Quite a long time ago while tinkering around with new ideas for my 3d printer, I made up a quick bookcase sculpt. This took maybe 20 minutes and I used Google Sketchup for this. It was a quick sculpt and I made multiple prints at different sizes, some I made for my kids' doll houses which were a hit (then quickly forgotten).
After having these sculpts for maybe 2 years I decided to finally make a mold of one suitable for 28mm scenery. I think it is a great, simple sculpt as it is suitable for a variety of time periods.
Seeing as I was molding it anyway I thought I would do a blog-post about how to make 1 piece mold.
It really is quite simple and doesn't require any specialised tools (although you might want to buy some things specifically for this use if you are going to mold things fairly often).
Step 1: Building the Mold box.
The molding box is important because, well, it holds the sculpt you are trying to mold and stops your gooey rubber from going everywhere.
You can build your own molding box or purchase ones specifically for the making molds. I initially bought a molding box but found it quite tricky for making small molds as it was quite tall and was hard to tape up gaps so I went out to a cheap toy store and bought some imitation Lego blocks for a little bit of cash. These are great because they allow you make the mold box to your own size and the pieces fit together quite tightly. You do get some leakage from it but there are ways to combat that easily.
Firstly I make a base to fit the sculpt. I make sure there is enough room around it so that it will still be firm when pouring in my resin for casting at a later date. I make this two layers thick so it is quite firm and doesn't fall apart. A thin sheet of MDF would do the job just as well although it may not be to the correct length and width but that might not prove a problem if you just use some sturdy rubberbands to hold it in place.
After this I put some clingfilm over the top to stop any rubber leaking below, and I wrapped this underneath.
Now I build the walls up around the outside. This snaps easily over the top of the clingfilm. You need to make sure that when you build the walls that they go well above the height of the sculpt you are molding. As with the walls you need a good thickness of rubber underneath so that when pulling out your casts they do not rip the bottom out of the mold. The height of my bookcase was about 1.5cm and I built my walls up 4cm.
The last step here that I do is to put some tape around the outside. If there are any gaps this will stop any leakage from spilling out.
Step 2: Molding
First I fix my sculpt to the bottom of the moldbox. For this I just used superglue which held quite nicely.
There are a lot of different molding agents out there you can use. In the past I have used Pinkysil and Oomoo which are a 1:1 volume mix ratio so eay if you are starting out. They cure fairly quickly as well so if you are keen to mold and cast within a day, using something like this might be useful. For this cast I am using Moldmax. It has a longer curing time of about 4 hours but has the benefit of being able to withstand high temperatures. So if you are wanting to cast in pewter this could be useful.
I measured the inside space of my moldbox and found it could take about 150mls. So I estimated about 120mls of rubber plus my bookcase would fill it up nicely.
After mixing your rubber together you usually have a good quarter of an hour or so to use it which is plenty of time. Vacuum chambers can be used to remove all the bubbles from your rubber but these can be expensive especially for general hobby use. A slow pour can help to remove some of the bubbles that may have occurred during mixing.
I start pouring in the corner where it hits the bottom of the mold box first and then gradually gets higher.
Pouring straight onto the sculpt you are molding can cause bubbles to be trapped, ruining your casts.
When the rubber reaches the top of the sculpt I am molding I make sure to pour extra slowly so that the rubber can reach all the gaps without trapping any air.
After you have finished the pour, tapping the sides of the mold box can help release any trapped bubbles.
After this it is just a matter of time, waiting for your mold to cure before you can demold and begin casting.
Step 3: Demolding
This is a simple step. All I do is remove the tape and take apart the molding box brick by brick. Some bricks can be quite stuck when molding rubber gets underneath them.
Following this I had to remove the bookcase from the mold. This step can get tricky if you haven't quite managed to seal the bottom of your sculpt but a scalpel can cut it away fairly easily.
Final Product
The final product I made using polyurethane resin. The resin I used has been sitting around a while so is a bit discoloured but will serve the purpose nicely.
As I had not actually intended on using this sculpt for casting there is one change I would have made to it so that casting would come out better. The gap between where the tops of the books and the shelves above are very thin but deep in some places. This has caused it to rip after only a few castings. I would make the depth of the less if I did this again. However these are just for myself and the flaw is not largely visible so it is not too much of an issue.
All in all I would call it a successful mold and I can imagine I will get a lot of use out of it building some sort of miniature library.
A quick note on 2 piece molds
Two piece molds are very similar to this but I generally use an oilbased clay to embed my sculpt into. And make some air channels and a pouring channel for the resin to flow through and the air to flow out. One side is molded first and then the oil based clay is removed and the sculpted cleaned up. The sculpt is put back into the first piece molded and then the second molding piece is poured on top. Make sure you use some mold release on this so that the two pieces separate easily. I've made this mistake before and it is an absolute pain.
