This process should work for many different types of items or models. Note that the original item must have a parting line without undercuts. That means the model must be able to be separated into two halves from which the new molded part can be easily removed.
Here are the steps:
Step 1: Frame for Holding the Epoxy.
Epoxy is a two-part liquid that eventually hardens into a solid material. To create a mold from epoxy, you need need a frame to hold the liquid epoxy until it hardens. The mold will consists of two halves, so the frame must also be constructed as two halves that fit together.
This is a pre-made aluminum mold frame that I purchased from LNS Technologies at www.techkits.com.
I could have tried to make my own frame, but this one was affordable and already had steel alignment pins to align the two halves.
Step 2: Mount the Original Model Into the First Half of the Mold Frame.
I super-glued a round piece of plastic to the model (a topical fish). This was used to suspend the fish in the cavity of the mold frame. It also forms a channel (called a sprue) that allows the melted plastic to enter into the mold during the final injection step.
Note: this mold frame had cutouts for sprues on all 4 sides, so I had to block the other three cutouts. But, I noticed that the epoxy mold frames on the www.techkits.com website now only have a single sprue cutout, so this may no longer be necessary.
Step 3: Mix & Pour the Epoxy for the First Half of the Mold.
I purchased a quart of two-part epoxy from Tap Plastics (www.tapplastics.com). It comes in two 8 oz bottles marked Part A & Part B. I mixed equal parts of A & B, being careful not to introduce air bubbles into the mix. Tap also sells colorants that can be added to the epoxy. I added black colorant to better see the mold surface after curing.
Then I carefully poured the epoxy mix into the mold frame to surround 1/2 of the tropical fish. Again, pour slowly to avoid creating air bubbles in the epoxy.
Step 4: Let the First Half of the Epoxy Mold Harden.
I left the mold frame with the model & epoxy sit undisturbed overnight. By the next day, the epoxy was completely hardened & was able to remove the model to check this first half of the mold.
Before starting on the second half of the epoxy mold, I had to coat the first half with automotive car wax. This prevents the second pouring of liquid epoxy from bonding to the first half. Without this step, I would have ended up with a solid block of epoxy instead of two halves that can be separated!
Then I put the fish model back into the frame & set the second half of the mold frame onto the the first half. The second half of the mold frame has a top removable plate that allowed me to mix another batch of epoxy & then pour it over the first half. (Sorry, I didn't take a photo of this second pouring step)
Step 5: Now I Have the Second Half of the Epoxy Mold.
Again. I allowed the epoxy to harden overnight. Then I was able to pry the two frame halves apart (thanks to the automotive wax) and now I had the second half of the mold.
I hope you can see in the photo that the epoxy was able to pick up even the tiny printing details that were on the side of the original item.
Step 6: My Bench Model Plastic Injection Molding Machine.
I had previously purchased a Model 150A injection molding machine from LNS Technologies. It was affordable & it is easy to use.
I placed the two halves of the epoxy frame mold together. The frame has steel alignment pins, which automatically aligns the two halves. I clamped the mold into the Model 150A & injected some yellow melted plastic into the epoxy mold.
Step 7: A Nice Copy of the Original Item.
Well, this worked even better than I had expected!
Opening the epoxy mold revealed that the newly molded part was basically identical to the original item (minus the painted stripes, of course).
I have injected several more copies of the fish using this epoxy mold & have not yet noticed any degradation in the details. I don't expect that epoxy molds would be a durable as solid aluminum or steel molds, but it certainly seems like a simple low-cost alternative for making limited number of molded items.