A novel sandwich structured wheel to achieve mirror finish in grinding of a mould steel

A novel manufacturing tool called sandwich flexible elastic grinding wheel is developed to achieve high material removal rates and machined finish of a mould steel. This tool is made of aluminium matrix, ethylene propylene rubber layer and abrasive layer. Ethylene propylene rubber layer has great elasticity and produces elastic deformation when contacting with the workpiece. With the existence of an abrasive layer, rigidity at small scales allows grinding to occur, which can effectively remove the residual height of the surface and improve the surface quality. In this paper, influences of grinding process parameters of the sandwich elastic grinding wheel on the key characterisation parameters of surface integrity are studied, and the surface roughness and material removal rates under various process parameters are investigated. Then, the process parameters that are suitable for mirror machining are selected by experiments and are demonstrated on an S136 mould steel workpiece. Mirror finish can be achieved by single processing. The surface roughness of the mould steel converges from the original Ra142 nm to Ra7.42 nm, the surface texture is more compact and regular, and the surface integrity is significantly improved.


Introduction
Precision components are increasingly required in various fields due to social and technological advancements.These components, such as optical products, require high surface quality for optimal performance.However, achieving this high-quality standard often involves multiple processing steps, resulting in lengthy processing time and high costs [1].Therefore, there is a need to research and develop more efficient processing tools and methods to reduce processing time and costs while maintaining the required high-quality standards.
Traditional machining operations for the mould steel parts can be classified into two major categories: grinding and polishing.Grinding operation is used to obtain the accurate shape and size required, whilst subsequent polishing operation further improves the surface finish of the moulds for the high quality moulding process.The required surface roughness of general moulds is about Ra 0.4 μm or less [2].The surface roughness of the injection mould needs to be around Ra 0.1-0.25 μm.For the moulding of high-end polymer optics, S136 mould steel is typically used, mirror surface finish (e.g.Ra 0.01 μm or less) on the moulds is often required.
To meet the quality and production rate requirements for the machining of mould steel parts, research works have been carried out investigating different process approaches to improve the surface finish of mould parts.W.Y.H. LIEW [3] used coated cemented carbide tools to machine S136 mould steel in order to increase the durability of ultra-precision cutting tools and reduce tool wear, achieving a roughness of up to Ra0.7 μm.Hao Ni [4] achieved a surface roughness of up to 7.9 nm by using ultrasonic-assisted vibration diamond cutting to cut S136 mould steel with sensors as intermediaries.However, ultra-precision cutting requires high requirements for machine tools and cutting tools.
Zhou [5] performed magnetic finishing of S136 mould steel with magnetic abrasives prepared.The roughness was reduced from 0.341 μm to 0.1 μm.J.H. Oh [6] used magnetic abrasive finishing (MAF) to process S136 mould steel, reducing the surface roughness from 300-450 nm to 8 nm.Similarly, Pan [7] employed electrochemical-mechanical polishing (ECMP) to polish S136 stainless steel, achieving a roughness value of Ra0.08 μm from an initial value of 0.34 μm, using a new electrolyte combined with a high-frequency narrow pulse power supply.Additionally, Zhang [8] utilized chemical mechanical polishing with 20 nm silica abrasives to achieve mirror-like finishing on the S136 steel surface.The root-mean-square roughness was reduced from 37.47 nm to 3.01 nm, and the material removal rate was 187 nm/min.Although the use of these polishing technologies results in high surface quality, the small size of the abrasive particles increases the cost of the process.
Wu's team [9,10] used rubber-based elastic abrasive tools to grind and polish hardened SKD11 steel, reducing the roughness from 0.8-1 μm to 0.056 μm.Liu et al. [11] first used 1 μm diamond abrasive (polishing head: softwood) to rough-polish the workpiece surface and then used 0.25 μm diamond abrasive (polishing head: nylon-coated steel ball) to conduct fine polish.The roughness was reduced down to 8.5 nm in 99 min.Fang et al. [12][13][14][15] machined a flat surface achieving an average roughness of 16.7 nm by ball grinding, ball-burnishing and ball polishing.In order to take into account the surface hardness of the workpiece, the parameters of steel ball rolling were optimized.Under the condition of ensuring good surface hardness, the roughness can be reduced from 0.85 μm down to 79 nm.Yuan et al. [16] used alumina abrasives with grain sizes of 1000# and 4000# for rough grinding and ultra-precision grinding, respectively, of S136 mould steel, and finally conducted polishing with 7 nm silica polishing powder to reduce the surface roughness to 5 nm.Anthony et al. [17] proposed a novel manufacturing method for making free-form optical moulds, i.e., shape-adaptive grinding technique.The technology includes rough grinding using 9 μm and 40 μm Ni-based corundum pellets, and finish grinding with 3 μm resin-based corundum pellets, which can reduce the surface roughness of S136 mould steel down to 0.81 nm.
Most of the methods mentioned above require step-by-step processing to reduce the surface roughness, i.e., combining grinding and polishing process to form a process chain, whilst different grinding tools and polishing tools are needed during different stages of the machining process.Polishing belongs to free abrasive machining method, the machining outputs including surface finish and material removal rate, strongly depends on the particle size, density and uniformity of the abrasives in the polishing fluid, whilst the material removal rates of various polishing processes are generally rather low.It would be ideal to develop a new process with special tool design, which integrates the roughing and finishing steps, achieving a good balance between the machining quality and process efficiency.
In this paper, a novel sandwich flexible elastic grinding wheel was developed to enhance the material removal rate and machining finish of mould steel.The grinding wheel was applied to a fiveaxis CNC tool grinder.Grinding experiments were conducted under a wide range of experimental conditions to check the grinding performance of the developed grinding wheel.The feasibility of the grinding wheel for mirror polishing of S136 mould steel was verified by experiments.

Sandwich flexible elastic grinding wheel tool
The sandwich elastic grinding wheel adopts a three-layer structure, including the grinding wheel matrix, an elastic interlayer and an abrasive layer, in which the grinding wheel matrix material is aluminium alloy.The overall structure is shown in Figure 1.Firstly, the hard elastic material is used for the elastic interlayer material to ensure its material removal rate.Secondly, the grinding process generates grinding heat, so the temperature resistance of the elastic material should also be considered.The impact elasticity of ethylene propylene diene monomer (EPDM) is good, and the rebound rate can reach 60%.It can be used for a long time at 150 °C and heated for a short time at 200 °C [18].EPDM rubber sheet is the hardest of the rubber types.Therefore, to further increase the hardness of this elastic material, fabric-EPDM rubber is selected as the elastic layer material in this paper, as shown in Figure 2. Its material performance parameters are shown in Table 1.The abrasive layer adopts 3M's 237AA series pyramidal abrasive belt (3M Trizact™), as shown in Figure 3. Trizact abrasives provide a precision surface with a microreplicated structure.The microscopic view of a pyramid in a Trizact abrasive belt is shown in Figure 4, where a pyramid consists of abrasive grains, an epoxy bond and trace amounts of alumina minerals [19].The grinding wheel dressing system designed in this paper is shown in Figure 6.The dressing process was divided into two parts: the dressing of the elastic sandwich layer and the dressing of the abrasive layer.The grinding wheel used for dressing is a WA40 ceramic CBN cup wheel.To reduce the wear of the dressing wheel and increase the service life of the dressing wheel, a relatively low speed (n=1000 rpm) and a relatively fast feedrate (f=200 mm/min) were selected.After dressing, the circular runout error of grinding wheel was controlled within 10 μm.
A non-contact 3D white light scanning laser interferometer (Zygo New View 7100) was used to measure the surface roughness.The surface micromorphology of the workpiece and the grinding wheel was detected using the ultra depth of field (VHX-1000) three-dimensional microscope system.The surface profile was measured by taking the mechanical stylus profilometer (Taylor Hobson).The weight method is used to calculate the material removal quantity of workpiece processing to obtain the material removal rate.Before the workpiece was weighed, surface oil stains and wear debris were wiped with anhydrous ethanol and then the workpiece was cleaned in the ultrasonic cleaning machine.The relevant experimental conditions and equipment specifications are shown in Table 2. Ultra-depth field microscopy VHX-1000

Results
The grinding process is a complex procedure.Characteristics of machine tools, workpiece materials and shapes, grinding wheels, grinding parameters, and grinding fluids all have a significant impact on the surface quality of the ground workpiece.Under existing processing conditions, a single-factor experimental method was used to investigate the effect of grinding parameters, including the grinding depth of down-pressing, wheel speed, feedrate, and abrasive grain size, on the surface quality and material removal efficiency during the grinding of S136 mould steel using a sandwich elastic grinding wheel.Single factor experimental parameters are shown in Table 3. the increase in the grinding wheel speed, the plastic bulge of the machined surface increases and the surface roughness increases.This phenomenon is not consistent with the general law of rigid grinding wheel because when the depth of downpressing is small (ap < 7 μm), due to the elasticity of the grinding wheel, the grinding process removes the original shape that is the residual peak.At a depth of 4 μm, the original shape of the workpiece surface is not completely removed.The roughness tends to decrease with the increase in the downpressing depth until the roughness reaches the minimum when the original shape is completely removed.Thereafter, as the downpressing depth increases, the pressure between the grinding wheel and the workpiece surface increases, the depth of the abrasive grain cutting into the workpiece surface increases and the grinding depth of single abrasive increases (the undeformed cutting thickness increases).At the same time, the contact arc length between the grinding wheel and the workpiece increases accordingly, and the plastic deformation of the workpiece being processed increases, thereby enhancing the surface roughness of the workpiece.

Wheel speed.
Figure 8 shows the surface roughness for the grinding wheel speed.Under specific grinding parameters (abrasive grain A30, downpressing depth 7 μm, feedrate 200 mm/min and path distance 0.1 mm), with the increase in the wheel speed, the plastic bulge of the machined surface increases and the surface roughness increases.This phenomenon is not consistent with the general law of a rigid grinding wheel because the presence of its elasticity causes the actual grinding depth to not coincide with the downpressing depth.At a given downpressing depth, the elastic grinding wheel is in contact with the workpiece at a specific pressure.As the surface material is removed from the workpiece, the grinding wheel compression decreases, and the layer removal is achieved through the slow recovery of the grinding wheel elasticity.When wheel speed is increased, the tangential resistance of the abrasive grain increases, and the elastic action duration of the grinding wheel decreases.That is, the elastic effect is weakened.Although the number of abrasive grains per unit time in the grinding area increases, the thickness of the layer in the layer removal increases so that the thickness of single-grain undeformed cutting is not reduced but increased, and therefore, the roughness increases.

Feedrate.
The relationship between workpiece feedrate and surface roughness is shown in Figure 9.The experimental results show that with the increase of the feedrate, the plastic bulge (machining marks) increases slowly and the surface roughness of the workpiece increases slowly for the given parameters (abrasive grain A30, wheel speed 2000 rpm, down-pressing depth 15 μm, path distance 0.1 mm).This result occurred because as the workpiece feedrate increases, the number of abrasive grains involved in grinding at the same grinding point per unit time decreases, and the cumulative time for material removal decreases, thus increasing the surface roughness.

Abrasive grain size.
A wide range of surface roughness was observed for the various abrasive grain sizes, from 9.584 nm to 19.402 nm (Figure 10).The experimental results show that with the increase in the abrasive grain size, the surface plasticity of the workpiece becomes progressively obvious and the surface roughness rises with the increase in the abrasive grain size, given that the other parameters remain unchanged (wheel speed 2000 rpm, down-pressing depth 7 μm, path distance 0.1 mm).This result occurred because as the abrasive grain size increases, the number of abrasive grains in contact with the workpiece surface decreases, the number of abrasive grains involved in cutting per unit time at a certain grinding point decreases and the average pressure on a single abrasive grain becomes larger.As a result, the cutting thickness of a single abrasive grain increases and the abrasive cutting marks on the surface of the workpiece being processed increase, which finally leads to an increase in surface roughness.

Effect of grinding conditions on material removal rate 3.2.1. Depth of downpressing.
Figure 11 shows the change in the material removal rate at different downpressing depths.The material removal rate increases as the downpressing depth increases because the contact area between the grinding wheel and the workpiece surface rises with the increase in the downpressing depth, and the number of abrasive grains involved in the same grinding point per unit time increases.At the same time, the pressure between the grinding wheel and the workpiece increases, and the depth of the abrasive grains cutting into the workpiece surface increases, thereby increasing the material removal per unit time and the material removal efficiency.

Wheel speed.
The relationship between grinding wheel speed and material removal rate is shown in Figure 12.As can be seen from the picture, the material removal rate increases as the grinding wheel speed increases, keeping the other parameters constant, because the elastic wheel is characterised by slow recovery of its elasticity as the material is removed from the workpiece surface, thus achieving layer-by-layer removal.At the same time, as the wheel speed increases, the cutting capacity of a single grain increases and the number of abrasive grains involved in the same grinding point per unit time increases, resulting in faster material removal and an increase in the actual removal depth.As a result, the total amount of material removed per unit time increases and the material removal efficiency increases.

Feedrate.
The grinding wheel is processed at different feedrates on the surface of the workpiece to observe the change in the material removal rate, as shown in Figure 13.The experimental results show that the material removal rate increases as the feedrate increases, because as feedrate increases, the number of abrasive grains involved in grinding at the same grinding point per unit time decreases, but at the same time, the grinding processing time decreases significantly.This time reduction is faster than the reduction in the number of grits involved in the same grinding point per unit time.Finally, the material removal efficiency increases.

Abrasive grain size.
Different sizes of abrasive grains were selected for processing to observe the relationship between the particle size and the material removal rate.Figure 14 shows that the material removal rate increases with the increase of abrasive particle sizes, given that the other parameters remain the same because a large abrasive grain size corresponds to less abrasive grains on the abrasive layer that actually participate in the surface contact of the workpiece.As the average pressure on a single abrasive grain becomes larger, the number of abrasive grains in the sliding and ploughing stages is relatively reduced.The number of abrasive grains in the cutting stage increases, and thus, the material removal rate increases.

Discussions
To further explore the relationship between the actual grinding depth ap' and the surface roughness Ra after workpiece processing, the S136 mould steel was ground with different grinding parameters (f = 100-500 mm/min, n s = 1000-3000 rpm, a p = 4-20 μm) under different abrasive grain sizes.The experimental results are shown in Figure 15.The figure shows that within the given parameter range, the surface roughness range of the workpiece processed by three kinds of abrasive grain sizes is approximately below 45 nm.The surface roughness of the three kinds of abrasive grain size can reach about 15 nm by selecting the appropriate combination of parameters, and the surface roughness of the workpiece processed by an abrasive grain size of 16 can even be as low as one nanometre order.
Therefore, to obtain a high-quality surface with a relatively high material removal rate, appropriate grinding parameters should be selected according to the original surface of the workpiece.When the original surface of the workpiece is rough, a relatively high wheel speed, a larger abrasive grain size and feedrate, and a higher downpressing depth need to be selected.When the original surface of the workpiece is good, a relatively moderate wheel speed, a smaller abrasive grain size, a faster feedrate and a lower downpressing depth need to be selected.When the appropriate processing parameters are selected, the surface grinding of S136 mould steel mirror may be completed.The original surface roughness of the workpiece used in the mirror grinding experiment is Ra 142.762 nm.The experimental process parameters are selected as Table 4.The ground surface can be achieved mirror finish by machining in raster trajectory mode (Figure 16).The sandwich elastic grinding wheel corrected the S136 mould steel from Pt 2.7893 μm to Pt 0.1964 μm (Figure 17).The surface roughness (Figure 18) was improved by nearly two orders of magnitude (Ra 142.742 nm to Ra 7.472 nm), which also proved the effectiveness of the tool.Figure 19 shows that the surface of the workpiece is not burned and no obvious grinding groove was created by using the new grinding wheel, which indicates that the grinding state is good, thus ensuring the uniformity and consistency of the grinding process.Compared with previous research works (as shown in Table 5), we can see that the minimum roughness value obtained by sandwiched elastic grinding wheel grinding in these research methods is considerable.Moreover, the grinding tool has a coarse abrasive grain, fewer processing steps, and relatively low processing conditions, indicating that using a sandwiched elastic grinding wheel for grinding has certain advantages.However, the grinding wheel still has some drawbacks.Its oil resistance is poor, which requires the use of oil barriers or water-based grinding fluids.Additionally, since the adhesive bonding method is used, high temperatures can affect the overall stability of the grinding wheel.Therefore, it is necessary to control the grinding temperature and enhance the cooling capability.
The above analysis reveals that because the grinding wheel has certain elasticity, the grinding force reduces.Under the effective cooling condition, the use of the new sandwich elastic grinding wheel can avoid workpiece surface burns and obtain high-quality and high-precision processing surface for S136 mould steel.At the same time, it can reduce the processing procedure, the processing time, the processing cost and improve the processing efficiency, which has great potential in the grinding of S136 mould steel.

Conclusions
This paper presents an innovative sandwich elastic grinding wheel that can be used for grinding hardened S136 mould steel to obtain a mirror effect.The composition and design of the tool are introduced in detail, mainly by adding an elastic layer between the substrate and the abrasive layer to ensure the flexibility of tools.The grinding performance of sandwich elastic grinding wheel is investigated from the point of technological parameters.The new grinding process has the following characteristics: (1) The sandwich elastic grinding wheel is simple in structure and low in cost, and it has low requirements for machine tools.
(2) In the grinding process, a grinding wheel and the workpiece undergo squeeze contact, the intermediate elastic layer is shaped according to the shape of the workpiece so that it can be closely fitted to the workpiece and maintain a good fit.Therefore, high-precision grinding can be achieved even with poor rigidity of tool and NC machine tool.Furthermore, a sample statistical analysis was conducted, which revealed that the surface roughness of S136 mould steel with a hardness of HRC48~50 can reduce from an original surface roughness of Ra0.1-0.2 μm to below 45 nm by using a sandwich elastic grinding wheel.
(3) The effects of various process parameters on surface roughness and material removal rate were successfully investigated.The results show that the material removal rate can be improved by increasing the depth of down-pressing, the abrasive grain size, the grinding wheel speed and feedrate, but the increase of material removal rate will bring the corresponding increase of roughness.In the single machining process, the corresponding process parameters should be formulated according to the original surface condition of the workpiece.For example, using the grinding parameters listed in Table 4, a small surface area of S136 mould steel with an original surface roughness of Ra 142nm was processed, resulting in a surface roughness as low as Ra 7.472 nm, thus obtaining an excellent mirror effect.
This novel grinding process is very advantageous compared with the existing grinding technology, being able to obtain the target surface through a single processing.In the machining process of S136 mould steel, the use of sandwich elastic grinding wheel is a promising technology that can replace the existing tools.

Figure 1 .
Figure 1.Structural diagram of the sandwich flexible elastic grinding wheel.

Table 1 .
Fabric-EPDM rubber material performance parameters.Parameters Value Elastic modulus (MPa) 68.05 Linear elastic compressibility (mm) About 0.5 Density (g/cm 3 ) 1.0 (20.0 °C) Shore hardness (HA) 78 Thermal diffusivity (mm 2 /s) 0.146 (200 °C); 0.130 (300 °C) Coefficient of linear thermal expansion (mm/°C) 0.23-0.25 2.2.Experimental setup S136 mould steel is a high-quality super mirror corrosion-resistant mould steel produced by ASSAB (Sweden).S136 mould steel is medium-carbon high-chromium steel with high toughness, poor thermal conductivity and low elastic modulus.After quenching, the hardness of the workpiece is 48-50 HRC, and the original surface roughness of the workpiece is Ra 0.1-0.2μm.The grinding machine used for the experiment was a TG-200A five-axis CNC tool grinder manufactured by Taigang CNC Precision Machine Tools Co, which is cooled by grinding oil with pressure of 0.8Mpa and flow rate of 40L / min, as shown in Figure 5.

Figure 7 .
Figure 7. Effect of downpressing depth on surface roughness.

Figure 8 .
Figure 8.Effect of wheel speed on surface roughness.

Figure 9 .
Figure 9.Effect of feedrate on surface roughness.

Figure 10 .
Figure 10.Effect of abrasive grain size on surface roughness.

Figure 11 .
Figure 11.Effect of downpressing depth on material removal rate.

Figure 12 .
Figure 12.Effect of wheel speed on material removal rate.

Figure 13 .
Figure 13.Effect of feedrate of workpiece on material removal rate.

Figure 14 .
Figure 14.Effect of abrasive grain size on material removal rate.

Figure 15 .
Figure 15.Relationship between actual grinding depth and surface roughness.

Figure 16 .
Figure 16.Surface appearance of the workpiece before and after processing.

Figure 17 .
Figure 17.Surface contour of the workpiece before and after processing.

Figure 18 .
Figure 18.Surface microstructure and roughness of the workpiece before and after processing.

Figure 19 .
Figure 19.Surface microstructure of the workpiece before and after processing.

Table 2 .
Related experimental conditions and equipment specifications.

Table 3 .
Single factor experimental parameters.
3.1.1.Depth of downpressing.The relationship between the depth of downpressing and surface roughness is shown in Figure7, along with the surface conditions.Under specific grinding parameters (abrasive grain A30, downpressing depth 7 μm, feedrate 200 mm/min and path distance 0.1 mm), with

Table 5 .
Related parameters and roughness values of each processing method.