Build my own DIY Powder Coating Oven or buy professional?
Let me begin by saying that there is a serious threat of fires, explosions, personal harm and/or death with an unsafe oven installation. It is imperative that installers, operators, maintenance personnel, and managers recognize these threats and act accordingly. Over the past 15 years, I have seen three installations go up in smoke due to unsafe ovens and one person needlessly died.
Do you want to build an oven? Can you install and do you understand the following safety devices? Motor Overloads, Fan Proving Air Cells, Purge Timers, Powered Exhaust, Deviation Control Programming, Guards for Moving Parts, Explosion Relief Doors/Hatches High Limit Control, Door Switches, Safety Shutoff Valve, High/Low Gas Pressure Switch Combustion Safeguard System Sure, for a hobby powder coater, go ahead and buy an old oven for your parts. You could even build your own oven using low-watt density Incoloy (or similar) sheathed heaters – remember to follow UL guidelines and ensure your heater loads are broken into circuits no higher than 48 amps each, use 16 gauge aluminized steel for the interior shell (aluminized is important for reflectance).
Insulate your oven with 3-5 inches of 6 # mineral wool and top off the outer frame with heavy-duty structural steel. Make sure you take into account NFPA 86, which requires that all fuel-fired and/or class A process ovens are equipped to provide adequate explosion relief (1ft sq/15 ft cubed oven volume). Can you design in explosion venting latches on the doors along with an explosion venting panel in the roof of the unit?
How much heat do you need? It is not as simple as getting some toaster oven elements, wiring them up, and plugging it all in. Figure out your requirements with this equation:
Parts Being Finished = Workload per hour (lb/hr) × specific heat of the parts (Cp) × the temperature difference between the parts and the solution (in this case air) (F) divided by 3,412 (BtuH/KW)
DIY Powder Coating Oven
Powder coating is increasingly accepted as the preferred finishing process for many applications. Increasingly stringent environmental regulations, rising costs in all areas, and demands by consumers for better quality and more durable products are among the challenges facing today’s finishers. Powder coatings provide a solution to these challenges and others. Powder coating is the technique of applying dry paint to the component.
The powdered paint is normally applied by using a powder feed system and gun to electrostatically charge and spray the powder onto the part. For some applications, the part being coated is dipped into a fluidized bed of powder. The coated part is then heated in an oven, or via infrared panels, to melt and cure the paint. During the curing process, a chemical cross-linking reaction is triggered and it is this chemical reaction that gives the powder coatings many of their desirable properties.
Want to be a real powder coater? Here is some information about DIY Powder Coating Oven
Then you need to take this seriously and realize you need professional equipment. Almost 90% of the oven issues our technicians have seen were with novice-built ovens (fabricated with insulated steel panels and electrical
heat elements – much like your oven at home). Sure your shop can build the best widgets in the industry, but do your engineers understand how to precisely control airflow and velocity? Do they know how to ensure effective heat transfer that ensures accurate and uniform temperatures along and across the parts?
If not you will have discoloration, orange peel and under/over bake problems. If you need to save money and still get quality parts you need to build a quality oven or purchase a well-used oven. By building an oven using the suggestions on the prior pages and incorporating a professional burner box you will have a much better system than one using electrical elements to heat the oven. However, even these ovens can have air issues unless you have first-hand knowledge of the baffle design and airflow requirements of your particular oven configuration.
Gas or Electric (convection and IR)
Gas is significantly less expensive to operate than electric (for both convection and IR ovens). A significant portion of electric energy costs for ovens derives from the monthly demand charges imposed on energy consumed during periods of high demand. For purposes of comparison, analyze the energy costs of an electric system with a demand capacity of 392 kW and a 300 kW average usage level operating eight hours a day, 22 days per month.
With these figures, the estimated monthly electrical energy cost is $7,168.24 – of which almost 60% was attributable to demand charges. Compare these operating costs with those of a 1.6 million BTU/hr. gas system. With the same usage per month, gas charges are estimated at $1,047.55. The significant savings were possible because there are no utility demand charges for gas usage. Thus, energy-related operating costs for the proposed larger system were estimated at about $6 per hour vs. almost $41 per hour for the previous system. Are you going to run your oven during the day (when demand charges are high) or only on the third shift?
Oven Efficiency
Oven efficiency is the ratio of the heat input into the product vs. the energy consumed by the oven. Electric radiant elements typically have a radiant efficiency (the ratio of radiant energy emitted vs. energy consumed) of 60 to 90%. Gas burners typically have radiant efficiencies of 40% to 60%. In each case, the remainder of the energy input (that which is not converted directly to radiation) becomes heated air within the oven.
Engineers design ovens to use this heated air to provide additional heat to the product and offset losses that typically occur through the exhaust and enclosure. The moving air improves overall oven efficiency, ameliorating the inherent radiant inefficiency of gas (when compared to electric). The additional convection heating system supplements the preheated air, helping to heat the poles more rapidly and uniformly than is possible with radiant heating alone.
What about UV and Electron Beam?
UV powders have been available for about 10 years. In fact, I was one of the original formulators of UV coatings as they exist today back in the early 1990s. The first successful UV application of powder coatings was by Baldor USA for their electric motors. UV is still however in it’s infancy due to the high costs of the curing equipment and powder coatings.
It is however an excellent choice for highly heat-sensitive substrates such as preassembled parts such as shocks and electric motors as well as for plastics. You can cure a UV powder in as little as one minute! The following comparison shows how dramatically curing time can be reduced by moving from convection to infrared and finally to UV curing for a free radical 100% UV solids operation. In one particular analysis, the cost reduction from converting from 100% heating to 100% UV solids resulted in a savings of over $250,000 per year on electric energy. – ($/ft2)
What about Temperature and Energy?
It has been documented (Powder Coating, October 1996, p. 33) that within a commercially applicable cure oven temperature range of between 284F (140C) and 410F (210C) the energy consumption increases by an average of about 6% for every 18F (10C) temperature increase:
Temperature: Energy Consumption Increase (%)
284F (140C) 0.0%
302F (150C) 6.0%
320F (160C) 12.1%
338F (170C) 19.1%
356F (180C) 26.2%
374F (190C) 33.8%
392F (200C) 41.9%
410F (210C) 50.4%
284F (140C) was chosen in this example as the baseline temperature, however, any given temperature can be used as a baseline.
Many powder coatings are capable of curing at this temperature, make sure you specify this temperature with your supplier! Mathematically, energy consumption changes can be expressed as: Cure Oven Energy Consumption Change in % = 100 x (1.0033(+ Temperature Change in Fahrenheit) -1)
Notes:
- Temperature changes can be positive (temperature increase), or negative (temperature decrease)
- The resulting values from the equations shown above are estimates. Oven design, insulation, airflow, and other factors may change the 6% figure for an 18F (10C) temperature increase somewhat. However, these equations offer a reasonable assessment for energy consumption changes related to oven temperature changes.
Application Conditions in DIY Powder Coating Oven
We have reviewed earlier that powder coatings are:
- Hydroscopic (tend to absorb moisture easily)
- Sensitive to heat and hot environments.
- Are fine particles and inhalation exposure should be minimized.
Some points to keep in mind:
- Control temperature. Recommend 80F 27C or less. Remember that powder requires minimal storage space. For example, a semi-tractor-trailer-sized area can accommodate 40,000 lbs of powder which is approximately equal to 15,000 gallons of liquid paint.
- Efficiently rotate the stored powder to minimize inventory time, so that the powder is never stored for a period exceeding the manufacturer’s recommendation.
- Avoid having open packages of powder on the shop floor to preclude possible moisture absorption and the high risk of contamination
- Precondition powder prior to spray application by providing preconditioning fluidization as is available on some automatic systems or by adding virgin (unused) powder through the reclaim system. These techniques will break up the powder if minor agglomeration has occurred in the package or during storage.
- Maximize powder transfer efficiency in the booth to avoid the problems associated with the recycling of large quantities of powder.
- Minimize the amount of powder coating material held on the shop floor if the temperature and humidity of application areas are not controlled.
Excess Heat in DIY Powder Coating Oven
Powders must maintain their particle size to allow proper handling and application. Most thermosetting powders are formulated to withstand a certain amount of exposure to heat in storage. This will vary according to types and formulation but can be estimated at 100 – 120F. When these critical temperatures are exceeded for any length of time, one or all of the following physical changes may happen.
The powder can sinter, pack and /or clump in the container. The pressure of powder weighing on itself, (i.e., large tall containers, can accelerate the packing and clumping of the powder toward the bottom of the container). Unless exposure to the heat has been excessive and over an extended period of time, a powder that has experienced such changes can usually be broken up and rejuvenated by passing it through a coarse (~ 60 mesh) screen.
Powders with very fast or low-temperature curing mechanisms may undergo a chemical change as a result of exposure to excess heat. These powders may partially react or “B stage”.
Even though these powders may be broken up, they will not produce the same flow and appearance characteristics as unexposed powders. These powders will have and will irreversibly retain restricted flow even to the point of a dry textured appearance after curing. Protect from Humidity, Water, and Contamination Water and powder do not mix when the intent is to spray as a dry powder.
Exposure to excessive humidity can cause the powder to absorb either surface or build moisture. This causes poor handling such as poor fluidization, and poor gun feeding which can lead to gun spitting and eventually blockage. High moisture content will certainly result in poor electrostatic behavior which can result in changed or reduced transfer efficiency and in extreme conditions will affect the appearance and performance of the baked coating film.
Because powder coating is a dry coating process, contamination by dust or other powders cannot be removed by filtering as in liquid paint. It is imperative, therefore that all containers are closed and protected from plant contaminants such as airborne cardboard carton fibers, packing materials, grinding clouds of dust, aerosol sprays, etc.