Quotes for anodising can vary a lot. Understanding why helps you specify the work accurately and then compare suppliers on an even basis.
Alloy Grade and Oxide Quality
The alloy you are going to anodize can produce vastly different results in terms of the finish that is produced. High silicon casting alloys (LM25 being a typical example) can produce a grey / mottled finish, which can limit the range of dye colours that can be used to colour the anodized surface. The process parameters for the anodizing bath may also have to be adjusted to try to produce a consistent finish. Wrought alloys (6082 for example) anodize in a much more predictable manner, and are generally the cheapest to anodize due to the aforementioned reasons.
Part Geometry and Racking
Parts with deep recessed areas, blind holes or with very thin walls require special jigs. These jigs are set up before the processing of the first part and therefore increase the processing costs for such parts. Simple flat or cylindrical parts are generally cheaper to anodize than parts with unusual geometries.
Specified Coating Thickness
Sulphuric acid anodising is specified by thickness between 5 and 25 microns. The thickness specified for anodising has a direct bearing on the amount of time an item will spend in the anodising bath. For example, processing to 25 microns may take double the time to process to 5 microns thickness. The closer the specified thickness tolerance then the closer the process needs to be monitored to achieve the required thickness.
Batch Size and Rack Utilisation
A rack that is filled to maximum capacity will receive lowest charge per part compared to an empty rack or partially filled rack. It is therefore beneficial to place orders in such a way that complete use is made of the available space on the racks.
Sulphuric acid anodising is covered in more detail at https://www.poeton.co.uk/surface-treatments/anodising/sulphuric-acid-anodising/.
Pre-Treatment Steps
Chemical etching or brightening of the surface of an item prior to anodising is generally charged as a separate process. A heavy etch removes more material, and can cause an item to shrink in size. A light detox and desmut is a fast process and therefore is cheaper.
Sealing and Dyeing Add-Ons
It’s best to price up sealing and dyeing as separate items as they will usually cost a lot more than the anodising and can vary a lot depending on the colour required and the final finish required. For example an un-anodized coating offers little corrosion resistance and is only suitable for applications where the item will not be exposed to the elements. A hot-water seal or a mid-chrome seal can provide a good level of corrosion resistance and are often specified as an additional process.
Knowing what to specify for anodizing and having it quoted accurately will also save you from potential rework charges later.