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.

Driving this trend in the market, is the desire for exposure systems that are less energy intensive, give faster exposure times and achieve a better 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 LED’s provide a more collimated light source and there will be less crossover of the light.

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 and 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. Breakdowns on press can also be improved by moving to modern mesh that has been plasma treated. With plasma treated mesh you will get superior stencil adhesion, resulting in less stencil breakdown on press, delivering longer print runs far beyond other conventional treated fabrics.

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 (who have manufactured metal halide exposure equipment) will not have the correct output for the LEDs. However, those screen printers that have designed and built their own LED exposure units may end up 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.

Sensitivity cure relationship with wavelength

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 LED’s are 405nm, you will get the best penetration of the light into the emulsion and it will give 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 even when achieving a step five or six with a 21-step wedge. 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.

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