Surface Finishing Options For Your Casting

After carefully planning the design of your casting, you’ll be confident that it’s mechanically sound. But what about its aesthetics?

Various methods of surface finishing will allow you to alter the look of your casting and – if your casting will require painting – provide a good pre-treatment, too.

Surface finishing can also improve the performance of your casting in application, for example by improving corrosion resistance.

Sand Castings

The sandcasting process may leave some graininess on your product. In order to gain a smoother finish and provide a better base for painting, shotblasting can remove any excess sand and ensure that your casting has fine textured look and feel.

Gravity and Pressure Die Castings

Where these castings may experience burrs on their surfaces, vibratory finishing will ensure that they are efficiently cleaned and smoothed.

Again, this won’t just improve their appearance initially, but make any necessary paint much easier to apply.

Alochroming (Chromate Conversion)

A common treatment for aluminium alloy castings, alochroming involves immersion in chemical baths, containing a solution that chemically reacts with the alloy to produce an etched surface - or to form a chemically converted protective film.

This chemical conversion process offers attractive and corrosion resistant finishes, but for most applications they are generally too easily abraded to be of practical use alone. This pre-treatment creates a great key for subsequent coating.

Powder Coating, Painting, Silk Screen Printing & Enamelling

High bake powder coating is a low-cost process for items requiring a high degree of impact and abrasion resistance. The resulting finish is tough and durable and is available in varying levels of gloss and texture.

However, there is a catch, as masking on items to be powder coated is difficult and time-consuming; so, if masking requirements are high, it may become too expensive a process to be viable.

When considering this process, you should also bear in mind that small surface blemishes (which are not easily detectable on raw castings) become much more noticeable after powder coating.

To this end, wet sprayed coatings can be considered where a higher standard of surface finish is required. A proper surface pre-treatment, such as the application of a suitable primer such as zinc chromate, is essential if the final paint coating is to be truly protective.

Stove enamelling temperatures are not usually high enough or of sufficient duration to influence the properties of aluminium castings, but should be considered with castings that have been fully heat treated.

Where necessary to add lettering or other artwork to painted castings, this can often be achieved by silk screen printing.

Electroplating

Coatings such as copper, nickel, zinc, chromium, silver and gold are often applied to zinc and copper castings for decorative purposes. To a lesser extent, they are also used on aluminium castings – but more importantly of all, the surface of a casting must have a high degree of soundness in order for electroplating to be applied.

Anodising

Anodising is the electrolytic process for producing a thick oxide coating on aluminium.

Anodising takes advantage of the natural oxide film that is formed spontaneously on the surface of aluminium when exposed to the air; this natural oxide film, which is extremely thin, protects the underlying metal, giving its well-known properties of resistance to corrosion.

Although there are many positives to anodisation, there are also some less-desirable effects to be considered.

With time, the oxide film will go grey in colour. This is acceptable in some applications, but not in situations where appearance is important. In aggressive atmospheres the film can become sooty, and sometimes even powdery.

Anodising thickens this oxide film electro-chemically to give a hard, transparent coating, which greatly increases corrosion resistance and has the advantage of protecting any polished, chemically brightened, etched or similar surface finish.

In addition, unsealed anodic films are microporous and can be dyed to an even more impressive finishing affect.

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