Advice on expanded polystyrene / hello from the UK.

Discussion in 'Lost foam casting' started by TiMiP, Nov 14, 2022.

  1. TiMiP

    TiMiP Copper

    Hello everyone!

    UK based lost foam enthusiast here. Ive taken a lot from this forum over the last year and so thought it was time I share some of the things I have cast with all that help and information!

    See some pictures of various things I have built / cast below.

    I also have a query on casting with expanded polystyrene. I keep having failures when casting large pieces which are solely expanded foam and wondered if it was to do with temperature or the way in which I am feeding the parts. See next two pictures.

    These are the only photos that I have by way of explination, they were a test for a lamp base. I really like the bobbly texture and have been trying to use that as a feature. I have has success when using it in addition to XPS but when done alone I have had very limited success.

    The first attempt I realised that I needed to pour a lot faster than when using XPS.

    I then made some much larger pouring cups so that I could empty the contents of an A20 crucible into the cup before the sprue had even burnt away. No luck. I tried using a sprue made from the densest XPS I have. No luck. I tried a very very long sprue, a thin one, a thick one. Every time the part still only fills halfway / three quarters before collapsing in on itself. I have tried vibrating for 30 minutes solid. Feeding from lots of locations. etc etc.

    I cant really think of how else to approach it. Any advice welcome!



    Some bits in no particular order.

    Lamp - single piece casting. The shade is made by casting onto a piece of laser cut aluminium which took a fe attempts.


    Newish casting barrels. I have two, both with trapddors in the bottom for emptying / filling with forklift assistance.



    Two person lifting and pouring tongs for an A25, the largest crucible that will fit in my gas furnace. Copied / inspired by this forum. Really effective. I modelled up them up in Solidworks to get the correct tipping point.


    In use. Please excuse the lack of PPE.


    Some experiments with laser cutting XPS.



    And some sculptures. Both cast as single pieces. Top one measures about 350mm tall. The second one is about 600mm.


    d0723604-cdf0-44fc-9e95-8eaae8912236 (1).jpg

    Attached Files:

  2. Al2O3

    Al2O3 Administrator Staff Member

    Just to make sure we are using the same terminology, XPS is eXtruded PolySytrene (typically foam insulation board) and EPS is Expanded PolyStyrene (Molded Beads). I use both, but primarily XPS (typically 1.3-1.5lb/ft3) for the patterns because it machines and finishes much better, and EPS (though available in a wide density range, I most often use <1lb/ft3) for the feed features. I use lower density EPS for the sprue/feeed system because isconsumes less melt energy and delivers hot metal to the pattern quickly. Most people don't like EPS because of the modeled/beaded an artist, you are not so constrained.

    It's usually the other way around for me, XPS evaporates more slowly. Most of my castings are machine parts with very high surface area/volume ratio. Bulky massive parts are often the most challenging for lost foam, a sphere for example.

    Was the failed casting in the second picture actually cast from one of the patterns in first picture? Or a different but similar pattern?

    You mentioned a very large pouring cup (nearly the volume of the casting) but unless what is seen in the picture is just what settled out of a much taller cup, it is not large. One thing I can say with fairly high certainty is the mold has become destabilized in the vicinity of the sprue and experienced localized collapse. This is usually caused by an interupted pour. If you had a large cup that was rapidly filled, it's possible a large amount of gas burped back through the sprue and caused same.

    The sprue looks a bit small for the size part it is feeding. If the sprue is too small, the metal ingress can melt the pattern faster than the sprue can supply metal and the result is mold collapse. A larger taller (more head pressure) sprue helps. I'd also suggest some taper to the sprue, but still square or rectangular cross section. Lost foam doesn't typically follow the conventional empty cavity sand casting norms when it comes gating and spruing conventions because the metal velocity is usually limited by how fast the foam evaporates, as opposed to gravity and sprue/runner/gate cross section. In the case of LF metal velocities are usually much less than conventional casting. However, if you have a large part volume beneath a small sprue, metal velocities in the sprue can become quite high once it has become evaporated, thus a tapered sprue helps avoid turbulaence (so does ratangular vs circular cross section). A longer sprue/head pressure also helps force evaporated pattern gas through the refractory coating and mold media versus buoyantly bubbling back through the sprue. I can't say it happens every time, but in my best designed feed systems, I experience no smoke or fire when pouring.

    Any idea what your metal pour temerature is?

    What are you vibrating with? From the pictures, your flask looks like a 55 gal drum which would be fine except it would be VERY heavy (~600-700lbs) and require something very energetic (high power) to effectively vibrate and pack it, especially as it becomes full, thus the top of the mold near the sprue will be loosest sand. To test your packing density, if you just pour sand into you flask, then vibrate it, and measure the setlling, you should get at least 10% reduction in height and better yet 15%. This is what I typically observe with silica sand. 90% of packing happens in the first minute. If full packing is not achieved in the first 5-10 minutres, it usually won't happen at all.

    Nice work on your castings BTW.

  3. TiMiP

    TiMiP Copper

    Hi Kelly,

    Thanks for your reply and kind words!

    Yes we are on the same page. I typically use XPS for most of my lost foam castings and EPS for sprues but in this instance have been trying to experiment with using solely EPS for the textural qualities.

    Sorry, this was my poor wording. The EPS evaporates much faster and I found I had to adjust my pouring technique. Large bulky parts are where I seem to be having issues. How would you go about feeding a sphere for example? I have been trying to limit the number of feeding sprues when using EPS in what I am calling 'clouds' (parts made from EPS where the beads make a bobbly exterior surface without flat surfaces) this so that there is less finishing work as the bobbles are very difficult to mimic.

    The two pictures are the same pattern / casting, the first attempt if I remember correctly. This failure was because of an interupted pour and led to subsequent attempts with larger pouring cups. Up until this point I had very few failures (having followed knowledge from all of your YT videos) and so have added sprues and runners in the most logical way normally using 1" square EPS. As mentioned I added some XPS (2-3") into the sprue just before the part to try and gain some extra time before the metal enters the part.

    I will try again with a larger sprue and let you know how I get on.

    This is something I am not sure of as I dont have a probe or thermometer, I usually work on the principle that hotter is better, waiting until the crucible is full and fully molten and then turning up gas pressure to full power on the furnace and waiting another minute or two. Is there an upper limit in LF castings where the material is considered too hot and affects performance?

    Yes, the 55 gal drums are heavy (hence the forklift) but mean that I can get 2-3 large parts or sometimes 7-8 smaller parts into the same flask. I am vibrating with 4 x GT-20 pnuematic vibrators. I have a 13bar screw compressor that I use to feed a metal laser cutter that has the throughput to vibrate the flasks adequately. The laser cutter also supplying most of the material that I use for casting with.

    Thanks again for the advice and I will let you know how I get on with using a larger sprue and feed system!

  4. Al2O3

    Al2O3 Administrator Staff Member

    Interupted pours are the kiss of death for a lost foam casting......avoid this at all costs.

    I'd suggest you make or buy a contact pyrometer. They are pretty easy to make and there are threads here on the forum for doing so. The biggest reason to do so is consistency in results. As you gain experience you'll know what is needed for your type of castings. I make machine parts and in general, you want to pour at the lowest temp that consistently yields success because super heated aluminum is far more likely to produce porosity, defects, and poor surface finish. As an art caster, you have considerably more lattitude and in general hotter is better for first pass success.

    My castings are most typically 1/4" wall with very high surface area/volume ratios. My ordinary pour temp is 1425F. LF generally requires +100-150F compared to open cavity sand casting. I have poured as hot as 1750F for a very thin (.060" wall) castings. This was an ornamental part. I'd never pour a machine part that hot. Here are a couple. Closest I come to art. Every time I try art they come out looking like machine parts.–-the-lost-foam-edition.1020/

    It depends on how large it is because the volume of a sphere goes up as ^3 power of the radius. For spheres up to 3-4" in diameter, probably no particuarly effort, just a sprue into the top but as size increases, I'd increase sprue size and contact area on the pattern. It's hard to give one-size fits all answers for spruing and gating lost foam castings.

    Well, you likely have enough compressed air at the source, but I'd hazard a guess to say you are still grossly under powered for your 55 gallon drum. However, there are things you can do to help. For reference, you may have seen my rig.

    It's based upon a 30 gallon barrel but I routinely use a height extender that gets me in your size/weight range.

    In retrospect, the G8s vibrators on my rig are toys. I replaced two of the G8s with G25s so now have three G25s. That flask sits on heavy springs. The springs make it unstable and easier for those vibrators to excite that mass. Also, even though you have a screw compressor and presumably adequate compressed air capacity, what size hose are you feeding those four G20s with? If it is the normal utility sized hose, it's very unlikely that it's flowing enough. Put a pressure gage right at the inlet of one of your vibrators and obsevre the feed pressure when it is actually operating. I had to install a larger air hose (1/2" ID) and am very near my 80 gallon compressor (surge) tank.......and still experience a lot of pressure drop at the vibrators.

    Don't load the flask full of sand and then vibrate. Vibrate as you fill. It will be much lower mass partially full and that is where your featured patterns are. The sprues and feed system are less critical and easy to pack.

    Best advice is my previous comment........measure the settling you achieve......then you'll know. If you cant see the sand moving on the surface of the flask, nothing is happening.

    On a final note, for doing multiple pours from the same crucible, you might consider using multiple smaller flasks and making a vibe platform they rest on and can be removed from so you can pack multiple flasks. I always choose the smallest flask that will accomodate a part.....much easier to handle and you can still pour multiples from a single melt.

    My smaller rig that does what I describe is also discussed in the link above.

    .....and larger pouring cup! That classic lost foam pause can get many casters, especially if you are struggling to see through smoke and flame. A cup >25% of part volume will help.

  5. Tobho Mott

    Tobho Mott Gold Banner Member

    Looks like a fairly thick EPS pattern, I can imagine at the base of the sprue the metal had lots of room to spread out really quickly, so the sprue had to be able to pass enough metal to keep up with that to keep the mold from collapsing where the sprue gated into the part. Maybe different gating is the way to fix it. Kelly makes good use of long runner/gates on some of his lost foam cast parts which you can see in some of his threads here; maybe something like that could help.

    Or if you're trying to fill up a big pouring cup fast enough to build up a reservoir that helps keep the whole mold full when casting thick EPS patterns gated as shown and still having trouble keeping up with it, maybe an XPS sprue would buy you just a little more time to fill the cup before the metal really starts draining full speed into the EPS? I've had similar things happen often when trying to cast some bigger (for me) EPS shapes and I'm never really confident they'll fill when I do try pouring them.

    Good luck!

    Nice work too by the way :D


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