Tech note: molded plastic parts — which defects can a 2D camera not see?
Flash, pits, spots and broken hooks are detectable in 2D. Sink marks show almost no difference in flat images; with 3D laser measurement, a 33–40 μm height difference becomes obvious.
Injection-molded parts such as cosmetic containers and switch housings can have more than ten defect types. In our tests, some are easy for a 2D camera and others never show up no matter how they are lit. Here is what two projects taught us.
Detectable in 2D
- Flash: detected once it protrudes more than about 5 pixels
- Pits on flat surfaces: detected at 5 pixels or more (about 163 μm in area)
- Spots, burn marks (colour camera with colour extraction), short shots (with a mirror to see the side)
- Broken hooks and long gates: judged from reflections under external coaxial light and shadows under bar light
Not visible in 2D, or only under special conditions
- Sink marks: no visible difference front or back; stereo 3D fails too, because it also relies on flat images
- Bumps: on reflective flat areas they do not show, only on edges
- Water marks, silver streaks, ejector marks: invisible under ordinary lighting; detected only with multi-angle, time-multiplexed lighting in specular or shape images
- Oil stains: not visible at 57.5 μm; detected at about 18–22 μm
3D laser: the answer for sink marks
Scanning with a 3D laser displacement sensor (0.5 μm Z resolution), good parts show a smooth height profile, while rejects show a 33–40 μm step, so sink marks can be judged directly by height.
A note on yield statistics
In one test of 396 containers, using “visible to the eye and larger than 9 pixels” as the criterion, 315 were judged defective — a yield of about 20%. That usually means the specification needs to be redefined with the customer: before automating, agree on what counts as a defect.
