Any wristwatch has three major components, namely,
Watch case is a subassembly with glass, back cover and case pipe (to guide stem meant for time adjustment using crown). This subassembly is carried out at the in-house watch case plant that has five major shops–
In watch case assembly, case center, back cover (BC), case pipe (CP), and glass are assembled on the watch case blank (made of metal) and later tested for joint-strength and water resistance (WR). The WR test is a mandatory statutory requirement for watches and is done through a water immersion method where watch cases are soaked in the water for 10 minutes with the pressure of 5 Kg/cm2 or 7 Kg/cm2, based on specification.
To attain this WR property, all the joints mentioned above should have specified strength i.e. glass joint to withstand a minimum of 4 Kg/ cm2, BC joint to withstand a minimum of 5 Kg/ cm2 and CP to withstand minimum pressure of 3 Kg/ cm2. This strength will ensure the mating parts are intact when the watch is worn while riding a two-wheeler, running, jumping, etc., as well as resisting entry of water/moisture during testing/use because of outside pressure being higher.
As per horology standard and know-how, a glass joint is made by either of the following methods:
Each of these methods will have a certain advantage for each kind of design or configuration and quality. Based on the design of the watch case, the type of joint mechanism to be employed is chosen as follows:
- Conditions for choosing Gasket :
1. A round model with flat glass
2. If the model is shaped, it should have wall/housing for the glass
3. Corner radius should be minimum 2 mm
In the gasket type joint, a minimum gap of 0.35 mm is present. Many customers don’t prefer this gap between components and glass. Also, we can’t use a gasket if the corner is sharp.
- Conditions for choosing Adhesive :
1. If the model case is shaped and there is no housing/wall for the glass
2. Corners are sharp
3. Glass is having a gradient
With adhesive type joint, moderate WR is achieved. Also, the failure of watch cases is more prominent in this type.
- Conditions for choosing Sticker : Stickers are Pressure-sensitive adhesives that need a minimum 1.5 mm metallization band.
Thus, we see that each design demands a different method of glass joint/assembly with the desired quality. This puts pressure on the production system to maintain the tolerance, and on the designer to forgo his design ability because of this constraint. Also, the above joint methods have been in practice over decades. Hence, the challenge is to ideate and implement an alternate innovative method of joints between glass and metallic parts, which is universal for all designs.
This will result in the flowing: