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  • Question of "pressure tight" in sand castings

    Discussion in 'Metal casting & moulding' started by B Douglas Foundry, Jun 1, 2011.

    1. I am wondering about the meaning of "preasure tight" when addressing mechanical abilities in castings,what does this mean? specifically the alloy 535.2 as opposed to A356.2
      Thanks,
      Ben
       
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    3. GarethW

      GarethW Chief Clicker Staff Member

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      Re: question of preasure tight in sand castings

      Could it be something to do with porosity? I just had a quick look around on the web and "pressure tight" does seem to be a term that's used commonly with some sand casting alloys. Perhaps being a low-pressure process some alloys could be less prone to porosity when sand-cast...

      That's my best guess, but I hope that somebody can give us a definitive answer.
       
    4. Julian Harding

      Julian Harding New Member

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      Re: question of preasure tight in sand castings

      "Pressure" Tight?
      As in castings to hold hydraulic or gas pressure. You might look at post casting process such as HIP or impregnation.
      HIP – Hot isostatic pressing, closes porosity in castings but reduces overall size (allow for shrinkage) but can reduce surface quality.
      Impreg or double impreg, forces a resin into the porosity filling the voids, I have used with manufacture of aluminium sumps of automotive engines.

      Julian
       
    5. Re: question of preasure tight in sand castings

      Hello All, it is indeed an issue of porosity. Did some checking follow-up from a customers question. Basically preasure tight is a castings ability to hold a gas or liquid without leaking. All boils down to alloy castability.Some aluminum alloys cast easier than others.The difficult alloys tend toward defects,therefore less preasure tight.
      The test for preasure tight is to seal the casting under preasure then submerge it,look for bubbles,too easy.
      One interesting item, hydrogen porosity does not effect preasure tight.
      My thoughts when asked originally had along the lines of whether certian gases would come thru the walls,some molicules being so small. All of that is beyond me.
      By the way, Sold the job,a brass conversion,to aluminum. cut the price per part about 75%.
       
    6. alex39

      alex39 New Member

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      Re: question of preasure tight in sand castings

      Pressure tightness referred to an ability of the casting to hold specified internal pressure during specified time. It is usually important for a liquid or gas caring components. Gas or oil leaks can occur for a several reasons. During metal flow into the cavity of the die, small gas bubbles get trapped and form gas porosity. Then during solidification of the casting, thicker areas of the casting are the last areas to solidify. If these areas are surrounded by solidified regions, thermally induced porosity is formed. Usually due to higher solidification rates of the outside of the casting, porosity is formed inside of the casting. But sometimes porosity regions during solidification get connected to each other by small channels. Theses channels can get exposed to an outside of the casting, both sides of the wall. In this case leak path is created. Depending on the nature of the porosity, leak tight casting can by achieved by proper gate, overflow and ventilation system design, proper thermal management of the mold, or combination of the two. More expensive and less desirable way is to use impregnation of the casing. It add cost of extra handling of the part plus on more operation.
       
    7. lmurali

      lmurali New Member

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      Re: question of preasure tight in sand castings

      "Presure Tight"
      It refers to the avereage specific pressure on the casting during the whole process..,
      In usual (low pressure) sand die is just expected to constrain topographically to achive desired shape, as the melt stresses are less than binding strength sand die. As the melt solidifies to cast solid its strength also increases exerting surface pressure on the sand die, this being unevenly distributed depends on the topography will distort the shape, create low pressure pockets (porous holes), etc., many such strenght related issues leading to structurally inferior casting. To over come these, the high compressability of sand is leverage by constrain the sand as well using higher strength binding agents, to squeeze the solidify melt into the die, that reduces the stress distribution range as the part gets solidified.
      In simple terms as presure tight casting is structurally superior to low pressure one.
      Muralidhar Lakkanna
      Bangalore, India
       
    8. mddolson

      mddolson Member

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      In a former position we produced Explosion proof (CSA Class 1 Div 1, ExD rating) enclosures for level measuring devices. We needed to pass an 800 psi pressure (hydrostatic, 1minute) test. We found porosity was a problem with sand & permanent (cast iron) moulded aluminum castings.
      The only way we were confident, was to vacuum impregnate (after machining) as this would expose otherwise closed porosity.

      MIke D
       
    9. Hello MIke,
      That is facinating, Two questions, first, do you recall the alloy used and second,I am not familiar with vacuum impregnating,can you give an overview?
      Thanks,Ben
       
    10. mddolson

      mddolson Member

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      It has been a while, but the alloy was A356 .
      Simply:
      The parts are imersed in a resin, and a vacuum is drawn to pull the resin into the pores.
      The excess ressin is washed or centrifuged off, then the parts are baked to cure the resin.

      Here's a link: http://www.saranindustries.com/impregnation.html

      regards

      Mike Dolson
       
    11. PWASS

      PWASS Well-Known Member

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      I have also used vacuum impregnation in the past, it was performed as Mike describes.

      It was used on an alumininium casting which was used to attach and electric motor to a hydraulic pump and part was used as a hydraulic manifold which was designed to work at pressures up to 200 Bar.

      It had oilways drilled through it, which in places, were less than 10mm apart, without the impregnation most of them would have leaked between the holes but with the impregnation we had a 100% pass rate.
       

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