Two part resin mold making

Making a two part resin mold from a plastic toy model

A couple of weeks ago, I showed what happens when you try to mold a plastic dinosaur toy like a traditional flat backed item.  Molding a model that has undercuts and no large flat surface can be a bit challenging.  In order for me to make a mold of this dinosaur, I am going to have to do it as a two part mold.

Side note:  I’m not going to lie, when I started this molding this dinosaur, I wasn’t quite sure what to expect or how it will turn out.  There is a lot to consider and assuming it goes well, I will have learned a lot.  Once I get the technique down, I will be recording a video of the process as well.

Looking at the dinosaur, I had to mold him with the end in mind.  When it comes to making two part molds, you need to think of it as molding ‘mirror images’ of the model.  I also needed to mold him in such a way that there would be a large opening at the top for bubbles to escape the casting.  (Think about a large tree.  You want where you pour into the mold to be at the trunk, not the end of one of the branches.)  I decided the head would be the best place for that.

To do this project, I once again turned to the Composimold remeltable molding material.  I wasn’t sure how the casting was going to turn out and loved the idea that I could remelt the mold in case it didn’t turn out the way I wanted it to.

dinosaur model two views

For my mold box, I cut the bottom off a paper cup and set it onto the sticky side of contact paper.  I didn’t want the dinosaur to rest on the bottom of the mold, so I poured a small amount of Composimold into the cup and let it solidify.

pouring melted Composimold into a mold box

In holding the dinosaur, I decided (at least for now) that the head would be the easiest and best place to let the bubbles escape.  To make it easier to pour resin into the mold, I added some Alumilite modeling clay to make a sprue for the head.

add clay to the model for a sprue

Once the first layer of Composimold solidified, I placed the dinosaur with the clay into the cup.  I made sure to get the clay up next to the edge of the cup.  I poured enough Composimold to come approximately halfway up the dinosaur.  I then placed the mold cup into the freezer.  Note:  I did this upon the suggestion of a Composimold video on making a two part mold.

Not shown:  Once the mold was thoroughly cold (approximately 45 minutes), I removed it from the freezer and applied a couple of light coats of Petrolease mold release.

While the mold was still cold, I poured another layer of melted Composimold to coat the entire dinosaur.  I then let the mold cool at room temperature.

pour Composimold over model

My hope by doing it this way was that the two halves would separate simply by pulling them apart.  Unfortunately, it didn’t work out that way.  I had to use a scalpel blade to separate the two halves, although I did have a nice clear line to follow while doing so.

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Parting line estimate to cut for resin mold

I cut down the mold, across the dinosaur’s back and tail, until I was able to pull him out.

removing dinosaur model from resin mold

The tricky part with this mold is making sure both parts come back together snugly. Otherwise, there will be flashing of the resin that I will have to clean off.

bringing mold halves back together for casting resin

You can see the dinosaur void through the Composimold (another nice feature to using this molding material) and how he cast with a lot of his detail.  It’s going to be tricky to cast enough for the head, but not overdo it for the sprue.  (Although if you overdo it, it’s not a big deal — it just means more sanding and finishing to get it to look like the original.)

Dinosaur mold detail showing line between head and clay sprue

Speaking of the sprue, there is also much more room for me to pour the resin as compared to trying to use his feet in the previous casting.  (You can see how that went down in the video on the Resin Obsession you tube channel Basic Moldmaking Principles.)

The real test in all this is how well the dinosaur casts.  Stay tuned….

resin mold with sprue opening


Unpublished Blog Posts of Resin Obsession, LLC © 2015 Resin Obsession, LLC



Very interesting. I am looking forward to seeing how the resin pour goes. Thanks for posting this!


Drat! Talk about cliff-hangers! Please don’t leave us in the dark too long! This is like reading a book with no ending!


I mentioned some excellent videos by Smooth-on for professional results, 2 part mold making in another comment.
Here’s one of the links.
It’s a very good down and dirty, quick, video
(I do not work for smooth-on, I just like their How To videos and their silicon products (I use Oomoo to make my molds for my resin pieces.)

Laura F.

Question: If you pour one side and let it cure, then lay saran wrap over the dry part and smooth it out, and cast the remaining side, would you be able to separate the 2 halves and still have a clean mould?


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