These mold rubber strips from Castaldo were made to provide no shrinkage when casting.  The strips come in a five pound box, with each strip measuring 2 7/8 inches by 18 inches.  The strips must be vulcanized at 310 degrees Fahrenheit for 7.5 minutes per layer.   These strips will give you successful molds and casts every time.  Order today at Stuller.com/tools.