powerandpassionI wonder what this method of blasting does to the structural integrity of the materials?
ExBrat, a most interesting question. I don’t know the answer, across the range of materials encountered. Some thoughts :
The temperature of the stream coming out of the nozzle is not the temperature of dry ice. The bulk of the stream is compressed air, which post aftercooling, might be considered ambient. In the name of science I applied the nozzle to my leather boot, to see if I could snap freeze my toes, but I did not. Given the dwell time on most of the samples described so far, I would doubt that there would be much temperature change in the object. In thinking through the logic, an airliner and its range of metal and non metal materials of construction would be exposed to consistently lower temperatures for longer time periods, at height, without adverse affects, or we would know about it.
I did ruminate about ‘heat treatment’, in respect of permanent changes to the grain structure of metallic materials, via ultra cold temperature. In this case, the very short dwell time and the highly localised ‘temperature treatment’, in comparison to total mass of the object, at ambient temperature, would mitigate against any permanent change. Heat treatment requires hours of soaking, in general. I retain an open mind on this issue.
In simple terms ultra cold materials may become temporarily more brittle, so care should be exercised. I think the greatest physical affect is the condensation of moisture in the air on cold materials, after an object has been treated.
For the operator, the entrapment of particles of paint and materials in an aerosol ‘blowback’ require a respirator and safety glasses, as well as adequate ventilation. I do think you get a greater mouthful of CO2 after a few hours of exposure.
A word of warning ! If you do take some CO2 grains home to impress the kids with smoking drinks etc, while Mum is away, think about the potential for a ceramic toilet bowl to crack from rapid differential expansion, when you create a smoking toilet, because it is not easily explained later to Mum, and expensive to fix, as much as it thrills the kids. Only the rapid attendance of multiple urine ‘foam trucks’ dissolved the hissing mass of CO2, saving the day !
powerandpassionI would just like to say that I find this thread most enjoyable and please continue to do dives and somersaults off the high board. For the fat kid floating in the water the odd ripple of logic penetrates and the more ripples the better. Good show !
As with most interesting things in science and industry, you get to realize that ‘they were making it up as they went along’.
I would be very happy to receive any info on the Peregrine, all in good time.
I am specifically after detail on the additional cylinder hold down bolts which I believe came in with the Kestrel XVI, traveled into the Peregrine and early Merlin up to Merlin III one piece cylinder bank. These bolts held the cylinder ‘up’ against the top, to manage coolant leaks into the top of the cylinder. The giveway are removable blanking covers on the side of the bank that allow you to insert a spanner to tighten the end of the bolt. Most ideally I am after the remains, just a handful with at least one intact bolt, of any Kestrel XVI, Peregrine or early one piece Merlin bank to observe how the bolt was integrated into the casting and the metallurgy of the bolts. Sorry for going off topic. Pictures of the feature attached.
powerandpassionY alloy piston
This heavily damaged piston with carbon deposits suffered until the dry ice cleaned most of the carbon away. The good news is that you can blast away at the carbon without marking the aluminium. The beauty of the process is that fine details such as part numbers, which might be affected by traditional blast media, are unaffected.
powerandpassionCorroded magneto
This magneto lay for many decades in the mud, with its bakelite distributor facing down. The aluminium body is quite ‘fuzzy’ and oxidised, with parts of the body eaten away. The distributor and points housing was caked in mud and ferrous components heavily oxidised. Aggressive blasting was used to clean away the mud, grime and oxide. The bakelite distributor, brass points cap and brass label on the side of the magneto body was unaffected, while the unit was cleaned up.
powerandpassionStainless steel fish plate
The following items were subject to far more aggressive blasting, by holding the nozzle close and keeping it on, to see whether it was possible to remove iron oxide, paint or heavy deposits. In this piece, it was desired to leave the stainless steel unaffected, while trying to remove heavy rust from the carbon steel component. Traditional soda blasting will ‘matt’ the stainless steel, while a polished surface is desired to minimise fatigue cracking. This is a wing spar fitting from a Hawker Hind, similar to Hurricane, where the object is often found with heavy corroded carbon steel fasteners, while the stainless steel fishplate is sought for recovery, without damage. In this case heavy treatment did not mark the stainless steel, but also did not entirely remove the iron oxide to show clean metal. In this case, clean carbon steel is irrelevant, as that part is not ever usable. Dry ice blasting did remove loose rust, which would allow other processes access to removal of the stainless steel fish plate. I would be interested in any insights in ‘undoing’ heavily corroded carbon steel fasteners, such as those shown on this piece.
powerandpassionPneumatic brake unit
A Dunlop pneumatic brake unit was cleaned up without affecting an anodized aluminium label or introducing blast media into a finely constructed unit. I would consider dry ice blasting suitable for cleaning carburettors, generators, pneumatic and hydraulic components where the introduction of blast media is unacceptable, but these antique components invariably arrive covered in the grime of the ages.
powerandpassionMagnesium throttle box
A cast magnesium Mosquito throttle box with brass labels, wiring, string and bakelite features was tested to remove small traces of white magnesium oxide and accumulated grime. The magnesium was unaffected by a moderate pass nor were the bakelite or brass components. I would consider the process suitable for cast magnesium.
powerandpassionSpruce timber component
A small wing component with a spruce infill, duralumin structure and red fibre lead was tested. The antique spruce was quite soft and damaged. The dry ice obviously cleaned it but also removed the soft wood fibres between the grain of the timber. Dry ice treatment may be suitable for new, sound timber, but great care should be taken with antique timber. The red fibre lead was also damaged, albeit cleaned up. The aluminium was unaffected.
powerandpassionWebbing
Dry ice blasting can quickly destroy cotton insulation, fabric and other soft, friable materials. This experiment involved a quick, brush pass over webbing, where mould and moisture stains affected the woven cloth and steel buckles had surface oxide. I wondered if the printing would be removed, but a careful pass did not affect the object, while cleaning it up. It may be possible to spruce up leather and fabric components, with lower air velocities and smaller dry ice blasting units.
powerandpassionWing light fitting with oxide and fabric
This wing light fitting was made from soft, pure aluminium with elements of heavy, feathering oxide, doped fabric, carbon steel, timber. By varying dwell time the item was cleaned without destroying the doped fabric or driving off the heavily oxidised aluminium. In this zone, a previously unseen part number was made visible. The advantage in using dry ice blasting for ‘incremental’ exposure of features, much as an archaeologist uses a paint brush to work around a piece of ancient pottery is useful, in comparison to traditional media blasting. This part number, above the rusted screw in the last photo, would have been lost with plastic bead or soda blasting. The only disadvantage with the archaeological approach is wearing a topee and dealing with Lara Croft while you do it.
powerandpassionCopper lead engine bearing
A copper lead engine bearing was blasted aggressively to see if there were any affects on the soft bearing material, which there were not. The removal of gum deposits in small oilways on copper, white metal and silver engine bearings may be a suitable application for dry ice blasting. The removal of gumming from nitrided crankshaft surfaces may be a suitable application fro dry ice blasting. This steel backed bearing has a carbon steel backing with surface rust. Dry ice blasting cannot remove iron oxide, it is not sufficiently aggressive as traditional soda or grit type blasting. In the case of conrods and steel bearing caps, a smooth surface finish assists in managing fatigue cracking. I would consider a polishing wheel to slowly wear off iron oxide from the steel backed bearing, after dry ice blasting, but that is another experiment.
powerandpassionPotts Oil Cooler
A Vickers Potts Oil Cooler from Anson is made from brass sheet, zinc coated, with rivets coated in soft solder. The aluminium fittings are die cast with a smooth finish. On this piece aggressive blasting was used to remove paint, oxide and grease without harming the brass, zinc coating, solder or diecast aluminium surface. Mr Potts would be happy. The dry ice did not remove staining from the die cast aluminium fitting, which I assume might be from acids or other engine combustion byproducts in old engine oil. In this case a careful chemical treatment with phosphoric acid may deal with the staining, but that is another experiment.
powerandpassionPetroflex hose
A few more experiments with soft materials before progressing to paint stripping…1930’s petroflex hose where a consistent, long blast was played upon the metal fittings with a brush over the fabric hose component to clean it up. What was not apparent when the hose was dirty was a number of very fine earthing wires run laterally under the wire spirals. I would punt that these copper wires are soldered to the metal end fittings to provide earthing. It is the capacity of dry ice blasting to bring out details such as these that I find the best attribute of the process.
powerandpassionAlex, the setup requires a high capacity, trailer mounted air compressor, 400 CFM, with after cooler. If there is no aftercooler the compressed air is ‘too hot’. The dry ice blasting machine is a small unit, about the size of a large MIG welder, with a hopper taking about two buckets of dry ice ‘grains’, about the size and shape of rice grains. The dry ice comes in a 175kg chest. As there is no ‘dust’ associated with using blasting media we did this in the open air. It is quite noisy, sounding like a ‘broken air hose’, so hearing protection is absolutely necessary. The dry ice stream is not particularly ‘dangerous’, in the sense that particles of blasting media are being flung back, because the dry ice turns to gas near the exit nozzle. Basic safety glasses sufficed.
The only dangers I can think of are inhalation of particles of paint, cadmium coating etc or CO2 build up in an enclosed space.
Both the compressor and dry ice blaster are hired, for a minimum week hire, basically an AUD2,000/GBP1,000 exercise with dry ice ‘grains’ costed at AUD2 per kg. You can use up 20kg in 5 minutes, depending on mixture settings. It’s not cheap. I used over 1000kg for the industrial purpose, perhaps 100kg for the items seen here. The investment makes no sense unless you can line up 50 -100 items you wish to blast clean in a week, in which case the per item cost becomes a more reasonable individual amount. There are some people that go around with these units to do blasting, but not many. I chased one guy for months to come and do a job, which, in the end, stimulated me to hire the equipment myself. A dry ice blasting unit can cost AUD30,000 -50,000 to buy, so there are not that many around.
For me, the results that can be achieved speak in favour of the method, where the equipment may be hired once very six months to work through 200 accumulated items. Costs of AUD3,000/200 items = AUD15 each. The variety of materials that can be treated, the avoidance of blasting media, the avoidance of damage to soft metals, ‘clean’ work to grapple with, the convenience of ‘in house’ treatment are all good things.
My mind is drifting to ‘CO2 sessions’ once every six months in Melbourne where other folk can come in and blast away for an hour, paying a contributory fee, including beer tax.
powerandpassionMagneto
The next curiosity was a weathered magneto, with exposed windings, points and diecast aluminium body. It would be unattractive, again, to ‘wash’ or use any blasting media on such a piece, but by varying the distance and time across the various parts a good result was obtained, making it much easier to start to work on it. The windings were covered in dust and grease and an original coat of sealing paint or shellac on a fabric covering. The gentlest of passes removed the dust and grease, without affecting the fabric. It required great care. The points were cleaned up without damage and the die cast aluminium was cleaned without ‘matting’. A fine brass, mesh gauze for ventilation was unaffected.
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