By Eng. Gregorio Garza*
There are several types of furnaces for painting curing, two of the main technologies are conventional and infrared, and although there are thousands of manufacturers and it is very difficult to find two similar ovens (since most of the time they are built for specific processes or parts), the principles and way of operation is practically the same.
It is very important that an oven works perfectly and that in case any fault or symptom is present, it is stopped operating immediately, since in most cases, a failure can end in explosion or fire. When a failure is suspected, you have to resort to a specialist, regardless of the cost this represents.
It is incredible the number of cases in which a plant catches fire or a conato of fire occurs because of a furnace in poor condition.
In the following paragraphs I will explain how each furnace works to better understand its failures and its causes.
Conventional Ovens
There are several types of conventional ovens; two of the main ones are electric and gas, but their principle of operation is the same: heat air so that it in turn and by means of recirculation raises the temperature of the pieces and in this way the paint is cured and although its principle is the same; its operation is something different so below I describe it and also the possible failures of each one.
It should be noted that it is important to distinguish two types of failures: process and mechanical or control. Process failures are generated by incorrect parameter adjustment or inadequate designs and many people confuse them with mechanical or control failures, that is, they think that some component is damaged. Mechanical or control failures occur when there is indeed a damaged component.
Electric Ovens
They generate heat by means of an electrical resistance which is heated to the passage of the electric current, an air recirculator is responsible for taking the heat from the resistance and taking it to the parts so that the curing of the paint is carried out.
Process Failures
1. The oven takes a long time to reach the process temperature
This is a very common fault and is regularly linked to the design of the furnace. The temperature that most ovens of this type read is that of the air: if the oven takes time to heat the air, it is most likely designed incorrectly. It should be noted that the shorter the reaction time of the furnace, the energy consumption and cost of the furnace is higher, since to heat the air in less time more energy is required and this increases the size of resistance, conductors and control equipment.
My recommendation is that furnaces be designed to reach process temperature in a range of 10 minutes for not very high temperatures of 60-70°C and 20-30 minutes for temperatures of 180-250°C, unless your process is high volume and does not allow you these times.
Another cause why the furnace takes time to heat can be damaged resistors, open fuses and / or low voltage.
2. Painting does not totally cure
This failure can be linked to the following causes, mainly:
a) Poor distribution of air inside the furnace. If the air does not have a homogeneous flow, the temperature in the oven is not homogeneous, so the paint does not cure the same.
b) Physical properties of the parts. Some physical properties of the parts such as: shape, heat transfer coefficient, mass distribution, etc., cause the part to gain heat faster in some areas than others; so the paint may not cure in certain areas, this is regularly corrected by increasing the residence time in the oven.
c) Low process temperature: Increase the curing temperature as far as your part and paint specifications allow.
3. Brightness or different hue after baking
a) Erroneous flashing and curing process: match the flashing and curing conditions emitted by the paint manufacturer.
b) Very high temperature. Some people increase the temperature of their ovens above the temperature recommended by the manufacturer, this in order to reduce the residence time; this is not recommended if your piece is not homogeneous in shape and mass distribution, since not being homogeneous, it gains higher temperature in certain parts and this causes the paint to burn thus changing its hue and brightness.
Mechanical or Control Failures
1.Damaged Pressure Switch
The pressure switch is a device that verifies that the recirculator is effectively recirculating air, it is a diaphragm that on the one hand connects to the low pressure side (suction) and on the other hand, to the high pressure side. When the recirculator is turned on, it drives the diaphragm against a spring and activates some contacts, which give permission for current to circulate through the resistance and thus generate heat. The pressure switch may fail for the following reasons:
a)Broken diaphragm.
b) Guide mechanism and stuck spring.
c) Broken, bent or destroyed connection hoses.
(d)Leaky recirculation ductwork.
2.Burnt cables and terminals
A very common failure in electric furnaces are burnt terminals and cables, these are regularly caused by loose terminals and / or poorly made connections. When in a resistance of 440V and high current there is a false contact, a "sizzle" is caused that really is a physical effect called plasma where very high temperatures are generated that are capable of melting any type of metal, therefore, it is always recommended to make sure to tighten the connections to the maximum and use the appropriate terminals.
Another cause of the above, is the use of cables and terminals smaller than those permissible since when circulating current through a small area a resistance is generated and when circulating current through this resistance heat is generated, which in some cases is so high that it burns the insulation and plastic bases of the terminals.
3.Burst resistors
This is another common flaw. Resistors usually short circuit and this is only because the recirculation of air is not enough to take away the heat that the resistor generates, so the resistive element rises in temperature until it melts and produces a short circuit. you should evaluate air velocity through your resistance bank and this should be 500 ft/min. in low temperature applications 60-70° C and 1,500 to 2,000 ft/min. in applications from 160 to 250° C.
* Thierica. [email protected]
Leave your comment