Tuesday, June 11, 2013

Waterjetting 10b - Introduction to Abrasive use

In recent articles in this series I have written about the processes that occur as a high-pressure waterjet impacts on a surface and then begins to penetrate and cut into it. However, as I noted in the last post, one of the problems with using plain water as the cutting medium is that it can pressurize within the cut and exploit any surrounding cracks, to the point that the edges of the cut are cracked and fractured, often back up to the top surface of the material.

Figure 1. High-pressure waterjet cut along a sheet of Plexiglas, note the fracturing along the sides of the cut.

This is not usually desirable, and what is needed is a way of cutting into these materials, so that the cut edges remain smooth, and the risk of shattering around the cut line is much diminished. The way that is usually used for this is to add small amounts of a fine cutting abrasive into the waterjet stream, and use this to cut the slots in the material, with the water there to add cutting power.

Figure 2. Abrasive waterjet (AWJ) cuts through safety glass. Note that there are two sheets of glass with a thin plastic sheet attached between the two.

This can be of particular advantage if you are faced with trimming, for example, safety glass (as shown in Figure 2). Cutting and shaping this glass used to be a significant problem in the industry, since the presence of the plastic sheet, between the two glass layers meant that it was not always possible to get both to break to the same plane if scribed with a glass cutter. Failure rates of up to 30% were described, to the author, as common when the technology switch to AWJ took place. And with the abrasive in the water, the jet cuts through both layers without really seeing that there was a problem. (And complex contours can also be cut).

The combination of abrasive and high-pressure water has many advantages over existing tools. Among other things it removes the majority of the heat from the cut zone, so that in almost all cases the Heat Affected Zone (HAZ) along the edges of the cut disappears and the quality of the cut surface becomes, when properly cut, sufficient to require no further processing. This can lead to a significant savings in certain forms of fabrication.

There are many different ways in which abrasive can be added to a high speed stream of water, and Dr. Hashish illustrated some of these in the introductory lecture he gave at an early WJTA Short Course, as follows:

Figure 3. Some different ways of introducing abrasive into the cutting stream of a high-pressure waterjet (After Hashish, WJTA Short Course Notes).

The top three (a, b, c) involve mixing the abrasive and the water streams at the nozzle, while the fourth (d) is a relatively uncommon design that is used in cleaning surface applications, and the fifth has never been very effective in any trial that we have run. The sixth (e) technique has become known by a number of different names, but for now, to distinguish it from the more widely used Abrasive Water Jet cutting (AWJ) I will give it the acronym ASJ, for Abrasive Slurry Jetting. It has a number of benefits in different circumstances, and I will write more about it in future posts. In more recent alternative designs to that shown by Dr. Hashish the flow to the abrasive holding tank is more commonly through a diverted fraction of the total flow from the pump or intensifier.

Figure 4. Very simplified illustration of the circuit where abrasive is added to the flow from the pump/intensifier before the nozzle. Obviously the abrasive is held in a pressurized holding vessel – the optimal design of which is not immediately obvious.

When fine abrasive is added to a narrow waterjet stream, and that jet is moving at thousands of feet a second, there are a number of considerations in the design of the mixing chamber, and those will be discussed in future posts. But one early conclusion is that, if the jet is going to be small, then the abrasive that will be mixed with it will also have to be quite small, though – as will be noted in a future post – not too small.

Figure 5. The simplified and generic components of a mixing chamber that mixes abrasive with high-pressure water in an AWJ system.

There were a number of problems with the early systems, such as that shown in Figure 5, at the time that systems first appeared on the market, and I will write about some of these as the next few posts continue to focus on this subject.

There have been a number of different abrasives used over the years, and it depends on the needs of the job as to which is the most suitable in a given case. In some cases discriminate cutting is required, and so an abrasive can be chosen that will cut the desired layer on the surface, but not the material behind it. In other cases the target material is extremely tough, and so abrasive may be selected that will rapidly erode the supply lines and nozzle, but which can still prove economically viable in certain cases.

Figure 6. Various types of abrasive, that can include (from bottom left going clockwise) blasting sand, copper slag, garnet and olivine.

There are many different properties of the cutting system, and the abrasive which control the quality and speed of the resulting cut. Some of these will be the topic of the next few posts, others will be discussed in further posts at a more distant time, when we discuss different cutting applications and the changes in a conventional system that might be made to get the best results in those cases.

Abrasive properties are not just a case of knowing what the material is. There is a difference, for example, in cutting ability between alluvially mined garnet and that mined from solid rock. There is a difference between different types of the nominally same abrasive when it comes from different parts of the world, and there are differences when the shapes of the abrasive differ. Glass beads and steel shot cut in a different way that glass and steel grit, for example. So there is plenty to discuss as we turn to a deeper discussion of abrasive waterjet cutting.

Figure 7. Parameters controlling the cutting by an abrasive waterjet system. (After Mazurkiewicz)

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