Material selection for the production of injection moulding tooling by Additive Manufacturing

As one of the first major markets for metal additively manufactured products, the importance of the tooling industry has long been recognised. There is still, however, limited information available on what mechanical properties can be expected for the various materials used. This report by Harish Irrinki, Brenton Barmore, Kunal H Kate and Sundar V Atre reviews the published data on various steel powders and processing conditions as well as the mechanical properties that have been obtained using the Selective Laser Melting process. [First published in Metal AM Vol. 2 No. 2, Summer 2016 | 35 minute read | View on Issuu | Download PDF]

Material selection for the production of injection moulding tooling by Additive Manufacturing
Fig. 1 Injection moulds manufactured using the SLM process (a) Maraging steel mould (Courtesy I3DMFG) (b) 17-4 PH stainless steel mould (Courtesy Materials Innovation Guild)

The manufacture of tooling for injection moulding is a key business sector that has a significant impact on overall costs and lead times in the product development cycle. Long production lead times, design constraints and the need to cut manufacturing costs have driven the injection moulding industry to look to new technologies for fabricating moulding tooling.

Among the various innovative processes for the production of tooling, the Selective Laser Melting (SLM) process has the potential to address many of the challenges faced by the tooling industry. The SLM process is capable of producing defect-free parts from a variety of steel materials as well as offering the unique ability to introduce conformal cooling. However, it is important for a tooling design engineer to know the material options and corresponding process parameters to examine the suitability of the SLM process in order to obtain mechanical properties that may compare well with conventional handbook data. To this end, this article reviews the published data on various steel powders and processing conditions as well as the mechanical properties that have been obtained using the SLM process.

The current status of the injection moulding industry

Injection moulding is a $170 billion global industry for the manufacture of a multitude of consumer products [1]. In 2010 alone the US plastics industry produced an estimated 7 billion kg of injection-moulded products for applications in packaging, electronics, household goods and biomedical areas [1]. Common materials that are injection moulded include thermoplastics, thermosets, elastomers and filled polymers. More recently, Metal Injection Moulding (MIM) and Ceramic Injection Moulding (CIM) technologies have further expanded the materials design window for the process.

Materials for manufacturing tools for injection moulding are selected depending on the type of polymer, production volume, mould cavity complexity and the type of tool component. Table 1 summarises several types of steels used for manufacturing tools, including carbon steels (1020, 1030, and 1040), tool steels (S-7, O-1, A-2, D-2, H13, and P-20) and stainless steels (420 and 17-4 PH). Additionally, the type of steel selected depends on mechanical properties requirements for the tooling components, such as ejector pins, clamp plates, inserts, cores, spruce bushings, gate inserts, support pillars, mould base plates, lifters, sliders and interlocks [2–10].

Material selection for the production of injection moulding tooling by Additive Manufacturing
Table 1 Steel materials used in making mould by traditional processes [2], [5], [8]–[11]

Injection moulding tools are most widely manufactured with conventional processes such as milling, lathe or CNC lathe. Over the years these conventional manufacturing processes have developed with the onset of computer aided technology used for designing tools, high-speed machining, improved precision and process automation. Although this has led to the faster production of tools, product development cycles are still long and expensive. Tooling costs account for 15% of injection moulded part costs [12]. However, considering global competition and the requirement for shorter manufacturing times, innovative manufacturing methods for tool production such as Additive Manufacturing have been explored to manufacture tools for injection moulding [13–21]. Moulding cycle times account for 35% of the part cost [11, 21], and innovative mould designs and materials using Additive Manufacturing appear to offer the promise for further impacting the cost-per-part produced by injection moulding [22, 23].

One such Additive Manufacturing process used to manufacture tools for injection moulding is known as the Selective Laser Melting (SLM) process, alternately known as Laser-Powder Bed Fusion (L-PBF), Selective Laser Sintering (SLS) and Direct Metal Laser Sintering (DMLS) [12, 16, 20, 24]. Fig. 1 shows an example of a tool manufactured using the SLM process for the injection moulding of plastics. The tool was fabricated using a maraging steel powder and is used for making injection moulded plastic cable connectors that are complex in shape and difficult to manufacture using conventional techniques [13].

In order to manufacture injection moulding tools using SLM, it is critical for the design engineer to have an awareness of the various material options and the corresponding process conditions to obtain useful mechanical properties from the process. Variations in powder characteristics and process parameters will affect the mechanical properties of tools [15, 18, 25, 26]. Many independent research studies have shown the successful fabrication of fully dense components using the SLM process for various steel powders by changing process parameters [28–32]. This report reviews over a hundred sources from the literature that cover different types of steel powder and SLM process conditions to successfully manufacture parts. Further, material properties typically obtained from the SLM process such as density, hardness, yield strength, ultimate tensile strength and elongation are compared to properties obtained from Metal Injection Moulded and wrought components. Additionally, SLM process conditions such as laser power and scan speed that are typically used for various types of steel powders in order to obtain competitive mechanical properties of fabricated components are summarised. It is hoped that this report will provide a convenient starting point for a tooling design engineer to select material and process options for fabricating injection mould tooling using the SLM process.

Materials

Material selection for the production of injection moulding tooling by Additive Manufacturing
Fig. 2 Relative emphasis of steels reported in the literature using the SLM process

The pie chart in Fig. 2 represents around a hundred SLM studies that have used steels powders of various compositions. It was observed that the most researched steel powders were 316L and17-4 PH stainless steels followed by H-13 and M-2 tool steels. In contrast, only a limited amount of SLM studies have been reported on using P20, T15 and A6 tool steels. The material compositions of steel powders used in the SLM process are listed in Table 2.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Table 2 Material composition of steel powders used in different AM processes

Powder characteristics

Table 3 summarises powder characteristics (shape and size distribution) for five types of steels from 25 sources and presents typical sintered densities (represented as % theoretical) obtained from the SLM process when different types of powder production routes and particle size distributions are used.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Table 3 Densities obtained for various types of gas and water atomised steels manufactured with the SLM process

It can be seen that, for various types of steels, densities between 95 and 99% are achievable for parts processed with the SLM process. For parts fabricated from 316L stainless steel powders, most research groups studied gas-atomised powders with powder size distribution of 0-60 µm and obtained 99.5 ± 0.3% density. In the case of 17-4 PH stainless steel, gas and water-atomised powders were used with powder size distribution of 0-45 µm and theoretical densities of 98.5 ± 1.3% were obtained. In contrast, a coarser particle size distribution of 50-150 µm has been used to used manufacture parts from H13 tool steels with the SLM process resulting in densities of 90 ± 3% and 80 ± 3% for gas and water atomised powders, respectively. For M2 tool steel powders, densities of 99 ± 0.8% and 95 ± 4% were achieved when gas and water-atomised powders of powder size distribution 50- 150 µm were used.

The extent of influence of powder production techniques, namely gas versus water atomisation, on the sintered density obtained from the SLM process showed conflicting results. For instance, parts produced from 17-4PH stainless steel using gas and water atomised powders had a similar density of around 98.5%, but parts manufactured from M2 tool steels showed that the use of gas-atomised powders resulted in parts with higher density (99 ± 0.8%) when compared with water-atomised parts (95 ± 4%). Therefore, it can be noted that the composition of steel and powder characteristics could largely affect the densification and consequently material properties of SLM parts. It was evident from the survey that an important knowledge gap exists in the SLM literature regarding the influence of particle size distribution, alloy composition, surface chemistry and packing density on process conditions, microstructures and mechanical properties.

Hardness

The most common mechanical property reported in the literature for various steels was hardness. Fig. 3 shows the hardness of various steels obtained using the SLM process. Data collected from nearly 70 studies were compared to the corresponding data obtained from wrought and MIM products. It was found that the hardness values of 316L stainless steel and M2 tool steel were the most reported data in the literature. Components fabricated using the SLM process exhibited comparable hardness values to those of MIM and wrought parts for all alloys with the exception of A6 tool steel. Fig. 3 also shows that 316L stainless steels components have the lowest hardness values and M2 tool steels have the highest hardness values. Additionally, P20 and H-13 tool steels, that are typically used in manufacturing injection moulding tools, also showed comparable hardness values for SLM, MIM and wrought parts.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Fig. 3 Literature data on the hardness of steels fabricated using the SLM process

Table 4 summarises the average and standard deviation of hardness values for SLM, MIM and wrought parts based on the above data. It was observed that A6 tool steel had a rather low hardness of 260 ± 40 HB when fabricated using the SLM process [67]. The hardness values of SLM samples fabricated from 316L and 17-4 PH stainless steel were 230 ± 40 HB and 360 ± 40 HB respectively and were comparable to the wrought and MIM hardness values. Among stainless steels, 420 stainless steels had the highest hardness value of 470 ± 50 HB when processed using SLM. Among tool steels, M2 had the highest hardness (730 ± 30 HB) when processed using SLM. Moreover, M2 and H13 tool steel showed suitable compatibility with the SLM process since it was possible to achieve hardness similar to the wrought and MIM values.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Table 4 Literature data on the hardness (HB) of steels produced by wrought, MIM and SLM processes

Ultimate tensile strength

Fig. 4 shows the ultimate tensile strength of various steels fabricated using the SLM process. The data were collected from nearly fifty studies and the strength values were compared to data obtained from wrought and MIM processes. 316L and 17-4 PH stainless steel strength values had the most reported data in the literature. Stainless steel components fabricated with the SLM process exhibited comparable ultimate tensile strength values to those of MIM and wrought parts. Fig. 4 shows that 316L stainless steels components have the lowest ultimate tensile strength values and H13 tool steels have the highest strength values. Additionally, 420 stainless steel and H-13 tool steels that are often used for manufacturing tooling for injection moulding also showed ultimate tensile strength values using SLM that were comparable to MIM and wrought parts.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Fig. 4 Literature data on the ultimate tensile strength of steels fabricated using the SLM process

Table 5 presents the average and standard deviation of ultimate tensile strength values for SLM, MIM and wrought parts. The ultimate tensile strength of SLM parts fabricated using 316L and 17-4 PH stainless steel samples were 550 ± 20 MPa and 1080 ± 30 MPa respectively and were comparable to the wrought and MIM ultimate tensile strength values. Among stainless steels, 420 series stainless steel had an ultimate tensile strength value of 1600 ± 50 MPa when processed using SLM. Among tool steels, H13 tool steel had the highest tensile strength value of 1850 ± 25 MPa when processed using SLM.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Table 5 Literature data on the ultimate tensile strength of steels produced by wrought, MIM and SLM processes

Yield strength

Fig. 5 shows the yield strength of various steels compiled from nearly fifty studies that used the SLM process. These values were compared to yield strength values obtained from wrought and MIM processes. The majority of reported yield strength data were for 316L and 17-4 PH stainless steels. Stainless steel components fabricated with the SLM process exhibited comparable yield strength values to those of MIM and wrought parts, with the exception of 420 stainless steel which showed lower values. Fig. 5 shows that 316L stainless steel has the lowest yield strength values and H13 tool steel has the highest yield strength values. Additionally, H-13 tool steel that is typically used in manufacturing injection moulding tools also showed yield strength for SLM parts that were comparable with MIM and wrought parts.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Fig. 5 Literature data on the yield strength of steels fabricated using the SLM process

Table 6 summarises the average and standard deviation of yield strength values for SLM, MIM and wrought parts. The yield strength of SLM fabricated 316L and 17-4 PH stainless steel samples were 350 ± 20 MPa and 700 ± 30 MPa respectively and were comparable to the wrought and MIM yield strength values. Among stainless steels, 420 stainless steels had the highest yield strength value of 800 ± 150 MPa when processed in SLM. Among tool steels, H13 had the highest yield strength value of 1450 ± 25 MPa when processed by SLM.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Table 6 Literature data on the yield strength of steels produced by wrought, MIM and SLM processes

Elongation

Fig. 6 shows the elongation (%) data of various steels compiled from nearly 50 studies obtained using the SLM process. The data were compared with elongation values obtained from wrought and MIM processes. The majority of elongation data from the literature were obtained for 316L and 17-4 PH stainless steels. Stainless steel components fabricated with the SLM process exhibited comparable elongation values to those of MIM and wrought parts with the exception of 420 stainless steel. Fig. 6 shows that 316L stainless steel had the highest elongation values and H13 tool steel had the lowest elongation values. Additionally, 420 stainless steel and H-13 tool steel that are typically used in the manufacturing of injection moulding tools also showed low elongation values for SLM comparable to MIM and wrought parts.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Fig. 6 Literature data on the elongation of steels produced by the SLM process

Table 7 presents the average and standard deviation of elongation (%) values for SLM, MIM and wrought parts. The elongation values of SLM fabricated 316L and 17-4 PH stainless steel samples were 20 ± 10% and 15 ± 5% respectively and were comparable to the wrought and MIM elongation values. Among stainless steels, 420 stainless steel had lowest elongation value of 2 ± 1% when processed by SLM. Among tool steels, H13 had an elongation value of 6 ± 2% when processed by SLM. However, no conclusions can be drawn for other steel samples fabricated by SLM due to a lack of data reported in literature.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Table 7 Literature data on elongation of steels produced by wrought, MIM and SLM processes

Microstructures

Studies that examined the microstructures of SLM fabricated steel parts are summarised in Table 8. The purpose of the table is to show the typical microstructures observed in SLM fabricated steel parts to achieve the desired mechanical properties mentioned in Table 8.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Table 8 Typical microstructures observed in SLM fabricated steel parts and their effect on mechanical properties

In 316L stainless steel parts fabricated by the SLM process a duplex microstructure with austenite and ferrite was typically found. This duplex microstructure resulted in parts with improved tensile strength and ductility. In SLM fabricated 17-4 PH stainless steel parts, the microstructures typically had a presence of martensite and metastable austenite that may have contributed to the tensile strength and hardness but produced parts with lower ductility. Heterogeneous martensite, austenite and ferrite phases were typically found in 420 stainless steel parts and such microstructures resulted in improved tensile strengths. In H13 and M2 tool steels, the SLM fabricated parts generally displayed both martensite and austenite phases. Additionally, carbide phases were generally found in the microstructure and resulted in the production of parts with desired properties. However, not much research has been reported on the effects of size, morphology and packing density of the powders on the microstructures and mechanical properties of steel parts.

Fig. 7 shows examples of quite different microstructures obtained for parts manufactured with the SLM fabricated parts when different powder sizes and shapes were used under the same processing conditions, to illustrate the importance of the scientific gap that needs to be addressed in the future.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Fig. 7 Microstructures of the SLM 17-4 PH stainless steel samples produced from (a)gas-atomised powder D50 = 13 µm and (b) water-atomised powder D50 = 24 µm (Laser power: 150 W, scan speed: 1550 mm/s) [60]

Process conditions

Process parameters reported for the SLM process for various steels were examined from around nearly a hundred studies in order to associate them with the obtained mechanical properties. The most common SLM process conditions that were reported were laser power, scan speed, scan spacing, layer thickness and laser beam diameter. Fig. 8 provides a comparison of laser power and scan speed that were reported for various types of steels in order to identify starting points for specifying process condition window.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Fig. 8 Laser power and scan speed explored in the SLM of steel powders

From Fig. 8, it can be seen that the reported values of typical laser power ranged from 50-200 W and scan speed values varied from 50-1200 mm/s for various types of steels. Additionally, it was observed that, for slow scan speeds (<350 mm/s), typically low laser powers (<100 W) were used and, with additional increase in laser power, a wide range of scanning speeds was used to selectively melt the steel powders. Out of all the process conditions reported for steel powders, the most broadly studied process window was observed for 17-4 PH stainless steels, while the least number of studies were for H13 tool steel. Within the dataset of reported process conditions, a relatively higher laser power was used for fabricating components from 420 stainless steel and M2 tool steel compared to 316L and 17-4 PH stainless steels.

Table 9 summarises the typical mechanical properties that can be obtained for four types of steel powders for laser power of 50, 100, 105, 195, 200 W and scan speed values between 50 mm/s and 1200 mm/s. In order to understand the evolution of mechanical properties of printed parts with process conditions, the majority of the studies focused primarily on laser power and scan speed. To standardise comparisons for process parameters used to print a part with the SLM process, beam diameter values of 30 ± 5 µm, scan spacing values of 100 ± 15 µm and layer thickness of 50 ± 20 µm were taken as a basis. It was noted that the majority of the studies failed to report powder characteristics of the steels and, hence, the influence of particle attributes on process conditions and mechanical properties could not be considered in this analysis.

Material selection for the production of injection moulding tooling by Additive Manufacturing
Table 9 Summary of mechanical properties of steels with corresponding process conditions in terms of laser power (W) and scan speed (mm/s)

For 316L stainless steel powders, when the laser power was varied between 35-100 W and scan speed between 50-800 mm/s, the ultimate tensile strength values ranged between 500-600 MPa, the yield strength was between 300-450 MPa, and the elongation was between 10-30% . For 17-4 PH stainless steel powders, when the laser power was varied between 35-200 W and scan speed between 50 -1200 mm/s, the ultimate tensile strength values ranged between 1000-1100 MPa, the yield strength was between 550-700 MPa and the elongation was between 5-20%. For H13 tool steel powders, at a laser power of 200 W and scan speed between 500-800 mm/s, the ultimate tensile strength ranged between 1750-1900 MPa, the yield strength between 1200-1500MPa, and elongation between 4-9%. For 420 stainless steel powders, at a laser power of 200W and scan speed varied between 500-1000 mm/s, the ultimate tensile strength values ranged between 1500-1650 MPa the yield strength was between 700-900 MPa and the elongation was between 1-3%. However, for M2 tool steel powders; when the laser power was 200 W and the scan speed varied between 50-200 mm/s, the hardness was between 550-850 HB.

Conclusions

This report surveyed the use of SLM for the fabrication of components using tool steels (H13, M2, A6, P20, T15) and stainless steel (316L, 17-4 PH, 420) powders. It is evident that steel powders processed by SLM can attain mechanical properties comparable to wrought or MIM properties.

Only limited sets of processing parameters have been reported in the literature that provide a useful starting point for studying any steel alloy. However, a detailed understanding of the influence of process parameters on mechanical properties and microstructures of SLM steels is clearly lacking.

The SLM of steel gas-atomised powders has received a lot of interest. However, there have been relatively few studies reported using water-atomised powders in the SLM process. The main difference between the two types of powder is their particle shape. However, the accompanying influences of particle size distribution, surface chemistry and packing density on ensuing microstructures and mechanical properties have not received much attention. Steel powders vary widely in size and shape. As a consequence, processing conditions in the SLM process would need to be adjusted in order to obtain desired properties. Choosing the optimum parameters for a desired application can reduce the production time as it reduces the number of trial experiments. However, based on this review, the selection of process parameters depending upon variation in powder characteristics is another scientific gap that needs to be addressed in the future.

Acknowledgement

The authors thank the Walmart Foundation for funding this work.

Authors

Harish Irrinkia, Brenton Barmoreb Kunal H. Katea and Sundar V. Atrea

a Materials Innovation Guild
University of Louisville
Louisville
KY 40292
USA

b Mechanical, Industrial and Manufacturing Engineering
Oregon State University
Corvallis
OR 97330
USA

Contact: Sundar V. Atre, [email protected]

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