Surface Treatment & Finishing

Surface Treatment & Finishing

Galvanising

Galvanising image

Diagram is the same for the first 6 steps really

Don't ignore step 7, it is still required and is important

The flux step is not always required but you might as well put it in if you can remember it

The purpose of hot dip galvanising is to protect steel from corrosion

Advantages: Produces a very tough and corrosion resistant surface finish that is long lasting

Disadvantages: Very high initial cost for the equipment, long process, only suitable for large products e.g. lampposts, limited materials, requires trimming and inspection etc


Anodising

Anodising image

Similar to electroplating, but things are different. This time, the workpiece is the anode.

Anodising has 2 cathodes (the coating) and one anode (the workpiece)

Also anodising uses an acid electrolyte solution


Grinding

Abrasive wheels cut away the surface of the metal, leaving a shiny/scratched surface. Takes off a lot of material and doesn't give the best surface finish


Polishing

Provides a better finish than other methods, makes the material shiny, looks better than grinding

Can be done with a cloth wheel or a cloth, sometimes uses metal polish

Provides a smooth finish


Painting

As engineering A level is about working with metal, this is the process for painting metal:

1) The surface must be cleaned completely first. No dirt, dust, grease, grime or corrosion can be left on it.

2) The metal must then be sanded down so it is as smooth as possible. Remove dust once done.

3) Apply a coat of primer to the metal, leave to dry.

4) Apply another coat of primer, leave to dry.

5) Apply a coat of paint, leave to dry

6) Apply another coat of paint

7) Leave to dry for 36-48 hours before using


Electroplating

Electroplating image

A tank is filled with an electrolyte solution. This solution allows electricity to pass through it

The anode (positive) and cathode (negative) are put in the solution and connected to the power supply, linked to the positive and negative terminals accordingly

When the power supply is switched on, a current passes through the electrodes and the solution. The positive anode (the material that is being used to coat the object) loses its metal ions

The positive ions (anions) pass through the solution and are attracted to the negative cathode

The anions bond with the cathode, and so the cathode (negatively charged object being electroplated) gets covered with the metal

Advantages:

Protects the object from corrosion, makes it aesthetically pleasing

Disadvantages: Can only be used with metals, can only be used on small scale products, it is mainly for aesthetics and so the surface isn't durable, it is too expensive and requires too much energy to do on a large scale


Plastic Coating (Dip coating, powder coating)

Galvanising image

Used for metal. Workpiece is cleaned to remove dirt and grease.

It is then placed in the oven and heated to between 250 and 400 degrees celsius. Removed from oven when it is hot enough.

The fluidising tank (fluidiser) is filled with plastic powder and the blower is turned on which blows low pressure compressed air into the tank from the bottom

The air blows through the tiny holes at the bottom of the tank (porous ceramic tile) into the plastic particles and they appear to be fluid and moving around.

The hot workpiece is dipped into the plastic on a hook and left for a few seconds before being removed. The plastic powder melts onto the hot metal

The workpiece is hung up to cool

Advantages: No waste, tougher finish than spray paint, makes material corrosion resistant, very aesthetically pleasing

Disadvantages: High initial cost for the machine, only provides a thin coating that isn't very durable, frequently doesn't work perfectly, limited range of materials, can't use on products with holes as it covers everything


Self-finishing

Self finishing materials are (obviously) materials that don't require any further surface finishing from their raw surface finish

Examples include non-ferrous metals that won't corrode, such as aluminium and brass. All plastics are self finishing. So are composite and smart materials