More RTV rubber questions

I started a thread about a month ago asking for info on different resins and molding compounds and the info was very helpful.

I can't say I went with the best choice, I decided to purchase the Alumilite product carried by Hobby Lobby mostly because I could use their coupon and get 40% off.

My issue is this...

The RTV is thick! If you want to mix small batches at a time, and if you have to pour it back and forth from one container to another trying to get the amount you need, most of it sticks to the container, which is a big waste. I tried scooping some out, I tried pouring directly from the main container, all of it created a big mess and each time I tried it, I lost a small amount of material.

So I'd like to ask, is there a way of getting the rubber out of the main container that doesn't create loss while transferring? What do you guys use?

My only other option is to mix the whole "kit" up and once, but then I need ALL of the items I want to mold, ready to go all at once. I'd rather not have to do that.

dropshipbob said:

I started a thread about a month ago asking for info on different resins and molding compounds and the info was very helpful.

I can't say I went with the best choice, I decided to purchase the Alumilite product carried by Hobby Lobby mostly because I could use their coupon and get 40% off.

My issue is this...

The RTV is thick! If you want to mix small batches at a time, and if you have to pour it back and forth from one container to another trying to get the amount you need, most of it sticks to the container, which is a big waste. I tried scooping some out, I tried pouring directly from the main container, all of it created a big mess and each time I tried it, I lost a small amount of material.

So I'd like to ask, is there a way of getting the rubber out of the main container that doesn't create loss while transferring? What do you guys use?

My only other option is to mix the whole "kit" up and once, but then I need ALL of the items I want to mold, ready to go all at once. I'd rather not have to do that.

Click to expand...

Hi, most rtv is pretty thick. Not sure I follow why you're pouring it back & forth but if you're concerned about getting the right volume you can fill your mould box with water, pour it into a container that your going to mix your rtv in so you know the exact volume you need (you can only do this if the thing you're molding won't be damaged by water & it must be completely dry before you pour in the rtv).
Alternatively you can mix up less than you think you need, pour it into the box then mix up some more until it's covered (doesn't matter how long between it, can be straight after the first batch or even once the batch has set.
Also if you're trying to avoid wastage, once the leftover rtv in your mixing container has set you can cut it up/ shred it & add it into future rtv moulds as filler.
Hope this helps.

Sent from my SM-N910T using Tapatalk

I have been down a few of these roads and as I am also new to Silicon and have been learning some hard lessons.

The first problem you have already identified, which is the product. Alumilite does make a decent Resin that you can purchase for a low price. It isn't the best plastic ever but is great for prototyping and test cures.

For Silicon, I think that the Polytek products and Smooth On are the best out there. They are not inexpensive, but you can pour the amounts you need and save the rest for later. Polytek's Platsil 73-20 is a great rubber that works in most situations. It isn't cheap, but you will get a very good product and product life for your money.

dropshipbob said:

The RTV is thick! If you want to mix small batches at a time, and if you have to pour it back and forth from one container to another trying to get the amount you need, most of it sticks to the container, which is a big waste.

Click to expand...

Do you mean you're pouring Parts A and B into separate mixing containers, then pouring a little bit back into the product container to get equal amounts? I won't do that, especially if it's humid. You don't want to introduce moisture into the rubber components. It will shorten your shelf life.

Instead, try this. Check the viscosity of your Parts A and B. If one of them is thicker than the other, start with the thick one. Pour the approximate amount of that part into a mixing container. Use a clean craft stick or stir stick to wipe along the rim of the container and "cut" the rubber flow away from it. That way you can control the amount more accurately and avoid making a mess on the side of the container.

Repeat with the other part but pour a bit less of that part than you need. Use another container, so you can compare amounts. Then using the same wipe/cut action add just enough to get the amount you need. If you're close, you might just need to add the small amount stuck to the end of the stick. Just clean the stick on the edge of your mixing container and make sure to transfer all of that part to the other mixing container. Do not reuse the stick from the first step. Use a clean stir stick to avoid cross contamination. If your rubber has a 1:1 mix ratio, you should have equal amounts of both parts.

This works for me. It minimizes waste and mess.

The rubber I'm using is made up of the base (rubber) and the catalyst (which is sort of like water).

I'm having to pour it back and forth because too much comes out of the rubber container and I'm trying to get the right amount so that I don't have a ton of it mixed and nothing to mold. I know that pouring the stuff back and forth is a bad idea, which is what I was asking what you pros do to cut back on this. I was trying to pour out the stuff slowly but it comes out pretty thick no matter what.

itsgreg said:

Hi, most rtv is pretty thick. Not sure I follow why you're pouring it back & forth but if you're concerned about getting the right volume you can fill your mould box with water, pour it into a container that your going to mix your rtv in so you know the exact volume you need (you can only do this if the thing you're molding won't be damaged by water & it must be completely dry before you pour in the rtv).
Alternatively you can mix up less than you think you need, pour it into the box then mix up some more until it's covered (doesn't matter how long between it, can be straight after the first batch or even once the batch has set.
Also if you're trying to avoid wastage, once the leftover rtv in your mixing container has set you can cut it up/ shred it & add it into future rtv moulds as filler.
Hope this helps.

Sent from my SM-N910T using Tapatalk

Click to expand...

I find that dry rice does the same job in measuring volume (and you don't have to worry about waterproof bucks)

Well, looks like it was much a do about nothing. I tried it again last night, just poured really carefully, and I managed just fine. I also prepared two others items to be molded so that if I did have extra rubber, I could go ahead and pour something without simply tossing what was left over into the trash.


Now another question. Is there anything I should be doing to the molds when I'm done with them?

God opens a door...and closes a window.

So I had an item I wanted to mold, and had it molded half-way before I ran out of rubber. So about a week later I go out and buy more, mix it up and pour. The previous batch was an off-white and pretty rigid.

This new stuff is slightly PINK and really squishy!!! I wanted the item to be molded in one big piece and then I was going to simply cut it in part-way in half and pop the item out. I went ahead and did this but some of the new rubber hasn't fully cured...just a few spots where it's still gummy. And there's a few places where the two rubbers meet where they haven't bonded.

It sounds like I got two different types of rubber, but I've never seen Hobby Lobby do that...plus I threw away the packaging of the previous batch, so I have no idea. My other thought is that I didn't mix the two parts good enough or I didn't measure them out correctly BEFORE mixing them....but how would I go from an off-white to pink? I can imagine the trouble this will cause if I try and cast from this mold, I should probably start all over again.