Beware of some types of foam for pattern making

Discussion in 'Lost foam casting' started by Bob10, Sep 21, 2022.

  1. Bob10

    Bob10 Copper

    I'm so impressed with the many examples of Lost Foam parts being made by people here.

    Kelly has been outstanding with the videos created for the deep dive explanations in making foam patterns and casting the parts. These are very helpful, although I haven't seen any more presented lately. Hope all is well.

    I've been reading most threads in this forum especially LF subjects, but don't recall seeing anything discussed about what foam to not use.
    It's somewhat of a safety caution with a type of foam that could actually explode when casting. Absolutely don't want anyone casting parts to become injured. If the subject had been written and posted here before I apologize for not reading it or for not commenting on it on another's thread.

    Anyway, I was reading an article and thought I'd share it from Modern Casting magazine.
    https://www.moderncasting.com/artic...n-diversify-lost-foam-casting-negligible-cost

    Hope this article is helpful to those just getting started or even the seasoned molders.
    Keep up the awesome threads and examples.
     
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  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Hi Bob, all is well. I don't cast as much in the Summer and have been concentrating on improving my CAD skills so I can machine better patterns.

    I'm an AFS member and had previously read that article. I've also bought a few of their tech pubs on LF. I find many of the AFS writings to be seemingly written by academics (perhaps having visited or interviewed practitioners) not practitioners which I find to be a strange criticism of a professional society pub. There were a few things in that article I took note of.

    In my experience this is indeed the best foam density range for LF patterns.

    I searched Clearcast foam and couldn't find a thing. The name seems to suggest a castable polystyrene. That would be useful if it existed but the comment about block form availability makes me suspect. In any case it would need some kind of blowing agent which is a very finicky process to control foam density, especially for a hobbyist. I have no idea how that would be helpful to controlling carbon content in foam. My understanding is polystyrene is not the preferred pattern material for iron, let alone steel, in the LF processes. Several other foams are preferred but they're not practical for hobbyists.

    I've been using the Owens Corning and DOW commercial PS XPS board for many years now. I went back a read the product material and although I didn't find any comments specifically saying flame retardants, the specs cite low flame spread and smoke development specs and I'd be surprised if their were NO additives that would qualify as flame retardants. I have never had anything that I would characterize as an explosion during LF casting. The worst I've experience would be metal bubbling or burping back through the pouring cup. I'd say the threat of steam explosion from wet sand in conventional bound casting is greater than LF with dry sand.

    Amen

    I thought this was pretty interesting. I primarily top feed my aluminum castings and sometimes side feed with large contact area. Haven't done any Non-ferrous work yet.

    I've found this to be very true but not always practiced by the commercial foundries who have exceptional compacting equipment and practices.

    Very consistent with what I have found compared to conventional open cavity bound sand casting.

    They must having been referring to the scope of their article because most commercial LF casting equipment do utilize vacuum. I almost never do with aluminum LF casting.

    I think optimally designed gating systems exhibit no or very little flame (catch fire) and in this case the foam doesn't actually burn. I associate fire/burning with combustion. In y experience, when PS burns it produces black smoke and flame. When it is merely melted and vaporized in the absence of oxygen, it produces liquid and decomposes to a white vapor that can be observed when demolding. If you have flame in your cup, which most do LF casters do, it's usually from the initial melting of the sprue which is free to burn in air. My best gating designs and pours have no flame, but flame is by no means a gauge of casting success or failure, just something to strive for.

    Best,
    Kelly
     
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  3. mytwhyt

    mytwhyt Silver

    Kelly- I've read all your sand compaction with vacuum assist threads.. Always impressed with the quality and consistent results. When you say "I almost never do with aluminum LF casting" . Can I assume that you're getting the same results without the vacuum, and only compaction of the sand?
    Fredo
     
  4. Tops

    Tops Silver Banner Member

    Fixed it for you Kelly. No need to thank me... ;)
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yes, I think it's fair to say that for aluminum I'd just use some additional sprue height if needed and don't (almost never) use vacuum except for some extreme circumstances such as very thin walls or very long metal travel distances. High vacuum actually can create other unintended problems.

    This would be an extreme example where I used vacuum:

    http://forums.thehomefoundry.org/index.php?threads/saucer-cup-spoon-–-the-lost-foam-edition.1020/

    However, I'm uncertain about whether the same would be true for iron, because the gas evolves so much faster and vacuum assist seems to be the norm in commercial LF iron foundries.

    Best,
    Kelly
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    In lost foam it's all stinky!

    Best,
    Kelly
     
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  7. Tops

    Tops Silver Banner Member

    Touché
    :)
     
  8. Bob10

    Bob10 Copper

    Hello Kelly,

    Glad all is well.

    Modeling skills are ever changing, it's called keeping up with the revisions made to the software.
    What software are you using?

    Look forward to seeing more videos from your shop, whether its pattern making or casting, you do an outstanding job describing the process.

    Regards,
    Bob
     
    Last edited: Sep 26, 2022
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Alibre Design Pro v25

    Thanks Bob. Very kind of you to say. There will definitely be more vids. I have a number of intake manifolds on the drawing board.....make that computer screen.

    Best,
    Kelly
     

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