Blog February 27, 2014

Methods of tank container surface grinding – Part Two

By Nicola Byers

In the first part of this two part blog series, Intermodal Eye looked at the first three methods of tank container grinding (click here to read part one). Now we look at the next four methods including automated grinding and polishing.

Method 4a2

4. Manual grinding with a flexible axle machine

Regarding surface quality and stock/product removal, this is by far the best process to be used for manual grinding. Unfortunately not many users are aware of its existence. It requires some training and muscular strength to handle the powerful equipment. The weight is not an issue.

• Risk of high voltage machinery (None)

As these machines are only 48 Volts, there is no risk of high voltage. As the design uses a transformer only, it is advised to have a special transformer / inverter designed, allowing the main on-off switch of the machine to become a low current switch, switching relays in the inverter only. The relays become high current switches. This brings some extra costs but prevents the main switch from burning and resulting in machine malfunction.

• Stock/Product removal (Good)
Stock removal by this method is very good as there is optimal surface contact. Good stock removal consequently increases the speed per job.

• Risk of surface damage (Low)
The risk of surface damage is low although working with this machine takes some training in order to keep the belt straight on the surface instead of on its side (cutting).

• Waste of abrasive (Low)
There is minimal waste of abrasive as the belt is used on its full width and length and the power of the operator is enough to have grits (pellets) cutting instead of rubbing.

• Surface quality after treatment (Good)
As grinding lines are vertical and parallel with the flow of liquid, the quality is very good. The end result however depends on the polishing procedure. If polished with a disk, the quality is lowered but if polished with a flap-brush or belt the quality is increased Method 4

5. Manual polishing processes, grinding and not grinding related

Disk or flap-brush?
There are two ways of polishing that influence the quality of the end result. The first is circular movements with a disk and the second is vertical straight lines by a flap-brush (or belt). Circular movements always result in horizontal scratches and pores obstructing the product from running down. This increases product pollution after discharge. Vertical lines form “rivers without pores” guiding product downwards and this helps total discharge and decreases product pollution.

Polishing with a smooth grinding disk, grinding related (grit 360 and higher)
In theory it is possible to polish with such a disk (small grits >360 / very high rpm / very low contact pressure). Why do so though if better materials are available? This process will increase pores and scratches because the material is aggressive. Some are still using this method due to lack of information on other processes.

Polishing with a smooth grinding disk, not grinding related (grit 360 and higher)
See previous.

Polishing with a soft non-woven disk, grinding related (scotch brite)
As the disk is very soft and without any backing it will easily be torn apart on a rough surface and is therefore unfit for polishing after grinding work.

Polishing with a soft non-woven disk, not grinding related (scotch brite)
In combination with a smoothening soap it can be used on a smooth surface in order to remove light material discolorations. Since each sharp spot or particle will tear the disk apart it cannot be used for product removal. As the disk is very soft the 90 degree scratches may be minimal.

Polishing with a hard non-woven disk, grinding related (scotch brite with backing)
This is the most common way of polishing used worldwide. It is rather fast and relatively cheap but has a poor end result. To the eye however, the surface may appear smooth.

Polishing with a hard non-woven disk, not grinding related (scotch brite with backing)
This way of polishing can be used to remove deeper discoloration and also crystalized product remnants. The end result here is relatively poor even though to the eye, it may seem like a smooth looking surface.

Polishing with a flap-brush, grinding related
For manual polishing purposes, this is by far the best for quality. The process takes a bit longer just as the disk process does and is therefore slightly more expensive. The quality is superb.

Polishing with a flap-brush, not grinding related
This can be used for removal of deeper discoloration as well as product removal. The same points apply as with the grinding related method.

Polishing with a belt
Polishing with a non-woven belt is the most sophisticated way of polishing. The results are amazing as it not just smoothes the peaks of grinding lines but it also widens the valleys (“rivers”) allowing more liquid to run down smoothly.

Method 5Method 6

6. Automated grinding and polishing process

Having explained all of the manual processes from the worst (the flap-disk followed by disk polishing) to the best (a small belt used on a flexible arm machine followed by flap-brush polishing), there is a way to eliminate the negative consequences of even the best manual processes and at the same time improve on the positives of manual treatment. This is automated grinding and polishing (in this example we have used the robot produced by Robogrind Oy).

• Risk of high voltage machinery
Although the outside unit of the robot is connected to 380Volts, the risks of electrocution are eliminated by the use of low voltage circuits on the inside unit and various circuit breakers and safety systems as required according to European safety laws for electronic equipment.

• Waste of abrasive
As with the manual small belts, the waste of abrasive is minimal. The constant same pressure and speed make the belt very durable. After intensive grinding, belts can be used again for a second time. This may be needed if a surface is severely discoloured.

• Risk of surface damage
Risk of any surface damage is minimal. The position of the robot is auto-correcting so belts will always contact at a straight position.

• Stock removal
Stock removal is extremely good and limited only by the properties of stainless steel and the quality of the end-result required.

• Surface quality after treatment
The surface quality is unequalled by any other method. The constant contact speed along the circumference, the same contact pressure, the same belt speed (rpm) and the vertical straightness of the movement, consequently results in equal cutting during grinding and a finish at the highest possible quality. It is only limited by the total time a job may take.

The two best methods of automated polishing

Re-fining with an abrasive belt
Although this method of polishing might give the optimal result in multiple step treatments, it is less effective in a single step treatment. As the abrasive is thin and stiff, only the highest peaks in the surface are touched. The belt will not get in between the peaks to reach the lower ones in the so called “under groove”.

On a more technical note, it will not reduce the number of peaks (thus valleys) per cm but will just flatten the top of a few. In order to flatten more peaks, multiple steps of treatment are necessary whilst decreasing the grit size per step.

Refining with a non-woven belt
As in 99.9% of all jobs, if a single treatment is sufficient then this may be the ultimate belt to use. The belt is built up of a thick layer of nylon-like abrasive fibres on a thick backing. As it is built out of fibres, air pockets make the belt flexible and compressible. These properties ensure that the abrasive fibres pushed towards the lowest parts of the “under groove” encase each singular peak on the surface and “attack” it from all sides.
On a more technical note, it reduces the number of peaks (thus valleys) per cm and takes away the edges of the remaining ones. Less valleys per cm results in the enlargement of remaining valleys (=“rivers” guiding product flow)

To conclude, there is a third way of re-fining – the compound wheel
A compound wheel is, as the name says, a wheel built out of abrasive grit in a flexible resin. Depending on the hardness of the resin, the grits are pushed deeper into the “under grooves”. As the resin becomes softer the wheel will also wear out faster. This is one of the reasons a compound wheel is less suitable for a roughened surface (from grinding) as you need a soft compound to push grits deeper in between peaks but a harder compound is required to have it survive the rough surface.

7. The polishing and re-fining diagrams

Stainless steel is a flexible, ductile and tenacious alloy more than it is brittle and hard. These properties make stainless steel very difficult to cut. For that reason not many processes are fit for grinding and polishing / re-fining as we have already seen in previous notes 1 to 6 (including part one of this two part blog feature.)

Disk polishing
Whilst we polish with a disk (either abrasive or non-woven) we work crosswise on the peaks created by vertical grinding which has the same effect as when we polish with a disk after grinding. For the reason mentioned in the introduction, peaks will bend over in the free space behind them due to the pores created. The surface looks smooth to the eye but in fact is not at all. The pores increase possible pollution as liquids may get trapped in them and become difficult to remove.

Abrasive belt polishing
Polishing with an abrasive belt of a fine grit means that you can’t push it into the irregularity of the surface. For that reason only the higher peaks are flattened. The peaks, however, will not bend over as we work parallel with them making cutting a real possibility. (There is no free space. Free space is only at the sides).

Flap-wheel polishing
Polishing with a flap-wheel causes the abrasive flaps to wear around the peaks. Due to this wearing, the abrasive goes deeper but not totally into the irregularities. As a consequence, peaks are flattened and sharp side edges are cut off.

Non-woven belt re-fining
Polishing or refining with a non-woven belt has the best end result. Abrasive fibres are pushed deep into surface irregularities enclosing all peaks and attacking them by cutting from all sides.

Method 7