How do I get a metallic finish?

Written by: Tim Plunkett | Published:

Metallic finishes can be achieved in a variety of ways depending on the substrate material, quantities involved and goal of the treatment. The following is a short review of the more common options that are commercially available.

If starting with a metallic component:

Polishing
Whilst potentially time consuming, as it is mainly a manual process, the use of polishing wheels can help. The result is a smooth, high gloss sheen. Flat or 'open' surfaces polish well, but intricate shapes can give access problems. Care is required to retain sharp edges and clearly the better the surface quality of the component, the quicker a good finish can be achieved.

Vibratory Finishing
This is machine based and can run with batches of parts at a time, improving the cost effectiveness. The process is often used to deburr components and uses a drum filled with an abrasive media. Combinations of media, speed and magnitude of vibration give rise to a variety of finishes.

Sand Blasting (or beadblasting)
A simple process that projects a fine media in a high velocity airstream at the component. It provides a flat uniform surface and can be used as a pre treatment to other processes.

Aqua blasting
Basically similar to sand blasting except that the media is carried in water. This softens the impact and results in a smoother surface. (Frequently used as a precursor to anodising).

Brushing
This creates a uniform parallel texture on the component surface and is quite effective at smoothing out imperfections. It is usually applied by wire brush or belt.

Metal Plating
There are two options: electroplating where an electrical current is used to coat the substrate and electroless plating that relies on the substrate catalising a reaction. Plating thicknesses can vary depending on the requirement and usually invoke the deposition of an alternative material to the substrate. Benefits can go far beyond purely cosmetic, but rely on having a good quality finish on the substrate as plating is not good at hiding imperfections. Most commonly deposited materials are copper and nickel, but chrome gold, silver etc. are available.

Anodising
Anodising converts the aluminium surface to a decorative, durable and corrosion resistant oxide finish. It's not an additional layer like paint, but a surface conversion process. There are a large range of colours that can be achieved as well as clear.

However, if your goal is to achieve a metallic finish on a plastic component, then the choices are:

In Mould Effects
There are a couple of options. One is 'in mould decoration' (IMD) where a metallic finish can be applied to a thermoformed 'shell' that is placed in the mould before the component is injected. Another is to use the latest metal effect materials such as Hostaform Metal LXTM. For economic reasons these tend to be applied to the higher volume parts as the tooling can be expensive.

Plating
This requires that the plastic is conductive - and with the right materials this can be achieved by an initial electroless plating operation. Parts go through an etch, activation and accelerator to gain a dull metallic finish. This is followed by an electrolytic plating process to create a bright chrome or other effect.

Vacuum Metalising
Typically completed with aluminium, but possible with other metals as well. The process takes place in a vacuum chamber and uses an electrical current to vapourise the metal being deposited. Layer thicknesses are typically a few micron and a range of finishes are possible.

Painting
A highly effective post treatment that is very suited to low volumes, but with the proper production equipment can be applied to higher quantities. Different paint systems are available from simulated mirrored chrome to metallics and metal flake effects.

Hot foil printing
This is usually applied when localised areas require a metallic finish. Heat is used to deposit the metallic finish from a foil onto the component.

Tim Plunkett is director of Plunkett Associates.


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