The layer must be printed in one pass.
Printing cannot be interrupted.
Keep this in mind for your designs.
Explanation of the term "infinity"
Unfortunately, our extruder head used in 3D printing of concrete is not optimised to pause the flow of material and allow the head to move to another location and then resume printing. By comparison, in the case of printing with melted plastic (FDM), this is possible.
The nature of the concrete type means that it would harden inside the head during the pause, especially if we are printing with a combination of a two-component head and printing material. (See more in the article Printing Materials and Printing Head).
In such a case, with two-component material, the beginning of the mixture's hardening occurs a few seconds after it leaves the printing nozzle. Hopefully this will explain that when designing an object, we must consider that one layer of the object will have to be "drawn" by the extruder head in one pass, without interrupting the flow of concrete.
Below are some techniques for achieving this.
The start and finish of a print
At the beginning of a print, the robot operator will manually remove the material flow from the extruder head nozzle, for example with a trowel. This happens when the extruder head moves to the starting point of the print on the print bed.
Similarly, when the head reaches the final point of the printing trajectory and moves to the parking position above the waste bin, the material flow is also manually deflected to prevent the printed object's surface from being affected by excess uncontrolled concrete flow during this transition.
The ideal situation is when the print trajectory starts and ends at the same point after circling around the entire perimeter of your object. It then moves on to print the next layer (see the article "Transition between Layers" for more information). Typical use cases for this technique are vases and generally circular and rectangular enclosed objects.
Print tracks meeting
Sometimes, for the purpose of increasing strength or achieving a specific design, it is necessary for the print tracks to intersect at certain points. In such cases, we must ensure that both tracks slightly overlap, with the materials going "over each other" by approximately 3-4 mm. This is particularly important because 3D concrete exhibits dimensional shrinkage of around 1-2%, and we want to avoid any gaps at the intended points of contact.
Double print track
When printing an open (non-closed) object and there is a need to return to the starting point of the print on a trajectory similar and parallel to the initial print path, a double printing track is created.
This technique is also used when a thicker wall for an object is required, more than the maximum diameter of the printing nozzle allows in one pass. In this case, it is necessary to ensure that the second track slightly overlaps the first one (approximately 3-4 mm), to prevent any gap between the tracks during curing and shrinking. Typical object forms with a double printing track are those with a U or V shape.
Typically, this is used when combining the above-described double printing track and a closed circular object. To print in one pass, it is necessary to ensure that the outer track and the inner track "exchange" at a given point.
This crossing also occurs in objects of the "figure-eight" type when the tracks need to cross at one point.
When the crossing occurs at the same height or layer, applying the printing material twice at the crossing point can result in a less desirable outcome. To avoid this, it's preferable to include either a seam, where the track transitions to a new layer, or a spiral transition between layers (vase mode) at this point, depending on the printing needs of the entire object.