Thank you for making this insightful comment, it got me thinking. You are on to something very interesting about the choice of materials. In the case of Azuki, she is constantly producing and reproducing the simulacrum of Meguru to get closer to the idealized form from her memories. Polymer clay can be baked to harden into plastic, but that's something she never does, since she is always one step away from the perfect body, unable to even make a head for the model. Every time one accesses a memory one rewrites it and that is exactly what is happening here, Azuki even forgetting the details of Meguru's face. The lability of the polymer clay corresponds with the lability of her memory of Meguru. There is also the correspondence between the sense-memory of having sex with Meguru and the sensory experience of molding and groping the polymer clay into the desired form. Now that she's unknowingly using the sensory experience of Remu's body to rewrite the memory, everything is getting confused and mixed up with her budding desire for Remu.
Remu's choice of plastic is illustrative of the ideal feminine form that she desires, a very generic, almost mass produced feminine ideal form. Fitting then that she then chases this form in mass-produced product that is very stable in a material sense, while never actually reaching that ideal (or seeing her face). However, the plastic dolls can never satisfy her precise desired ideal, so she is like Azuki in this way, but different in that she is chasing this unattainable ideal through consumption. She says she puts the plastic wrap over the clay to disguise the taste, which makes sense on the surface if you've ever tasted polymer clay, but there's something deeper here. She wants the labile clay to be as close to the stability of plastic as possible, the unchanging comfortable ideal. But ideal forms are as labile as memory, and now that a labile doll material has been introduced along with sensory experience with Azuki, I expect that ideal to drastically shift.
The contradictory neuter/sexualizaton of dolls makes so much sense for both of these characters, where they want the sexualization and sense experience of another body in a completely controllable way without the threat of sex and all that comes with it. I wonder if that will change now that they've both found people they can truly trust with their whole selves.
