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Urethane is in, and I am beginning to run tires. Doing some tests to see what combination of vacuum de-airing the uncured rubber, and pressure casting gives the best results. I do need the pressure pots to fill orders, so there is a bit of a bottleneck there, but it looks like both vacuum and pressure is best. I did learn you need to let the rubber fullly cure before relieving the pressure, or you get some odd little balloons on the tire. So tires get run last thing, and sit under pressure overnight (16 hour cure time).
I thought I would lay out the steps of how the patterns and molds were made.
First step was to measure the Thunderslot wheel and tire, and then make some 3D files for printing.[attachment file=”Wireframe tire.jpg”]
This was exported as a .stl file, and set up for printing on my resin DLP printer I gave the tire a little standoff and placed it on a base, so that with any luck the back side of the tire won’t be to hard to tidy up.
[attachment file=”Mold Tray Render.jpg”]
Here is the printed part on the build-table
[attachment file=”02-part on build table.jpg”]
Now to make a silicone mold. I drilled some vent holes in the base in hopes it will ease de-molding. Remember to tape them up, though, or the silicone will run out.
[attachment file=”03-vent hole.jpg”]
ABS sheet is used for mold walls. Taped together flat like this, so the walls can be folded up and taped for a re-useable mold box.
[attachment file=”04-mold box.jpg”]
Time for RTV
[attachment file=”06-silicone poured.jpg”]
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The next morning the mold is opened
[attachment file=”07-mold open.jpg”]
Urethane is next. This is a 30 shore Urethane from Smooth-On
[attachment file=”08-urethane.jpg”]
Black colorant is added, parts a and b are mixed, de-aired in a vacuum chamber and poured (very slowly) into the molds
[attachment file=”09-urethane pour.jpg”]
Next step: See if they fit.
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This all looks like fun…I’d love to experiment with creating tire compounds – but wanted to be sure you realize the rules for ThunderSlot say this:
- Rear: OEM Treaded Tires (TYC002R) or Upgrade slick 19X10mm (TYR004R standard) allowed.
Therefore, home-made tires would not be allowed without a rule change (which may be warranted due to the lack of tire availability). 🙁
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“This all looks like fun…I’d love to experiment with creating tire compounds – but wanted to be sure you realize the rules for ThunderSlot say this:
Rear: OEM Treaded Tires (TYC002R) or Upgrade slick 19X10mm (TYR004R standard) allowed.
Therefore, home-made tires would not be allowed without a rule change (which may be warranted due to the lack of tire availability). ????”Yeah, I do realize, but it was the lack of availability (and the poor quality of the OEM tires) that prompted me to do this. Unfortunately, they can’t be cranked out quickly enough to have a supply ready for everyone right away. Still working out kinks, it looks like the most time consuming method is the only viable option.
The rubber needs to be fully cured under pressure, which takes up a pressure pot needed for producing figures.
Even with de-airing the rubber, some little bubbles remain that create weak spots.
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My Dad the Radial tire engineer discovered that the tires did not cool like bias ply tires. The hook conveyor usually used to move hot tires from the curing mold machines would leave the radial tires out of round. It required the addition of a machine at the press called a post cure inflater that kept the tire round during cool down, and a ring around the tire blew cool air on the outside. Getting the slot car tires solid and round probably is one of the major failures we see in our scale tires. Good luck.
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First – super-cool post! Next – do you have a 3D-Printer? If so – it may be interesting for you to collaborate with Alex, Steve and Russel to 3D some custom chassis for a future series (e.g. where Scalextric, Fly or Carerra offers a cool body but no decent chassis are available.)
TIRES – Marc if you could duplicate some F30s size tires they would be good to replace the crappy tires on the rental cars that are stashed in out pit shelves. -
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I have left a supply of Thunderslot tires (not legal for racing, but good for practice) in the club room on the same shelf as the power supplies. Please help yourself but don’t be piggy, and please let me know when supplies run low.
Along with the tires, are some mounting supplies. Some measuring cups, stirrers, and toothpicks for application of glue.
The glue is Smooth-On Ure-Bond III a two part flexible urethane adhesive. It has a pot life of 5-10 minutes, depending on temperature, and a cure time of 24 hours. Mix is 1:1 by volume. I have been using the mixing cups to pour out 5ml of each, which is a lot for a couple tires, but any less and it might be tough to get proportions right. You can always try blobbing out a bit of A and B on a card, or other clean surface like you would epoxy. Like epoxy one of the components is more viscous than the other, just something to be aware of.
Once you mix, the clock is ticking, so make sure you have everything prepared in advance.
Uncured glue can be cleaned up with isopropyl alcohol. Once it’s cured it can be a bear to remove. so prepare a work area with some newspaper or similar.-Marc T.
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