New type of extraction system for 5-axis machining of composites
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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
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 . 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  worked on the numerical simulation of air currents and particle trajectories in extraction hoods. Blecken  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 . 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.
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.
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.
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 %.
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.
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.
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