Finding Cracks in Polypropylene

Is visual inspection restricted to using the naked eye? If not, you might look into the possibility of using a magnification system such as those by people with low vision. Some of these systems use a TV camera feeding a magnified image to a large screen monitor.

If simple magnification isn't enough, then maybe a system using infrared cameras would work. The cracks would probably generate "shadows" that would show up on infrared as dark areas.

If neither of these options is viable, then maybe simply exposing the impellers to vibration for a time before inspection might make the cracks more visible.

Is it any trend in the way cracks develop? Do you find a repeatable pattern?

It could help to find a good solution.

Immerse impellers in a tank of water.

Bubbles should appear coming thru or around the cracks

if they are not all the way thru (i.e. hair-line crack).

UFG

I don't have the answer to the problem but I would have to wonder if polarized light would make them easier to see. I do remember playing with polarizing filters as a kid and seeing weird colors and lines being seen that way.

This would be a good answer for clear Polycarbonate parts - two pieces of polarized film held 90 degrees to one another and looked through shows up residual molded in stresses in PC very well -

Jim

You could purchase a dye penetrant kit to look for cracks. Check out the following website:

www.magnaflux.com

Immerse the part in a tub of dark concentrated dye dissolved in propyl alcohol. The dye should not stick to the smooth PP surface, but will accumulate (due to surface tension) in any cracks. Then air-dry the part, and the cracks should become visible. To remove the stains, soak the part in the alcohol solvent with no dye.

I have not tried this -- I am merely brain-storming. But in any case, the alcohol will do absolutely no harm to polypropylene.

A questions for the original poster.

Are the cracks the result of material flow (weld lines) or are they from some mechanical damage to the polypropylene?

We use Tonnes of PP each year. Detection of weld lines would have a different strategy than detection of mechanical damage.

Also, What is the base colour of the material? (Again, the pigment will have some impact on the visual appearance of the defect when inspected.)

Is there any "filler" or re-inforcement in the material? Some fillers are fibrous and some just provide additional bulk or density.

What size are the parts you are investigating? The strategy for small parts (less than 1Kg) will be substantially different than for large parts.

I know that you are saying "Absolutely no chemical to be added to the components." and I believe that there will be a method. I'll go and check polarised light on some parts now, just for my own interest.

I'm assuming that cracked impellers are rejected because they might fail when used by the end user.

You might try obtaining a motor that will spin the impellers at a much higher speed than what they are normally expected to encounter in their design use.

If the impeller does not fail when spun up to 150 or 200% of rated speed then it's probably OK, even if a crack is present.

If you aren't allowed to use dye penetrant, then your idea of vibration testing might be the only way besides actual testing. Methinks testing will be more cost effective, just spin it extra fast and if it doesn't break it's good.

Have had a look at three different PP materials with polarised light. Absolutely no luck, so that avenue would seem a dead end.

From your description "impeller" I'm guessing that there is a hollow portion where the shaft would be attached and there are multiple vanes.

Is the problem cracking in the vanes or in the boss around the shaft area? There are different potential causes for those conditions.

If the problem is that the boss around the shaft is "cracked", then it may be a "weld line" where the molten plastic meets as it flows around the core in the tool that creates the detail for the inside bore. Weld lines are very predictable in position and they represent a localised weak area that will fail once strain is placed onto the area. Depending on the number of parts to be inspected, you will notice there to be a "nick" in the surface where the crack will propagate from. You can closely inspect that area, but the real solution is to modify the material flow in the tool so the problem does not happen.

If the problem is not a weld line, then any localised weakness in PP will show as the part is stressed to a condition near the yield point. The localised weakness will show as a white streak on the component surface and the mark will remain there after the strain is removed. The way to check for this is to "flex" the parts to a known deflection that does not damage a "good" part and hold it there for a short time to allow the material to begin to yield. A smart strain gauge would be able to discriminate "normal" and "abnormal" parts either by checking force at a fixed deflection or by comparing the force/deflection curve to known good parts.