1 Introduction

3-Dimension (3D) printer technology began to be commercialized by developing a functional photopolymer system that forms three-dimensionally, beyond the existing two-dimensional printing method by irradiating light on a cross section and laminating it layer-by-layer. Initially, it was used only in high-tech fields, such as aerospace and automobile industries, but now it has started to be generalized by being distributed to individuals, private companies, and each industrial field. As a result, it is in the spotlight for various high-tech industrial applications, ranging from simple prototype mock-up production to medical fields, such as skulls and artificial organs; machinery industries, such as car bodies and internal combustion engine parts; bridges and 3D printing housing construction; and even space rockets and satellites [1,2,3,4,5]. As 3D printers have become more common, production costs and operating and maintenance costs have been reduced, and research is actively underway to improve quality and increase production efficiency [6]. However, for use in various high-tech industries, inspection methods are required to determine the durability of 3D printers and the presence or absence of defects. The 3D printer has the advantage of being able to quickly manufacture products and replace parts needed in the industrial field, because it has small manufacturing process time and cost, and can freely produce shapes that are not allowed in the cutting method using existing machine tools. However, there are disadvantages, in that it is weak in impact strength and vulnerable to heat, because the lamination process technology has not been much developed. In addition, due to low reliability and durability, bending of the output and internal fracture are major problems. Although research on process technology is being actively conducted, due to the characteristics of multi-layer laminates it is difficult to evaluate internal defects and manufacturing completeness. Stereo-lithography apparatus (SLA) and fused deposition modeling (FDM) methods are common in 3D printers, and both processes are manufactured by projecting lasers to cure liquid fuel (Resin) in layers. Although the output speed is fast and has high precision, the production of air gap and strength in the output is reduced due to various variables in the laminating process, such as the precision of the device, and the external environment when laminating liquid fuel. Since defects can be a direct cause of brittle fracture, it is very important to detect and evaluate them in terms of the safety and reliability of 3D print outputs. Although 3D printing is being expanded as temporary replacement parts for various equipment, fatigue failure due to the continuous accumulation of fatigue damage in industrial sites can cause great casualties in the operation stage. In some industrial fields, 3D printers are used to manufacture and operate temporary parts, but the inspection standards are not clear, and reliability problems due to deformation and cracking have been raised [7, 8]. In order to solve these problems, various studies on the safety of 3D printer output have been attempted domestically and internationally.

First, considering the research trends related to 3D printer fabrication and evaluation, S. Park et al. manufactured a tensile test specimen in the SLA method, conducted research on the mechanical characteristics according to the lamination thickness and direction, and confirmed that the tensile strength was higher than the vertical test specimen [9]. P.G. Ikonomov designed a skeletal structure model without additional support to use 3D printers in the medical field, and studied 3D printer selection and imitation design structure methods suitable for actual human bone fabrication using various 3D printers [10]. In addition, various studies have evaluated the strength measurement and output state that change according to various conditions, such as output temperature, internal filling amount, output pattern, and lamination direction [11,12,13]. Even in the non-destructive field, by grafting the existing non-destructive inspection technique to a 3D printer, research in the field of defect detection and real-time defect detection of laminate manufactured products according to the output method is in progress. P. Lawely conducted defect detection on 3D printer output with ultrasonic C-Scan, and compared it with simulation results [14]. M. Elsaadouny and co-authors used synthetic aperture radar (SAR) to detect defects in polylactic acid (PLA)-type 3D printer output, and also verified the non-destructive test through machine learning and convolution neural network (CNN) learning using the acquired data [15,16,17]. In addition, many researchers are studying how to improve the surface of 3D printer output using ultrasonic [18,19,20].

As such, most of the studies evaluated the strength and output state according to the production conditions, such as the output temperature of the 3D printer output, the amount of filling inside, the output pattern, and the direction of lamination [21]. In addition, research on the application of non-destructive testing techniques was conducted only to determine the presence and form of defects [22]. The evaluation of strength and output condition only measures the strength of the product and cannot prevent accidents due to defects occurring during use. There is a study of micro-defect detection due to the layer-by-layer deposition process, such as near-surface and surface defects of a Laser powder deposition monolayer using laser ultrasonic [23]. In addition, there is a lack of research cases that approach ultrasonic non-destructive testing techniques and ultrasonic basic theories or evaluate the ultrasonic propagation characteristics of printed materials in consideration of laminating of multi-layer outputs [24]. Defect detection without considering the ultrasonic propagation characteristics cannot estimate the exact location of the defect. To verify the lamination rate and reliability of the 3D printed output after production is complete, it is necessary to apply a method that can inspect the laminate without destroying it. As an alternative, a non-destructive inspection technique should be applied. There are various inspection methods for the non-destructive technique, and suitable inspection techniques for inspecting laminates include radiographic testing (RT) and ultrasonic testing (UT). Artificial defect geometries produced by metal additive manufacturing are often inspected with RT [25], but due to radiation regulations, RT has many limitations in evaluation and the inspection process takes a long time. That's why UT is a good alternative for inspecting 3D printed output. UT has a variety of techniques for controlling residual stress and evaluating the initial state and micro-defects of materials [26,27,28,29,30,31].

This study, rather than focusing on defect detection using ultrasonic waves, compares theoretical and experimental values through the correlation between the thickness of laminated outputs and ultrasonic waves according to the laminating direction using a commonly used SLA 3D printer. Therefore, the ultrasonic wave propagation characteristics according to the lamination thickness and angle of the acrylonitrile butadiene styrene resin (ABS Resin) output of the SLA method 3D printer were studied. Based on the study of ultrasonic propagation characteristics, defects were manufactured inside the specimen, and it was confirmed whether defect detection and defect location could be determined. As a result, a non-destructive evaluation method of 3D printer output using ultrasonic defect detection technique was presented to check the deviation of the ultrasonic propagation velocity, to enable reliable on-site inspection for various defects existing in laminated products.

2 Theory

2.1 Pulse Echo Technique

The ultrasonic wave causes a vibrator to vibrate by any vibration, which is converted into mechanical energy by the piezoelectric effect, and is converted back into electrical energy, which can be confirmed on the oscilloscope.

Ultrasonic waves are classified into longitudinal waves and transverse waves, as particles vibrate parallel to or perpendicular to the direction of wave propagation, respectively. In addition, because the bonding force between particles of each material is different, the velocity of ultrasonic waves varies depending on the propagating medium.

The ultrasonic velocities of the longitudinal and transverse wave are expressed by Eqs. (1) and (2), respectively:

$$C_{L} = \sqrt {\frac{E}{\rho } \times \frac{1 - \nu }{{\left( {1 + \nu } \right)\left( {1 - 2\nu } \right)}}} = \sqrt {\frac{{K + \left( {{\raise0.7ex\hbox{$4$} \!\mathord{\left/ {\vphantom {4 3}}\right.\kern-0pt} \!\lower0.7ex\hbox{$3$}}} \right)G}}{\rho }}$$
(1)
$$C_{s} = \sqrt {\frac{E}{\rho } \times \frac{1}{{2\left( {1 + \upsilon } \right)}}} = \frac{\sqrt G }{\rho }$$
(2)

where, \(\rho\) is the density, \(V\) is the Poisson’s ratio,\(E\) is the elastic modulus,\(K\) is the bulk elastic modulus, and \(G\) is the shear modulus.

The ultrasonic waves generated by the probe pass through a couplant (Glycerin) for contact between the probe and the specimen, and then return by passing through the specimen, and reflecting from the bottom of the specimen. The returned ultrasonic wave is converted by the transducer into an electric pulse. At this time, since the transmission speed of ultrasonic wave is different for each material, in this experiment, the sound speed of ABS resin and acrylic material was inputted, respectively. The wave velocity wave can be calculated if the thickness information of the object and the return time after reflection from the bottom are known. Equation (3) below expresses the velocity of ultrasonic wave:

$$c = \frac{2 \times d}{{\Delta t}}$$
(3)

where, \(d\) is the thickness of the specimen, and \(t\) is the measurement of the reciprocal travel time of the ultrasonic wave that passes through the specimen, and is reflected back from the bottom.

In this experiment, the pulse–echo method was used among ultrasound techniques. An experiment was conducted to measure the position of a defect and the thickness of an object by injecting an ultrasonic wave into the output of the SLA method 3D printer, and receiving the reflected ultrasonic wave. Figure 1 shows the pulse–echo method. If the thickness d of the specimen is known and the reciprocal travel time t of the ultrasonic wave reflected from the bottom surface is known, the ultrasonic velocity of propagation inside the material can be measured. By measuring the ultrasonic velocity of a 3D printer product having each thickness, it is possible to know the physical ultrasonic velocity that changes according to the layer thickness compared to the ultrasonic velocity that can be physically derived from the raw material in theory.

Fig. 1
figure 1

The principle of Ultrasonic testing pulse–echo

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2.2 Ultrasonic Wave Reflection Coefficients that Propagate Vertically Within a Multilayer Structure

Figure 2 shows that when ultrasonic waves pass through a multi-layer structure, the longitudinal wave velocity and acoustic impedance of ultrasonic waves vary, depending on the material properties and the direction of the boundary layer, and there is a difference in the reception time and energy ratio.

Fig. 2
figure 2

Ultrasonic waves propagate vertically within the multi-layer structure

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When ultrasonic waves passing through one material move to a material with different physical properties, some of the ultrasonic waves are transmitted, and some are reflected at the interface. Therefore, the reflection coefficient, which is the ratio of ultrasonic energy reflected at the interface to the input energy, is calculated.

Equation (4) shows the wave equation for longitudinal wave:

$$\frac{{\partial^{2} u_{x} }}{{\partial x^{2} }} = \frac{1}{{c_{L}^{2} }}\frac{{\partial^{2} u_{x} }}{{\partial t^{2} }}$$
(4)

Defining acoustic impedance as \(Z = \rho c_{L}\), reflection and transmission coefficient can thus be weitten as Eq. (5) [32].

$$R = \frac{{Z_{2} - Z_{1} }}{{Z_{1} + Z_{2} }}, T = \frac{{2Z_{2} }}{{Z_{1} + Z_{2} }}$$
(5)

Based on the formula derived above, the acoustic impedance and energy ratio reflected from the multi-layer structure can be obtained.

3 Experimental Setup and Specimen

3.1 Experimental Setup

In this study, pulse–echo method was used, and Fig. 3 shows a schematic of the experimental set-up. A high-voltage pulser (Ritec's RAM-5000) was used to transmit and receive the pulser echo method, and a (1, 5, 10) MHz single transducer (Olympus) was used to select the optimal frequency. At 1 MHz, the wavelength was longer than that of 5 and 10 MHz, so the specimen was not suitable for use in thin specimens, and it was confirmed that it overlapped with the main bang signal. 10 MHz had good resolution, so reception was good at 1 mm, but at 8 mm, the reception signal was very weak due to signal scattering and attenuation due to the characteristics of the ABS resin material, a polymer composite. Finally, in this study, 5 MHz frequency was selected and the experiment was carried out. Ultrasonic was transmitted and received in 1 cycle to avoid overlapping signals due to thin specimens. In addition, the signal was saved using an oscilloscope (Lecroy’s Wavesurfer 3074). Figure 4 shows that the prepared specimens used an acrylic delay line to avoid the main bang signal and interference when detecting the bottom or defects near the surface, and below a certain thickness. Through this, the correlation between the thickness measurement and defect detection of the 3D printer laminated output and the laminating thickness was confirmed.

Fig. 3
figure 3

Experimental setup for 3D printer material by ultrasonic wave

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Fig. 4
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Comparison of signals with and without delay line

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3.2 Specimen

3.2.1 Intact Specimen with Horizontal Lamination Layer Direction

In this study, an SLA-type 3D printer (UnionTech’s Lite600) was used for the experiment, and it was fabricated in a horizontal laminating method, which is generally manufactured. Unlike the FDM method, which generally heats filaments and extrudes and laminates them through a nozzle, SLA is a photocurable resin molding method, and consists of a water tank, laser, and mirror. By reflecting the circular laser with a mirror, heating the resin in the water tank, curing it, and laminating it layer-by-layer to form it, the output time is slow compared to FDM, but the output error and surface roughness show excellent precision. Each specimen was produced by the SLA 3D printer method, and used an ABS resin material (JS-UV-2016-B of Shenzhen Kings 3D Printer Co., Ltd.), which is a liquid photocurable resin. Table 1 shows the main properties of the ABS resin material used in this specimen:

Table 1 Material properties of JS-UV-2016-B
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Figure 5 shows that a total of 3 sets of specimens were produced to secure reliability of inspection. The intact specimens were used to check the correlation coefficient of the ultrasonic velocity for each layer thickness to check the material properties change according to the layer thickness of the 3D printer. The 50 mm × 50 mm specimens were prepared with different thicknesses of 1, 2, 4, 6, and 8 mm to set the thickness as a variable. When the 3D printer was laminated once, the thickness of the layer was set to 0.1 mm, and the inner filling density was set to 100%, without additional support. At this time, to manufacture each specimen uniformly, 1 set per bed was manufactured.

Fig. 5
figure 5

Intact specimen with horizontal lamination layer direction

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3.2.2 Intact Specimen with Horizontal Diagonal and Vertical Laminating Directions

To understand the correlation of ultrasonic waves according to the lamination direction, specimens with different lamination directions were manufactured by applying the same material and processing conditions as previously manufactured. The laminating directions were horizontal (\(0^\circ\)), diagonal (\(45^\circ\)), and vertical (\(90^\circ\)), and each of the 3 sets were manufactured according to the direction. The size of the specimen was manufactured as shown in Fig. 5. Figure 6 shows the laminating direction photographed using LV-SEM equipment.

Fig. 6
figure 6

Intact specimen with laminating direction. a Horizontal, b Diagonal, c Vertical

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3.2.3 Defects Specimen

The defect specimens are produced by depicting artificial cracks that may occur due to lamination defects, such as pores, and various factors that occur due to problems in the 3D printer process. Their size is difficult to realize in real situations, so they were manufactured as shown in Fig. 7 with the purpose of detecting defects in the output of the SLA 3D printer. The size of each specimen was 150 mm × 150 mm, and the thickness was 8 mm.

Fig. 7
figure 7

Defects specimen

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Defects were manufactured by emptying four spaces without using additional supporters inside the specimen during the additive manufacturing process, and parts except for defect parts were manufactured by laminating without empty spaces. Figure 7 shows the size and location of the defect. Defects were made at (1 and 4) mm from the surface.

4 Experimental Results

4.1 Experimental Result of Intact Specimen with Horizontal Lamination Layer Direction

The SLA defect specimen with constant lamination conditions to be measured was made of ABS resin material, and at this time, each specimen was subjected to repeated experiments to reduce the standard deviation. Figure 8 shows the ultrasonic measurement results according to the number of laminations, and as can be seen from each result, the first shows the main bang signal, the second shows the signal from the 20 mm acrylic delay line, and the third shows the signal of the specimen manufactured by 3D printer. At this time, the thickness of the specimen can be estimated by considering the pulse interval and the ultrasonic velocity in each medium. Basically, the ultrasonic longitudinal wave velocity in ABS resin is \(\mathrm{2,250} m/s\). Since this study is to investigate the correlation behavior of ultrasonic waves in laminate manufactured products, the magnitude of the amplitude is not meaningful.

Fig. 8
figure 8

Experimental result

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Figure 9 shows the results of the experiment of 3 sets of intact specimens laminated in the horizontal direction as a velocity graph. Table 2 shows the error rate compared to the reference wave velocity of ABS Resin. Error rates of 16.72 and 10.65% were generated in specimens of 1 and 2 mm, respectively, and it can be confirmed that the reference ultrasonic velocity and large error values were generated. When the thickness was more than 4 mm, it was close to the average value, and gradually converges to the average value. This means that the SLA 3D printer laminates 0.1 mm at a time, so it is judged that the change in physical properties or the internal filling amount cannot fill 100%, or if the thickness is thin, deformation occurs in the process of hardening after lamination.

Fig. 9
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Wave velocity by thickness

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Table 2 Wave velocity error rate
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When analyzing the results of the intact specimen, there is no additional signal reflected from the inside of the specimen made of acrylic delay line and ABS resin. This is a very different result from metal, because in the case of ABS resin, since it is a polymer composite, the composition of the material is not completely uniform. In addition, the SLA method was used, which among the actual 3D printer methods is very sophisticated, but looking at the LV-SEM imaging results, in the commercial 3D printer, the thickness of the layer entered into the production program and the actual thickness were not uniform, so there was a slight difference. Also, inclusions and pores were present on the surface or cross-section. The ABS resin material is a composite of acrylonitrile, butadiene, and styrene, and is a heterogeneous material composed of particles with different density and elastic modulus. For these complex reasons, each impedance generates resistance, and the signal emitted by the pulsar is scattered and attenuated, which has a great influence on LV-SEM imaging, as well as ultrasonic testing. When the specimens were laminated once, the thickness was fixed at 0.1 mm, and when the laminated direction was horizontal to the bed, different number of laminates was set as a variable. At less than 4 mm, the ultrasonic wave was slower than the standard wave velocity, and it was confirmed that there was a deviation even with the same thickness. Due to the nature of the SLA 3D printer process, it repeatedly expanded and contracted during the curing process using a high-temperature laser, and could not be laminated consistently. This is thought to be because the inflow of foreign substances, such as dust or micro-pores, affect the mechanical properties of the ABS resin material, which has weak bonding strength. In the case of ≥ 4 mm, it was confirmed that the ultrasonic velocity converges to the average value. These experimental results show that the error in the average lamination thickness confirmed in the LV-SEM results did not significantly affect the ultrasonic experiment. The difference in acoustic impedance is caused by the bonding force between particles according to the laminating direction, and it is judged that the difference in impedance has a greater effect on ultrasonic wave propagation. The lamination angle experimentally proved the ultrasonic basic theory that as the elastic modulus of the material increases, the ultrasonic velocity increases. Tables 3 and 4 show the error range for the laminated materials in the diagonal and vertical directions.

Table 3 Wave velocity error rate (Diagonal)
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Table 4 Wave velocity error rate (Vertical)
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4.2 Experimental Result of Intact Specimen with Horizontal Diagonal and Vertical Laminating Directions

Experiments with specimen with different lamination directions were also tested with the same test set-up as for specimens with the same lamination direction. Figure 10 shows a graph of the velocity and deviation for each thickness of specimen in which the lamination directions are not the same. This result proves that the lamination direction of the 3D printer output affects the mechanical properties. In general, since ultrasonic waves are greatly affected by the bonding force between particles, when the ultrasonic progress time and speed according to the laminating direction were compared, it was confirmed that when the laminating conditions were constant, the ultrasonic progress speed gradually converges within the standard data error range. In addition, in the case of the horizontal laminating direction in the SLA method, it was lower than the standard velocity of \(\mathrm{2,250} m/s\) at less than 4 mm, but converged to the standard velocity at 4 mm or more. However, in a previous study, when the tensile test was conducted with different laminating directions, the actual velocity of ultrasonic wave was also different, because the Young’s modulus of the material was changed. Based on the above results, it was confirmed that the laminated thickness did not significantly affect the ultrasonic wave, but the number of laminates and the laminating direction had a great influence on the ultrasonic behavior. This can be seen as a difference from the reflection coefficient propagating within the multilayer structure. Therefore, the ultrasonic behavior of the ABS Resin material linearly converges to the ultrasonic standard velocity at the number of times of laminating over a certain level, and if a laminating direction has a certain angle, the standard value is different.

Fig. 10
figure 10

Wave velocity by thickness

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4.3 Experimental Result of Defect Specimens

Based on the results of the intact specimen, a thickness close to the standard data was selected. Artificial defects were processed on a specimen with a thickness of 8 mm, and defects were detected. Defect specimens were laminated horizontally by 0.1 mm in the same laminating direction as the intact specimens. For defects, the amount of filling was 0% at 1 and 4 mm in the depth direction from the upper surface of the specimen, 10 mm × 10 mm wide and 2 mm thickness.  Figure 11 shows the experimental results.

Fig. 11
figure 11

Experimental result of defect specimen

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Equation 3 can be used to estimate the position of the defect based on the signal reflected from the defect, and the reflected \(\Delta \mathrm{t}\) from defects #1 and #2 are \(0. 99 \mu s\) and 1\(. 03 \mu s\). Converting to ultrasonic velocity based on the reflected time yields \(\mathrm{2,202 }m/s\) and 2,116 m/s, respectively. In addition, the values of \(\mathrm{2,281 }m/s\) and \(\mathrm{2,222 }m/s\) were measured for defects #3 and #4. This becomes a value that cannot be estimated without knowing the thickness of the 3D printer output and the location of the defect. As can be seen from the results of the previous horizontally laminating intact specimen, as it converges to the physical ultrasonic velocity of the raw material at 4 mm or more, accurate data can be extracted only when the \(\Delta \mathrm{t}\) of the reflected signal after passing the delay line is \(3.45 \mu s\) or more. When the \(\Delta \mathrm{t}\) value is less than\(3.45 \mu s\), the thickness of the defect can be inference by comparing it with the data measured on the defect specimen as shown in Fig. 12.

Fig. 12
figure 12

Comparison of experimental result. a Data from intact specimen 1 mm and defects #1 and #2, b Data from intact specimen 4 mm and defects #3 and #4

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5 Conclusion

In this study, ultrasonic-based material characteristics were analyzed according to the lamination manufacturing conditions to verify the reliability of the existing 3D printer product strength and defect measurement techniques.

  1. 1.

    When the laminating direction is horizontal, theoretically, the ultrasonic velocity of the ABS resin material and the experimental results were compared and analyzed. When the laminating direction is horizontal, it can be confirmed that the reference signal is gradually converging at 4 mm or more, and it is confirmed that the ultrasonic velocity is low when the laminating direction is less than 4 mm. It was determined that the internal properties of the thin specimen changed due to heat generated during the lamination process, and the difference from the reflection coefficient propagating in the multilayer structure showed a large error with the actual reference ultrasonic velocity.

  2. 2.

    As a result of experimenting with different laminating directions, it can be confirmed that the velocity of the standard ultrasonic wave is faster than the standard ultrasonic speed. As for this tendency, the error in the average laminating thickness confirmed in the LV-SEM result does not have a significant effect on the ultrasonic experiment, and It can be seen that the coupling force has a greater effect on ultrasonic wave propagation due to the difference in acoustic impedance. Based on this, the basic ultrasonic theory that the lamination angle becomes higher as the elastic modulus of the material increases was experimentally proved.

  3. 3.

    Although the test results of the defect specimen confirmed that the presence or absence of the defect could be determined, it was impossible to accurately target the position because the signal reflected from the delay line was less than \(3.45 \mu s\). However, it was possible to estimate the defect value by comparing it with the intact specimen signal.

Through this study, it was confirmed that when manufacturing a 3D printer product, the laminating direction and thickness should be considered according to the characteristics of the product to be manufactured. In addition, it was confirmed that the ultrasonic speed, which varies depending on the thickness, needs to be corrected and applied in order to find the exact location of the defect when the defect occurs inside the initial product and during use. In this study, the relationship between the thickness of the output and ultrasonic waves when applying the ultrasonic non-destructive testing to 3D printer products, and presented the reference value of the ultrasonic reflection signal when applying the ultrasonic testing. This will be of great help in terms of product stability by securing reliability and saving time and cost due to damage by manufacturing products applied to major high-tech industrial facilities.