How Tempered Glass Screen Protectors for smartphones are made.

I’d like to show how Joly Joy tempered glasses are made.

There are two major ways to create tempered glass.

"Heat-treated glass" is made of normal glass heated to extreme temperature and than cooled rapidly.

Alternatively, the glass can be treated chemically. In this case normal glass submerged in a salt solution and heated for hours in an oven, but temperature in the oven is lower than in the first case.

This last process is applied in manufacturing of Joly Joy tempered glasses.

What we call tempered glass screen protector actually has four layers:

  • Oleophobic Coating - protect against fingerprints, smudges, oils and over lubricants.
  • Tempered glass
  • Silicon layer. In case of damage of the tempered glass it keeps small pieces from falling out
  • Glue layer for assembly on the phone
  • (Additional layer) Protective membrane to protect the glue from dust before application.


Here is the step by step process how this layers come together.

1.   Incoming Material

Factory purchases the glass from subcontractors from Japan in big sheets.

Each sheet inspected for defects before cutting.


2.   Board Cutting

Before undergoing the toughening process the glass parts must be cut to size. Any additional machining must be completed before the glass is toughened, as it would shatter if it were cut in its toughened state.

Each big sheet cut in to rectangular blanks.



3.   Shaping to the form of actual phone screen

CNC (Computer Numerical Control Engraving) Machines used to shape the blank glass to the sizes as close as possible to the actual phone models.

2.5D edge curve is also formed here.


4.   Polishing

Already cut pieces of glass polished on both sides to remove any scratch on the surface.


5.   Cleaning

The glass is washed to remove any grains of glass deposited during polishing, as well as any dirt that may interfere with the tempering process.


6.   Tempering

Our manufacture uses chemical process to strengthen the glass. A piece of glass is drowned into an oven containing a potassium salt (KCl) at ~752°F / 400°C for 6-7 hors.

The heated glass is then subjected to rapid cooling, causing the outer surfaces of the glass to cool and contract faster than the center. Center then cools, it forces the surfaces and edges into compression, giving the tempered glass its strength.

This stage is the most crucial for the scratch resistance of the future tempered glass. So named 8H or 9H strength can be reached only if the heating lasts for at least 6-8 hours.

Some manufactures are trying to save time and cut this process to 3-4 hours. As a result the glass does not reach required toughness. At the end of the post you can see the difference between the two.


7.   Cleaning

Glasses are cleaned again to remove any residuals of the potassium salt.

8.   Laminating

On this stage the glass covered with oleophobic coating on one side and silicon layer on the other.

9.   Die Cutting

Cutting a peace of membrane of exactly the same shape as the glass with layer of Japanese glue on it.


10.   Plying-up

On this stage all semi-finished parts are aligned together and heated up under pressure. As a result AB epoxy glue covers equally silicon layer, the glass, and a membrane, so in the future the tempered glass can be applied to the phone smoothly and without any bobbles.


11.   QC

Last step is quality control



The difference between correctly tempered glass and low cost tempered glass.

Because of the high amount of energy stored in residual stresses, tempered glass shatters into a large number of very small pieces when broken. 

The broken pieces are not as sharp and hazardous as those from ordinary glass.

Low cost one breaks almost as a normal untempered glass.

On the next picture a correctly tempered glass in on the left, and low cost one on the right.

Little more techy stuff about tempered glasses

In the production of flat glass the molten silica-based mix is cooled slowly under carefully controlled conditions. This annealing procedure removes undesirable stresses from the glass. Cooling occurs in an annealing; hence, the glass is termed "annealed" or "ordinary" glass. Annealed glass which has been heated to a temperature closer to its softening point (~1,200°F / 650°C), and forced to cool rapidly under carefully controlled conditions is described as "heat-treated glass."

This leaves the center glass thickness relatively hot compared to the surfaces. As the center thickness then cools, it forces the surfaces and edges into compression. Wind pressure, missile impact, thermal stresses or other applied loads must first overcome this compression before there is any possibility of fracture. So this type of tempered glasses usually used for side and rear windows in vehicles, entrance doors, racquetball courts, microwave ovens, skylights, and bulletproof windows.

Alternatively, for products such as tempered glass screen protector, the glass can be chemically strengthened by a surface finishing process. A piece of glass is drowned into a bath containing a potassium salt (KCl) at ~752°F / 400°C, which causes sodium ions in the glass surface to be replaced by potassium ions. As a results from chemically treated glass has increased toughness compared with thermal toughening, and can be applied to glass objects of complex shapes.