Thread: How to: Duplicating a radio bezel.

Now that my silicone RTV rubber has arrived, I can put the master part in the box, and seal its edges with modeling putty.

You have to seal the edges of the piece (unless it is perfectly flat, and even then I would still use clay), otherwise the poured rubber will flow underneath the piece, and you don't want that to happen for a 2-part mold.


In this case, I am using an oil-based modeling clay from Van Aken, that never dries out, and is compatable with silicone rubber.

That last part is important - the clay must not contain sulphur, because sulphur can interfere with the silicone rubber's curing process.

The stuff I got is pigmented (colored), and in retrospect I should have gotten pigment-less clay, as the pigments leave a mess on my hands, and the oil-base is a bit of a pain to wash off.

There was also wax-based clays available, but they didn't explicitly say "silicone compatable", so I passed.


The picture shows the master part mostly sealed.

I've applied a 1/4"-1/2" snake of putty to the edges, pressed it in to seal, and trimmed it with a special modeling razor blade.

The blade that I used was a Flexible PolyBlade from AMACO (American Art Clay Co), made of super-flexible stainless steel 6" long, and can shave off slices of clay super thin.

I got it from Michael's arts and crafts for about $5, well worth it. I also use it shave off thin slices of posterboard, but I wouldn't use it for heavy cutting.


The rest of the part will be sealed off, and the casting and venting channels will be created out of more modeling clay, to be cast into the rubber as channels.

I used about 1/4 pound of the modeling clay to create the pouring and venting channels in the box.

Rather, the clay will form the impression of the channels into the molding rubber.

When this side of the mold cures, those channels will be filled up again with molding clay, and the other side will be poured.

The result will be hollow channels in the mold, ready for resin to flow.


The channels were rolled out of 1/4" molding clay snakes, pressed to the posterboard, and trimmed down with the shaping blade.

The venting channels up top were made thin and narrow on purpose, as air doesn't need a lot of room to escape, and narrow channels mean less resin to take up space.

The pouring channels on the bottom were made a little wider, because the resin is thicker than water, and needs a little space to flow.


I picked up a few cheap supplies from the local Dollar Store for mixing the molding rubber, basically consisting of a large plastic bowl, a scraper spatula, and a cheap tupperware-like container for mixing the resin.

The large plastic bowl will be for mixing the molding rubber, and the spatula is for mixing and scraping the sides to get all the rubber out. The silicone rubber when cured won't stick to the plastic bowl, so you can just peel off any leftovers.

The resin, however, will stick to the regular plastic bowl, so the tupperware-like container will be used for the resin.

The tupperware-like container I believe is made out of polyethelene, and the resin shouldn't stick to that.
Any leftover resin should pop out, but in case it doesn't, it's not a big loss.

The stuff cost me like $3, so it's cheap.

I'll be mixing and pouring the molding rubber tonight, so more pics will be posted later.

The first half of the mold is complete, and I've taken it out of the box to air out.

The silicone RTV produces alcohol while it cures, so you should let it air out for 24 hours before using it.


There's a lot of pictures here, and they're all hosted from my cable connection, so please bear with the load times.

First, I modified the box some more to reduce the amount of rubber I would need to pour. The fact that it isn't a square anymore doesn't matter.


In order to measure out the rubber, the mixing bowl was placed on the scale, and the scale was re-set to 0, to ensure an accurate measurement.


I measured out 3 pounds of rubber to start, and measured 0.3 pounds of catalyst on the digital scale on the left.
Actually, I used Google to convert 0.3 pounds to grams, and measured it out in grams, as it's a bit more accurate that way.


Here's what the base materials look like:


And mixed:


As you can see, 3 pounds wasn't quite enough to cover the piece. Not a problem, as I had 60 minutes to work with, so I mixed another 1 pound.. then another.. and finally another, making 6 pounds total to cover.


About 8 hours later, and the rubber has cured enough to de-mold the part.

But first I have to get it out of the box - and the easiest way to do that is to cut the box apart at the seams.
Easy enough to do, and the box can be glued back together for the second pour, or a new box constructed.


Some of the excess rubber was trimmed off the edges, and the mold peeled right off of the posterboard.



The blanks on the back of the bezel were popped off, and the part was re-inserted into the mold face down. I also sliced the modeling clay venting channels off the posterboard, and re-inserted them back into the rubber.


All of the blue sections you see above will be later sprayed with a lacquer-based primer (without the part installed), the box will be reassembled with the mold face down (as shown), and the second half of the mold will be poured.

The lacquer primer will prevent the two halves of the mold from sticking together, and is called a barrier coat.

I don't have too much rubber left in the bucket, but I won't need that much, as the rubber doesn't need to cover too much of the back side of the mold.

rock and roll! that's so sweet. I have a feeling your experience and the wisdom that you're sharing with us is going to come in handy for me in the future. A lot of those things I would never have found out any other way but hard way. awesome.

to bad this costs a lot. I wonder if RTV gasket sealer will work what is the cost of the plastic? if its cheap i may need to try that.

Originally Posted by killerfishes

to bad this costs a lot. I wonder if RTV gasket sealer will work what is the cost of the plastic? if its cheap i may need to try that.

I don't know about RTV sealer.. might work if it's silicone, but you'd need a lot of sealer to cover.

There are also brush-on rubbers that can do the same thing, but you need to build a support structure for it.


The plastic, in this case Poly 15-3, is $35 for a 5 pounder, $80 for a 20 pounder, including shipping. I could probably get away with a 5 pounder, but I may get a 20.. I can always use more.


The rubber actually cost me about $86 instead of $116, due to the new customer free-shipping deal.

good thread...

I've done a 2 cast aluminum mold with sand, that was interesting. This seems like a very interesting project....cant wait to see the end result.

Originally Posted by integramodder

good thread...
I've done a 2 cast aluminum mold with sand, that was interesting. This seems like a very interesting project....cant wait to see the end result.

Yeah, I did a 2-part aluminum mold with sand ages ago in high school, where you compacted this black sand down hard into the shape of the master part.

I still have the large aluminum boat tie-down that I made, too.


The other thing you might find interesting is that they have resins that are filled with aluminum, for heat-resistant parts.. they're also machinable, though you can do that with normal resin too.

How flexable is the rubber mold? Would you ever need some sort of stiffening material in it?
Good Job!