Picture of Make a two part reusable mold using plaster
Before I start, I'd like to note that this is my first instructable so please excuse any potential gaps in logic I made and some of the blurry pictures. I tried to take as many photos as possible and will explain the process as thoroughly as possible, but should you require additional help feel free to mention it in the comments below.

So What I've done here was develop a cheap way to create molds for various casting projects. Depending on what materials you already have available, the total cost for creating a single two part reusable mold will range from 0-25$.

The advantage of a two part mold over a one part (where molten material is poured into an open mold) is that much more structurally complicated objects can be duplicated in this way. To demonstrate the process for the instructable, I have chosen a sea shell since it is an object that cannot be duplicated with a one part mold system but is very streamlined - it lacks advanced features such as protrusions.

Plaster is an ideal material for this project as it is very malleable in its unhardened shape, and can withstand very high heat when dry. Therefore, finished molds will work for almost any casting material - I preferably like to create projects out of wax, tin, silver, and gold as these materials are very easy to work with. For this project, and for cost issues, I will make a casting in wax.
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Step 1: Materials

Picture of Materials
The materials listed here can be easily interchanged with others of similar properties. Get creative! instead of buying what I listed here try to find alternative supplies

Materials needed for making the mold:

an object to duplicate - make sure it is something you can sacrifice in case of a screw-up :)
a generous amount of plaster of Paris
Hot glue, or any glue that can be easily removed when dry
Play doh or modeling clay, or any other material with the same consistency 
Soap (to act as mold release) - there is also professional mold release spray that can be bought from eBay or amazon
paintbrush, or any tool to spread mold release
a plastic box, or four strips of plastic or other material that can be glued together to make a hollow box
portfolio paper cover, or a paper plate, or anything other than newspaper to serve as an overspill guard

Additional materials to make a wax based duplicate:

a large candle, or any excess wax
cooking pot and stove
boiling water

Step 2: Making the mold perimeter

Picture of Making the mold perimeter
Whatever perimeter you choose, make sure that it will be big enough to contain the object you chose to duplicate.

If you don't have a spare plastic box, try creating a perimeter out of strips of plastic - a good place to get those would be out of plastic packaging for products. Here I used an old GLAD brand food container.

The overall shape of the perimeter does not matter, as long as it is taller than the height of your object.

Cut out the bottom of the box and glue it to your overspill guard. I prefer to use only four small glue applications, one on each side, as it simplifies detaching the guard later.

Step 3: Play Doh Time!

Picture of Play Doh Time!

Use your Play Doh, or substitute material, and pack in down inside your perimeter. Then take your object and press it roughly halfway in the play dough. Make sure to cover any interior crevices before inserting your object (I glued up opening of the shell)

Note- push back the Play Doh so it will not engulf the object any more than half its height. Not doing so will greatly complicate removing the object later, and you will likely have to break either the mold or the object to make either usable.

Step 4: Protrusion and Lubrication

This step will detail the preparations before casting.

If your object does not have a flat base or is not touching the perimeter, it is important to create an artificial tube through which molten material can be poured once the plaster dries. I created mine by cutting and gluing four sections of a drinking straw and placing  them at the top of the shell. As shown in the picture, it does not have to completely touch the edge. It is easy to break off whatever thin layer of plaster will accumulate there.

Using the end of the paintbrush, or a pencil, make indentations in the play doh. These will come up as bumps on one of the halves of the mold and will make for easy alignment of the two parts (think of two Lego bricks locking together).

Now spread a generous amount of soap all over the play doh, object, and sides of the perimeter. This will serve as a non stick layer and allow for easy removal.

Step 5: Pouring the first half

Picture of Pouring the first half
Mix your plaster to an even consistency with as little clumps as possible. Tap your mixing container repeatedly on a table or other hard surface to remove air bubbles - remember that air bubbles trapped around your object will distort the final shape and give it a tumorous look.

When pouring, pour only at one corner of the perimeter and let the plaster slide naturally over the object. Pouring directly over the object will heighten the chances of air bubbles forming in crucial areas. 

Top off using as much plaster as possible without spilling.

Step 6: Pouring the second half

Picture of Pouring the second half
Detach the perimeter from the overspill guard, flip it upside down, and re glue.

Remove the play doh - be careful to not knead it back into its container before washing so it can be reused for later projects

give the mold and sides a generous coat of soap before pouring the plaster, using the same method as the first pour.

Step 7: Observe your creation

Picture of observe your creation
Separate your two halves and remove your object.
If needed, break off the thin layer of plaster covering your pour-access tube.

This step concludes the mold making part of the instructable. From here, I will describe how to make a duplication using wax as a medium.

Make sure to wash out any remaining soap as it will distort the shape of your duplicates.

Step 8: Gather Materials for wax

Picture of Gather Materials for wax
Melt your wax inside the pot. If your candle has a wick, fish it out using a toothpick or Popsicle stick.

Tie your mold together with string or hold it together with rubber bands.

Boil your water in preparation for cleanup

Step 9: Pouring

Picture of Pouring
I did not take pictures of the actual pouring as I needed both hands for the procedure.

Place your mold over a spill guard, with the hole facing up.

My pot had a prebuilt spout in it which made pouring easier. If you do not have one, pour as carefully as possible. I highly suggest to not hold the mold while pouring to avoid an accidental spill over your fingers.

Wax shrinks in volume as it hardens- continue pouring and topping off the mold every few minutes until the reduction ceases.

I recommend letting the mold sit for at least 20 minutes to ensure that the wax has hardened.

Pour any excess wax into another container for future use.

After the wax hardens, pour boiling water over the mold to melt off any wax that spilled down the sides. Try to remove as much wax as possible from the half line as this will make separating the two halves a lot easier.

Unfortunately, my cast broke when I separated the two halves. Perhaps lubricating the insides will assist in preserving the duplication's integrity but I highly recommend against lubricating the mold when using mediums hotter than wax, such as silver. It's ok though, as I can always scrape out the wax and remelt it for another attempt.

Thanks for viewing my first instructable. Feel free to ask questions and add recommendations in the comments.
shaddoty2 years ago
can this plaster take 480F
armorlord (author)  shaddoty2 years ago
I use these plaster molds to cast molten silver and gold, which I use oxyacetylene to melt. Yes it can take extreme temperatures - just make sure to heat it gradually, preferably using an open flame torch. If you subject plaster to an extreme and sudden temperature change it will crack.
This was my question, i was wanting a resueable molds that could hold upto at least zinc, but silver and gold even better.

Are you saying before melting my metal warm the mold? Then melt the metal and pour? I have been plan to do all heating with torch either propain or acetylene i have both.
armorlord (author)  mcshannon1 month ago

Before pouring your metal make sure your mold is properly heated. Otherwise the sudden heat shock will cause cracks. Depending on the mold complexity it may require sticking the whole torch inside the mold. With some practice you'll get the preheating concept right. Until then - pour over sand!

ChrisJ251 month ago

One thing I would recommend when making the mold is to incorporate some kind of vent to allow air to escape as 'molten' material enters, to avoid air bubbles in the new finished item. We did something like this when I was in 8th grade and the vent(s) were required as we were pouring pewter.

joemellin10 months ago

Thanks for making this. The bit about the over spill guard is a bit confusing to me. It looks like from the picture of you glueing (because of the reflection) that you are glueing two boxes together, when it seems it is just a 4 sided box and basically making it so that you can switch which of the other 2 sides is inclosed.

I think a picture where you can tell it is being glued to something / no reflection would be a great help!

Thanks again for making this, I am gonna go get my mold on :)

framistan2 years ago
I once molded a chess pawn similar to your method. Fortunately, it was not valuable, because the "professional" mold release spray didn't work at all. My pawn was sealed halfway into the plaster like LUKE SKYWALKER in carbonite! I notice you used SOFTSOAP. I will try something like that next time.

FIY it's Han Solo in carboite

FYI it's FYI

i know sorry

Could you use this technique in making ballistics gel molds?

Excellent instructable, I am eager to give this method a try. Would you recommend using a mold release on the plaster when using metals instead of wax?
armorlord (author)  troopersmachine2 years ago
If you can find a temperature resistant mold release, It would definitely be a good idea.

For me, since I already built enough experience to make perfect molds without imperfections, I just use them as one timers. Cast in the metal, then break the mold. I find it easier to create a new mold than mess around with trying to preserve it in perfect shape, especially for complicated shapes.
Yeah, that's so true, when i do a one time cast, I make single-use molds, but when I make a mold that I want to cast several times in, I have to go with a 2 part, reusable mold. Another minus with single-use molds, if you poured the metal incorrectly into the mold, you can't cast it again, you just wasted: plaster, time, and metal, but with a reusable mold, if you messed up the metal pour once, you can always try again. I tend to make reusable molds for small casts like marbles, but for big intricate casts, I make single use molds, since I know I don't need more than one. Also, what metal do you usually use, I use pewter, zinc or lead, all of them have a minus though, zinc is nontoxic, strong, but it decreases in size greatly as it solidifies, leaving a bad cast, also, zinc is hard to melt on a stove, I prefer a propane torch on a crucible, it leaves the metal molten for long enough to pour cArefully. Pewter is nontoxic, safe, easy to melt, but its very soft and it easily is bent and pewter at times can be pricy. Lead is easy to melt, pretty much cheap, not too expensive, but it is very toxic and carcinogenic, resulting in a cast that you cannot play with to much with your hands. Sorry for long post, thAnks for reading, goodnight to you.
Peter Angel2 years ago
Nice instructable.

An important step is to wet the plaster mould with water than drain it before pouring in the hot wax - otherwise the hot wax will stick to the dry plaster.