Introduction

In core-intensive casting processes, organic or inorganic bonded sand cores shape highly complex hollow structures in metal casting products.1 Two common ways to produce sand cores are through core shooting,2 where the mixture is fluidized by air pressure and blown into a core tool or 3D printing,3 where the binder is selectively printed into the sand bed layer by layer. Sand cores are destroyed after the casting has solidified to remove them from the casting (lost cores). The removal of the sand core from the casting is called decoring.4 The decoring process is characterised by hammering and shaking.5

Stricter environmental laws drive the trend towards sustainable production in the European Union.6 The metal casting industry, too, has to meet the demands of a more environmentally friendly production. In core-intensive casting processes, inorganic binders gained more attention and returned to the focus since the turn of the millennium.2 In this regard, water glass-bonded sand cores do not develop harmful emissions due to the absence of organic ingredients. However, poor decoring behaviour is a limiting characteristic for casting applications due to the high residual strength after casting compared to organically-bounded cores.7

A study investigated the decoring behaviour of water glass-bonded cores in 3D printing. Geometrically artificial artefacts, such as predetermined breaking points, were printed into the sand core. The results show that the imprinted artefacts lead to better decoring behaviour than sand cores without imprinted artefacts.8 Till now it was impossible to integrate fully hollow structures in water glass-bonded cores to improve the decoring behaviour.

The new methodology presented in this study demonstrates that complex water glass-bonded hollow cores, like in 3D printing, can be produced in a conventional core shooting process. The basic idea is to use a sacrificial ice core (SaIC) shelled with a conventional water glass sand system by core shooting. Various tasks and questions arise in producing hollow cores from SaIC and will be addressed in this paper. One interest is the core shooting setup for hollow core production. Next, challenges are addressed in integrating the SaIC into the core shooting process in a non-destructive manner. Further, the curing process after core shooting is investigated whether migration processes occur between the SaIC and the primary core (PrC). The challenges are investigated in detail using a bending test bar. The mechanical properties, as well as the decoring behaviour of the bending bars, are presented. The application potential is shown using the example of a complex near-series prototype core. The reusability of the SaIC contributes directly to more resource-efficient sand consumption and limits the amount of material used. This study promotes a broader application of water glass-bound sands in the metal casting industry.

Materials and Methods

Materials

Derived from the aims of this study, two material systems are used. The first material system is used for the SaIC and consists of sand and tap water as binder. The second material system consists of sand and a water glass binder which generates a persistent bonding compared to the first system.

The sand used in this study is a commercially available silica sand H32. The medium grain size is 320 µm (Quarzwerke GmbH, Frechen, Germany). The sand is used for the primary core (PrC) and the sacrificial ice core (SaIC). To produce the PrC, H32 is premixed for 1 minute with a liquid inorganic binder of Cast Clean VC (Peak Deutschland GmbH, Nossen, Germany) or a dry inorganic powder binder of Pulver Binder-S (Peak Deutschland GmbH, Nossen, Germany). Additionally, tap water was added to the dry powder binder. To produce the SaIC, H32 is only premixed with tap water for 1 minute. Table 1 summarises the mixtures used.

Table 1 Used Mixtures in This Study
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Fabrication of Hollow Cores

Basic Process

Figure 1 shows an abstraction of the multi-step process to produce the PrC. This process is realised on a Morek LUT-1 core shooting test rig (Multiserw-Morek, Brzeźnica, Poland). The fabrication of hollow cores comprises the following steps: First, the shooting head is filled with the PrC material and compacted. Next, the SaIC is inserted into the aluminium tool and centred with the bullet insert (see Figure 3). The SaIC has a diameter of 14 mm and a length of 195 mm. Lastly, the core shooting process starts with 2 second shooting time and a pressure of 5 bar. The PrC is shot around the SaIC.

Figure 1
figure 1

Core shooting rig and process.

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SaIC Production

Figure 2 shows the production steps to produce the SaIC. The proportions of the SaIC mixture are stirred in a mixer for 1 minute. The SaIC mixture is pressed manually into a separatable aluminium tool and cooled at −18 °C for 24 h. A separating agent, Demotex A (Technofond Gießereihilfsmittel GmbH, Harthausen, Germany), is sprayed on the tool before. After freezing at −18 °C for at least 24 h, the SaIC is unpacked and placed in the core shooting tool. The SaIC has a length of 195 mm and a diameter of 14 mm.

Figure 2
figure 2

SaIC production steps.

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Application of the Process for the Production of Hollow Test Bars

Figure 3 illustrates the process steps of core shooting and further processing as described in section "Basic Process". The further processing involves inserting the core shooting tool in a resistance-heated laboratory furnace at 200 °C for 20 min. During this process, the water glass-bonded core material hardens. At the same time, the SaIC loses strength due to the evaporation of the water. After the curing process, the dried SaIC can be removed by simply pouring the sand out of the tool. After this step, the PrC can be unpacked from the tool.

Figure 3
figure 3

Hollow core production steps.

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Three-Point Bending Test

Three-point bending tests are carried out on a Zwick universal testing machine Z020 (ZwickRoell, Ulm, Germany) with a maximum test load of 20 kN to measure the bending strength in N/cm2 (Figure 4). The test speed for the hollow test bars and the reference samples is set at 1 mm/min. The measurement starts until a pre-load of 0.3 N is reached. The support distance is 150 mm. The test ends when a force cut-off threshold of 90% of the maximum measured test force is reached. The outer dimensions of the bending test bars are 22.4 x 22.4 x 170 mm. Reference samples without bullet insert were used for comparison. Three samples are produced per mixture for testing.

Figure 4
figure 4

Three-point bending test.

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Diffusion Tendency

The inner diameter of the bending test bar is evaluated to investigate possible interactions between the SaIC and the PrC material. Figure 5 shows the sample preparation and measuring of the inner contour of the hollow bending test bar schematically. A sample of 20 mm width is cut out at the vent side and shooting side of the bending test bar. The inner diameter and anisotropy for each hollow specimen is determined four times with a digital calliper gauge to determine the mean value and the standard deviation. For each mixture, two samples were measured. One taken from the vent side, the other from the shooting side.

Figure 5
figure 5

Schematic representation of the sample preparation and the measuring principle.

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Measurement of the Decoring Behaviour of the Bending Test Bars

The decoring behaviour of the cast-in bending bars was carried out following the test setup in,4 Figure 6. The weight of the ram is lifted manually using a crank and falls onto the sample in free fall at the highest position. The ram applies an impulse to the clamped specimen. The impulse depends on the height and the weight. The height and weight of the ram are set at 50 mm and 6.8 kg, resulting in an energy of 3.3 joules per impact. A spring mechanism ensures that the spring back triggers no other impulse per crank rotation. 20 impulses are defined as one impact series. Bevor decoring, and after every impact series, the weight of the samples is weighed to determine the amount of decoring impulses. The sample is considered decored when the sand core is completely removed. The decoring behaviour was investigated on 3 hollow test bars with the composition of 6 wt.-% tap water in the SaIC and 2 wt.-% liquid water glass, as well as on 3 reference test bars made of 2 wt.-% liquid water glass. The casting temperature of the aluminium melt was set at 750 °C.

Figure 6
figure 6

Production of decoring samples and decoring test rig according to et al.4.

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Production of a Double Tube Core as an Application Demonstrator

To investigate the application potential of this novel process, a double tube core was selected as a demonstrator for testing, see Figure 7. The SaIC mixture contained H32 Sand with 6 wt.-% tap water, and the PrC contained H32 Sand with 2 wt.-% liquid water glass binder. The production of the double tube follows the same principle and parameters as shown in section "Fabrication of Hollow Cores".

Figure 7
figure 7

Production of an application demonstrator.

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Results

Bending Strength of Hollow Test Bars

The results of the bending strength tests of the hollow cores compared to the reference samples using a liquid or dry inorganic binder are given in Figures 8 and 9. The upper line of the box-plot diagram represents the maximum value, the x represents the mean value, the bottom line represents the minimum value, and the line in the box represents the median value. For each mixture, three samples were tested.

Figure 8
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Bending strength of test bars with liquid water glass binder.

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Figure 9
figure 9

Bending strength of test bars with dry water glass powder binder.

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Figure 8 shows that increasing liquid binder content of 1 wt.-%, 2 wt.-% and 3 wt.-%, the strength of the hollow cores and the reference bars increases. The reference test specimens show significantly higher strength compared to the hollow cores. The increase in the strength of the hollow cores is moderate despite of the different amount of water glass added. One exception is the hollow core with 6 wt.% tap water and 2 wt.% water glass binder. The strength here is at the level of the hollow cores with 3 wt.% binder content at about 300 N/cm2. The bending strength of the dry powder binder shows that the strength of the hollow cores and that of the reference is at the same level, Figure 9. The strength of the hollow cores with a dry powder binder is significantly lower than that of the hollow cores with a liquid binder.

Inner Contour Sharpness of Hollow Cores

Figures 10 and 11 show the measured diameter of the hollow bending test bars as a function of the mixture of tap water added to the SaIC, the water glass binder type, and the water glass binder amount. Each bar represents two samples from one bending bar. One sample is taken from the shooting side, the other from the vent side.

Figure 10
figure 10

Measured diameters of the hollow bending test bars with liquid binder.

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Figure 11
figure 11

Measured diameters of the hollow test bars with dry powder binder.

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The liquid water glass binder results show that with a binder content of 1 wt.-%, the cavity has good geometric accuracy to the SaIC. In contrast, the hollow core diameter decreases significantly with 2 and 3 wt.-% water glass content. The reduction in the cavity diameter is about 0.5 to 1.5 mm. One idea would be that some liquefied water in the SaIC migrates into the PrC material, diluting the water glass binder. The diluted water glass binder binds a part of the SaIC's sand particles, resulting in a smaller diameter. With increasing water content in the SaIC, this effect decreases as the more considerable amount of water pushes the binder back more. In the case of the powder binder, there is no significant change in the hollow core diameter as a function of the water addition in the SaIC and the binder amount. One idea would be that the powder binder is saturated by the amount of water added and cannot bind additional water from the SaIC.

Decoring Behaviour of Bending Test Bars

Figure 12 shows the result of the decoring behaviour of hollow cores with 6 wt.-% tap water and 2 wt.-% liquid binder compared to reference cores with 2 wt.-% liquid binder. As described in chapter 2.5, twenty impulses are defined as one impact. In total, three samples were tested per mixture. The hollow cores need 1 impact (20 impulses) till they are fully decored. In comparison, the solid cores require at least 4 impacts (81 impulses) to a maximum of 6 impacts (120 impulses) until complete decoring. Due to their geometry, the hollow cores have significantly better decoring behaviour than the reference cores.

Figure 12
figure 12

Number of impacts until complete decoring of hollow bending test bars versus reference bending test bars.

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Discussion

Notes on the Preparation of Hollow Cores by Using SaIC Cores

The preparation of SaIC plays an important role. This includes safe demoulding of the SaIC from the tool. A frost-resistant separating film, such as a separating agent or a tool coating, is required to separate the SaIC from the tool safely. The same applies to removing the PrC from the hot-box tool after curing. After the curing step, the SaIC must be completely dried-out and the PrC completely hardened to remove the PrC from the tool. It is imaginable to lift the PrC out of the hot-box tool via ejector pins after removing the dried-out SaIC. In this study, the PrC was taken from the tool by lightly tapping on the tool with a rubber hammer.

Further, the water content in the SaIC and the correct centreing in the core shooting tool is vital for hollow core production. The frozen water turns into ice and acts as a binder between the sand particles, strengthening the SaIC for handling. Sufficiently high water content is essential for handling to have enough time for placement in the core shooting tool. However, the water content in the SaIC should not be too high. Otherwise, the curing of the water glass binder will be impaired. This is shown in Figure 13a. Here, an attempt was made to produce a hollow core with 10 wt.-% water content in the SaIC. The ice present liquefies during the furnace process. It penetrates downwards from the shooting side to the venting side by gravity, hindering the hardening of the sand particles surrounded by water glass.

Figure 13
figure 13

Exemplary defect patterns in the production of hollow cores using SaIC cores.

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This principle is also applicable by misaligning the SaIC in the mould. The correct centreing of the SaIC in the tool is essential to produce a hollow core. A SaIC misaligned cannot be homogeneously shelled, thus adequately impairing the outer contours of the water glass-bounded mixture, Figure 13b.

The methodology shows that the bullet insert geometry is suitable for producing hollow cores. Other insert geometries are imaginable if they have good permeability for the PrC mixture. Without an insert, producing hollow cores is impossible since the contour of the SaIC is destroyed by the inflowing PrC mixture during the shooting step. It must be noted that the insert supports the centring and protects the SaIC from abrasion through the inflowing sand mixture.

The dried-out SaIC can be reused an infinite number of times. This process contributes to resource-saving production, using less sand and water glass binder.

Bending Strength, Decoring Behaviour and Inner Diameter of Hollow Bending Test Bars

Compared to the reference specimen, the bending strength comparison with liquid binder shows that sufficiently high strengths can be achieved with about one-third less material in the hollow core bending bar. The mixture of 6 wt.-% SaIC and 2 wt.-% liquid binder is to be regarded as the ideal mixture, as it gives a good compromise between the handling of the SaIC and bending strength. The strength stays the same with a liquid water glass content greater than 2 wt%.

The strength comparison with powder binder shows no differences between the hollow core and the reference, regardless of the composition of the mixture. However, it should be noted that the hollow core bending bars have the same strength level despite the lower material content compared to the reference. Material savings are advantageous for resource-saving production. According to the manufacturer's mixture instructions for the powder binder, the water addition was specified as around 50% based on the amount of powder binder used. Compared to liquid binders, they have significantly lower strength.

Overall, the bending test results showed that the water content in the SaIC does not influence the strength. The low strength of hollow cores compared to the reference cores has a lower strength despite the same binder content. This is due to the lack of support material in the hollow core, as more sand particles are bound inside than the reference core and thus provide a higher strength contribution.

One idea for the decreasing hollow core diameter with 2 and 3 wt.-% liquid binder is that some neighbouring sand particles of the SaIC migrate into the PrC material as the water dilutes the water glass binder. For better illustration, this theory is shown in Figure 14. T1 shows the initial state of the test specimen after the shooting process. Here, the PrC material surrounds the SaIC. T2 shows the specimen in the furnace. Two processes take place in the furnace. The water present in the SaIC begins to change from a solid to a liquid state as the drying time increases. The liquid water dilutes the surrounding water glass in the peripheral zone of the SaIC. The diluted water glass migrates into the SaIC at the interface between the PrC material and the SaIC. As the curing time progresses, the water glass binder hardens and binds the sand particles near the SaIC interface, reducing the hollow core's diameter, T3. At 1 wt.% binder content, the amount of binder present in the PrC material is too low to bind other sand particles from the SaIC. Unlike the liquid binder, the powder binder does not bind other sand particles from the SaIC. One theory is that the dry powder binder does not go into solution with the water from the SaIC and, therefore, no longer binds sand particles compared to samples with liquid water glass binder.

Figure 14
figure 14

Model of the binder interaction of water glass in the main moulding material and water in the SaIC.

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New Possibilities and Challenges for Industrial Application

Figure 15 shows detailed images of the production of a double tube core. The Production follows the same method as described in Chapter 2.2. The methodology indicates that comparable core geometries can be produced as in 3D sand printing. However, this is not a real core geometry for a cast component. But it can be shown here that more complex core geometries can be produced compared to a cylindrical bending bar. Future work must investigate how even more complex cores can be produced using the present methodology.

Figure 15
figure 15

Detailed view of the double tube core production.

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In principle, the most significant advantage of hollow cores is their better decoring behaviour compared to solid cores. Therefore, the application in iron casting is of particular interest since; in this area, the decoring capability is one of the essential characteristics of the moulding material. An exhaust manifold, for example, may be of interest as a possible application.

Nevertheless, the increased decoring capability of hollow cores also offers advantages in light metal casting, such as in the aluminium sector. One possible application would be a rear axle beam. Another possible application can be seen in hand moulding. Here, simple shell-like structures can be produced to form outer moulds of castings to save material resources.

However, all these possibilities also bring challenges in manufacturing, especially in the core pocket area of SaIC. In the case of very complex cores, it is likely to be ensured that the SaIC remains stable and secure in the core pocket for a sufficiently long time until they are reshot with PrC. One possibility is to increase the water content in the SaIC to extend the stability in the hot-box tool over time. An alternative possibility would be water cooling in the area of the core pocket or some ceramic insert or coating to protect the SaIC from excessive heat input. The precise inserting of the SaIC and closing of the hot-box tool requires the manual skill of the foundry workers.

Another challenge can be seen in the fabrication of the SaIC. In this work, a SaIC was fabricated with a cycle time of 24 h, which is far from industrial application. With a liquid nitrogen supply, a higher manufacturing cycle time is conceivably compared to the present variant via an ice box. A further alternative would be using dry ice to chill down the tool to create a SaIC. Thermocouples in the tool and the moulding material could determine how long the freezing time is until the SaIC has sufficient strength.

After the curing process in the oven, the dried SaIC is poured out from the hot-box tool and can be a problem in a series process because the sand that leaks out contaminates the core shooting tool for the next shot. In this case, adjustments must be made to the design of the core tool, or plugs must be provided via robots to prevent the sand from running out.

Conclusion

This methodology shows that SaIC cores can be used to produce hollow water glass-bonded sand cores in a conventional core shooting process. Core complexities can be produced that were previously only possible in 3D sand printing. Inserts are necessary to protect the SaIC from abrasion during the shot and to centre it in the tool. Geometrically the inserts should have a favourable permeability for the water glass-bound mixture during core shooting. The hollow sand core bending strength achieves sufficient strength, and the decoring behaviour is significantly better than the reference samples. Lower material consumption for sand and binder through the SaIC reusability proves the given method's sustainability. It can contribute to more environmentally friendly production in the metal casting industry. The present methodology could also produce moulds in shell structures with low complexity in the hand moulding area. Challenges arise in handling the SaIC and setting shorter drying times. The SaIC’s processing time is limited from when it is removed from the cold chain at warmer ambient temperatures. Further, as the mass of the SaIC increases, the drying time to empty the SaIC from the moulding tool also increases. Further aspects regarding logistics along the production chain and energy consumption in the casting plant need to be investigated. Intelligent logistic solutions under economic and practical aspects are required.