Old RTV Molds

Darkov · Post by **Darkov** » Tue Sep 12, 2006 8:44 pm

I have a question on what to do with old worn out or no longer needed molds. I have heard that you can grind them up and use them as filler. What do you use to grind them up? Do you just add the ground up bit to the rubber you are going to pour or do you place it in unimportant parts of a new mold?

Thanks,
Pete

tonyG2 · Post by **tonyG2** » Wed Sep 13, 2006 2:48 am

I chop 'em into tiny cubes and use them as filler to reduce the amount of RTV in the next mold.....

Antenociti · Post by **Antenociti** » Wed Sep 13, 2006 4:10 am

I use an old meat grinder - cast iron - grinds it down into tiny pieces without much effort at all.

Not sure that the modern day plastic ones would be strong enough though... you soon find out that rtv is quite tough.

Antenociti · Post by **Antenociti** » Wed Sep 13, 2006 4:12 am

I use an old meat grinder - cast iron - grinds it down into tiny pieces without much effort at all.

Not sure that the modern day plastic ones would be strong enough though... you soon find out that rtv is quite tough.

modelnutz · Post by **modelnutz** » Wed Sep 13, 2006 7:52 am

Meat grinder for me as well.....the silicone beats them up pretty well tho....

Guess I'm glad that I can get them at a local flee market for 2 or 3 dollars each !

Modelnutz 8)
"the name sez it all"

Darkov · Post by **Darkov** » Wed Sep 13, 2006 12:25 pm

Thanks Guys......

Now once you have it all ground up do you just add it to your mixed RTV?

Won't it mess up the detail of what you are trying to make a mold of.

modelnutz · Post by **modelnutz** » Thu Sep 14, 2006 6:32 am

I ran into problems when I just cut the silicone into chunks.....the edges of the chunks would show themselves in the mold and cause issues with the castings.
Perhaps it was the slightly differing hardnesses that created the problems ?

Since I started grinding it into small chunks ( 1/8" sq. or so ) I find that I get no issues or impressions in the castings.

Grinding with the meat grinder works good, but it really cuts down the life of the grinder. I've had to buy 3 this year!
I find that it also helps to cut the silicone into strips no bigger than 1/2" square in cross section.

Hope this helps!

Modelnutz 8)

CheddarMelt · Post by **CheddarMelt** » Thu Sep 14, 2006 8:35 am

What if the pieces were shreaded in to micro bits? Would this work?

modelnutz · Post by **modelnutz** » Thu Sep 14, 2006 10:35 am

Empireman....Yup. Should work fine.
How would you go about shredding the silicone ?

Another thing about adding old silicone to new...it tends to speed the cure of the new silicone as well as making it thicker.
I usually have to pressureize the mix to prevent air traps near the master.
I typically reserve this trick for not so important molds....

M'nutz 8)
"it's not what you think......"

CheddarMelt · Post by **CheddarMelt** » Thu Sep 14, 2006 4:34 pm

modelnutz wrote:Empireman....Yup. Should work fine.
How would you go about shredding the silicone ?

Another thing about adding old silicone to new...it tends to speed the cure of the new silicone as well as making it thicker.
I usually have to pressureize the mix to prevent air traps near the master.
I typically reserve this trick for not so important molds....

M'nutz 8)
"it's not what you think......"

Well I was thinking about using a grader to shread it into small strips,I have not tried it yet,but what do you think? Would it work?

Antenociti · Post by **Antenociti** » Thu Sep 14, 2006 9:06 pm

Get an olde worlde "Spong" meat grinder if you can find one - they are nigh on indestructible.

I mostly use old rtv as a "filler" to bulk out the back of the mould or to fill in large empty spaces and always paint on the new rtv to get all the fine detail, then leave that to set up for an hour or two, then put in the re-mixed stuff over that.

modelbaseguy · Post by **modelbaseguy** » Fri Sep 15, 2006 10:50 am

I cut mine into small chunks but first I pour a skin coat of silicone over my master and let it cure. This helps to ensure none of the little chunks of added silicone interfere with the shape of the castings. If I have a larger areas of void space I will use larger chunks of old mold silicone.

modelnutz · Post by **modelnutz** » Fri Sep 15, 2006 12:52 pm

Empireman...I tried a rotary grater and it did'nt work that well...the silicone just rode over the "teeth" and never really shredded.
The cast iron style meat grinder really works the best.

The only thing that might work better ( I'm told by a fellow engineer) is a set of intermeshing gears to (in effect) crush the silicone into small bits.
Sounded like an idea till he told me what kind of horsepower I'd need to run the beast.....not interested in running 3 phase power to my shop just to grind a bit of silicone.

By the way...doing it with a 40 grit sanding belt on a combo sander is definatly not a good idea !

Antenociti...Tell me more about the "spong" grinder of which you speak...I've already killed 2 of the old cast iron type.

As always.....good luck !

Modelnutz 8)