Method for forming multi-layer laminates

A method for the formation of a multi-layer laminate of thermoplastic materials, which comprises:

• (1) introducing a melt of each thermoplastic material into a separate feed channel;
• (2) passing the stream of material issuing from each feed channel to a back pressure cavity wherein transverse flow occurs under conditions of heat and pressure to stress-relief the molten thermoplastic material and release strain in said material and provide equal pressure distribution;
• (3) directly passing each stream of stresss-relieved thermoplastic material issuing from a back pressure cavity into an elongated flow restriction channel wherein shear stress is induced, said material exerting equal pressure along the length of the elongated flow restriction channel;
• (4) said flow restriction channel establishing a delta pressure on said thermoplastic material which is related to the viscosity of the thermoplastic material and further is related to the shear stress in accordance with the following equation:
• ##EQU3## where .DELTA.P is the difference between the pressure entering the flow restriction channel and the pressure exiting the flow restriction channel, h is the width of said flow restriction channel, and L is the length of the flow restriction channel;
• and
• (5) converging the streams issuing from the flow restriction channels into a melt-laminate at the exit end of each of said flow restriction channels;
• whereby at the point of convergence, the viscosities and velocities of the converging streams of thermoplastic materials are substantially equal;
• said back pressure cavities being of greater cross-section than said feed channels and being of greater cross-section than said flow restriction channels.