Welded joints on corrugated PP sheet get tested by looking at them, which tells you nothing useful. A bad weld and a good weld look identical from the outside, and the difference only appears under load. Fortunately, the meaningful test costs you one scrap part.
The peel test
Cut a strip either side of the weld, grip both ends, and pull them apart. A sound weld either tears the parent material away from the joint or requires real force before separating. A poor one peels cleanly along the fusion line with the two faces parting neatly – the signature of insufficient fusion.
The key observation is where the failure happens, not how much force it took. Failure in the parent material means the weld was stronger than the material, which is the result you want.
Pull test for spot and ultrasonic welds
For discrete welds, try to separate the parts by hand or with a lever and note whether the weld nub pulls material with it. A nub that comes away clean, leaving a smooth circle, indicates poor energy transfer. A nub that tears a disc of surrounding material indicates a joint stronger than the substrate.
Bend test tells you more than you expect
Bend the joint sharply, progressively, until something happens. Sound welded polypropylene tolerates considerable bending before failing. A joint that cracks open at a modest angle was under-fused, overheated, or contaminated – all three look the same at rest and behave differently here.
Sample rate and timing
Test one part at the start of a run and one at the end, not thirty. Weld quality drifts as tooling heats and as material varies, and the two samples catch most drift. Where parameters are set per batch rather than per shift, re-test when the material roll or batch changes.
Do not skip visual inspection entirely
Discolouration, excessive squeeze-out and visible voids all indicate the same thing – too much energy – and they are free to spot. Where signal appears, test destructively rather than assuming. Details on joining methods are in welding corrugated PP sheet.
The honest limit
These tests are comparative, not quantitative. They tell you whether batch A is worse than batch B and whether a joint properly fused. They do not give you a strength value anyone should design against, and no amount of workshop testing changes that – if you need a number, it needs a lab.
What a good failure looks like
When a welded joint is loaded to destruction, the failure should occur in the parent material away from the weld line. If it fails exactly along the fusion boundary, the joint was not properly fused regardless of how much force it took – and it will fail the same way in service, usually at the worst moment.
Keep the broken samples with notes on where they failed. A small collection of failures is more persuasive in a supplier conversation than any report, and it makes the difference between “the welds look bad” and “the welds are failing at the fusion line on every sample”.
What we can do
We weld polypropylene assemblies with recorded parameters per batch and are happy to supply test coupons from the same run for your own destructive checks. Ask for them at the enquiry stage and they cost nothing.