STREAM FINISHING - the green alternative to chemically accelerated processes

Now we know the criteria, we need to identify which process comes out top where it really matters. One option is manual or mechanical robotic smoothing and rounding. Here the processing quality through-out the tool’s life cycle does not remain constant, as progressive wear on the grinding wheel or brush re-sults in an ever more imprecise surface finish over time. Both methods produce very uneven results when processing inner geometries because the tools can accommodate different components only to a very lim-ited extent, and are difficult, if not impossible, to position in tight places. Components with very narrow tol-erances lay bare the limitations of these methods, which cannot process them effectively. Current technology often favours trough vibrators for processing. Using certain acidic process liquids can help to reduce process times. The types of liquids used include mixtures containing phosphate salts, phos-phoric acid, oxalic acid, sodium oxalate, sulphate, sodium bicarbonate, chromate, sodium chromate, chro-mic acid and sulphuric acid. Most of these substances are hazardous and employees require special pro-tective equipment when handling them. (Sodium) chromate and chromic acid have been classed as sub-stances of very high concern by the REACH regulation (Registration, Evaluation, Authorisation and Restriction of Chemicals) as they are carcinogenic, mutagenic, and in the case of sodium chromate, toxic to reproduction. Along with sulphuric acid, oxalic acid and phosphoric acid, they are also skin sensitis-ing and, in some cases, toxic. For this reason, the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) has listed the following properties for most of these substances:

• Acute toxicity
• Hazardous to aquatic environment
• Various other health hazards
• Skin irritant that is corrosive to metal and may cause serious eye damage

Whilst this process might tick all the boxes in terms of finishing requirements, the process liquid can gen-erally only be used once and must be replaced once processing is finished. This results in increased pur-chasing, neutralisation and disposal costs, meaning the process is far from cost-effective. As we have al-ready seen, the use of these hazardous substances also brings with it health and safety and environmental risks. 

Dry electropolishing processes are another option available on the market for polishing metal workpieces in a single operation. They use electrolyte granulate to reach and process even inner geometries. However, not all granulate is suited to the workpiece shape, and electrolyte can get jammed in the workpiece. One of the biggest drawbacks is that this electropolishing method cannot remove the waviness caused by the forming process of grinding as its abrasion level is simply too low. The average process time ranges from 30 minutes to several hours. What is more, by nature of the process, only the exposed areas can be properly finished. The electrolyte granulate must also be disposed of as hazardous waste once it has reached the end of its given service life, and buying new granulate to replace it is an additional financial burden.