Finishes, Coatings and Surface Treatments

Surface preparation

Many finishing processes depend on the condition of the surface beneath them.

Oil, oxidation, scale, dirt, scratches or previous coatings can affect the result.

Surface preparation may involve cleaning, sanding, polishing, blasting, chemical treatment or other processes depending on the material and desired finish.

A coating cannot necessarily hide poor preparation.

In some cases it makes defects more visible.

Powder coating

Powder coating applies a dry coating material to a prepared surface, commonly using electrostatic attraction.

The coated part is then heated so the powder flows and cures into a continuous finish.

Powder coating is widely used on metal products because it can provide a durable and attractive surface in many colours and textures.

The coating has physical thickness.

That matters around:

• close-fitting joints;
• holes;
• threads;
• bearing surfaces;
• mating components;
• precision interfaces.

A designer may therefore need to decide which areas are coated and which are masked.

Anodising

Anodising is commonly associated with aluminium.

Unlike ordinary paint, anodising is an electrochemical process that modifies and thickens the naturally occurring oxide layer at the surface.

The resulting surface can improve corrosion resistance and wear behaviour and can also be coloured.

Because anodising modifies the surface rather than simply covering it with conventional paint, designers and fabricators need to consider it as part of the manufacturing process.

Painting

Paint remains one of the most versatile finishing systems.

Different primers and coatings can provide colour, environmental protection, chemical resistance or specialised surface properties.

The appropriate system depends on the substrate and service environment.

As with powder coating, paint adds material to the surface and may need to be kept away from precision interfaces.

Polishing

Polishing improves a surface primarily by mechanically refining it rather than applying a separate coating.

Successively finer abrasive processes can reduce visible scratches and produce anything from a smooth satin finish to a highly reflective surface.

Because polishing removes small amounts of material, aggressive polishing can affect sharp edges, fine features or dimensional tolerances.

Blasting

Abrasive blasting directs particles against a surface.

It can be used to clean material, remove coatings, create a uniform texture or prepare a surface for another finish.

Different blasting media produce different effects.

Blasting therefore becomes another manufacturing operation rather than merely cosmetic cleaning.

Plating

Electroplating and related processes deposit a layer of another material onto a component.

Plating may be used for corrosion protection, appearance, electrical properties, wear resistance or other specialised purposes.

Like other coatings, plated layers have thickness and can affect dimensional interfaces.

Laser marking and etching

A laser can also be used as a finishing or identification process rather than simply as a cutting machine.

Depending on the material and laser process, the beam may remove surface material, alter an existing coating or produce a visible change in the material itself.

This allows permanent text, serial numbers, graphics, scales, logos and identification marks to be added without applying a printed label.

The result depends strongly on the underlying material and surface treatment.

A laser setting that produces excellent contrast on anodised aluminium may behave very differently on bare aluminium, painted steel, timber or polymer.

Design for the finished part

A CAD model often represents the nominal geometry of the manufactured component.

The real finished object may also include:

• coatings;
• oxide layers;
• polishing;
• plating;
• surface texture;
• masked regions;
• etched or marked graphics;
• bonded materials;
• assembly interfaces.

That means the designer should think beyond the cutting operation.

A hole that fits perfectly before coating may become tight afterward.

A thread filled with powder coat may no longer function correctly.

A surface intended for bonding may need to remain free of certain finishes.

A laser mark may need a particular coating underneath it to produce the desired contrast.

The manufacturing process is not complete until the part has reached the condition in which it will actually be used.

Bringing the Principles Together

Fabrication is the interaction of geometry with the physical world.

A drawing specifies intent.

A machine provides controlled motion or energy.

A tool performs an operation.

A material responds according to its physical properties.

Workholding keeps the material in the expected position.

The manufacturing process introduces real widths, forces, temperatures, tolerances and surface effects.

Finishing changes the part again.

The operator or manufacturing system has to account for all of them.

This is why good fabrication begins before the machine starts.

The more accurately the design represents what the manufacturing process can actually produce, the more predictable the result becomes.

That is also the reason PRE-FAB pays attention to geometry that may appear insignificant on a screen.

A tiny gap, duplicate line, impossible corner, incorrect scale or unsuitable feature can become a very real problem once software turns that drawing into machine motion.

Understanding fabrication makes CAD easier to understand.

Understanding CAD makes fabrication easier to control.