Barrel plating

Drum electroplating is a term used in metalworking.

So-called mass-produced goods such as screws , nuts , rivets or even small parts that cannot be attached to frames or can only be attached with great effort are processed in drum electroplating devices. The drum with the parts to be processed is transported from the treatment bath ( degreasing , pickling , rinsing, galvanizing , post-treatment such as chromating ) to the treatment bath in automatically controlled systems . For the final drying, the material to be treated is placed in a centrifuge or it is dried directly in the treatment line in a dryer.

In smaller companies u. a. small hand drums are also used, which are then guided through the various treatment baths by crane or by hand.

Contacting

Due to the rotational movement of the mass-produced goods in the drum, the flow of current must be ensured by suitable contacting during the electrolytic coating. Various methods have been developed for this:

• freely movable clapper (contact cable with conductive cap within the drum volume) [1] ,
• permanently installed contact rod (along the axis of rotation of the drum),
• and others (e.g. contacting via suitable contact points in the drum wall)

The selection of the suitable contacting method is largely based on the sensitivity of the mass-produced goods to be coated.

• Clappers move as free cable ends in the drum volume and, due to their own weight, can hit the bulk goods in jerks during rotation; therefore they are particularly suitable for robust, symmetrical mass-produced goods (e.g. balls, nuts ...). Clappers have the advantage that, thanks to their adjustable length, they can protrude far enough into the drum volume in order to contact even a small amount of material to an acceptable degree.

• For sensitive, filigree geometries, a gentler contact z. B. recommended by a contact rod. Here, only the weight of the filling quantity affects the individual bulk goods. In order to ensure the necessary current flow, the filling quantity of the mass-produced parts has to be chosen significantly higher due to the lack of proper movement of the contact rod, so that sufficient parts always touch the contact rod during the drum rotation.

Statistical distribution of layer thicknesses

The distribution of the galvanic coating on a defined body depends largely on its geometry (keyword "bone effect " / "field line image " DOI: 10.7395 / 2015 / Frey_Stieler ). Due to the current field applied during the galvanic coating, the coating takes place fastest on tips and corners, relatively fast on edges and slowest on the middle of the surface, depending on the geometry. [Further explanations should follow here or suitable links should be set.] Because of these properties, a defined measuring point is strictly recommended when defining galvanic layer thicknesses in the specifications between the customer and the galvanizer. DIN EN 1403: 1998 (attention: unfortunately withdrawn according to Beuth-Verlag!) Describes options for the case that no measuring points / areas are defined.

The distribution of the galvanic coating in the barrel process over all parts of a filling follows statistical rules. During the drum rotation, the individual bulk goods move statistically through each other. As a result, individual parts can contact longer during a coating process and thus obtain an increased layer thickness or vice versa. If all parts of a filling quantity are measured (at the same measuring point!), This results in a statistical distribution of the layer thickness with corresponding fluctuations. The type of distribution (Gaussian, Lorentzian, completely different) seems to largely depend on the mobility of the parts during the rotation of the drum. This in turn is determined by the geometry of the parts in relation to the drum size and therefore has to be determined empirically for each individual electroplating task. Even if their use is obvious, especially for the proper handling of complaints (/ defense), the expected effort in a cost-benefit assessment prevents the actual implementation.

literature

• Bach, Möhwald, Laarmann, Wenz: Modern coating processes . Publisher John Wiley & Sons, 2006, ISBN 978-3-527-60889-8 , pp. 120 f . ( Google Books ).
• ISBN 978-3-9814687-3-1

Individual evidence

1. ↑ DIN EN 1403: 1998-10 . beuth.de. Retrieved April 29, 2020.