The present invention provides for an improved device for molding keyhole sipes larger in size, i.e. sipes having a wider passage, than currently being formed in treads that will avoid damaging the tread during formation and removal thereof from a mold. The device includes an elongated deformable body member having a blade extending in a direction away therefrom and substantially along the length thereof. The body member also includes a stiffening member securely fixed in position therein that extends substantially the length thereof. The stiffening member provides a desired rigidity to the deformable body member, thereby limiting flexion along its length under the stress of a rubber formulation when the mold is closed during the molding process. Also, the deformable body member provides low stress on the tread when a slit of the keyhole sipe is pulled around the body member during removal of the tread from a mold.