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Sil_x
09-26-2009, 04:08 AM
Hi everyone! I have been building molds for a short while and am looking to those with more experience for pointers and critique. I will post my build steps so tell me what you think.

1. Build a gear with your favorite cad/cam tool. Nice pretty tool paths

2. Cut the mold. I like to use wax since its easy to machine and does not need any release agent. I pack the edge with clay

3. Mix up some silicone and pour

4. Into the pressure tank for curing. I use about 50 psi.

Sil_x
09-26-2009, 04:14 AM
5. Cured silicone. Pretty good cast, not too many bubbles at the top.

6. De-clay the mold. easy enough.

7. Mix up your favorite variety of quick curing urethane and pour.

8. After the urethane is cured, I drill a small hole into the wax and pop out the new mold with compressed air.

Sil_x
09-26-2009, 04:18 AM
9. Make a hub. I drill dimples for extra grip.

10. Mix up some epoxy and some secret fillers :)

11. Pour the epoxy and back into the pressure tank to cure over night.

That's all for tonight. I will post how the casting comes out tomorrow.

Thanks

mcphill
09-26-2009, 05:48 AM
Thanks for sharing! Very interested in your results, as well as your proprietary "fillers"...

posix
09-26-2009, 07:48 AM
I'm more interested in what do you actually do with cast gears? wouldn't tolerances be way out for them to be usable for anything "precise"? why not just simply cut them from a block of your secret recepie expoxy mix?

Sil_x
09-26-2009, 10:18 AM
Thanks for sharing! Very interested in your results, as well as your proprietary "fillers"...

Alumina hydrate, strontium carbonate, grog, kyanite, and blue chalk line for coloring. Most of it is left over refractory components from building a forge. Most significant is kyanite. It has needle shaped grains that should provide structural stability. I spent too much time reading the EG thread.

Sil_x
09-26-2009, 10:25 AM
I'm more interested in what do you actually do with cast gears? wouldn't tolerances be way out for them to be usable for anything "precise"? why not just simply cut them from a block of your secret recepie expoxy mix?

Good question. I suppose i could cut it out of my epoxy mix or aluminum although the epoxy is a little abrasive. The idea is to make something that is easily repeatable and make a bunch with minimal work. Who knows if I get the accuracy good enough I might sell a few on ebay. As for precise, I have a 2:1 set on my z axis that are way out of tolerance and I am still about .001 accurate.

posix
09-26-2009, 10:29 AM
good then. so what does the finished item look like?

Sil_x
09-26-2009, 10:35 AM
12. Its done cooking and time to de-mold

13. Looks nice, right?

14. Into the lathe for a bit of boring and cleaning up the back

Sil_x
09-26-2009, 10:40 AM
15. The accuracy from cog to cog looks pretty good. My biggest issue with accuracy is the shaft being concentric. Right now I am using really soft silicone. I think it is shore 10A and changing to a much harder shore 40A or 50A should give me my best results.

16. rinse and repeat.

Janos
02-07-2010, 08:05 PM
The only thing I would probably do differently is cure it under a vacuum. I have had some exposure (Enough to be dangerous) and used a vacuum chamber to remove any air that had gotten into the part during mixing and pouring of the media. You have to be careful doing this because if you just flip the switch and hit it with the full available vacuum any air bubbles that is in the part will get EXTREMELY large REALLY FAST and boil out of the mold. To counter this, place it under the glass bell and slowly bring the air pressure down. A few small bumps of the vac. switch and watching it carefully will take every tiny void out. Then when released the part will actually be under pressure at zero atmospheres. This makes a very nice consistent part.

You can buy a bell from most any thrift store. They sell them as covers for bread or cheese. No point of spending for one if you can get one on the cheap. Then a little work with a lathe to face off a piece of 1/2" aluminum with a gasket groove and your on your way. (Not to mention the pump) Here is a vid of what I was talking about. http://207.234.149.99/preparing/VacDegas3e-500.wmv I snagged it from a DeGassing thread posted here. I believe that on a part like yours where the top is open enough to allow the air out without having to pressurize the material into a void would work well.

ynneb
02-08-2010, 12:36 AM
A great thread here. I have been talking with a friend for years about doing just this.
Maybe if you cut the teeth deeper than they need to be, and turn the outside in a lathe to ensure the centre is concentric with the outer.

ynneb
02-08-2010, 12:44 AM
Oh and another thought. you really should let it cure under normal atmospheric conditions.
You vacuum so that the bubble rise out of the mould. But if any are still caught, they become the size of pin pricks the moment the vacuum is released. You win with both processes that way.

judleroy
02-08-2010, 01:07 AM
You can also use an air release agent to remove the air from castable polyeurthane without the need of a vacuum. Silicone rubber molds are great for making short runs of parts (500-1000 pcs). The parts are suprisingly accurate and repeatable. I have also had extremly good results cutting multiple pc molds from polypro for polyeurthane casting.They require little or no mold release agents to demold and allow you to easily make injection mold style parts with castable resins. This is a great way to prototype parts that would normally require tedious jigs to cut and allows you to make many parts for real world testing in a resonable amount of time. There are a few tricks to designing the molds to allow the parts to fill properly but with a little tweaking and experimenting you can get close to injection molded part quality at a fraction of the cost.
Judleroy

Sil_x
04-23-2010, 12:25 PM
Oh and another thought. you really should let it cure under normal atmospheric conditions.
You vacuum so that the bubble rise out of the mould. But if any are still caught, they become the size of pin pricks the moment the vacuum is released. You win with both processes that way.

I really aught to vacuum the silicone to get rid of the bubbles but since I don't have the equipment, pressure casting is my best alternative. Pressure casting the resin works really well to fill any open voids. I have to pressure cast the silicone because any bubbles become pits when I use it to pressure cast the resin.

Sil_x
04-23-2010, 12:33 PM
You can also use an air release agent to remove the air from castable polyeurthane without the need of a vacuum. Silicone rubber molds are great for making short runs of parts (500-1000 pcs). The parts are suprisingly accurate and repeatable. I have also had extremly good results cutting multiple pc molds from polypro for polyeurthane casting.They require little or no mold release agents to demold and allow you to easily make injection mold style parts with castable resins. This is a great way to prototype parts that would normally require tedious jigs to cut and allows you to make many parts for real world testing in a resonable amount of time. There are a few tricks to designing the molds to allow the parts to fill properly but with a little tweaking and experimenting you can get close to injection molded part quality at a fraction of the cost.
Judleroy

The biggest trick I have learned to keep the parts accurate is the quick cast urethane shell around the silicone. without it the silicone deforms under its own weight and the casting resin. I am really happy with the accuracy of these parts, I am using several on my mill as a 3:1 reduction. I recently discovered my accuracy problem was in my mill. My lead screw was worn out and I had uneven backlash.

CarbonKevin
05-05-2010, 01:08 PM
A few ideas for you...

Vacuuming doesn't have to be an expensive process. An old fridge compressor will pull 28" of vacuum with no modification. You can buy a venturi vacuum generator that uses compressed air to pull vacuum. You can even get a hand pump.

Something to consider would be degassing the resin BEFORE you pour it into the mould - get most of the bubbles out - then vacuum it again afterward.

But if you're mixing resin, you're definitely getting air inside it, and these voids are weak points in the final product. I definitely encourage you to consider it further.

Good luck!

Kipper
05-05-2010, 04:16 PM
Could you make the mould with the pilot bore and boss already made? Nice Idea and no doubt perfectly good enough for it's intended use....I have a foundry with wax vinamold resins silicon refractory plaster etc....Just never considered the saving to be worth it....A 40t pulley is £5 and that's at rip-off Britain rates :D