By Bernd Duchstein and Bill Clancy, Institute for Machine Tools and Factory Management, Berlin Institute of Technology
A machine tool viewing panel. On the left of the window the impact of a chuck jaw can be seen.
Photo: SEGE viewing panel Gmbh & Co. KG
The use of polycarbonate in safety installations, for example as viewing panels in machining centers, has increased rapidly with the advance of high-speed machining and is now a standard feature of the field. The positive characteristics of this material are its high ductility, and with that its strong impact resistance. The absorption of the impact energy takes place mostly in a large elastic deformation, but also partly in a smaller plastic deformation. However, under the influence of certain processes, the material ages quite rapidly and can thus carry a significant danger for the user.
Currently polycarbonate is the only known transparent material of which a single panel is strong enough for use in safety screens without the need to mix materials or create a sandwich structured panel with several sheets. Due to its excellent penetration resistance a 12-mm sheet of polycarbonate can withstand the impact test DIN EN 12417 that has a 100-g projectile and a force of 1,125 Nm, whereas a similarly thick piece of polymethylmethacrylate (PMMA) could only withstand 31 Nm1. In 2008 the Institute for Machine Tools and Factory Management (IWF) began running trials on polyethylene terephthalate (PETE), reinforced PET, acrylic glass, and PMMA to investigate whether additives yield significant improvements. The additives were not found to have a significant effect, and so the use of polycarbonate is still the sensible solution.
Effects of aging
Over time the material’s strong resistance to penetration decreases due to the harsh environment of the machine tool. Aging is caused by UV light, the emulsion of coolant and water, improper cleaning, and the addition of harmful additives — which increase tool life and productivity, and give a better surface finish, but are harmful to polycarbonate. The aging leads to a significant reduction in penetration resistance, thus putting machine operators in danger. Aging causes the panels to lose their ductility, and so to break with a fraction of the force of a new panel (Fig. 1).
|Figure 1. A) High-speed photo shows how the projectile penetrates aged polycarbonate, but B) a similar view demonstrates how a new polycarbonate panel prevents penetration. |
Since the end of the 1990s, the IWF has been trying to solve the aging problem caused by the cutting fluid/water emulsion. Initial research into the problem was carried out by the BGIA Institute for Occupational Health and Safety and the German Machine Tool Builders Association (VDW)2, from whom we take the first aging graph (Fig. 2). Further testing has been carried out by Bold, which confirms the results in general3. The procedure for these tests involved rapid aging of the plates using a dip tank with the cutting fluid/water emulsion.
The IWF is currently running a project aimed at integrating the effects of aging of polycarbonate into an FEA impact simulation. For this, polycarbonate plates are to be aged both chemically and through thermal oxidation. The aging process aims to give a closer replication of the real working conditions of polycarbonate viewing plates used in industry. The plates are kept at 50ºC and an 8% coolant emulsion is sprayed on one side of the plates for one-minute periods, with one-minute breaks in between. The sheets are taken and analyzed at regular intervals so that records can be made about the effects of short and long-term aging.
The impact tests to determine the decline in resistance to penetration will be carried out in line with the standards set out in DIN EN 12415 (with a 2.5-kg projectile) and DIN EN 12417. Furthermore, finite element models were created to simulate the impact behavior of new and aged polycarbonate sheets. The material behavior in the model is characterized by linear elastic and visco-plastic properties of deformation, damping parameters and failure criteria. Then the aging conditions can be added to the model from a material profile in the form of dynamic flow curves and failure strain4.
|Figure 2. The effect of aging on the penetration resistance of protected and unprotected polycarbonate panels. |
Although the initial research occurred quite a long time ago and there has been no fundamental changes in the area, it is now more relevant than ever. This can be seen especially in the not yet published industry standard for machine tools safety in turning machines (DIN EN ISO 23125) in which an aging curve has been included after international discussions with standards committees and requests from both industry and research institutes.
The solution to the problem of aging due to UV light, coolant emulsion and improper cleaning is quite simple: protect the plates. Protecting the plates, however, is quite a complicated process. The 2002 paper from the VDW, “Polycarbonate viewing panels in machining centers,” described a simple hard coating, however this coating is not as effective as it is widely believed to be 5. To get a durable and long lasting protection from PC in this environment you need to protect it with a laminate, glass or surface coating. This information is taken into account in the new standard for safety of machine tools (DIN EN ISO 231125), whereas in the old standard this information was not considered.
By protecting the PC sheets you can keep the desirable properties, i.e. ductility and resistance to penetration, while allowing it to operate in the machining center environment without aging. Multi-layered structures that have a combination of glass and plastic layers also can be used. If a multi-layered structure is being used, polyethylene terephthalate can be used instead of polycarbonate. Producing the multilayer structured panels is very complex, so they should be produced only by experts.
Despite these protective measures there can still be a risk if highly reactive additives are used in the coolant. Although manufacturers give detailed instructions on which coolants and cleaning agents can be used, many operators will not follow these guidelines. The consequences of this are varied: from corroded hoses to machinery rusting, to the liquefaction of viewing panel seals. The use of these harmful additives leads to a shorter operating life of the machine or, with respect to the liquefaction of viewing panel seals, significant danger for the operator. As many operators are unaware of the long-term effects of using these additives, action must be taken to increase operator awareness of the problem. In order to increase awareness and to develop solutions for the safe use of polycarbonate, the IWF at the Technical University Berlin and the Fraunhofer-Institute for Production Systems and Design Technology are conducting both fundamental and industry related research.
The authors are research engineers at the Institute for Machine Tools and Factory Management, Berlin Institute of Technology.
Citations:  DIN EN 12417: Werkzeugmaschinen – Sicherheit – Bearbeitungszentren. March 2007.  Mewes, D.; Trapp, R; Wahrlich, H.-J.: Trennende Schutzeinrichtungen an spanenden Werkzeugmaschinen. Dsseldorf, Sonderdruck fr die Vereinigung der Metall-Berufsgenossenschaften, 2005  Bold, J.: Trennende Schutzeinrichtungen fr Werkzeugmaschinen zur Hochgeschwindigkeitsbearbeitung. Dissertation from the IWF at the TU Berlin, 2004  Uhlmann, E.; Duchstein, B.: FEM- Aufprallsimulation fr die sicherheitsgerechte Entwicklung von Werkzeugmaschinen. In: wt-online. 98 (2008) Nr. 7/8, P. 533-537 Wrz, T.; Kuhnmnch, K.-P.; Mdden, H.: VDW-Merkblatt “Polycarbonat-Sichtscheiben in Werkzeugmaschinen,” Frankfurt am Main, Verein Deutscher Werkzeugmaschinenfabriken (VDW) e.V., 2002.