ULT highlights the importance of air handling technology for PBF-LB Additive Manufacturing
September 10, 2020
Additive Manufacturing offers many benefits compared to conventional production processes, but the technology involves some special challenges with regards to extraction and filtration technology, particularly during Laser Beam Powder Bed Fusion (PBF-LB) metal Additive Manufacturing.
PBF-LB AM takes place in an enclosed build area under an inert gas atmosphere, usually comprised of nitrogen or argon. The process results in the production of fume and fine dust in the build area, which partly consists of extremely tiny nanoparticles.
Boris Frühauf, Key Account Manager Laser Technology for ULT AG, headquartered in Löbau, Germany, spoke to Metal AM to explain the importance of air handling technology, and the research yet to be done in the area, to ensure the safe disposal of fume and particulates generated during PBF-LB.
ULT has been a vendor of fume extraction technology for Additive Manufacturing processes for some years, providing a wide range of experience and technical expertise in this area. “One could say that we are the pioneers in this field, constantly enhancing our technology,” Frühauf commented.
“To guarantee a stable construction process, the entire system must be designed to enable a uniform laminar flow over the powder bed, capturing fume and particulates but not the powder bed material,” explains Frühauf. “Finally, airborne pollutants are collected in a filter. This is definitely a technical challenge.”
Safe disposal of highly reactive dust
ULT’s gas flow systems use specially developed cartridge filters, in which particles are retained on the surface of the filter elements. At programmable intervals, supported by sensors, the cartridges are cleaned with a compressed air counter-flush and the dust is completely gathered in dust collectors. Afterwards, the filters can resume their work. This guarantees, among other benefits, suitability for long build jobs and a long filter life.
ULT attaches great importance to occupational health and safety – for employees, machinery and products. Due to its small particle sizes, the dust produced in PBF-LB can be highly reactive and inflammable. As a result, safety precautions must be taken and safety guidelines followed. It is also important that filters are able to be exchanged without contamination, or at least, with a low level of contamination.
However, there are not currently any uniform standards or rules on correct dust disposal for PBF-LB. That might be why so many metal Additive Manufacturing users struggle to find the right disposal company; one with sufficient experience in that area, which knows how to deal with reactive materials. “ULT has compiled special passivation opportunities and disposal options,” explained Frühauf.
In filtration technology, there are two basic techniques: cartridge filter, which can be dedusted, and storage filters, which at some point become saturated and must be disposed of. Storage filter disposal also requires correct handling, as dust may ignite through exposure to the air or the vibration of the cartridge. This can be prevented by passivation.
Cartridge filter solutions are used commonly in the industry because of the equipment’s longer lifespan, which can be some months depending on the materials being processed. Furthermore, ULT fume extraction systems are equipped with differential pressure sensors; these measure pressure differences and recognise saturated or maximum-loaded filters. Required filter exchanges are then signalled in time to prevent system stops.
Post-processing and powder handling
Fume extraction technology is not only utilised in Additive Manufacturing machines; it is essential for the entire workflow. Solutions for post-processing steps such as removing support structures and for powder drying are also required. In addition to portable solutions, stationary units can be utilised.
“We are constantly striving to meet the requirements of Additive Manufacturing by continuously enhancing our modular systems,” Frühauf stated. “‘Extraction. Filtration. Persistence.’ is ULT’s corporate motto. In this instance, ‘persistence’ means knowledge on the latest technologies, which we have successfully implemented in various solutions for laser processing, for example. That requires high commitment to research and development.”
During material exchange, the entire construction chamber must be cleaned. For instance: if the production of a stainless steel component follows the production of an aluminium component, the machine must not contain any aluminium particles when the stainless steel build begins. Mobile fume extraction systems or wet separators are used to capture and bind residual material from the process, but in the case of non-oxidised aluminium, there is the danger of reaction with water, which can produce hydrogen. If hydrogen gas escapes and comes into contact with a spark, it may explode.
“So far, there is little experience with that matter. Yet, the first Additive Manufacturing companies have begun to measure the hydrogen content in an attempt to determine the critical explosion magnitude and force ventilation or waste air outlet to the outside,” Frühauf explained.
The extraction of residual material is not the final link in the disposal process chain. The exhaust systems must also be cleaned; to do this, contaminated water is poured into containers, in which the particles slowly sediment. The remaining metal sludge must then be disposed of at regular intervals.
Today, these containers are partly positioned within the production facilities. “The risk here is that the hydrogen content in these halls increases. Therefore, ULT recommends placing the containers outside to enable an outward degassing. At this point, much educational work is needed,” Frühauf commented.
Many potential scenarios in Additive Manufacturing are not yet determined. There are many technical challenges, and even more where new powder types and materials are used.
Many standards for Additive Manufacturing are still in development, for example on health and safety, emissions and powder handling. There is, as yet, no information available on material mixture reactions on the nanoscale. Additionally, there are no numbers available on the explosivitiy of nanoscale materials < 0.5 µm.
Material classification – whether related to hazardous goods or not – and disposal are hard to determine for system operators. Each disposal company places different values on hydrogen generation of powder in combination with water; this area is little studied.
In conclusion, Frühauf stated, “That is why ULT works on solutions together with other companies, institutions and associations within a large research network. Our goal is creating standards within Additive Manufacturing.”