This is a such a good question (and an important one). It is definitely important to know the pros and cons of each mold type before making your resin castings.
Both types of molds can work with a variety of resins, but knowing the limitations of each is essential to making sure you get the results you want.
These reusable resin molds are usually made from polypropylene or polyethylene sheet plastic. They are generally made from a template and are partially flexible.
Inexpensive. Plastic molds generally can be purchased for less than $10 each. (Many for under $5!) This is also good when you damage a mold, you’re usually not out a lot of money to replace it.
Easier to work with. Plastic molds are a great place for new resin crafters to start. They are not generally complex (nooks and crannies) and are easier to care for when compared to silicone. (wash after use and allow to dry)
Castings will not be shiny bright. The surface of the casting that is touching the surface of the mold will NOT be shiny once it is removed from the mold. You will either need to recoat with another layer of resin or cover with a layer of resin gloss sealer spray.
Can easily be scratched. While it is helpful that the plastic is flexible, the softness that allows this can also allow you to scratch the surface. Something like a toothpick or a fingernail can leave a noticeable gouge if you’re not careful.
Difficult to make your own. Unless you have a high tech system that allows you to mold the plastic, it will be almost impossible for you to make your own resin molds.
These reusable resin molds are made from two part silicone and can last for many years if cared for properly.
Very flexible. Silicone molds allow you to do very complex castings that can be removed much more easily as compared to plastic molds.
Custom molds. You can purchase two part silicone yourself and mold anything you want to (assuming you have a model).
Castings may come out shiny. I say may here because the original model used to make the mold must be shiny. If it is (like with our geometric silicone resin molds), then your casting should come out just as shiny as the original.
Expensive. Silicone molds are at least 2 to 3 times the price of a plastic mold. Silicone costs much more than plastic.
Requires more care. Silicone molds must be cleaned after use. They should also be stored flat in a cool, dry area.
May require vacuum casting. Unless you’re using a clear silicone mold, you might not see all the bubbles in your resin when casting it into silicone. You might have to resort to using a vacuum casting setup to make sure all your bubbles are out before the resin starts to cure.
So which kind of resin mold should you choose?
If you’re a beginner, I would definitely go with a plastic mold. They’re inexpensive and if you make a mistake (like your resin doesn’t cure), you’re not out a lot of money.
Once you get more seasoned, a silicone mold is a great ‘collection piece’ to add to your mold collection. It will last for hundreds of castings. Hint: if you make a casting from your plastic mold that you really like and you want it to be shiny, complete the casting, get the shine you like, then mold it in silicone.