Subtleties of Injection Molding

Dear Reader,

There will be a few back to back updates here, there’s been a lot happening in a short span of time.

Shortly after our last update I noticed an issue with the enclosures I had received.  These were the first enclosures to come out of the new mold, our CM made a few as a test so we could give approval before proceeding to mass production.  The plastic near the connectors had cracked.  Structurally the enclosures felt totally sound, and honestly if the plastic had not been transparent I’m not sure if I would have noticed them, but it was still a problem that needed investigating.  Here’s the funny thing: only the enclosures that had been sent through the mail had cracked, not the ones that the CM had kept.

Hairline cracking in the connector bosses

The area of the part where the cracks appear is definitely the most vulnerable to cracking.  In an injection molded part, you want all the plastic to cool and contract at the same rate, because when plastic contracts at different rates it creates areas of internal stress in the part.  The area where we found cracking is doubly vulnerable because it’s (a) at the junction between a thick and a thin wall and (b) next to an overmolded brass part (the connector).  Both of these will contribute to internal stress in the part.  This would not be a problem with the original plastic we designed with (a softer ABS blend) but we had to switch to a harder polycarbonate during the regulatory process, and polycarbonate is more vulnerable to cracking.

We asked our CM to put the parts they had retained in the freezer overnight, and sure enough a few of them cracked in the same place.  The parts were coming out of the mold with internal stresses right on the hairy edge of cracking, freezing them added thermal stresses that pushed them over.  Most likely the ones we received were shipped to us intact and cracked in the frigid cargo hold of the aircraft.

We reviewed the problem with the CM and asked them to make another small batch with the mold parameters changed to reduce internal stresses.  They shipped us the new batch, which arrived without cracks, and I subjected them to heating, cooling, even freezing in a vacuum chamber.  They did not crack, so we gave the go-ahead for full production!  Our injection molding subcontractor has been working diligently.

The full quantity of enclosures should be ready very soon.  Exciting times.

Thanks for reading!

5 Responses to “Subtleties of Injection Molding”

  1. Seth September 18, 2014 at 8:05 am #

    Glad you guys found the issue before the issue found us :) I’m also glad the fix was fairly easy to implement as well. Thanks for the update!

  2. Beat September 18, 2014 at 3:12 pm #

    Thanks for the update, very much appreciated to get news ! :-)

    Keep the professional work going !

  3. Terrel Jones September 19, 2014 at 5:05 pm #

    What a process! you guys are amazing!

  4. Vincent Royer September 24, 2014 at 6:10 pm #

    Do you have any news?

    Is there somewhere to get an estimated shipping date?

    • James September 24, 2014 at 9:32 pm #

      Hi Mr. Royer,

      The enclosures are in customs and the PCBs are being assembled as we speak. After that it’s time for final assembly, calibration and fulfillment. We’re presently forecasting shipping in the second half of October.


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