The increased use of turbochargers has become an important trend in car manufacturing. This trend has been driven by requirements to design smaller, more powerful engines that also reduce fuel consumption.
Modern vehicles contain hundreds of electric control units (ECUs). These control functions that range from operation of the engine to keyless entry. An ECU receives input from the vehicle then communicates with an actuator to perform a specific action. For example, a door lock ECU would activate an actuator to unlock or lock a door. OEMs and manufacturers are introducing more ECUs into vehicles to meet demands in features and performance.
A common frustration we hear from manufacturers is that their vacuum impregnation process may seal porosity, but some parts still need to be scrapped due to sealant contamination (Image 1). Most of these manufacturers assume this is normal. The fact is that this is not normal, nor should it be accepted as a typical outcome of the vacuum impregnation process.
Image 1: This feature has excess sealant. The excess sealant will cause quality and assembly issues.
There are a number of reasons why sealant contamination is a problem. Excess cured sealant can affect assembly and part quality. Cured sealant found in tapped holes or blind passages can cause assembly issues or prevent fluid flow in oil passages or water channels. Excess cured sealant on a part’s surface can affect secondary operations, such as painting, plating, or machining. Nonfunctional parts are unusable and must be rejected.
During the impregnation process, uncured, liquid sealant is drawn in the part’s porosity. Residual sealant can also remain in openings such as oil passages and water channels. This is not a problem, as long as the sealant is properly washed from those openings. If that wash does not happen, or it is inadequate, the sealant will cure to a hard polymer and cause problems in those areas of the casting.
The root cause of this contamination may be one, or a combination, of these failure modes:
To eliminate sealant contamination, one first must determine its cause. For an existing program, a manufacturer should review sealant control, then process parameters, and finally equipment capability (Image 2).
Image 2: Proper fixturing enables sealant to be properly washed from machined features.
For a new vacuum impregnation program, one must be aware of the requirements of the parts to be processed. Each program should be evaluated for equipment, sealant, and process as no two programs are the same.
For example, a manufacturer should not use the same process to seal macro-porosity in powder metal as for micro-porosity in aluminum die casting. The powder metal process uses just a passive wash; a more aggressive wash is needed to remove the sealant from small features in the aluminum casting and decrease the risk of contamination.
This simplified blog provides a better understanding of sealant contamination. It’s a starting place to look for the root cause and identify how to correct it. Sealant contamination does not have to be accepted. By eliminating it, manufacturers can maximize their number of parts in production.
There exists a misconception on testing requirements for vacuum impregnation sealants and vacuum impregnation processes. Testing impregnation sealants for application compatibility and testing the impregnation process effectiveness are grossly different.
Vacuum impregnation is a process that seals porosity in metal castings. If left untreated, then the porosity creates a path for fluids and gasses to leak from the part. When performed properly, vacuum impregnation seals the porosity, but it is undetectable on the surface or in the machined features of the casting.
Before vacuum impregnation is applied in production, operators often request indisputable evidence that the process is capable. This is done by measuring key process characteristics of the vacuum impregnation process. Common processes that are tested are:
The sealant gel time test produces a test slug. Often the sealant slug is discarded after the sealant gel time. But before it is discarded, the operator should examine the slug’s color and clarity for other conditions. What is optimum is to have the sealant coming out of the system looking like the clean, clear sealant that originally went into the system. Below are three common reasons why the sealant may not match its original state:
If any of these occur, then the possible action plan can include:
Vacuum impregnation is a process that seals internal pores in metal castings. Sealing the porosity allows the part to hold gas and fluid under pressure. Measuring the key process characteristics and the sealant slug provide traceable, quantifiable and actionable data to keep the vacuum impregnation process effective.
Vacuum impregnation in metal castings and powdered metal parts refers to the sealing of leaks resulting from porosity. In this post, we will discuss in detail one of the selection variables, the three different types of vacuum impregnation processes.
Vacuum impregnation seals porosity and leak paths in metal castings and powdered metal parts that form during the casting or molding process. The process is done by filling the pores with a sealant under pressure to stop fluids or gases from leaking under pressure. Vacuum impregnation stops casting porosity and allows manufacturers to use parts that would otherwise be scrapped.
To say that all vacuum impregnation processes are equal would be to say that every die casting process is the same. Nothing could be further from the truth. The vacuum impregnation process will have a direct impact on the sealing quality.
Ever since metal casting was first discovered, casting porosity, an area of sponge-like internal structure in an otherwise sound metal part, has been a problem. Porous castings may be caused by internal shrinkage, gas cavitation, oxide films,inclusions and combinations thereof. It can be found in virtually any type of metal casting or part, and is a particular problem in castings made from aluminum, zinc, bronze, iron, magnesium, and other alloys. Porosity is always present in powdered or sintered metal parts because of their structural nature.
Various methods have been used to attempt filling casting porosity openings in parts designed to contain liquids or gases under pressure. One of the first materials used for impregnation was “water-glass” or sodium silicate. In addition to sodium silicate, tung oil, linseed oil, pitch gum and many other materials were used with little success. Shortly after World War II, the development of thermosetting plastics, to be used as impregnants, became an effective and economical means of sealing porosity within the walls of metal castings, especially when used in conjunction with vacuum pressure impregnation techniques.
Vacuum impregnation in metal castings and powdered metal parts refer to the sealing of leaks resulting from porosity. The impregnating sealant, as a liquid, is introduced into the voids or porosity within the wall of the part usually using vacuum and pressure. The sealant is then solidified, filling the porous openings and making the part pressure tight.
Related: Watch How Vacuum Impregnation Seals Porosity
Impregnation of powdered metal parts not only seals parts for pressure applications, but also improves plating or finishing, since bleedout or spotting due to entrapment of plating solutions in the pores is eliminated. Extended tool life is another benefit when machining powdered metal parts.
The part was impregnated prior to coating. If it was not impregnated prior to coating,then coating would breakdown.
When castings have blind or continuous porosity areas, impregnation prior to painting or plating improves and protects the final surface finish from bleedout and blistering.
Impregnation technology seals leaks on all ferrous and nonferrous metals, including die castings, sand castings, investment castings, pressure castings, powdered metal parts as well as forgings or weldments. Iron, bronze, aluminum, zinc, magnesium, steel, sintered metal, as well as alloys of these metals can be impregnated. Other non-metallic materials, such as wood, plastic, and ceramics can also be impregnated.
Related: 3 Reasons to Impregnate Powder Metal Components
When casting porosity causes leakage problems, “bad” parts are often sorted out by testing and inspection. The “good” parts that are sent to production are often as porous as the “bad” parts, but the porosity is blind and not completely interconnected. Subsequent machining, mechanical or thermal shock, or stress often breaks the thin membrane which keeps the blind porosity from being continuous, thus causing a “leaker”. Impregnation fills porosity from both sides preventing leaks even if the membrane does break. Therefore, impregnation improves and enhances quality, while inspection only sorts out leakers.
The value added to metal parts by machining, handling, and assembly may range into the hundreds or even thousands of dollars. This value is lost when a metal part is scrapped because of porosity and leaking. Impregnation costs are small fractions of the costs of remelting, recasting, re-machining and part overruns. Impregnation allows the manufacturer to save time, money, energy and insure quality by salvaging parts which would otherwise have to be rejected. The elimination of scrap and rework substantially increases productivity. In addition, 100% impregnation of metal parts sometimes eliminates the need for expensive leak testing, and often results in a dramatic reduction of field rejects in products such as transmission cases, air-conditioners, pumps and other metal parts.
Related: What is the Cost of Vacuum Impregnation?
Impregnation of powdered metal parts provides the added benefit of prolonged tool life (up to 100 times) because Godfrey & Wing sealants serve as lubricants as well as supporting the individual powered metal particles. Lubricity eliminates the chatter effect during the machining process of unimpregnated powdered metal parts.
Because of the proven effectiveness and economies of impregnation, many engineers specify its use for all types of metal parts that must contain liquids or gases under pressure. It is now common for impregnation processes to be incorporated directly into production schedules to insure quality, rather than to be used strictly as a salvage operation.
There are two general classifications of porosity found in metal parts: macro-porosity in the form of large flaws in the part which may be visible to the naked eye; and micro-porosity in the form of very small, almost invisible voids. In powdered metal parts, the structure of the metal results in a condition similar to macro-porosity in castings having low density, and micro-porosity in high density castings.
Related: 3 Types of Casting Porosity
Porosity can be found as “through, blind or totally enclosed” .
Through porosity (highlighted in red) stretches completely through the wall thickness of a metal part causing a leakage path.
Blind porosity (highlighted in blue) is connected only to one side of the part wall.
Totally enclosed porosity (highlighted in green) is totally isolated within the wall thickness of a part.When castings are machined, both blind and totally enclosed porosity are often “opened up” becoming continuous porosity and causing leaks.
There are three common methods of impregnation are dry vacuum-pressure, dry vacuum, and wet vacuum.
The most common process is the dry vacuum-pressure process. The steps to the dry vacuum-pressure are:
Step1:
Parts are loaded into a dry impregnation chamber, and the vacuum is applied until a predetermined setpoint is achieved. This vacuum setpoint has been specified in US military specifications to be no less than 29” of mercury (23.4 Torr or 31mbar). There is no liquid present in the vessel to impede air removal from the porosity. All parts see a uniform vacuum pressure. This is the “Dry-Vacuum” portion of the process.
Step2:
When the vacuum end point is reached, the transfer valve is opened. The sealant is de-gased and pulled from the reservoir to the impregnation vessel while the vacuum is maintained.
Step 3:
Next, the vacuum is released, and overpressure is applied (typically between 70-90 PSI). The pressure is then held to allow the sealant to penetrate the porosity. The transfer valve is re-opened and the sealant is transfered back to the storage reservoir. The parts are removed to be washed and cured.
Related: Types of Vacuum Impregnation Processes
The United States Department of Defense has established military specifications MIL-276A and MIL-I-17563C that outline the requirements for impregnating processes and sealants.
MIL-276A is the military standard for impregnation of porous metal castings and powdered metal components. This standard covers the requirements and tests for the impregnation of structurally sound castings and powder metal components in aluminum, magnesium, copper, iron (excluding steels) and zinc alloys.
MIL-I-17563C is the military standard for impregnation of cast or powder metal components. This standardcovers the requirements for impregnants suitable for use in sealing the voids found in cast or powder metal components which cause leaking of contained fluids.
In order to meet the standards required to produce pressure-tight castings, the ideal sealant must be capable of penetrating and filling the porosity and then solidifying completely within the porosity of the metal parts. The sealant should be a polar, low viscosity liquid containing no inert solvents, no filterable solid materials in suspension and producing no gaseous or liquid by-products on curing or transforming into an impervious solid. These properties allow the sealant to penetrate the tiniest openings and deepest recesses of porosity by capillary action. That is, such a sealant can be drawn in by capillary forces, where it may not be possible to push it using hydraulic pressure alone. In addition, a sealant should be stable, have a long pot life, be easy to handle and test without introducing unacceptable health and safety hazards in the work environment.
We are sometimes told by customers that they are unaware of what vacuum impregnation is. Ultimately, they have a casting with porosity that is leaking whatever is passing through the part (i.e. air, gas, water, etc.), and they need a solution to their problem. Here are some of the more common questions that we receive in regards to vacuum impregnation and porosity. Our intent is that you can use this information as a guide when you need to correct castings with porosity.
Would you like this information in a printable form? Of course! Click here to download it as a pdf.
What is porosity?
Porosity is an area of sponge-like texture in an otherwise sound metal casting. There are two types of porosity:
Porosity is typically caused during the casting process by internal shrinkage, gas cavitation, oxide films and inclusions and the many combinations thereof.
What is vacuum impregnation?
Vacuum impregnation (also known as “impregnation” and “impreg”) seals the porosity in metal castings, thus making leaky castings pressure tight.
What is the process of vacuum impregnation?
In a nutshell, the impregnation sealant is introduced into the voids within the wall thickness of the casting through vacuum and/or pressure methods. Subsequent processing solidifies the sealant.
What are the types of porosity?
There are three types of porosity:
How does impregnation improve product quality?
When casting porosity is reviewed during inspection, “good” castings are as porous as the “bad” castings because the porosity is blind and not completely inter-connected. Subsequent mechanical or thermal shock or stress often breaks the thin membrane in the “good castings” which keeps the blind porosity from being continuous, thus causing a “leaker”. Impregnation fills porosity from both sides preventing leaks even if the membrane does break. Therefore, impregnation salvages castings and improves quality, while inspection only sorts out leakers.
What materials can be impregnated?
All ferrous and non-ferrous metals, whether sand cast, gravity die, pressure die castings or forgings can be impregnated to eliminate porosity. Iron, bronze, aluminum, zinc, magnesium, steel, sintered metals and plastics, as well as alloys of these metals can be impregnated.
Can cracked castings be fixed through impregnation?
No. Impregnation will not increase the strength of a casting. The cracks will reopen when the casting is under pressure.
Can vacuum impregnation cure surface flaws?
No. This is because impregnation is within the part and not a surface treatment.
Should impregnation occur before or after machining?
Impregnation occurs after a casting is machined. This is because machining may potentially uncover additional porosity.
Will impregnation discolor or damage the casting?
No, because impregnation occurs within the walls of the casting. There is no film or coating on the part surface that will change any dimensional tolerances.
These are not the only questions we receive in regards to vacuum impregnation and porosity.
To discuss your specific questions, or situation, contact us at 330-562-1440 or send us an email.
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