Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 27 Aus Wissenschaft und Forschung * Dipl.-Chem; Dr. rer. nat. Heinz Dwuletzki Carl Bechem GmbH, Hagen, Germany, Head of R&D Minimum Quantity Lubrication (MQL) versus Traditional Flood Application: A Break-Thru or only for Niche Applications? H. Dwuletzki* Eingereicht: 9. 3. 2017 Nach Begutachtung angenommen: 1. 8 .2017 Minimalschmierung (MQL) im Wettbewerb zu klassischer Überflutungsschmierung - Ein Durchbruch oder nur eine Nischenanwendung? MQL-Anwendungen, speziell von OEM und deren First Tier Lieferanten haben vermehrt Einzug auf die Shop Floor-Ebene gehalten. Konsequenterweise fügt die letzte Version der DIN 51385 (2013-12) die Gruppe der Minimalmengenschmierstoffe ebenfalls ein. Es wird eine kurze Übersicht der historischen Entwicklung dieses Verfahrens gegeben. Der Vergleich der schmierstoffbezogenen Fertigungskosten zeigt, dass nur ca. 1,6 % der Kosten dem Schmierstoffbezug zuzurechnen sind, während alle weiteren Kosten vorwiegend aus der Anlagentechnik resultieren. Die wesentlichen Applikationssysteme für MQL-Anwendungen werden vorgestellt, wobei 1- und 2-Kanalsysteme auch in der Praxis dominieren. Die Anforderungen an MQL-Medien werden diskutiert und deren Schmierungseigenschaften verglichen. Die Umstellung von klassischer Überflutungskühlschmierung zu MQL-Fertigung wird anhand eines Beispiels der Hochleistungsaluminiumzerspanung im Einzelnen vorgestellt und diskutiert. HSE-Anforderungen, die die Verwendung von MQL- Systemen möglicherweise erschweren können werden ebenso besprochen wie die Nachteile des MQL-Verfahrens. Zusammengefasst kann festgehalten werden, dass beide Methoden ihren berechtigten Platz in der Fertigungskette nebeneinander besitzen und beide Verfahren neuerdings durch die mögliche Einführung von Additiven Fertigungsverfahren zukünftig an Relevanz verlieren können. Schlüsselwörter Minimalschmierung (MQL); Schmierstoffe nach DIN 51385 (2013-12); Historische Entwicklung der MQL- Technik; Applikationssysteme: 1- und 2-Kanalsprühtechnik; Kostenvergleich klasische Überflutungstechnik und MQL bei der Aluminiumbearbeitung; HSE- Anforderungen der MQL-Schmierstoffe; Anforderungen an MQL-Schmierstoffe MQL has made its way onto the shop-floor of many well reputated manufacturers, mainly OEM’s or first tier suppliers to them. As a result, the latest version of German DIN standard 51385 (2013-12) puts MQL lubricants in the same rank like metal removal media, metal forming media and multifunctional fluids. The driving force for the implementation of this new technique has always been the idea of reducing costs of fluid delivery by the use of a consumable lubricant which does not need any surveillance and nearly no field service. As well the desire to reduce the over-all costs of equipment for tank systems, filter units and last but not least cost of disposal for used coolants led many technical universities to evaluate the feasibility of this idea. In the early times of use of MQL-techniques, many obstacles were to overcome, e. g. mainly the questions of application, health and safety issues, danger of exposure to aerosols and/ or mist of coolants. In the mean-time many producer of large volume parts tried the MQL-technology and are able to answer the title question more comprehensively. A detailed comparison between classical, wet cutting machining of an aluminum alloy is given from the field of aluminum machining. The long term experiences of the use of emulsions is described and contrasted to the use of MQL-application in machining of the same parts. Keywords Minimum Quantity Lubrication (MQL); Lubricants acc. to subgroup of DIN standard 51385 (2013-12); Survey of historical approach to MQL; Techniques of application: 1-2 channel systems; Requirements of lubricants for MQL; Cost comparison in aluminium machining; HSE-requirements of MQL-lubricants; Kurzfassung Abstract T+S_1_18 06.12.17 12: 19 Seite 27 28 Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 1 Systematic approach to Minimum Quantity Lubrication (MQL) Following the latest version of DIN 51385 (2013-12) [1] the systematic description of processing fluids for forming and machining of materials includes: • Processing media for machining of materials SC: Cutting operations • Processing media for forming of materials SF: Forming operations • Multifunctinal Oils: MFO • Processing media for Minimum Quantity Lubrication applications: MQL As shown in picture 1 Reasons for the development of this new class of lubricants are manifold [2]: • General demand for the reduction of main costs of production • Legislational aspects • Soft parameters like, image questions of manufacturers and public interest in modern technology The detailed evaluation of cost sources for the manufacturing process show the following distribution: • Main cost drivers are: • Invest costs for equipment • Labour costs • Additional costs: - cleaning of parts - chip handling - controll of emissions In most of the evaluated cases (e. g. mass production of sensitive aluminium parts) it becomes clear, that coolant related costs contribute to approximatively 15 % of the total process costs [2]: See picture 2 The directly related lubricant costs are generally very low by approx. 1,6 % of total process costs only. This could be compared to tool costs of approx. 4 %. It becomes quite clear, that the quality of used coolant directly influences the proportional costs of tools significantly, thus being already 260 % of the lubricants’ costs! Most of the overall costs are based upon the invest of equipment and energy. Consequently the demand was put forward to a lubricant-free machining which turned out to be a „mission impossible“. The Technical Universities of Aachen and Dortmund, Germany underwent a great deal of experiments and evaluations to find out a minimum volume limit of lubricants for machining processes in a „nearly dry state“. From this scientific approach the following definition of MQL was brought to the stage and remains still state-ofthe-art: MQL: Target oriented total-loss lubrication at lowest lubricant volume (< 50 ml/ h) applied by spraying devices in metal working applications 2 Requirements for MQL - application To fulfill the requirements of MQL applications it is obvious that special devices for the appropriate supply of lubricants are needed [3]: In a most simple way to differentiate the application ways the MQ-lubricant could be applied to the out-side Aus Wissenschaft und Forschung Picture 1: DIN 51 385 (2013-12): Processing fluids for forming and machining of materials Picture 2: Coolant related costs T+S_1_18 06.12.17 12: 19 Seite 28 Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 of a tool (direct supply) or internally through the tool [4,5]: This leads to the following survey: Picture 3 External (direct) supply of MQ-lubricant means: - most simple application - best for simple setup change (retrofit) - no change of tool-design - applying the lubricant on the tool - applying the lubricant on the work piece - spraying of the lubricant by specially designed nozzle/ s - positioning of nozzle/ s is most important The more sophisticated internal MQ-lubricant supply needs more care and attention: - direct MQL-supply to the point of action - no adjustment of nozzles - constant supply of MQL - low MQL-losses - adjustable dosage of MQL-air mixture - high costs of implementation to existing machines - use of tools with internal coolant supply - a must for drilling with high l/ d-ratio: mainly deep drilling (gundrill) applications For the dosage of MQ-lubricants exist as well different systems: See picture 4 The easiest way of spraying the neat lubricant directly to the tool normally results in much higher consumption and does not allow to use the lowest amount of lubricant to apply. Better results are achieved by mixing the MQlubricant with air for dilution and transportation to the point of action. These pressurized spraying systems transport the media through the spindle of the tool machine by a socalled one-channel or two-channel application technique. 1 channel-systems: The externally prepared MQL-aerosol is transported through the tool and in 2 channel-systems: Air and MQL are transported separately through the tool and merged together right before the point of action of the tool Based upon the results in practice it is worth knowing the differences in application: 1 channel-systems: • lubricant feed depends on rotational speed N • N max. 16.000 rpm • max. feed depends on air / bore size • slower reaction time (tool change) • MQL-viscosity: max. 50 mm 2 / s • min. air pressure > 5 bar • often used in transfer lines lower MQL-volume required (max 200 ml/ h) and 2 channel-systems: • lubricant feed is not depending on rotational speed N • N max. 40.000 rpm • Not depending on air / bore size • Very fast reaction time (tool change) • MQL-viscosity: max. 100 mm 2 / s • Min. air pressure > 4 bar • Mainly used on machine tools (CNC-Centres) • Higher MQL-volume required (> 400 ml/ h possible) 29 Aus Wissenschaft und Forschung Picture 3: MQL-supply systems Picture 4: Dosage systems for MQL application T+S_1_18 06.12.17 12: 19 Seite 29 30 Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 3 Requirements for MQ - Lubricants The comparison of the results of 1to 2-channel spraying systems indicate that some requirements to MQ-lubricants do exist [5, 6] e. g.: Basic formulation: • similar to non-water soluble lubricants • Non-watersoluble base fluids • low mist and vapour emitting fluids • most frequently used base fluids: alcohols and esters • Viscosity: V 40 °C =104 F > 20,5 mm 2 / s • Flash point (COC): > 150 °C = 302 F • Evaporation loss: < 65 % acc. to Noack-test (250 °C = 482 F) On the other hand some restriction within the use of fluids and additives are known as well: Less suitable lubricants: • water miscible MRF (concentrate) • water diluted MRF • non-treated bio-esters (e. g. rape seed oil): due to glueing, staining and residue formation of untreated natural esters on parts or/ and machine components • smelly lubricants • low viscous, mineral oil based lubricants: danger of ignition and fire hazard Of course, so far the question of the most interesting characteristics of a lubricant hasn’t yet been touched: The inherent power of lubrication. Not really surprising it was shown on all conventional frictional lab testers that MQ-lubricants behave as „normal“ lubricants, but the most challenging test as always is test in the field. See picture 5 4 Results from the field In various applications on steel workpieces the following data were gathered: See picture 6 Machining results of steel parts with MQL-application [5] The data clearly show that MQL-techniques offer a new pathway for the manufacturing of steel parts mainly used in the automobile industry. Aus Wissenschaft und Forschung Picture 5: Tribological test methods for MQ-lubricants Picture 6: Machining results of steel parts with MQLapplication [5] T+S_1_18 06.12.17 12: 19 Seite 30 Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 In the case of aluminium machining the corresponding data result for the most known applications like a the machining of cylinder heads: See picture 7 Machining results of aluminium cylinder heads with MQL-application [5] For an informative comparison the machining of aluminium valve body plates, the most critical parts of an automatic transmission will be discussed in detail: • Disadvantages of MQL: • new machine technology needed • high cost of air supply and emission control • high cleaning costs of machine and parts Another aspect definitively needs attraction as well: The health and safety aspects have always being critically discussed due to the danger of aspiration of aero-sols and mist of MQL-application. So another point needs more interest: • Disadvantages of MQL: • HSE-requirements • CLP-problems of spraying usage due to European Regulation of chemicals: 31 Aus Wissenschaft und Forschung Picture 7: Machining results of aluminum parts with MQL-application [5] Picture 8: Comparison between conventional flood coolant and MQLapplication in aluminum machining See picture 8 Comparison between conventional flood coolant and MQLapplication in aluminium machining 5 Minimum Quantity Lubrication versus Traditional Flood Application The above discussed change from traditional flood application to MQL-technique allows a general judgement: • Advantages of MQL: • less space requirements • less costs for coolant supply, filtering • less water, energy consumption and T+S_1_18 06.12.17 12: 19 Seite 31 32 Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 • Substances need to be registered for specific applications, e. g. spraying • 31.05.2018 REACh 3 rd. Restristration deadline is valid for registration • So far, only very little substances are registered for the application of spraying (PROC 7 and PROC 11: Industrial and non-Industrial spraying) • This leads to the demand of further raw material testing by the additive suppliers • For safety evaluations, the inhalative toxicity of raw materials need to be tested It seems to be sure that in future MQ-lubricants need to be tested for the inhalative toxicity. 6 Summary: MQL - From niche application to mass production MQL-techniques had been looked at as a „nice technology“ without any real relevance in practical application. This was state-of-the-art till the end of the 20 th century and in many heads it is still the case. But in reality within the last 10 years many big shifts from classical flood coolant application to MQL could be watched on the shop floor and clearly indicate the potential for mass parts manufacturing: See Picture 9 MQL making its way from single machine to general shop floor application Finally, there is no one-way to decide between MQL and flood application, both technologies have their benefit and will be used side-by-side in future manufacturing. It is not a nor or neither decision it will be a together! 7 Literature [1] DIN 51385 (2013-12), Processing fluids for forming and machining of materials, definitions [2] Trockenbearbeitung und Minimalmengenkühlschmierung, K. Weinert, Springer Verlag 1998 [3] GfT Gesellschaft für Tribologie e.V.: Arbeitsblatt 8.1 und 8.2 Minimalmengenschmierung; 2003 [4] Fachausschuss Maschinenbau, Fertigungssysteme, Stahlbau: Low emitting metal working with minimum quantity lubrication, Sheet No. 6, 07/ 2005 [5] DGUV Deutsche gesetzliche Unfallversicherung: Minimalmengenschmierung in der spanenden Fertigung; BGI/ GUV-I 718, 11-2010 [6] H. Sefrin, Minimalmengenschmierung. Einsatz in der Praxis, Tribologie und Schmierungstechnik, 3/ 1999, Nr. 46 S. 9-14 Aus Wissenschaft und Forschung Picture 9: MQL making its way from single machine to general shop floor-application Anzeige Nutzen Sie auch unseren Internet-Novitäten-Service: www.expertverlag.de mit unserem kompletten Verlagsprogramm, über 800 lieferbare Titel aus Wirtschaft und Technik T+S_1_18 06.12.17 12: 19 Seite 32