Rigid means pre-assembled with glued dowel construction. Flat pack means that metal scam and dowel fasteners are used. If these units are assembled with a bead of PU glue in the joints they are almost as strong and rigid as the rigid units.
I don't know if you guys have this product in the UK, but LePage's over here makes a moisture cure polyurethane construction adhesive they call "PL Premium" that achieves a bond strength 3 times as strong as the conventional construction adhesives marketed by other companies, and it achieves that bond strength within 24 hours.
For something like wood cabinets, which you won't be taking apart once assembled, I'd use PL Premium as the glue to assemble them.
Different market, different suppliers. Any reasonable PU will do, the type that comes in caulking gun cartridges work better on this type of job (less foam out), but the PU only adds to the strength of the KD fittings and cannot replace them. A carcase assembled using KD fittings is still quite strong, but dowelled construction (rigid) is far stronger in any case
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local