Basic mold making tips

Basic mold making principles

When I’m getting ready to make a mold of something, I like to start with the end in mind.  Using two examples, I’m going to walk you through my basic mold making tips why the same molding technique isn’t one size fits all.

Before you get started, here are three things I like to consider:

1.  Do I have or can I create the proper size mold box?  This is important because molding material can get expensive.  You need a mold box that is large enough to hold your model, but doesn’t have a lot of extra space that you have to fill with silicone.  For example, imagine you are trying to mold a bottle cap.  While a small dixie cup and a plastic gallon jug container could both hold the bottle cap, you will use much more molding material by going with the gallon jug container.  Try to find a mold box that will let you get by with something that will comfortably hold your model without wasting a lot of space.  When selecting a size, I also like to make sure the mold box is large enough that I have at least 1/2 inch of molding material extending beyond all sides of the model.  Hint:  I love using food containers for mold boxes.  They essentially free and tend to be made of thin plastic that can easily be cut with sharp scissors or a razor blade.

2.  Whatever you use for your template will impart onto the mold finish.  For example, if your template is rough, it will cast a rough finish into the mold.  Keep this in mind to avoid creating work for yourself.  Try to select something that will take out any finishing steps, especially if a super shiny, polished finish is what you want.  Hint:  Polished metal and acrylic makes great templates for silicone molds and will impart that same shiny finish onto resin castings.

3.  Don’t forget the mold release!  If you have an object that is porous, like bone or wood, the mold making material may adhere to it and which may make it not be able to be demolded.  Smooth surface objects shouldn’t be a problem, but when it doubt, a couple of light coats of a good mold release can save you a lot of aggravation.  I like to use the Petrolease non silicone mold release.  Hint:  If you are using silicone and your model is made from silicone, you must use a mold release, otherwise you will create one big block of silicone.  Extra hint:  If you are using a polyurethane rubber to make a mold, always use a mold release.  I have found that polyurethanes just don’t play well with most anything!

Now that we have some basic guidelines in place, here’s what I’m thinking when I go to make a mold:

Using a block piece as a model:

Lego model underside

This is a pretty straightforward item to mold.  While the bottom has openings in it, the perimeter of the bottom is even and will adhere nicely to the bottom of the mold box.  (If I was worried about resin seeping underneath, I could always fill in the underside with clay, glue, etc.)  It will cast with a large opening for me to pour resin into.

Like this post? You may be interested in  Casting resin in candle molds - Resin experiments

Lego block labeled

The bottom of the model is also similar to the to the top of the model.  I don’t have any strange bends, turns or angles to have to worry about casting.  If you are wondering how molding and casting this Lego block went, you can see the results in the blog post Reusable mold making material for resin casting.

Applying the principles we just used to small dinosaur model aren’t necessarily going to work.

dinosaur model underside

The contact points are limited, so molding material will run underneath our model.  It is also only going to create small holes to pour the resin into once the model is demolded.

dinosaur model

The model is also not flat and varies significantly from top half to bottom half.  Molding the dinosaur the same way we mold the Lego block is not going to work very well.

In fact, I show you in this video what exactly happens when you try to do that way:

What mold making problems have you run into?


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



Thanks for posting this video. I am really curious how you would actually cast the dinosaur. Can’t wait for the video showing the proper way to do it!

Cindy Tesler

Thanks for mentioning that you need the proper size mold box since molding material can get expensive. You also said that you can also use food containers for mold boxes. I think it’s a good idea to choose a mold making company that gets their raw materials from reputable sources.


I want to take some of my old college text books and make a small table out of them. I was thinking of getting or making a small box that is just a little bigger then the books, then putting the books I want as my table in the box and filling it with a clear resin so that i can still read the side covers of the books. what are your thoughts on this project and do you have any advice for me. thanks in advance


I want to make a mold of what is essentially a rectangular open box. How would you go about it to make the mold? I am specifically concerned about the hollow part of the box ( for reference think shadowbox pendant). I have plexi glass that I will be building the model out of and am using a latex rubber based molding material. Thanks for all your great knowledge, prior posts and vids have been very informative 🙂


I am considering making a mold of a deceased pet paw for a memorial item (as opposed to cremation expenses). I would also incorporate pet hair into the pour. Do you have any tips that I should consider first?

Stella jones

What about creating a reusable mold for lifesize animal antlers?… any suggestions on how to go about that?

Katherine Swift

Hi Stella, I want to give you the best possible answer to your question and it’s the kind of question that I can’t answer properly in a blog comment. I would be happy to set you up for a 15 or 30 minute consultation call. If you want to send an email through our contact page, I am happy to respond with prices and availability.

Pete Ramon

This is my first time and im very interested in resin now. Iv seen only 2 sites and wow your very informative.
I just saw this video and i have a question. Can you use a disposable neddle kit to inject the resin in quickly it would also eliminate bubbles and you can get in those tight areas even if you have to puncture the mold to get to certain areas. Just an idea. Like injecting a turkey to get that juice in lol. Hey theres no silly Questions i kind of figure.

Katherine Swift

It’s hard to use a needle and syringe to inject small amounts of resin without introducing more bubbles. What you need to do when building the mold is to create vent holes to allow the bubbles to escape.


I’m having issues with the mold I made using Oomoo 30 mold maker with two part resin and I’m hoping someone can give me some pointers. The mold came out fine and the pieces I made using fast curing resin turned out fine. However both times I tried using two part resin in it, the piece come out cured but sticky. I’m pretty sure I measured and mixed properly because I poured some from both batches into some plastic molds and those turned out fine. Its been 43 hours since the first pour and those are still sticky too. The mold isn’t old either, I’ve only used it 4 times.

Katherine Swift

Hi Cam, this sounds frustrating! Quick-curing resins are incredibly moisture sensitive. If your castings are coming out sticky, it means there is moisture somewhere. What else are you using in the resin?


I have made some attempts with molding but I can figure out how to make a flat back piece stay in place. I always end up with silicone in the bottom side. I’ve tried glue but its not very convenient. Also I can use glue in some pieces because they have “holes” in their surface. Help ?

Katherine Swift

Hi Nantin, are you using a sticky surface (like tape) to apply your models to? That should help with leakage.


I used tape, it seemed pretty sticky to me but obviously it wasn’t enough. Is there something more specific I could use? I was trying to make a mold for a pendant, it was a “word” so I couldnt use glue.


Katherine, I’m going to make silicone molds of light switch covers and plug ins. Do I plug the holes or tape them off or What? I plan on using hot glue for the rest of the bottom. I’ve got the plastic molds and am no big fan of them, even with several coats of mold release. Your input would be greatly appreciated.

Lesa Vaughan

Thank you for the information.

I have been making moulds from latex for years and am transitioning over to Silicon. It is interesting how different the moulds are. The super stretch of the latex, I had taken it for granted. Silicon has some challenges, but thats what all these online resources are great for.
Thank you.


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