Designing for Electroplating and Anodising
For optimum performance, consistency and efficiency of processing, it is advisable for designers and engineers to consider the factors which will have an effect on electroplating or anodising.
Engineers and designers will frequently select a type of surface treatment for its desired properties with little idea of how that coating is obtained. However, some knowledge of plating processes can help inform design decisions to give a better final outcome.
All parts for electroplating or anodising need to be held or suspended during processing. This provides the dual function of holding the component and providing electrical contact.
As the anodised layer is electrically insulative, the contact point must remain free from anodising. We will keep these contact marks as small and inconspicuous as practicable, but there are limitations on what is possible whilst holding the part securely. Any instructions about where contact marks are not permissible must be communicated via the drawing or purchase order.
Sometimes it is necessary or desirable to add additional tooling holes to the design of a part specifically for jig contact so that anodising can be continuous over all other surfaces and the part can still be held securely.
We use a number of different designs of jig, including several standard metric thread sizes, and we would be happy to advise on the jigging options for a part during its design. Parts can also be held using wires.
Airlocks and Cavities
The geometry of parts for electroplating or anodising can lead to airlocks, where air (or gas evolved during processing) cannot escape. This can prevent the formation of anodised coating, limit coating thickness and inhibit dyeing.
Another issue which can be caused by the geometry of parts is entrapment of chemical in cavities during processing. This can cause severe quality issues, such as staining or corrosion, as entrapped chemical seeps out slowly after processing. Entrapped chemical could also react adversely in subsequent chemical treatments, leading to material damage. Rinsing at every stage in the process is a key factor in achieving a high quality finish and part of our standard process, but sometimes parts are designed in such a way as to make effective rinsing impossible.
For the avoidance of both airlocks and entrapment risk it can be advantageous to add drainage (or air escape) holes to the design of a component. The positioning of drainage requires knowledge of jigging methods – please email us with drawings if you require advice.
Blind holes, spot welds, folds and inserts also pose a risk of chemical entrapment and it may be wise to consider whether these features can be avoided or added at a later stage (after electroplating or anodising).
Unless they are affected by an airlock, blind holes or deep bores will generally anodise well. Electroplating however has a limited “throw” into blind holes and recesses and may require special jigging and an internal anode if plating is required in these difficult locations. Again, please contact us with drawings if you have any special requirements that you would like advice or costings for.
Incomplete welds pose a high risk of chemical entrapment as a very small cavity is created which can prove impossible to rinse and dry effectively. When welds completely enclose a large void this can also be a source of problems if the entrapped air causes the part to float in our processing tanks!
It is necessary for any weld flux to be removed mechanically before items are sent to us for treatment. Pretreatment solutions are not designed to remove this type of contamination and it is not possible to satisfactorily electroplate or anodise over/through the weld flux.
Heat migration around welds can cause local alteration to the microstructure of the base material and this can result in uneven colouration of the coating in this area.
See also: Welding Zinc Plated Components.
Sometimes the function of a part requires that a certain area is free from protective treatment, either for reasons of dimensional tolerance or electrical conductivity for example. This can be achieved by a number of different methods, selection of which is dependent on the cost or practicality of the method.
|Customer locally removes coating after treatment||Cost effective.||Not possible for more complicated geometry.|
|Masking||Flexible – there is a suitable method for most parts.||Will increase cost of processing.
May increase turnaround time.
Not suitable for all processing solutions.
Choice of base material
See also: Aluminium Alloys For Anodising
The condition of the base material will have a strong influence on the appearance of the coated part. Anodising or electroplating will not hide scratches, pits or marks in the base material. Any mechanical finishing should be carried out before submitting parts for coating. It is important to note the potential for introduction of foreign materials to the base metal, for example if dirty linishing belts are used, as this can have a very detrimental effect on anodised coatings. Separate linishing belts should be used for ferrous metals and aluminium to avoid contamination issues.
Inserts of different materials can cause severe problems. For example, steel inserts in an aluminium part which is to be anodised will be attacked with such ferocity by the processing solution that the surrounding aluminium can be damaged as well as the insert being completely destroyed!
Mixing different alloys in an assembly which is to be anodised should be avoided. Please let us know if a batch of work brought in for treatment contains different alloys.
Electroplating is a deposit on the surface of the base material, so changes in dimensions of the plated part can easily be calculated. For example, an internal diameter will decrease in size by twice the plating thickness.
An anodised coating extends into the base material as well as growing from the original surface. For most alloys, the anodised coating is approximately 50% ingress and 50% growth. The growth of coating from the original surface is therefore half the stated coating thickness. Internal diameters will get smaller by approximately the coating thickness and external diameters will grow. You can use our anodising growth calculator to estimate the size of a feature after anodising. We recommend that you let us check your calculations.
Anodised coating thickness may vary dependent on alloy and treatment specification. Typical coating thicknesses are as follows:
Up to 5µm for chromic acid anodising
5 to 15µm for clear (natural) sulphuric acid anodising
10 to 25µm for dyed (coloured) sulphuric acid anodising
20 to 70µm for hard anodising, with thicknesses up to 100µm achievable on some alloys.
More Info and Specialist Advice
Please email us if you would like advice on designing for electroplating or anodising. This page just gives general suggestions and our specialists will be able to provide more in-depth information.
This page is provided for information only, it should not be considered advice and we cannot accept any responsibility or liability for your use of the information on this page. The information on this page is used and relied on at your own risk and you bear the sole responsibility for any outcomes. E&OE.