Method for making a video disc
AssigneeDiscovision Associates
InventorJohn R. Holmes
Life of this patent
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Abstract
A video disc member is described having a uniform value of birefringence over the entire information storage surface of the disc. This value of birefringence is the same at the inner information storage radius as at the outer information storage radius. The preferable composition of the video disc is a specially prepared polymethylmethacrylate, referred to as PMMA. The method of molding the improved video disc member includes an extended drying time for said composition and increasing the melting temperature. The increase in melt temperature provides a low and uniform value of birefringence. The improved drying cycle eliminates splay marks, blisters and other surface imperfections as compared to manufacturer recommended drying times.
Claims13
A method for forming an optically readable information bearing structure, comprising the steps of:
- processing a clear polymethylmethacrylate plastic to a melt flow rate of about 20 to about 23 grams per 10 minutes;
- heating the plastic to a temperature falling within the range of 510.degree. F. to 560.degree. F.;
- and
- using said heated plastic in an injection molding cycle.
A process for making an optically readable information bearing member of substantially low and uniform birefringence, comprising the steps of:
- (i) processing a clear polymethylmethacrylate plastic to a melt flow rate of about 20 to about 23 grams per 10 minutes;
- (ii) thoroughly drying the plastic;
- (iii) putting the dried plastic into an injection molding machine wherein the temperature of the melt is maintained between about 510.degree. F. and about 560.degree. F.
Dependencies are inferred from the claim text.