|Graphics have been applied to appliance panels throughout the years. Most of these graphics require further processing with heat to bond the art to the panels. While painted panels are decorated with organic inks, many porcelain panels use ceramic pastes for improved durability. A flexible processing system is required when both painted and porcelain parts are needed for assembly.
While flexibility in manufacturing is desired and is almost a requirement to stay in business in this day and age, it can also create problems when multiple product lines are brought together in the manufacturing arena. The problem can be magnified when these product lines have diverse production requirements. Paint, porcelain, and stainless steel are three such product lines.
Paint, porcelain, and stainless steel have long been used in the appliance
industry to protect and beautify a variety of parts. None of those parts
are as critical to the consumer as the control panel. Traditionally,
the control panels are decorated with graphics that inform the user of
the desired settings for the operation of the appliance. After application,
many of the graphics require an additional heating process to bond the
graphics to the panel. Painted parts, which are decorated with ink, require
a curing process temperature between 300°F and 400°F. Paste-on porcelain needs to be fired at approximately 1,400°F.
The stainless steel panels have special requirements due to the reflectivity
of the surface and the tendency for the panels to discolor when too much
heat is used.
Having individual systems to meet each of these requirements is not acceptable in today's manufacturing environment. Floor space is at a premium. Capital expenditures are tightly controlled. The need for efficient processing systems is a necessity. Rapid response to production requirements and smaller run quantities are expected of any new process.
Electric IR technology can meet the aforementioned requirements with the flexibility needed to process a variety of decorated parts. Heating with IR is mainly through radiation, which means that most of the energy transferred to the parts is direct, line-of-sight heating. While this may present problems when trying to cure return or hidden flanges, this is a perfect fit for heating graphics applied to panels. The graphics are almost always on a portion of the panel that can be hit with the direct radiation from the IR elements.
IR radiation is the band of the electromagnetic spectrum that is just below the visible light range. IR energy travels at the speed of light, through space or air, to objects that are within the line-of-sight. Heating of panels over short distances with IR radiation results in an effective energy transfer from the emitters. The overall effectiveness is related to a variety of issues. Some of the most important issues include the wavelength of the radiation, the distance between the emitter and the object to be heated, and the characteristics of the object.
All objects give off IR radiation. The warmer the object, the more energy is radiated. The IR range of the spectrum is further divided into three segments with varying wavelengths - short wave, medium wave, and long wave (see Figure 1). The shorter the wavelength, the higher the temperature and the more penetrating the radiation.
Figure 1. The electromagnetic
A wide variety of IR heaters are manufactured to meet particular process requirements. The biggest requirement for a finishing system that combines paint, porcelain, and stainless steel is the high temperatures needed for firing the porcelain paste.
Short wavelength IR penetrates the coating and reaches the substrate where it is turned to heat. The wavelength emitted by a bulb is determined by its construction and the energy input to the bulb. Short wavelength IR is required for the high temperatures needed to fire porcelain pastes.
A series of quartz lamps with a tungsten filament has more than enough
watt density to bring the temperature of a panel to 1,400°F. These are
called T-3 lamps and are capable of outputs of 100 to 200 W per sq in.
These lamps can be connected to an SCR to control the energy output.
T-3 lamps respond very quickly to voltage changes, and full power output
can be reached in a matter of seconds. The output voltage from the SCR
can be varied to reduce the energy output to the lower levels needed
for the painted parts.
An inert gas, usually halogen, is sealed in each T-3 quartz lamp. The
gas helps improve the life expectancy of the filament by redepositing
tungsten that evaporates from the wire. The process of redepositing the
tungsten is only maintained above a bulb temperature of 480°F. Operating
the bulbs below this temperature will result in darkening of the bulb
and shorter bulb life. Life expectancy for T-3 bulbs operated above this
level is 5,000 hr. The lower temperatures required for the painted and
stainless panels must be addressed in the design of any system to prevent
reduced bulb life.
Last year, one of Maytag's range plants in Cleveland, TN, U.S. had processing requirements similar to those described above. A redesign of the manufacturer's surface-cooking units added to the already existing complexity of the silk-screening process. The graphics on most of the control panels at Maytag's plant required additional heat processing. Inks would be applied to painted and stainless parts, and pastes would be applied to porcelain parts.
Initial investigation of alternative firing methods for porcelain through the Porcelain Enamel Institute (PEI) led to trials with some electric IR suppliers. The trials yielded favorable results, and a decision was made to install an electric IR oven. The decision was based on several factors.
System flexibility was one of the most important factors. The system had to be able to run small quantities of each type of part with a minimum of changeover time. Since the T-3 lamps have such a quick response time, porcelain parts can be run within a few minutes of the painted parts. Because of the low thermal mass of the oven, heat dissipates quickly, and painted parts can be run shortly after the oven has been at a high heat for the porcelain parts. A silk screen operation is intermittent. Required production quantities can be run in a short amount of time, but production comes to a halt while the screen is cleaned and a new part is set up. This type of production cycling is ideal for a T-3 IR oven.
The ability of the IR oven to react quickly to the changes in demand increases the system's efficiency. This is another factor that led to the selection of an IR system. The quick response time of the T-3 lamps allows the oven to shut down when no parts are passing through. The shut down reduces the electrical demands and increases the bulb life. It also reduces the overall heat load in the surrounding area. A part sensor brings the oven up to operating output just seconds before the parts enter the firing zone.
The electrical load for a T-3 IR oven can be quite high, especially
when parts must be heated to 1,400°F. The ability of a system to quickly
react to production requirements and shut down all heating elements when
no heat is required means a reduction to overall operating cost.
Floor space is the third factor that led to the decision. An IR system can fit in a much smaller space than a gas or electric furnace. In the case of the Maytag system, the control panel for the IR oven is nearly the same size as the oven.
CCI Thermal of Greensburg, IN, U.S. designed an IR oven to meet Maytag's production requirements. Initial testing at the company's test facility revealed the ability of the T-3 lamps to bring the panels to temperature very quickly. In fact, the required temperatures were reached so rapidly that it caused problems with the panels.
The problems were evident as the panels were heated. Since the IR heat is transferred through radiation, the portions of the panels that are exposed are heated quicker than the hidden portions, and this uneven heat distribution leads to warping. The porcelain paste was completely fired, but the rapid heating caused stress fractures in several places on the panels. The T-3 lamps were capable of heating the panels to the desired temperatures, but the rate of heating had to be controlled.
Firing and curing temperature curves from existing processes helped determine the times and temperatures required for an acceptable product. The size and shape of the parts and the production requirements led to an oven with 12 control zones. Each zone is controlled by an individual SCR that can vary the lamp output from 0 to 100 percent. The SCRs are controlled by a PLC, and recipes can be programmed into the PLC to allow the operator to set up the oven for a particular part with the touch of a button. As ware enters the oven, a part sensor starts the sequence, and the control system brings each zone to the desired output. Control timers shut off the power to each zone after the parts exit.
Theoretical power requirements can be calculated for a volume of parts to be
fired. The following is an example:
design specifications: 6 parts per minute and 4 lb per
part to total 24 lb per minute or 1,440 LB/hr, with a firing
temperature of 1,400°F and a starting temperature of 70°F,
which equals a difference of 1,330°F.
= Weight/Time x Temperature change x Specific heat x Conversion
specific heat of steel is 0.125 BTU/LB-°F, and for porcelain
it is 0.25 BTU/LB-°F.
After adding the system specifications and the units, the equation becomes:
Kilowatts (kW) = (1,440
LB/hr x 1,330°F x 0.25 BTU/LB-°F)3,413
The equation shows that a total of 140 kW is needed to raise the temperature
of the parts to 1,400°F. Additional power is required to start the oven,
and an energy transfer efficiency must be estimated. In general, these
efficiencies are near 50 percent. Assume a startup demand of 10 kW and
50 percent efficiency for a total of 300 kilowatts.
Therefore, an oven with T-3 bulbs that have outputs of 100 W per in would require 3,000 in of bulbs. A standard bulb length is 25 in. The theoretical requirements call for 120 bulbs.
The oven should be designed above this theoretical value to allow for lamp outputs less than 100 percent for better control over the temperature curve. Estimates can be made for each zone, if necessary. The power requirements for the example system when supplied by a 480-V service would necessitate an electrical current of 625 A.
Operating costs can be calculated based on the kW-hr rate from the power supplier and the time required for processing the parts through the oven. After the parts are fired, the high energy demand of the oven drops significantly.
As this paper has demonstrated, an electric IR oven has the flexibility and control required for firing and curing a variety of silk screen applications. Paint, porcelain, and stainless steel can all be processed through the same system.
This paper is an edited version of a presentation given at the Porcelain Enamel Institute (PEI) Technical Forum held in Nashville, TN, U.S. in May 2003.
Gwaltney is a manufacturing engineer with Maytag-Cleveland Cooking
has been with Maytag's
plant in Cleveland, TN, U.S. for