a drawn lightguide fiber is coated with a layer of a plastic material by moving the fiber through a container of the coating material and a die . the die is made of a relatively flexible material and is mounted so that a lower end extends through an opening of a diaphragm which is adapted to apply substantially coplanar forces to the lower end of the die to reduce the size of an exit orifice . the coating on the fiber is measured and if necessary , as a result of that measurement , the diaphragm is adjusted either manually or automatically to change the size of the die orifice and provide a coated fiber having a predetermined diameter .