Use of Molding Mixtures for the Production of Cast Porous Metals

Abstract

This paper aims to present the possibility of producing cast porous metals (or metallic foams) in a low-tech way by the use of conventional foundry technologies, i.e., the common procedures and materials. Due to the technological and economic complexity of the production processes of cast metallic foams, research into this material currently focuses on the development of less demanding technologies. The introduction of such production processes may help to exploit the full application potential of metallic foams. Within the framework of our proposed procedure, molding and core mixtures are used for the production of molds and filler material (space holder), also called precursors. It is the shape, size, and relative position of the individual precursors that determines the shape of the internal structure of the resulting metallic foam. The core mixture for the production of precursors is evaluated in terms of changes in properties with respect to storage time. Attention is focused on one of the most common bonding systems, furan no-bake. Casting tests are carried out for the possibility of making cast porous metals from aluminum alloy with different shapes of internal cavities depending on the different shapes of the filler material. The collapsibility of the cores after casting is evaluated for the test castings. The results show that even using commonly available materials and processes, cast metallic foams with complex internal structures can be produced.

1. Introduction

Nature has been a well of inspiration for mankind since time immemorial. People have always been inspired by what they see around them. Today, the phenomenon of inspiration from nature has its own professional name: bioinspiration. In recent years, this concept has often resonated in scientific publications [1,2,3].

Nowadays, there are several scientific articles [4,5,6,7,8,9,10,11,12,13] and literatures devoted to the problematics of metallic foams, ranging from older sources that describe the basic procedures of their production and evaluations of their properties [4,5,6,7,8] to current knowledge that describes the specific applications of this material [9,10,11,12,13]. In short, metallic foams represent a material that has received enormous attention in the last decades.

The motivation for the experiment presented in this paper is the current absence of metallic foam producers both in the Czech and European markets. There is a high rate of production of castings in the Czech Republic, and the introduction of a new type of material into the production portfolio would increase the competitiveness of the manufacturers. Therefore, the main objective of the experiment is to determine a low-cost process for the production of metallic foams based on conventional foundry processes and materials (molding and core mixtures).

The foundry way of producing this material presents a number of advantages; it is a time- and energy-saving process with the possibility of producing shaped porous parts from a wide range of materials (ferrous and nonferrous alloys). In these respects, even advanced metallic foam technologies such as 3D printing or powder sintering [14,15] cannot compete with foundry processes.

1.1. Properties and Applications of Metallic Foams

Thanks to their porosity, which can reach up to 97% depending on the production technology, these materials have a low density. At the same time, they retain excellent mechanical and physical properties thanks to the base metal material. The resulting properties and, consequently, the application possibilities of these materials depend in particular on the porosity, the degree of interconnection of the individual pores, and their distribution and size [16].

From the metallurgical point of view, it is highly recognized that the heat transfer in distinctive foundry stages of metallic mold materials [17,18] has an important role upon the resulting mechanical properties of the material [18,19,20,21,22]. This is provided by the imposed operational conditions that provoke distinct cooling rates [19,20,21,22], which can be planned in order to attain desirable microstructural array and, consequently, the resulting properties (e.g., mechanical property and corrosion behavior) [19,20,22].

The most important advantages of these unique materials are their low specific weight [23] (possibility of reducing the weight of the structure), their ability to absorb impact energy [24,25] (e.g., in crumple zones of vehicles), and their damping capacity [26] (acoustic barriers). The use of metallic foams is optimal when two or more of their advantages are used simultaneously [6]. In this respect, these materials are particularly suitable for so-called multifunctional applications [27]. Below are selected areas of application of metallic foams based on their properties.

One of the biggest requirements for cars produced today is the reduction of fuel consumption and the associated reduction in emissions and environmental impact. The simplest way to do this is to reduce the weight of the vehicle, i.e., to use materials with low weight or high specific strength, such as aluminum [28] and magnesium alloys [29]. However, when designing smaller cars, we encounter a problem, and that is the reduction of the length of the crumple zones. The need to reduce acoustic emissions from cars has led to a demand for new silencers. Metallic foams, in particular, offer a possible solution to some of these problems. Due to their excellent ability to absorb impact energy, these materials can be used in vehicle crumple zones [30,31].

Metallic foams can also find a wide range of applications in the construction industry. Most of the materials used today are too heavy, or their use is problematic from a fire protection point of view.

Metallic foams could be a suitable material for earthquake-resistant building construction [32]. Materials and composites based on metallic foams have also found widespread uptake in this field of human activity in order to provide the low-cost thermal management of buildings [33].

Depending on the amount, size, and openness of the pores, porous materials exhibit either high (open pores) or low (closed pores) thermal conductivity [34]. This can be used, for example, in the application of metallic foams as heat exchangers [35] or, conversely, in thermal insulators or noncombustible heat shields [36]. It is also possible to let a liquid flow through the open pores of the material, which can accelerate cooling or heating.

In addition to absorbing noise [37], the aluminum foam components can also dampen vibrations very well. The sound wave that hits the porous material is partly reflected and partly enters the structure. Part of this wave is absorbed, and the rest is transferred away. If the average depth of the foam cell is in the millimeter range, this mechanism is only effective at relatively high frequencies (on the order of >300 kHz). With all mechanisms, metal foams can achieve relatively high absorption levels of up to 99% for certain frequencies [38].

1.2. Production Possibilities of Porous Metals

Currently, there are several technologies for the production of metallic foams. In general, these are time-consuming and economically demanding processes, i.e., they increase the final price of the material. This is the reason why, to date, the wide application potential of this material remains untapped [39].

Regarding the state of the starting metal material, the technologies for the production of metallic foams can be divided into four main groups [40,41]. Porous metal materials can be made of powdered metal [42,43], liquid metal [44], metal vapor [45], or metal ions [5].

Currently, the attention of scientific teams is focused on the use of additive technologies in the production of metallic foams [46,47]. These technologies allow the production of materials with very complex internal structures [48]. These technologies are certainly interesting and can be used in a wide range of applications, but there are a number of limiting factors (material, size, time and complexity of production, use of special materials and equipment). Therefore, materials produced in this way cannot be used in mass production.

2. Materials and Methods

The entire experiment presented in this paper is based on foundry technology for the production of metallic foam, specifically, the technology of infiltrating liquid metal into a mold cavity filled with precursors (space holder material). This technology is based on a technology in which a water-soluble material is used as the mold filler material, which creates a complex internal structure in the resulting metallic foam [49,50,51]. Salt or NaCl is the most used precursor material in this technology. The advantage of this process is that the filler material is easily removed from the complex internal structure of the metallic foam, being simply washed away after the metallic foam has been made. However, the problem is the use of NaCl, which can adversely affect the properties of the resulting material [52] (e.g., corrosion resistance). In addition, the issue of disposal of the salt leachate after the leaching of the precursors needs to be addressed.

For this reason, we have proposed a porous metal fabrication process that maintains the principle of liquid metal infiltration into a mold filled with precursors (Figure 1). However, instead of salt particles, precursors made from conventional core mixture are used in this process.

In the framework of previous experiments [53], many materials for precursors have been verified, e.g., material based on Al₂O₃∙SiO₂, shards of cores (Croning technology, SAND TEAM, spol. s r.o., Holubice, Czech Republic), and furan no-bake mixtures (Eurotek Foundry Products Ltd., Elland, West Yorkshire, UK) (Figure 2). The problem with these precursors was the material from which they were made (ceramics), creating an irregular shape (Croning technology, furan no-bake mixture). Based on these experiments, it was determined that the furan no-bake core mixture is a suitable material for precursor production, exhibiting good casting resistance and excellent postcast decay, which is crucial due to the very complex cavity from which the precursor must be removed.

In the experiment, it was first necessary to design and validate a process to produce precursors that would exhibit the same shape and size. The shape and size of the pores has a key influence on the final properties of the metallic foam [54]. Therefore, mastering the technology of filler material production is essential to be able to predict the properties of cast metallic foam [55,56].

2.1. Composition and Preparation of the Mixture for the Production of Cores and Precursors

In order to make the results reproducible, the conditions for the preparation of the furan no-bake core mixture—its composition and mixing procedure—were first accurately determined. Subsequently, the conditions under which the test samples would be stored pending evaluation were also determined.

Precursors and cores (test beams) were made from a furan no-bake core mixture (Eurotek Foundry Products Ltd., Elland, West Yorkshire, UK). The individual materials and quantities selected are shown in

Table 1. Composition of furan no-bake core mixture.

	Basic Sand	Binder	Catalyst
	Foundry silica sand	Furan resin <75% FFA	Based on p-toluensulfonic acid
Name	BG 27 (Biala Góra)	Ecofur 2375	100T5D
Supplier	Sand Team	Mazzon	Hüttenes-Albertus
Dosage [wt. %]	100	1	45 (to the binder content)
Dosage [g]	5000 ± 5	50 ± 0.01	22.5 ± 0.01

.

The mixture of basic sand and catalyst was mixed with a laboratory paddle mixer (Kenwood KM 636 Major Classic, Kenwood, Havant, UK) for 1 min for better homogenization. After this time, binder was added to the mixture, and the mixtures were mixed again for 1 min.

2.2. Procedure for Evaluating the Properties of Cores and Precursors

The mixture was immediately used for the production of cores (test beams) and precursors. The service life of the mixture (so-called lifetime) was 5 min and was measured using a touch strength measuring instrument (DISA Georg Fischer +GF+, Norican Group Global, Taastrup, Denmark).

The handling strength for the disassembly of the cores was reached 2 h after mixing the mixture. The production of specific samples proceeded as follows:

• Beams—Standardized test specimens (beams) with dimensions of 22.5 mm × 22.5 mm × 170 mm (Figure 3a) were made by compacting the mixture into the core box;
• Precursors—For the fabrication of the precursors, 2 special 100-cell (10 × 10) core boxes were designed and 3D printed (Figure 4) to produce 100 cube-shaped precursors of 10 mm × 10 mm × 10 mm (Figure 3b).

A total of 20 beams and 1200 precursors were produced by these processes.

The samples were left in a laboratory environment at a constant temperature of 23 °C and humidity of 20%. Subsequently, selected tests were performed on these samples, which were three-point bending strength tests, abrasion loss tests, and the determination of collapsibility of the cores after casting.

2.2.1. Three-Point Bending Strength (Beams)

To evaluate the properties of the core mixture (furan no-bake), a test series of samples and, subsequently, a “validation series” of samples were tested. An amount of 15 test beams of each series were taken. These 15 samples were divided into 3 groups of 5 each. Each group was then tested under specific conditions at given times after the curing of the mixture (24 h, 240 h, 480 h—the selected core storage times represent the most common core storage times in practice before use due to production, storage, or delivery time). The possible change in strength properties after longer storage of the cores gives us an indication of the suitability of the core production technology for storage or direct use. The mechanical strength of the cores is characterized by a measurement known as the three-point bending strength. This measurement was carried out on a molding mixture strength measuring device (LRu-2e-Multiserw-Morek, Marcyporęba, Poland) under normal laboratory conditions (a temperature of 23 °C and a humidity of 20%). Measurements of the specimens were carried out using a load of 0.25 N·cm⁻². The size of the samples (22.5 mm × 22.5 mm × 170 mm) was given by measurement. A schematic of the measurements and the sample is shown in Figure 5.

2.2.2. Abrasion Loss (Beams)

The purpose of this test was to determine the effect of the storage time of the beam (24 h, 240 h, and 480 h) on the abrasion resistance. This parameter is important with respect to the manufacture of precursors by rubbing.

The evaluation of abrasion loss was performed on the specimens after bending strength testing (i.e., the test specimens were the resulting halves from the broken beam) using a laboratory apparatus with a perforated rotating sieve (Figure 6). The specimens were weighed and then placed in the 180 mm diameter perforated sieve and rubbed for 60 s at 57 rpm. After rubbing, the samples were weighed again. The resulting decrease in weight compared to the original weight of the measured samples gave the percentage abrasion loss values.

2.2.3. Abrasion Loss (Precursors)

In the case of the precursors, these were not standardized samples. The abrasion loss test was carried out on these samples to determine the possibility of changing the shape of the precursor from an initial cube with a 10 mm edge to a shape close to the ideal spherical shape.

The 1200 precursors produced were divided into several test batches of 100 pcs per batch. These batches were weighed and sequentially placed in a laboratory apparatus with a rotary sieve designed for the determination of abrasion loss. One batch was left completely without abrasion in its original state in the shape of a 10 mm × 10 mm × 10 mm cube. Each batch of precursors (100 pcs) was then rubbed in the device for a predefined period (20–140 s). The batch was then weighed again to determine the abrasion value. Rubbing the precursors for different time periods for each batch was carried out to obtain precursors of different shapes, approximating the ideal spherical shape. Because of the assumption of decreasing precursor size, two batches (i.e., a total of 2 × 100 precursors) were always chosen for longer rubbing times (than 80 s).

2.2.4. Collapsibility of Cores after Casting (Beams)

The determination of the collapsibility of the mixture (or of the cores) after casting is a very important indicator. If the core mixture is to be used for the production of precursors, one of the most important parameters that it should meet is that of good collapsibility after casting or the possibility of easy removal from the complex internal structure of the metallic foam.

Test castings were designed to determine this property; their design can be seen in Figure 7. In each casting, two beams were established to serve as cores. After casting, these cores were removed with a special knocking-out mandrel. This was attached to a standard sand ramming machine for making standard rollers from molding compounds (see Figure 8).

The composition of the molding mixture for the production of the molds that made the test castings can be seen in

Table 2. Composition of the molding mixture for the production of molds for the manufacture of test castings.

	Basic Sand	Binder	Catalyst
Name	Foundry silica sand BG 27	GEOPOL 618	SA 73
Supplier	Sand Team	Sand Team	Sand Team
Dosage [wt. %]	100	1.8	18 (to the binder content)
Dosage [g]	4000 ± 5	72 ± 0.01	12.96 ± 0.01

. The mixture of sand and catalyst was mixed in a laboratory mixer for 1 min (Kenwood KM 636 Major Classic–Kenwood, Havant, UK). The binder was then added to the mixture, and the mixture was then mixed again for 1 min. After molding, the mixture was allowed to cure for 20 min at normal laboratory temperature and humidity, and after this time, i.e., after gaining handling strength, the mold was disassembled, and the individual parts of the pattern were removed. The disassembled mold was cured in free air for 2 h before casting. After that time, the cores (beams) were placed in the molds.

Prefabricated test beams (24 h after curing) were used as cores and were shortened to a length of 70 mm for this purpose. Thus, two cores were made from each test beam for casting.

A total of five molds with established beams (cores) were cast to evaluate the collapsibility of cores after casting. The aluminum alloy AlSi7Mg0.3, according to EN 1706:2020, was chosen for casting. The material was supplied in the form of pre-alloy clusters with guaranteed chemical composition by the supplier according to the standard. The used alloy was not alloyed, modified, or refined in any way. The melting of the AlSi7Mg0.3 alloy was carried out in an electric resistance furnace in a 2 L SiC crucible. The melt preheating temperature for casting was set at 740 °C to ensure sufficient melt run-in and was controlled using a digital thermometer. The casting time was 4 s. The molds were disassembled, and the castings removed and prepared for further measurements 24 h after casting.

2.2.5. Evaluation of the Possibility of Using Studied Precursors for Metallic Foams Casting

A total of eight metallic foam castings were made to test the possibility of using the produced precursors as filler material. The casting conditions were identical to those for the castings with the cores (beams). The composition of the molding mixture for mold making was also identical (see Table 2).

The experiment obviously took into account the fact that the core mixture is an inhomogeneous system. Therefore, all properties of the mixture were tested on several samples.

Table 3. Summary of the number of samples for each test.

Sample Type	Total Pieces	Test	Time	Pieces per Test
Beams	20	Three-point bending strength test + abrasion loss	After 24 h	5
			After 240 h	5
			After 480 h	5
		Determination of collapsibility of cores after casting	After 24 h	5 (resp. 10 × 1/2)
Precursors	1200	Abrasion loss	For 0 s	100
			For 20 s	100
			For 40 s	100
			For 60 s	100
			For 80 s	200
			For 100 s	200
			For 120 s	200
			For 140 s	200
		Casting of metallic foam	0–140 s	approx. 100 pcs/casting

is attached to give a better idea of the experimental procedure in the evaluation of the molding mixture for different types of samples. All results were subsequently confirmed by a “validation series” of samples.

3. Results

After performing all the above tests, the properties of the furan no-bake core mixture and the test samples (beams, precursors) were evaluated.

3.1. Three-Point Bending Strength (Beams)

The fabricated test cores (beams) were subjected to mechanical properties tests. Each measurement was carried out on five samples. The shelf life of the cores was monitored for a period of up to 480 h after manufacture, while the beams were left in a normal laboratory environment with a constant temperature of 23 °C and a humidity of 20%. The suitability of the cores for storage was evaluated with changes in three-point bending strength as a function of storage time and external conditions.

A graphical representation of beam strength evolution as a function of storage time can be seen in Figure 9.

From the measured results and the graphical representation of the property evolution in Figure 9, there is a 51.6% increase in strength with longer storage times between 24 and 240 h, which decreases only slightly, by a total of 2.0%, as the storage time continues up to 480 h.

3.2. Abrasion Loss (Beams)

The results of abrasion resistance correspond with increasing strength during the time range of 24 to 240 h, with a 38.0% overall reduction in abrasion with increasing strength after 240 h. With further storage, up to 480 h after molding, the abrasion further decreases by a total of 6.6% despite slightly reduced flexural strength. Given the fact that, during storage, the strength first increases and does not change significantly with increasing time after reaching maximum strength and that, on the contrary, the amount of abrasion is further reduced, the mixture can be evaluated as suitable for storage as it does not change its properties significantly during a prolonged storage period. Average abrasion as a function of storage time can be seen in Figure 10.

More detailed data on the evaluation of abrasion, including the average weights of the specimens before and after the abrasion test, are given in

Table 4. Abrasion test results for beams.

Time [h]	Ø m of the Sample before Abrasion [g]	Sx [g]	Ø m of the Sample after Abrasion [g]	Sx [g]	Abrasion Loss [g]	Sx [g]
24	128.48	1.32	106.87	1.74	21.612	1.26
240	128.46	2.68	115.06	2.42	13.402	2.33
480	127.91	1.74	115.45	1.85	12.464	1.13

. The “Time” column represents samples measured at 24, 240 or 480 h after curing.

3.3. Abrasion Loss (Precursors)

Individual sets of precursors (100 each) were wiped in a specialized apparatus for a predefined period. For the average measured values, it can be observed how much material was removed from the precursors as the abrasion time increased. In the case of an abrasion time of 140 s, this is more than half of the precursor mass, as shown in Figure 11.

The detailed measured mass loss values for each precursor set in the abrasion loss test are given in

Table 5. Abrasion test results for precursors.

Time [s]	Ø m of the Batch (100 pcs) before Abrasion [g]	Ø m of the Batch (100 pcs) after Abrasion [g]	Abrasion Loss [g]
0	153.56	153.56	0
20	155.40	138.18	17.22
40	157.34	128.82	28.52
60	149.18	115.01	34.17
80	148.27	100.36	47.90
100	144.87	93.79	51.08
120	148.95	78.77	70.18
140	146.60	66.81	79.79

. The “Time” column indicates individual sets of precursors rubbed for a specified time (0–140 s). For each abrasion time, 100 pcs of precursors were rubbed at a time for both the test and validation series. The error range between the validation and test series ranged up to 5%, which was considered an acceptable error.

The shape of the precursor changed in parallel with increasing abrasion time. However, there was simultaneously no loss of material from the entire volume, i.e., all surfaces were abraded, but only a gradual rounding of the edges and corners of the precursor and thus a gradual change from a cubic to a spherical shape. The gradual evolution of the change in shape from the original cubic shape can be seen in Figure 12. The precursor after 140 s of abrasion is already close to a nearly perfect spherical shape, which is the most favorable for us in terms of the resulting porous structure of the cast metallic foam.

3.4. Collapsibility of Cores after Casting (Beams)

The collapsibility of the mixture (or of the beams) was evaluated 24 h after casting. The spontaneous initial disintegration of the core mixture was already observed during the first handling of the castings and during the removal of the sprues. For the actual discharge of the cores by means of a special jig, the so-called knocking-out mandrel, placed in the laboratory sand ramming machine, needed to be applied for 0–2 knocks, i.e., to exert knock-out work of 0–6.6 J. At a work value of 0 J, the thermally exposed mixture was discharged from the casting spontaneously when the casting was placed in the jig. The collapsibility of the mixture after thermal exposure can therefore be assessed as excellent. This statement was subsequently confirmed during the removal of the residual precursors after the casting of the metallic foams, where complete cleaning of all cavities was very easily achieved by gently agitating the surface of the precursors and then blowing them with pressurized air.

3.5. Evaluation of the Possibility of Using Studied Precursors for Metallic Foams Casting

To evaluate the possibility of using the studied mixture, or precursors made from it, for the manufacturing of cast metallic foams, a total of eight castings were made. Infiltration technology for transferring liquid metal into the mold cavity filled with filler material (precursors) was used for the production of the metallic foams.

It was verified that the precursor material fully meets the requirements for the filler material; in particular, it meets the requirement of good collapsibility after casting for the removal of the precursors from the complex inner cavity of the casting.

Metallic foam castings were made with different shapes of internal cavities depending on the shape of the precursor used. Evaluation of the influence of the precursor shape on the resulting properties of the metallic foams (porosity, etc.) will be the subject of further research. Figure 13 shows the final metallic foam castings, which have a porosity of up to 55% (depending on the specific shape of the precursor). Evaluation of the mechanical properties of the castings with respect to specific porosity will be the subject of further research.

4. Discussion

The experimental part of the presented work focused on testing the core mixture (furan no-bake) intended for the production of so-called precursors, i.e., space holder material, for the possibility of producing metallic foams by foundry method. Foundry technologies could offer economical, timesaving, and in many cases, ecological possibilities to produce metallic foams with a wide range of internal structure morphology in relation to shape, size, regularity, distribution, or degree of interconnection of the internal pores. In the experiment, therefore, special attention was paid to the technology of infiltration of liquid metal into a mold filled with filler material.

The main objective of the experiment was to determine the properties of the core mixture studied and to evaluate whether this mixture could be used to produce precursors. The properties of the mixture, such as bending strength, abrasion loss, and collapsibility after casting, were then considered as being key in this area. On the basis of the experiments performed, a so-called validation series was carried out. This series was subjected to the same procedures and showed values in the same range as the samples for that experiment.

The partial results of the used procedures are summarized in the following points:

• Bending strength evaluation area: Since the core mixture is an inhomogeneous system, the bending strength was always measured for five specimens. The measurements were carried out 24 h, 240 h, and 480 h after the curing of the mixture (or test beams). The measured values clearly show that the beam strength increases with increasing storage time and does not decrease further even after a long storage period (20 days). This is a significant result because one of the requirements for precursors is the possibility of storing them without affecting their quality;
• Abrasion loss area: This property was evaluated on two types of specimens: test beams and precursors. The evaluation of the abrasion loss of the beams aimed to verify the change in abrasion resistance as a function of storage time. It was found that the abrasion resistance increases with storage time (and increasing strength). The aim of the precursor rubbing was to verify whether it is possible to create precursors of different shapes in this way, especially precursors that are close to the shape of a sphere. It has been shown that this process can produce precursors of various shapes. Moreover, the assumption that the size of the resulting precursor would change significantly with increasing abrasion time was refuted. It has been shown that the corners and edges of the original cube are rounded during the rubbing process. The diameter of the resulting round precursor is almost identical to the edge length of the original cube;
• Area of assessment of collapsibility after casting: The material under study proved to be excellent in terms of collapsibility after casting. Thanks to a specially designed apparatus and procedure, it was found that the removal (knock-out) of the cores from the casting required a knock-out energy of 0–6.6 J. These results demonstrate a very good collapsibility of the studied material after casting. This is one of our fundamental results. To be able to cast metallic foams by liquid metal infiltration into a mold cavity filled with precursors, the precursor material must be easily removable from the complex inner cavity of the casting. If this is not the case, or if the precursors remain embedded inside the casting, it is already a different type of material, known as so-called syntactic foams [58];
• Area of cast metallic foam production using precursors: It has been verified that the proposed technology to produce precursors allows their subsequent use in the cast metallic foam production process. By changing the shape of the precursor used, we can easily change the shape of the internal cavities of the cast metallic foam and thus change its final properties. These parameters will be addressed by further research in this area. Future research will also focus on materials for the production of metallic foams (alloys), particularly aluminum alloys [59], which represent the greatest potential in this area.

5. Conclusions

The presented area of developing foundry technologies to produce metallic foams is very complex. It is, therefore, necessary to look in detail at all the individual parameters that constitute this process. These include, for example, the mold material (molding mixture), the precursor material (core mixture), the casting material (cast alloy), the desired design of the final casting, the shape, size, and distribution of precursors, and their contact with each other.

Due to the breadth of the subject matter, the present paper has focused only on one of these many parameters, that is, the precursor material and its evaluation. Furthermore, the experiment established a procedure for the fabrication of the precursors of different shapes using laboratory apparatus with a perforated rotating sieve. In the next steps, the ideal precursor shape will be determined. A perfect sphere shape would appear to be ideal, but the mutual contact of the spheres is only point contact, which is inappropriate in terms of the possibility of cleaning the casting or removing precursors from the casting cavity. Therefore, a shape closer to a sphere is preferable, where the mutual contact of the precursors will be in a plane and thus the cleanability of the resulting castings will be ensured.

As the molding mixtures are an inhomogeneous system, for each property measured, the mixture was evaluated over five samples. Given clearly defined experimental conditions (mixture composition and storage conditions) corresponding to normal laboratory and operating conditions, it is possible to reproduce these results or to produce precursors with the same properties.

It has been shown that complex metallic foam structures can be produced using conventional foundry materials and methods. Further developments in this area are expected to establish an economical and environmentally friendly process to produce metallic foams. These processes can be implemented in conventional foundry operations. This step should contribute to the full exploitation and application potential of these materials.

From the point of view of production ecology, in further experiments the focus will be on the production of precursors from core mixtures based on inorganic binder systems. At present, due to the increasing demands for the greening of production in the foundry industry, there is a trend to reduce waste production and reduce the use of core mixtures with organic binders. However, these materials remain indispensable in the production of castings.