Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Reexamination Certificate
1999-08-05
2003-11-11
Ortiz, Angela (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
C264S328900, C264S328110, C425S562000, C425S577000
Reexamination Certificate
active
06645417
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
While the invention is particularly desirable for products with optical surfaces which are free from objectionable defects such as those caused by gate marks, it can be used generally for the molding of thermosetting and thermoplastic materials.
Optical quality products are in wide use. They are needed generally for assays where test substances are subjected to examination by electromagnetic radiation, including visible light. Optical products are also used for instruments, such as microscopes and ophthalmic devices.
Devices that require optical surfaces originally were prepared by grinding glass members. Such products now increasingly employ plastics to expedite manufacture and reduce cost. In general, the demand for plastic optical products is now considerably greater than for glass.
The shift from glass to plastic has occurred primarily because plastic is lighter and often has superior qualities. In addition, protective coatings to provide scratch and abrasion resistance for plastics have become available. Plastic also comes in a wide range of gradient-density tints and colors.
Of the many advantages exhibited by plastics, their relatively light weight and durability have proved to be significant. For optical surfaces, the lens thicknesses are the same for glass and plastic. Consequently, the reduced density of plastic produces a product that is of lighter weight.
The reduction in weight and density is particularly important when high powered surfaces are required, or when large optical surfaces are needed.
Previously, devices with large optical surfaces, particularly those of high power, were typically manufactured by the casting of thermoset resins, for example acrylics that were peroxide cured. However, the availability of polycarbonates and related thermoplastics permits the replacement of cast thermoset plastics. This is because modern polycarbonates have low densities and high refractive indices. For the same optical thicknesses, polycarbonates have an even lower weight than cast plastics, and far lower than glass.
Additionally, since polycarbonates have great impact strength and breakage resistance, they permit the production of relatively thin optical members. Moreover, coatings for polycarbonates are available to provide abrasion resistance. Polycarbonates are particularly suitable for products with “single” optical surfaces, i.e., those with frontal convex and/or backside concave surfaces.
Optical surfaces are defined by two measures of the ray bending power of light or other waves. Spherical power produces magnification and/or reduction, while cylindrical power produces astigmatic corrections. The units of corrective power are in diopters. It often is desirable to have product available with a spherical power in the range from +4 (magnification) to −6 (reduction) diopters, and a cylindrical power in the range from 0 to +2 diopters. Within this range, a volume-frequency distribution can be plotted, centered at zero power. There is reduced frequency in the plot as spherical or cylindrical power increases or decreases.
To be competetive, injection molded products require high yields, with a reduction in scrap and secondary operations, such as trimming.
Additionally, it is desirable to run optical surfaces of differing powers at the same time, without sacrificing productivity, quality or yields. A four-cavity moldset, for example, quadruples the productivity of a particular molding machine. Two of the cavities can be used to mold common spherical and cylindrical power combinations. The remining cavities can be used for less common surfaces.
An illustrative optical surface is found on an optical disk for the laser reading and storage of information. Optical disks for video respond to analog signaling, while compact digital disks are for audio signals. There also is a wide range of computer program disks for information and data storage. These include the CD/ROM (Compact Disk/Read-Only Memory) which is irreversibly encoded with program information, DRAW (Disk Read And Write, i.e. “user write once”) and EDRAW (Erasible, i.e. “by the user”, Disk Read And Write).
Many disks are encoded during molding by a “stamper”, which forms a face surface in the mold cavity. The digital information is represented on the stamper by a spiral of tiny projections, which, in turn, form indentations in the plastic molded disk. A typical indentation has a depth of 0.1 micron and a length of 1-3.3 microns, with a track pitch of 1.6 microns for a spiral array that extends radially outward.
One requirement of high quality molding is intimate contact of the polymer melt with the stamper, without any voids or premature shrinkage. Contact is maintained from the time the cavity is filled with melt until cooling takes place below the glass-transition temperature of the plastic.
Another requirement for optical products is reduction of internal stresses, i.e. “orientation”, within the polymer. Ideally, the molding should be “isotropic”, i.e., exhibit the same properties in all directions, so that molded stresses and flow induced orientations are eliminated. Such stresses and orientations produce localized differences in ray bending power. The resultant nonuniformities in refractive index are measured in terms of optical path differences, commonly expressed as “birefringence”. Avoidance of birefringence is desired.
With surfaces that employ laser signal reading, any flaw which disrupts or deflects the laser beam causes errors. Other properties which require consideration are percentage of light transmission, percentage haze, and the index of discoloration. Localized flaws include opaque specks or clear areas, such as voids or bubbles, which have different refractive indices and optical bending power than adjacent material. Absolute planarity or flatness are often needed where localized warpage would induce prismatic effects and result in off-axis signal transmissions.
In the molding of many optical surfaces it is necessary to conduct operations in clean or “white” rooms. Such rooms provide particle free environments in the range from Class 1,000 to Class 10. Since workers are the biggest source of contamination, automation of handling and post-molding operations is desirable.
Furthermore, for efficiency, microprocessor or CNC (computer numerical control) control should be used. The molding machines also should have individual moldsets, temperature controllers and hopper dryers. A clean air shower is needed for the clamp open and part removal position, together with robotic part pickers.
While optical molding commonly employs a single cavity mold, that makes inefficient use of clean room floor space, and results in a high captial and equipment fixed cost per part. Consequently, it is desirable to produce optical quality parts with multiple cavity molds.
Despite the advantages that have been achieved in the injection molding of optical quality products, there has remained the disadvantage of the product deformity associated with the need for gating the plastic into the mold. At the point of gating the resultant product invariably includes a blemish that requires removal in order for the product to be generally suitable. Typically the removal of the gate blemish takes place by polishing which requires a significant manual effort and additional cost.
II. The Prior Art
(a) Straight Injection Molding
Early attempts to make acrylic or polycarbonate optical parts used injection molding with the mold cavity surfaces fixed throughout the molding cycle. This required long cycles, high mold surface temperatures approaching the glass-transition temperature of the plastic, along with high plastication and melt temperatures. Slow, controlled fill rates were followed by high packing pressures, which were held until the completion of gate, freeze-off.
Fixed cavity processes employ large gating and runner systems to permit appreciable packing pressure and delivery of material before gate freeze-off occurs. At that time no further transfer of molten p
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