The types of plastics you could use for casting your own lego custom parts. First thing you want in a plastic is for it to be liquid or runny when not cured. You want a liquid uncured plastic (mostly found in 2 parts) because it is pourable and will capture all the detail of the mould (this is where the quality of the mould will pay off). Now the list of kust some plastic/resins which would work fine.

1)Urethane Liquid Plastic: The Smooth Cast 320 Series consists of ultra-low viscosity/low cost casting resins that yield virtually bubble-free castings ( this is good as it is easy to pour and bubbles ruin casted pieces). Smooth Cast 320 Series resins pigment better and are lower in cost (this means they take on colours better which means you can cast different colour plastics. Easy to mix and pour, these resins offer the convenience of a one to one mix ratio (one Part A to one Part B by volume). Fully cured castings are tough, durable, machinable and paintable.

2)Alumilite casting resin: Alumilite Regular provides users with an extremely tough, high temperature resistant, plastic that is incredibly easy to use. It has a low visicosity (very thin and easy to pour and resists bubbles). This however is not able to take colours so you might have to paint the finished casted piece

3)Polyurethane Liquid Casting Plastics:Polytek polyurethane liquid casting plastics come in several varieties that can be poured, brushed or sprayed to create castings with unlimited versatility. Almost any color, size, shape, weight and appearance can be achieved by using these systems. Our polyurethane liquid plastics are able to be poured as supplied or can be thickened to a brushable consistency for making mold shells or hollow/lay-up castings. Certain Polytek plastics are available for sprayable applications where hardcoating foam or spraying-up large lightweight castings is required.

4)
Epoxy Casting Plastics:liquid plastics are excellent for casting small decorative objects, prototypes, duplicate masters and for decoupage coating and glass bonding as well as for making vacuum- and thermo-forming tools & molds. Polypoxy resins can be cured with various Poly Cure hardeners to suit a variety of applications.

Not moisture sensitive like polyurethane resins

Versatile epoxies – accepts fillers easily- which can overcome bubble problems

High hardness – temperature resistant

Pour or layup

If you have any questions just ask.