By Doug Kaufman
ENGINE BUILDER Magazine
One of the least pleasant parts of any engine builder’s day is the cleaning process.
Removing the oil, carbon and general gunk from blocks, cylinder heads and assorted parts before your inspection, machining and rebuilding process can even begin may seem as painful as a trip to the dentist’s chair.
But if you think that part of your day is bad, fail to pay attention to what happens AFTER you clean, and you could find yourself in an even more uncomfortable seat across the table from the EPA.
The U.S. Environmental Protection Agency (EPA) is the regulatory body that writes federal regulations to implement laws made by Congress. For example, the Resource Conservation and Recovery Act (RCRA) is the law under which EPA regulates all solid and hazardous waste. With a goal of protecting the environment from the hazards of waste generation and disposal, the law puts definite restrictions on how you manage your shop’s waste.
Have you considered how your preferred method of cleaning fits into the law? As a business person you obviously want to find the most cost-effective method available. But as a responsible citizen, you want to be conscious of your responsibilities to future generations.
Methods of Cleaning
Cleaning cast iron and aluminum, for the most part, represent two separate challenges. However, in a realistic world most shops cannot afford two different setups for cleaning these metals.
Most shops have some type of cleaning equipment, whether it is an automated jet spray washer or an oven and airless shot blasting system. Some shops have more than one type of cleaning system and therefore can take advantage of the best attributes of each. It’s accurate to say that there is not one perfect system. Each system offers benefits but may also leave some things to be desired in the end result if used on its own.
If you use a hot tank or a jet spray washer you are familiar with some of the chemicals that go into them. There are a variety of chemicals available from different chemical manufacturers, and those manufacturers should be consulted about the right solution for your shop’s needs.
If you clean a variety of metals, then a straight cast iron solution probably won’t meet your standards. According to manufacturers, dual-purpose chemicals will clean cast iron reasonably well and aluminum very well, and are the most popular with engine builders today.
Cleaning aluminum with an aqueous system requires using a compatible solution that won’t etch or discolor the metal. When exposed to certain chemicals, aluminum forms a protective oxide layer that must be broken down in order to effectively clean the aluminum component removing it takes a caustic solution. A chemical that contains inhibitors will protect the aluminum from oxidizing and turning black. The black oxidation can be removed with a de-oxidizer, which will then produce a very uniform aluminum surface.
One chemical manufacturer says to use a chemical with a high pH so you can clean heavy contaminants and also protect the aluminum surface. Another added benefit of a high pH solution is that it is safe for all metals. Therefore the best performance from a chemical solution can be achieved by using a high pH level with some type of caustic to cut through the oxidation layer and using an inhibitor to protect the aluminum surface after it has been cleaned.
Basically, you want to clean with as much strength as possible without attacking the aluminum surface. With a multi-metal chemical, it should be safe on aluminum yet strong enough to clean cast iron, so you can use one chemical and one system for all of your cleaning needs instead of operating and maintaining two separate systems.
A spokesman for a well-known cleaning equipment manufacturer says that while aluminum-safe chemicals may not be as aggressive as those used for cast iron, they are safe for both products. Consequently, your cycle times may last a bit longer on cast iron components, according to this manufacturer. The results will still be the same and in the end you’ll get the component clean – it’s just going to take a little longer.
In addition to choosing the right chemical for the job, the manufacturer reminds engine builders to use the proper temperature setting on your spray washer or hot tank and to be sure you have the right concentration of chemical as well. You want it strong enough, yet not too strong. It is possible to actually mix the solution with too much chemical, which will cause other problems like discoloration and etching. By monitoring the pH levels, you can be sure the solution stays in the 12- to 14-pH range in most cases.
Engine builders have used ovens to clean cast iron components for years. One of the biggest reasons for using a thermal cleaning system is that it’s relatively hands off. A technician can put the piece in the oven and walk away for a while until it’s done baking. Then he takes the work piece out of the oven and puts it into the airless shot blaster and walks away again.
Thermal systems also produce a work piece that looks more aesthetically pleasing than with some other systems. However, with aluminum there has always been some concern about heat ruining the component. Cast iron is not as susceptible to heat damage as aluminum, so it can run through an oven cycle for longer without losing its strength. But some engine builders have the perception that using an oven to clean aluminum is unsafe. According to our experts this is a common misconception in the industry. You can safely clean aluminum with the oven method but it requires careful attention to temperature and procedures.
According to some manufacturers, there are effective ways to clean both aluminum and cast iron in one setup, which they say is procedurally related. If you were cleaning with the oven and airless shot blasting system, some experts recommend using a stainless steel cut wire instead of stainless steel cast. And some caution against using standard steel shot like SS-170 grade because it’s a cast shot that tends to atomize very quickly.
Essentially what happens when you clean cast iron is the material has a tendency to adhere to the shot itself. In so doing, the shot will become coated by the cast iron piece you just cleaned. So when you introduce an aluminum component to the airless shot blaster after a cast iron piece has just been cleaned, the cast iron particles will smear or transfer onto the aluminum piece, causing it to become discolored. In addition, this will aesthetically challenge the end result.
The key to thermal cleaning especially with aluminum parts, say experts is to dry the oil and sludge to a dried ash state. There are various ways to do this and each manufacturer has its own unique system of heating the components. One way to do this is with an indirect heat as opposed to direct heat.
This is called a convection process and involves swirling the hot air around in the oven. It generally heats the component much quicker than conventional oven heating methods. The advantage to having this type of heating element is that the component is brought up to temperature more evenly, which our experts say is safer for aluminum. There are no temperature spikes, which may cause parts of the component to lose strength.
Compared to aluminum, our experts say cast iron can be cleaned much quicker and with higher temperatures because there is not the same concern with annealing that you have with aluminum. Without a metallurgical lab, though, it may be difficult to tell what composites and heat treatments are involved with today’s aluminum components. Therefore some experts believe it is better to stay on the safe side of temperature and time exposure unless you’re certain the aluminum component can handle it.
Heat cleaning aluminum may take extra time and care compared to cast iron, but that doesn’t mean you can be carefree about cleaning cast iron. In the old days it was much more straightforward to clean cast iron than it is today because bearings were softer, oil was thicker and bearing tolerances were much greater it just wasn’t as critical to have parts very clean. Today, a small particle of dirt embedded in a bearing can do a lot of damage.
Ultrasonic cleaning systems are seen more and more in today’s shops because of its ability to clean right down to the component’s bare surface. Ultrasonic cleaning is the process of creating tiny bubbles on the surface of the part, which then act as scrubbing agents to pull dirt particles away from the component surface. Like other aqueous systems, ultrasonic systems use a chemical solution to help get the part clean.
The difference with ultrasonic systems is that the bubbles create a cavitation effect that can reach virtually every area of the component that is exposed to chemical solution. Anywhere water can reach, it can clean. If you are cleaning both aluminum and cast iron you will have to use a chemical that is safe for both. The chemical you use may be the same as for the jet spray washer, however our experts say that concentration levels can be much lower with an ultrasonic system.
Some manufacturers of this equipment claim that it eliminates 90 to 95 percent of the labor involved with cleaning because you can set the part in and it will do all the work. Some areas may have to be lightly hand cleaned or brushed off but in many cases the part comes out with no need for further cleaning. Ultrasonic systems are also very fast, cleaning many components in 10 to 20 minutes. Engine builders we spoke to who use these systems say that they work extremely well on aluminum because of the ability to get into all of the galleys and blind passageways.
According to ultrasonic system manufacturers, engine builders are typically using these systems for precision cleaning. They use the jet spray washer to pre-wash the parts and remove heavy grease and then put it in the ultrasonic system for the final wash cycle.
Some stubborn grease and hard, baked-on carbons can only be removed by using some form of blast media. Manually operated blast systems like a glass bead machine, for example, direct a high-pressure stream of glass media at the surface of the part. Bead media does produce a nice and clean looking finish, but it can be difficult to remove the media afterwards. Glass media can also become embedded in the surface of aluminum components, due to its soft metal characteristics.
Soda blasting equipment that uses a form of baking soda media for removing baked-on carbons and sludge is another option for components that are too difficult to clean using other systems. System makers say their products can safety clean many components without damaging sensitive substrates.
These systems are "one pass," meaning the media is not reusable. Soda media is water-soluble so you don’t have to worry about blasting away in hidden passages. Soda breaks and becomes "fractured" when it hits the surface of the part, turning into a fine particle that can be simply washed away.
Some shops use a soda blast system for spot cleaning purposes. These shop operators tell us they blast the most difficult areas with the soda and then allow their other systems to clean the rest of the component.
Because it isn’t reused, the cost can be a little bit higher with soda but it also can save labor time due to the fact you don’t have to mask off anything or pre-wash the parts. And by minimizing how much they use the system, some engine builders say they are able to keep the cost down.
Wet Slurry Blasting
Wet slurry blasting, which has been used successfully in the aerospace industry for years, has been recently introduced into the engine rebuilding industry. One of the reasons it has not made it into this market sooner is that it’s often seen as too expensive for most two man shops; but now with as much aluminum castings as the industry is dealing with, it may be a more attractive option for some of the larger production shops, according to a manufacturer of this equipment.
The wet slurry blast system basically mixes water and an abrasive media together into one solution in a sump, which then sends the mixed solution to the nozzle. The slurry solution can be either used in a manually operated blast cabinet or in an automated system with multiple nozzles.
Unlike a sandblast or dry-blast process that strikes the part once and ricochets away, the slurry actually scrubs the components, says a manufacturer. Because the water surrounds the media it protects it from becoming embedded in the surface itself. The system can do extremely fine cleaning as well as remove scale, carbon buildup and get into all the nooks and crannies. The system can also handle parts with oil and grease on them, eliminating the traditional steps of washing and drying the parts before putting them into a dry blast cabinet. With a wet slurry system you can do that all in one process.
One of the advantages wet slurry blasting has provided to the aerospace industry is its capability for exposing very fine cracks in a casting. Blasting a casting with a traditional media blaster might actually push the top edges of a microscopic crack together, making it impossible to see even with magnetic particle inspection. With slurry blasting the water gets in there and it will actually clean out that crack, leaving it exposed for inspection.
You still must rinse and thoroughly inspect the part afterwards to be sure all the abrasive media has been removed. The lone manufacturer marketing this system to the industry says that it is addressing this problem by adding a secondary rinse system, which will be tied to the machine as part of the filtration package. It’s a closed-loop system that includes a filter to separate the oil and waste.
There are many alternatives for engine builders when cleaning cast iron and aluminum components. Finding what works best for your shop is the key. For some rebuilders using a combination of cleaning systems works best, and for others an old tried-and-true method like thermal cleaning works for them.
With aluminum becoming more and more abundant in the industry you have to work it into your cleaning process as seamlessly as possible without giving up any quality. Most systems offer ways to clean both metals but you may have to add more cycle time and adjust your procedures a bit to compensate.