Does anyone here have any experience casting rubber? I'm thinking about casting a rubber carburetor mount and I really don't know where to start. It would have two metal flanges separated by a short rubber tube, something similar to this.
Seems like a challenging project with the over-molded metal and rubber. Is the needed part out of stock? Could it be done as two cast flanges with a lip or boss to accept pre-made rubber tube?
We'd be using them to mount Weber side draft carbs on a race car, so far, I've been unable to find an exact match for the ones we have. There are other ways of mounting the carbs but these are our preference.
I've cast some polyurethane bushing parts for suspension before. I'm guessing this part wouldn't be too different. Make your mold (3d printing works well). Mold release and wax....multiple times!! Pour it and wait till tomorrow. Demold it. If you designed the molds right and mold released it enough it will take some gentle persuasion but it should pop free with no damage to the mold or part. If you didn't design the mold correctly (no underhangs!) or didn't unerstand apply mold release and wax multiple times... (like when you think it's enough...do it one more time) one of the two is going to die. I try to sacrifice the mold because the 3D printer can kick out another one of those. Best advise is hit Youtube and look for some resin casting videos (Frank Ippolito has some good ones, but there's 100's of them out there) casting is casting when it comes to plastics/rubbers at this size of parts. You get into statues or tables you have to worry about heat buildup vs cure time and bubble release. Good Luck!
Most of the castable elastomers don't do well in fuel rich environments, especially with elevated temps. What material are you going to use? Best, Kelly
That's one of the things that I was hoping to get some guidance on. I'm pretty much out of my depth on this, I'm trying to educate myself so I can decide if this is something worth pursuing.
Viton is the go to elastomer for bonded carburetor parts but not very practical for home/hobby one offs and hard to get in small qtys. For you, considering ease of purchase and workability, though over time there may be some degredation, I'd say polyurethane would be good enough. Which Elastomer Offers The Best Oil & Fuel Resistance? (wcrp.uk.com) It will tenaciously stick to clean metal and many other things so as CBB said, use mold release agents.......PVA and waxes. It can be purchase in many places. You will want mid to high duromometer as far as rubbers go. I get mine from Freeman Manufacturing but Smooth-on also stocks many. Best, Kelly
Earlier this morning I sent an email a manufacturer of castable elastomers describing what I hoped to do and, a few minutes ago, I got this response;
It's true, but as an intake manifold, you have a lot of cool air movement, so during use, it will be ok depending upon the interface temp at the cylinder head, which I think would be <180F. Out of curiosity, why would you want an elastomeric intake manifold instead of rigid metal? Motorcycle carbs? Best, Kelly
Thanks Kelly, I'll check with Freeman and Smooth -on. Weber DCOE sidedraft carbs, a flexible mount of some description is recommended. We have some that we like very much and we've been using for many years but we need more and thus far are unable to find new ones. BTW, the picture in my first post is the DCOE/ DHLA mount used on Alfa.
Yah, I'm sure they would respond as such, but you may want to try a sample of castable urethane and stick it in a jar suspended above some petrol in the Sun and see what you think. Not sure I follow why you'd prefer an elastometic over metalic material for an intake maniifld when they hard mount on each side. If it's heat isloation, a phenolic spacer would suffice. Those pieces could be lost foam cast in a blink Best, Kelly
I've had good luck with Smooth-On products in the past. Their tech dept seems to do a good job with questions. That Said I'd follow Kelly's suggestion and test a bit. For the carb mount you're going to want a medium to stiff durometer of poly. Hard is going to be like roller-blade wheels (believe it or not they deform ever so slightly at the point of contact) Soft is going to be radiator hose. If you want the poly to bond to the metal (on the flange) keep it clean. Wipe it with acetone, or denatured alcohol. a couple of holes in the flange would also be useful but make sure to generously chamfer the top of the hole. you want a smooth mating surface to mount the carb to so you'll need to trim the extra poly that come through the holes. But there needs to be enough left that it doesn't pull out of the hole. chamfer gives you that top is bigger than the hole keying effect easily. I have had to burn out some poly bushings at times....the right compound can take some heat. Stinks and smokes terrible too! I don't recommend breathing it or heck doing it... it was one of those it should take an hour to changes these.....4 hrs later I'm pissed and huffing poly smoke. Took an hour to complete the job after that Good luck Rocco! If you give this a go....I want pictures. I've got a friend that has a British sports car with a weber sidedraft. I don't think that thing has ran in 20yrs... terrible engineering (the rest of the motor, carb works ok as long as you can keep fuel to it) . This might be a good Idea to try when we get that thing on the lift for a 0 mile overhaul.
The reason I suggested you experiment, is I have used many materials in the induction systems I've built and have had quite mixed results. In some cases, I used materials that were supposed to be excellent with not so excellent results, and in other intances used materials that were supposedly just ok to marginal that worked out great. For example, Buna N (Nitrile) is supposed to be excellent in the presence of petroleum. I used polyimid fiber reinforced sheets of it for quite a while for carb and intake manifold gaskets. After time in use, they deteriorated significantly. I don't use it for gasket material any more. I've used hard thermoset urethanes for fillers and inserts for intake manifold plenums and they've held up well. If you paint the urethane with a thin layer of epoxy, even better. If you try to use the materials rated as excellent such as Buna-N (Nitrile) and Fluoroelastomer (Viton), you'll find the ability to locate and buy small quantities to be frustrating and even so, they will further require additional hard-to-find/buy regents to cure them, often at higher temps requiring metalic molds. -Not impossible but not very practical. So why elastomer? Best, Kelly
Not exactly sure. I'm not the carb expert, the owner of the car is and we're hoping to replicate what he's been using successfully for decades.
Just a data point not related to casting but might help for the part described? 3D print using TPU filamanet. I've used it to print carb gaskets that are NLA and so far it has worked well. Have a sample in some E85 gas that has been there for several months with no sign of swelling or degradation. With a bit of trickery you can print around embedded metal parts.
Does it HAVE to be real actual rubber? If it's a smallish part, what about using a high temp silicone gasket compound to make the part? That crap is already fuel resistant. Several years ago I made a silicone boot for my 3D printer's hot block using a multi-part 3D printed mold and high-temp silicone gasket material. I don't know about fuel resistance, but I've got hundreds of print hours on that boot and it's still in great shape so it's definitely high temp. Don
Slicones handle heat vary well but supposedly have poor fuel resistance. I say supposedly because this is another one where you may find it does ok even though the charts say it is poor in the presence of petroleum. Fluorosilicones are a special case and excellent as far chemical resistance, but as I mentioned earlier, harder to buy and use at hobby level. The gasket material is usually RTV (room temp vulcanization), and actually cures with the presence of water vapor, but casting thicker sections in an enclosed tool won't usually work without a catalyst. How did you make the boot? Coat a one-sided tool? Best, Kelly
Nope, the boot mold was a totally enclosed 6 part mold with core, but the mold did have several vents,. I needed somewhere for any extra air/RTV to get out of the mold. I don't think the casting thickness is much over 1/8" - maybe 1/4" in the ring around the heat-break. I split the boot through that ring so that I could remove the boot to work on the hot-block without having to tear it completely apart. The main reason I made the boot was that I was tired of burning my fingers when I wiped the drool off the nozzle, right before I started a print. I read somewhere that glycerin would aid in curing the silicone. I got the cheapest liquid water based personal lubricant I could find. They used KY, I got the generic stuff from Walmart. I brushed it on the mold as a release agent, it worked like a charm because everything released cleanly. I got a few air inclusions, but it was my first attempt at 3D printing a mold, my first attempt at molding anything, and my first attempt as using that stuff as a release agent. The Gods do sometimes smile upon idiots - because everything worked. The water in the lubricant probably aided in the RTV cure. If/when I have to make another boot there are things that I'd do differently to try to get rid of the air inclusion. But honestly, I don't think I added enough silicone before I closed up the mold. There are other ways of messing the the cure of silicone RTV, Google "oogoo", and/or "Making RC tires". (But it's got to be the 100% silicone stuff, not the siliconized latex stuff.) Don
