Mold making tips and techniques

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Greg Bryan
Posts: 100
Joined: April 19th, 2019, 9:15 pm
Location: Moriarty New Mexico

March 9th, 2020, 5:53 pm

A recent project has had me focusing on the skills I have learned about casting my own resin parts. This made me decide to share some of the tricks and techniques I've picked up since I've started doing this. Now this comes in real handy when you need to make multiple identical parts for a project.

Knowing how to make my own rubber molds for casting your own resin parts has also saved my butt on more than one project. Most notably a 1/72 scale space shuttle that I bought that was missing one of it's engine bells. I was able to simply use one of the remaining two to make a mold and cast a replacement for not only the missing one but all the others so I was sure they were identical.

Now I do prefer working with one piece molds whenever possible. I've always had the issue of resin leaking when I've made two piece molds in the past. This usually results in a big mess and I switched to one piece molds to remedy the situation. However there is also limitations on what you can do with a one piece mold. So I've spent a lot of time thinking about how to better design my two piece molds.

Now as I said before the biggest issue I've had is resin leaking out from the seam between the two parts of my molds. Mostly due to the seam running around the top bottom and sides of the mold while the resin is poured. While on a one piece mold there is no seam only an opening in the top. So there isn't any sort of opening for the resin to leak out of but it's also a challenge getting resin into some parts of the mold let alone getting the cast parts out of the mold at times.

This is where a two piece mold is the better choice but for the sake of not ending up with everything including myself covered in resin I've been forced to stick with one piece molds while thinking of a solution to the problem. Recently I tried out a new design that's been floating around my head that solves this problem and I'll be showing how it worked out.

Before this my two piece molds were basically just a top and bottom piece of rubber that I turned on their side when I poured the resin into them. Unfortunately I never got a good seal between the two halves and of course they leaked. I'll also admit I had a tendency to combine multiple parts in a mold and that also helped contribute to the problem. Now I'm focusing on only having one or two parts per mold in an effort to keep the molds as simple as possible.

Now I could go on and on about the many mistakes I've made. Instead I'm going to go over the basic method I've used up until now. I've always started a mold with the container or form I was using. The bottom half of this was completely filled with modeling clay and the masters was pressed into the clay. Once the master was in place pour and vent spouts were added usually coming out of the seam on one side. I then mix up the mold material which I will refer to simply as the mix and pour it into the form.

Once the poured mix sets up I remove everything from the form and after cleaning the first half of the mold it receives several coats of release agent. I next place this half back in the form in the place of the clay. Place the master, pour and vent spouts back in place then pour more mix over the first half of the mold. Unfortunately this method also leaves a seam that runs completely around the finished mold.

My new method moves this problematic seam from the sides to the top. Essentially the bottom half of the mold acts like a bowl. While the top half of the mold forms the bowl's lid. Leaving no seams along the sides and bottom for the resin to leak out of. Also any resin that leaks is contained so the only mess you end up with will be due to spills.


Now to start with in need several tools and supplies. The first being Alumilite's "Supper Casting Kit" I picked up at one of my local Hobby Lobbys. It's normally $69.99 but thanks to the 40% off coupon that reduced the price to $42. Ideally I would have just got the Quick Set mold rubber and a bottle of release agent. But Hobby Lobby doesn't sell the release agent by itself and I was getting low on resin so this works out in the end.
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Next on the supplies list is thin cardboard for making the forms as well as some airsoft pellets, poster putty, Scotch tape, and Elmer's glue. Tools consist of a pair of scissors, my hobby knife, putty shaping tool, a pencil, and a ruler.

Now the process begins with the part you wish to cast replicas of. In this case I'm using the gun from my Alex Gundam. After placing the part on a piece of cardboard I mark around it to make a template of sorts. This way I can get the basic shape of the modeling clay base shaped out as well as figure out what size of container I'll need for the form of the mold.
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I shape out the model clay on the template to get the general shape. I also make sure it's between a quarter to half an inch thick but there will be a little variation since I'm also placing the part in at angle so the resin flows down to the end of the guns barrel when the mold is completed. Note this also can be done by placing something under the form container to tip it slightly before filling with the mold material but you have better control with the other method.

With the modeling putty ready I press the part into it. After being sure several key details like the trigger guard and handle is seated in the clay I add pour and vent spouts made from the poster putty.

I also lightly press in airsoft pellets around the gun to create locking tabs and slots to help hold the two parts of the mold together. As well as angling the sides of the clay for the same purpose.
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At this point I'm ready to place all of this into the form of the mold. This I made myself by glueing a cardboard cylinder to another piece of cardboard. Also once I finish pouring I will reinforce the form by placing a strip or two of tape across the top. Now I will mention that I made this form bigger than I should have for this mold and future molds will address this mistake.
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I'm able to pour the first part of the mold at this time. This mostly is getting your mix ready according to the instructions in the kit then carefully pouring the mix into the form. I then add a strip of tape across the top of the form for reinforcement and wait for it to set up.
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Of course Mr Murphy's laws also made their presence known to me at this time. It turns out my master floated. This required me to take first the mixing stick I prepared the mix with then a toothpick to hold it down. In the future if there's a possibility of what I'm using as a master floating I'll have to make a point to address this during the pouring of the mix by doing it in two stages. First stage i would only pour enough mix to partially surround the master. Then once the first stage had set up make more mix and fill the form the rest of the way.

Now due to the master floating I had to basically cut it out of the first half of the mold. As well as cut new pour and vent spouts. However without that problem the next stage consists of the following.

You remove the modeling clay, the master, and the pour and vent spouts. I then clean up any residue left by the clay from both the mold half and the master.
Once clean I apply several coats of the release agent from the kit with a paint brush.
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Note this is a critical step and you need to be sure to coat every nook and cranny where your planning to pour more mix. Otherwise the new mix will bond with the old and form a solid block of rubber.

Here you see how the first half of the mold looks once it has set up and everything has been removed. I've gone ahead and blushed several coats of the release agent I mentioned before and I've put the master as well as poster putty vents and pour spouts back in place in preparation of filling the cavity with more mix.
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Last edited by Greg Bryan on March 9th, 2020, 6:03 pm, edited 1 time in total.
Greg Bryan
Posts: 100
Joined: April 19th, 2019, 9:15 pm
Location: Moriarty New Mexico

March 9th, 2020, 6:01 pm

Making the second half of the mold

Also Mr Murphy kicked my plans in the butt again at this point. I made the form to big and used most of the suppied mix while making the first part of the mold. Not having enough mix to finish I had to come up with a solution otherwise I would have to by more mold material.
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Thankfully as I said before if it's not treated with a release agent old rubber will fuse to the new mix. So I gathered up all the trimmings from all the cutting I did earlier. After making sure the trimmings were in small pieces and started the second pour.
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I first carefully applied small amounts of the mix to the small cavities created by the airsoft pellets and around the master. this was to be sure I got mix into these. Next I poured just enough mix to cover the rest of the master.
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Now I sprinkled the trimmings into the main cavity lightly pressing them down. Before pouring more mix on top of them. Now towards the end I was literally scraping the bottom of my container of mold rubber and applied more catalyst than before resulting in the mix being darker. But this is only a paper thin section on the half of the mold and shouldn't effect it any.
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Now after the second half sets up I can pop the two parts apart and remove the master and the poster putty. I now have a two piece mold but it's necessary to give it look over. If you need to carefully trim around your vents and pour spouts some more. Sometimes a thin amount of mix gets between them and the master and it will need cleared away before you do a test casting to check for other problems.
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