Contact Lens and AnteriorEye, Vol. 20, No. 2, pp. 49--55, 1997 © 1997 British Contact Lens Association Printed in Great Britain ANALYSIS OF THE CRIMPING METHOD OF PRODUCING TORIC SURFACES IN CONTACT LENS BLANKS USING FINITE ELEMENT AND EXPERIMENTAL METHODS Girma J. Orssengo*, David C Pyet and Arthur Ho~ (Received 28th October 1996; in revisedform 9th January 1997) Abstract ~ One of the most common methods of producing a toric surface in a contact lens blank is crimping. This method has the advantages of being simple and cheap, but the toric surface dimensions obtained, especially at higher cylinders, can be less accurate. The relationships between the crimping parameters used to produce the toric surface and the dimensions of the toric surface obtained are not well known. If these relationships are known, improved toric surface dimensions may be obtained. In this paper, finite element analysis and experimental results obtained for these relationships are presented and discussed. KEYWORDS: contact lens, manufacturing, toric lens, crimping, finite element model Introduction obtain optimum fit with the cornea or to correct l the refractive errors of the eye, contact lenses that have toric back surfaces are used. 1,2 One of the most common methods of producing toric surfaces is crimping),4 Figure 1 shows a schematic diagram of a crimping tool, which consists of an adjusting nut, a crimper body, two push pins, and two blank stops (the second pin and blank stop are not shown). The two pins lie in the yz plane and are located opposite to each other about the z axis. The two blank stops lie in the xz plane and are located opposite to each other about the z axis. In Figure 1, when the adjusting nut is tightened, the pins are displaced axially, and this pushes the lens blank out of the crimper body in the yz plane. However, in the xz plane, the lens blank is prevented from moving Crimoer Parameters: w~ yp = Pin position from z-axis Push pln ~ . = Blank stop,,,,idth,2 mm V .~ /ILIA b = Blank stop depth, O. 4 mm / ~ / ~ Crimper / ",.(//y//~// ~r/-///~..."-~/ body X/I~ YZ~J ~ ~x/ Adjusting ~ished ~ ~'~'-Blankstop b lank Blankstop - ~ ~-- b Figure 1. A schematic diagram of a crimping tool. * BTechnol, MASc, Grad. Dip. in Comp. Sci.; Research Fellow, School of Optometry, UNSW. t BOptom, MOptom; Director of Clinics and Senior Lecturer, School of Optometry, UNSW. BOptom, MOptom, PhD; Senior Project Scientist, Cornea and Contact Lens Research Unit, UNSW. out by the blank stops. Thus, the pin displacement w changes the spherical surface, of initial back optic zone radius ro, of the lens blank into a toric surface. This toric surface of the crimped blank is then cut into a spherical surface, which becomes a toric surface when the lens blank is released from the crimper. The Crimping Process To analyse the crimping process, the crimping parameters that affect the dimensions of the toric sur- face obtained must be identified, and are considered next. For the modelling of the crimping process, the geom- etry and material property variables that define a lens blank are given in Figure 2. From experimental results (Figure 3 and Table 1) the relationship between the back optic zone radii of a crimped blank and the pin displacement were found to be linear. Therefore, the crimping process may be rep- resented as shown in Figures 4 and 5. Lens blank geometry variables: dl= flange diameter d2 = lens diameter ro = initial back optic zone radius I tl = blank centre thickness h = blank flange thickness dl ~ .... d2 Lens material properties: E = modulus of elasticity , v = Poisson's ratio I __] Lens For polymethyl methacrylate 5,6 blank E = 3.3 GPa and v = 0.35 Figure 2. Geometry and material property variables that define a lens blank. 49