Mold for optical thermoplastic high-pressure molding

[54] MOLD FOR OPTICAL THERMOPLASTIC HIGH-PRESSURE MOLDING

[75] Inventor: Steven M. Maus, Osseo, Minn.

[73] Assignee: Galic/Maus Ventures, Columbia Heights, Minn.

[21] Appl. No.: 109,228

[22] Filed: Oct. 16,1987

[51] Int. CIS B29D 11/00

[52] U.S. CI 264/2.5; 264/219;

425/256; 425/542; 425/808

[58] Field of Search 425/256, 808, 542;

264/2.5, 1.3, 219, 225, 338, 337, 106, 297.8;

249/135

[56] References Cited

U.S. PATENT DOCUMENTS

2,292,917 8/1942 Williams 425/407

3,408,429 10/1968 Wichterle 425/808 X

3,632,695 1/1972 Howell 425/808 X

4,017,238 4/1977 Robinson 425/808 X

4,095,772 6/1978 Weber 425/808 X

4,116,753 9/1978 Tojyo et al 249/135 X

4,246,207 1/1981 Spycher 425/808 X

4,364,878 12/1982 Laliberte et al 264/2.2

4,434,113 2/1984 Neefe 264/2.5 X

4,479,910 10/1984 Kurokawa et al 264/2.5

4,573,903 3/1986 Boudet et al 425/808 X

4,664,854 5/1987 Bakalar 264/297.8 X

FOREIGN PATENT DOCUMENTS

61-92452 5/1986 Japan 264/2.5

Primary Examiner—Richard L. Chiesa

Attorney, Agent, or Firm—Merchant, Gould, Smith,

Edell, Welter & Schmidt

[57] ABSTRACT

An improved method and apparatus for high-pressure injection or compression molding of optical thermoplastic parts such as lenses or optical disks, wherein the optical surfaces of a molded part are formed by intimate contact with suitably contoured or profiled and lapped surfaces of one or more optical mold insert elements. The optical surface of such mold element is of a different composition metallurgically than is the bulk composition of the rest of the mold element, thus allowing for optimum selection for materials-of-construction parameters (such as heat transfer, load-bearing quality, and ease of fabrication) in the bulk properties of the substrate metal, without necessarily trading off against optimum optical surface characteristics (such as imperviousness to chemical oxidation or attack, resistance to scratching and abrasion, ease of generating optical surface therein). Thus, a one-piece construction (not mechanically joined from separate elements) of a seamless nature, generally consists of at least one layer of relatively thick electroplating (nickel or chromium) onto a beryllium-copper alloy substrate of certain specified mechanical and thermal characteristics. Heat transfer is further improved by means of machined flow channels provided for circulating liquid coolants. A most preferred mold element construction consists of, first, a machined beryllium-copper substrate onto which a thick watts nickel plating was deposited, followed by abrasive lapping to create the specified surface contour to a high level of microstructure perfection and smoothness, onto which a final thin hardcasting of either vacuum-deposited TiN or flash plate of chromium is deposited.

14 Claims, 3 Drawing Sheets

MOLD FOR OPTICAL THERMOPLASTIC
HIGH-PRESSURE MOLDING

BACKGROUND OF THE INVENTION 5

1. Field of the Invention

The present invention is directed towards a method and apparatus for producing better molds for making precisely-contoured thermoplastic products of ectremely consistent surface profile and smoothness, in general. In particular, optical plastic products, such as vision-corrective lenses (implant interocular, contact lens, prescription spectacle lens, reading glasses, magnifier lens), and information storage optical disks (digital audio compact disks, video disks, interactive video disks, CD/ROM, and write-once or erasble computer memory storage disks), as well as any number of photographic and instrument lenses requiring precise magnification or demagnification are specifically included in 2Q the field of the present invention.

2. Description of the Related Technology Optical plastic products are typically formed in one

or more of the following ways:

(1) Low-pressure molding or casting processes in 25 which typically a low-viscosity prepolymer or monomer mixture is polymerized while contained within a mold cavity;

(2) High-pressure molding or casting processes wherein the incoming plastic polymer is already of 30 suitably high molecular weight and essentially complete structure and is heated to sufficient temperature to provide a flowable, viscous mass which can be formed into its ultimate configuration while being held under high pressures within-a mold cavity, and cooled to an appro- 35 priately low temperature for solidification therein;

(3) Direct machining processes from a preformed shape or plastic stock material.

For either low-pressure casting or high-pressure molding processes, some of the requirements for the 40 mold elements which form the optical surfaces of the final plastic product are the same; such part-forming optical mold elements must, of course, have precisely the desired contour or profile (including a shrinkage compensation factor therein) and an extremely smooth, 45 highly polished microsurface (free of flaws and discontinuities). In addition to these optical quality requirements of the part-forming mold surfaces, which are requirements common to both low-pressure and highpressure molding processes, there are additional special 50 requirements for high-pressure thermoplastic molding:

(1) Much greater need for rigidity and dimensional stability because of the high pressures exerted within the mold cavity which quite commonly reach 5-10,000 PSI, and fast injection fill times which may take only 55 0.1-0.2 seconds, thus creating a Shockwave in the mold cavity.

(2) Desirably high rates of heat transfer because inherent to high-pressure thermoplastic molding processes is the need in each molding cycle to remove large 60 amounts of heat from the plastic as it goes from a temperature well above its melting point to a dimensionally stable temperature below its solidification point or glass-transition temperature point. Therefore, the partforming mold elements commonly contain coolant-flow 65 passages wherein suitable heat-transfer fluid is continuously circulated to assist in this function of rapid cooling.

2

(3) Desirably impervious mold surface resisting corrosion and scratching.

In addition, repairability and service life are considerations in the selection requirements for suitable partforming optical mold elements.

Thus, some materials of construction and types of construction well suited for low-pressure molding and casting optical processes are unsuitable for high-pressure optical molding processes. For example, glass and certain ceramics are widely used in optical die construction for cast plastic lenses and sheets formed from crosslinking reactions (polymerizing the plastic while contained in such mold cavities via heat-initiated or radiation-initiated polymerization reactions). Glass and related ceramics are readily fabricated into almost any optical shape and surface of interest by very well known and established, relatively low-cost means. However, these glassy materials are not useful in highpressure optical molding processes because they are heat insulators and relatively soft-surfaced, easily damaged materials of unsuitable mechanical properties.

Another class of optical mold materials widely used for low-pressure molding and casting processes include the electro-formed metals (usually nickel), which is produced from well-known die-replication processes off of a mirror image master or model piece. The heattransfer rate of such electro-formed nickel mold surfaces is, of course, quite superior to glass, but these electro-formed mold elements are economically limited to relatively thin sections (30-150 mils) and, thus, are unsuited for high-pressure molding processes unless somehow assembled into a composite with a highstrength metal-backing material which usually also contains the coolant-flow channels. Combining such an optically acceptable part-forming surface element with such a reinforcing and cooling backing element is far from being a trivial challenge. First is the need to precisely mate the adjoining surfaces, to prevent stress concentrations and to provide a good interface for optimum heat transfer across that boundary. Also, since the nickel surface is of only moderate hardness and, thus, susceptible to scratches or other surface damage rendering the mold optically unsuitable without repair, some sort of provision for removing this surface element at a later time is required. The easier the replacement of such surface elements becomes, however, the more often compromised is the stability of the resulting composite assembly or its desirable heat-transfer rates.

For these reasons, high-pressure optical thermoplastic molding processes have, most commonly, employed monolithic metal (varying grades of tool steels, in particular) for materials of construction for the part-forming mold elements. These mold elements are fabricated in all the usual ways well known to the metalworking industry, including metal removal via milling, lathe turning, fly cutting, or spark erosion by electrical discharge. Once the nominal dimensions, shape or contour of the fabricated steel mold element have been attained, then the part-forming surfaces are abrasively lapped by successively finer abrasives in a manner well known to those skilled in the art until these contoured surfaces reach satisfactory degrees of smoothness and polish.

An anomaly in this mold fabrication process is that in the first stages of machining processes directed towards achieving nominal dimensions and contour, a relativelysoft, easily-worked material is desired (or required in some cases, such as single-point diamond turning—most tool steels cannot be fabricated in such a way, this fabri