the next article.