In large-format additive-subtractive manufacturing of polymers, iterative alternation of printing and machining is often required to achieve dimensional accuracy and geometric complexity. However, this strategy is limited by the low strength of interfaces between sequentially manufactured segments. When a new segment is deposited onto a cooled and machined substrate, a cold joint is formed. In materials sensitive to thermal history, this can lead to segment separation already during manufacturing. The aim of this thesis was to design a segment joint that reduces the negative effect of the cold joint and to describe the influence of its geometric and process parameters on the interface strength of PP30GF.
The experimental validation was carried out on a robotic hybrid manufacturing cell that combined large-format pellet-based additive manufacturing with machining. The proposed strategy was based on creating a mechanical interlock in a thin-walled printed segment and subsequently filling it with molten material during the following additive operation. Tensile tests of model specimens were used to evaluate the influence of groove width, groove depth, controlled over-extrusion and manufacturing accuracy of the robotic workstation on the resulting joint strength.
The best group of specimens with the hybrid mechanical interlock achieved a strength of approximately 8 MPa. This corresponded to an approximately fourfold increase compared with a conventional cold joint and to the recovery of approximately 65% of the strength of continuous printing. The results further showed that, within the tested range, a larger residual substrate width relative to the groove width was decisive for higher load-bearing capacity, because critical failure occurred mainly in the machined segment. Groove depth affected the location of critical failure, while controlled over-extrusion increased the load-bearing capacity of the joint.
In addition to describing the influence of the investigated parameters using response surface methodology, the thesis identified key limitations of robotic hybrid manufacturing that significantly affected the resulting strength of the segment joint. The results provide a methodological basis for the use of segmented and iterative hybrid strategies in the production of large polymer parts.