Curing Oven Basics
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.
- Assume a production rate of 600 parts per hour.
- Assume each carrier holds two parts.
- Required number of carriers per hour 600 ¸ 2 = 300 carriers per min.
- Required number of carriers per min 300 ¸ 60 = 5 carriers per min.
- Assume a carrier spacing of 36 inches or 3 ft.
- Five carriers per min × 3 ft = 15 fpm.
- Ware centers × required production = conveyor length
- 2 ft × 1,000/shift = 2,000 ft/shift
- Conveyor length, production time = conveyor speed
- 2,000 ft per shift/7.5 hrs per shift divided by 267 ft per hr/60 min per hr = 4.45 fpm
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.
- Product Ware Weight
- Unit Hanger (carrier weight)
- Conveyor Weight
- Design Conveyor Speed
Conveyor speed × 60/Ware center = units/hr
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.