Molly screws are screws with additional pieces of metal or plastic that are designed to expand once the screw is tightened into a piece of wood or drywall. Once installed, a molly screw will expand behind the drywall and spread out the weight on the hanger or screw, preventing the screw from slipping back out through the hole. Molly screws can, however, be removed easily by unscrewing--which removes the molly end inside the wall.
Molly screws are typically used in drywall applications where there is no stud, or piece of wood, behind the installation site. Drywall and Sheetrock is highly susceptible to screw slipping, or having a screw hole increase in size as the screw is tightened down, leading to decreased grip on the drywall and potential hanger failure. If a stud is present behind the wall, a molly screw is not necessary as the wood will give sufficient grip. Molly screws expand behind hollow walls and prevent the screw from pulling back through its hole, increasing the amount of weight that can be hung on drywall or Sheetrock alone.
Molly screws are constructed in such a way that the act of tightening down on the screw causes the anchor to expand behind the wall. This acts like a grappling hook on the other side of the wall, preventing the screw from loosening. These work best in drywall and thin plywood or paneling applications, as the expansion anchor needs room behind the surface of the wall to expand. The screws will not function in cement or thick wood applications. The expansion anchor portion of the molly screw must be longer than the material you are installing it in in order to function correctly.
Molly screws come in a variety of designs, with varying installation methods. There are varieties that are designed to have their anchors tapped into a pre-drilled pilot hole, and then the screw is tightened into the anchor. There are also self-drilling molly screws and anchors that are intended to be drilled in to drywall or Sheetrock using a drill in a similar fashion to a screw. Installation style varies widely based on the application; consult a home improvement store associate for help in selecting a molly screw.
Special Purpose Molly Screws
Special purpose molly screws are made for applications other than drywall and Sheetrock. A toggle bolt molly has a large set of fins that only unfold in one direction. This allows the wings to be folded against the screw and slipped through a pre-drilled hole. However, when the screw is slid back out through the hole, the wings catch the inner wall of the drywall and unfold out, often an inch or more. Toggle bolts can be used in wood applications as well as drywall, and are usually rated for several times the weight limit of other molly screws.
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