It seems as though venting would be a good idea. How about leaving a bump on top of the foam pattern. After coating it run a drill bit into the bump down into the pattern as a starter passage. Then when you bury in the sand poke a paper or plastic straw in the hole to provide the vent above the sand. Thoughts?
Straws have been used as vents in lost foam before (I've done it myself following other examples) It seemed like a good idea but how much it helps I don't know, after all the sand needs to be permeable and unlike green sand you are not having to deal with steam/moisture. Why the bump on the pattern? or is that the same size as your straw vent?
The idea of the bump was simply so the end of the straw isn't in the part where it might create a void. There isn't any steam as you say but the foam itself is turned into a large volume of vapor that needs to escape. It would seem the quicker that happens the better the cavity will fill. Since you destroy your pattern with every pour it would seem you want to ensure success every time.
I think vents are one of those things that does not translate from conventional sand casting to Lost Foam casting. The best way to explain why, is there simply is no cavity to vent: The closed cell foam pattern is not porous. As Peedee said, the entire surface area of the pattern contacting the mold media should be considered a vent. This is indisputable with uncoated patterns and certainly should still be the desire for any coated pattern. In LF, a vent is not a vent until all the foam between it and the advancing molten metal front has been evaporated. At that instant, the straw/vent will fill with aluminum and cease to be a vent. Best, Kelly
I work with water filled pipes and a 1/8” vent hole lets air out at a huge rate while hardly any water flows through that hole. My thinking was that before you insert the straw you actually run a drill bit down into the foam almost to the bottom so that the vent channel opens the instant the foam vaporizes and the hot gases would keep it open until the molten metal rises into the straw. My grandson and I have been working to get everything set up for casting. He leveled an area in the yard today and made a pad of pavers to set up the furnace and burner on. We have a framed area next to it with sand in it where we will pour. I modified the tongs so we don’t need to transfer a hot crucible. We can lift it, pour, and put it back in the furnace.
I used 12x24 (sandstone?) pavers from Lowes to make a 4x8 work area. I toss handfuls of sand down and sweep it around before a melting session. I've had drops and plops of metal as hot as bronze with no pops. Any way to avoid entrapping steam is a good decision. My dogs would make a mess out of a sandbox, and the local felines would probably make messes in it!lol.
It's another counterintuitive thing about foam but air voids of any kind in a lost foam casting are counter productive and will most likely cause flaws and/or localized mold collapse instead of benefit. Even hollowing the sprue is counterproductive. I can post some links if you're interested in the why, but I'd suggest save the effort and enjoy better results. Spruing/gating will be the next challenge to your intuit......again, not the same ball game as sand casting. You can gate the top of the pattern and run pressurized feeding systems in lost foam whereas in sand casting, bottom gating and non-pressurized feed systems are the norm.....at least in aluminum. Best, Kelly
Kelly, Thanks for the tips, trying to get up to speed quickly. My grandson is on the high school FIRST robotics team and it might be neat if we can cast some parts to use in the robot. The next challenges details are released this Saturday then have just 6 weeks to build and program a robot. Likely going to need to just start pouring random parts to see what happens.
If your patterns are relatively simple and quickly reproduced, a lot can learned through experimentation but if you have time invested in the patterns, pouring molten metal on hours of invested time without a successful result can be a bit unrewarding. Best advice for starting out: Avoid large underhung surfaces by placing patterns at angle in the mold, Vibrate the mold for good packing, Top feed the casting, Pour hot....1600F. That's ridiculously hot by sand casting standards but not LF. If it's not a critically stressed part, I wouldn't worry about that at all and take the much higher probability of successful pour that comes with higher LF pour temps. A pouring ("Kush") cup can be helpful to insure an uninterrupted feed of molten metal. Have fun, make a stink, ....and some castings. Best, Kelly
