Tuesday, February 26, 2013

Waterjetting 6c - Cutting foam and testing with it.

Last week’s post discussed a simple test which helps to show not only how to compare the effect of different operating conditions (varying abrasive type, nozzle design, AFR etc) as a way of finding a possibly better and cheaper cut. It is also often handy to know when a nozzle is starting to wear out, so that different cutting operations might be scheduled to allow the nozzle to continue to work, without threatening the quality of critical product.

Figure 1. Change in the cutting depth of a jet stream, at 50,000 psi, when traversed over ASTM A108 steel as a function of the time that the nozzle had been in use.

While we have found that nozzles from a given manufacturer roughly agree in cutting performance and times before they wear out, the pattern of wear and performance change differs from one nozzle design to another. Also there is some variation in performance between nozzles even of the same design and under the same conditions.

There are also times when cuts are made without abrasive, or when the cutting/cleaning jet is hand held – what to do in those cases? Mainly we have used foam as the cutting target, set up so that the jet won’t cut all the way down through the foam all the way along the cut, so that, as with the steel, some idea of not only cutting depth but also cut quality can be seen.

Figure 2. Cuts through thick stiff packing foam. Note the rough edge at the bottom of the extracted pieces, but the good initial quality of cut that was achievable for some 14-inches.

There is a caution in cutting foam, in that some of the softer varieties are going to fold into the cut, and give a slightly inaccurate measure of true performance, although for a quick comparison to see how a nozzle is lasting that is not a real issue. When cutting thicker material, and also when going for higher quality cuts, that is, however, something that should be borne in mind.

The white expanded foam that is used as a packing material is also very easy to cut, even with the pressures that can be found with a pressure washer type of system. Thus, if you are going to clean a deck or other surface it helps to check, by swiping the jet across such a piece of material, to be sure that you have a good nozzle on the end of your lance before you start.

This may seem fairly logical, after all you just went to the hardware store and bought a new packet of nozzles. Well, as with the other nozzles we have looked at, quality is only assured after testing. In this particular case we ran as many different variety of fan nozzles as we could to see how they would perform when cutting across a piece of packing foam. It is not hard to cut packing foam with a high pressure jet. And since domestic cleaning is usually carried out at either 1,000 psi or 2,000 psi we ran tests at both levels.

Figure 3. Results from a good, top, and a poor nozzle with cuts at 1,000 and 2,000 psi. and with the foam moved through the jet at a distance of 3 inches. The number identifies the nozzle and note that at 3 inches number 18 could barely remove the top of the foam.

A fan jet is defined by the amount of water that it will allow to pass at a set pressure, and by the angle of the cone with which the jet spreads out from the orifice. In passing we found that the cone angle that the jet actually spread at was a little larger than that designated on the package.

The worst nozzle design that we found had difficulty in cutting into the foam, even at a very close range:

On the other hand the best nozzle was still able to cut the material with the nozzle held some nine inches from the foam.

Figure 4. Cutting result with the good nozzle held at nine inches above the foam target. At this distance the jet is removing as much material as the poor jet did at a 3-inch standoff.

A very typical result would have the jet fail to cut into the foam much beyond four inches from the nozzle. (I’ll use some photographs in a couple of weeks to explain in more detail why that is). And as a short editorial comment to those of you who clean around your house with a domestic unit, how many of you hold the nozzle that close to the surface? (Or at the car wash?) If you don't you are losing most of the power that you are paying for, and you are in the company of most of the students that I ran this demonstration with in my classes).

However there is one other feature to the photographs of the cuts that I would point out. Fan jets distribute the water over a diverging fan shape. But the results of the design fell into two different types, one where most of the water still concentrated in the middle of the jet, (as in Figure 4) and those where it was focused more on the side.

Figure 5. Cutting pattern with the jet streams more at the side of the flow. (arrow points), note that the two pressure cuts are on the other sides of the sample here).

The benefit of using foam is that it allows this picture of the jet structure to be easily seen, with very little time taken to swipe the nozzle over a test piece of material at the start of work, to make sure that the jet is still working correctly.

This is both an advantage and a disadvantage. Because the foam is relatively easy for a jet to cut, even at a lower pressure, this means that the cut can become more ragged with depth, where deep cutting is required.

One of the programs that we ran, some years ago, looked at how deeply you could cut into the stiff packing foam that is used in some industrial plants, where the item being packed needs to be held firmly, yet will be released easily when needed. This requires that the foam be cut to a very tight tolerance, and at the time, pieces were still being cut by hand and then glued together. (Figure 2 above)

We found that we could cut up to about a foot of material, before the small cut particles became sufficiently caught up in the cutting jet that the edge quality of the cut fell below specification. But in order to get to that depth we did have to add a small amount of a polymer to the cutting water. This helped to hold the jet more coherent over a greater distance, and also reduced the amount of particulate that got caught up in the jet, allowing the greater cutting depth.

Foam works as a simple sample to give some sense of the jet shape, where the pressures are lower. When they are higher then a stiffer material is needed, though it should still be cuttable by water without the need for abrasive. Plywood is a useful target in this case, and I will write about those tests next time.

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