Requirements on additives for tomorrow’s metalworking fluids
The world’s manufacturing industry is currently facing several radical changes which also have a strong influence on the manufacturers of metalworking fluids, industrial lubricants and lubricant additives. Some of these changes are initiated by upcoming global trends like electromobility, near-net shape machining technologies or the increased use of lightweight design to improve fuel efficiency and reduce greenhouse gas emissions. Others are driven by changes of the base oil market, by an increasing demand for ecologically acceptable lubricants or by more restrictive national or regional legislation. This article gives a short introduction into different aspects of the current and future requirements on additives for metalworking fluids (MWF’s), however without any guarantee on completeness.
Trends in design and manufacturing The broad implementation of electromobility in car industry will lead to a replacement of many chip removing metalworking processes as they are used for crankshaft, camshaft or engine block manufacturing in favor of stamping and fine blanking operations to produce rotor and stator sheets or engine cases for electric motors. Gear boxes may be necessary even in electric cars however they will probably be less complex and producible with lower manufacturing requirements. Whilst the above-mentioned chip removing processes like grinding, milling or drilling are predominantly run by using water mixed metalworking fluids, separating processes like stamping or fine blanking are commonly using neat metalworking fluids based on mineral oils or solvents. The most important properties of stamping and fine blanking fluids are the prevention of cold welding and the reduction of tool wear. Thus they require high concentrations of suitable extreme pressure and anti-wear additives.
By the introduction of lightweight design, several new materials like high-strength steels, special titanium-, magnesium or aluminum alloys but also fiber-reinforced composites like carbon fiber reinforced plastics were established. Most of them require new or adapted machining techniques. The machining of composites by means of traditional ways for instance leads to a lot of defects and surface roughness problems. Therefore finding an effective way for machining and manufacturing of composites will be a major research area in the next years. Also the machining fluids and the additives which are used to optimize the chemical and tribological properties of these fluids have to be selected according to their influence on the machining processes and materials which have to be machined.
In near-net shape industrial manufacturing techniques, the initial production of a part is very close to its final shape. Thus, metal removal processes like cutting or grinding are substituted by forging, casting or forming. This switch to metalworking processes with higher operating temperature and/or higher pressure and mechanical stress creates new demands on
metalworking fluids, like improved thermal stability and enhanced extreme pressure properties.
Furthermore, the trend to increase the productivity of industrial machining processes is still continuing. Beneath organisational measures like changes in working procedures, this can be achieved by increasing the material removal rate of cutting processes through higher machining speed and / or higher chip thickness. However, also the pressure of the lubricant which is applied to the cutting zone has to be strongly increased to achieve stable process conditions. In modern high performance grinding machines for instance, the achievable cutting speed can be 100 m per second and more. The grinding oil is splashed on the wheel with pressures up to 12,000 kPa (1740 psi). These parameters cause a high turbulence of the grinding oil and a very fine distribution of air bubbles in it. Because of its very poor cooling and lubricating properties the embedded air has a strong negative effect on the performance of the grinding oil. It additionally may cause pump failure due to cavitation. The tanks for the metalworking fluid in modern machine tools however are quite small. They usually contain only 300 to 800 liters. If the air is not separated quickly from the metalworking fluid the oil / air mixture will be continually picked up by the coolant pumps and re-introduced onto the cutting zone. If the air separates fast from the oil it forms foam which has to break fast. Antifoam agents may prevent the formation of foam for a couple of days but because of their high surface affinity they will be filtered out within a short time. Many antifoam agents also have a negative influence on the air release value. The best way to formulate low foaming metalworking fluids with good air release is to select only additives with good foaming and air release properties and to avoid any antifoam agents.
Changing base oil availability
Metalworking fluids can be divided into neat oils that are used without further dilution, and water-miscible concentrates which are mixed with water before use. Depending on the content of mineral oil, water-miscible MWFs are further divided into soluble oils (> 35% mineral oil), semi-synthetics (5 – 35% mineral oil) and synthetics (mineral oil free).
Neat oil metalworking fluids are typically composed of additives and 70% to 99% base oils. According to the American Petroleum Institute (API), these base oils are categorized into the following five categories (API 1509, Appendix E):
• Group I: Solvent-refined base oils • Group II: Hydrotreated base stocks • Group III: Hydrocracked base stocks • Group IV: Polyalphaolefins (PAO) • Group V: All other base oils, e.g. esters or polyglycols In the past, group I oils were by far the dominant base stocks for industrial lubricants and metalworking oils. This has been
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LUBE MAGAZINE NO.139 JUNE 2017
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