Croft Additive Manufacturing Ltd was recently approached by its sister company Croft Filters Ltd, both based in Warrington, Cheshire, UK, to assist in the development of a new metal filter range that combines Additive Manufacturing and conventional materials.
Croft Filters manufactures bespoke metal filters for many industrial sectors. One of the major challenges in this industry is delivering the required filtration level (aperture size) and strength whilst retaining a maximal open area for efficient filtration. Last chance filters, for example, commonly made from solely woven wire mesh, boast a high open area, but with reduced overall strength.
For most filter media whose filtration portion is formed from shaped woven wire mesh, the woven wire mesh must have sufficient strength to support the shape of the filter in its final format. This is usually achieved by producing a mesh wire diameter with sufficient strength to be formed into the required shape; however, this decreases the size of the apertures. Another solution is to add a perforated support layer, which increases the pressure drop across the filter, thus reducing filtration efficiency.
Croft Filters sought a solution that would increase both the strength and stability of mesh-only, woven wire filters, through the addition of localised supports. Traditionally, mesh is cut and formed into a cone with the help of a jig, then spot welded along the seam. A cone filter manufactured from stainless steel 316L mesh holds its shape, but can easily be distorted by operator and operation. The wire ends are free and not held in position, which may also affect the aperture size and shape of the final product.
In Croft AM’s research, stainless steel woven wire mesh was spot welded directly onto the AM machine’s build plate to secure the mesh in position. Layers of stainless steel 316L were then additively manufactured using Laser Beam Powder Bed Fusion (PBF-LB), according to the CAD design. The mesh and supports were then removed from the AM build plate and the filter body and ends cut out, then reformed using another jig.
This method ensured that the mesh shape was cut without distortion of the mesh, and the ends of the mesh held in place. Finally, the support material seam, end and fixings were laser welded to create quality, improved performance products. Initial trials demonstrated the need to tailor the laser power and duration during the melt phase in order to prevent destruction of the mesh, but no other issues were reported.
These ‘EXOstructure’ filters can be made in a number of shapes, as the support structures are added to the woven wire mesh in specific patterns to suit the design of the filter, and extra supports can be added to suit operational pressure requirements. The addition of a metal support frame around the edges of the woven wire mesh firstly secures the loose ends of the wires, holding them in place, thus preventing loose wires from being dislodged during operation and delivering solidly welded edges.
The mesh filter shape is thus strengthened and retains its shape during operation. In addition, fewer support areas are necessary compared to traditional perforated filter supports, meaning the maximum open area of the mesh is obtained, delivering a greater filtration efficiency.
This manufacturing process enables a variety of different ends and fixtures to be secured to the filter mesh more easily and with greater precision. The EXOstructure filters can be manufactured to a more consistent specification than the previous manufacturing process.
Further studies are ongoing to explore the use of finer wire and thicker wire mesh through the optimisation of build parameter settings.