Those big rubber Halloween knife castings, some had an aluminim armature, other had a section of bandsaw blade with the teeth ground down as an armature. The bandsaw blade allows a thin rubber casting with bend and give while not destroying the prop itself when in use. All armatures run through the handle.
For an as is thin blade casting you can use a very firm rubber but im not certain its a material available outside the professional industry and even then its only available in five gallon kits. Ive made a fair share of prop cast knives with non thickened blades out of MPK90 high impact plastic. Some survived multiple throws at walls, the ground and at actors/stuntmen while fighting even up camera close where rubber, although would have worked fine, a hard casting was much more suited to the task. Ive never needed an internal armature with the MPK90. Not even for thin bladed knives.
You can thicken a blade in various ways from adding wax sheet to the existing blade, casting the blade alone then using a skin of that applied to the blade for the knife moulding, or simply bo a two part box mould with a hard split board. The hard split board is important as when moulding the second side you simply tilt the entire knife upwards, blade stays down, to cheat thickness of the blade while keeping a sharp and thin knife edge. Its very simple and ive never had an issue.
Keep in mind a knife mould is considered an advanced task due to the blade itself. Pulling a casting is the same. Its not something youll want to penny pinch on materials with cheap hobby resin or brittle silicone. You would just end up with a junk mould or unusable casting.
