The object of the necessary pretreatment before wet painting and powder coating is first to clean the surface efficiently and, possibly after intermediate treatments such as desmutting, to apply a high quality conversion coating that, in the case of aluminium, fulfils all the criteria of QUALICOAT/GSB, thus guaranteeing the long life of the product by ensuring good adhesion and protecting from corrosion. The quality also depends upon the essential rinsing stages between the separate applications, especially the final rinses.
Two principal conventional methods of application are used:
The immersion process is carried out with cranes moving up and down over the separate process baths, transferring the load to the following process stages. The inner areas are inevitably filled or covered with bath liquid so treatment of hollow sections requires considerably higher chemical consumption, increasing the waste-water contamination. In addition, the products must be jigged with greater care than necessary for the electrostatic application of paint because the mechanical strain on the products caused by the vertical movements of the crane and the liquid displacement in the baths is quite considerable.
The spray process occurs in chambers arranged in tandem which jigs on appropriate conveyor systems pass through longitudinally. This means that, between each process and rinsing stage and between each rinsing stage and the next process stage, there must be a ,,drip – dry“ area equal in length to the longest product or jig. If this is not the case, liquid running along the length of the sections will either contaminate the next rinse or dilute the next process chemicals.
In addition, there must be suitably long ,,drip - dry" areas at the beginning and end of such a pretreatment line. Just a 2-stage continuous spray pretreatment (e.g. cleaning and chromating with only one intermediate and final rinse for each) for 7m sections requires a plant length of more than 63m. This type of plant construction is used only for a few special cases, mainly vertical jigging. For this reason, spray applications for horizontally jigged materials are only carried out in closed chambers as this is the only way to eliminate the ,,drip – dry“ areas that take up so much space.
Another, often used alternative is to combine the chemical application and following rinse in one chamber. The disadvantage is that the chamber walls are inevitably rinsed too during the rinsing stage, so more than three times the actual area of the load is rinsed. As a result, both the chemical consumption and chemical contamination of the rinse water is also more than threefold. Due to the dual use of the chambers, the time spent in each chamber is also significantly longer. Often an unnecessarily high spray pressure is used in an endeavour to reduce the treatment time, which in turn increases the demands on the jigging system.
Job coating plants often need to acquire additional orders and still be able to pretreat different materials optimally, such as:
Conversion coatings that need to be applied are:
and are often specified by the customer. For aluminium building components, the trend is towards other variations of the conversion coating process. Besides the yellow and green chromate treatment, coatings incorporating
have been developed. It is not known which process will be specified and which trend will be successful in the future.
in addition, the old-fashioned and outdated so-called ,,etch cleaners" can be replaced by
cleaners which are more efficient and far more economical in consumption. This is also the only way to simplify the process since the modern cleaners are equally effective on different substrates.
When planning new plants and extensions to existing facilities, people should take these options into consideration to cater for their different possibilities in the future.
The new transverse flow technology allows space-saving, efficient and variable pretreatment before powder coating and wet painting (this system technology has now been patented in all major industrialised countries.)
In this type of system, the treatment and rinsing areas are positioned side by side. The jigs are moved transversely through the plant. Each area has spray units moving vertically. The products are sprayed at low pressure and covered from an optimum angle. This prevents aerosol formation. After each treatment stage, the spray units automatically come to rest in the lowest position and the separation walls between the separate treatment areas are lowered. The jigs are then moved to the next treatment area through a simple side-step, therefore minimising the transfer times.
Given the length of the product, the space required for such a system then depends only upon the number of treatment stages and the specific width of the jigs. Normally the width of each area is about 1m.
A system for 4 different treatment stages, each followed by a rinsing stage, i.e. 8 areas, for say 7m long sections would need to be approx. 8m wide and less than 10m long.
This system even requires less space than a comparable immersion facility because each rinse can automatically be controlled as cascades in each rinse area so no extra space is needed to save water. In addition, the height of the system is much lower than an immersion treatment system because the jigs do not need to be moved vertically by a crane.
It could be said that the
transverse flow low pressure spray pretreatment
requires the least possible space of all, making the system ideal for being built into already existing plants.
New plants can save costs by constructing smaller buildings. The compact construction allows the use of multiple pretreatment chemicals, depending upon the substrate and customers' requirements. This means the pretreatment can also be applied to other metals, such as steel or galvanised steel, with all their special requirements for cleaning and conversion treatment (e.g. phosphating). The little additional space needed for such systems means that the plants could also be adapted for future developments in the pretreatment chemicals. The compact construction of such systems results in shorter transfer times which, in turn, increases the number of jigs processed per hour.
Small loads can also be pretreated efficiently, i.e. single loads can be separated by colour and commission order before pretreatment which eliminates the extra work required to rearrange the loads prior to coating or to sort the products prior to packing.
In addition, the advantages of this system‘s low pressure spray application with vertically
movable nozzles should be emphasised:
The products are covered with quite large amounts of liquid followed by a short penetration phase and another spray phase. This improves the efficiency of both the chemicals used and the diffusion-controlled rinse processes.
The vertical motion of the spray nozzles in relation to the horizontally hanging products means there are no dead areas and the products are covered from every angle.
The low pressure reliably prevents aerosol formation.
There is no mechanical strain on the product. This means that the same simple jigging can be used as needed later for the contacts for electrostatic coating. There is no need to change jigs.
Typical system design:
A system for pretreatment before powder coating of aluminium building components (and/or steel construction parts) typically consists of 7 pretreatment stages. The following alternatives for their efficient use are listed here as examples (it is quite conceivable that the number of possible uses rises drastically with each additional treatment area):
The storage tanks with a volume of approx. 8m3 are shallow and located under the treatment areas. The tanks under the rinsing areas are separated into up to 3 chambers to allow cascade rinsing.
All the storage tanks are made of nickel-chromium-molybdenum steel, a type of stainless steel withstanding all corrosive demands and thus suitable for all chemicals. Plastic gratings cover the storage tanks so the whole plant is accessible. Each storage tank has the suitable number of adjustable chemical pumps connected; these are either immersion pumps or valve-controlled.
Each area is separated by a high quality elastic curtain, in some cases double, each having an automatic lowering mechanism.
The overhead structure consists of a self-supporting steel construction carrying the hoisting frames with drive motors and transfer tracks. Each low pressure spray unit consists of double spray nozzle rings made of nickel-chromium-molybdenum steel.
The hoisting frames for the spray unit and the curtains are driven by synchronised geared motors - connected on both sides with an axle across the length of the chambers, carrying the strain on both sides. The propulsion motors are controlled by limit and regulating switches, as well as process-controlled frequency transformers. The hoisting frames are fixed on all sides in vertically positioned gliding tracks; they are supported by high-duty belts made of a special plastic fibre.
The whole system is encased in either stainless steel sheets or plastic. Every single area has a side door with windows and lights. The doors are secured by light barriers.
The whole system is computer-controlled and can be freely programmed (SPC). The central control unit includes all the measurement and control instrumentation, e.g. frequency transformers, temperature measurement and control units, locking devices, spray time programming.
Summary / conclusion
The characteristics of a
transverse flow low pressure spray pretreatment system
and its advantages over conventional technology are:
requires little space
so ideal for building into existing plants.
can be used for many applications
chemicals can be chosen to meet special requirements
so geared to the future.
* Environmentally friendly
low water consumption / waste-water volume
optimum use of chemicals
so improving quality.
tested mechanics / electronics
each area can be accessed and looked into
short transfer times
more lots per hour
possibility of integrated jigging
automatic program execution
* * *