Repairing Wiper Arm

The rear wiper arm of our Hyundai Santa Fe ends in a clip which snaps over the shaft of the wiper blade assembly. It's made of a very tough plastic but ours broke during a blade change.

The local dealer wanted $80 for the part. Auto parts stores don't stock it. No Internet vendors carry that exact part, and they wanted $50-$60 for similar parts for other models. So I decided to try to fix it.

What broke was a kind of claw that went under the shaft, from left to right in this picture. I did not think I could replicate that exactly with the required strength. 

But I noticed there are two holes in the arm, quite parallel, one T-shaped on the left and one D-shaped on the right. I could make a part that would go into both holes and fully trap the shaft. With the right dimensions, the friction fit should hold it securely. Chamfers on the edges of the post help avoid excessive friction in the corners.

PLA wouldn't do because it's not terribly strong and does not tolerate heat. Nylon is tougher and more heat tolerant, and is somewhat bendable and resilient. (See my bicycle part.) I have a spool of Taulman Bridge Nylon which should do well. Here's an article about printing with nylon. 

I used a 0.25mm nozzle because these dimensions are pretty small. The key point was to make the T-post and the D-post just the right size to fit tightly, but be removable for future blade replacement. I designed this parametrically in Fusion 360, and I defined a "tolerance" added to the dimensions of each post. That makes it easy to tweak the size. I printed just one half of first unit so I could clearly see how the posts lined up. Then I printed four iterations getting the sizes just right so they would fit with just the right friction. The filament cost per part was about 8 cents, and it took 30 to 40 minutes to print, so no big deal to try a few times getting it right.

The picture shows a lot of stringing, typical of nylon in my experience. That really does not matter for this application so I did not work hard at cleaning them off. Usually I use a metal bit in a Dremel tool to clean up, but it really doesn't work on nylon. So I just snipped the worst and left the rest.

Printing it as shown, the posts were prone to layer separation. I did not want to try to print it on its side and deal with support. I counteracted the layer issue with three methods:

1. 100% infill. For a tiny part like this there's no reason to make it hollow.
2. Higher print temperature to try to bond the layers.
3. Heat treating the finished part. I had never tried this before. See this article. I did this on just the final iteration, and the posts seemed really strong after this.

Here's how it looks installed inside the wiper blade, holding the shaft in place.  

And here it is installed on the car. The base is only a few millimeters thick, so it doesn't get in the way of anything.

The total material cost was about 35 cents. Much better than $80 for a whole new arm!

UPDATE: It didn't work. The next time she drove through the car wash, the force of the brush mechanism popped it right out of the arm. I ended up buying a whole new arm.