A panel comprising at least one layer of thermoformable plastics material
having separate regions of different thickness, is obtained by a method
comprising the steps of: extruding of a sheet of plastics material with
the addition to the said material of an expansion agent, the extrusion
being conducted in conditions such as to avoid expansion of the material
or to cause only partial expansion thereof; heating the thus-obtained
sheet to a temperature such as to cause post expansion of the material;
and thermoforming the sheet in a thermoforming cavity of complementary
shape to the desired panel so that the thermoformed panel produced has in
the regions of greater thickness an expanded cellular structure where the
material has a lower density than the density of the material in the
regions of lesser thickness.
