Since all the caps are printed in ABS, there is a nice method to smooth out the caps. Acetone vapour smoothing is a easy method to do.
I got a acetone proof lid(can be any material that isn't affected by acetone), and put a wood screw into it. Then I screwed on a stem cap.
Then inside a glass cup, I poured in some acetone and put the lid on. The stem cap sits inside the cup above the acetone.
To speed up the process, the cup can be heated up to accelerate the evaporation. I did this on the heated print bed of the printer. You can do the same with a double boiler style setup, or whatever other method you want. Do keep in mind that acetone is flammable, and we're dealing with vapour, so being in a well ventilated area would be good.
Out comes a smoothed out cap. Leave it for a minute or so for the acetone to evaporate away before handing the cap. With a bigger container, you can do multiple at a time, however there could be some colour contamination of working with lighter colour plastics together with darker.
With a simple shape of the stem cap, there's not much detail that is lost. The dimensions of this gold cap was 19.70mm diameter insertion with the largest diameter of the cap at 23.40mm. Afterwards, the cap came out at the same dimensions. The ridge is not affected at all. Only the sharp edges became a little bit rounded, but it's a very minimal difference.
Things I've discovered:
• The top left cap was sanded, and the final surface finish is semi gloss. The process only takes less than a minute for sanded caps. Leaving them in there for longer makes no difference.
• The top right cap had the seam(position where each new print layer starts) sanded away only, so only that area is semi gloss. The rest of the cap was untouched before acetone smoothing, and is very glossy.
• The bottom right cap was untouched before the smoothing. I found that the longer it's in there, the more melted it looks. This one was left in the cup for about 30 mins whilst it was being heated.
• The bottom left cap is an original cap.
For solid colours, the process works very well. For the metallic gold cap, it did smooth out the surface, but as the metallic plastic is printed layer by layer in a round path, the metallic flakes stay in place and it still looks as if there are layers.
I have 3 red, one white, and one gold cap that are smooth. They are going to the people who helped out with the project if they want it. If you want a smoothed cap, get in touch.
For those sticking to the hand sanding/buffing, I find that a fingernail sanding/buffing stick that is foam backed works the best. The foam helps prevent you from sanding in a flat spot. The walls are 1.2mm thick, so keep in Ming how much material you're removing. Once all the layer lines are removed, you can work your way up to finer grits before using the buffing sticks. An alternative to using a buffing stick is using a bit of toothpaste/paint polish on a cloth. Stretch the cloth on a hard enough surface with one hand, and then rub the cap really fast on the cloth to buff it up. Or a clean buffing wheel will work too. It's a bit more work compared to using acetone, but it can result in a very similar surface finish.