This 30 aperture blade consists of 30 symmetric open spaces and 30 symmetric closed spaces. Hence the "open" time is the same as the "closed" time. The blade is made of sheet aluminum with thickness that range from 0.025-0.07". The blade is made with an electrical discharge machining (EDM) process where a laser cuts the blade pattern onto a thin aluminum sheet. The blade is then anodized black. There are three screws at the center that mount it to the chopper shaft coupler. 
 
Technical Specifications:
Blade thickness: 0.025-0.07"
Mount: three screws to chopper motor shaft coupler
Outer blade diameter: 4"
Inner aperture diameter: 3.5"
Inner circle diameter: 1.5"