A couple of Amazon things

Two recent Amazon purchases got me thinking about design and manufacturing.

Let’s start with design. On the recommendation of a couple of friends, I bought this LitraTorch LED light cube.


It’s a nice compact little light that generates fairly uniform coverage at three intensity levels (and there’s a strobe mode, which I find useless). Also, it starts at the lowest intensity so you don’t get immediately blinded when you turn it on in a dark room. It’s not cheap, but it’s a professional tool for the inspections I do at work. Overall, I’m happy with it.


It has a permanent battery that recharges through a micro USB port. I always struggle getting the orientation of the plug and port to match, and working with this one is going to be even more trouble than I’m used to.

Litra micro USB port

Every port I’ve run across before is shaped like the plug in sort of a curved trapezoid. Not this guy. The only way to know which end is up is to see the black connector part inside the black rectangle on the back of a black cube. I assume the decision to go with a rectangular port was driven by cost, but it’s an unfortunate choice, especially in an $80 flashlight.

Now let’s move on to a manufacturing problem. I also bought this LIHIT LAB pen case to hold my pens, pencils, erasers, and tweezers.

LIHIT LAB pen case

It’s a simple case and not expensive, but the first time I tried to unzip the main compartment, the zipper pull came off in my hands.


The little hook on the slider that holds the pull had snapped off.

Broken hook from slider

Since I was at work, I took the broken hook into the lab to have a look at it under the microscope. Please forgive the fuzziness; it’s hard to get good depth of field in optical microscopy.

Zipper hook fracture surface

So many problems in how this was made. The immediately obvious flaw is the bubble in the center. This is a cheap casting made of cheap pot metal and the casting left a huge (relative to the size of the part) bubble in the center of the hook.

But I doubt that was the main cause of the failure. The hook is loaded mainly in bending, and the bending strength of a hollow part isn’t usually significantly different from that of a solid part. Many parts are deliberately made hollow to save material without reducing strength much.

You may also notice the crack extending inward from the edge on the right side. That, too, is a flaw, but didn’t lead to the failure. That crack is perpendicular to the final fracture surface and would not be affected by the bending stresses that broke the hook.

Now look at the black areas at the upper right and upper left corners of the cross-section. This is paint, and because it’s on the fracture surface, we know that cracks in those areas were already present when the hook was painted. And because they’re on the outer edges of the cross-section, where the stresses are high during bending, they are the flaws that were the most direct cause of the failure. I’m guessing the casting cracked during cooling but was not weeded out during whatever quality control process the manufacturer used.