Local heat is the usual way to put a bend into corrugated polypropylene without a crease line. Done well it gives a smooth radius and a strong corner. Done badly it gives a corner that looks perfect and fails at the first knock, because the heat took more from the material than anyone could see.
The window
Polypropylene does not have a sharp melting point in the way metals do. It softens progressively. There is a band where it forms easily – roughly around the point where it becomes flexible rather than rubbery – and above that it degrades: surface oxidation, loss of mechanical properties, eventually discolouration and a burnt smell.
The forming band is narrower than it feels. Heating until it “seems soft enough” is how most people end up above it, because by then the sheet has already absorbed far more heat than needed and keeps conducting it inward after the heat source moves away.
Even heat, not more heat
A strip heater or a hot-air bar that covers the whole bend line evenly will form at a lower temperature than a spot source, because you are not compensating for cold patches by overheating others. Uneven heating also gives an uneven bend radius, which looks like sloppiness but is really a heat distribution problem.
With corrugated board specifically, the flutes create uneven contact with any heated surface. You heat the crowns first and the valleys lag. Expect to spend longer than you expect on a heat bar and much less with hot air.
Overheating is invisible
This is the part that catches people out. Mildly overheated PP looks fine – same colour, same surface, same dimensions. Its impact strength and tear resistance have dropped, sometimes considerably, and there is no inspection short of destructive testing that will show it. The evidence arrives months later as corners that crack in cold weather.
Discolouration means you have already gone well past acceptable. Treat visible yellowing or surface gloss change as a rejected part, not as a cosmetic issue.
Thermal limits elsewhere
Every thermal exposure adds up. A panel that was heat-formed during manufacture and then lives in a hot container behaves differently from one that was never heated – the point we made about how panels behave in hot shipping containers applies doubly to locally heated zones.
Bend radius with heat
Heat lets you take a tighter radius than cold forming, but there is still a minimum. Below roughly a couple of times the sheet thickness on many grades the outer face thins noticeably and becomes the weak point. Forcing tighter than that for appearance produces a part that looks elegant and breaks at the first drop.
Cooling under restraint
Hold the bend until it has cooled below its softening range or it will partly open – the same springback described for folded corners, amplified by being warm. A simple jig does this properly and costs nothing; holding it by hand does not, because people let go too early.
What we can do
We form corrugated PP sheet parts and control heat bending with jigs rather than judgement, because it is the one operation where the damage does not show until much later. Tell us where the part will live – freezer, hot truck, outdoor yard – and we will size the bend radius and method to suit, or suggest a welded corner if the angle is too tight to form safely.