Introduction: Casting a Two Part Silicone Mold of a Custom Joby Gorillapod Cold-shoe Camera Adapter Part:1
Do you own a Joby Hybrid or SLR zoom GorillaPod, the one with the big Coldshoe? Have you ever wanted to use the quick release clip foot that comes with the GorillaPod on or with a regular tripod or other camera system you own? You can't because you don't have a Cold-shoe adapter to thread onto the 1/4"-20 screw on your tripod, since Joby doesn't make one!!! Well, guess what.... I made one and I am going to show you how I made it.
There are a lot of ways to make Silicone molds of a master part for duplication. I will cover how to make a two part mold, similar to a traditional injection molded tool, that can be used to cast a urethane plastic or resin part. The beauty if this kind of a mold is that it lasts a long time and has even greater flexibility since it is not stressed as much during the de-mold process when the cast parts are removed compared to a one piece mold. It also has other advantages not covered in this tutorial. It's my preferred method of casting an encapsulated part that does not have a flat side to pour into.
I will cover the set up involved in casting the part and vacuum degassing the silicone so that the cast parts can be pressure cast in a pressure pot for a high performance bubble free part. I will not cover how to make the reusable mold box, or the casting of the part in this instructable.
You can watch the YouTube video here
Step 1: What We Need to Make a Two Part Silicone Mold
I created and will use a reusable 3/4" plywood mold box that I will use to pour the silicone over the part to make the two part mold. I also need a 1/2'-3/4" spacer block to raise the part to the correct height in the box. I made this out of a scrap piece of wood, any smooth material can be used for this. Last but not least I used some GI 1100 Silicone from innovative-polymers Inc. a vacuum tank and a mixing cup with about three times the volume of the silicone that I need. I will explain the reason for that a bit later. Since this is a reusable mold box the sides are labeled so I can reassemble the mold box the same way in the future after I take it apart and use it again.
Step 2: Prepare the Master Part to Be Cast
I take the master part and clay the areas that I do not want to cast onto this side of the silicone mold and use the clay to hold the part to the wood insert. It's important that the clay holds the part in place with out destroying the finish on the part. Also of great importance is the type of clay you use. You will need to use a sulfur free clay so that the silicone does not have any issues curing against the clay. Ideally you want to keep the master part encapsulated (do not take the part out of the cast silicone before you cast the second side) when casting the two part mold to get the highest quality reproduction. I did not do this in this Instuctable, but it is what you want to try to achieve if possible. Unfortunately I was forced to remove the master from the initial cast for clean up before casting the second half of the mold. I am using a NSP soft modeling clay (Sulphure-Free plasteline) from Chavant. you could use play-doh as well. The play-doh has the advantage of being soft and easily removable, it however is not nearly as sticky as the clay I am using. If the part is not held in place firmly and securely it will float away when the silicone is poured into the mold box. You can also use a very thin double-sided clear Scrapbooking tape from Scotch 3M
I add a Sprue (A place where the resin can be poured into the mold for casting) made of some white polystyrene. The larger triangle is the Sprue. The Sprue has beveled edges as it touches the master part to form a gate, and allow it to be snapped off easily once the cast part has cured. The other white strips are exits or vents for the resin and air to escape.
Also note, how the part is twisted on the wood insert. I will be pouring resin into the low side of the part, this forces the air in the mold out the high end so as no to trap any air in the final cast part. These little details are key to producing bubble free parts later. This kind of thing come mainly from trial and error and experience over time.
Step 3: Prep the Mold Box and Insert Your Part
I drill some shallow holes in the wood with a 1/4" drill bit that will act as locating pins so the two halves of the silicone mold can be lined up easily when they are reassembled. Next, I clay the inside edges and corner of the molding box so the silicone does not leak out, just to be safe. I use a plastic clay tool to get the clay in the right place and trim the excess. No release agent will be need to be applied to the part, we want an exact reproduction. You can add some Johnson's paste wax to the wood if you want, but it is not needed.
Step 4: Mix Your Silicone Well
It's time to mix the silicone. You will need some silicone of course. For this mold I used GI 1100 Silicone with a shore A hardness from innovative-polymers Inc. GI-1100 is a high tear strength, tin-catalyzed RTV-2 silicone rubber. They sell a few different kinds of Silicone depending on the application. You can call them and talk to one of the techs and they can guide you if you tell them a bit about your project. GI-1100 is a really nice all around silicone and I find it to be very reasonably priced as well.
You will also need an accurate scale. I have a flat digital glass top scale from Taylor, so that it can be easily cleaned. I have it set to grams as that is easier to use than ounces.
A digital kitchen timer is also useful to keep track of time for mixing and degassing.
Pour the silicone into a cup with three times the volume of the silicone so that it does not overflow during vacuum degassing. (large yogurt cups or plastic ice cream buckets are good) I am guesstimating the amount of silicone I need for my project. You can use rice or corn to visualize the volume of silicone needed, or even use some simple volume math as well. Make sure you tear the cup on the scale so you get an accurate weight of the silicone you are pouting into the cup. Next, add the appropriate amount of catalyst to the silicone and begin mixing. Use a nice strong stirring stick, silicone tends to be thick. The Silicone will be uniform in color when it is ready to be used. The GI-1100 has working time 1.5 to 2.5 hours.
Step 5: Degassing Your Silicone in a Vacuum Tank
Next you can vacuum degas your silicon if you have a vacuum tank. Degassing your silicone will take the air out of your silicone that you added into it while you mixed the catalyst. Degassing does several things, is will give you a better chance of having a silicone mold with out any bubbles and it will also allow you to get better mechanical properties out of your silicone and then allow it to be used it in a pressure tank for casting so you don't get cast parts with the mumps or pin holes.
You can see the images of the silicone expanding as it is being degassed under the -30in/Hg (inches of mercury). When it's finished It will be smooth and no bubbles will come to the surface anymore when the silicone has been degassed. If you pull a vacuum on the silicone for about 20 minutes it should be bubble free and ready to pour, your mileage may vary of course depending on the silicone you use and how strong the pump is.
Step 6: Pouring Your Silicone to Make the First Half of the Mold
Once you have let the air back in your vacuum tank you can take your container of silicone and slowly pour into one side of your mold box with your master part. Pour slow and steady in one place. Let the silicone flow over the part naturally. (if you don't have a vacuum tank, then this step is of vital importance. It will help eliminate some of the trapped air in the silicone when you pour it in a thin stream) I did pour a little into the center thread area here first to let the silicone flow into the hole first because it is a threaded area. Be patient here keep pouring into the same spot and let the silicone flow all around the part so you don't get any air bubbles trapped anywhere. Fill the mold box a good 15mm above the part.
That's it for the first side. Let the silicone cure over night so it is not tacky. Make sure you have a nice level surface to let the silicone flow out and cure, that way everything is square.
Step 7: Unbox and Prep for the Second Half of the Mold.
Take the sides of the wooden box of from your mold box and you will have something that looks like the first image. A silicone block with your part trapped inside and a base still attached. Ideally you want to leave the part in the silicone and just remove everything else that is not needed, the base and the clay. I had some issues with my part and thought it would be best to remove the part so I could trim some of the silicone that creeped onto the part, see the second picture. This also allowed me to remove the clay easier. You want to try to leave the part in the silicone if you can, however this may not always be possible, as it was in my case. I trimmed my excess silicone with a pair of fine scissors and reinserted the master part back into the silicone and make sure to add the Sprue and the exit vents back in exactly like they were.
Next I reassemble the four sided mold box around the silicone as shown in the last picture. You can add the bottom just in case, but it should not be needed. I added it to keep everything square and so I don't loose the extra part. I also waxed the wood, it's not absolutely necessary but you can do that if you wish.
Step 8: Get Ready to Pouring the Second Half of the Mold
Silicone does not stick to much other than it's self. So, it is super important to add a release agent to the silicone surface that will be exposed. I use something that I can brush on so I can control the release agent and it does not get onto my master part. I used Vaseline that was thinned down with Naphtha (you can get it at your local hardware store, it's a fast drying thinner for oil-based paints) I would use no more than a 90% Naphtha to 10% Vaseline ratio. The idea is to let the solvent evaporate so you are left with a very thin coat of Vaseline to prevent the silicone from sticking to it's self. You will need to apply two coats to be safe. You can also dye the mixture if you have a very complex part to make sure you cover all the surfaces.
Step 9: Pour the Second Half of the Mold
At this point you need to repeat the previous steps 4,5 and 6, mixing, degassing and pouring your silicone, to make the second half of the mold. Let the silicone cure over night and demold your master part.
Step 10: Ready to Cast Some Nice Resin Parts
You should be able to pour some resin parts at this point. I will cover the casting of the parts in my next instuctable. Here are a few images of some of the hot shoes I cast in various colors and materials.
Thanks for checking out my first Instructable, I hope to put up many more. Good Luck
$5.99 plus shipping for a cold shoe, PM me
I have lots of other design related videos on You tube if your interested.