Hiya,

I'm having this weird interaction between this mod and Scaling Machine mod. The plastic scaling recipe is not available for any chemical plants at all.

Tested by unchecking all mods except the 2 scaling mods. New game. Search for plastic tech, the scaling recipe is "made by" empty list.
I've also tried to console give me the scaling version of the machine and the recipe still doesn't show up.

There is no problem on my end. The scaled recipe for plastic bar is usable on chemical plant +3 or above.

I think the problem comes more likely from the mod settings.
First, be sure under settings of Schall Machine Scaling, you have the option Highest overtier Chemical Plant to some large number. The default is 3 (giving you +1, +2, +3 machines).
Second, check the settungs of Schall Recipe Scaling. The default settings are:
Exclude recipe category : rocket-building, creative-mod_*
Exclude recipe : creative-mod_*
Machines using scaled recipes : *-MS-[3456789]$
By using default settings, chemical plant +3 should be using the scaled recipes.

Oh, I missed the part where the scaling recipe only supports machine +3 or higher. D'oh.

You can modify the settings for your uses.
For example, changing "Machines using scaled recipes" to *-MS-[23456789]$ allows you to use scaled recipes on all +2 machines (including furnace +2, centrifuge +2).
Or change to *-MS-[3456789]$, chemical-plant-MS-2, if you want to allow chemical plant +2 exclusively.