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.
Coating trough selection
The quality of your coating trough will aect 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 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.
This coating trough from Fotec has angled cheeks for the best coating performance
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 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.
The coating pressure will aect 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 will affect the volume of liquid deposited
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.
Coating emulsions at speed will lead to air bubbles within the coating trough and in the mesh openings due to incomplete encapsulation of mesh fibres. This is more likely to happen on coarse mesh counts of 77 threads per cm or lower. Conversely, coating emulsions too slowly can lead to high coating thickness from excessive push-through of the emulsion.
Coating at 1+1 is done by screen printers to save money related to emulsion costs. This creates a false economy as it only creates problems with pinholes and breakdown of the emulsion on press. Increasing the number of coats to 1+2 or 2+2 improves the ink opacity on the garment and the durability of the stencil. Always follow the coating guidelines of your emulsion manufacturer. Whatever coating regime (1+2, 2+1, 2+2, etc) you choose, always start with coating on the print side and finish off from the squeegee side. This will ensure the emulsion is pushed through to the print side, where you want the emulsion build to guarantee the ink deposit and durability of the stencil. If you finish your coating by scraping off the emulsion either on the print side or the squeegee side, you will introduce air into the coating; basically, you should have chosen a different coating regime from the start, ie 1+1 or 2+1.
It is important to remember not all emulsions expose at the same time, and all the above factors can influence the correct exposure. A step test exposure is recommended to find your optimum.
Coating 1+1 can trap air bubbles in the mesh opening that will cause pinholes during printing. One way to avoid air bubbles being trapped in the mesh is to coat from the print side until you see a gloss appearance on the squeegee side of the mesh. This should force any bubbles in the emulsion (from the first coating on the print side) to be forced out of the mesh opening and dispersed when applying the final coats from the squeegee side.