Simply heating up the substrate does not cure the coating. There are many variables to consider when choosing the best cure oven for your application...
The final step in the paint process is curing. The cure oven raises the product mass and coated material to a specified temperature and holds this temperature for a set time. Typically, 25 to 35 min at a set temperature is needed to achieve a minimum curing temperature for 20 minutes.
Some product configurations trap liquids and may require zoned ovens. The oven exhaust, if insufficient to handle volatile materials released during curing, can negatively impact the cure and final part appearance. The amount of volatile material also depends on the product being used.
The amount of exhaust and the type of heat source can affect product color. Poor exhaust and gas-fired ovens typically cause coating color to darken and/or yellow. The amount of color drift varies with product type.
Prior to entering the cure oven, the product is usually cleaned, rinsed, dried, and coated.
Time spent in the oven is determined by the coating suppliers, who specify the required time at a given temperature to thoroughly cure the coated product. However, line speed, product window size, hanging spacing and product weight/conveyor weight must be defined prior to designing a cure oven.
To allow for variation in production requirements, it is advisable to set a maximum speed of about two times that calculation. A variable speed with a speed range of about 3:1 is the most common.
Moving product/conveyor load weight.
Units hr × Ware weight = _____lbs ware/hr
Unit hanger weight × Units/hr = _____lbs hanger/hr
Conveyor weight/ft (in lbs) × conveyor speed fpm × 60 = lbs conveyor/hr.
When calculating oven heat loads, the above lbs/hr should be kept as a separate number due to different temperature rises and specific heat requirements.
Oven heat load calculation. Parameters to consider when determining oven heat load include: radiation loss through enclosure panels; product heat absorption; conveyor/hanger heat absorption; heat losses through air seals or openings; and fresh air requirements for burners; continuous exhaust for insurance requirements; or coating material releases.
Thermal heat T (supply - recirculated). Supply air temperature minus oven design temperature of a gas-fired oven should be as follows:
125F will give ±10F in oven
100F will give ± 5F in oven
Curing coatings varies. Infrared (gas and electric), radiant wall, conventional convection and high-velocity convection curing are some of the options. Most often direct-fired conventional convection curing is used. Infrared or radiant-wall designs are often incorporated for preheating only.
A convection oven has five major components.
Oven Enclosure (shell) contains the environment necessary for the curing process, including a support structure; insulated panels (enclosure); and product openings/air seals.
The oven support system should be designed to carry the enclosure weight and the product conveying system. Structural steel must be connected with slotted hole connections to allow for expansion.
Insulated Panels (enclosure) contains the heat of the process. Panels 30 to 33 inches wide with fiber insulation (one inch of four-lb density insulation for every 100F) sandwiched between aluminized metal skins are used. The assembled panels are tongue-and-groove for easy installation. The outer skins are connected with formed metal channels. These channels form a through-metal condition, allowing significant heat loss at the joint. This panel joint can become too hot to touch, so the channel is slotted to reduce the area available for heat migration. This technique reduces joint temperature to less than 100F in a 450F oven, without losing the structural integrity of the channel.
Corners present another problem with panel construction. At the edges of the oven, panels do not fit tightly together and leakage occurs. Void areas are filled with loose insulation and the areas are jacketed with sheet metal flashing. This is not sufficient to stop the escape of cure products that condense and stain oven walls. Continuous gaskets must be used to create a proper seal along horizontal seams.
Personnel access must be provided. The door and hardware must seal the opening without using a positive latching device. Any panic hardware with positive latching features must allow the door to be opened from the inside. Locate access doors so that an exit is never more than 25 ft away. Oven doors with windows are easier to locate.
A great source of oven problems is enclosure openings where products enter and exit. These openings are designed using minimal clearance for the ware. Bottom entry/exit designs make use of the natural sealing features of hot air and present no real problems. Openings in the sides of ovens require mechanical air seals to contain the environment.
To seal an opening, it is best to draw hot air from the oven and force it back into the opening. For this to work, a significant velocity must be developed at the center of the opening. Additionally, the oven must run negative relative to the production environment. These two requirements draw factory air into the oven. This pressurization is relieved by exhausting the enclosure, resulting in a considerable source of heat loss.
An alternative to traditional construction methods is an oven module. When the design allows for shipping, 20-ft-long completely assembled sections of the oven can be fabricated. This construction includes all-welded interiors that eliminate areas for dirt to collect; steel buried in the panels to reduce interior surface area; fewer joints with through metal for less heat loss; and speed and ease of assembly at the customer's factory. Despite the many positive features, these ovens are rarely practical because of their configuration.
Heater units. The second system at work in an oven is the heater unit. The heater generates the energy for curing and begins the distribution of energy. The most significant components of the heater are the burner, supply fan and filters.
To properly size heater equipment, a detailed heat load must be calculated. Energy losses for the ware load, conveyor load, enclosure and exhaust requirements must be considered. These losses, expressed in Btu's/hr, are used for selecting the burner and corresponding electrical devices necessary for burner control. The burner is most often a direct-flame device that provides the energy for curing.
The heat load calculation also provides information for selecting an oven supply fan. The heat required to maintain good oven temperature is delivered by heating the supply air to no more than 100F above the oven operating temperature and distributing this air to the oven. The fan volume must be expanded for the elevated temperatures. The supply fan should turn over the oven volume about two times a minute. Because the fan is a constant volume device, the fan motor is sized for cold starts to avoid overloading. This provides an oven temperature profile better than plus or minus 10F throughout the enclosure.
Many heater units have filtration systems to continuously clean the oven environment. Filter efficiency varies with the application, but the types modified for the elevated temperatures used to filter final makeup are most effective. Filters require much lower velocities than in normal heater units. When filters are used, heater unit size must be increased. Oven filters continuously clean the air and, as a result, load very slowly. It is not necessary to pre-filter high efficiency filters.
Sometimes the products of combustion are not compatible with the coating. In these cases, indirect-fired heater units are an option. These use air-to-air heat exchangers and may require one third more energy to operate.
Supply Air System. Another problem occurring when the products of the cure and combustion combine and come in contact with a direct flame is the production of NOx. When this becomes a problem, it is overcome by introducing large amounts of fresh air into the heater. This lowers the temperature of the flame-heated air to a point where NOx is not produced. This, like the indirect oven, is applied at a significant cost of energy.
As the heater unit discharges the supply air, it is directed into the oven supply system. The purpose of the supply system is to deliver and distribute the energy developed in the heater unit. The rectangular supply duct is constructed of aluminized metal. For proper operation, velocities in the duct should not exceed 2,500 fpm. This assures good laminar flow and good temperature control. Supply ducts should be along all walls and between every other conveyor run to eliminate cold spots.
Recirculated air systems.The recirculating system returns oven air to the heater unit so that energy is continually added to the oven. This is accomplished using the duct with the supply fan to create a negative pressure condition within the enclosure. The oven air naturally migrates to the areas of low pressure, where it is captured in the duct system and returned to the heater.
Recirculating duct is fabricated in much the same manner as the supply duct. The duct is designed for slightly lower velocities. The velocity in the duct is held at 2,000 fpm and openings are 20-25 pct greater than the supply.
It is poor design to use the recirculating duct to provide control over the oven environment. The influence of suction pressure is negligible, even at short distances from the source. While air naturally moves to the areas of lower pressure, this movement cannot be easily controlled. It is better to place a small amount of recirculation in the hottest part of the oven and let the supply air do the work. This assures that the design requirements will be maintained.
Exhaust air system. Every oven must be exhausted. Exhausts create a negative environment so that air seals operate properly and remove VOCs and other cure products from the oven. Additionally, the exhaust purges the oven prior to start-up. The requirement for purge is to change the enclosure atmosphere four times in approximately 20 min prior to ignition.
The main job of the exhaust is to maintain a safe environment. A good rule of thumb is to exhaust 10,000 cu ft of air (expanded for elevated temperatures) for every gal of solvent driven off in the oven. This assures the oven atmosphere will be maintained below the lower explosion limit of the solvent with a safety factor of four.
Exhaust also eliminates smoke build-up. Smoke is produced when curing some electrocoatings and powders. The exhaust requirement here is best recommended by the coating supplier.
The flexibility of convection curing keeps it popular with today's finishers, despite pressures to increase quality and reduce the space required for paint shops. A properly designed and installed convection oven requires little attention relative to pretreatment and application processes. It runs effectively with simple controls. It can be combined with other curing methods. Filtration or indirect firing can be added to improve quality. Because the exhaust can be controlled so well, abating oven gases is reasonably achieved. To conserve on factory space, ovens can be elevated, located outside or on building roofs.
Typical oven maintenance and inspection schedule. A typical weekly maintenance schedule would include inspection of the following: Flame failure detection system; Ignitor and burner operation; Burner air filter; Burner blower impeller cleanliness; Burner blower motor cleanliness; Heater shell interior cleanliness; and Fan lubrication.
A monthly schedule would include a check of the following: Fuel safety shutoff valves; Fan and air flow interlocks; Time delay switch settings (purge); Conveyor interlocks; Limit switches; Explosion relief latches; Gas drip leg; High and low fuel pressure interlocks; Interior ductwork cleanliness; and Interior oven cleanliness.
Annual inspection and maintenance schedules should include: Ignitor and burner components; Combustion air supply system; Flame failure system components; Piping and wiring components; Combustion control system; Instrument calibration; Automatic fire checks; and Operating sequence tests.
The understanding of oven system requirements will lead to a successful implementation when the end user, coating and equipment suppliers work as partners in developing the oven curing system right for you.