Here at SGS Polymer Solutions Inc we take our coffee seriously. There is almost always one pot at the ready and another one brewing in our commercial coffee maker. A lot of the time we even freshly grind the beans just before brewing. That’s why it was such sad news when an email went out this morning to the All Staff listserv saying that the coffee machine is out of service and has been unplugged.
As with every weekday, one of the first people on site went to the break room and started a pot of coffee. When they returned at the end of the brewing time to fill their cup, they found coffee water all over the counter. Maybe it was operator error. Was the lid not on right, or was the rinse water not all dumped out before brewing this pot? A second pot was perfectly prepared to accept the next batch and the Brew button was pressed. Another person came in for their first caffeine fix of the day to find that the carafe had overflowed. The machine was unplugged and the word was sent out that coffee could only be brewed in case of an emergency.
That emergency came at 8:35 when those two batches had already been consumed. Concerned for their coworkers, an altruistic employee carefully set up another carafe and stood by at the ready to turn the machine off right at the end of the brewing cycle. Still with 90 seconds left in the cycle, the pot began to overflow. This time the mess was isolated to the base of the machine.
An investigation was launched into the root cause for the spillage. The machine has an advanced flow part that makes the water drip through lots of little holes all over the coffee in the basket. This is supposed to control the pressure from the boiler tank and make sure that the grounds are evenly saturated. The part was loose and easily pulled out of place.
(photograph of the part)
The faulty part was further examined with our Keyence digital microscope. The part was manufactured in November of 2014, as shown by the date wheel. This is a common feature of plastic items that is produced by adjustable and/or interchangeable inserts in the mold. The fitting that attached the part to the machine has failed in the leading thread. The top section along the first part of the thread has fallen off and was likely thrown away in some used coffee grounds. The circumferential fracture continues along the thread for about ½ of a rotation of the part, and a radial crack has also formed. Fractography indicates environmental stress cracking (ESC) as the culprit. This is a common failure mode for plastic parts which has characteristic thumbnail patterns due to progressive micro-cracking.
(micrograph of the failure)
~~ Jarret Wright, M.Sc., Lab Manager and Senior Scientist