Method for producing optical disc substrates

Abstract

Provided is a method for producing optical disc substrates having a low degree of birefringence. The method has good mass-producibility, in which the pattern transferability on the substrates produced is good. In producing optical disc substrates having a diameter of from 80 to 120 mm and a thickness of from 0.5 to 0.7 mm, through injection molding or injection-compression molding, a resin for the substrates is injected and charged into the cavity of a mold at a resin filling rate of not lower than 65 cm3 / sec, said resin filling rate being obtained by dividing the cavity volume (cm3) of the mold, into which the resin is charged, by the time (sec) taken from the start of resin injection through the tip of the nozzle of the injection-molding machine to the arrival of the resin at the deepest end of the cavity of the mold.

Description

FIELD OF THE INVENTIONThe present invention relates to a method for producing optical disc substrates.BACKGROUND OF THE INVENTIONOptical discs which have heretofore been developed include read-only discs such as compact discs (CD), laser discs (LD), etc.; rewritable discs such as magneto-optical discs (MO discs), etc.; and write once discs such as recordable CD (CD-R), etc. FIG. 3 is a perspective view schematically showing one example of such conventional optical discs. As in FIG. 3, in general, an optical disc comprises a substrate with guide grooves (or pits) 1, 2, each having a track pitch P of a predetermined size and a predetermined depth D. For reading or rewriting the information recorded in the optical disc, a laser ray having been focused through a lens to have a wavelength of about 800 nm is irradiated to the pits or to the recorded information on the guide grooves. The information capacity of conventional optical discs is about 640 MB / disc.Recent developments in the mult...

AI Technical Summary

Benefits of technology

The present invention has been made in consideration of the problems noted above, and its object is to provide a method for producing optical disc substrates having a lowered degree of birefringence, in which the pattern transferability onto the substrates produced is high and the mass-producibility of the substrates is also high.

Problems solved by technology

However, conventional CD and MO discs could not meet the requirements in the market, as their memory capacity is insufficient.

However, the 2P method for producing optical disc substrates is problematic in that the mass-producibility therein is lower than that in an injection-molding method for producing substrates, and that the production costs for it are high.

The problem of the reduction in the pattern transferability due to the formation of the cooled and solidified layer in the injection-molding method noted above is seen also in injection-compression molding for producing high-density optical disc substrates.

However, the cooled and solidified layer formed in the method brings about the problem of retarding the pattern transferability and increasing the birefringence of the substrate formed, whereby the quality of the substrate is lowered.

Where high-density optical disc substrates having finer pits (or guide grooves) than those of conventional ones are produced in the injection-molding method, the problems to be caused by the formation of the cooled and solidified layer will be more serious.

Specifically, with the increase in the density of the pattern for pits (or guide grooves) formed on the surface of the stamper to be used, the resin could hardly enter the depth of the hollows of the fine structure of the pattern thereby often causing transfer failure.

For example, even when substrates for DVD-RAM, which are for recording on both the inside of the guide grooves (hollows) and the top between the adjacent guide grooves (hills) through phase conversion, are intended to be formed under the conventional injection-molding conditions, the resin could hardly enter the space to be between the adjacent guide grooves and therefore good hills could not be formed on the substrates.

Even if the substrates with such no good fills thereon are used to produce optical discs, good recording and reproduction on the discs produced is impossible and the discs are failed products.

However, for the method, the substrate material is limited to only a specific molding resin.

Moreover, still another problem with the method is that the cooling time in the method must be long and the molding cycle is prolonged.

However, there is found no literature that suggests the relationship between the pattern transferability and the resin filling rate in injection-molding for producing high-density optical disc substrates, such as DVD substrates which are more small-sized and thinner than conventional optical disc substrates.

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