The metering variations are obtained by changing the speed of the metering belt according to the Set value. The gravimetric preparation of the glue mixture is effected with 5 components suitably regulated. For each component one appropriate pump, driven by a frequency converter, provide to blow in the system a continuous and regulated flow. The flow meters (mass for the glue and volume for the other components) will measured and control the precise dosing of the components in the system. The correct mix of the five components is obtained by using appropriate recipes, that are insert in the PLC, in a simultaneous mode for the surface and core layer.