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Mold Making & Casting Basics: Making A Block Mold

Mold Making & Casting Basics: Making a Block Mold

Hello and welcome to the first part of my mold-making tutorial series! Creating molds can be a very challenging process, but it is also a very useful skill for a cosplayer to have.  So over the next several months, I hope to share all the knowledge I’ve gained in mold-making, to help give you the head start I wish I had!

Before We Begin:

Consider for a moment that molding & casting materials can sometimes be pretty costly, so if you only need one or two it might be more cost-effective to shape it out of plastic or foam instead.  This isn’t a hard & fast rule, though! Sometimes the item has a lot of intricate details that would be easier to sculpt in clay for casting, instead.  After you make your first couple of molds, you’ll start to get a feel for when a mold is a good idea.  Practice is key!

Also don’t forget that sometimes things already exist without needing to go through all the work!  For example, if you need a bunch of light weight silver studs, you could create a block mold and cast dozens of them out of resin… or you might also buy a giant bag of googly eyes, paint ’em silver, and be done!  This is a trap even experienced mold making fall into all the time! I regularly catch myself thinking about how I might mold something for my costume, when there’s a much easier way already available to me.

So what is a block mold, exactly?

A block mold is one of the most basic types of molds, and is typically used for casting small or simple hard props out of resin.  It’s called a block mold because of how it looks: it looks like a solid block of rubber!  Block molds are great for starting out because of how few steps there are to creating one; there’s only a handful of things to watch out for when you make yours.

There are plenty of other types of molds as well, so when you are considering molding and casting, there’s a couple things you want to look at to help you decide if a block mold is the right way to go:

Silicone Block Molds Making Horns A couple block molds in action! Making a whole bunch of horns.
  • How large is the item you are casting?
    • (If the item is too large, it will be very expensive to make a block mold for it.  A different type of mold that uses less materials might be better!)
  • What shape will it be?  Will it be a fairly simple shape with some details, or will it be crazy, curvy, or complex?
    • (You’ll need to think about how you’ll wiggle the prop out of the mold once it’s cured.  The simpler the shape, the easier it is to pop out.  More complex shapes might need a different kind of mold that has seams and openings to help get it out.)
  • How many of the item will you need?
    • (The more you need, the more useful a block mold becomes.  For example when I do my Zabrak Horns, I need at least 5 of them at a time, so I made my block mold with all 5 horns in the same block.)

WARNING! Recasting is Stealing!

Before we get into the steps of making a mold, let’s talk about recasting.  Recasting is when you take someone else’s prop or item, that they put a lot of time and effort into making, and make a mold of it yourself so you can make copies.  This is considered stealing, and you could get in a lot of trouble for it!  The costume/prop community is very good at policing for this sort of behavior and they do not treat recasters kindly.  You don’t want to be caught doing it!

As a general rule, as long as you are the person who fully made the original item (or at least heavily modified a store-bought item) you will be fine.  But making copies of someone else’s work is never okay without their express, written permission!

Step 1: Make the item you want to cast

There are plenty of ways to make the item you want to cast!  You could sculpt it in clay, shape it in plastic/worbla, or anything, really! (I once made a mold of an actual banana!  The banana… didn’t survive the experience…)

The most common of course is to sculpt it in clay.  If you want to give that a shot, the clay to purchase will be oil-based clay. Oil-based clay is a type of clay that really doesn’t dry out ever, so it’s easy to re-use in multiple projects.  Some oil-based clays get softer when they heat up, and harder when cold, which allows you to control it with temperature as well as with sculpting tools.  This comes in handy!

Clay Sculpture of Cyborg Eye A clay sculpture of a Cyborg Eyepiece. Perfect for a block mold because it's flat on the back to make it easier to glue to your eyebrow.

As for brand, one of the best brands out there is Monster Clay, but there are plenty of other brands and each has their pros and cons.  I may write a guide some day explaining the brands I’ve tried and what I like about them.  But for now, get yourself some clay and sculpt away!

For block molds, your best bet is to stick with simple sculpts of things that will be flat on their backside.  Some examples that I’ve made have been pendants, small horns, or gemstones. They’re all flat on the back which is something block molds are great for.

Step 2: Put the prop in a container with high walls

You want a container that is as smooth as you can find, and place the flat bottom of the prop against the flat bottom of the container.  If you can’t find a container big enough, you can glue one together using scraps of plastic and hot glue.  Make sure the walls of the container are at least a half inch or more higher than the highest point of your prop.  You want to make sure that for the next step, your prop will be fully covered by the liquid you’re pouring on top of it.

Step 3: Pour your molding material into the container
These items were being made in resin, so I made the molds out of silicone!

The type of molding material you use will depend greatly on what your prop is.  As a general guideline, if your prop is:

  • Hard (such as resin)
    • Make the mold soft/flexible (such as silicone)
  • Soft (such as latex)
    • Make the mold hard (such as plaster)

There are plenty of other materials as well and it’s hard to cover the uses of all of them.  Your best bet is to speak with the retailer you can buy materials from and ask them what materials they recommend for the prop you want to build.  They are always happy to answer specific questions!

Some other tips for pouring the block mold:

  • Pour carefully and don’t pour it directly onto the prop.  Pour it on the side
    • Pouring can cause air bubbles, especially near fine details on your prop.  You want the liquid to slowly seep into those details without accidentally trapping air bubbles around them.
    • The thinner the liquid, the easier it is to capture details.  But if you made your own container, thinner liquids will find any leaks you may have missed! Best to use a pre-made container if available, until you’re comfortable making water-tight containers.
  • Keep in mind how quickly the liquid sets!
    • Some liquids take 24 hours to start to get hard.  Some set as fast as 15 seconds!  When you are buying, double check how much “working time” you have with you.  You don’t want to be pouring and have it suddenly come out in globs!
  • Pour enough to fully cover the prop, plus a bit extra
    • You want to make sure there’s a thick enough layer around your prop that the mold is strong and won’t break or tear by accident.
Step 4: Wait for the mold to fully cure

This could take minutes, hours, or days.  It all depends on what you made the mold from.  Many silicones cure within a few hours, while plasters can take 24 hours to fully cure.  There will always be instructions about this on the product you bought, make sure to check for that!

Step 5: Pop the mold out of the container and clean it out!

It might take a bit of wiggling, but the mold should come out of the container without too much fuss.  You maaaaay potentially break the container though, so make sure you didn’t make the mold in something valuable!

Cleaning it out shouldn’t be too hard.  If you sculpted it with oil-based clay like mentioned above, I have a little trick that makes cleaning it out pretty easy.

  • Put the mold in the freezer for a couple hours, let the clay harden up.
  • Using sculpting tools very carefully, pop the clay out of the mold.  Because it’s hard, it should come out cleanly without any bits left inside.
  • Use small tools to scrape out any remaining clay left inside, but be careful not to be too forceful.  You don’t want to damage the mold by denting or scratching the inside
  • Use mineral spirits and a papertowel to wipe down the inside of the mold and clear out any remaining residue
Step 6: Cast your first prop!

It’s time to cast!  Follow the instructions provided to you by the retailer to make sure you mix & pour your casting material correctly.  Whether you’re working with latex, resin, foam, silicone, or anything else you might cast with, every material is slightly different and the directions are very delicate!

Don’t forget Mold Release!

Mold Release is something you put into the mold first to make sure that your cast doesn’t accidentally stick to the mold!  The worst thing that can ever happen is if your cast fuses with the mold.  In some cases, you may be able to pull it out and save the mold but sometimes you’ll accidentally break or ruin the mold in the process!

Mold Release helps make sure the cast comes out easily and your mold stays nice and clean.  Don’t forget to use it!

Block Mold Making Horns A set of horns I am making using my block mold!

After waiting for the cast to properly cure, you can pull it out of the mold, paint it up, take pictures of it, sell it, use it, whatever!  Its yours now!  And you can make as many as you want!

Some last minute advice:

  • Never be afraid to ask questions from other prop-makers!  We love to help!
  • The stores that sell molding & casting materials are experts, too!
  • There are plenty of guides on youtube that might be able to show you specific types of molds

Thanks so much for reading this guide!  I hope it was helpful, and I’ll be adding more guides like it as soon as I can!

Amber Brite

Amber Brite is a professional prop-maker and cosplayer from Arizona. She has years of experience building large props and prosthetics, armor crafting, sewing, bodypainting, and more. She also loves to share her knowledge through tutorials on her site, as well as panels at the conventions she attends.

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