Overhang and bottom layer problems

[CatsandGoats 0]

I am trying to print custom Lego-compatible blocks using PLA on an Ultimaker 2 for an engineering project. The blocks are designed to have the male Lego studs along two adjacent edges and female along the other two, as shown below.

Obviously, the precision needs to be quite high for these pieces to mesh together, and I am having problems in two areas:

1. The first problem is with overhangs. If I print the piece lying flat as shown in the CAD drawings above, drips form on the underside of the rim above the female studs that prevent the male studs from fitting in, as shown below on the left. The same general issue also occurs on the studs themselves, which will drip from whichever is the lower end when they are printed. I have tried using Cura’s automatic supports, but without any luck. They do not support the right areas, and are essentially impossible to remove. What else can I try?

2. The other problem is what I believe is called an elephant foot in my first few layers. If I print the piece standing upright on its narrow edge, the bottom layer that it sits on becomes spread out slightly. As shown below in the brick on the left, it makes the studs and rim thicker than they ought to be, so that they cannot mesh with other bricks. I have tried decreasing the temperature of the heated bed, thinking that maybe the bed is keeping the PLA too soft and allowing it to deform under the weight of the layers above it, but so far that has not helped. What else can I try?

I think that the dimensions of my model itself are fine, since any studs that are printed on the top surface and therefore protected from dripping mesh perfectly with real Lego bricks.

Thank you for your help!

[gr5 2,172]

There's no easy answer.

1) If you only care about 2 of the 4 sides then print with one of those sides facing down and the other facing up. You need of course to adjust leveling 10X better than normal so don't use the normal leveling procedure and instead adjust by turning the 3 leveling screws equal amounts up or down until the skirt laid down is exactly .4mm wide (assuming your nozzle is a .4mm nozzle).

2) You could print the part in a few pieces and glue them together or have them press fit together.

3) You can measure the error accurately and then change the shape in cad to account for this. This works much better than you would think. Basically remove a few layers off the bottom of each of those protruding cylinders. It sounds crazy but it works pretty well if you are printing consistently.

4) You can get rid of that stringing on the sides (but not the bottoms) of the cylinders by making sure fan is at 100% well before it gets to those "bumps". Also lower printing temp by 10C or 20C to around 190C or 200C and also consider a different filament. This filament appears to be one of the softer formulations that "string" more than average.