The desirable property of plastics
The property of &quot;moldability&quot; can be based on several properties of plastics
There are broadly two types of polymer commonly used. Polymers that set and thermoplastics that can be molded when hot. Both of these types can have the desirable property of being liquids that can fill a complex mold before becoming rigid (though by far the commonest is the thermoplastic that merely requires heat to make it flow).
Thermoplastics can be supplied to manufacturers as beads of the already polymerised compound that can be melted in the feed to the moulding machine. Examples include polystyrene, polypropylene, polyethylene and many others. These form flowable liquids when hot enough so that a mold can be filled easily during manufacturing and the polymer will set into a rigid shape when cooled. This makes for very easy manufacturing.
This is how a very wide variety of complex shaped can be made. PET bottles, Lego bricks, toy soldiers, plastic shells for computers and printers, car bumpers and many, many other complex components are made this way.
But other complex components can be made from plastics that set from chemical reactions and cannot be thermally molded. These work as long as the setting reaction is slow enough and the components are liquid. There are a variety of such materials which usually involve mixing a (fairly) liquid monomer or oligomer (examples include methyl methacrylate, some silicones, epoxies and some polyesters. Usually the process involves mixing a controlled amount of the monomer (eg methyl methacrylate which was often the basis for home modelling kits) with another ingredient that triggers polymerisation (often a radical initiator). With the right mix of the two ingredients, the polymerisation speed can be controlled so the speed of polymerisation is slow enough that the liquid can fill a complex mold before starting to set. This process is more often used in small batches as it isn't always well adapted to speed because the curing process can take minutes to hours.
The thing that matters in both thermoplastics and thermosets is that the components are liquids that can fill complex molds before setting rigid. In some cases the setting mechanism is based on cooling; in others it is based on chemical reactions.