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Porosity sealing

  (Redirected from Metal impregnating)

Porosity sealing, is the process of filling a porous substrate to make it airtight. Porosity is sealed through the process of vacuum impregnation. Vacuum impregnation is the preferred method for sealing parts to prevent fluids or gases from leaking under pressure. Vacuum impregnation seals porosity and leak paths that form during the casting or molding process. It does so without changing the castings' dimensional or functional characteristics and allows manufacturers to use parts that would otherwise be scrapped. There are three types of porosity; Flow Porosity, Gas Porosity and Shrink Porosity. Porosity is found in many cast metal components, but it commonly found in aluminium and magnesium die castings. [1]

Contents

ProcessEdit

Porosity sealing is a four-step process:

  1. In the impregnation chamber (also known as an autoclave, pressure vessel or vacuum vessel) air is evacuated from the leak path in the part by using a deep vacuum. The evacuated leak path is filled with sealant by covering the part with the sealant and applying pressure. More energy is required to penetrate the porosity with sealant than to evacuate the air.
  2. In the recovery station (also known as drain station, centrifuge) excess sealant is recovered for reuse.
  3. In the wash/rinse station (also known as surge station, rolling rinse or pump over station) residual sealant is washed from the part's internal passages, tap, pockets and features where sealant is undesirable.
  4. In the cure station (also known as standing, rolling cure or pump over station) the sealant, impregnated into the walls of the part, is polymerized in the leak path.[2]

Vacuum Impregnation MethodsEdit

There are a variety of methods that can be used to impregnate metal parts. The method chosen depends on a part requirements, specifications and sealant that will be used. All methods used are effective and once porous metal parts are sealed and cured, they will be sealed indefinitely unless the sealant material fails in the future due to exposure to chemicals, excessive heat, wear or other damaging conditions.

Dry Vacuum and Pressure (DVP): Dry vacuum and pressure impregnation is the most robust and thorough method of impregnation. DVP is used when a casting or component’s sealing requirements are critical or when parts have a design that makes them difficult to seal using the other impregnation processes. Dry vacuum and pressure is the preferred method for impregnating parts and components manufactured for the automotive, aerospace, and defense industries. The process steps are as follows:

  1. 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.
  2. 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.
  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 transferred back to the storage reservoir. The parts are removed to be washed and cured.[3]

Dry Vacuum (DV): Dry vacuum impregnation is an effective method to seal porosity and leak paths. However, since it lacks the final pressure stage to assist in sealant penetration, it is less thorough and requires much longer cycle times. The process steps are as follows:

  1. Parts are loaded into a dry impregnation chamber, and the vacuum is applied. As in the DVP system, vacuum is held until all of the air is evacuated and upon completion of the vacuum cycle the transfer valve is opened, and the sealant is de-gassed and pulled from the reservoir to the autoclave while the vacuum is maintained.
  2. Next, the vacuum is released. No pressure is applied. Sealant penetration is solely dependent upon atmospheric conditions and the vacuum created within the pores of the casting. Hence the rate of penetration is much slower and may not seal all types of porosity. The sealant is then transferred back from the autoclave to the storage reservoir, and the parts are removed, washed and cured.

Wet Vacuum/Pressure (VP): This cycle requires only one tank that contains the desired sealant. The process steps are as follows:

  1. Parts are loaded into a process basket and placed into the sealant tank.
  2. Vacuum is activated in the sealant tank to remove all air, including air within the pores of the parts.
  3. Vacuum is released and desired pressure is added using compressed air.
  4. Pressure is released from the tank.
  5. The processing basket full of parts is removed from the tank and all parts are washed and final processed according to the metal material and specifications of the.

Wet Vacuum (WV): This cycle is the quickest impregnation method. It requires only one tank with the desired sealant. The process steps are as follows:

  1. Parts are loaded into a process basket and placed into the sealant tank.
  2. Vacuum is activated in the sealant tank to remove all air, including air within the pores of the parts.
  3. Vacuum is released and tank is vented to atmospheric pressure.
  4. Parts are soaked in the sealant for desired amount of time.
  5. The processing basket full of parts is removed from the tank and all parts are washed and final processed according to the metal material and specifications of the part.[4]

Common applicationsEdit

Die castings and permanent mold castings commonly contain internal porosity. This porosity is generally localized to the deepest cross-sections of the part and does not extend to the outer skin. However, if the part is also machined, the internal porosity will be exposed and the part will leak if pressurized. Machined die castings that need to hold fluids (intake manifolds, coolant connectors, transmission cases, pump housings and fluid power components) are routinely sealed for life using acrylic resins. Because the sealant is internal to the part, the exterior dimensions and appearance of the part are unchanged.

Powder metallurgyEdit

Powder metallurgy (PM) components are sealed prior to plating and to reduce internal corrosion. Plating operations typically involve submerging the parts in acid solutions. After plating, residual acid internal to the part can promote corrosion and/or preclude an acceptable plating finish. The solution to this problem is to seal the internal voids prior to plating. As explained above, the porosity is saturated with monomer and is then rinsed completely clear of the surface. The resin cures to a durable polymer. Thus, the exposed surface metal is free to be plated while the interior spaces are sealed dry.

The porosity in powder metal parts becomes a liability when the part must resist a differential pressure. PM applications for compressed air, fuel handling or hydraulic housings are common and effective; however, they must be sealed first. The polymer does not add structural strength to the physical part, but it will hold high pressures without creeping. If the wall thickness of the part exceeds 1/4 inch, the leak pressure is typically on the same order of magnitude as the burst pressure of the part.

Powder metal is also impregnated to enhance machinability. PM parts are generally difficult to machine and some compositions may not be machinable without ruining the cutting tool. Porosity sealing improves the life of cutting tools by ten to one-hundred times. Resin impregnation is more effective than compacting additives and can be selectively applied for near net pressed parts.[5]

ReferencesEdit

  1. ^ Ultraseal, International. "What is Porosity?". Retrieved 2017-11-13. 
  2. ^ Ultraseal, International. "The Impregnation Process Explained". Retrieved 2017-11-13. 
  3. ^ Ultraseal, International. "The Impregnation Process Expained". Retrieved 2017-11-13. 
  4. ^ Henkel Corporation (2005). Porosity Sealing by Design. pp. 9–10. 
  5. ^ Ultraseal, International. "Porosity In Powdered Metal Components". Retrieved 2017-11-13.