Merge pull request #15976 from SandPoot/duct-piping

Several duct pipe fixes/qol

code/modules/plumbing/ducts.dm

@@ -388,7 +388,7 @@ All the important duct code:
 	singular_name = "duct"
 	icon = 'icons/obj/plumbing/fluid_ducts.dmi'
 	icon_state = "ducts"
-	custom_materials = list(/datum/material/iron=500)
+	custom_materials = list(/datum/material/plastic = 400)
 	w_class = WEIGHT_CLASS_TINY
 	novariants = FALSE
 	max_amount = 50

code/modules/research/machinery/_production.dm

@@ -120,7 +120,7 @@
 	return round(A / max(1, all_materials[mat] * ef))
 
 /obj/machinery/rnd/production/proc/efficient_with(path)
-	return !ispath(path, /obj/item/stack/sheet) && !ispath(path, /obj/item/stack/ore/bluespace_crystal)
+	return !ispath(path, /obj/item/stack/sheet) && !ispath(path, /obj/item/stack/ore/bluespace_crystal) && !ispath(path, /obj/item/stack/ducts)
 
 /obj/machinery/rnd/production/proc/user_try_print_id(id, amount)
 	if((!istype(linked_console) && requires_console) || !id)