Hello from Vermont

Welcome SRHacksaw. Sounds like you're a practitioner with some great experience. You should fit right in here. Welcome.

Best,
Kelly

 

Thanks SR, lots of different casting displines practiced by members here. I'm sure you'll find the forum and members helpful. BTW, that looks like a nicely done Gingery. You'll have to show us you bigger lathe.

Best,
Kelly

 

No, in fact, iron is probably the most common alloy poured commercially using the LF method. The vast majority of commercial practice is in Asia. Because of the much higher heat, different coatings and mold media are used compared to non-ferrous casting and vaccum assist is the norm. The coating generally uses a higher refractory base material like Al2O3 or Zirconia to handle the heat, and higher coating permeability is preferred. Even so, the substantially higher heat more rapidly evaporates the foam so vacuum is used to more rapidly drive that vapor across the coating barrier. This is why I made all my flasks vacuum capable and made a vacuum cart.

http://forums.thehomefoundry.org/index.php?threads/vacuum-forced-air-cart.810/

However, I almost never use it for aluminum, because it's just not needed and can cause as many problems as it solves, and I have so many aluminum projects I still want to take on, iron parts haven't made the priority list.....yet. It's a bit more involved of a discussion but the rest of the processes are esentially the same. How important vacuum is to home/hobby practice I can't say for sure, but I would say, it's likely just a matter of getting the right process combination that adequately evacuates the vapour. I've found more head pressure, (deeper molds longer sprues) makes for very tranquil pours in Aluminum, and since iron is so much more dense, that added pressure would likely be beneficial as well.

There was one member here that did a LF iron cast, no special coatings, no vacuum.

http://forums.thehomefoundry.org/index.php?threads/cast-iron-lost-foam.615/

I borrowed a lot from what I observed in commercial practice. This was one of the most interesting videos. If you watch and listen closely, there are a lot of valuable process details.



I made reusable pouring cups out of moldable ceramic fiber for aluminum. For iron, I'd probably just mold/make them from sodium silicate or chemically bound sand and consider them as an expendable.

Yes, low temp alloys can be more of a challenge for LF. I've done lead several times. The lower temps cause the vapour to evolve more slowly, but like with Zinc, the higher densities provide some additional driving force via increased head pressure. Because the foam evaporates more slowly, the molds fill and molten metal front advances more slowly so they lose more of what little heat they have to the mold media. When demolded you tend to see a lot more liquid driven across the coating and sand lumps bound with decomposed polymer.

Best,
Kelly

 

Haven't tried Elmers rubber cement. Most of the foam safe contact cements are latex. Still don't think they machine or sand well. Still prone to loading cutters and due to the hardness difference, dont blend well at the joint with abrasives. I have some FoamTac brand, just havent tried it.

The other problem laminating foam, unless it's the smooth exterior surface of the face, as opposed to a machined and sanded surface, the cellular nature of the foam always makes the joint thicker than you'd prefer. I was thinking CA glues might be an alternative because they chemically cure. Havent tried that either. Dont know how well they'd burn out and think I'd give that vapour plume with due caution.

Best,
Kelly

 

Rubber cement is a generalism. What is the cement and what is the solvent it is suspended in for the composition your referring to?

The most common glues referred to as "rubber cements" are indeed latex suspended in volatile solvents that fairly aggressively attack/reduce polystyrene though other elastomers are also common. I've tried a number of adhesives referred to as rubber cements and contact cements, including those specially formaulated for foam like 3M 77 and although they will certainly stick foam together, in my opinion they all are very poor performers for my purposes........they load cutters ruining cnc patterns in the coarse of cutting and will instantly load and foul a sanding belt/drum and wont abraid evenly with the foam when hand sanded......but it all may depend upon your expectations. In my experience Shellac is best for a joint that needs to be machinable but even though it is very thin, it's hard and also will not abraid evenly with foam when worked with abrasives by hand but better than elastomeric cements. It doesnt burn out as well as low melt point glues and elastomers but there is so little of it, it can be a fair trade.

Best,
Kelly

 

I should also mention PVA, as in PolyVinyl Alcohol as opposed to Poly Vinyl Acetate as a candidate for laminating foam. Alcohol is the solvent so it doesn't attack foam and has much higher vapor pressure than water based glues so you at least have a chance of drying out laminates with time and/or modest heat. More here:

http://forums.thehomefoundry.org/in...yrene-foam-gluing-techniques.1000/#post-24785

I know we've discussed it a bunch of times. Two sided tapes are also an option if your just sticking boards together but again, anything with very low melt temps or that remains tacky at room temp won't machine well. I have accumulated a half dozen new glues that I was going to test for machining, sanding, hot wiring, and burn out, etc......just havent gotten around to it because on most, I think I can predict the outcome. Maybe in the next few weeks I'll get a chance and we can move the discussion out of your intro thread.

Best,
Kelly

 

Not trying to be combative or argumentitive, just informed. Now if you tell me you have been using Elmer's rubber cement to laminate sheets of polystyrene foam, and you run a 1/4" diameter cutter through feet of the glue joint at 20+krpm all day and it never sticks to or fouls the cutter, and you can make shallow angle cuts through the glue joint and it sands like the joint isn't even there, you've got my attention, unless I discover it's just some elastomer supended in toluene.

Elmers is a brand not a formulation. I did go and look at a half a dozen ads, and there is no mention of the formulation. The official Elmers website is not helpful. Rubber cements and contacts cements are generalisms for elastomers suspended in solvents. Contact cements have more solvent and less elastomers. IMO, they all suck as far as being machinable and sandable. The reason I asked for composition is I don't need to test different brands if they are the same composition because having already done so many times I know how those formulations will behave. Just like when someone asks me if I've tried a certain brand or type of foam product for patterns......my reply is always "what's the base polymer and what's the density?"

I've learned I'm better off sourcing thicker foam or designing the part in pieces such the the glue joint doesn't need to be worked.....just assembled.

After you get your casting process for lost foam iron down, it will become all about the pattern materials, design and construction methods, gating and positioning in the mold. If you're just trying to cast general shapes you can get away with a lot, but if you want professional looking and functioning castings, these things matter.

Best,
Kelly