Varistem® stemming plugs are manufactured through a process called dip moulding. This process, in combination with the material type, makes the Varistem® plugs flexible with a high tear strength. When a blast is detonated, an initial pressure shockwave is released that precedes the burning front of the explosion.
Due to Varistem®’s flexibility and high tear strength, once this initial shockwave energy exerts force on the plug, it is forced upwards into the stemming material, and creates a temporary pressure seal in the blasthole. As a result, more of the shockwave energy/explosive gas travels into the microfractures in the surrounding rock, weakening the rockmass.
The initial shockwave is quickly followed by the burning front of the explosion, which destroys the Varistem® stemming plugs. But as a result of the rockmass being weakened by the improved retention of the initial shockwave energy, more of the blast energy travels into the rockmass (even though the Varistem plugs are destroyed in the process).