Efficiency Research of Conformal Channel Geometries Produced by Additive Manufacturing in Plastic Injection Mold Cores (Inserts) Used in Automotive Industry

Abstract

In the production of geometries that traditional methods cannot produce, it is seen that additive manufacturing (AM) technology, which has come to the fore, has been used extensively in conformal cooling channel (CCC) applications in recent years. This study, conducted within the scope of CCC's use of applied mold cores in automotive industry plastic part production, aimed to reduce the cycle time in the injection printing process. The v1 geometry, which gives the analysis results for ideal printing quality from the channel geometries developed with three different design approaches, is produced with direct metal laser sintering, which is an AM laser sintering technology, and the injection printing cycle time has been reduced by 38%. CCC applied the study's primary motivation to develop duct geometry to provide balanced cooling in the automotive industry's mold cores produced with AM. It is known that the Computer Numerical Control machining process in traditional mold methods does not allow the processing of the channels in the internal geometries, and the deep areas where the heat is concentrated cannot be cooled sufficiently. In the study, CCC geometries where AM design parameters are used effectively and the balanced cooling performance expected from the die core can be achieved. The effects of different geometries on production are discussed.

Introduction

Plastic injection molding (PIM) is a highly efficient and manufacturable manufacturing technique highly preferred in plastic part production.^1–3 The injection molding method is used to produce parts of a wide variety of sizes and complex shapes. Depending on the design of the mold and the geometry of the plastic part to be produced, product quality can be improved and cycle times reduced in the injection molding process.⁴ This technique is preferred especially in various fields such as automotive, aviation, biomedical, packaging industry, electronics, and toy production.^5,6 Although many parameters affect the injection molding cycle, it is known that the most crucial parameter economically is cooling.^7–10

In traditional methods, straight holes are drilled in solid molds for cooling purposes. In the use of cooling channels in molds, methods such as drilling, electrical discharge machining, and boring tools are known to be used. Conventional cooling channels used in injection molds are generally made by opening inclined or straight parallel holes on the base surface. While this method is used effectively in surface cooling in these systems and does not cause any problems, the core part is insufficient cooling.

The extremely high temperature in the core parts must be lowered to a certain temperature for the plastic part to be produced successfully. The uneven heat distribution between the surface and the core increases the mold material's stress and product distortion. The product's compliance and quality to the dimensional tolerances directly depend on the mold's temperature cycle profile. While conventional cooling channels are sufficient for simple geometries, they fall short for complex geometries.^11,12 In addition to the cooling rate, the cooling process will also affect the fluidity and filling ability of liquid plastic, the appearance quality, and the dimensional accuracy of plastic products.¹³

Conformal cooling channels (CCCs) offer an excellent alternative for problems that traditional methods cannot answer for complex design cooling channels. In CCC applications designed following the mold geometry, homogeneous cooling can significantly reduce the cooling times. The CCC dramatically improves the performance of molds.^14,15 Additive manufacturing (AM) technology for CCC production stands out as an up-and-coming technology.

AM technology, also known as three-dimensional (3D) printing, has been essential in recent years.^16,17 With the advances in AM technologies, this technology has also been used in mold production.¹⁸ It is possible to redesign molds with AM technologies. Thus, higher quality products can be produced with the design of higher performance molds.¹⁹ CCC, which cannot be produced traditionally with AM technologies, can be produced quickly. Also, recent advances in 3D printing technology make the CCC manufacturing process more economical and commercial.

AM processes can also facilitate injection molding process optimization.^20–22 CCC containing molds or cores (inserts) can be successfully produced using methods such as selective laser melting (SLM) and selective laser sintering.^23–25 The direct metal laser sintering (DMLS) method is an SLM technology where products are formed by sintering metal powders layer by layer using direct computer aided design models.²⁶ In this study, the CCC applied to die core production was made with DMLS.

With CCC systems produced with AM methods, it is expected to replace straight hole cooling molds in the future, as part quality can increase significantly, increase production rates, and reduce production costs. Plastic molding processes, such as injection molding, are an essential industrial manufacturing method that produces most of today's plastic products. Unique cooling systems are required to improve the cooling performance of the molds. CCC stands out at this point.

The CCC concept will play a significant role in producing injection molded parts in variable thicknesses, sizes, complex shapes, partially thick volume flat parts, complex large automotive parts, and (local) thin-walled parts. Besides plastic molding, metal mold forming is another vital production area where CCC plays an essential role in formation quality and cycle time. The application areas of metal mold forming mainly include die extrusion, casting, hot forging, hot stamping, and sheet metal forming. Using CCC molds in a metal die casting, the cooling capacity effectively increases, thus reducing the need for extra spray cooling.

The use of air instead of water as the refrigerant can also provide an excellent cooling effect. Therefore, the casting process of Al can be safer due to the elimination of potential explosion hazards. Also, the use of CCC molds can improve the fabricated part's tensile porosity and surface quality. The primary purpose of using CCC is to provide uniform and rapid cooling. Even if the ducts are compatible, an appropriate design is essential to achieving this goal, as an improper design may not improve cooling performance. Using CCC can reduce cycle time by up to 70% as well as significantly decreasing shape deviations. The more complex the core mold, the more difficult it is to cool with conventional cooling channels and the greater the potential to reduce cooling time with CCC.²⁷

Products with functional fluid channels are widely used in various fields, including aerospace, automation, and biomedical applications. The production of these products with complex fluid channels often harrows conventional manufacturing methods; therefore, some fluid flow functions must be sacrificed to realize a viable and smooth production. AM provides a new way to produce products with complex internal channels. It allows the desired design for existing products with normal internal channels currently limited to traditional production methods.²⁸

Studies have shown that using CCC in PIMs reduces the cooling time by ∼22%, and the total cycle time is reduced from 44 to 37 s. It has also been shown that the amount of distortion is reduced by 50% by using a CCC.²⁹ Given the benefits of AM technologies, the rapid development of AM has attracted significant attention. For example, a 2019 study of the 3D printing industry found that China's 3D printing market scale reached Renminbi 2.36 billion in 2018, and that figure has increased by nearly 42% since 2014. The Wohlers report shows that AM application rises every year in the automobile, aerospace, and industrial machinery industries. The automotive sector is one of AM technologies' most crucial usage areas, with a share of ∼20%.³⁰

The choice of cooling method is one of the essential parameters in injection mold design. However, an improper cooling system causes many consequences, such as differential shrinkage and warping in the molded part. Injection molding is the most typical method used for plastic materials. Components obtained from PIM are used mainly in the automotive industry in the modern world. For this reason, a high-performance cooling system is essential for efficient heat removal for a mass production mold. It is well known that rapid cooling reduces the production cost by shortening the cycle time, improving the quality by minimizing the differential shrinkage, internal stresses, and warping on the manufactured parts.

However, it has been challenging to achieve minimum cooling cycle time and maintain uniform temperature distribution through the cooling system's conventional design. Today, research in this area can be divided into two main groups: optimizing conventional cooling channels (straight hole cooling channels) and designing a new cooling channel architecture for the injection mold (compatible cooling channels). Research shows that one effective way to reduce thermal stress on molded parts is to design and produce compatible cooling channels.^31,32

Much professional software such as commercial simulation software brands have been developed for CCC design and cooling performance. CCC, cooling processes, product quality, and tool life can be optimized.^33,34 The design of cooling systems is a fascinating topic for both industry and academic studies. These studies' primary purpose is to shorten the cooling cycle times, provide stable part surface temperature, and increase product quality.³⁵ Cooling the mold core (core insert) is one of the biggest problems in injection molding applications. Although baffles, bubblers, and metal inserts are used to cool the core, these methods remain structurally weak.

At this point, AM technologies provide a significant advantage.³⁶ Automotive companies, in particular, have significantly improved their sustainable production practices by applying new methods in product design. It has been found that the automotive industry is shortening product platforms and constant design changes have the most significant adverse environmental impact in all sectors. This is due to the shortened life cycles of motor vehicles and the need to design and manufacture new tools, dies, and molds necessary to produce these new models and discard old tools, dies, and molds. Therefore, CCC is essential not only in terms of cost but also in terms of sustainability and the environment.³⁷

In this study, the effects of three different channel geometries (CCC) designed to be used in the injection mold core of a plastic product used in the automotive industry on productivity were investigated. The study, in which three geometries in different parameters are handled comparatively, includes selecting the model with the highest efficiency values and producing with DMLS.

Experimental

In creating channel geometries, the channel volume value of 8.8 cm³ has been kept constant in all duct designs based on three different approaches. In the V1 channel geometry determination, efficient cooling is aimed with linear channel geometries; modeling was done with the simplest possible channel geometry selection. In V2 geometry, it is aimed to create more coolant channel volume in the upper part of the mold, which is the surface where the most intense heat accumulates; In the V3 model, a turbulent-intensive modeling method was preferred (Table 1). According to the Moldflow analysis results, Model 1, which has the most efficient result in many parameters, was produced with maraging steel (MS1) material in the DMLS production system. MS1 is a steel material used extensively in mold production. The mechanical and thermal properties of the material are given in Table 2. If it is necessary to return to research, the study mold core dimensions produced by AM can be given as 79 × 84 × 66 mm and its volume as 236.56 cm³.

Table 1.

Maraging Steel Mechanical and Thermal Properties

Tensile strength in the horizontal direction (XY)	1200 ± 100 MPa	Tensile strength in the vertical direction (Z)	1100 ± 150 MPa
The yield strength in the horizontal direction (XY)	1100 ± 100 MPa	The yield strength in the vertical direction (Z)	930 ± 150 MPa
Modulus of elasticity in the horizontal direction (XY)	150 ± 25 GPa	Modulus of elasticity in the vertical direction (Z)	140 ± 25 GPa
Elongation at the break in the horizontal direction (XY)	% (12 ± 4)	Elongation at the break in the vertical direction (Z)	—
Hardness	33–37 HRC
Thermal conductivity (as build)	15 ± 0.8 W/m°C	After heat treatment thermal conductivity	20 ± 1 W/m°C

From EOS.³⁸

HRC, hardness rockwell.

Table 2.

Three Different Conformal Cooling Channel Geometries Developed Within the Scope of the Study

Design process

In the automotive industry, injection mold was used in plastic part production, and CCC modeling studies of the core part to be cooled were performed in Catia V5^© Part Design and Generative Shape Design modules. The rib command was used in the Part Design module to create the channel geometries. In the rib process, the center curve was created with the Generative Shape Design module, and the canal section created in the Part Design module forms the basic canal geometry. An adequate approximation of the channel geometries to the regions to be cooled is provided with the help of a 3D curve. Since the 3D corner feature is not available in the Part Design module, the Generative Shape Design module, a surface modeling module for center curve drawing, was preferred.

Some parameters are essential in the production of internal channels without support in metal AM systems. Although there is the possibility of the unsupported output up to 8 mm in channel geometries (Table 3), there may be distortions in the channel geometry that adversely affect the fluid flow due to the sagging problem.³⁹

Table 3.

Direct Metal Laser Sintering Design Parameters

The ratio between the section and height of the object to be produced should not be more than 8:1	The outside-to-outside dimensions of the design should not be larger than the platform size	Alternative geometries should be used for sizes larger than 8 mm, which are desired to be produced without support42
The minimum hole diameter in the design should not be less than 0.5 mm	The maximum hole diameter to be produced without support should be 8 mm	The minimum diameter for the cylinder geometry used in the design should not be less than 1 mm
Unsupported angles should not be less than 30°	Part dimensional tolerance: ±0.1 mm	The minimum producible wall thickness should not be less than 0.4 mm

From EOS,⁴⁰ Crucible,⁴¹ and Çalışkan.⁴²

Considering the negative effect of large-diameter channel discharges in the mold core on the mechanical strength, it was preferred to determine the channel diameter of 4.4 mm in the first stage. After the analysis results, the desired cooling rate was evaluated, and the channel volume value of the first channel geometry, which was 8.8 cm³, was taken as constant. Since the center curve values are in different lengths in all three models, the canal diameter value was determined to be variable in the range of 3.98–4.4 to fix the canal volume value. In the measurements made, the channel volume value of 8.8 cm³ was obtained with a channel diameter of 4.4 mm in the v1 and v2 models and a channel diameter of 3.98 mm in the v3 model.

In this case, by ensuring that the three models are equalized in terms of volume, the analysis process of the study, in which the three models are compared, has been started. Channel geometries are modeled on the core section and shown in Figure 1a. The part outside the core, shown in gray in Figure 1, is important in the mechanical fixing of the mold to the system and the bedding of the core section. In addition, cooling liquid connections are provided in this section.

FIG. 1.

(a) Plastic injection mold and CCC-applied (insert) core are shown in blue. (b) The usage view of the mold core and noncore part used for fixing the core in the injection system. CCC, conformal cooling channel.

Mold components consisting of fixed and movable parts appear together in Figure 1a. The left side indicated in Figure 1b with a red frame is the fixed side of the mold in the automotive industry; the right side is defined as the moving side. In addition, the movable mold side (Fig. 1b, right) forms the inner part of the part to be produced, while the fixed side (Fig. 1b, left) forms the outer part.

Flow analysis

The cooling process in plastic injection systems involves a complex heat transfer process. Mold cooling analysis mainly aims to find the temperature distribution on the produced and mold cavity surfaces.⁴³ In analysis studies, it is expected that the heat generated in the mold during the production process and the heat removed from the mold through cooling channels are in balance. This situation is calculated by the heat balance equation⁴⁴ Eq. (1):

$${\dot{Q}}_m+{\dot{Q}}_c+{\dot{Q}}_e=0$$ (1)

In the formula, ${\dot{Q}}_m$ is the heat content of the melting plastic; ${\dot{Q}}_c$ is the heat lost by the refrigerant; ${\dot{Q}}_e$ represents the change in the surrounding heat flux.

In injection systems, the molten plastic material is injected into the mold and cooled in the mold, and a solidification process occurs. The rapid solidification process may cause deformation, in part. Therefore, the balanced and homogeneous development of the cooling channel cooling is essential for the risk of distortion and shrinkage of the produced part. The following equation is used to calculate the average temperature distribution of a molded plastic part in the in-mold cooling process³⁵:

$$k\left(\frac{{\partial }^{2}T}{\partial X^2}+\frac{{\partial }^{2}T}{\partial y^2}+\frac{{\partial }^{2}T}{\partial z^2}\right)=0$$ (2)

Flow problems include momentum, mass, and energy conservation solutions. In this context, the following equations are essential⁴⁵:

$$\rho \frac{\partial v}{\partial t}=-\nabla .\rho +\left[\nabla .\eta \underset{\_}{\gamma }\right]-\rho \left[v.\nabla v\right]$$ (3)

$$\frac{\partial \rho }{\partial t}+\left(\nabla .\rho v\right)=0$$ (4)

$$\rho .C_p\left(\frac{\partial T}{\partial t}+v.\nabla T\right)=\beta T\left(\frac{\partial p}{\partial t}+v.\nabla p\right)+\eta {\gamma }^2+\nabla \left(k\nabla T\right)$$ (5)

The formula is as follows: ρ is the material density; η is the viscosity; C_p, specific heat; t is time; β, thermal expansion; T, temperature; k represents thermal conductivity.

CCC applied to the mold core in PIMs refers to the cooling channels developed by the mold's region geometry to be cooled. The water or cooling liquid passing through the channels in the determined parameters is designed to ensure a balanced development of the heat output required in the mold. Accordingly, the following formula is used in calculating the heat transfer coefficient of water⁴³:

$$\alpha =\frac{31.395}{d}{R}_e^{0.8}$$ (6)

Moldflow software was used in plastic flow analysis of three different geometries determined within the study's scope. In a comparative analysis, in three different geometries, the ideal geometry was determined by investigating the effect of plastic flow on part quality and cooling channel efficiency. In the analysis study conducted in this context, the primary analysis input parameters are given in Table 4.

Table 4.

Moldflow Analysis Input Parameters

Part raw material type	PA66 GF50
Part raw material identification	Ultramid A3WG10
Inlet pressure	3.5 Bar
Environment channel water temperature	70°C
CCC water temperature	18°C

CCC, conformal cooling channel.

In this study, besides the mold core produced by AM, other mold components are produced by traditional mold methods of this core (Fig. 1). Apart from CCC applied inside the core, different channel applications for cooling purposes can be mentioned in other mold components. External cooler ducts seen in the analysis images in Table 8 were used in all three models with standard geometry and parameters.

Table 8.

Moldflow Analysis for v1, v2, v3 Geometries

The analysis study started with the Reynolds number's determination to determine the three geometries' solution model. The following formula is used in the study:

$$Re=\frac{\rho VD}{\mu }$$ (7)

If the Reynolds number is higher than 4000, the solution model is turbulent, the transition between 2300 and 4000, laminar between 100 and 2300, and stagnated type of coolant flow model if it is less than 100.⁴⁵

Reynold's value of the v1 geometry, 20,609; 16,327 of the v2 geometry; the v3 geometry was calculated as 4475. After this calculation at the beginning of the analysis, the entire analysis process was carried out over the Moldflow software. In the mold core cooling process, balanced cooling creates turbulence in the mold during the geometry determination phase. In this context, it is seen that v1 geometry with the highest Reynolds value obtained in geometry determination is more suitable than other geometries. This finding is confirmed by the analysis studies carried out in this study.

The analysis study was carried out at the stage of mold insert material definition; since the thermal, mechanical, and physical properties of AM MS1 material were not included in the material library, the material definition was carried out by changing the parameter of the standard steel material in the software library. At this stage, the technical specifications given in Table 1 were updated on the steel material and completed the definition. After defining the mold and plastic part, the liquid inlet, and outlet than the boundary conditions, the meshing process was started.

In the mold insert, the CCC fluid inlet pressure was determined as 3.5 Bar. The temperature of the channels, defined as the external cooling channel in Figure 2 and located around the mold core except for the CCC, and which is not considered within the scope of variability in this study, is given as 70°C. These channels are standard in all three models and are modeled in the analysis to simulate the actual situation. However, it was not mentioned in the article because it was not the subject of this research. The temperature data of 18°C were determined as the liquid temperature in the CCC channels, and the meshing stage was started.

FIG. 2.

Structure of mold insert, plastic part, and internal and external cooling channels.

In the mold insert analysis phase, mesh structure quality is essential in terms of simulation results and simulation duration. To resolve an appropriate mesh for the simulations, a mesh independence study was performed for three models (Fig. 3). Mesh data of v1, v2, and v3 geometries are given in Table 5.

FIG. 3.

Computational domain and details of mold insert (a) and plastic part (b).

Table 5.

Mesh Information and Densities of Three Mold Insert Geometries

	v1	v2	v3
Triangles	52,214	102,676	53,464
Connected nodes	26,107	51,336	26,732
Invisible triangles	536	52,214	536
Volume by element types (cm3)	236.784	473.596	236.884
Average aspect ratio	1.57	1.56	1.58

DMLS production

The mold core, whose analysis was completed, was produced using the MS1 powder with DMLS standard production parameters (Table 6).

Table 6.

Direct Metal Laser Sintering Production Parameters

Laser power	Scan speed	Hatch distance	Layer thickness
285W	960 mm/s	0.11 mm	40 μm

Preheat temperature	Laser intensity	Scanning strategy
40°C	67.4716 J/mm3	Rotation by 67° for the next layer

Laser type	Atmosphere	Global beam offset	Part beam offset
Ytterbium fiber	Nitrogen	0.166 mm	−0.055 mm

Maraging steels are an important mold material that shows high strength at high temperatures (up to 500°C), high resistance to cracking, and does not show dimensional changes after heat treatment.⁴⁶ The chemical composition of the powder used is given in Table 7.

Table 7.

Chemical Composition of Maraging Steel Powder (wt %)

Ni	Co	Mo	Ti	Al	C	Fe
17–19	8.5–9.5	4.5–5.2	0.6–0.8	0.05–0.15	≤0.03	Balance

From EOS.³⁸

In mold core (insert) production, the part is positioned vertically on the production platform (Fig. 4).

FIG. 4.

The direct metal laser sintering production process, the final stage of production, and the mold core mounted on the injection machine.

After the production, which has a total production time of 24 h, the parts of the part except for the support area, with 70–110 μ glass beads; by using a steel shot with a diameter of 1000–1400 μ and 8 bar air pressure for about 2 min, the production process was completed without applying heat treatment after the production.

The leading mold components other than the mold core shown in blue in Figure 1 were carried out using traditional mold techniques. AM includes only the production of the mold core. It was preferred to limit the number of AM components to only the CCC-applied part and reduce molds' total cost.

Results and Discussion

In the analysis study, the die core was evaluated in two stages with three different channel geometries. In this context, cooling channel geometries in addition to the first evaluation phase directly related to the channel geometry such as water flow rate, water outlet temperature, outlet pressure, and friction coefficient (Table 9); in the second stage, product-oriented parameters such as mold temperature, solidification time, shrinkage rate, and distortion rate (Table 9) are discussed.

Table 9.

Moldflow Analysis for v1, v2, v3 Geometries

Among the v1, v2, and v3 channel geometries, the geometry that gives the best results in the relevant parameters is indicated with a black frame in the tables. Accordingly, v1 geometry gave the most efficient value in six evaluation parameters, while v2 geometry gave the most efficient value in two parameters. Parameters where v1 parameter is the most efficient, water flow rate (4.550 lt/min), water outlet temperature (20.39°C), outlet pressure (6.844 kPa), friction coefficient (0.0428), shrinkage ratio (8.580), and warpage rate (0.2536 mm) can be expressed as the parameters; where v2 geometry is most efficient are the mold temperature (39.70°C) and solidification time (5.09 s).

Although the water outlet temperature of the v3 geometry resulted as 23.29°C, it was considered positive data in the result evaluation. Still, as a result of other factors and the assessments of the industrial engineers, it was concluded that this situation was caused by residual water accumulation caused by the inconvenience of the flow. For this reason, it is not considered the best value.

The first group evaluation stage directly related to the mold geometry is essential in choosing the v1 channel geometry as the best model in the CCC application (Table 8). In this context, water flow, liquid outlet temperature, outlet pressure, and friction coefficient values are important in terms of the flow and cooling efficiency of the determined geometry. The second stage shows (Table 9) that the v1 channel geometry, which gives the lowest value in the shrinkage ratio and distortion ratio parameters, primarily considered in terms of part quality, also has the expected cooling efficiency in terms of the part quality produced. Therefore, the v1 channel geometry, which gives the predicted cooling values in six basic parameters, is determined as the most suitable geometry for the CCC application.

The distortion ratio given in Table 9 represents the deformation rate of the plastic part produced with the CCC mold due to insufficient heat transfer during production. Accordingly, it is seen that the lowest deformation rate occurs in v1 channel geometry with 0.2536 mm. Other results obtained with the analysis study are given in Tables 8 and 9.

In the evaluation of the analysis results in Table 9 in terms of the quality of the produced plastic part, it can be stated that the part cooling balance is taken into account. Accordingly, it is seen that the green color distribution is more intense in the v2 design in the solidification time color map in Table 9. It can be stated according to Table 9 data that this solidification time advantage, which is caused by the intensive use of v2 channel geometry in the upper part of the mold core, does not occur in the later parameters. Accordingly, in the evaluation of the color map in the shrinkage rate line in Table 9, the red color zones on the outer borders of the plastic parts produced from the v2 and v3 geometries draw attention.

In the color distribution in this line, the more balanced blue and green color range of the v1 geometry is essential in terms of part quality. In addition, considering the part function properties, the red region in the v2 and v3 geometries at the outer boundary is green in the v1 geometry, important for the v1 geometry selection.

Production trials have been carried out by producing the mold core using the v1 geometry (Tables 8 and 9), where the most efficient analysis results are obtained within the scope of production efficiency. Within the scope of field tests, it is seen that the intensity of the red field in the mold core is significantly reduced in the CCC mold core produced by AM in the images obtained from the temperature measurements made with a thermal camera (Table 10) by printing with the mold core produced with computer numerical control connected to the injection system.

Table 10.

Thermal Images of the Conformal Cooling Channel Mold Core and Postprinting Plastic Part Produced by CNC and Additive Manufacturing

Besides, thermal images taken on the printed parts show that the parts' temperature values and cycle time are significantly reduced (Fig. 5).

FIG. 5.

The temperature comparison of the mold core produced by the traditional method and the mold core produced by CCC-applied additive manufacturing. CNC, computer numerical control.

AM production properties and production parameters affect the thermal and mechanical behavior of metallic materials. The effects of process parameters on degradation, residual stresses, and porosity have been investigated in previous studies.

It has been said that increasing laser power and scanning speed increases residual voltage magnitudes and distortions. High porosity can occur in SLM parts due to overheating or insufficient heating. Excessive heat can cause excessive melting, while insufficient heat leads to poor interlayer bonding. Increasing the layer thickness reduces both residual stresses and distortions and causes an increase in undesirable porosity.⁴⁷ It has been reported that a higher layer thickness and/or scanning speed cause a decrease in density and lead to a decrease in hardness.⁴⁸

In general, parts produced with AM have a finer microstructure than those obtained by conventional processes (e.g., casting); therefore, AM parts retain approximately the same ductility, while static mechanical properties (yield and tensile strengths) are generally higher.⁴⁹ Considering the mold steels produced by the powder bed fusion (PBF) method (MS1), high mechanical properties can be achieved (1047.8 ± 56.9 MPa).⁵⁰ AM/As-built specimens have a hardness of 330–403 HV, considerably higher than conventionally produced maraging steels (280 HV).⁵¹ The hardness of the forged maraging steel was found to be 328 (±13) HV. The hardness value of the sample produced with DMLS was determined as 570.5 (±15) HV on average. The forged and heat-treated maraging steel sample measured hardness values are 565.8 (±9) HV. It is seen that the hardness of the samples produced with DMLS is very close to the hardness of the heat-treated forging. It is possible that maraging steel samples produced by AM do not significantly differ in terms of hardness when heat treatment is applied compared with conventionally produced maraging steels.

In addition, there was no significant difference in hardness between the scanning and construction area of the samples produced with DMLS. It has also been said that maraging steels produced with DMLS show better wear resistance than conventionally produced maraging steels.⁵² However, post-treatments (heat treatments, surface treatments, etc.) can significantly change the material's mechanical properties.⁵³ As-built, MS1 tensile properties exceed several hundred MPa of forged maraging steel, while Young's modulus and ductility were comparable and toughness values were reduced.

When aging heat-treated printed samples, hardness and tensile strength were comparable with aged forged maraging steel, while ductility and toughness decreased. The properties of aged or heat-treated maraging steel closely resemble those of aged forged or other conventionally produced maraging steel.⁵⁴

Unlike laser photons, electrons are almost completely absorbed at the initial contact position with the powder and penetrate the material with almost no reflection. Regarding energy absorption, powder bed fusion-electron beam (PBF-EB) is thermal property sensitive. That is, the thermal properties of powder beds play essential roles in PBF-EB. A dust layer in AM technologies limited the energy absorption and melting efficiency due to increased emission. It decreased the thermal conductivity of the material compared with melting on a solid substrate.⁵⁵

In AM/metallic powders, repeated heating and cooling create a steep temperature gradient in the material due to local heating and low thermal conduction. The steep temperature gradient and thermal expansion cause stress to occur in the component. As a result of thermal stress, residual stress, part deterioration, and crack initiation can degrade final part performance. The final performance of the part is highly dependent on process parameters such as laser power, layer thickness, scanning speed, and scanning range.⁵⁶

Future Trends

It is thought that the cost disadvantage of AM technology, which is high in mold production compared with traditional production technologies, can be solved by the core production method in molds, which is the subject of this study. Instead of producing the mold entirely with AM, using AM as a secondary method only in the region where CCCs will be applied will provide advantages in cost and efficiency. In geometries where heat transfer is essential, internal modeling, which traditional methods do not allow, is advantageous in AM technology.

In future studies, developing different channel geometries and determining different geometries in channel sections instead of cylinders can add depth to the subject. The more effective use of AM process parameters in producing inner channel geometries³⁹ and the research done within the surface roughness improvement of the inner channel area are essential in improving the flow and cooling performance. To ensure more widespread use of the core production method, developing techniques that will produce mold cores more economically in AM systems and research to be made in this context. Some production process parameters such as layer thickness can be considered in this context.

Conclusions

Within the study's scope, three different CCC geometries produced by AM were compared in terms of productivity and part production quality, and the data obtained are shared in tables. After the analysis study, the v1 geometry gave the most suitable values for use, and the mold core with this geometry was produced in the DMLS system, and the study was terminated.

In the light of the data obtained through the analysis study within the scope of the research, the following information was obtained within the scope of v1, v2, and v3 geometry comparison:

• It can be said that the v1 geometry is the most suitable duct geometry to work within the scope of the most efficient values given in the parameters of flow rate, water outlet temperature, outlet pressure, friction coefficient, shrinkage, and distortion ratio.

• Reynolds values also confirmed that the most efficient CCC geometry was v1 geometry.

• The difference between the water inlet and temperature was compared, and it was determined that the v3 geometry was not suitable for CCC.

• When the water pressure losses were compared, it was determined that v1 geometry showed the lowest loss, whereas v3 geometry gave the highest loss.

• When Moldflow analyses are examined, it can be stated that v1 geometry, which provides the least volumetric shrinkage rate, is preferable in terms of homogeneous and balanced cooling.

• Within the scope of the study, the most suitable geometry for the mold core with CCC applied is v1 geometry, and v2 geometry is a geometry that creates a cooling advantage but needs to be developed due to the rapid effect. It was concluded that v3 geometry is an inefficient geometry.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received.