Print Reinforcement Method - Proof of Concept Fail

Spoiler alert : My idea didn't work. I'm publishing this test because I think it's important to know what does not work, as well as what does. Perhaps someone else will have more success with the idea than I did, or perhaps it will inspire another better approach. If you play around with this and have more success than I did, please let me know in the comments.

This simple print was created to test an idea for adding reinforcement to a print to make it less liable to break along filament layer lines. I was only marginally successful in getting my idea to work, but I thought it was worth publishing for others to look at and perhaps improve.

The concept starts with the idea that FDM prints are stronger in the X and Y directions, and weaker in the Z direction of layer stacking (presumably due to imperfect bonding between layers).  This can sometimes create printing issues when the optimal print orientation and the optimal strength orientation are not the same.

The basic idea is to create a print with vertical cylindrical holes in it, and then fill those holes with extruded filament. The test print is a simple hollow cylinder (with a side vent hole near the bottom, to allow air to escape as filament is extruded into the top). 

I used PrusaSlicer to generate gcode for the base print, and then modified the gcode so that when the basic print has completed, the following occurs :

1. Print head positions itself over the center of the cylinder
2. Extrusion temperature is increased from 210°C to 260°C (to make the filament more “liquid” and promote bonding to the base print)
3. Print head is lowered .5mm (to “seal” the nozzle to the top of the print)
4. Filament is injected into the top of the cylinder

I experimented with varying the parameters for items 2, 3, and 4 above until I got something that worked reasonably well, but there is likely room for improvement in the values I selected.

My gcode modifications were based on the following assumptions, which may not be correct in all cases :

1. The print head needs to be positioned above the hole to be filled (pretty obvious)
2. The filament extrusion temperature needs to be increased for these reasons :
   • to reduce melted filament viscosity
   • to keep the filament from solidifying to quickly
   • to help bond the filament to the inside of the hole
3. The filament extrusion speeds needs to be reduced (this is empirically true based on testing)
4. A vent hole is needed at the bottom of the cylinder to allow air to escape as filament is injected
5. The print head should be lowered slightly prior to filament injection to help ensure that all injected filament enters the cylinder

Here is the gcode I added to the base gcode. Both the base gcode and the modified gcode are included here.

The method I used is as follows :

1. Generate gcode using PrusaSlicer
2. Use a text editor to insert the code below into the generated code. The code is inserted at the point where the print is completed.

Note that if you generate your own gcode using a different slicer, the X,Y center coordinates  (X125 Y105) may need to be changed.

; **************************************************************
; **************************************************************
; ***** Move to center of cylinder and extrude filament
G1 X125 Y105 Z10.5 E0 ; move to center of cylinder
M104 S275 ; set extruder temp
M109 S275 ; wait for extruder temp
G1 X125 Y105 Z9.5 E0 ; lower head
G1 E30 F500 ; extrude filament, reduce extrusion speed - smaller F-number = slower extrusion
G1 Z15 ; raise head
; **************************************************************
; **************************************************************

Some observations :

Extrusion speed appears to be the most critical factor. With very fast extrusion speeds, the filament does not melt fast enough. With very slow extrusion speeds, the filament solidifies too quickly and blocks the flow of additional filament into the hole. I experimented with various intermediate speeds, but was not able to fill the cylinder hole more than about half full. I tried raising the extrusion temperature to as high as 290°C but it did not seem to make much difference.

Print Instructions

Print in PLA. The gcode for this print is in the “-mod” gcode file. You can edit that file to change parameters in the code section shown above.

CAD

The OnShape 3D CAD file for this is here :

https://cad.onshape.com/documents/ff559b49ba5d90d0d8ae09b5/w/e012d2e77ee2370748ffc36c/e/ebc60841af4fb2fd27d1c607