Non destructive testing on small plastic parts.

[gregjwilson]

We need to cut sprues on a small plastic piece without damaging the piece. I need a method to confirm the piece (a medical device) isn't damaged.
Does anyone have experience with or can anyone point me in the direction of resources/references that can help me find an efficent way to do this?

Thanks!

 

[gerhardl]

What is small? What is your definition of damaged? Plastic what? More details please...and beeing not native to English, could you also please explain to me what sprues actually means in this context?

 

[gregjwilson]

Small is about 12mm x 3mm. There is an end cap on a small neck that snaps off, but for manufacturing ease 2 channels or legs have been added from the body to the cap to allow plastic flow.

I need to cut those legs without damaging the center neck piece. As my cutting blades or device gets dull or damaged I need to be able to determine if the center neck piece is getting stretched or broken.

 

[FACS]

A microscope or magnifying glass?

Charlie

http://www.facsco.com

 

[gregjwilson]

I'd really love to automate this : we'll be going through a lot of them.

I'm already handling the items to cut them, I was hoping to find a vision process or something (xray? ??, sonic?? ) that can see a break or stretch in the plastic at the neck.

The problem is identifying damage on such a small item in an area that's already designed to break.

 

[MikeHalloran]

It might have been better to extend extra runners to the cap from the main sprue, rather than trying to flow plastic into the main body, back out again, and into the cap. Then you could use submarine gates and eliminate all the trimming. Probably too late for that now.

Maybe a hot knife, or a vibrating knife, but it's still skill- intensive and needs to be done under magnification, which is itself exhausting. At least buy a television based microscope.

Sellers of ultrasonic machines will mumble sunshine promises. It never worked worth a crap for me; maybe your mileage will vary. Insisting on a money- back performance guarantee for machine and tooling will considerably cool their ardor.

Could you weaken the ends of your pseudo- runners so that you can snap them off at assembly? Or make them flexible enough to leave in place? Yeah, you should be so lucky.

Or, weaken them _away_ from the body and cap, so when you cut them or snap them, you leave a stub on the actual parts? Let me guess; it would be a puncture hazard, or would interfere with some tight clearance.




Sorry; I was in a bad mood _before_ I looked at your problem ... and I don't see a magic bullet anywhere for you.



Mike Halloran
Pembroke Pines, FL, USA

 

[gregjwilson]

thanks for your input, Mike, I hadn't seen an easy way either.

Changing the part design is out at least short/medium term. Production is already underway.

The problem is from mgmt/corp. committee-think risk mitigation. These things are being cut at the manufacturer, but shipped and handled before they get to us. I've now got to come up with a way to cut these things in house in the event the manufacturer has a problem and the heads start breaking off in shipment. We could get them to stop the cutting and do it in house.

 

[Strong]

Greg,

Here are a couple of ideas:
1) Monitor cutting with ultrasound (say 40 kHz region) to detect abnormal cut or cutter wear/breakage.

2) Assuming the general shape of the object is like a flattened soda bottle, then clamp the bottle head to hold object as a cantilever beam. Apply impact (object or air jet) and measure natural frequency(s). A change in naural frequency would indicate a damaged/miss-shappen structure.

Walt