By now you have heard of running your 3d printer in vase mode (and if not go check it out now!). In vase mode your 3d model becomes a 1 layer thick shell. The 3d printer gradually raises its z while printing creating a spiral print that has no vertical breaks. The result is beautiful, seamless, and fast printing parts that use up less plastic (aka cheaper to make). The only problem is they are structurally weak prints.
Step 1: Challenge 1: Vase Mode With Holes
I will be using a part from another project as an example.
I have a part that will be printed in vase mode but I would like to have a clearance hole for a bolt to pass thru it. The problem is a hole in the part will create a break in the vase mode print. The 3d printer nozzle will stop extruding, retract, move, and start again. Leaving strings, globs, and not as strong of a print. On top of that, 3d print software never seems to choose the best position to start the next layer.
Fill in your holes
The easy fix for simple holes is to fill the holes in for the print and cut them out later. Now the extruder will never stop and you will have smooth strong prints.
I have found it is good to leave a small indent where the hole will go. This will leave a witness mark for when you are looking to cut them out later, and helps strengthen the part.
Step 2: Challenge 2: No Internal Structure
After filling in the holes I can get a beautiful continuous print, but as you can see on the last picture there is no internal structure. If I were to mount this arm on with a bolt there is nothing to keep the outer wall from being deformed until it comes into contact with the inner wall.
Also the bottom of the countersink has created a feature that is impossible to print so the 3d printer left an opening.
More CAD modifications
The countersink gap is easily solved by adding draft to the feature.
To add internal structure without ruining our vase mode print I added some thin cut features to the model. The biggest challenge with this is having the cut feature thick enough to be recognized by the 3d print software, but not too thick or too close to another feature. If it is too close the 3d print software will try and merge the print path. I found 0.040" to be the magic number for feature thickness, and 0.080" the magic number for keeping the lines from merging.
Step 3: Post Print Cut Out
The last step is to cut out the hole after the print. Depending on the hole size you can either use a knife or a power drill.
Looking inside the part you can see the features are close but do not touch. Now when the part is bolted into position the outer wall will no longer collapse inward.
As an added bonus all the zigs and zags and change of direction of the added CAD features strengthens the plastic. Feeling around with your fingers and you can tell the plastic around the holes is noticeably tougher and less resistant to cracking.