Powder coating two colours on precision metal components.
We’ve been working with a leading train manufacturer to powder coat two colours on a single product.
We were asked by Bombardier if we could look at powder coating a part for them which they were having difficulty achieving the quality they require from their existing powder coater. This part required powder coating two colours on the same item.
The part, an aluminium casting which forms the outer case for a train disabled toilet alarm is required to be in two colours. The main colour red and a top strip in white with a very clearly defined straight line between the two different colours.
In our initial trails we used standard masking tape and naively thought it would be quite straight forward to achieve a result using this method. We soon found out this was not to be the case, the masking tape did not stand up to the high temperature of the oven and resulted in a sticky baked on mess.
Method two was to source a high temperature masking tape which would withstand the 200-degree centigrade oven temperature necessary to bake the powder coating being applied.
Preparing the Casting for Powder Coating
The casting was prepared, the necessary parts masked off and powder coated white. We did not run the items through the oven when the masking tape was applied prior to powder coating as the heat of the oven would cause it to peel away and allow powder onto internal surfaces which were required to be paint free. Once powder coated the masking tape was completely removed.
Painting the First Colour
The casting was now painted white fully on the outside and would require the bottom section to be powder coated red leaving the top white.
The process now required us to mask off a straight line which would be the demarcation line between the two differing colours. When this was in place the rest of the top which was to remain white and the inside of the casting had to be re-masked too. Great care must be taken at this point to ensure that there are no small holes in the masking which would allow red powder to go beneath the tape and onto the white.
Painting the Second Colour
Now the unit was ready to be powder coated red, this however was not as straight forward as we thought!
On our first attempt, upon removing the masking tape following baking we discovered that the painted surface beneath the masking tape had distorted. The paint under the masking tape had become soft in the oven and creased beneath the tape making the paint finish completely unacceptable. This meant we had to totally strip the units and start again. When the paint had been removed and the white reapplied we looked again at a method of masking without disturbing the base coat with the high oven temperature necessary to cure the powder.
We came up with the following: –
Mask off the white area not required to be painted red and any other parts not requiring painting with the heat resistant tape. Without running the units through the hot oven Powder coat the red parts. Reduce the oven temperature and run the parts through the oven at the lower temperature. This serves to solidify the powder to the extent that it can be carefully handled when cooled but would not affect the existing cured powder beneath the masking tape.
We then carefully removed the masking tape and using gentle compressed air cleaned the unit of powder parts which flaked off from the masking tape.
The castings were then rehung on the line and baked at the usual temperature for the required time to cure the powder properly.
Following this quite long and involved process we are pleased to say the resulting finish was very well received by our customer and is a process we can continue with consistently and there are over three hundred to do.
Powder Coating Nottinghamshire
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