I understand that Hardlex is used in the Marinemaster due to its shatter resistance. My question is why doesn't Seiko use an acrylic crystal in this case? Acrylic crystals are shatter resistant and unlike Hardlex can be buffed if scratched. Could a Sapphlex crystal be used in this application?
The materials currently used as watch glass and its main characteristics are shown in the following table.
Since the acrylic resin (1) has soft hardness and a heat-resistant temperature is very low, concerning the glass of PROSPEX watches which have high durability performance, such as a MARINEMASTER of this question, we couldn't use acrylic glass in which a lot of scratches and deformation are occurred on the surface.
Although (2) and (3) glass are used by SEIKO with the trademark of "HARDLEX" (glass which raised mechanical intensity by special chemical strengthening processing), they are used for standard watches.
On the other hand, sapphire glass (4) is used abundantly for the dress watches and high-class water-resistant watches and also diver's watches, such as CREDOR and Grand SEIKO and also some PROSPEX watches, such as LANDMASTER watches, SCUBAMASTER diver's watches of SEIKO brand, and hardness and theoretical intensity are excellent, there is also a problem of being unable to make as theoretical strength by the mechanism in destruction of the glass with which a cut angle control is important and a micro crack being detected in microscopic examination in case of large and thick glass.
In SEIKO watches, in addition to the above-mentioned general materials, "High- quality HARDLEX" (the high intensity and highly efficient glass strengthened on (3) Borosilicate glass) and original "SAPPHLEX"(the high cost performance combined glass which adhered thin plate of (4) on (2) or (3) glass) are put in practical use, and those are used according to the aim and specification of the watches.
"High- quality HARDLEX" which is the high intensity and the highly efficient article of the glass are used for the PROSPEX watches and ordinary sports watches. Also "SAPPHLEX" glass is used for the sports watches or the sporty watches,
In case of diver's watch which are required so many characteristics totally, such as resistance to pressure, water resistance, shock resistance, wear resistance, heat resistance, and durability etc., we usually use "High- quality HARDLEX" glass having the most excellent cost performance and sometimes use sapphire glass as a glass of some PROSPEX diver's watches.