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Precision Cleaning, Passivation,
& Finishing Systems

FAQ – Frequently Asked Questions

This page is comprised of various commonly asked questions Best Technology is often asked about part cleaning, passivation, electropolishing, wet and dry processes while researching new equipment to fit a customers manufacturing process needs.

Part Cleaning Equipment & Process FAQs

What is degassing and how to degas ultrasonic cleaning tanks?

Naturally any water that comes out of a pressurized water supply will contain dissolved gases and these gases need to be removed or degassed from the solution for maximum ultrasonic cleaning performance.  The reason for removal is gases are easier to compress than liquids; therefore, when the ultrasonics cavitate the solution, some of the cavitation energy is absorbed by the gases.  

Degassing solution is easily achieved by either letting the solution sit out for number of hours.  This is why a glass of water tastes “different” when first out of the faucet vs. drinking it hours later.   Running the ultrasonics in a tank will expedite the degas process significantly typically down to 5-10 minutes.  Keep in mind solution only needs to be degassed when first dispensed from a pressurized supply.

Although 5-10 minutes is much shorter than hours achieved by letting the solution sit, 5-10 minutes is still too long to wait for our automated cleaning and passivation systems to degas each time the solution is pumped from the storage tank to the process tank.  Our system feature a fast-degas feature at the start of the ultrasonic cycle which allows the solution to degas in a matter of seconds vs. minutes.

The fast degas feature can be heard in the video below (note high pitch of ultrasonic degas and tuning amplified for video demonstration)

How to tell if a solution is degassed or not?

Solution de-gases by simply releasing the dissolved and entrapped air in solution.  During a degas process with ultrasonics,  fine bubbles will suddenly appear and begin to rise to the surface of solution (similar to that seen after first pouring a glass of beer) This implosion or cavitation of solution with dissolved gases can result in high pitched audible sound from the ultrasonic tank until the solution is degassed as heard in the video above.

Once a solution or fluid is degassed either by letting it sit, ultrasonic cavitation energy, or heating, it does not need to be degassed again unless the solution replaced with new fluid.

 

Why are automated systems easier to process validate than manual equipment?

Process control and stability are critical aspects to regulated medical device and aerospace processes.  It is important to ensure that a process has input and output variable limits which are defined and fully tested during process design, Equipment Qualification (IQ), Operational Qualification (OQ) and Process Qualification (PQ) validation testing.  Setting up a proper DOE (Design of experiments) to test these limits is also important as it result of the DOE will give statistical confidence intervals of the limits.

Being that operators and employees perform various process operations different no matter how instructed in work instructions, the variation of operators must also be captured during process qualification (PQ) validation.  An automated system typically eliminates many of the operator variability in the manufacturing process and this process “input” elimination also allows for tighter process output controls.

For example, in our automated passivation system, the elimination of relying on an operator to move the parts basket from stage to stage ensures that the parts remain in the appropriate (wash, rinse, acid passivation, etc) solutions for the process defined times and in accordance with the proper ASTM A967, AMS2700, etc specification.  If a parts basket is immersed in the acid passivation solution too short or long duration, the passivation can likely fail and be outside specification limits.

Why does spotting occur on parts after washing and DI rinse?

There are three ways that spotting can occur:
  1. If there is soil introduced with the rinse (ie: contaminants in the DI bath),
  2. If soil is introduced in the air stream (ie: either present in the atmosphere and blown onto the parts or circulated from the air supply into the heater and blown onto the parts), or 
  3. If soil is left as residue from the wash process (this could either be soil that was originally on the parts and not completely washed off or it could be residue from the cleaning chemistry that is not completely rinsed off).

What’s the difference between solvent based cleaning and aqueous cleaning?

There is an old saying when it comes to parts cleaning: “Like dissolves like”.

This comes from the world of chemistry, and is really quite a simple and useful phrase to remember. In chemistry molecules are described as being polar or non-polar. (Think north and south pole on the Earth)  Polar molecules have a polarity that causes them to attract other molecules that have polarity, while non-polar molecules do not.

Water is a polar molecule.  Oil is not.  At the molecular level this is why “oil and water don’t mix”.  Chemically they are dissimilar and cannot absorb each others molecules.  By contrast salt IS polar; this is why you can dissolve salt in water.

Solvent based cleaning systems (like Vapor Degreasers) are used when you need to clean true oils from your manufactured parts.  Aqueous Cleaning Systems are used to clean water based materials from your parts.

When parts are washed the parts themselves, as well as the basket they are in, carry some of the wash with them into the rinse tank. This “drag out” means that the rinse solution has to be constantly replaced or will simply become less and less clean over time. The biggest issue is not that the parts will be rinsed off, but that when the parts are withdrawn from the rinse tank, they may have soil redeposited on them. Once the parts are dried this soil can cause spotting on the surface of the otherwise clean parts. A second rinse bath produces a much cleaner final product by rinsing off the soil that is redeposited during the first rinse.

Often times, the second rinse tank includes a heated facility water inlet which constantly overflows the second rinse tank with small amounts of water to ensure water cleanliness.  The second rinse tank overflow is sent to rinse tank 1 and then rinse tank 1 overflows to drain.  This cascade overflow process ensures constant water quality over time no matter the amount of drag out on the parts and baskets.

How does a rotary drum parts washer work?

A Rotary Drum Parts Washer is essentially a large “inside-out” screw.  Small parts are fed into the opening and the continuous rotation or tumbling of the barrel causes the parts to travel along this auger from one end of the barrel to the other.  As the parts travel through the inside of the barrel they pass through a wash area, a rinse area (or areas) and finally drying areas.

Small perforations in the side walls of the barrel allow the cleaning solution and rinse(s) to envelope the parts.  In addition, high powered spray jets spray on the parts during both wash and rinse cycles to provide even greater cleaning action.  Finally, the action of the parts traveling along the auger causes a tumbling action of the parts so that not only the aqueous cutting fluids are cleaned from the parts, but also small particulates are brushed off and drained away.

Once the parts are completely washed they travel through the dryer area of the barrel where a combination of heat and blowing air dry them before they are transported out the end of the barrel for final packaging.  These rotary drum parts washers are ideal for stamped parts, die-cast parts, cold heated parts and many more parts where high volume throughput are required.

How does a vapor degreaser work?

If you’ve ever been wearing glasses as you walked into an air-conditioned building on a hot summer day, you already have a good understanding of part of the Vapor Degreaser process works. (For those in colder northern climates, walking outside while wearing glasses on a cold winter day is an even better example)

A vapor degreaser has two tanks (sumps) of solvent inside.  One boils the solvent (boil sump) which creates a vapor or mist of the solvent. The second sump (ultrasonic sump) is heated but not to the boiling point and is used as the second cleaning stage.  The vapor degreaser also has bands of cooling coils inside just above the level of the sumps. These coils cause the vapor to return to a liquid state and fall back into the sump.  The effect is like small “clouds” of the solvent are formed between the top of the sumps and the cooling tubes.

As parts at room temperature are lowered through the cooling area into the vapor, the vapor from the boil sump condenses on the parts just like moisture in the air does on your glasses in the examples above.  This condensation contains the solvent that dissolves the oils on your parts, and the beading action creates droplets which run across the surfaces of the parts and fall back into the boil sump. The parts are then moved to the ultrasonic sump which contains heated but non-boiling solvent.  This allows the parts to be lowered into the sump so that any blind holes or internal features are also thoroughly exposed to the solvent.

Finally parts are raised into the cooling coil area to allow the solvent to quickly dry and and then raised through a second layer of freeboard coils near the very top of the vapor degreaser that insure complete drying and the recapture of the solvent from the parts.

The below video demonstrates the process:

Is desiccant required in a vapor degreaser?

Desiccants are used to absorb the water found in humid conditions to reduce or eliminate condensation. It can also be added directly to liquids to absorb the water content from the fluid.  We are used to seeing the small white bags of desiccant found in packaging for everything from shoes to electronic equipment.  Most of this desiccant is silica – typically in gel form.  Other common substances used as desiccants are activated charcoal and calcium chloride.

The desiccant used in vapor degreasers is 3 Angstrom Molecular Sieve, small pellets of zeolite clay. Like all desiccants, the zeolite clay adsorbs water from the solvent, and may be reused by baking it dry. Desiccants are most often used in a vapor degreaser if the solvent contains an alcohol. This is often the case with solvents used for defluxing processes on soldered boards and leads. Water found in the separator extracts the alcohol from the solvent and in turn the water and alcohol is absorbed by the zeolite clay. If a degreaser is operated in a very humid environment a desiccant may be need to effectively remove the water from the solvent.

 

Part Passivation Equipment & Process FAQs

What is the Best Method of Passivation?

Have you ever noticed that many day-to-day things seem to get divided into two opposing camps?  Things like asking what’s the best computer quickly become the PC versus Mac debate.  Phones? iPhone versus Android.  So, passivation?  Yup, nitric versus citric.

There are real advantages to each nitric and citric passivation, but that is a topic for other FAQ’s because once you pick one you still have several choices to make.  Choices like: which standard do I use? which method do I choose?

There are two primary standards that address passivation. The SAE Aerospace “Passivation of Corrosion Resistant Steels” (AMS2700) and the ASTM “Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts” (A 967).

Each allows for citric or nitric and each allows for several options within each citric and nitric.  So what’s the difference? Each standard is copyrighted (and protected) and has to be purchased from the publishing agency, so getting into a lot of detail is not allowed. But here’s a short version of the differences in the various methods:

Concentration of the specific acid – each method varies based on how concentrated the citric or nitric acid is – and in the case of nitric if the nitric acid is used alone or with sodium dichromate.

Temperature of the bath – again, each method specifies a temperature range that must be maintained for the duration of the passivation bath.

Time – the length of time a part must remain within the passivation solution is spelled out as well.

What seems complicated becomes really simple.  The standards spell out the concentration, the temperature and the time.   Each method simply varies one or more of those three items.

 

What’s Involved in the Passivation Validation Process?

Within the medical device world there is a need to validate the passivation process.  But what does that mean, and how does that work?

Validation is the process of insuring that the passivation process you use will reproduce repeatable and predictable results every time a batch of parts is run through the process. By validating the process you are able to forego subjecting every part to testing to prove that it is properly passivated.

Typically you will here the validation process broken down into three distinct parts: the IQ, the OQ and the PQ.  Let’s look at each part.

The IQ or Installation Qualification is the first part.  It is developed by describing the machine – what is it? what does it do? etc. It also looks at what the components on the machine are, gauges, switches, PLC, etc.  It provides a description of the machine and its parts – what is it and how does it work?

The OQ or Operational Qualification is the second part. It essentially help you verify the IQ – does the machine operate as it is supposed to?  Do the components do what they are designed to do? etc. – does everything work as intended?

The PQ or Process Qualification is the third part. If the IQ is the theory of how things SHOULD operate and the OQ is the practice of how things DO operate, then the PQ defines how CONSISTENTLY the machine operates.  You create a DOE (Design of Experiments) that tests the equipment at the top end and bottom end of allowable ranges and run parts to verify the results across the entire range of the variables. Now when the machine operates within the specified range of time, temperature and/or acidic concentration you know that your parts will meet the specs.

Why are automated systems easier to process validate than manual equipment?

Process control and stability are critical aspects to regulated medical device and aerospace processes.  It is important to ensure that a process has input and output variable limits which are defined and fully tested during process design, Equipment Qualification (IQ), Operational Qualification (OQ) and Process Qualification (PQ) validation testing.  Setting up a proper DOE (Design of experiments) to test these limits is also important as it result of the DOE will give statistical confidence intervals of the limits.

Being that operators and employees perform various process operations different no matter how instructed in work instructions, the variation of operators must also be captured during process qualification (PQ) validation.  An automated system typically eliminates many of the operator variability in the manufacturing process and this process “input” elimination also allows for tighter process output controls.

For example, in our automated passivation system, the elimination of relying on an operator to move the parts basket from stage to stage ensures that the parts remain in the appropriate (wash, rinse, acid passivation, etc) solutions for the process defined times and in accordance with the proper ASTM A967, AMS2700, etc specification.  If a parts basket is immersed in the acid passivation solution too short or long duration, the passivation can likely fail and be outside specification limits.

What is passivation?

A non-electrolytic process typically using nitric or citric acid which removes free iron from the surface and forms an inert, protective oxide layer that in turn renders the stainless steel more rust-resistance due to lack of iron to react with atmosphere. During machining manufacturing processes, the stainless steel parts may have imperfections from iron being embedded or smeared onto the surface from the machining tool steels. These free irons on the surface of the stainless steel need to be removed to prevent a corrosive reaction can occur between the two different metals. The passivation process chemically removes these free irons and forms a passive oxide “film” layer which further improves corrosion resistance. When exposed to air, the stainless steel undergoing passivation will form a chemically inactive or inert surface. This is one advantage of using citric acid vs. nitric acid for passivation. More detailed information on our passivation page

When parts are passivated with citric or nitric acid, the parts themselves, as well as the basket they are in, carry some of the acid  into the rinse tank. This “drag out” means that the rinse solution has to be constantly replaced or will simply become more and more acidic over time. The biggest issue is not that the parts will be rinsed off, but that when the parts are withdrawn from the rinse tank, they may have soil redeposited on them. Once the parts are dried this soil can cause spotting on the surface of the otherwise clean parts. A second rinse bath produces a much cleaner final product by rinsing off the soil that is redeposited during the first rinse.

Often times, the second rinse tank includes a heated facility water inlet which constantly overflows the second rinse tank with small amounts of water to ensure water cleanliness.  The second rinse tank overflow is sent to rinse tank 1 and then rinse tank 1 overflows to drain.  This cascade overflow process ensures constant water quality over time no matter the amount of drag out on the parts and baskets.

What’s the difference between nitric and citric passivation?

Years ago, many avoided citric acid due to potential organic growth and molding issues.  Today, citric acid has improved leaps and bounds with new formulations for biocides to prevent any organic growth in solution.  See our complete write up on the advancements of citric acid (specifically Citrisurf).  These new advancements in citric acid have allowed smaller manufacturers which had little to no experience with chemical handling and processing to bring their passivation needs in house. Today, many companies are taking a second look at citric acid due to its ease of chemical handling & disposal and safe for employee exposure all of which are more difficult with nitric acid.

What’s the difference in passivation and electropolishing?

As a non-electrolytic process, passivation uses solutions like citric and nitric acids instead of an electrical current used in electropolishing to create a inert oxide layer / film and remove free iron and foreign matter from metal surfaces. Electropolishing, a non-mechanical interactive process, can be used on objects with complex geometries.  The electropolishing  process uses a combination of electrolytic chemicals and an electrical current to carefully eliminate imperfections and contaminants of metal part surfaces. This particular process is an alternative to abrasive fine polishing.  Electropolishing can be used to polish, deburr, and smooth metal components whereby doing so also causes material removal to occur including the free irons on the surface of the part. Electropolishing would be the more appropriate approach when  aesthetics are necessary.  A smoother, polished surface can result from electropolishing. Passivation, on the other hand, does not change or brighten the surface appearance and is not an effective method for removing surfaces that has been welded with oxide heat effect scale or heat treated.

 

Part Electropolishing Equipment & Process FAQs

What is electropolishing?

Electropolishing  is an electrochemical process that removes material from a metallic part, typically stainless steel or similar alloys.  The part is immersed in a temperature controlled bath of electrolyte (typically mixtures of sulfuric acid and phosphoric acid) and the part serves as the anode as it is connected to the positive terminal of a DC power rectifier DC and the negative terminal of the rectifier is attached to the cathode which is typically made from titanium or other alloys which do not dissolve during the electropolishing process.

Visit What is electropolishing? How does electropolishing work? for more detailed information on electropolishing

What’s the difference in passivation and electropolishing?

As a non-electrolytic process, passivation uses solutions like citric and nitric acids instead of an electrical current used in electropolishing to create a inert oxide layer / film and remove free iron and foreign matter from metal surfaces. Electropolishing, a non-mechanical interactive process, can be used on objects with complex geometries.  The electropolishing  process uses a combination of electrolytic chemicals and an electrical current to carefully eliminate imperfections and contaminants of metal part surfaces. This particular process is an alternative to abrasive fine polishing.  Electropolishing can be used to polish, deburr, and smooth metal components whereby doing so also causes material removal to occur including the free irons on the surface of the part. Electropolishing would be the more appropriate approach when  aesthetics are necessary.  A smoother, polished surface can result from electropolishing. Passivation, on the other hand, does not change or brighten the surface appearance and is not an effective method for removing surfaces that has been welded with oxide heat effect scale or heat treated.

How many amps of current will it take to electropolish a part?

The current or amps required to electropolish a part is primarily based on the surface area of the part or total surface area if multiple parts are desired to be electropolished at the same time.  More information on electropolishing can be found on What is electropolishing? How does electropolishing work? page

Visit the Electropolishing Current Required Calculation Spreadsheet to calculate the approximate current amperage required for electropolishing

How long will it take to electropolish a part?

The cycle time required to electropolish a part is primarily based on the current / amps and surface area of the part or total surface area and rectifer current if multiple parts are desired to be electropolished at the same time.

Visit the Electropolishing Current Required Calculation Spreadsheet to calculate the approximate cycle time required for a part or total parts for electropolishing

What is electropolishing? How does electropolishing work? contains more in depth information on electropolishing.

 

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