Grinding work: fettling and fine fettling before the powder coating
If a workpiece is to be powder-coated after being hot-dip galvanised, grinding work is always necessary between the two processes. This is done to roughen the surface in ideal preparation for the next finishing steps. Components that are not processed further after galvanising are fettled, for example to eliminate the risk of injury from run-off projections.
Fundamentally, we have to meet various requirements with the grinding work. To ensure that we can permanently guarantee the required qualities, batch galvanising in accordance with the familiar DIN EN ISO 1461 standard and powder coating are both subject to a recurring and standardised process at our company.
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Fettling in compliance with DIN EN ISO 1461
Batch-galvanised components are reworked and fettled in accordance with DIN EN ISO 1461. To minimise the risk of injury, all run-off projections are removed or smoothed down. Any zinc ash residue is brushed off the components and suspension points and run-off edges are reworked.
Material or processing-related unevenness, such as rolling marks, spliced material, rough surfaces or protruding weld seams, is not treated. In order not to jeopardise the main purpose of batch galvanising (protection against corrosion), the zinc layer is not damaged or worn down by grinding during this process.
On delivery, the raw components should have a uniform surface with regard to the degree of rust, scarring or roughness to obtain an even galvanised surface. To achieve the smoothest and thinnest possible zinc coating for subsequent powder coating, steels in the low silicon range (category A steels) or alternatively in the Sebisty range (category B steels) should be used.
For general anti-corrosion applications, the option "hot-dip galvanising" must be selected when ordering steel in accordance with the applicable product standard.
Fine fettling before the powder coating
When fine fettling before the powder coating, we process and grind the components significantly in excess of DIN standard requirements. However, the edges of the delivered raw components should already be broken and, in the case of laser edges, additionally ground, as these can later cause adhesion problems within the coating. In addition, any imperfections in the galvanising must not be repaired with zinc dust paint to rule out any adhesion problems later on. This naturally also applies for delivered galvanised components. Additionally, the design should include appropriate suspension possibilities which can be easily reworked or are located out of sight.
To satisfy our customers’ requirements for a practically smooth and even surface after the powder coating process, we remove any roughness such as run-off traces or spots from the accessible surfaces to the extent allowed by the design and machinery capabilities. Any material defects that occur are smoothed or removed as far as possible. For this work we mainly use rotary grinders, which slightly reduce the thickness of the previously applied zinc coating.
To avoid a negative impact on the components’ stability, we do not grind any protruding weld seams caused by the galvanising process. Material-induced unevenness, such as concave tubing or mat filling bars, cannot be completely smoothed out either.
The increasingly sophisticated requirements of our customers often mean that more grinding work is necessary. The challenge in such cases lies in fulfilling their specifications while keeping to the zinc layer thickness required by the DIN standard. Our workforce undergoes intensive training in grinding work and is capable of achieving a great deal in this respect. Nevertheless, compliance with DIN EN ISO 1461 cannot always be guaranteed with finely fettled surfaces.
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