**3. Results**

#### *3.1. Cycle Times Part Output and Failure-Modes*

As can be seen in Figure 12, the recorded temperature near the critical region of tensile specimen Bluestone, aluminum, and steel tools are depicted. The final cycle times are determined through the critical mold temperature of 30 ◦C. As soon as the critical temperature is reached, a new cycle is started.

**Figure 12.** Recorded temperatures near critical region of cavity within Bluestone, aluminum, and steel tools for tensile specimens.

As can be seen, through the immense thermal conductivity of aluminum tools, the temperature in the depicted region fell even lower than 30 ◦C, close to room temperature, before a new cycle could be started. The results show that for every two Bluestone tool cycles, around seven steel cycles and eight aluminum cycles could be run. This directly translates to the increased thermal conductivity of steel and especially aluminum compared to Bluestone. In general, for every Bluestone tool type over 100 parts could be manufactured for all represented nominal fiber contents which includes tool trial runs. However, after a varying number of cycles depending on the tool type, different failure modes could be detected which were assessed as discard criteria or could be temporarily overhauled. The most prominent failure modes and deteriorations are depicted in Figure 13.

**Figure 13.** Failure modes and tool deterioration during specimen manufacturing: demolishing of mold at areas with strong staircase effect (**a**); clogging of channels at fiber contents of 60 wt.% (**b**); abrasion of mold material at the edges resulting in flash on parts (**c**); penetration of mold through fibers and PP at regions of high impact (**d**); contamination of parts through scraped mold material (**e**).

A typical challenge with additive manufactured components is the occurring staircase effect for steep geometries especially around angles of 45◦ paired with high temperatures. Figure 13 shows the critical regions with increased staircase effect (a), which caused demolishing of the mold especially at high fiber contents. For fiber contents of 60 wt.%, the demolishing eventually leads to clogging of channels at critical regions (Figure 13b). A typical deterioration for all nominal fiber contents is the abrasion of the mold material at the cavity edges (Figure 13c), which causes flashes on manufactured parts. This is either caused through underestimation of the clamping force, the flexibility of the mold or as an effect of repeated demolishing through material abrasion and heat exposure. A similar effect can be seen near the center of the mold and the melt entry point. After a non-specified number of cycles, these regions of high impact showed penetration of the mold through fibers and plain PP (Figure 13d). In extreme cases, the scraped mold material was found in the injected molded components (Figure 13e). Parts which showed signs of foreign particles and contamination were declared as rejects and the mold was eventually discarded. For aluminum and steel tools, none of the aforementioned failure modes or deteriorations could be detected.

## *3.2. Mechanical Properties*

Generally, the results for the Young's modulus (Figure 14) and the tensile strength (Figure 15) are increasing for rising nominal fiber contents, while the elongation at tensile strength (Figure 16) is

declining. Typical for brittle materials, the elongation at tensile strength and the elongation at break are basically equal. The highest variation of the results is visible for nominal fiber contents of 60 wt.%.

**Figure 14.** Young's modulus for tensile specimens from Bluestone, steel, and aluminum tools for nominal fiber contents of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.%.

**Figure 15.** Tensile strength for tensile specimens from Bluestone, steel, and aluminum tools for nominal fiber contents of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.%.

**Figure 16.** Elongation at tensile strength for tensile specimens from Bluestone, steel, and aluminum tools for nominal fiber contents of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.%.

The results of the Young's modulus show that comparable values toward experimental data supported by the manufacturer are reached for every tool type and nominal fiber content. Furthermore, Bluestone tool specimens of 60 wt.% nominal fiber content are showing slightly higher values than steel and aluminum tools, although the increased variation of the results must be considered. An almost linear increase of the Young's modulus can be detected with increasing nominal fiber contents, with maximum values around 12,000 MPa.

The results of the tensile strength also meet nearly identical values for each tool type and nominal fiber content. However, there is no detectable increase in tensile strength from 40 wt.% toward 60 wt.%, with maximum values around 110 MPa at 60 wt.%. As described before, the deformation analysis is performed at higher testing speeds than the analysis of the elastic behavior. This must be considered for the comparison of the tensile strength as well as the elongation at tensile strength toward the manufacturers' data.

The elongation at tensile strength shows a constant decline toward higher nominal fiber contents since the material gets increasingly brittle. Aluminum and steel tools show almost identical results. However, test specimens of Bluestone tools show lower values than aluminum and steel tools, especially at nominal fiber contents of 40 wt.% and 60 wt.%. In further analysis, μCT data were able to show an increased number of voids for these nominal fiber contents, which are possibly caused due to limited processing conditions (Figure 17). The polymer matrix is responsible for an improved elongation behavior since stresses in lateral direction can be minimized. Through voids, the elongation and elastic behaviors are locally reduced.

**Figure 17.** μCT image from spattered voids in Bluestone tool specimens for a nominal fiber content and 60 wt.%.

## *3.3. Fiber Length*

The results of the fiber length analysis for a nominal fiber content of 40 wt.% are shown in Figure 18. All other results for 10 wt.%, 20 wt.%, and 60 wt.% are represented in Appendix A. Analogue to the results of Goris [17] for steel tools, the length average of the fiber length reduces with increasing nominal fiber contents. The same phenomenon is visible for aluminum and Bluestone tools. Processing causes severe fiber breakage, which causes the number of long fibers to reduce. With increasing fiber contents, the shear stress and fiber interlocking increases, which causes increased early state fiber breakage. Therefore, the reached average values are far below the initial fiber length of 15 mm, with higher values for the length average compared to the number average. The highest length average variations are visible close to the gate and the entry point of the melt (Location A). This is most prominently visible for steel and aluminum tools at all nominal fiber contents. A possible explanation is the highly disoriented state in which the melt enters the cavity, thus creating a broader variety in fiber length. This theory can be further supported by the length average values, which show higher varieties than number average values. As described before, the length average values tend to be more highly impacted by the existence of a small number of relatively long fibers.

**Figure 18.** Fiber length (N = number average; W = weight or length average) for different tool types (for better representation in two diagrams) with 40 wt.% nominal fiber content at different specimen locations A, B, and C in accordance with Figure 7.

Depending on the locations A, B, and C, the depicted fiber contents show di fferent behaviors with an increasing melt path. In general, the length average values of each tool type show higher fluctuations over the melt path than the number average values, which show a nearly stable length around 0.6 mm for all nominal fiber contents and tool types. This value directly translates to the results of Kim et al. [32], who declared a fiber length between 0.5–2 mm as typical values for long fibers after injection molding processing. Prominently stable results for the length average of each location are visible for Bluestone tools at nominal fiber contents of 40 wt.% and 60 wt.%, with a slight increase over the melt path (location A to C). Steel and aluminum tools show contrarian behaviors for nominal fiber contents of 40 wt.% and 60 wt.%, with maximum values at location A, followed by a decrease over location B to C. For a nominal fiber content of 10 wt.%, the samples of Bluestone tools show a decline in length average from location A to B, while increasing to maximum values of around 1.8 mm in location C. This behavior is also present for steel and aluminum tools, although to a lesser extent and with maximum values for the length average at location A. For 20 wt.% nominal fiber content, samples of Bluestone tools show contrarian behavior compared to 10 wt.%, with high length average values at locations A and B and a prominent decline in location C. A similar behavior is visible for aluminum tool samples, while samples from steel tools show almost identical behavior for 20 wt.% compared to 10 wt.%.

Despite the discussed di fferences in length average for di fferent tool types, the results show that globally similar values for the average fiber length are reached depending of each tool material. Therefore, Bluestone tools can be declared viable for the use in functional validation of steel tools.

#### *3.4. Fiber Concentration*

The results of the fiber content analysis for a nominal fiber content of 40 wt.% are shown in Figure 19. All results for 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.% are represented in Figure 20. The schematic results for 10 wt.%, 20 wt.%, and 60 wt.% are shown in Appendix A. As can be seen, Figure 19 schematically depicts the results for an initial nominal fiber content of 40 wt.%, which show a characteristic increase in fiber content for all three tool types. This phenomenon can be seen for all initial nominal fiber contents. The lowest fiber concentration can be seen around the disc center with values mostly below and around the initial nominal fiber concentration. With increasing flow path, the fiber content increases until reaching above average values within the disc edges. This general increase of the fiber content with increasing melt path is visible for all nominal fiber contents, which is identical to the results of Goris [50]. A possible hypothesis for this behavior is the increase of the shear stress between liquid melt and solidified regions. Two e ffects occur in these regions: fiber breakage and fiber pullout. Through shear forces in the border region, the fibers are carried away by the melt flow and are shortened in this process. In retrospective to the results of the fiber length in Section 3.3, this phenomenon is especially visible on aluminum and steel tools. However, the results of Section 3.3 often show a limited or contrarian fiber shortening behavior for Bluestone tools toward metal tools. This is possibly caused by the slowed solidification and cooling process, as well as the limited processing conditions for Bluestone (injection speed, pressure, and clamping force). The fibers are more likely to be pulled out and carried o ff, with limited fiber breakage due to a more gradually transition between solidified material and the flowing liquid melt.

**Figure 19.** Characteristic fiber content distribution pattern for disc segments of 40 wt.% initial nominal fiber content: steel tool (**a**), Bluestone tool (**b**), and aluminum tool (**c**). The color scheme describes values below (blue) up to values near (white) and values above (red) the initial nominal fiber content (in accordance with Figure 8).

**Figure 20.** Fiber content distribution for disc segments of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.% initial nominal fiber content at different locations (1–33 in accordance with Figure 7) for steel tools and Bluestone tools.

The comparison between Bluestone tools and steel tools for all initial fiber contents is represented in Figure 20. For the comparison of steel and aluminum tools, see Appendix A. As can be seen, for an initial nominal fiber content of 10 wt.%, the overall values for Bluestone tool samples are below the values of steel tools. This can most likely be explained through inaccuracy in the before discussed gravimetric mixture of equal parts 20 wt.% fiber-reinforced PP and plain PP. Regarding this, Bluestone tools and steel tools show nearly identical values and behaviors up to 40 wt.% initial nominal fiber content, with low standard variation. 60 wt.% samples from Bluestone tools show a higher spread toward minimum and maximum values compared to steel tools. This phenomenon is possibly caused by the gentler processing conditions for Bluestone tools. In comparison to the results of aluminum tools (see Appendix A), the results show that the best representation of steel tools for functional validation is through aluminum tools. However, the global fluctuation of the Bluestone tool samples is still low enough to be declared viable for functional validation of steel tools, especially for fiber contents up to 40 wt.%. The characteristic zones are identical for all tool types.

#### *3.5. Fiber Orientation*

As described in Section 1.2, seven characteristic regions can be determined in injection molded parts. Figure 21 shows the orientation tensor components over the relative sample thickness exemplified for a nominal fiber content of 60 wt.% at location B of a Bluestone tool specimen segmen<sup>t</sup> (in accordance with Figure 9). As can be seen, the characteristic zones can be identified (1, 2 and 3).

**Figure 21.** Orientation tensor components (*<sup>a</sup>*11, *a*22, *a*33) over relative sample thickness for a nominal fiber content of 60 wt.% (Bluestone tool; segmen<sup>t</sup> B), with characteristic zones after Osswald and Menges [36] (shell layers = 1, core layer = 2, transition layers/skin layers = 3).

In addition to the identification of the characteristic zones, it is visible that the tensor component *a*33 is constant over the entire relative sample thickness. This is valid for all examined segments. For further analysis of the fiber orientation, only components *a*11 and *a*22 are described because of their relevance toward the identification of the characteristic zones. Figure 22 shows the results for a nominal fiber content of 60 wt.% for Bluestone tools, steel tools and aluminum tools at locations A, B, and C. The results of all other nominal fiber contents can be seen in Appendix A.

**Figure 22.** Orientation tensor components (*<sup>a</sup>*11, *a*22; without *a*33 for better overview) and relative sample thickness fora nominal fiber content of 60 wt.%. Specimens were created with Bluestone, steel, and aluminum tools, with samples from regions A, B, and C (in accordance with Figure 9).

a11 a22

As can be seen, the quality of the results is highly dependent on the sample location. Since B provides the most developed, unhindered, and oriented state of the melt flow, the characteristic zones are identifiable without further complication. Location A provides the most inconsistent transitions compared to locations B and C. A possible explanation for this behavior can be given based on the melt flow state. Since the melt flow is in a highly disoriented state when it enters the cavity, it is most likely that this region shows the highest inconsistencies toward fiber orientation. This behavior is present for all tool types and nominal fiber contents. Location C shows more consistent results than location A. However, due to the material stagnation at the edges of the disc, slight inconsistencies that hinder clear identification of the characteristic zones are still visible. Therefore, the decision was made to concentrate on location B entirely for the identification of specific trends dependent on increasing nominal fiber content, as well as different tool materials. The results can be seen in Figure 23.

**Figure 23.** Orientation tensor components (*<sup>a</sup>*11, *a*22; without *a*33 for better overview) and relative sample thickness for nominal fiber contents of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.%. Specimens were created with Bluestone, steel, and aluminum tools.

It can be seen that there is a global increase of the shell layer thickness with increasing nominal fiber contents. A possible hypothesis for this phenomenon is based on different crystallization behavior for increasing fiber contents. Each fiber in the melt acts as a nucleating agen<sup>t</sup> during cooldown. When more fibers are included, the crystallization process and solidification take place faster, which benefits increasing shell thickness. In terms of applicability for functional validation of steel tools, the Bluestone tools show almost identical fiber orientation compared to steel and aluminum tools. The thickness of the specific zones and the range of the orientation tensors are almost identical for all used tool types, which is essential for the qualification of Bluestone tools for functional validation. However, further repeated experiments are necessary for a sufficient qualification, since only one specimen of each tool type and nominal fiber content was examined during the experiments. Furthermore, the different thermal conductivities of all tool types impact the viscosity of the melt, which must be analyzed to solidify the aforementioned hypothesis.

#### **4. Discussion & Conclusions**

The discussion of the results of this investigation concentrates on two different aspects:


The results of this investigation successfully illustrate, that specimens manufactured with Bluestone tools show comparable results toward metal tools for the mechanical properties and the fiber configuration, which includes fiber length, fiber concentration, and fiber orientation. In terms of the cycle times, the part output and the tool durability, Bluestone tools are not able compete with metal tools. As expected, zones of high impact and choke points proof to be critical for the Bluestone tool durability. A possible solution would be the use of modular concepts that make use of overall usable metal inlets, which can be applied at highly stressed regions. However, for the use of Bluestone tools in spare part and small series production, the reached part output of over 100 parts proves to be sufficient. Due to the similarity in fiber configuration and the mechanical properties, the use of Bluestone tools for functional validation is nearly qualified. Further experiments toward the fiber orientation need to be conducted to solidify the existing results.

Multiple characteristic phenomena could be detected, which are applicable for polymer tools, aluminum tools, and steel tools. For all tool types, the extent of the nominal fiber content influences two aspects: the thickness of the characteristic zones defined by Osswald and Menges [36], as well as the fiber length average. The analysis of the fiber orientation shows that with increasing nominal fiber content, the thickness of the shell layers increases respectively, while the thickness of the core layer decreases. A possible hypothesis for this phenomenon is based on the altered crystallization mechanism for increasing fiber contents. When more fibers are included, the crystallization process and solidification take place faster, which benefits increasing shell thickness. The analysis of the fiber length illustrates that the length average for specimens of each tool material decreases with increasing nominal fiber contents. The number average stays nearly stable for all tool types around 0.6 mm. This value directly translates to the results of Kim et al. [32], which declared a fiber length between 0.5–2 mm as typical values for long fibers after injection molding processing. Another phenomenon is determined through the analysis of the fiber concentration. Analogous to the results for rectangular specimens created through steel tools described by Goris [50], a gradual decrease of the fiber content is visible with increasing melt flow distance. This behavior is visible for steel tool specimens and now for Bluestone tool and aluminum tool specimens as well. A characteristic maximum at the edges of the disc specimens can be declared.

All in all, additively manufactured Bluestone proofs to be a viable approach for the rapid creation of polymer tools for injection molding processing. Further investigations will be centered around the improvement of the polymer tool lifecycle, as well as improved cycle times through efficient cooling methods. The use of modular concepts that make use of overall usable metal inlets needs to be further investigated. At last, the cost efficiency of polymer tools toward metal tools needs to be analyzed for a specific example in a comparative study.

**Author Contributions:** This investigation is based in equal parts on the scientific work of L.K. and R.S., under the supervision of D.R., K.W. and T.O. The work was divided in the following categories: Conceptualization, L.K.; methodology, L.K. and R.S.; software, L.K. and R.S.; validation, L.K. and R.S.; formal analysis, L.K. and R.S.; investigation, L.K. and R.S.; data curation, L.K. and R.S.; writing—original draft preparation, L.K. and R.S.; writing—review and editing, L.K. and R.S.; visualization, L.K. and R.S.; supervision, D.R., K.W., T.O.; project administration, L.K.; funding acquisition, no additional/external funding. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding. The APC was funded by Polymer Engineering Center, University of Wisconsin-Madison, Madison, WI 53706, USA.

**Conflicts of Interest:** The authors declare there is no conflict of interest.
