Understencil Cleaning Revisited Print E-mail
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Written by Clive Ashmore   
Wednesday, 30 March 2011 18:26

Don’t assume the software presets are optimal for your process.

If you’re a regular reader of this column, you know where I stand on the issue of understencil cleaning. For those new to this space, let me give you the two-second brief: Understencil cleaning is a necessary evil, a process fix that should not be required if all printing inputs are spot-on. Pausing every 10 (or fewer) prints to clean the stencil is a huge overhead in time and consumables costs. (You have to feed your cleaner a lot of fabric and solvents or it isn’t happy.) Sadly, most printing inputs are not perfect, and just about everyone cleans.

Given this unfortunate reality, we must make the best of the understencil cleaning process. I’ve previously discussed the use of high-quality chemistries and fabrics in combination with ultra-fast understencil cleaning systems to remedy some of the defects and throughput overhead cleaning can induce. This month, I’d like to address the capability of the understencil cleaning software and how program optimization can significantly reduce costs.

Surprisingly, many operators do not use – nor are even aware of – the full range of the understencil cleaning software. Most assemblers tend to use the default settings supplied by the printer manufacturer, and these usually err on the conservative side. Let’s start with speed, as an example. There are three things that can be done with an understencil cleaner: a dry wipe, a wipe with vacuum and a wipe with solvent. Each of these can be independently programmed to run at different speeds. In general, the dry and wet wipes tend to be set at moderate speeds, and the vacuum cycle is generally set a bit slower so that it has time to draw material out from the apertures. The factory presets for our company’s systems are 40mm/sec. traversing speed for wet and dry cycles and 10mm/sec. for the vacuum cycle. Again, this is set conservatively. There are many applications where all these cycles could be run much faster, which would have a tremendous impact on throughput. My advice? Do some DoE, some process engineering, and find out – just as you would with the print speed – the optimum speed for cleaning. hat’s more, all these recipes can be recorded and easily switched back and forth based on the job that’s running.

Next on the DoE agenda is consumables. If the fabric used is of good quality with high absorbency, the simple fact is that less solvent will be required. With high-quality fabric, when solvent is released from the solvent bar, only a very small amount is needed, as a highly absorbent fabric will ensure good wicking across the length of the fabric roll. Again, most printer OEMs tend to set solvent delivery at a rate that will ensure coverage that spans the length of the cleaning roll – whether with superior or inferior fabrics. So, evaluating the fabric that’s being used and running some tests may enable the use of less solvent if fabric absorbency and wicking capability is good. Likewise, the fabric advance function within the understencil cleaning software can control the quantity of fabric that will be moved forward before the next operation. Like all the other settings mentioned previously, the factory pre-set on fabric advance is also pretty conservative at approximately 20mm (about 1"). Again, if solvent dispensing is managed properly, a small amount of solvent will wick across the fabric, create a thin bead of wet material, and, realistically, a fabric advance of 15mm should be ample. If, however, the fabric is oversaturated, or you are using a sub-par fabric, the advance may need to be set at 20, 30 or even 40mm until the next section of dry fabric is reached. Managing the consumption of solvent and fabric is purely about doing some process engineering, setting up samples and optimizing the understencil cleaning operation per job. It’s that simple, and it’s easy money saved.

Aside from these examples, there are a gracious plenty of options that can be selected for the most cost-effective understencil cleaning process. For example, on our company’s systems, six cycles of various combinations of wet, dry or vacuum can be run. The out-of-the-box standard and safe approach is a wet cycle, followed by a vacuum cycle, followed by a dry cycle. But, by setting up two different clean modes (e.g., mode one as just a dry mode and mode two as a full wet, vac, dry), the process can be even more optimized. So, for example, after every tenth board, you may run a dry-only cycle (mode one) and every 20th board a full, three-cycle clean (mode two). Instead of throwing in the big boy clean every tenth board, which equates to more time, less production, more cost (and less profit), a strategy with a bit more thought behind it yields far better returns.

When it comes to understencil cleaning, we must make the best of a job that shouldn’t be required. When life hands you lemons (less than perfect inputs = requirement to clean), make lemonade (utilize understencil cleaning software to optimize process = less cost)!

Clive Ashmore is global applied process engineering manager at DEK International (www.dek.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears bimonthly.

Last Updated on Thursday, 31 March 2011 17:19
 

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