The Screen Print Column
Understanding the limitations of LED exposure systems for screen printing
LED exposure systems is a continuing trend in the screen printing market, especially for screen printers that are involved in textile screen printing. William Shorter, of SPT Sales + Marketing, dives head first into this trend.
D
fine line detail. LED exposing also offers longer life cycles in comparison to conventional systems such as UV tubes (one year) or metal halide (three months when not using an integrator) exposure system, whereas LED have a working life of 10,000 to 50,000 hours depending on the source of the LEDs.
Throughout this year I have been called into many screen printers in the UK that have moved away from conventional exposure systems and added LED exposure to their screen making process and have subsequently experienced technical difficulties with their emulsions during the processing, printing or reclaiming stages and various causes have been identified.
Weakness and pin holing in the stencil during printing The space between the LEDs and the distance that the LEDs are positioned from the glass can have an impact on the exposure of an emulsion. If the LEDs are two wide apart this can cause areas that are not fully exposed, especially as LEDs provide a more collimated light source and there will be less crossover of the light.
riving this trend in the market, is the desire for exposure systems that are less energy intensive, give faster exposure times and achieve a better
purchasing LEDs that do not have the correct wavelengths and peaks for exposing all screen printing emulsions. Depth of light penetration into coating = cure depth. Stencil adhesion and durability are directly related to cure depth. Cure
depth is heavily dependent on wavelength. ● 365nm – low sensitivity – only surface cures – stencil too opaque. ● 385nm – cannot fully penetrate thicker coatings – poor latitude. ● 405nm – penetrates deeply – optimum cure and latitude. ● 425nm – too weakly absorbed – stencil too transparent. Whenever
purchasing a LED exposure unit or LEDs for a DIY exposure unit, make sure that they are admitting peaks between a
wavelength of 360nm to 420nm. This would then give you a peak at the 395nm for Diazo based
emulsions and 405nm for photopolymer emulsions. More importantly if they say and can prove that the LEDs are 405nm,
Columnated vs multi light source (uncolumnated) and how this can affect undercutting during exposure
This means that you can have areas of the emulsion that are exposed fully and others that are under exposed and cause defuse shadows. You can check this by conducting multiple exposure checks across the glass to see if there are differences in exposure between and directly over the LEDs. Always remember to check exposure times using a 21-step wedge or an exposure calculator. For Dual Cure emulsions you are looking for a step seven and on photopolymer emulsions a step five to six for full exposure when using a 21-step wedge.
UV wavelength – Sensitivity vs absorbance (intensity) It is well recognised within the industry that the use of a metal halide lamps with a built-in integrator offers the best cure for screen printing emulsions. The reasons for this, these lamps have an excellent overlap in absorbance and sensitivity and more importantly have the peak at 405nm which means that the light penetrates deeply into the emulsion to give optimum cure and latitude. A fully cured stencil is quicker and easier to reclaim than an emulsion that is under cured. LED exposure systems have been developed to replicate the metal halide lamps and it is very unlikely that a LED exposure unit from a recognised company in the industry will not have the correct output for the LEDs. However, screen printers that have designed and built their own LED exposure units may end up
| 26 | September 2019
Sensitivity cure relationship with wavelength
you will get optimum cure and latitude for whatever emulsion you decide to use for your stencil making.
The intensity of light admitted by a LED exposure unit can be very low in some circumstances. When a photopolymer emulsion is coated on a coarse 48 to 77 threads per cm yellow mesh, it can cause problems with through cure. This can normally be detected by touching the squeegee side of the emulsion after washout, in these circumstances the emulsion will feel soft to the touch and you will notice the emulsion colour on your fingers. To resolve this, you are best moving to a white mesh, which will also give you faster exposure times (less absorbance of the light from the mesh), better through cure and improve throughput in your screen making process. Under cure can also cause problems with mesh staining, which subsequently results in ghost images that will appear in the image areas of your next print run. Before moving away from conventional to LED exposure, do your homework and wherever possible ask the reseller or the manufacturer of the LED exposure unit to allow you to see the equipment in operation with your mesh and stencil systems. This will help you identify any of the many technical problems that I have referenced above, before purchasing. More importantly, remember a LED exposure unit with an output of 405nm will always give you the optimum cure, latitude, durability, speed of exposure and reclaim of your chosen stencil system, whether it be a diazo, dual cure, triple cure or photopolymer emulsion. With the correct LED exposure system and correct exposure times (exposure times are dependent on the mesh count, mesh colour and coating regime) you can improve throughput in screen making and remove many of the technical complications that are related to conventional exposure with UV tubes, spot lamps etc.
www.printwearandpromotion.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92
