This a random shape made of three pieces of foam cut on a scroll saw and glued with a low temperature hot glue gun. The XPS foam is Bastion brand (made in Turkey) from the hardware shop that rhymes with Cool Runnings. I used Gyprock Multi-Purpose Joint Compound as the coating. The aluminium is A356 recycled from industrial gearbox pistons. The piece weighs 2kg. It turned out better than I was expecting although I'm not sure what the little rice bubbles are on the bottom of the casting (last photo). I think I'll use a hot wire cutter next time as there are noticeable lines from the scrollsaw's cutting action.
Hi John, impressive first post, welcome to the forum. The forum's resident lost foam expert is Kelly (username Al2O3), he's posted tons of useful information here and his youtube channel, if you haven't already, check them out. The "rice bubbles" you've got is what we here sometimes refer to as "leakers", do a search on that term for ways of reducing them. Changing the orientation of the pattern in the mold so that there are no horizontal surfaces on the underside of the pattern will improve the way the sand packs the mold and go long way to reducing the leakers.
Hi John, I agree with Rocco, very well done first time through. At first I thought they looked like air bubbles in the drywall coating and couldn't be certain that it was just "Leaker" but due to the positioning in the mold and since they are on a flat bottom side, most likely. Leakers occur in areas where the dry sand is insufficiently packed/compacted. In those areas the coating becomes unsupported by the sand, suffers a small fracture, and the aluminum penetrates (leaks) into the sand. Rocco is also correct, positioning the pattern n the mold so the surfaces are slanted and not horizontal/flat overhangs will cure that problem. Are you using vibraion to pack the mold? As far as leakers go, they are extremely minor and can usualy be knocked off with a light tap on a screw driver or chisel because there is actually still coating between them and the pattern surface. If they are caused air bubbles in the coating, they will be more tenaciously adhered to the casting. It looks like it was good casting stock but how do you know your scrap aluminum stock is A356 alloy? 356 is a common casting alloy but there are many others. The "A" actually designates a higher and more closely controlled standard for the composition of the alloy. It's a minor point and it probably doesn't matter at all for casting ornamental pieces and maybe not even mechanical pieces depending upon what the are to be used for, but mybe more so if you are going to heat treat or machine the finished casting. Once you have your other processes working well (and you are off to a great start), LF casting becomes greatly about your pattern work. The castings will never be better than the pattern and coated foam patterns can reproduce very fine detail. ....and so the obsession begins......welcome to the forum. Welcome to the forum. Best, Kelly
Thank you Rocco and Kelly. Based on your posts I think the bubbles are air trapped in the coating. When I mixed it I used an electric drill and paint stirrer and this introduced a lot of air. The attached photos shows the slurry today (two weeks after mixing). The way I dried it the slurry accumulated at the bottom surface (location of bubbles) and was a thicker coating than everywhere else. I packed the sand with a reciprocating saw without the blade. I did this once I had filled the bucket. I think this worked ok but I'll try doing it in steps next time. Re the A356, I was given a manufacturing drawing for one of these about 20 years ago. I run these through a hot tank before I melt them so they're pretty clean and there wasn't much dross to scoop off before the pour. I'll try casting two of the attached P&L logos next (10cm diameter). I haven't worked out the gating yet.
Are you brushing or dipping the coating? If you mix some dawn dishwashing soap with water (maybe a TBSP/gal) and either dip or spritz the foam pattern with it, the Propylene Glycol in the soap acts as a surfactant and helps break the surface tension, especially when dipping. Adding the same to your coating helps too. Brush coating tends to more naturally do so. Always mix the slurry immediately before dipping, but try not to do so so affressively that you entrain air. Applying vacuum to the slurry before hand is great albielt a bit impracctical. Vibrating the slurry canalso have a degassing affect. If I haven't used my commercial coating for a few weeks or more it noticeably thickens and degrades. I mix it once a day for 3-5 days before I use it and that ressurects its properties. The packing mechanism doesn't have to be fancy just effective. As a test, fill your flask by just dumping sand in, then vibrate and observe the settling. 10-15% settling of the flask height should get decent results. Vibing while it is partially full helps a lot because it's much easier to excite the lower mass and the portion of the flask that contains the pattern is much more important than that which contains the sprue and on up. In general, vertical vibe excitation seems more effective than other directions but you need to sit or hang the flask on springs or on an unstable surface to get the vertical action. After you pack if you drop the full flask on the ground from an inch or so and it settles more you'll see what I mean. Good on you....good casting stock. Good luck on that next go. Best, Kelly
Here's the latest, hopefully I'll pour this tomorrow. One of the sides fell off when I tried dunking it in the slurry, I don't think I let the spray adhesive dry long enough on that side. I'll see how it looks in the morning once its dried out. I tried spraying the parts with the dishwashing spray and adding dishwashing detergent to the slurry. It didn't seem to work out that well although a lot of the bubbles on the surface have popped. I made one of the sprues around twice the cross section of the other one so I'll see how that goes.
Did you hot wire that pattern? Maybe, the buoyant forces when you dip can be quite strong. On fragile patterns sometimes ladling or spooning the slurry over the pattern is a better strategy. I use Dawn brand. The stuff is a wonder but it only breaks the surface tension and help the slurry lay down on the foam pattern. It won't degas the slurry. If you cant use vaccuum or vibration, another way to degas the slurry can be to plunge a rod or stick into it and let it drain off in a thin stream back into the vat. Do this many times and stir very gently so as not to entrain more air. The positioning of the pattern looks pretty good. The challenge with that part will be getting the thin ring to fill. Gating into the two more massive areas where the letters touch the ring and attatching both of those to the sprue would be my starting point. If it was a pattern I had a lot of time invested in, I'd have a perimeter ring gate like I did at the end of my saucer cup and spoon video series and thread. That's the take no prisoners approach but more work to degate. Saucer Cup & Spoon – The Lost Foam Edition | The Home Foundry Good luck.....Pour that one hot!! Best, Kelly
No, I used the scroll saw. They took around 30 minutes each. If I get time I'll try making some using the hot wire cutter today. Yes, I think your right. I pushed down on the sprue rather than the actual part. I don't think Dawn brand is available in Australia. I'll check if I can get it online. That looks like the way to go. I'll pour this one as is but try more complex gating next time. I gave the items a second coat last night and tried drying with a fan heater but one of them got too hot and started to melt.
When making a complex shape with a hot wire, I first cut the shape out of a piece of cardboard, pin to it to the foam blank then run the hot wire up against the cardboard. This makes the foam cutting process quick, easy and repeatable.
It turned out quite well. There's one spot where the sand didn't fill but otherwise it looks quite good.
Here are a few more photos of today's effort. A bit of porosity and some room for improvement in the surface finish. I'll try a wax coating on the foam next time.
