Playin' Around

[ScaleMotorcars]

Hey Steve, wheres the pic's. Please use the attachments for photos its allot more reliable.

[strevo]

Hey Steve, wheres the pic's. Please use the attachments for photos its allot more reliable.

I edited my last post to fix the pictures.
-Steve

[hot ford coupe]

I've got to figure out how to make tires also. I have some stuff from Micro Mark but its a few years old. I guess it's experimentation time.

[xken]

Here is a link to visit all kinds of materials for casting . I have used some of their products and it is first class stuff. Some where in their product listing is tire material.

Model Making | Smooth-On, Inc.

You will just have to explore for it.

[strevo]

I also recommend the Smooth-On products. I have also used Rhodia V-340 silicone from Freeman Supply with great success. Although Freeman doesn't sell in small quantities, 5 gallons and up. After my experiments with casting a thin-walled basket out of silicone, I can say that it is possible to get thin-walled parts out of silicone, although it is not easy. You will definitely need a vacuum can and pressure pot, and something like a caulk gun to inject the silicone into the mold. Pouring doesn't work when the walls are that thin. But the nice thing is the mold can be almost anything since the part is silicone.
-Steve

[hot ford coupe]

Thanks guys for the input. Making the tires is the one thing that made me hesitant to make the Coffin in the larger scale. I can also turn some tires on the lathe if I can find plastic pieces big enough.

[Don Garrett]

Guys...hook up with Rick and Tim. TDR has had an RP prototype made for a modular tire and wheel package that will allow you to set your width and backspace for both the tire and wheel, it's got some bugs but with your help Rick and Tim can help us speed up the the process to make it available to you guys.

[strevo]

Thanks guys for the input. Making the tires is the one thing that made me hesitant to make the Coffin in the larger scale. I can also turn some tires on the lathe if I can find plastic pieces big enough.

Jeff,
I've got a bunch of random chunks of renshape foam. It turns pretty nicely on a lathe. If you want, I can dig you out a piece to try out. The only issue is it does take a little extra work at the end to fill all the pinholes, but nothing a good high-build primer can't handle. The one thing that has kept me from thinking about turning some tires is cutting the tread. Well, that and I don't have a lathe.
-Steve

[strevo]

Just wanted to chime in that I've had a lot more experience molding silicone parts now. I've done several at work that were used as prototype parts for testing on an actual diswasher by a major appliance OEM. They used the parts to test the "washability score" of the dishwasher which involves measuring any pressure loss.
Basically, the method I have found that works the best is to design the molds on the computer including any alignment features, gating, and vents. Then I print the molds out on my 3D printer at work. Once they are cleaned up the real work begins. In order to get a good part, there is a lot of smoothing required. I usually spray the parts with a good sanding sealer first, then sand and fill and sand a few more times. Once it's starting to feel smooth, I spray on a primer coat so I can see any pinholes remaining. I fill those and sand one more time. Finally, a coat of crystal clear smoothes it out even more and helps avoid any interaction with the silicone. You have to make sure that the entire rapid prototyped part is covered because the silicone will react with the superglue in the mold and cause it to not cure properly. Once the mold halves are finished, I spray them with a release agent that is safe for the silicone I use and put them together. I run electrical tape tightly around all of the seams, but leaving the vents open. The next step is to measure out the correct amount of silicone and mix it. Then I throw it in our degassing vacuum chamber. Once degassed, I pour it into a self-loadable caulking gun type thing that I use to inject it into the mold. I squeeze it in until it is coming out of the vents. Then I quickly put it into a pressure pot and crank it up to 78psi. I let it sit under pressure overnight. The next day I release the pressure and take it out. After cutting away all of the electrical tape it's time to start prying the mold open. When it's been cured under pressure the silicone tends to really vacuum to the mold surface so it can take quite a bit of force to get the mold apart. Once it's open the only thing left is to trim the gates, vents, and flash and you have a finished thin-walled silicone part. I've made parts this way from very small gaskets smaller than a penny(I didn't sand those as well as I would have liked to, but I was under tight time contraints) to large baskets that were 11"x4"x6".

[strevo]

As a side note, Dan, or anybody else using RTV silicone, how big is your vacuum chamber, how low do you go with your vacuum, and how long does it take you to get there? I use a 55 gallon drum about half filled with plastic pellets and a 12CFM vacuum pump. I try to take it to about 28-28.5 inHg vacuum, and it takes around 10-15 minutes to get there. We just upgraded from a Robinair 6cfm pump to the new Viot 12cfm pump. I'b just wondering if you're getting similar results. I've seen some videos of vacuum chambers that seem to be able to pull 29 inHg almost instantaneously, but I have a feeling the video has been sped up.
-Steve

[strevo]

Does anybody out there have any specs on their vacuum chambers? I just want to make sure that mine is working the way it should.
-Steve

[ScaleMotorcars]

I built mine from 3/8" steel and had it arc welded down at Billy Bobs welding. Those guys did a great job and beveled all corners then welded not only the outside but inside also. Well worth the cost in my opinions. By the way be careful with the drum, Ive seen those implode under to much vacuum and even the smallest dent or imperfection can mean disaster..

So anyway the Vac chamber is 20 x 14 x 14 and has a 1 1/2" thick plexy door. Vacuum comes from an industrial grade pump I picked up on eBay.

Plumbing , is all NW-25 with 2" industrial vacuum ball valves. Ive got it plumbed so I can use the chamber as a vacuum tank on my 18x14 vac forming table.

The entire unit pulls a -30 hg in about 4 to 5 minutes. Usually its faster then the RTV and I have to break it down to 8 to 10 minutes so the mixing pot doesn't boil over.

Nice thing with the fast degas is with some resins with a short pot life. Faster degassing means more work time and less waist.

[strevo]

4-5 minutes for 30inHg is pretty quick. I think I'm losing some vacuum through the hose connections and the gasket on the lid of the drum. Thanks for the note about the drums imploding. The one I have is heavy walled, but I agree, it wouldn't take much to make it implode. The lid actually indents inward when the vacuum gets strong enough. I've been playing around with the idea of mounting the vacuum pump to the lid and running hard lines to help with the vacuum loss. Out of curiosity, do you know the CFM rating of your pump, or the manufacturer?
-Steve

[ScaleMotorcars]

Here's the specs from a rebuilt one online.
Condition: [B]Rebuilt[/B]
Warranty: [B]90-Days[/B]
Part Number: [B]P101978[/B]
Price: [B]$1,275.00[/B]

Link


[B]Welch 5 8915 DirecTorr Direct Drive High Vacuum Rotary Vane Dual Stage Mechanical Vacuum Pump Rebuilt Refurbished[/B]

The Welch 5 8915 DirecTorr Direct Drive High Vacuum is a durable vacuum pump with a superior vapor handling capability. The Welch 5 8915 Direct Drive High Vacuum dual stage rotary vane mechanical vacuum pump offers an excellent ultimate pressure of 1x10-4 Torr. These pumps are commonly used for vacuum furnace evacuation, backing turbomolecular pumps, backing diffusion pumps, rotary evaporation, refrigeration system evacuation, epoxy degassing, vacuum sterilization, backing mass spectrometers, freeze drying, vacuum centrifugal applications, vacuum distillation, space research, and other vacuum processes. These Welch 5 8915 DirecTorr Direct Drive High Vacuum pumps have a pumping speed displacement of 5.5 CFM.