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Molding, Etc.

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Posted by Robert May on September 28, 2004 18:13:21 UTC

Molding (moulding, casting and so forth) glass generally doesn't give a surface that is anywhere near accurate enough for using as even a lens surface. Plastics do a lot better and the molding process can be used for low quality lenses if so desired. Also, glass will adsorb the mold material if the right stuff isn't used and will change the chemistry at the surface of the glass.
The general accuracy for mirrors of good quality is 1/20th of a wave accurate surface to the desired surface shape. You can obtain cast mirrors on a special mold that are basically 1/4 wave accurate but they do suffer from problems of various kinds. The plastic surface is also a bit pressure sensitive.
Thus the grinding process is the best way to get the surface shape desired. The roughing, refining and polishing of the surface to the desired shape ends up being the cheapest way to get the glass to the desired shape. The final step of figuring is generally the most difficult as the shape is not that of the spherical surface which can be stably polished into the surface but rather some other shape which will be a dynamic shape process.

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