2.1.2. Materials

The AM tools are made from Accura Bluestone by the company 3D Systems®, which is processed on a Viper si2 System (SLA). Accura Bluestone is a ceramic filled epoxy composite, which is characterized by relatively high heat deflection temperature of up to 284 ◦C and a tensile strength about 8000 MPa [47]. Due to its material characteristics, high imaging accuracy and low surface roughness, Kampkar et al.[21] recommend Accura Bluestone as an AM tool material.

The thermal conductivity coefficient of Accura Bluestone was determined for this investigation at approximately 0.781 W/mK, which is 50 times lower than the thermal conductivity of conventional steel (34.5–49.3 W/mK) [48] and 150 of aluminum (130–160 W/mK) [49]. Aside from the cooling system, all other tools are identical in their geometry and design. Table 1 shows a brief comparison of the material properties of the different tool materials. As a reference, the same tools are conventionally milled from aluminum (EN AW-7075) and steel (C45 U).

**Table 1.** Comparison of the material properties of the different tool materials (Bluestone, aluminum, steel).


To increase the heat deflection temperature, the Accura Bluestone mold insert is tempered with a temperature profile as recommended by 3D Systems. This is done through thermal post curing for 2 h at 120 ◦C, which increases the deflection temperature from 65–66 ◦C to 267–284 ◦C [47].

Representative for the most used LFT, the glass fiber-reinforced PP injection molding material STAMAX from SABIC is used in different fiber weight percentages of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.%. The specific fiber contents are chosen in accordance with the results of Goris [50], which showed the highest contrast and a clear differentiation for interpretation. Ratios of 30 wt.% and 50 wt.% were not considered to keep the experimental volume adequate. STAMAX is a certified product series for the automotive industry and is already available pre-mixed in many different glass fiber contents. Accordingly, the mixing ratios 20 wt.%, 40 wt.%, and 60 wt.% for the investigations can be obtained directly. The mixing ratio 10 wt.% is gravimetrically prepared from STAMAX 20 wt.% (20YM240) and pure PP, which the manufacturer SABIC uses as a base matrix material. In addition to its wide industrial usage, the material is characterized by its particularly good processability. PP has a wide

processing temperature range with a low melt viscosity, melt temperature, and adhesivity. This is beneficial to reduce the thermal and mechanical stress on the polymer tools and to ensure the possibility to produce enough samples for the evaluation. For the specimen fabrication, a Boy 25 E injection molding machine from Dr. Boy GmbH & Co. KG is chosen.

## *2.2. Experiment Methodolgy*

## 2.2.1. Mechanical Properties

To analyze the mechanical properties, tensile testing after DIN EN ISO 527 is performed. The results are analyzed toward tensile strength, elongation, and Young's modulus. For the calculation of the standard deviation, five samples of each specimen type are analyzed as recommended by DIN EN ISO 527. Nominal fiber contents of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.% are analyzed. In accordance with DIN EN ISO 527, a testing speed of 1 mm/min is used for determination of the elastic properties and, more specifically, the determination of the Young's modulus. For the deformation properties, the testing speed is increased to 50 mm/min for the practical purpose of decreasing testing time. Although not in accordance with the norm, this is a common approach in the field of tensile testing. However, this effect must be considered for the discussion of the test results.

#### 2.2.2. Fiber Length Analysis: Epoxy Plug Method

The epoxy plug method is centered around a down-sampling step after Kunc [40], which reduces the fiber count from <1,000,000 to a representative amount around 15,000–60,000 fibers. The detailed experiments steps are depicted in Figure 4.

As can be seen in Figure 4, the matrix material of the sample is initially removed. A sample diameter of at least twice the initial fiber length must be chosen to avoid measuring fibers that crossed the cutting plane during extraction. An initial fiber length of 15 mm results in a diameter of 30 mm. The matrix removal is performed by pyrolysis at 500 ◦C for 8 h in an industrial oven. Rohde et al. [46] studied the impact of matrix removal by pyrolysis or chemical decomposition on the morphology of single fibers. The results show that performing pyrolysis at 500 ◦C for 2 h is optimal for a PP sample. However, initial test runs showed that the pyrolysis time needed to be extended to 8 h to su fficiently remove the matrix.

For this investigation, the selection of the experimental fiber analysis test method was based on two factors: to provide su fficient repeatability and comparability toward previous investigations, as well as being able to generate an adequate output with a reasonable experiment duration. Therefore, the developed method by Goris et al. [17] (compare Section 1.2) was chosen. Based on the investigations of Rohde et al. [46], a thermal matrix removal is chosen instead of a chemical removal, since the fiber length could be negatively altered. Next, the down-sampling step is performed. A defined amount of UV-activated epoxy B0027N07MM liquid-glue by the company BONDIC (Aurora, Canada) is injected in the exposed fiber bed. The diameter of the injected epoxy varies from ca. 4–7 mm. After UV-curing of the glue and careful removing of non-attached fibers, a second pyrolysis is performed at 500 ◦C for 8 h. The subsequent fiber dispersion step is performed within a dispersion chamber, which can be seen in Figure 5. The turbulent dispersion is performed through small amounts of pressured air, performed 3–4 times at 1 bar for around 0.5 s. The fibers are than dispersed on a 210 mm × 255 mm × 4 mm glass plate, which is positioned on an EPSON Perfection V800 Photo scanner of the company Seiko Epson (Suwa, Japan) to create a digital image at 2400 dpi. For image enhancement and threshold, Adobe ® Photoshop ™ is used. Threshold levels are approximately 40 at the image center and 60 at the edges. This variation is necessary due to inhomogeneous illumination. Then, a MATLAB-based algorithm is used for fiber detection, which was developed at the Polymer Engineering Center (Madison, WI, USA) [51] and is based on the work of Wang [52]. The fiber detection is automated and even detects stacked and bent fibers.

**Figure 5.** Dispersion chamber after Goris adapted from [50].

Two di fferent averages are calculated for investigation of the fiber length: number average and length average. The number average is calculated using (compare nomenclature Table 2)

$$L\_N = \frac{\sum\_{i=1}^n (N\_{i\cdot} l\_i)}{\sum\_{i=1}^n (N\_i)} \tag{4}$$

and the length average (compare nomenclature Table 2)

$$L\_{\mathcal{W}} = \frac{\sum\_{i=1}^{n} \left( N\_{i} \cdot l\_{i}^{2} \right)}{\sum\_{i=1}^{n} \left( N\_{i} \cdot l\_{i} \right)} \tag{5}$$

**Table 2.** Nomenclature.


Long fibers have a more significant impact on the length average than short fibers. This e ffect is described as nonuniformity, of which high values are usually preferable in technical applications. Another e ffect that must be considered during the epoxy plug method is the preferred pickup of long fibers during the down-sampling step. This phenomenon can lead to a distorted average fiber length and is schematically represented in Figure 6. Five fibers are visible, from which only four are picked up by the down-sampling region. The fifth does not contribute to the experimental analysis aside from a similar alignment of the center of mass of each fiber. Therefore, Kunc et al. [40] introduced the so-called Kunc correction, which determines a corrected fiber frequency *Ni* in favor of shorter fibers. It is calculated as:

$$N(L\_i) = \theta(L\_i) \cdot \left(1 + \frac{4 \cdot L\_i}{\pi d}\right) \tag{6}$$

**Figure 6.** Illustration of the Kunc correction procedure, adapted after [40].

For this investigation, the analysis of the fiber length focuses on the analysis of the fiber-reinforced discs. Three segments—A, B, and C—with a diameter of 30 mm ± 0.5 mm are therefore removed by sawing from discs as depicted in Figure 7. The diameter is chosen based on the initial fiber length of 15 mm for STAMAX pellets. For the calculation of the standard deviation, three samples were analyzed for each location. Nominal fiber contents of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.% are analyzed.

**Figure 7.** Disc segments for fiber length analysis.

#### 2.2.3. Fiber Content Analysis: Pyrolysis

The analysis of the fiber content is determined gravimetrically. For this, a pyrolysis in accordance with DIN EN ISO 3451 is performed for each sample within an industrial oven. Each sample is kept at 100 ◦C for 30 min before the matrix is removed at 625 ◦C for 3 h. This represents a less gentle and therefore faster temperature program compared to the pyrolysis steps of the epoxy plug method, since the fiber quality is not important for this experiment. The fiber content is then calculated in accordance with DIN EN ISO 3451 as (compare nomenclature Table 3):

$$
\alpha = \frac{m\_F}{m\_{Total}} \cdot 100 \text{ wt\%} \tag{7}
$$


**Table 3.** Nomenclature.

For this investigation, the analysis of the fiber content focuses on the analysis of the fiber-reinforced discs. The discs are therefore quartered and segmented in 33 squares with an edge length of 11.5 mm ± 0.5 mm as depicted in Figure 8. The samples were then cut out with scissors, resulting in 22 segments with identical in volume for each nominal fiber content, as well as 11 segments from the disc edges with varying volume and sample shape. The varying shape does not constitute a problem for later calculation of the fiber content, since the results are normalized by the individual total sample mass. For the calculation of the standard deviation, three samples were analyzed for each location. Nominal fiber contents of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.% are analyzed.


**Figure 8.** Segmentation of fiber-reinforced discs for fiber content analysis.

2.2.4. Fiber Orientation Analysis: Micro-Computed Tomography

For fiber orientation analysis, μCT scans are used, which are performed with a GE v|tome|x m 240/180 by the company General Electric (Boston, MA, USA). The scan parameters can be seen in Table 4.


**Table 4.** μCT scan parameters GE v|tome|x m 240/180.

The analysis of the μCT data is performed with VGSTUDIO MAX 3.3.0 64 Bit by the company Volume graphics GmbH (Heidelberg, Germany). The software was chosen in accordance with a comparative study by Goris [17], which compared results of the conventional method of ellipses to μCT-data analyzed with three di fferent algorithms: VGSTUDIO MAX 3.3.0 64 Bit, slit method (SM) algorithm, and Mimics (proprietary to SABIC and Materialise MV). The results showed that a minimal resolution of 19 ± 1 μm could be used to su fficiently analyze the fiber orientation using VGSTUDIO MAX. For this investigation, the analysis of isotropic fiber orientation is performed with plane projection. The direction of the normal vector is identical to the thickness direction. Table 5 shows the detailed analysis parameters. As can be seen, the fiber material is defined by specific threshold, which is identical for all samples.

**Table 5.** Analysis parameters VGSTUDIO MAX 3.3.0 64 Bit.


For this investigation, the analysis of the fiber orientation focuses on the analysis of the fiber-reinforced discs. As depicted in Figure 9, a rectangular sample of 18 mm × 70 mm (±0.5 mm) is removed with a saw from each disc. Three squares—A, B, and C—with an edge length of 12 mm ± 0.5 mm are then analyzed within VGSTUDIO MAX. Nominal fiber contents of 10 wt.%, 20 wt.%, 40 wt.%, and 60 wt.% are analyzed. Due to limitations for the experimental amount, only one sample of each fiber content and tool type is analyzed.

**Figure 9.** Segmentation of the fiber-reinforced discs for fiber orientation analysis; Segments A, B, and C (blue) are analyzed within VGSTUDIO MAX.

#### *2.3. Processing Parameters and Tool Design*

The processing parameters for STAMAX are based on the processing guidelines [53] recommended by SABIC. Within these guidelines, a distinction is made between minimum, moderate, and maximum parameters. For this investigation, moderate parameters are targeted to generate comparability to the results of Goris [50]. In accordance with the guidelines, the injection pressure is chosen at 800 bar with linear decline down to 700 bar for steel tooling, with moderate injection speeds of 70 ccm/s to 60 ccm/s. The holding pressure is recommended at 50–80% of the injection pressure. Therefore, a holding pressure of 400 bar is chosen with a linear decline to 350 bar. Since aluminum and Bluestone provide lower mechanical properties than steel, the processing parameters including the clamping force for these tools were lowered accordingly. Table 6 gives a detailed overview of the processing parameters of each tooling type.


The melt temperature is chosen at 250 ◦C with a flat temperature profile within the screw cylinder. As can be seen, the mold temperature is set to 30 ◦C. The cooling process is accomplished through a water-cooling cycle of the same temperature. As soon as the implemented temperature sensors near the cavity reach the targeted temperature, a new cycle can be started.

As described before, geometrical limitations for the cooling channels are small due to additive manufacturing. This is represented in Figure 10, which shows the difference in possible cooling cycle alignment for ejector half metal disc tools (a) and additively manufactured disc tools (b).

**Figure 10.** Ejector half disc tool designs: aluminum and steel (**a**) and Bluestone (**b**). Ejector pins are marked (A).

As can be seen, sealing plugs were used to close the boreholes of the metal tools. For the fixed mold half, the cooling path is analogue to the ejector half. The cooling channel geometry was determined through iterative calculation steps based on fundamental theorems of heat transfer and through calculation of the divergence in mold temperature. Moldflow simulations were used to guarantee a homogenic average mold temperature of the developed concepts, to minimize the effect of different cooling channel geometries on the results. As an example, the simulative results of Bluestone tensile specimen-tools are depicted in Figure 11.

**Figure 11.** Moldflow simulation results of the average temperature for tensile specimen-tools out of Bluestone at 30 ◦C coolant temperature.

As can be seen, the average temperature ranges from approximately 30 ◦C up to 125 ◦C. The highest temperatures can be seen in the stagnation points of the melt at the tensile specimen shoulders. Since for Bluestone, a uniform temperature could not be guaranteed within the whole part, the focus lied on creating a homogenous average temperature within the functional regions, which is the gauge for tensile specimens. The stagnation points (red) were defined as critical regions, which will be discussed further in the following chapter. In case of the tensile specimens, parabolic runners with a diameter of 5 mm and a non-specified gate are used only on the ejector half. For the disc tools, the melt enters the cavity directly through the nozzle without the use of runners or a gate. The striking point of the melt also represents a critical region, especially for disc tools.
