Universal Glass Joint For Watch Cases

Challenge Details

Any wristwatch has three major components, namely,

1. Watch case, which is housing the mechanism,
2. The mechanism or module with dials & hands,
3. The strap to wear the watch on hand.

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–

1. Press shop
2. Machine shop
3. Polishing shop
4. Plating shop
5. Watch case assembly.

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:

1. Plastic gasket
2. Adhesive
3. Sticker
4. Ultrasound welding in case of acrylic/plastic.

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:

1. Productivity increase
2. Reliability ensured
3. Design support
4. No conditions imposed on watch designers and watch case assembly units.

Options Tried:

• Sticker type joint was tried where the pressure-sensitive adhesive is used. It demands metallization (metallic coating of the plastic component) band-width of 1.5mm.
• Hot melt glue, which forms a sticker type joint when it is heated at 120 ^oC for 1 hour, was also tried. However, such high temperatures have an adverse effect on plating quality.

Solution Requirements:

• The alternate method of glass assembly should be universal for all designs and meet all desired requirements.  
• It should not pose any restriction or constraint on the design or shape of the watch case.
• It should provide high WR (up to 25 ATM for diving purposes) even in asymmetric shapes.
• Any shock load shouldn’t induce hairline cracks or other failures in WR.
• The new joint type should provide higher quality at optimal cost with ease of production for the manufacturer.
• Also, this alternate method should prevail throughout the world, i.e., it should have no geographical constraints.
• The customer should be able to use and operate all types of watches, without knowing the preconditions of any watch type.