Normally aluminium plates have two functions: 

• Separate the wheels from each other to prevent adhesion when curing in the oven

• Keep the shape of the grinding wheels by a spring force from top of wheel stack.

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Simply speaking, for depressed grinding wheel, one wheel need one aluminium.

and for cut-off wheels, according to users' processing technology, every 5-10 pcs wheels need one aluminium plate and some separating net.(Teflon or fiberglass net)