A tool for curing and bonding a honeycomb stack with void filler, characterized by the tool comprising: a first tool piece and a second tool piece, the first tool piece having a first cavity that receives at least first layer of unfilled honeycomb material; the first tool piece having a first end, the first end having a first aperture and a first seal, and a second end with a second aperture, the second end being opposed to the first end, the first cavity being accessible from both the first aperture and the second aperture; the second tool piece having a second cavity that receives at least one layer of void-filled honeycomb; and the second tool piece having a third end with a third aperture and a second seal, and a fourth end with a fourth aperture, the fourth end being opposed to the third end, the second cavity being accessible from both the third aperture and the fourth aperture; a compression seal arrangement that creates a gas-tight seal between the first seal and the first side of a barrier layer and the second seal and the second side of a barrier layer; a third seal that covers the second aperture so as to form a first gas-tight sealed volume including the first cavity; a pressure plate that covers the fourth aperture so as to apply pressure to the void-filled honeycomb; and a fourth seal that covers the pressure plate so as to form a second gas-tight sealed volume including the second cavity. a first tool piece and a second tool piece, the first tool piece having a first cavity that receives at least first layer of unfilled honeycomb material; the first tool piece having a first end, the first end having a first aperture and a first seal, and a second end with a second aperture, the second end being opposed to the first end, the first cavity being accessible from both the first aperture and the second aperture; the second tool piece having a second cavity that receives at least one layer of void-filled honeycomb; and the second tool piece having a third end with a third aperture and a second seal, and a fourth end with a fourth aperture, the fourth end being opposed to the third end, the second cavity being accessible from both the third aperture and the fourth aperture; the first tool piece having a first cavity that receives at least first layer of unfilled honeycomb material; the first tool piece having a first end, the first end having a first aperture and a first seal, and a second end with a second aperture, the second end being opposed to the first end, the first cavity being accessible from both the first aperture and the second aperture; the second tool piece having a second cavity that receives at least one layer of void-filled honeycomb; and the second tool piece having a third end with a third aperture and a second seal, and a fourth end with a fourth aperture, the fourth end being opposed to the third end, the second cavity being accessible from both the third aperture and the fourth aperture; a compression seal arrangement that creates a gas-tight seal between the first seal and the first side of a barrier layer and the second seal and the second side of a barrier layer; a third seal that covers the second aperture so as to form a first gas-tight sealed volume including the first cavity; a pressure plate that covers the fourth aperture so as to apply pressure to the void-filled honeycomb; and a fourth seal that covers the pressure plate so as to form a second gas-tight sealed volume including the second cavity.