Just about every screenroom I visit uses some kind of pressure washer during screen production and reclamation. These tools are available in all kinds of configurations, and like most items in recent years, their cost has come down considerably as more and more companies move their manufacturing to less expensive locations. Anyone can go down to their local superstore and pick one up for almost nothing. But will it get the job done? More importantly, what should you look for in an industrial pressure washer?
High-pressure washers are used for several distinctly different purposes at different stages. This month, I would like to go over some of the guidelines for successfully selecting and using pressure washers and properly processing your screens.
Some very simple practices can make a huge difference in how much you spend monthly on screenmaking and screen-cleaning chemicals. But before dissecting your screenmaking process, it's important to understand exactly what high-pressure washing is.
Line water pressure is usually about 75 psi in the US. Taking the line pressure and focusing it down with a nozzle will increase the pressure, but not more than a couple hundred psi. This is not high pressure.
Line water pressure is most commonly used to wash out the image area after exposure. Many screenmakers believe that using low pressure prevents damage and preserves delicate image detail. They are misinformed. If the screen is properly dried, imaged, and rehydrated, then little damage is done by using fairly high pressure during the final washout. More on this later.
The very inexpensive pressure washers (often less than $100-300) available in hardware and lawn and garden stores usually operate in the range of 500-1750 psi. Those pressures are certainly better than nothing, but they're limited when it comes to doing any real work. Additionally, these units are built for intermittent use. These light-duty machines work well for image washout, but they're not appropriate for mesh prep or reclaiming.
You need way more than 1000 psi in order to really do some work. My personal comfort zone is between 2400-3000 psi. Pressures this high require the use of a multistage pump driven by a 5- to 10-hp motor. These industrial-grade machines are designed to work hour after hour in continuous operation. The cost range can be $400-4000, depending on the working pressure, duty cycle (how long the unit can be used non-stop before it must be allowed to rest), and the size of the motor being used.
The journey begins with stretching the mesh on the screen. Virgin mesh is notoriously unstable during the first round of preparation, imaging, and reclaiming. The tension changes constantly (even with very good low-elongation mesh). In addition, various machine oils and silicones may remain on the mesh from the weaving process. Proper practice dictates that you remove any of this foreign contamination before degreasing and coating the screen with emulsion. How you handle the screen during this phase can permanently affect the life of the mesh and the tension of the screen.
For those of you who use stretch-and-glue static frames, the stability of the mesh tension is directly affected by the use of a high-pressure wash. Mesh tension can only go down once it's glued to the frame. When mesh tension was first studied in the early 1980s, researchers thought the initial drop in mesh tension after stretching was caused by a phenomenon called cold flow. This is a process whereby the polymer molecules align themselves in the direction of the tension vectors. During alignment, they slide past each other, and the tension drops. This phenomenon served as a major argument in favor of retensionable frames, which allow continuous adjustment of mesh tension to maintain a consistent target value.
The development of high-density, low-modulus polyester polymers greatly reduced the initial drop in tension, but they did not cure the problem. Work done at the University of Wales, Swansea in the 1990s led to the observation that mesh tension is not uniformly transmitted across the screen. In fact, researchers found that knuckle friction at the intersection of the warp and weft threads restricted the transfer of force over the screen. Only with time, or additional physical assistance, would the tension be smooth, even, and stabilized.
Your screens can experience a 10- to 15-N/cm drop in tension the first time you clean them with a high-pressure washer. Water pressure greater than 2400 psi forces the knuckles open and lubricates the mesh thread, causing the thread to slip and the tension to drop. You can observe the same phenomenon during the first 150 prints on brand new mesh. The continual off-contact fatigue cycle causes the same knuckle release to occur, resulting in a large tension drop.
You should adjust the screen to its final tension after abrading and a high-pressure wash. My shop has experienced a marked improvement in virgin mesh's performance by following this practice. You can reduce the initial tension drop on press by up to 80% by using a high-pressure rinse, retensioning the mesh to its target level, and then degreasing. If you use static frames, you won't be able to make corrections. You'll simply have to live with the drop in tension.
My shop recently changed screen-washout practices with noticeable improvement to image quality and moiré reduction on high-density stencils and halftone images. Like many screenmakers, we were under the impression that extreme care was necessary when washing out exposed images—and I have always been very strict about how we prepare our screens, the environmental condition of the screenroom, and the quality of the exposure systems we use.
We now soak our exposed screens in a water tank for two minutes or longer after exposure in an effort to more thoroughly rehydrate the unexposed areas. We also use a power-assisted spray to wash out the screens and further improve the irrigation and removal of any unexposed resin. We use 500 psi at about 9-12 in. when washing from the print side.
The changes my shop made to the stencil-developing routine resulted in noticeable improvements to edge definition, stencil bridging, and halftone sharpness. Line water pressure simply doesn't have enough energy to dig down into the very fine capillary crevices of the mesh's knuckle intersections or into the areas where the round stencil dot edge intersects with the mesh. We're now fully exploiting these areas and greatly enhancing print quality.
Moiré reduction is one of the most significant improvements. Using a powerful wash allows us to develop a more fully formed halftone dot by clearing all available stencil transfer openings and fully developing the gasket-cavity profile by removing all unexposed emulsion resin.
A high-pressure wash plays a huge role in reclaiming and ghost removal. Some printers think 3000 psi is overkill during the reclaiming process. They point out that ink degradents and stencil removers soften or dissolve the emulsion. They're right, but these products should be the foundation of the process, not the only part of the workflow.
Working with very high pressure at the reclaiming stage forces mesh knuckles apart and strips out any trapped emulsion or ink. The result is a much cleaner mesh with little, if any, ghosting. The higher the initial printing tension, the better the results will be.
Another objection I frequently hear is that high pressure rips the mesh. Screens can tear during reclaiming, but high-pressure washout isn't necessarily to blame. Screens rip during reclaiming as mesh tension drops. The lower the mesh tension, the greater the probability of a tear. Low-tension screens are more elastic, sort of like a trampoline. When you hit a low-tension screen with a high-pressure wash, the mesh will snap back as it absorbs the force of the water. This rapid snapping shocks the mesh and initiates the tear. On the other hand, high-tension screens (25 N/cm and more) exhibit very little elasticity. A high-pressure wash essentially applies a cutting or shearing action to high-tension mesh, which is why ghost images are so effectively stripped away.
To minimize the chance of ripping a screen during reclaiming, start at the bottom of the screen with the washer's wand at least 12 in. away from the mesh. With water pressure on, bring the wand toward the mesh until the washer's tip is 1-2 in. from the screen's surface. Finally, begin a deliberate, side-to-side sweep up the screen until the image is completely removed.
The combination of high mesh tension and high water pressure in the screenroom can result in longer screen life, better image quality, and less moiré. You will also benefit from more consistent printing tension and less ghosting on future runs.
© 2005 Mark Coudray. Republication of this material in whole or in part, electronically or in print, without the permission of the author is forbidden.
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