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:
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.
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.
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.
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?