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, Volume 10, Issue 5, pp 48–53 | Cite as

New type of extraction system for 5-axis machining of composites

  • Andreas Gebhardt
  • Peter Miller
  • Andre Schulte-Südhoff
  • Michael Hauck
Production Machining Of Composites
  • 188 Downloads

Machining fibre-reinforced plastics and wood products releases large amounts of chips and dust that need to be reliably extracted. Conventional systems of extraction often fail in this task. Fraunhofer IPA and Schuko developed a new approach that combines blowing and extraction and enables maximum collection rates — even for 5-axis machining.

Machining as an Essential Step in Adding Value

Machining is an essential step in the value chain when manufacturing components from fibre-reinforced plastics (FRCs) or composite wood products. Despite near net-shape production, FRCs often require edge trimming to remove resin protrusions and to meet dimensional and form requirements. Further processing steps include drilling and cutouts as well as preparing fitting surfaces [1].
Figure 1

Particle deposits during CFRP machining (© Foto: Fraunhofer IPA; Fotograf: Rainer Bez)

In wood processing, it is mainly sheet workpieces that are machined to trim edges or divide up in a nesting process. Complex components requiring 5-axis machining can be found in the construction of windows and doors as well as in the areas of stairs and moulded parts.

Different Challenges but the Same Need for Extraction

Common to both groups of material is the need to collect the particles that result from the process in a reliable manner in order to eliminate any risk for humans or machinery and to ensure the safety of this production process.

FRC machining causes predominantly fine particles in the range of 1 μm to 1 mm. This means that a large part of the particles can be inhaled and can sometimes penetrate as far the pulmonary alveoli. Workplace exposure levels must therefore be complied with and personal protective equipment must be worn in the event of any direct contact with these particles [2]. With carbon-fibre-reinforced plastics, there is the additional aspect, that fragments of fibre have very good electrical conductivity and can therefore damage electrical or electronic component assemblies. In addition, the fibres, like glass fibres, are very abrasive and result in increased wear as a result of dual chipping on the tool or other friction counterparts, for example in guides and bearings.

Machining wood and composite wood products results in comparatively larger particles with a certain proportion of fine particles present. These primarily comprise hardwood dust that, like beech and oak dust, can be viewed critically, as they are classified as carcinogenic.

Extraction Technology for 5-axis Machining often Deficient

Various extraction systems are used to collect particles. In wood, processing the use of extraction systems — located near the tool in the form of extraction hoods enclosing the machining area — is widespread. They encircle the spindle and seal the machining areas from the workpiece using fins or brushes. The main job of the brushes is to slow down the accelerated, high-mass particles so that they can be drawn off by the air current in the extraction hood. This works particularly effectively with sheet workpieces. It is usually not possible to form this seal in 5-axis processing so that there is a reduction in rates of collection and considerable deposits of particles on components and in the machine as a whole.

High collection rates can only be achieved with high effort on the extraction system.

The forms of components made from fibre-reinforced plastics are usually complex, which makes the reliable extraction of particles significantly more difficult. Suction systems are very often used that require high-volume airflows to vent the entire processing machine, which, however, results in high power consumption. The ventilation openings or hoods are usually located at the edge of the enclosure, meaning that, in contrast to extraction hoods located near the tool, there is little risk of collision with the workpiece or the machine. However, these systems often only allow fine, airborne particles to be collected so that a large proportion of the particles settle on the component, on the jig or in the machine.

High rates of collection can be achieved if extraction is integrated into the jig. In this case, the particles are extracted along the milling path or around openings, and drill holes via ducts or bays in the jig. However, the manufacturing costs for such systems are several times higher than for conventional jigs, meaning that they are primarily used in large-scale production.

As in wood processing, extraction hoods can also be found that enclose the spindle and sometimes only the tool. As FRC components are generally complex structures, NC control is required to adjust the height of the hood in order to avoid collisions as well as to allow the particles to be collected. Owing to the increased programming effort involved, these systems are also mainly encountered in large-scale production.

Suction systems with high power consumption and generally low particle collection rates are thus the principal form used in industry — as a result of the high investment and set-up costs. Nevertheless, uncollected particles are not just a health and technical risk; they are also a significant cost factor. The processed components have to be manually cleaned following machining, which is a time-consuming process. And then there is the cleaning of the jig for the next components as well as the cleaning of the entire system at the end of the shift. All in all, cleaning work often accounts for a considerable proportion of total working time.

New Type of Extraction System for 5-axis Machining

In the past, various research projects looked into the optimisation of particle collection for wood and other composite materials. Common to all projects was the aim of improving collection near the tool. For example, Dressler [3] worked on the numerical simulation of air currents and particle trajectories in extraction hoods. Blecken [4] conducted studies into the design of extraction hoods and developed a prototype for the 3-axis processing of composite wood products. The Institute for Machine Tools (IfW) at the University of Stuttgart tested particle and dust collection using a tornado-like airflow [5]. However, the idea proved to be unsuitable for the removal of particles.

Only a few projects look into room extraction, which is widespread in FRC machining. Deserving mention here are studies by Hesel [6, 7] that deal with the use of blower air currents aiming at the sedimentation of particles in the processing machine. In this process, air is drawn off from the work room and used as targeted blower air in order to move particle deposits on the workpiece or on flat surfaces of the machine tool, for example towards a particle conveyor.

This process makes use of the effect that a current of blower air has 30 times the range of a current of extracted air [8]. The reason for this is that a current of extracted air causes a falloff in pressure around the extraction opening (extraction hood), and air flows there from all directions in the room. Blower airflows remain focused over a longer distance, owing to the effects of mass inertia. This can be seen in Figure 2.
Figure 2

Diagramme of the range of suction air and blower air currents based on the example of a radial fan [7] (© Fraunhofer IPA)

In the extraction area, the flow speed vs at the simple distance of a nozzle diameter D decreases to a tenth of the value directly at the nozzle (v0). In the blower area vg, this value is reached at a distance of around 30 times the nozzle diameter D.

Extraction alone is therefore not suitable for collecting particles effectively using suction, since the airflow speeds are too low for particle removal, owing to the large distance between the extraction hood and machining point. For this reason, a new type of extraction system was developed, relying on a combination of blower air and suction in a two-year, publicly funded research project carried out by Schuko and the Fraunhofer IPA. The idea behind it is that, in a systematic closed-loop of extracted air, particles are transported towards the extraction hood by means of blower air, where they are drawn off. The system, for which a patent is now pending, was christened ARS (air return system).

A current of blower air has approximately 30 times the range of an extraction air current.

In contrast to the studies conducted by Heisel [6, 7], the blower air is taken from the flow of extracted air and not through additional extraction openings in the ceiling of the work room. This increases the suction output and ensures a targeted current of air through the work room. The process air is cleaned of a large proportion of the particles in an initial filter step. With large particles of wood, this can be effected, for example, with a cyclonic separator, or with fine FRP dust using an M-class baghouse dust collector. After filtration, the airflow is split: around 80 % of the extracted air is used as process air, while the remaining air is led to a fine filter and subsequently released outdoors or in the production hall. The difference between the air extracted from the work room and the blower air results in slight underpressure, which causes fresh air to flow in from the production hall and prevents dust-laden process air escaping. If personnel need to enter the work room — for example to change the workpiece — the flow of process air is stopped and all the extracted air is led to the fine filter. Figure 3 shows the schematic layout of the extraction system.
Figure 3

Diagramme of the airflow of the ARS (© Fraunhofer IPA)

Compared with current technology, the extraction system offers two main benefits: improved particle collection and significant energy savings compared with traditional work room extraction.

By the new extraction system, manual cleaning work and fine dust emission could be significantly decreased.

The Extraction System Improves Particle Collection and Lowers Power Requirements

Improved collection of particles is achieved by the flow of blower air above the component to the extraction hood. Flow speeds in the range of several m/s allow particles to be transported reliably without them settling on the component. These high air speeds cannot be achieved with extraction alone. Particle collection can be improved with a suitable tooling concept. For example, a positive angle of twist of the milling tool can cause particles to be ejected upwards into the airflow. Compressed air via ICS or an external feed to the machining point assists removal by dispersing the particles.

The power consumption of an extraction unit is determined by the volume of air moved and the pressure loss of the extraction system, with the overall pressure loss consisting of pipeline pressure loss, filter pressure loss and miscellaneous pressure loss (for example, due to fire protection flaps or restrictors). Long pipelines, high airflow speeds and increasing filter classes raise the amount of pressure loss and hence the required fan power. The ARS uses upstream coarse filtering of process air, meaning that only around 20 % of the extracted air needs energy-intensive fine filtering. Moreover, coarse filtering is performed optimally when local, i.e. directly at the processing machine, meaning that the pipelines are kept short and additional energy is saved. The use of process air has the additional effect that only a part of the finely filtered air is released into the environment. This reduces energy requirements for air conditioning in the production hall, particularly in winter, as 80 % less warm hall air is released into the environment. Furthermore, filtered air still contains a small amount of fine dust. This share is also 80 % lower due to the use of process air.

Pilot Plant Used in Series Production Shows Suitability for Industrial Use

These benefits convinced Polytec Composites Germany, based in Kraichtal in the Baden region. Polytec is a leading manufacturer in the field of fibre-reinforced plastic components for the automotive industry. At its plant in Kraichtal, it mainly produces glass-fibre-reinforced plastic parts in an SMC process. A major step in the process is machining, which is performed by several machine tools and robot machining cells. The collection of particles in the two robot cells is particularly difficult, as collection around the tool is not possible due to the many different work pieces. Polytec thus became the pilot customer for the first ARS unit.

An analysis of the work pieces, the machining processes, the particles produced and the geometric circumstances of the robot cells supplied the input variables for calculating the required volume flows and the airflow simulation in the work room. Owing to the different component forms, it turned out that height-adjustable blower nozzles combined with a large-area extraction hood provided the best results. This led to five blower nozzles — lockable and height-adjustable with catch bolts — being installed in consultation with the production staff and management at Polytec. The layout of the robot machining cells is shown in Figure 4.
Figure 4

Filter unit of the new extraction system (left); view of a robot machining cell (right): component on jig (1); machining robot (2); extraction hood (3); height-adjustable blower nozzles (4) (© Fraunhofer IPA; Photographer: Rainer Bez)

Furthermore, the extraction unit was connected to the robot cell control system, allowing process air infeed to be interrupted and only extraction to continue directly before the end of a processing programme and while the workpiece is being exchanged by an operator.

The use of process air reduces the proportion of fine dust in the air by 80 %.

The new extraction system has been in multi-shift use for almost a year, delivering convincing performance in industrial conditions. Dust deposits on the components and jigs could thus be decreased by well over 90%, reducing the manual cleaning work that the operators have to perform to a minimum, Figure 5. It was also possible to reduce dust loading in the workplace and to raise workplace acceptance considerably.
Figure 5

Reduction of dust deposits on the components and jig through the ARS (© Fraunhofer IPA)

Current development work on the part of Schuko and the Fraunhofer IPA involves further optimisation and automation of the ARS for other problem areas in machining, such as window and stair production and straight plastic machining. The ARS can also enhance particle collection and lower energy requirements of the extraction system in these areas.

Notes

Thanks

The investigations described here were undertaken as part of the project “Development of an energy-conserving and particulate-reducing dust extraction system for machine tools processing wood and plastic”. The research project was supported by funding from the German Federal Ministry for Education and Research (BMBF) as part of its “Central programme for Innovation for SMEs” (ZIM) under the funding code KF2916810DF4. The installation of the pilot plant at Polytec Composites Germany was undertaken with funding from the ReTech-BW programme, which is supported by the State Ministry for the Environment, Climate and Energy Industry of Baden-Württemberg and coordinated by Umwelttechnik BW. Special thanks go to Leichtbau BW GmbH for making this article possible.

References

  1. [1]
    Schneider, M.; Birenbaum, C.; Forbes, A.; Mayer, T.; Burkhardt, J.: Spanende Bearbeitung von Leichtbauwerkstoffen. Einführung und Überblick. Stuttgart: e-mobil BW GmbH — Landesagentur für Elektromobilität und Brennstoffzellentechnologie Baden-Württemberg, 2012Google Scholar
  2. [2]
    Bearbeitung von CFK Materialien. Orientierungshilfe für Schutzmaßnahmen. Fachbereich Holz und Metall der DGUV (2014)Google Scholar
  3. [3]
    Dressler, M.: Simulation von Späneerfassungsvorgängen in Absaughauben bei holzbearbeitenden Maschinen. Stuttgart: Univ., Inst. für Werkzeugmaschinen (Berichte aus dem Institut für Werkzeugmaschinen / Konstruktion und Fertigung, 30), 2007Google Scholar
  4. [4]
    Blecken, J.: Optimierung der Staub- und Späneerfassung in stationären Holzbearbeitungsmaschinen. Braunschweig: Univ., Inst. für Werkzeugmaschinen und Fertigungstechnik, Vulkan-Verlag, 2004Google Scholar
  5. [5]
    Heisel, U.; Tröger, J.; Müller, S.; Haag, M.: Späneerfassung an Bearbeitungszentren Teil 1. In: HOB — Die Holzbearbeitung 53 (2000), pp. 90–93Google Scholar
  6. [6]
    Heisel, U.; Tröger, J.; Müller, S.; Dressler, M.: Späneentsorgung durch Sedimentation. Deutsch-tschechische Fachtagung anlässlich des Tages der deutschen Umwelttechnik. In: Innovative Technologien des Umweltschutzes, (1998), pp. 87–103Google Scholar
  7. [7]
    Heisel, U.; Tröger, J.; Schneider, M.: Späneerfassung in 5-Achs-Bearbeitungszentren Teil 1. In: HOB — Die Holzbearbeitung 6 (2007), pp. 62-65Google Scholar
  8. [8]
    Pfeiffer, W.: Absaugluftmengen von Erfassungseinrichtungen offener Bauart. In: Staub — Reinhaltung der Luft 42 (1982), pp. 303–308Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Andreas Gebhardt
    • 1
  • Peter Miller
    • 2
  • Andre Schulte-Südhoff
    • 2
  • Michael Hauck
    • 3
  1. 1.Fraunhofer IPAStuttgartGermany
  2. 2.Germany
  3. 3.Polytec Composites Germany GmbHKraichtalGermany

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