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Suppose I have a detective pikachu mold tray, some hot glue sticks and a small cooking pan.
Can I melt the hot glue sticks in the small cooking pan and use it as a resin, to be pour into the detective pikachu mold tray?
Will it cause any damages to the cooking pan, or the mold tray?
You can melt hot glue in a pan, but it tends to turn yellow and eventually brown if it gets too hot. whether the mold survives the pouring depends on the material.
The real problem is that hot glue shrinks quite a lot while cooling down. The mold surface is cooler than the hot glue, so it sets at the outside first, while the center stays liquid for longer. As the center cools down and shrinks, it pulls all surface areas towards the center, warbing the object and possibly damaging the mold.
I've done this experience with a very small object: a papercut of a treasure chest, no more than 2 x 1 x 1 cm. I wanted to add weight to the object and filled the whole thing with hot glue. Once it was cool, the side walls were notably pulled in and there was a big (relatively speaking) hole at the top of the hot glue, where the top surface was pulled towards the center.
Casting resins have very different characteristics, like:
There is a long list of differences, and hot glue basically sucks at those characteristics. If you need the characteristics of resin, Elmy's right, hot glue probably wouldn't be a great choice. That said, "it's a big world out there"; not all casting requires those characteristics. Also, if you are aware of hot glue's shortcomings you can often compensate for them.
There is an entire arts and crafts industry built around melting embossing powder or hot glue in a pot and casting with it. Embossing powder has much better characteristics than hot glue, but it's also much more expensive. Hot glue is still good enough for a lot of purposes.
It is important to heat the material to the proper temperature. It you get it too hot, the hot glue degrades, changes color, etc. A high capacity hot glue gun is a good tool for dealing with hot glue because pouring a huge amount into a large mold generally won't produce good results. You can also use a heat gun set at a low temperature to keep the applied glue liquid if you are working on a large surface.
But if a melting pot is what you want, there are melting pots designed for the task. Ranger made one of the most popular models, with temperature adjustable for most meltable materials. I understand they have now discontinued it, but there are others. This picture of their unit will illustrate the concept:
Image courtesy Pinterest
Many people just use a cheap, non-stick electric skillet that has an adjustable temperature that goes low enough for the material. Just don't use it for food afterward (they're pretty cheap, though).
Relatively small and reasonably thin castings tend to come out fine, especially on the mold side. There can be a little shrinkage on the sides, but it shows up mainly on the backside, and can be dealt with if that's important to your application. On a larger item, precise dimensions isn't critical for some purposes.
So the typical use is for things like costume jewelry, small toys, hobby items, decorations, and the like. A thin layer of hot glue is clear enough that it can be used to embed objects in small pieces. It's used to cast molds of objects to use for casting with materials like resin or Plaster of Paris. Another use is similar to a 3D printer for purposes like modelling or prototyping. Sections of an item are cast and then assembled by methods such as using a hot knife to melt and join edges, or running a hot glue gun along the seam.
So back to the original question, depending on the mold material and size, what you need it for, and your expectations, casting with hot glue from a pan could potentially be a good solution for you. Or it could be completely inadequate for your needs.
One way it might work is to mix as little hot glue as possible with as much as possible of something with a lower coefficient of thermal expansion. Sand for example. This will also make it stronger. Hot glue isn't generally a decorative finish but if you're using one that is you might want to melt your glue, coat the inside of the mould, then add your filler. Certainly this would be effective in the case mentioned in @Elmy's answer (adding weight).
Melting in the oven might be better than on a stove as you can control the temperature (including testing a small batch). Some hot glues would actually melt in a pot immersed in boiling water but most need a higher temperature than that. 120°C would be a good place to start. I suggest using a dedicated container such as a clean food tin (or even a glass jar in the oven) as normal hot glue isn't intended for food use, and isn't easy to get rid of completely.
