PLA is often maligned for its poor temperature resistance and tendency to creep (deflect slowly over time). However, it's also super stiff by nature, and with a little carbon fiber it's way stiffer! That stiffness makes PLA particularly great for violin bodies, where a high stiffness to weight ratio means better sound.
[This blog is guest-written by David Perry, lead designer and printer wrangler at OpenFab PDX.]
You probably already know that Proto-Pasta's HTPLA can be heat treated, or annealed, to give it superpower-like resistance to elevated temperatures. But knowing this is one thing -- what happens when the rubber hits the road? What happens when I do something crazy? Crazy like, say, leaving my HTPLA-CF violin in a hot car.
I've left 3D printed fiddles in sun before, and with devastating consequences. I once left a fiddle, with a premium body printed in CFPLA, in the sun for 20 minutes on a 75 degree day. Here's what happened to the body:
I take these fiddles camping, and I need to be able to leave them outside without treating them like a newborn baby at the beach. So what to do? Some folks have printed in PETG and ABS, but either material will wreck my beautiful CFPLA sound.
Sure, theoretically, annealing HTPLA can give it resistance to elevated temperatures up to 155C (310F) (from product page), but what happens when the part is used under significant load in real life? Violin string tension is about 50 lbforce, which results in about 20 lbforce straight down into the body of the instrument onto what is called the top plate. The top plate of a Modular fiddle is about 2.5mm thick. That's a lot of load for a 2.5mm thick plate!
To minimize shrink during annealing I used Proto-Pasta's HTPLA-CF. I wanted to try the basic HTPLA as a baseline test, but Alex warned that I would have 3X the shrink as the carbon fiber filled HTPLA. 3X the shrink! No thanks. Plus, the added stiffness of the carbon fiber really does have an incredible impact on the acoustics of the violin body. I went with black for worst-case heating.
Plus, black CFPLA is just beautiful. Here's my not-annealed test instrument just prior to its demise.
With the minimal shrink of the HTPLA-CF, I did not need to scale my parts (although they did need some extra filing to fit together). In the future, I'll make a few small changes to the joint geometry to compensate for shrink.
Annealing is appealing but can lead to hard feelings.
The annealing process is simple. Heat up the part so that all the plastic is hotter than the glass transition temperature 60C (140F), let it hang out there for a period of time, then cool it off. I'll save more detail for another blog post, but for such a simple concept annealing is remarkably complex in implementation!
The Modular Fiddle body proved the most difficult to anneal. The thin sections in the top and bottom plates had a tendency to warp far out of shape due to internal stresses. I had my best results with the following process:
- As print is finishing, preheat oven to 300F.
- Allow oven to preheat, and let it sit at temp for at least ten minutes.
- As soon as print finishes, leave part on hot glass build plate, place on top of glass casserole dish (to buffer heat from metal rack) and transfer to oven.
- Turn off the oven.
- Allow part to heat for 10-30 minutes, depending on geometry.
- Remove from heat and cool in room temperature air.
- As part cools, gently shape important features as needed.
Step 7 is key. I was able to straighten the neck as it cooled by clamping it on either end. If I had tried to clamp it in the oven the clamps would have simply squished the part, but by clamping during cooling the plastic is firm enough to endure some pushing but still somewhat form-able.
For the body, I used a butter knife through my F holes to straighten the top plate. I don't need it to be perfect, but I do need it to be pretty good.
The HTPLA-CF Hits the Road
After weeks of testing print settings and annealing process, I finally had two fiddles built up. Both are built with HTPLA-CF. One uses all annealed parts, the other uses parts that have not been annealed.
I set them out in the sun and crossed my fingers. I knew the not-annealed instrument would quickly fail, but I also was pretty confident the annealed instrument would buckle under load.
To my surprise, the annealed instrument did great! After 30 minutes, during which the body temperature on both instruments leveled out at 130F, the annealed fiddle showed no warp and was just a bit out of tune. The other fiddle, however, did not fare well.
I took a time lapse, and you can view the time lapse in this video summary of the test. It's embedded here to start at the time lapse.
Not Just Hype
We all know that Proto-Pasta isn't blowing smoke when they talk up HTPLA and the temperature resistance properties, but I didn't expect it to be this good. I thought surely that under load and elevated temps, even annealed HTPLA would rapidly creep -- it is still PLA after all! But no -- when properly annealed, HTPLA-CF can make a fiddle with better sound than PET or ABS that can resist pretty much whatever environment you throw at it.
This annealed HTPLA-CF fiddle truly is a durable violin. Take it to the desert, leave it in your car -- it will still work. The power of annealing!
Well, maybe don't leave it in your car at the desert.