For TDC/Ductmate, the corner pieces help hold and locate the flange joint, but they are not what makes a crooked run acceptable. Get the duct sections level and aligned first, with gasket on one flange face only, then seat the corners and tighten evenly. A corner can look a little ugly and still work; a duct run that is twisted, racked, or out of level will keep fighting every joint after it.
Ask when connector type, gasket, cleat/bolt schedule, pressure class, or manufacturer detail is unclear.
Watch out
Do not beat corners flat, force one corner tight while the rest of the duct is twisted, or use corner hardware to hide bad alignment. Do not call the joint done just because the corner pieces are installed.
Check
Confirm gasket is on one flange face only and is continuous through the corners.
Bring both duct sections level and aligned before starting corner hardware.
Seat all four corners before final tightening.
Check that the duct faces match, not just that the corner holes are close.
Add required cleats/clips along the long spans. If no cleats are available, ask whether 5/16 self-tappers in the flange are approved for this shop/job.
Steps
Check all four corners before calling the joint assembled.
If a corner is missing, do not finish the joint. Get the correct corner, seat it, then continue the assembly.
If a corner is loose after assembly, check whether it can be re-seated. If the straight-side hardware is too tight to allow that, loosen the joint first.
Verify the corner type matches the flange system being used. Do not mix corner styles unless the approved detail allows it.
After corners are seated, check that the gasket is still continuous because re-seating corners can shift it.
Say this to your foreman
The corner clips on this flanged joint are [missing / loose] at [location]. Do you want me to get the correct corners and reassemble before we move on?