Laser welding thermoplastics

As the friction welding the weld is at the heart of rooms and not the periphery.

In practice the laser beam passes through the transparent part and enters the darkened room where it is absorbed.   There is therefore a transfer of energy that goes into some part transparent conductive. Merging the interface between the two parts creates a point of molten polymer which allows the fusion of strings.
With a laser source, energy intake is controlled at all times, including by focusing the laser at the heart of play. This can be perfectly consistent with welds holding force superior to those obtained by other methods.

If you had two dark rooms you would weld periphery and not in heart.