Embodiments of the disclosure are directed towards a composite stiffener that is incorporated into panelized products to create stronger bonding, greater adhesion between laminated layers and less warping sandwich panels. In addition, the resulting products incorporating composite stiffeners can be lightweight, yet stronger than other lightweight panels.

A door includes a first skin, a second skin, and a core. The core is disposed between the first skin and the second skin in a thickness direction of the door and spans between the first and second skins. The core includes a corrugated medium that is fluted with fluting running back and forth in the thickness direction between the first skin and the second skin as the corrugated medium spans between the first and second skins. The door also includes a compressed section where at least one of the first skin and the second skin is compressed into the core toward the other of the first skin and the second skin in the thickness direction.

Embodiments of the disclosure are directed towards a composite stiffener that is incorporated into panelized products to create stronger bonding, greater adhesion between laminated layers and less warping sandwich panels. In addition, the resulting products incorporating composite stiffeners can be lightweight, yet stronger than other lightweight panels.

A size-adjustable animal door is provided. The door includes a pair of opposing frame units configured to be secured to opposing sides of a door or other barrier. Each frame unit comprises slidably mating components that permit each frame unit to have (i) an adjustable height, (ii) an adjustable width, or (iii) both The slidably mating components of each frame unit form an opening configured to align with a pre-cut opening in a door, forming a passageway for an animal. The door also includes a flap unit fabricated from a flexible material having a series of cutting guides to facilitate the custom fitting of the flap unit in the door opening. The door also includes a pivoting connection member arranged to connect the flap unit to a horizontal, upper frame member along the first frame unit. The door may also include a panel unit that is gravitationally received in u-channels along one of the frame units to block the flap unit. A method of installing a pet door is also provided.