Making a multi-cavity RVT injection mold?

Questions about Making Your Own Lures

Making a multi-cavity RVT injection mold?

Postby yakfish » Mon Jul 07, 2014 9:44 pm

I have several aluminum injection molds but they are mostly single cavity molds. I'd like to be able to increase my production and thought I might try making a few multi-cavity RTV silicone molds. I would like to be able to inject the plastic instead of pouring since a couple of my baits have small appendages. My question is which silicone product should I use for this purpose? I have watched some of the mold making vids here and it looks like most of the silicone might be too soft to support an injector pushing down on it? I think I would need a fairly stiff rubber for the mold. also would I be able to get as much detail out of an RTV mold as I can out of my aluminum molds?

Thanks!
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Re: Making a multi-cavity RVT injection mold?

Postby Fomen38 » Tue Jul 08, 2014 12:56 pm

Obviously I'm not Larry or Mike, and I know they know infinitely more about mold making than I do, but I do have experience in making RTV silicon molds. There are several different types of RTV silicons that you can buy, each differing in flexibility and shore hardness when cured. The HS3 is the most flexible. It's perfect for making molds of things that have deep undercuts that would normally require a 2 piece mold. The HS2 is a little harder and a little less flexible. There are many types of silicone that Alumilite offer, and most (or all) will list the cured shore hardness. The higher the number, the harder the cured product. If you Google search "shore hardness scale", it will show you a graph of shore hardness, and comparable textures to give you a gauge of what the finished product will feel like. If you're making a 2 piece injection mold, the harder the better.

I have a cast piece that I inject casting resin into. It's a 2 piece silicon mold that takes a lead core insert. I then use a BBQ marinade syringe to inject the mixed casting resin into the mold. It works like a champ. My advice would be to make the sidewalls of the mold thick (at least 1" on each side). This will minimize contortion. Also, I use a couple of cut to fit wood planks to put on the outside of the mold, then gently squeeze them together with some ratcheting clamps. The wood pieces are the exact size of the bottom of both halves of the silicon mold. Don't over tighten or your piece will get contorted and not look like your original. You also need to make sure you add adequate ventilation ports. Remember, whatever direction is "up" on your mold, if you have a high spot and there isn't a vent hole, you WILL trap air there, and your plastic piece won't look exactly like your original.

As far as detail is concerned, the silicon is FAR superior to the aluminum. They use a CAD program and a CNC machine to carve out the negative in block of aluminum. Depending on the size of the bit they use, you can get decent detail. However, when you pour a silicone mold, it will pick up every detail- even fingerprints and hair. My advice to you to avoid any air bubbles in your silicone is this: When you mix your silicon with the catalyst (10 to 1 ratio) an stir it in, let it sit for about 5-7 minutes in the mixing container so all the air bubbles can rise to the top. If you have a vacuum pump and can degas the silicon, that's even better. But most people don't. So when you've let it sit for a few minutes, pour a paper thin ribbon of silicon over your original. Once it's covered, get a small, very soft bristled paint brush and paint the silicon in a very thin layer over the piece. This will eliminate small air bubbles. Make sure you tug on the bristles of your brush before doing this to remove any lose bristles. Once you paint the thin layer on, proceed in pouring the rest of your silicon into the mold box you've made. Since the layer covering the piece is bubble free, it doesn't matter if the rest of the silicone has bubbles in it. The bubbles will not penetrate that painted layer. Keep in mind, the brush will be toast after this, so don't buy an expensive brush. You will need 2 of them if you're making a 2 piece mold.

You'll be surprised how well your plastics look when using a silicon mold. Just remember, the properties of silicone are different than aluminum. You may have to let the plastic sit a minute or two longer before extracting it from the silicon. Also (to answer your last question) you should get hundreds or perhaps into the thousands of pours into the silicone. It is well suited to take the heat from hot plastisol. If you want to be absolutely safe though, and think heat might be an issue, get the high temp silicon. It will take over 500 degrees F, and it's very stiff when cured. It's perfect for 2 piece molds where you're injecting molten plastisol inside.

Hope that helps.
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Re: Making a multi-cavity RVT injection mold?

Postby aka anglinarcher » Tue Jul 08, 2014 6:01 pm

Again, not Larry or Mike, but if you are talking production type molds, I would suggest Plat 55. It will require vacuum degassing, but if you are talking production type molds, you should take the extra effort.

Check it out at the following: http://www.alumilite.com/Category.cfm?Category=Silicone Rubber

More specifically: http://www.alumilite.com/ProdDetail.cfm ... y=Silicone Rubber&Name=Plat55

I know that Larry uses the Plat 55 quite a bit, so it is worth the effort. Now I use the simple Quickset for a more dimensionally stable mold. I am not a "production" guy, so it does great for me, but if I want or need a stiffer Silicone Mold, that is my choice.

http://www.alumilite.com/ProdDetail.cfm ... y=Silicone Rubber&Name=QuickSet
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Re: Making a multi-cavity RVT injection mold?

Postby Mike - Alumilite » Tue Jul 08, 2014 7:46 pm

Sorry for the delay in chiming in but I must say Formen38 and Anglin' you both answered his questions extremely well! Great posts!

I'd like to first suggest the materials that anglin mentioned ... Plat 55 if you have the ability to degas and QuickSet if you do not. The main difference Plat 55 has a high tear strength and very thick viscosity whereas QuickSet has a lower tear strength and an extremely thin viscosity. Due to most 2 piece injection type molds not have drastic undercuts, the tear strength really should not come into play but the viscosity when pouring will if you do not have the capability to degas.

Formen38 made a great suggestion to make the mold flanges and thickness a little bigger than normal. This will allow you surface area between the two halves of the mold to allow for some decent clamping pressure and area to displace the pressure. Allowing you to inject pretty forcefully without having any issue of the mold blowing out or flexing. Putting wood braces that help with dispersing the clamps pressure points is also highly recommended just as Formen stated.

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