Two Barrel Carburetor

Guilty as charged!

It's a commercial lost foam coating called Poly Cap 600. It's discussed here in my dip coating rig thread. It can also be brushed.

http://forums.thehomefoundry.org/index.php?threads/lost-foam-dip-coating-rig-–-the-big-dipper.776/

I had good results with non-setting drywall compound too. Here's a thread that discusses dip coating with thinned drywall compound.

http://forums.thehomefoundry.org/index.php?threads/dip-coating-polystyrene-foam.573/

I like the Poly Cap 600, I'd say it may have a slight edge as far as self leveling and faster drying but with aluminum I think comparable results are achievable with the drywall compound. Dip coating is fast, produces a very uniform coating thickness (uniform permeability) and coats all surfaces internal and external.

Best,
Kelly

 

Looking back, I probably should have called the thread Casting A Windmill Gear, that way I wouldn't have had to make so darn many copies myself....

Best,
Kelly

 

Got the Main Well patterns completed and sprued up.



Also dipped them. Supposed to have good weather Saturday so they should be dry and ready to pour by then.

Best,
Kelly

 

Had a nice day for casting today. High 40sF so I got after the Main Wells.

Here are the coated patterns.



They were a tight fit in the 5-gal flasks.



14 patterns went into the sand, and 14 castings came out. I’m on a roll!



Degated and media blasted.



Next, on to the Air Horns.

Best,
Kelly

 

I have all the air horn patterns done but am having a little problem pulling a mold for a wax embossment I would like to add so I haven't dip coated them yet. More on that later.



In the mean time, it’s going to be 60F here tomorrow and I can’t miss casting on a day like that this time of year so I prepped all the venturis…….8-on a tree, 4 trees.



Best,
Kelly

 

Buy the lowest melting point hot glue you can find. Use a dimmer switch to regulate the heat on the glue gun. Apply the glue in lines as opposed to a glob, and smear the surfaces of the joint as you assemble them.

If you need long working time and it's only 5-10mm joint width try PVA glue (Elmer's or white/Yellow PVA wood glue) and tape the joint in place to dry over night. These wont work for laminating big surfaces though because they'll never dry.

Best,
Kelly

 

The foam safe CAs I looked at were quite a bit more expensive (like 3x-4x) than regular CAs (which already aren't cheap) and most I found were slow.

The typical accelerators are all suspended in solvents that attack the polystyrene foam. I've been putting together a tutorial on glues. The difficult application is laminating large sheets/areas with something that doesn't attack the foam yet still sandable and machinable.

3M #78 is a spray contact adhesive designed for polystyrene but it is terrible for loading up abrasives and cutters like any other contact cement.

I haven't tried this yet, but if you apply Polyvinyl alcohol to each surface (and let it dry), it may be sufficient barrier to protect the foam. Then you could just use regular CA glues and accelerators........this has promise for laminating since it is a chemical reaction and instant bonding. I'd use the thinnest CA in this case.

It seems the joint may be fairly hard so still have differences in how it sands across the joint but would seem much less likely to load up abrasives and cutter like hot glue or to a lesser extent Polyvinyl Acetate glues. It's also yet to be seen how it would burn out. Similar to most thermosets, I doubt it has a melting temp so probably just burns and if so, not the best candidate in this regard but if there is very little of it, it might work. This is the case with shellac as well.

Would you like to be the Guinea Pig? What kind of joint do you need to glue?

Best,
Kelly

 

It was a great weather day, and good casting weather so time to cast all those venturis. Here they are coated and ready to go:



They nested nicely in my 5-gal flasks.



Here’s how they came out of the sand….still on a roll, gott'em all.



….and after I cut them off the trees and ground all the gates off on the belt sander.



These venturis are pretty delicate and probably better suited for another casting method but I think I’m getting a lot out of lost foam.



I reasoned if the foam patterns fit well so would the castings and that did turn out to be the case.



I need to cast the air horns and also accelerator pump diaphragm covers and the casting part of the project will be done.

Best,
Kelly

 

I'm nearly done detailing the Air Horn patterns. These were the most laborious of all the carb patterns because of the additional pieces, precision positioning, and detail work for the boosters. I have more time into these patterns than the rest of them combined. I mentioned in a previous post that I was held up completing these because I was having difficulty pulling a mold for a wax embossment. Here’s a print of the embossment on the carb provided by our fearless leader (thanks David!).



The problem was the Platinum/Addition cure silicone was reacting at the surface of the 3D SLA print. I was surprised by this because the print material looked so slippery and benign. I was in a hurry so I didn’t test for silicone compatibility and the pieces are so small, there wouldn’t be much if any difference in size between a sample and the actual mold, so off I went. Here was the first go. All iterations were allowed 24hr cure time over a warm air heating vent. The script is only 1/32” thick on a 1/32” thick plaque, so the loss of definition was unacceptable. I took a clay impression of the print and pulled one wax from it but clay molds are only good for one use and the wax usually contaminates the clay……but still can be useful for a one-off quickie wax embossment.



After a little online searching I read latex was a habitual offender for inhibiting Addition Cure silicones to the point that even handling master pattern with latex gloves could inhibit curing. When I received the prints, they were packed with painter’s tape on their surfaces for shipping, and most of those tape adhesives are latex based. So I scrubbed one with alcohol and soapy water and tried again on one cavity. -Same thing….maybe even worse, reacted and uncured at the surface.



So then I figured screw it, I clear lacquered each side of each print to seal it and tried again. My silicone is somewhat old so I thought maybe that was a contributing factor. So, as a control, I took some of the leftover silicone from this mold batch and placed a small puddle on the surface of materials in my shop ranging from various metals, wood, concrete, and 5 different plastics. The resulting mold was better but still reacted and produced unacceptable mold quality. All of the other sample puddles fully cured. After I removed the prints from the mold, 24 hrs later the uncured surface did cure further confirming the material incompatibility. The resulting waxes were so-so….not good enough…..especially for how much time I had invested in the Air Horn patterns.



I’m stubborn when it comes to failure, so I relented and went to my Tin catalyzed Condensation cure silicone. It is very old (like 10+ years!) and way out of use date. It was viscous to the point of barely flowing (it would usually run through the thinnest crack) and I didn’t think it would turn out but it did. It’s lower durometer at only 20-30 Shore A whereas the Platinum catalyzed stuff is 40-55 Shore A and preferred (at least by me) for such a mold. This did not react with the print and fully cured, producing a good mold and waxes.



So here are the iterations…..fourth time’s a charm



Here’s the original print and the wax from the final mold.



I’ve made a lot of silicone molds and it’s been a long time since I’ve been bitten by this issue but I just thought I’d post this for everyone. These parts are very small, but if it was a large complex mold, it would have scrapped a lot of expensive silicone and cost an enormous amount of time cleaning uncured silicone off a large complex master. In this case it was only a 10-minute job to remedy this each time.

In my experience, Platinum/Addition cure silicones are more likely to react with the master pattern material and less likely to react and inhibit the cure of the material you cast in the resulting mold, like urethane thermosets, which in addition to wax, is what I most often mold in silicone. Tin/Condensation cure silicones are the other way around.

As common as 3D SLA printing is these days, I would think pulling molds from the photosensitive/UV SLA resin prints is commonplace and there’d be more out there on this on the web. I guess, testing for material compatibility before you mold a big job is always the safest course of action.

Best,
Kelly

 

That stuff is $90.00 a bottle !!!
Standard resins are only $20.00

 

You're probably right about that ESC and I did take a look at several sites that had equipment/materials that could 3d print silicone molds directly. Those printable materials sound relatively new but I will say I'd think it would be a difficult material to print and maintain all the benefits of the typical mold compounds.

Molding silicones can reproduce great finish and detail, I'd expect beyond what can be printed......but of course if you are pulling the mold off a print it's just a matter of what the material being printed is capable of. Aside from reactivity with some materials they are general self releasing with most things you would mold with them....especially the Platinum cure variety.

The mold making silicones are circa $100-150/gal. Some of the specialty compound even more so I've already taken care to minimize mold volume. Sounds like David may have some knowledge of the printable materials but sounds like they may set a new standard as far as material cost.

This mold was probably one of the better candidates for being directly printed because it was small and just used for wax not something that could react with a new mold compound. it's probably a case no new tricks from old dogs but I had the silicone materials on hand and experience using them so it was just a matter of me getting out my begging bowl with David

Take care,
Kelly