Method to reduce interlayer shear or delamination under tension

One of the parts I print is a fitting for attaching a carbon fibre rod to a PCB probe. In use it should be always under compression, it is printed with the Z axis aligned with the shaft axis, and has proven robust in field use over several months. However recently we have had a few failures, where the part has been sheared along the layer boundary. I've had various reports as to the cause, from "someone fell on it", to operators using the device to "hook" down into position multi-ton equipment suspended from a gantry crane, to what looked liked someone had used it as a golf club. I can't comment on these failures occurring around the time the company announced reduced Christmas bonuses due to low profits!

A quick way to overcome the weakness in tension/shear would be to incorporate a couple of 3mm ID tubes running through the Z axis, then inert 3mm screws and pretension, as is done with pretensioned concrete beams. However I need to reduce the weight at the head as much as possible.

It occurred to me that it should be possible to print the strengthening tubes in the Z axis, perhaps around 1mm ID, and then have the printer go back and inject plastic into them to tie the layers together. Unfortunately I'm overseas in Brazil and away from my workshop for some time, and so can't carry out the necessary testing, but perhaps someone had tried this idea before?

My visualized sequence, just picking some numbers, which obviously would require testing.
Say for a 0.1mm layer build, using a 1mm ID pinning hole, if we found we could inject plastic 5mm deep:
1. Build the layers with the pinning columns up to 5mm
2. Increase hotend temperature as required for more fluid plastic
3. Move to centre of each pinning hole in turn, drop head/raise bed to seal against layer
4. Inject plastic of calculated volume
5. Step and repeat to next hole

An enhancement would be to stagger the start/end of each pinning hole in the Z axis, so as to avoid any single weak layer.

If this were incorporated into Slic3r, there is then the question of whether the program selects the pinning locations itself, as with support material, or whether the information can be added manually to the stl model. In the latter case the designer has better control of location and depths - the pinning could be done as internal tubes of set diameter, with a Slic3r option to recognize these and add the pinning fill.

Further ideas welcome!

David

While I don't know what you are making, and I am fairly new to 3D printing (3 months) I can tell you a few things I have learned. First is that natural color ABS is stronger that colored (by 10-15%). Smaller layer heights are stronger. And dipping parts for a few seconds in acetone (1-3 seconds only, any more will melt the part) will strengthen the bond between layers. Also higher infill up to 100% will cause greater strength

And as you mentioned, using higher hot end temp.

@tmorris9 Thanks for you comments. I do intend to trial the methods you suggest, once I get back to my machine. I'll set up a standard testing part and a rig for tensile testing, and get some quantitative data of the effect of various print settings and procedures.

However I would still like to trial my pinning/injection idea, as I think it could greatly increase part strength.
A couple of problems I foresee are:
1. The hot-end bonding to the plastic when pressed down to make a seal. This could perhaps be overcome by using a non-stick/thermal insulating material washer around the hot-end tip, or perhaps a full seal would not be required.
2. Air locked in the tube unable to escape. This could be avoided by design, leaving a path for air to escape at the bottom of the tube, or perhaps by using a slit-trench design rather than a tube.

Interesting idea, but quite problematic. Arrange competition for it
Free nozzle dropout not neccessarily go straight downwars, maybe put nozzle lower but then it sticks with great amount of plastic after "injection".
Difficult maybe to move away without dragging with nozzle.

If you need to produce these over and over and getting them right is more important that printing them yourself, why not print one, make a silicone mold and fill with a strong plastic resin. If you need more info on this let me know, I have done this quite a few times.

Actually my original plan was to do just that, I didn't think that the printed plastic would stand up to the high temperature environment and I had purchased resin and was going to use the 3D milling machine to make a mold. I printed one off for the the client to trial for mechanical design, saying I didn't expect it would last in the production environment, but after they had been using it for some 6 months in production they said it worked well and wanted more of the same. It was quite some time later we started to have a few failures, I think due to different operators or operator practices.

Printing does give you the ability to quickly modify the design, which I like. We had a few PCB breakages, and it only took an hour or so to come up with a modified version that provided more support for the board. At this stage the most we have had to build is a batch of five, but if quantities go up I think the molding approach will be the way to go.