Dust/fume containment system
Mechanized plasma cutting produces a considerable amount of sparks, dust, smoke, gases and slag created during the process. Much of the smoke, for example, is a byproduct of mill scale or oil in the material, while some comes from the material itself. Without proper ventilation or other removal/containment measures, these contaminants not only affect the final quality of the part, but can quickly overwhelm a confined space, creating unsuitable working conditions for operators.
Since most manufacturers of CNC equipment do not recommend the outdoor use of their plasma cutting machines, it is clear that we must seek our own containment measures for confined environments.
To that end, there are two systems that address this issue and can be integrated with a cnc plasma cutting table. The choice of each one is subject to a series of considerations that we will detail later to provide a clear idea of what they offer.
But first let’s get to know what those systems are and how they work.
Vacuum-based system (“dry tables”)
It consists of a downdraft extraction mechanism arranged under the table and an extractor that sucks the air, leading it out through the sides of the table, or, in more complex systems, through a filtration system. of air with connection to the outside.
In this video we see a schematic representation of the operation of a suction table.
These tables present a simple and effective system to eliminate dust and smoke, although they require sufficient airflow to displace the particles through the duct network. If the location of the workshop or enclosure allows the fumes to be vented to the outside, then that is enough to dissipate them, although depending on the dimensions of the table, too much heat may be generated in the discharge area.
Some more expensive designs use a cold air conditioning duct strategically placed near the table to use outside air instead of room air. Even some larger tables use zoned sections with movable grids so that suction occurs only in the area of the table where the torch makes the cuts. There are also tables that use a powerful self-cleaning filtration system in the enclosure, filtering gases and smoke, and keeping all of the heat inside said enclosure. However, all of these air conditioning, zoned, and filter systems add cost and complexity.
The suction table is a widely used resource, but in general it must be configured to measure for each installation. This video shows us a vacuum table in action, as well as the installation required.
Water-based system (“water tables”)
It consists of a tray or, on more expensive models, a shallow table filled with water, which is placed below the cutting surface. When the torch does its job of metal cutting, the force of the plasma jet drives the sparks and dust into the water, where they are quickly attenuated.
The water trays have a depth of approximately 5 cm and allow the contact of the water with the underside of the material to be cut. They are widely used in low-end CNC equipment.
On the other hand, the tables with water are generally deeper and have the particularity of raising and lowering the water level through a simple and controlled pneumatic displacement design. In the lower part of the table, the system has an air/water tank that is filled with air, which moves the water towards the upper chamber, raising its level with respect to the material to be cut. The advantage provided by tables with water is that underwater cuts can be made, which, as we will see below, have some advantages for cutting steel and at high power levels, greater than 200 A.
Although underwater cutting also significantly decreases the noise levels of the drilling operation, plasma cutting, not all metals are suitable for this and also the water can splash, so the bottom of the table could rust after excessive or incorrect use.
The next two videos show examples of plasma cutting that use tables with water. In the first video we see a cut where the water level is below the part and the second video shows a cut completely submerged in water.
Both the suction tables and those that use water can also be equipped with a automatic slag removal system (resolidified metal on the underside of the plate), small products and waste material, transporting them to the end of the table. This movement is carried out by means of oscillating conveyors that extend under the table and when they reach the end of it, the waste is collected in a container. The slag removal system is highly advantageous as it improves cut quality, dramatically reduces cleanup time (and therefore machine downtime) and eliminates post-grinding.
The following video shows an example of how this system works.
Factors to consider when choosing a cutting table dust/fume containment system
When the user knows how suction tables and tables with water work, his decision for one system or another will be largely based on the following aspects.
a) Cleaning and maintenance requirements of each design
The space required must be taken into account. plasma cutting table in the workshop, where it will be installed and how much work it will require, which will depend on the volume of cut. Some workshops clean the table once a year, others once a month.
Tables with water make a mess and this can create a sloppy environment. That is, the pieces get wet, then move off the table and wet the floor. It can be a bit difficult to keep the surrounding area and parts clean. As a palliative measure, the operator can have a hose near the table and spray the pieces already cut to remove slag residue, letting the washing water fall on the table or tray. You can also try various methods to keep the water clean so it doesn’t collect sediment that could later settle on the parts. Depending on table design, cleaning may require disassembly of grate sections for transport to another location where they can be disassembled, slag scraped off, and reassembled.
In the case of a suction table, depending on its design, it is possible that the grids must also be disassembled to empty the slag.
b) Advantages of underwater cutting
The cnc plasma cutting under water submerges the plate between 5 and 10 cm of water, so that the tip of the torch and the entire arc are submerged. The greatest benefit of a table with water is the work environment. Not only does it keep the arc flash and noise low, but the parts also stay cool, preventing warping and, not least, the risk of operator burns.
Although safety glasses should still be worn, working on a plasma table Underwater cutting is a much more pleasant environment than working on a dry table. the noise of plasma cutting on dry tables it can reach 120 decibels. When cutting underwater, noise is typically below the 85 decibel limit, so operator exposure is dramatically reduced.
c) Possible oxidation of parts
At underwater plasma cutting the material is overheated, submerged and surrounded by a warm environment. Those factors are conducive to the rapid rusting of carbon steel parts, for example. However, there are additives that can be added to the water, which coat the pieces preventing them from rusting. The coating often adheres to the faces of the parts and actually prevents rusting even longer than parts that are cut on a dry table. This means that if we use a dry table in a humid environment, the cut pieces that we remove from the table could end up rusting more quickly than if we had cut them on a table with water provided with an anti-rust additive.
d) Type of material to be cut
This is an important question to consider and that leads us to the next question: can all kinds of materials be cut on a table with water? The answer is no.
The underwater plasma cutting It is highly recommended for stainless steel, for example. If we do this operation on a dry table, the molten slag that falls under the plate could adhere to it, to other parts and other components of the table. This requires grinding and therefore increases costs and takes more time. On the contrary, if that same piece is cut on a table with water, it extinguishes the slag, preventing adhesions.
The case of aluminum plasma cutting and non-ferrous materials under water is totally different, since a chemical reaction is generated that extracts the hydrogen from the water. This hydrogen gas is highly flammable and can cause a fire or explosion. Therefore, if this type of material is cut on tables with water, care must always be taken that the piece is above the water level and never submerged. Alternatively, the table must be equipped with an aeration system that destroys the hydrogen bubbles that accumulate in the water.
When aluminum is cut on a dry table, you have the problem of aluminum dust, which can be explosive, and therefore a dust collector is required. Many users also do not want to mix aluminum dust with other dust, so it is sometimes necessary to install a complex dust collection system, consisting of two separate collectors, one for cutting aluminum and one for cutting other metals. where the one destined to cut aluminum has to have ventilation grills and be located outside. Faced with such an outlay of money, a table with water is much simpler and costs significantly less.
If properly designed, both wet tables and pans and vacuum tables are effective in controlling fumes and fumes during the plasma cutting process.
Wet tables can remove up to 98% of dust and smoke (depending on water level), cost 40% less than dry tables, are very simple to install and minimize ambient noise. Since they also reduce the heating of the material, they are very suitable for controlling the warping and warping caused by heat in long and thin parts. However, table water affects the quality of the underside of the cut surface and this can cause problems in high definition plasma systems, for example.
Generally, vacuum tables are less efficient at removing dust and fumes (between 75 and 80%), but they are much more versatile and can be used to cut a wide variety of materials, such as galvanized steel, stainless steel, aluminum. and other metals where toxic fumes can be produced, allowing the best quality cut on steel. However, if the smoke is simply extracted outside, heat will also be removed from the room, which is not convenient in winter. Given this, it is advisable to use internal filtration systems, although these can add cost and maintenance.
While some manufacturers sell far more wet tables than vacuum tables, the sales of others follow the opposite pattern. As a conclusion, we can recommend that if our activity processes a variety of metals, a dry table may be the best option for a ventilated system. On the other hand, if we are mainly dedicated to cutting carbon steel, a table with water can offer us considerable savings.