Line Width

Is there a way of increasing the line width above 2.11mm.
I have previously used autocad for my printed circuit boards with the polylines for copper tracks.
I use track widths up to 5mm and this has not been a problem with autocad polylines.
As my free acad licence finishes soon I am looking to use another cad system and to transfer my drawings across.
I notice though that if I open an autocad drawing in librecad the polylines disappear.

No, there is no way. LibreCAD doesn't have polylines the way Autocad has. LibreCAD's polylines just mean a grouping of successive line segments to one entity.
You could try the free version of Draftsight. This seems to have the feature you are looking for.

Thanks for that; I will probably stay with librecad as it is open source.
I did also use medusa4 cad and the max line width on that is also 2mm.
For the pcb circuit I will look at using a pcb software such as Kicad and then use librecad for the mechanical parts.
What would happen if an external file with line width more than 2.1mm was put into dxf form and then imported into librecad. Would the line width still show above 2.1mm.
Also could a macro be written to increase the line width above 2.1mm

What would happen if....
I don't know, it's a matter of testing. But I think that even if an export of a pcb-file to dxf has a width above 2.1mm LibreCAD probably can't show it.
A macro..., I'm not a developer , don't know, but if it's possible no one will do it, developers for LibreCAD are scarce. Can you do it? You are welcome.
It also depends on what you want to have in the end. A print-out to tinker a pcb manually or a file for a pcb-service?
Using a dedicated pcb software might be the best option.

Yes I will try it.
I have done quite a bit of assembly programming, mainly on microchips, but certainly no expert.
I will see how I get on, depends on available time.
I have quite a lot of autocad drawings for amplifier pcb's which will lose the tracks when put into dxf due to the polylines disappearing so I have to redraw them.
The mechanical cad has been useful as there are many pcb's inside each casing so I have used it for overall layouts.
Thanks for your help

Not a solution but maybe a workaround.
What would happen if you copied the lines you want over 2.1 mm width and offset the copy by an appropriate amount. Just a fanciful thought.
If that was to work then perhaps a macro to automate the duplication - I am not a programmer, let alone a developer so I have insufficient skills to try this.