Images Digital Edition DEC 2018

www.images-magazine.com DECEMBER 2018 images 79 KB TIPS & TECHNIQUES along its full length. • The profile of the edge will influence the amount of emulsion deposited: a sharp-edged trough of 0.5mm radius will deposit less than a round-edged trough with a 1mm radius. • Any flex in the trough will cause an uneven deposit so careful consideration should be given to the materials used for coating troughs. Typically, coating troughs are made of either stainless steel or aluminium. • Placing a lid on the trough can help reduce airborne contamination, protect the edge from damage and slow down drying in the trough when coating multiple frames. Angle of the trough to the mesh The coating angle will affect the volume of liquid deposited onto the mesh: • If the trough is too upright, it will bow the mesh. If the trough is tilted too close to the screen, however, it will scrape rather than coat. • You can guarantee the correct angle for coating by using coating troughs that are already fitted with angled cheeks to give the best coating performance. These are typically at an angle of between 15-30° to the mesh Coating trough selection The quality of your coating trough will affect the coating quality and build – also known as emulsion over mesh or EOM ratio – of the stencil. Here are some pointers to choosing the right equipment for the job: • The edge of the trough must be straight with no ‘nicks‘ or other damage Emulsion coating essentials for standard processing. • It is important to maintain the same angle when coating because if your angle changes on each pass, the amount of emulsion deposited can significantly vary. Coating pressure The coating pressure will affect the volume of liquid deposited: • Too high a pressure could potentially damage the mesh, especially if you have sharp edges on your trough. • Too little pressure will lead to an uneven coating. • Coating by hand can be more complicated and is more prone to these issues – using an automatic coating machine will guarantee an even coating pressure from bottom to top. The coating pressure can also be influenced by the mesh tension: • Low mesh tensions will cause an uneven deposit (from outside edges to the centre) and will require higher pressures during the coating regime. The minimum recommended mesh tension for any screen is 20 N/cm. Speed Coating emulsions at speed will lead The coating pressure will affect the volume of liquid deposited This coating trough from Fotec has angled cheeks for the best coating performance Getting the basics correct in emulsion coating can save you money and reduce problems on press, says Mark Colenso, managing director of Colenso Screen Services. Here he shares some expert tips for making sure your emulsion coating is spot on every time