Stencil wiper roll parameters & buzz words

“Very strong, great cleaning characteristics, high level absorbent and extremely low in lint. “

Have you heard it before? Almost all stencil wiper roll manufacturer say the same. But are these parameters important and are they backed up by evidence? Let’s have a closer look at SMT stencil roll parameters.

Tags: hyperclean

Stencil cleaning roll parameters

"Very strong"
What is “very strong”? Is it needed? No, you don’t need a very “strong” fabric. There are relatively small forces on the fabric in most printers. The fabric will not break, even if it’s not very “strong”. Elongation is more important. This is expressed in per cent and tells you, simply put, how flexible the fabric is. The larger elongation at break, the more deformation (stretching) of the fabric when used. This is something we would like to avoid. We don’t want the fabric to behave like a rubber band.

"High absorbency"
The craving to brag about high absorbency comes from the original use of the raw materials used for making a majority stencil rolls – wipes for mopping up a liquid spill. In the under-stencil cleaning process, there is very little liquid to absorb, right? You don’t absorb the metal in solder paste and the ability to catch the metal particles is certainly not measured as absorbency.

The only liquid that needs to be absorbed is the cleaning agent. There is no need to absorb more than the fabric can convey to the stencil (with some margin). The amount of liquid that is transferred from the fabric to the stencil is basically only dependent on the stencil and cleaning agent properties.  

Only a few grams per square meter can be transferred from the fabric to the stencil. No matter how thick or soaked or whatever the fabric is. Some math will tell us that you theoretically need around 80 gram per square meter absorbency or 200% for a 40-gram fabric.  

Very high absorbency is achieved by using cellulose fibres. Fine for mopping up your coffee spill. In your screen printer, it only means that you soak the fabric with solvent that is not used to clean the stencil. Waste of money.

"Low lint"
Of course, low lint is desirable. Cellulose (paper) based fabric can only be made low lint by hard fibre compression during manufacturing, leaving a glossy and flat surface that is anything but good for stencil cleaning. A good example is the Sontara type of fabric. It’s low lint but not a good material for stencil cleaning.

Result from cleaning trials, starting with completely filled apertures. Cleaning cycle W/V/D in SJ Innotec printer. Sontara to the left and Hyperclean to the right.

A simple way to test whether your stencil roll is linting is to take a piece of adhesive tape, put it on the fabric, peel it off and look under a microscope if you see any loose fibres on the tape. In the pictures below, you can see the difference.

Lose fibres from fabric samples. Samples were pressed against Tesa ACX plus with moderate force. Microcare FP to the left and Hyperclean to the right.

"Great cleaning characteristics"
Air permeability is a very important parameter for cleaning performance. Air permeability describes how much air that flow through the fabric. You rarely see this in numbers of data sheets, other than in the form of claims such as "great vacuum performance". The reason again is that it’s not an important parameter for the off-the-shelf mopping paper that is used in most products on the market. The data is not available to the wiper roll manufacturer. Or they don't care or understand.

We want as much air as possible to flow through the fabric in order to make the vacuum cycle effective. But this is a tricky one. High air permeability can of course easily be achieved by making the fabric very sparse. But this simple approach will result in a fabric that with bad cleaning performance and it will cause rapid contamination of the vacuum system in your printer.

Fibre composition and process parameters are what builds the fabric and the fundament of good cleaning performance. Taking a standard fabric and offer it as a stencil wiper is not good enough any longer. It will be a "cheap" roll, satisfying many purchasers. But the actual cost for the "cheap" roll should be calculated from the cost for soldering defects, not purchase price per roll. 
Ask your supplier for data that counts and demand claim to be backed up by evidence! It’s that simple.

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