Dip Spin (Centrifugal) Painting
What is Dip Spin or Centrifuge painting?
Centrifugal coating is an excellent way to finish large numbers of small parts.
A centrifugal coater is simply a wire or perforated metal basket, for holding parts, which is then suspended above a tub of paint. In practice, the tub is raised, or the basket is lowered so the paint covers the parts.
The basket is raised above the level of the paint and spun at high speed, causing the excess paint to be flung centrifugally off the parts onto the sides of the tub allowing the liquid to drain down into the bottom.
Dip Spin (Centrifugal) Painting
The success of the dip spin, or centrifugal, painting process is dependent upon the integrity of the pre-treatment processes as well as the painting process itself. In order to present the components in a fit state for painting, they must first undertake a series of preliminary processes which are outlined in more detail on the subsequent pages.
Briefly the processes are as follows:
- Cleaning to remove all debris and lubricants from the components.
- Rinsing to remove any excess cleaner.
- Phosphating in order to change the surface and formulate a phosphate film on the surface which has the ideal grain configuration for painting.
- Rinsing to remove excess phosphate.
- The dip spin or centrifugal painting operation itself.
- Stoving through a gas fired oven.
Cleaning
In order to remove all debris, oils and contaminants, a series of different cleaning methods are used. These vary from a low strength ‘knock-off’ cleaner to a highly caustic powder cleaner using the electrophoretic process line; two bespoke cleaning machines one of which has the added advantage of a drying unit or using the iron phosphate machine which has a mild acidic cleaner in the iron phosphate solution.
All these methods and cleaners are effective on a wide variety of oils and greases.
Experience tells which cleaning method to use from the mild acidic cleaner in the iron phosphate to the stronger caustic cleaner in the cleaning machines and finally the heavy caustic cleaners on the electrophoretic line.
The high alkalinity of the electrophoretic cleaners helps shift some types of soil although the level of caustic is not sufficient to de-rust effectively.
Phosphating
There are two types of phosphate available namely iron phosphate or zinc phosphate as below:
Iron Phosphating
Iron phosphate deposits a fine crystalline phosphate surface at around 0.2-0.4g/M². The phosphate is applied by a spray process through a high pressure in-line production spray line which has the added advantage of cleaning the parts as well as applying the phosphate.
The chemical used is a liquid material which is a combined coater (heavier coating weight phosphate) and cleaner solution designed to simultaneously degrease and pre-treat ferrous substrates prior to painting.
Zinc Phosphating
Trication Zinc Phosphate deposits a fine crystalline zinc phosphate at around 1-3g/ M². The Trication formula comprises a mixture of zinc, manganese and nickel to make up the phosphate structure. The Nickel content is expressly excluded from the substances of concern list in the ‘End of Vehicle Life Directive – 200/53/EC’ This is due to the fact that currently there is no better way to phosphate automotive parts.
‘Tricats’ generally ‘sludge’ less than traditional calcium modified zinc phosphates and are favored by Honda, amongst others. Keeping the temperature low, if possible, helps minimise ‘mapping’ problems from uneven phosphate build up.
This process is in accordance with BS 3189 Type 4 and DEF STAN 03 – 11 Class IV
Rinsing
It is absolutely imperative that any excess zinc phosphate solution is removed prior to further processing. Consequently, following on from the zinc phosphate tanks are two rinse tanks similar to those following on from the cleaning tanks. Again, these tanks are filled with cold town’s water agitated by passing air through the water. The ninth tank has a steady flow of water feeding into the tank which subsequently feeds into the eighth tank. By using this method, the ninth tank is kept clean and free from contamination
Special points of interest
- Salt spray resistance to a 720 hour as per ISO 9227
- Heat resistant : corrosion resistance and appearance unaffected at 150ºC
- Friction coefficient: adjustable from 0.10 to 0.16 and from 0.12 to 0.18
- Flexibility without flaking
- Excellent chemical resistance.
- Adhesion in accordance with ISO2409 to GtO
- Pencil hardness in accordance with ASTM D3363-92A at 4H+
Dip Spin (Centrifugal) Painting
The parts are loaded into the processing baskets via the automatic ramp. This ramp carries the work contained in bespoke containers up to the head of the processing plant where they are automatically tipped into the processing basket. This basket is then swung into the machine, operating on an automatic cycle, the doors close and the processing basket is lowered into the paint. Once fully coated the basket is raised out of the paint and spun rapidly in order to remove the excess paint. This spinning operation is carried out first vertically and then canted over at an angle of 45º. Once the cycle is complete, the doors open, the processing basket is swung out and the painted work automatically unloaded into the containers used for curing the parts.
These containers are then manually loaded onto the conveyor to be transported to the oven where they are then stoved, or cured, through the oven at 200 degrees Centigrade for approximately thirty minutes. Once cured the parts are either re-cycled in order to receive a second coat of paint or loaded into the containers ready for despatch.
Sealing
This solution is a chromium-free, inorganic-based post rinse, formulated for use after the zinc phosphating pre-treatment process and prior to painting, or powder coating. Its use will materially improve the overall system performance. This solution can either be used as a spray or immersion system.
