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.
Step 1: 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