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Coating, plating and other kind of surface treatments

by Marcello Cazzaniga June 29, 2018

In this post we are going to list and define the most common surface treatments for zinc die casting and their effects, and we are going to present three case studies in which the application of a specific treatment helped in improving the overall performance of the component.

surface treatments chroming

ZINC TECHNICAL SPECIFICATIONS

The large majority of die castings requires specific processes to meet aesthetic appearance requirements, to enhance wear resistance or provide a protective barrier against corrosion: most parts have to be improved with secondary operations before being shipped.

Some metals are more suitable for undergoing galvanic treatments, varnishing and painting than others: one such metal is zinc. Zinc is a quite dense metal, really durable and stronger when compared to different materials such as polymers. Die casted zinc also allows to create smaller draft angles, smaller and longer cored holes and thinner wall sections, thus conveying the possibility to develop innovative shapes. This, together with the possibility to use unconventional surface finishes offers a competitive advantage over competitors. By choosing to work with zinc, designers can choose between a wide range of surfaces treatments, giving different finishes and surface improvements.

Designers and project managers are always looking for new ways to improve products: a collaboration with a supplier could help them identify criticalities in design or solve pre-existing issue. In a co-design kind of collaboration, the supplier should suggest its client the best way to optimize the product for finishing, based on its expertise.

 

CHOICE OF MATERIAL & RELATIVE FINISHES

During the collaboration, the first step is selecting the right type of alloy and evaluating its performances while taking in account the final use of the component. The first point to be cleared is whether the component is going to be used indoor or outdoor:  in second case, a corrosion resistance treatment shall be considered. There are different types of corrosion depending on chosen metal and the surrounding environment. You can learn more about corrosion by reading this post.

Another issue to take in account is the necessity for a cosmetic finish: depending on the final application of the component, clients could request varnishing, painting, enamel, chroming or plating.

Once analyzed every technical aspect, it is important to study advantages of different kind of finishing such as zinc black, C&E conversion coatings, sprayed and baked liquid coatings, copper-tin-zinc electroplate, copper-nickel-chrome electroplate, mechanical plating, and epoxy and polyester powder coatings.

Zinc Black

In this process a thick black phosphate is conferred to the casting as an obstruction against humidity and corrosive environments. It is usually a pre-treatment where zinc plating is followed by a black chromate conversion coating.

C&E Conversion Coatings

These treatments are chemical immersions that give a protective film on the zinc surface. Their aim is to protect parts during storage or in indoor environments. Usually conversion coatings are followed by waterproofing substances or varnish to widen the range of their applications.

Sprayed & Baked Liquid Coatings

These types of coatings include different chemistries such as epoxies, polyesters, phenolic and urethanes.

Copper-Tin-Zinc Electroplate

The outcome of this process is a dull and silvery finish on the zinc surface. It gives zinc a correct protection but the finish turns to a black appearance.

Copper-Nickel-Chrome Electroplate

This finishing is one of the best against corrosion. It protects zinc surface against the acidity of subsequent baths, then is able to uniform the zinc surface finish and assure it a good electrical conductivity.

Mechanical Plating

This procedure regards placing parts in a drum with metal powders desired and a chemical activator, then tumbling these parts until the desired thickness of coating is reached. The coating is created thanks to a mechanical and chemical action. The advantage of this process is that it can be applied on all surfaces, also on inside corners. Final colorations depend on different metal combinations.

Epoxy and Polyester Powder Coatings

These coatings are used as powders in dry electrostatic process and then fused in an oven. Not using solvents during the process is an advantage, since it prevents the formation of solvent-caused defects

This list is not comprehensive of all the possible finishing, but it is just a selection of the most common zinc die casting finishing.

Now we will present a selection of case studies where these technologies have been implemented with a tailor made cut, thanks to the collaboration between client and supplier.

 

 

CASE HISTORIES

The use of Zamak in die casting processes allows a series of finishes, which would be impossible to obtain with other alloys, leading to products with both high technical coefficient and important aesthetic value.

Bruschi has a 70 years-experience with clients among several industries, which include automotive, lightning engineering, household electrical appliance, doors and windows locks. Having an extensive know-how that covers different fields allows the designer to provide alternative solutions borrowed from other industries and always ensuring top quality in material, processes, manufacturing and products durability.

 

CASE STUDY 1 – New finishing to avoid blistering

An important automotive player asked to Bruschi to produce a component already produced by other suppliers. Client wanted new solution to develop this particular component, because it had a scrap rate of over 25% due to blisters appearing after treatment of galvanic coating.

Bruschi worked focusing its efforts, involving its supplier network, to avoid blistering. The solution has been achieved by changing the finishing: turning from galvanic coating to passivation. This means a great saving in terms of scrap rate caused by blistering, that has decreased from 25% to only a few parts per million.

 

CASE STUDY 2 – New finishing to improve SST resistance

An important automotive player asked Bruschi a solution to improve the salt spray test resistance of an existing component that was already made in Bruschi to be assembled on subcompact cars. For the new project, the component should receive better treatments in order to meet requirements of luxury cars. In general, regarding surface of the component, the SST resistance is given by a particular kind of finishing.

For subcompact cars the resistance guaranteed was 120 hours, thanks to a surface treatment called Fe/Zn: however, this treatment was not suitable for luxury cars. To reach the new goal Bruschi had to push the resistance to 1000 hours.

Thanks to Bruschi expertise and supplier network, the client reached its goal by implementing a special galvanic treatment, which allowed SST resistance for over 1000 hours to salt spray test.

 

CASE STUDY 3 – Special painting instead of mat chrome treatment

For an important client in locks sector Bruschi developed a different process in order to decrease the scrap rate. In details, client required a mat chrome surface treatment on its components but, due to their complex geometry characterized by a high number of angles, it was impossible to guarantee the requirements requested and the scrap rate was out of control.

In collaboration with its suppliers network, particularly with the painter and a chemical company, Bruschi developed special paint to assure the same characteristics to components without the mat chrome treatment. By changing the technology the client had the same components, but less scrap rate.

These case studies prove the importance of surface finishing and how a collaboration with suppliers can help with reducing scrap rate by finding alternative solutions or creating tailor made surface treatments to obtain the best performances out of a component.

 


If you are interested in the topic of metal finishing, you can read our other posts on the topic:

1- Focus on Surface Finishing

2- Processing techniques for metal finishing

And to always be up-to-date with die casting industry, subscribe to our blog.

 

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