Silicone Molding and Resin Casting

This is a little tutorial for resin casting. There are two parts for resin casting: making the mold and making the casts. The first step is to make the molds. I like to use legos to make the mold containers because they’re plastic and I can basically build a box of any shape and size. The master piece is measured and a box is built around the master. The master is then placed inside the box.

The material I’m using is RTV silicone rubber and comes in two parts, the silicone base and the activator. The base and activator is to be mixed by weight, at a ratio of 10 parts base to 1 part activator. Your standard digital kitchen scale is very handy here when measuring. Using some disposable cups, the RTV is mixed.

Once thoroughly mixed, the silicone is poured into the box. I have small bits of cut up old silicone molds that I use as filler, this saves on the amount of raw silicone I have to use. So as I pour, I fill with the bits of cured rubber. The rubber bonds very well with itself so it works as the perfect filler. Once the silicone is poured, you will notice little bubbles slowly surfacing. This is the escaping air that is trapped when the silicone is poured. Now if all these bubbles don’t escape, your mold will have small bubbles. To counter this effect, I have a pressure pot that is powered by my airbrush compressor. The mold is placed into the pressure pot chamber and sealed. The compressor is turned on and a constant pressure of 40 psi is filled. This is enough to force all the air out of the mold creating the most bubble free molds.

A pressure pot is not absolutely necessary, as you can get fairly decent molds without one. But for the cleanest and most bubble free molds, the pressure pot facilitates this. The mold is left in the pot over night to cure. The normal cure times for this particular RTV is about 6-8 hours. And this will vary depending on relative humidity and temperature.

A few days later, the mold is ready to be removed. However, after checking on the mold and removing it from the lego casing, the rubber didn’t feel completely cured. To lower the risk of ruining the mold, the mold is left to sit for a few days so that it can fully set up and cure.

After about a week, the mold has completely set up. The mold may not have completely set up due to a problem when mixing the silicone. If not enough activator is added, the mold will cure too slowly or not cure at all, so it is very important to measure out the correct mixture ratios. The mold is solid and we can now un-mold the clay master. Using a hobby knife, the mold is cut carefully down. Once the clay master is exposed, carefully cut around the master while slowly releasing the master from the mold. I don’t cut all the way through the mold creating a clam shell mold. Once the master is removed, the mold is cleaned up with an old toothbrush to remove any clay that may have been left from the demolding process.

The mold is then sprayed with some mold release that will help preserve the life of the mold as well as help in resin removal from the mold. The mold release is heat activated. So a hair dryer is used to heat up the mold, activating the mold release. With the molds ready the resin casting can begin.

The resin is two part urethane resin. They are mixed at a 1 to 1 ratio by weight, so using the kitchen scale and a disposable cup, the two parts are mixed. The two resin components react exothermically, and learning from an experience when casting in the middle of winter – it is a good idea to preheat the resin components in a hot water bath before mixing. I had once ended up with a mixture that never fully cured because the base parts were too cold when mixed. The reaction is fairly quick, so you need to work quickly after mixing the two ingredients and pour it into the silicone molds.

Once poured into the molds, the molds are quickly placed into the pressure pot. Again, the pressure pot is an optional piece of equipment, but to reduce the amount of bubbles that may form, the pressure pot is employed. The resin will cure very quickly, within minutes. A full cure will occur after about 15-20 minutes. I usually allow the pot to sit for about an hour before de-molding.

After about an hour, the part is de-molded. Since I have extra resin, I poured some into another mold, and removing that mold, the resin didn’t fully fill the mold, this happens. I can just mix more resin and pour another mold and continue until I get the perfect replicated part. The casted resin head came out nice. In hindsight, I could have marked off where the original master fit and cut the mold so that the mold lines are easily removed. But these castings are pretty clean. I can cast an unlimited number of these parts now.

Products and where to buy

I have picked up the silicone RTV rubber from various locations. I have seen aluminite at local arts and crafts stores as well as brookhurst; however, since these products do expire, I usually like to hit up my local plastic retailer: Plastic Depot located in Torrance. http://www.plasticdepots.com/ They also sell various resins, but the resin product I have been using is Silwhite from http://www.silpak.com/; and this company is located in Pomona where folks local to Southern California can pick up the products. Baring that, I would search your local plastics dealer and inquire about silicone RTV and urethane resin products.

Pressure Pot

THe pressure pot I picked up at my local harbour freight needed some modifications before being used as a pressure pot for resin casting. The first picture is the complete setup; there are a few things that need to be attached to the out of the box pressure pot. Several fittings need to be added to plug up the various ends of the pot and an optional modification will help make the pot easier to use for casting larger items.

Starting with the optional modification. The pot has a metal tube that runs down the pot for extracting paint. I cut this tube out with a pipe cutting tool. Since the tube runs down the pot, removing it allows for casting of bigger items. The pot can be used with the tube in place, but in my uses, I have found that the tube just gets in the way.

Next I plugged up the exterior end of where the metal tube is attached. This is the end where a hose attachment that leads to a paint gun is attached for spraying paint. So this end needs to be closed up. The connection end is a 3/8″ pneumatic male connection. Since I wanted to use a drain valve that is a 1/4″ fitting, I needed to get an adapter fitting from 3/8″ to 1/4″. From there I have a coupler for which the drain valve is attached.

Next up, is the regulator/gauge setup. I plugged both ends with quick connect female ends. These are both 1/4″ fittings. I can then connect from the compressor to one of these ends, and with everything plugged up tightly, I can start pressure casting.

About gamerabaenre

model building, anime watching, scuba diving, skiing, software engineering run of the mill guy.
Tagged , , , . Bookmark the permalink.

14 Responses to Silicone Molding and Resin Casting

  1. funaka says:

    I know you’ve told me this but that was a year ago, and I can’t be the only one who wants to know… How about covering where you buy all your materials (with links, preferably) and covering how you connect the pressure pot to your compressor.

  2. gamerabaenre says:

    Updated with requested information.

  3. George says:

    That’s silicone, not silicon

  4. George says:

    Also:

    First, YMMV but I have heard it’s a bad idea to use paper cups or wooden stirring sticks. The reason for this is because they can introduce moisture which can inhibit the curing process (of mold rubber especially). I use small plastic bathroom cups and plastic knives generally.

    Second, if you have issues with mold rubber curing, it can help to mix the rubber, then transfer it to another cup and mix it again before pouring. When mixing, the rubber on the sides of the container tend not to get adequately mixed: but when you pour this material will pour out. It may appear in the finished mold as streaks with noticeably different color and a different degree of shrinkage. (Or the streaks may not properly cure at all…) By transferring to another container before you pour, you wind up with mostly well-mixed material on the walls of the new container, so when you mix again the stuff that wasn’t well-mixed before gets mixed up and you’re much less likely to get streaks.

    Third, if you’re going to mold a head in a one-piece mold you may want to cut open the mold on a side without detail. (If the head terminates at the neck, cut open the mold at the blank surface of the neck. Silicone mold rubber will stretch enough that a fairly small opening will be enough to extricate a simple part like this… And as a bonus it means no seam line on the facial detail. Alternately, cutting the seam down the sides of the head instead of the face would also be preferable…

    I haven’t gotten any pressure-casting gear yet, but I’ve generally been a bit skeptical of the claim that pressurizing the mold can “push air out” of it. Pulling a strong vacuum can “pull” the air out of mold rubber (it needs to be pretty strong – and what it actually does is allow the air bubbles inside the mold to expand and increase in buoyancy, so that they tend to rise and burst… And then when the vacuum is released the remaining air becomes rather small due to air pressure.) – I would expect increased air pressure to make those air bubbles smaller, (and I’m hopeful that this alone will be a big improvement to my own molds) but not necessarily make them exit the mold rubber.

    One of the problems with air bubbles in general is that they can deform the mold slightly. A bubble pressurized more than the surrounding air will bulge out, and a bubble pressurized less than the surrounding air will cave in. If a pressure-pot actually doesn’t help that air to exit, then you wind up with small bubbles that are always pressurized… And if that air eventually finds a way out and the air bubble goes back to normal pressure, it will collapse when it’s pressurized during casting… One of these days I need to try pressure-casting for myself (I can’t justify the cost of a vacuum-degassing rig, so it’d be just pressure) and see how things play out. I do expect pressure alone (with no vac.) will still be better than casting without it.

  5. tgg_admin says:

    There were three spots where I misspelled silicone from the entire article, a little nitpicky aren’t we?

    The mold didn’t fully cure because I wasn’t completely accurate with the mixture ratio – not enough activator. In my experience, letting the mold sit for a little longer, it’ll cure enough for use. And I believe that showing mistakes and how to over come the mistakes is better than showing a perfectly filmed tutorial. A great deal of model building is over coming mistakes.

    If you had bothered to watch the videos, you’ll notice that I used a metal stirring rod to mix the silicone. A wooden skewer is used to mix the resin. Regardless, from my experience, it is better to use plastic cups, but I haven’t found any significant degradation in quality with using paper cups.

    I have created molds with and without the pressure pot, so because of this experience, I can clearly state that using a pressure pot definitely reduces the amount of bubbles in the molds/resin and significantly improves quality.

  6. George says:

    Dude, I did watch the videos. I guess I was mistaken about what you were using to stir the RTV.

    Knowing that you’ve had success with pressure-molding does make me feel better about the prospect of trying it myself. It’s not a huge amount of money (esp. compared to vac. degassing gear) but it’s still a fair bit of change, so it’s good to have some confidence ahead of time that it’ll actually work.

    Not trying to be a dick, here, I’m trying to provide useful info. I have done a bit of casting myself, after all.

  7. When you cure the urethane under pressure, the bubbles are simply “broken up” until they are barely visible or invisible to the naked eye. They DO NOT exit the urethane. The only way to get air OUT is to use vacuum, but urethane mixes usually cure too quickly for this to be done effectively, because it is best to do it before you pour, so you inevitably are pouring partially-cured urethane into your mold. Common practice is to vacuum chamber your silicone before pouring, because you have the time before it cures, and pressure cure urethane because you don’t have the time for vacuum.

  8. masoud davallou says:

    I would like to make more silicone mold , such as industries part , pipe , electrical part and etc, . please help me .

  9. jonny says:

    i have been using RTV silicone. Fortunately i have been able to mix up by eye, “TUT TUT” with great results. unfortunately on this occasion i was rushing and i didn’t add enough catalyst activator, and so its not cured!!

    Will it cure eventually, or is there a solution to help it cure? can i brush over it with a thickener? The mould has sat for about 48 hours and is still tacky.

    Thank in advance.

    • tgg_admin says:

      It may cure eventually, like after a week or so. In the above case, the mold wasn’t fully cured after a day and I had to wait a few more before it was fully cured because I didn’t mix the exact ratio per the instructions. I believe that given enough time, it will cure, which is different from resin where if it doesn’t fully cure, it won’t. I would recommend letting it sit and just check it every day or so and see if it firms up.

      I haven’ tried using the catalyst and brushing it on to the silicone, so who knows, that may work. If it does, let us know. I’m curious if that works as well.

  10. jonny says:

    the silicon has cured, thank for your help. i thought id try a test on some uncured silicone from a separate bach. i let it set for 24 hours and it hadn’t cured but the surface was tacky.. so on one part i tried brushing on some thickener additive. this had no effect and remained on the surface as a slippery gel. secondly i dried some catalyst to see if that would have an effect. it did do something but again left the surface with a residue. both ideas dont work. glad thats confirmed.

    cheers.

  11. Jason says:

    Hello,

    Great stuff. I was looking for information on how I need to modify the pressure pot (same one as yours) I just bought for resin casting as well as mold making.

    I have a couple questions. If I am using the pot for the silicone mold, can I still use Lego’s for the walls, like I have always done without the pot, or is that a bad Idea, and should I use something more heavy duty?

    Also, all the attachments I need to modify my pot like yours, are these fairly easy to find, at say like Lowe’s, and do you use anything special to seal the fittings?

    Thanks much, Jason

  12. gamerabaenre says:

    I use legos for my walls when I put the silicone molds in the pressure pot, works just fine. I found all my mod fittings at lowes or home depot in the pneumatic tools section. They are very easy to find and standard fittings.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>