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  Measurement, Test and Calibration
  Testing for Cracks

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Author Topic:   Testing for Cracks
Dawn
Forum Contributor

Posts: 245
From:St. Marys, PA
Registered: Sep 98

posted 25 November 2000 04:07 PM     Click Here to See the Profile for Dawn   Click Here to Email Dawn     Edit/Delete Message   Reply w/Quote
I'm having a problem finding a form of testing for molded parts which are very soft before they are cooked at a very high temperature to make them extremely hard. My question is; does anyone know of any test methods which can be used to test these parts for cracks when they are very delicate before they are baked?

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CarolX
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Posts: 108
From:Illinois, USA
Registered: Jun 2000

posted 28 November 2000 03:12 PM     Click Here to See the Profile for CarolX   Click Here to Email CarolX     Edit/Delete Message   Reply w/Quote
Dawn,

I would suggest some sort of Non-Destructive Test (NDT) method. I am not completely familiar with the methods curently available, but it might be an option for you.

Carol

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CJacobsen
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Posts: 48
From:Williamstown, MA
Registered: Oct 2000

posted 29 November 2000 06:51 AM     Click Here to See the Profile for CJacobsen   Click Here to Email CJacobsen     Edit/Delete Message   Reply w/Quote
For comparison:
I am not sure exactly what the part is you have, however, I offer this scenario:

A client currently uses a gel type substance to fill a mold. After 24 hours the molds are tipped so the pieces are on a sheet (similar to a baking sheet/cookie sheet)only heat tempered glass. These pieces are soft at this point and susceptible to damage and surface anomoly. Touching them would damage them. It is necessary at this stage to bake these pieces to harden them, however, if there are any breaks in the surface (small cracks or splits in the skin) the pieces deform when heated. Since they cannot be touched, the whole sheet is passed under an optical magnifier where the parts are magnified about 100x and the surface of each is closely inspected. Since they are on a glass sheet, the bottom surface can be viewed as well.

Not sure if this helps you. I realize the application I describe may not even be close, but the similarities caught my attention.

------------------
Christopher E. Jacobsen
cej@cjsys.net
CJSystems

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Geoff Cotton
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Posts: 34
From:Staffordshire, England
Registered: Jun 2000

posted 29 November 2000 09:57 AM     Click Here to See the Profile for Geoff Cotton   Click Here to Email Geoff Cotton     Edit/Delete Message   Reply w/Quote
Dawn,

Give us a clue, what are the parts and what's the material etc?

Geoff

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Dawn
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Posts: 245
From:St. Marys, PA
Registered: Sep 98

posted 29 November 2000 06:44 PM     Click Here to See the Profile for Dawn   Click Here to Email Dawn     Edit/Delete Message   Reply w/Quote
Thanks CJacobsen,
We are making a powder metal part which is very soft and easy to break. I need to find a test we can do to determine a crack inside the part before it is baked. They are baked on a sinter oven which means we have to put alot of parts through before we can test for this crack. Is there something out there that I can use such as eddy current, ultrasonic testing, etc. which will detect this crack earlier in the process. The part is soft enough that it will fall apart with alot of handling. (Thanks - you guys are great)

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Drew H
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Posts: 11
From:Kelowna, B.C., Canada
Registered: Oct 2000

posted 30 November 2000 01:27 AM     Click Here to See the Profile for Drew H   Click Here to Email Drew H     Edit/Delete Message   Reply w/Quote
Eddy current will only test for surface cracks, and it sounds like you are looking for internal flaws. Certainly, ultrasonic will do that, but this is a method that requires contact and acoustic couplant. You might want to investigate an x-ray type procedure, as this will find internal cracks without contact.
Maybe there is some researcher out there who has a clever technique for your application - you might want to browse through NDT journals to see.

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Geoff Cotton
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Posts: 34
From:Staffordshire, England
Registered: Jun 2000

posted 30 November 2000 08:11 AM     Click Here to See the Profile for Geoff Cotton   Click Here to Email Geoff Cotton     Edit/Delete Message   Reply w/Quote
Folks,

As we all know 'you can't inspect QUALITy into a product. Should we not turn this problem around and establish the root cause of the cracks. We may then be able to engineer the problem out using DoE etc.

Geoff

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Tim Mosher
Lurker (<10 Posts)

Posts: 5
From:
Registered: Jun 2000

posted 06 December 2000 06:25 AM     Click Here to See the Profile for Tim Mosher   Click Here to Email Tim Mosher     Edit/Delete Message   Reply w/Quote
I am an asnt level III, so if you can give me some more information(part size, size of defect you wish to detect, and so on) I may be able to offer you some advise. I have worked with sintered powdered metal before.
Thanks!

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Tim Mosher
Lurker (<10 Posts)

Posts: 5
From:
Registered: Jun 2000

posted 06 December 2000 07:00 AM     Click Here to See the Profile for Tim Mosher   Click Here to Email Tim Mosher     Edit/Delete Message   Reply w/Quote

I have a little more time, so I will give you some more information. First, have you verified that the internal cracks are present prior to baking? In my experience with powered metal, most internal cracks are caused by shrinkage during the sintering process. It is indeed a tricky process, and one which does not lend itself well to most non-destructive methods. If your part is not large(less that an inch or so in its thickest cross section)a low power real time x ray system is your best bet. There are many companies which produce low-power systems(less than 75 kv) for inspection of food products. These systems scan product on a conveyor belt, which would minimize handling on your part. Also, most low power systems as found in airports will do the job nicely for you, and they have image enhancement software which can make detection more accurate. There are many considerations however when looking at this method. Part geometry, indication size, cost of the system,(you may be able to rent one)
training, safety, and so on.
I agree that the best thing to do is improve your process, however without a quantative method to measure the severity of the problem, process impprovement is difficult with this manufacturing process.
Any more questions, please let me know!

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