The damage or failure of the plastic casing can generally be attributed to stress loading. Excess stress loading is caused when the two screws that attach the magnet to the window sill or shutter frame are over tightened. This type of damage is called ductile cracking.
Ductile cracking is a phenomenon in which the plastic material undergoes plastic deformation before breakage. With a ductile fracture, the crack grows slowly on a microscopic level. The addition of environmental stress such as high heat accelerate the propagation of the fracture resulting in total failure of the plastic casing.
To reduce the possibility of this damage it is suggested that you use a simple hand held screw driver during installation and to tighten the included pan head screws just tight enough so the magnet does not move when opening and closing your shutter panels.
Although doing a speedy installation with the use of a screw gun is tempting when installing your magnets, it is not recommended do to the likeliness that you can over tighten the screws.