The present invention relates to the molding of non-planar optical grade mirrors from plastic stock and particularly to the production of convex mirrors.
It is the current practice to utilize injection molding processes to form plastic parts which are metallized to provide mirrors such as convex mirrors used in various optical applications. However, such processes are generally high cost especially with the need for production mold tools and suffer from incomplete adhesion between the plastic of the parts and the metallization, and dimensional distortion and blemishes in the final product.
It is an object of the present invention to provide a low cost alternative process to injection molding which also provides dimensionally more accurate parts. It is also an object of the process to permit the production of fewer parts at a reasonable cost per part without the initial huge start-up cost of creating an injection molding tool.
It is a further object of the present invention to provide a process of thermo molding of mirror parts with less or minimal distortion and blemishes and better adhesion between the plastic and the metallization to provide a higher quality product with better durability in the field.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.
a and 2b depict the formed mirror product as produced and processed for use respectively.
Generally the present invention comprises a manufacturing process for producing arbitrary pre-determined three-dimensional shapes from plastic sheets, while preserving the integrity of the sheets. The finished products can have optical integrity which makes them suitable for mirrors, reflectors, signs, camera domes, and other optically sensitive applications. The shapes may be pre-selected by using three dimensional CAD software, and they can be manufactured using this process. Furthermore, since the process is based on thermally molding parts starting from an initially flat sheet, the tooling is much lower cost than would be the case if a person tried to injection mold a similar part. The overall steps in this process generally include:
1. Construction/design of molding tooling.
2. Selection of raw materials for the plastic stock
3. Heating, shaping, and cooling the part
Finishing the Part
More specifically the process of forming an optically accurate non-planar element from a plastic sheet, comprises the steps of:
4. selecting a predefined three dimensional shape to be formed;
5. machining a block of a microporous material, such as by entering the selected shape into CAD/CAM or similar software and using the software to drive a machining tool to machine the block, to provide the selected three dimensional shape on one surface thereof;
6. placing a sheet of plastic over the machined three dimensional shape and distanced therefrom;
7. heating the sheet of plastic and applying a vacuum through the microporous material to draw the heated plastic sheet to conform to the shape in the block; and for optical parts such as a mirror;
8. metallizing a surface of the conformed shape plastic to a form an optical mirror surface.
Tooling Construction and Design:
The molding tool used in the process of the present invention comprises a machined block of a microporous material, preferably aluminum, such as MetaPor, a microporous solid aluminum material supplied by the Edward D. Segen & Company. Other microporous materials made from other metals or non-metals such as ceramics may also be used, but microporous aluminum is preferred for its advantages of high thermal conductivity and its property of being polishable to a very smooth surface finish. The tool is machined (preferably by using three-dimensional CAD/CAM software or any suitable machining method) to produce any three dimensional shape desired, with the exception of undercuts. After machining, the block is preferably polished to a high polish using progressively finer and finer grit sand paper, or by using progressively finer abrasive non-wovens.
In a preferred embodiment, the block is fitted with elements for precise uniform temperature control such as copper water tubes (element 14 in
Raw Materials:
Optical quality sheet stock is used to achieve a finished part with optical quality sufficient for a mirror, preferably in thicknesses from 1 mm to 6 mm. Both Plaskolite and Degussa make such “mirror grade” sheet at this time from extruded acrylic, although the invention is not limited to materials from these two vendors, and is not limited to this thickness range. Mirror quality parts thermo-molded may also be made with polycarbonate sheet, with PETG sheet and other similar materials. It is understood that the present process will not eliminate original defects put there by the original sheet manufacturing process, and it is preferred to initially start with a high quality.
There is a preference for the sheets to initially have poly masking (2-3 mil thick) on both sides (with no residue leaving adhesives) during heating. The poly mask also protects the surface integrity of the sheet during shipping and handling. Furthermore, during the forming process the poly mask on the side of the sheet is left to contact the mold if possible. With acrylics, this works extremely well, and any tiny mold mark-off is absorbed in the poly mask. When the poly mask is eventually stripped off and disposed of, the mold mark-off disappears with it leaving a pristine mirror surface. With polycarbonate this is not as preferred if the required forming temperature might be too high for the poly mask. Accordingly, with higher temperature heated materials such as polycarbonate, the masking is preferably stripped from both sides of the sheet prior to forming.
Heating, Shaping, Cooling the Part:
As shown in
After heating, the part is shaped by pulling vacuum through the porous metal, sucking the part against the mold for final dimensions as well as for cooling. As opposed to a porous structure or one with a minimal number of holes, microporous materials have millions of tiny vacuum holes, with the amount of air being extracted being roughly equal over the entire surface of the mold. Thus, conventional thermoforming molds made with solid aluminum are commonly fabricated by drilling numerous little holes (typically smaller than 1.5 mm and typically larger than 0.1 mm) in solid aluminum molds. Drilled vacuum holes however produce small optical defects at the location of the drilled hole. These defects may be due to the air movement in the vicinity of the vacuum hole in addition to the fact that the plastic must literally bridge the hole. In mirrors and other optically sensitive parts, this produces unsightly defects in the finished product. The porous metal tool used in thermo-molding of the present invention produces no such defects since there are literally millions of microscopic holes (with each having, on average, a diameter smaller than 0.02 mm) over the entire surface of the mold.
With respect to temperature control of the mold (formed shape 13), the mold should be hot enough to permit the part to accurately conform to the mold dimensionally, but not too hot. With acrylics, the best results are obtained with a mold temperature greater than 120° F. but lower than 200° F. The mold temperature is controlled by passing controlled temperature water through the mold's water tubes (element 14 in
Finishing the Part:
After demolding, if manufacturing a mirror, the part is vacuum metallized to become reflective with the adhesion between the deposited metal and the plastic being excellent. This results from the surface being metallized being protected with a poly mask until just prior to thermo-molding, and even then never touching anything but hot or cold air until the metal is deposited. The metallized side is accordingly pristine as well as the side that contacted the mold, resulting in a beautiful finished mirror or other part. After metallization the part is typically back-coated to protect the metallization, and trimmed (
Though the Figures depict an apparatus for making a single part, the vacuum box can be constructed to be large enough to define in the mold block several molding cavities, for the simultaneous fabrication of multiple parts, for example, convex mirrors, in groups of 4, 6, 8, 10, etc. If desired, separate mold blocks can be provided in the vacuum box for creating each mold cavity, to attain reduced costs, easier servicing, repair and/or for better individualized control and regulation of the fabrication of each part in the same vacuum box. Multi-cavity molds have higher capital cost, but reduce labor content and thus reduce unit costs in mass-production applications.
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.
This application is based on and claims priority to U.S. Provisional Patent Application Ser. No. 60/862,092, filed on Oct. 19, 2006 and entitled PROCESS FOR THERMO-MOLDING CONVEX MIRRORS, the entire contents of which are hereby incorporated by reference herein.
Number | Date | Country | |
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60862092 | Oct 2006 | US |