Experimental investigation on magnetorheological fine finishing of EN-8 steel alloy mold surface for cylindrical plastic bottle caps

Abstract

The cylindrical mold is a key element utilized in the manufacture of different cylindrical components for a wide range of applications in industries like automotive, aerospace, medical, and among others. Fine finishing of the cylindric mold with high surface integrity has several benefits such as good fits and tolerance, increased product quality, and high load-bearing capability for components. Recently, a cutting-edge rotating magnetorheological honing (RMRH) method based on permanent magnet tools was developed for the fine finishing of interior cylindrical surfaces with great productivity. Furthermore, many industrial real-time cylindrical molds need to be fine-finished with high productivity to enhance their functionality. To meet these requirements, the RMRH procedure could be the better option. Therefore, the RMRH process is investigated in the current study to finely finish the inner surface of the real-time EN-8 steel mold for plastic bottle caps. To determine how well the real-time EN-8 steel cylindrical mold could be fine-finished using the current process, the response surface technique was used to know the best finishing parameters. The significant % drop in parameters of the surface roughness profile (Ra, Rz, and Rq) of the MR finished real-time EN8 mold surface is achieved as 86.05, 82.17, and 84.62, respectively, after 40 minutes of the RMRH process with the optimal parameters. Also, a considerable improvement in straightness is obtained as the final surface waviness on the inside cylindrical surface of the real-time EN8 mold for plastic bottle caps is decreased from an initial value of 120 nm to 40 nm. Further, circularity and surface morphology tests are carried out over the mold's finished inner surface. After providing the efficient MR finishing using the current process, results indicate that surface properties and functional efficacy of the internal surface of the real-time mold for producing plastic bottle caps have improved.

Introduction

In today's era, a lot of cylindrical components in industries related to automobiles, medicine, food, defense, etc. are manufactured using casting and injection molding processes. ¹ In the casting process, a solid substance is dissolved, heated at a sufficient temperature, and then poured into a mold or cavity which retains it in the proper shape throughout solidification. ² Further, in the injection molding process, the molten plastic material is injected into a mold which then cools and ejects a plastic part from the machine. ³ In both processes, the super-finished surface of molds with good surface integrity has various benefits for manufactured components such as good tolerance, and increased product quality. ⁴ Since the surface characteristics of the industrial components majorly decide their reliability and long life, it is essential to have a mold with a high grade of finish. Further, it is also essential for mold to have high dimensional accuracy. This is because inaccuracy in the dimension of mold may lead to an inaccurate component. This can further lead to wear and friction owing to which there are chances of component failure while functioning. ⁵

For finishing the molds, a variety of conventional methods are used such as internal surface grinding ⁶ and hand-held grinding.^7,8 However, in these processes, the finishing force is uncontrollable which may cause a variety of surface flaws and affects the dimensional accuracy of the product. ⁹ Therefore, several advanced finishing techniques have been developed to overcome these defects and provide fine finishing on the surface while maintaining control over the exterior finishing forces. The magnetorheological finishing (MRF) process is one such process. The magnetorheological polishing (MRP) fluid is used in the MRF process. ¹⁰ In the normal state, the MRP fluid is liquid in nature. In contrast, the electrolytic iron particles (EIPs) in the MRP fluid magnetize when exposed to a magnetic field. ¹¹ This causes MRP fluid to stiffen and create a columnar structure as a result of the increase in rheological characteristics. ¹² Different finishing processes for fine finishing with various shapes have been previously created based on the MRP fluid.^13,14 Singh and Singh ¹³ developed MR finishing using a rotating core technique for fine finishing the cylindrical exterior surface of workpieces with better productivity. The revolving tool core with a rectangular shape is effectively used to fine finish industrial applications like cold rolls and grooved drums.^14,15 Arora and Singh ¹⁶ utilized the hemispherical tip-based MRF process for the fine finishing of the UHMWPE acetabular cup for better functionality.

Abrasive flow machining (AFM) ¹⁷ and rotating-abrasive flow finishing (R-AFF) ¹⁸ were developed for the fine finishing of the cylindrical workpieces’ interior surface to overcome the constraints of conventional finishing procedures. The finishing surfaces, however, counter different surface flaws because of the abrasives’ unpredictable motion and uncontrollable forces in these operations. Further, magnetic abrasive finishing (MAF) ¹⁹ was used to provide controllable finishing forces while fine finishing the hard material surface. The risk of scratching soft material surface, however, was a limitation of this technology because it used dry mixed powder of abrasives and magnetic particles. Further, magnetorheological abrasive flow finishing (MRAF) ²⁰ and rotating magnetorheological abrasive flow finishing (R-MRAF) ²¹ were successively developed. However, the magnetic field source for these activities was located outside of the cylinder. As a result, the ferromagnetic workpiece's internal surface was covered with iron particles due to the increased magnetic field at these surfaces, preventing abrasive particles from making contact with the finishing surface. Therefore, these techniques are only capable of interior surface finishing of nonferromagnetic workpieces. To overcome these restrictions, electromagnetic ²² and permanent tool-based ⁶ magnetorheological finishing processes were subsequently devised, where the sources of the magnetic field were made inside of the cylinder. Therefore, both ferromagnetic and nonferromagnetic cylindrical workpieces could be finely finished using this procedure. Further, after the establishment of this procedure for finely finishing the cylindrical workpieces irrespective of the nature of their material, it was further realized to improve its finishing productivity. Because, even though the method was proven to be capable of finishing all sorts of cylindrical workpieces’ material, finishing productivity for industrial usage is still a crucial need. This led to the development of the rotary magnetorheological honing (RMRH) method ²³ which is based on the finishing mechanism with the shuffling of abrasives and their relative motion improvement with regard to the workpiece surface. ²⁴ During this RMRH procedure, the cylindrical workpiece rotates in opposition to the magnetic tool. This led to an increase in process productivity with relative motion improvement. As a result, the industries may find that this improved process efficiency is more suitable for the speedy super finishing of cylindrical type molds or any components of a like nature.

Therefore, in the present work, a real-time cylindrical mold that is used in the industry for producing plastic bottle caps is finely finished using the present RMRH process. To increase the functionality of the cylindric mold which is made of the EN-8 steel alloy, the interior surface has to be finely finished. Therefore, the best RMRH process parameters for it were determined through the parametric evaluation. Further, the real-time EN-8 cylindrical mold has been finely finalized using the optimum process parameters that could be obtained. Finally, the mold surface quality is analyzed with scanning electron microscopy, surface roughness, and waviness. Also, the improvement in dimensional accuracy (circularity) has been investigated.

Materials and methods

Material selection

The EN-8 steel alloy mold is employed in the current process for finishing. The advantages of EN-8 in the mold industries, such as its lower shrinkage capacity, excellent machinability, higher damping capability, higher wear resistance, and higher abrasion resistance while casting the products, have raised its significance. ²⁵ The chemical composition of the bottle cap mold material is obtained by a spectroscopy test performed on the spectrometer. The value in terms of weight percentage is reported in Supplementary Table S1. From this test result, it is found that the material is an EN-8 steel alloy. In the present study, mold is utilized with an inner surface having a length of 60 mm and a diameter of 38 mm as depicted in Figure 1(a).

Figure 1.

(a) Photograph of the real-time EN-8 steel alloy mold for producing plastic bottle caps and (b) plastic bottle cap as final end product produced from the EN-8 steel alloy mold.

The final bottle cap component generally used to manufacture from the present mold is depicted in Figure 1(b). Traditional grinding is initially performed on workpiece samples to remove uneven markings from the inner cylindric mold surface. After that, the workpiece is cleaned with acetone and properly maintained before being finished with MRP fluid. For conducting the present study on this component, the MRP fluid is used as a finishing agent.

Experimental setup

The present experimentation on the EN-8 steel mold is performed utilizing the RMRH process as shown in Figure 2. The machine structure, a workpiece holding fixture, and a permanent magnet honing tool make up the present setup. Further, the machine structure comprises three servo motors (S1, S2, and S3) which are used to provide the different types of motions in the process. The servo drives are used to operate the servo motors which are controlled by the programmable logic controller (PLC). The PLC ensures that there is accuracy in the movements of the tool and the work part surface. The current technique employs three different forms of motion to finish the internal cylindric surface of the mold. These include the rotary and reciprocatory motions of the magnetic tool, as well as the rotational motion of the workpiece. The real-time EN-8 alloy steel mold is held in place by a unique fixture with an adjustable height as shown in Figure 2. Thus, the internal surface of the real-time EN-8 alloy steel mold is productively fine finished with this experimental setup of the RMRH method.

Figure 2.

Experimental setup for internal surface finishing of mold using RMRH process.

RMRH process mechanism for finishing the interior surface of the EN-8 steel mold

The abrasion action in the RMRH process is caused by the active abrasive particles (AAPs) rotating and reciprocating along the internal surface of the revolving cylindrical mold. Furthermore, the AAPs take the helical route as depicted in Figure 3 due to the concurrent rotation of the magnetic tool and workpiece, as well as a reciprocation of the magnetic tool. The asperities from the mold's internal surface get sheared off along this helical path. The helical route length and number of revolutions of the AAP on the helical path increase as a result of the opposite rotation of the workpiece surface and magnetic tool. ²³ This results in a fine and uniform surface finish on the mold's inside surface. Furthermore, as shown in Figure 3, the removal of material from the mold's inside surface is caused by three types of forces acting on the AAPs in this process as tangential shear force (F_{t_mg}), magnetic indentation force (F_{n_mg}), and axial force (F_{a_mg}).

Figure 3.

Material removal mechanism for fine finishing of internal surface of bottle cap mold using rotational magnetorheological honing (RMRH) process.

When the MRP fluid is placed over the surface of the permanent magnet of the tool during surface finishing, the EIPs get magnetized and create a columnar structure. Nonmagnetic AAPs in the EIPs chain tend to travel away from high gradient magnetic flux density (MFD) and toward low gradient MFD (workpiece surface). The exterior surface of the permanent magnet tool has a strengthened columnar structure created by the EIPs and AAPs. The magnetic EIPs exert the magnetic force (F_{n_mg}) over the AAPs, and the stiffened MRP fluid acts as a unibody across the tool surface (Figure 3). Due to this F_{n_mg}, the workpiece surface's asperities are indented by the AAP. The micro plowing of the material around the indented abrasive marks is caused by the indentation of the AAPs. ²⁶

The indentation of AAPs into the workpiece's finishing surface is aided by this force. Further, the AAPs shear off the asperities from the interior surface of the mold workpiece as the magnetic tool rotates and reciprocates in tandem with the rotation of the workpiece. The AAPs encounter the tangential shear force (F_{t_mg}) and the axial force (F_{a_mg}) as a result of the tool's simultaneous rotation and reciprocation as well as the rotation of the mold workpiece, as indicated in Figure 3. The shear force's magnitude is strongly influenced by the relative speed of the AAPs. The relative motion of the AAP over the work part surface is increased by the rotational motion of the tool and the workpiece cylinder rotating in the opposite direction. The F_{t_mg} acts by an AAP on the surface of the work component which takes into account the tool and workpiece's rotational motions.

The magnetic tool has a reciprocation motion in addition to the tool and work component rotating at the same time. The AAP is subjected to an axial force (F_{a_mg}) due to the magnetic tool's reciprocation as shown in Figure 3. The F_{a_mg} helps the AAPs to clip off the asperities from the mold surface along the axial direction.

Further, the finishing of the workpiece surface's asperities relies on the resultant force (F _r ) from the F _{t _ m g} , F _{a _ m g} , and F _{n _ m g} . The removal of the asperities from the mold surface occurs due to the repetitive action of the F _r over the surface as shown in Figure 3. This results in a finely inside surface of the mold work component.

Experimentation

In the current study, the MRP fluid's composition has been chosen based on initial experiments and a literature review.^27,28 MRP fluid was made up of 60% carrier fluid, 20% electrolytic iron particles (EIPs) (500 mesh size), and 20% SiC abrasive by volume. In addition, the carrier fluid is composed of 20% AP3 grease and 80% paraffin oil by weight. The EIPs in MRP fluid are used due to their irregular form compared to the carbonyl iron particles’ (CIPs’) spherical form. ²⁴ The chain structure of CIPs may have a significant number of vacancies because of their spherical form. The ability of the CIPs chain structure to trap the irregular abrasive particles may thus be limited even though CIPs have a greater magnetic saturation capability than EIPs. This may result in a decrease in the MR finishing effectiveness. However, the irregularly formed EIPs structure may hold abrasive particles more firmly and endure more shear strength which improves the finishing performance of the process. Therefore, the current MRP fluid uses EIPs.

Further, using the newly developed RMRH process, a detailed study for the effective finishing of the internal cylindrical mold surface of EN-8 steel alloy has been done. In this work, three parameters namely magnetic tool rotational speed (T), tool reciprocation speed (A), and workpiece rotational speed (W) are selected. Each of these parameters has its own influence on the present process. The output of the process is taken as % increase in surface finishing (% ΔR_a). The % ΔR_a is calculated using Equation (1).

$$\text{\%}\mathrm{\Delta }{R}_a={\displaystyle \frac{(R_{ain}-R_{af})}{R_{ain}}\times 100}$$

(1)

where $R_{ain}$ is the initial surface roughness (SR) and $R_{af}$ is the final SR of the internal of the mold workpiece surface.

Additionally, preliminary experimentation was carried out on the interior surface of the cylindric mold workpieces to identify the range of the different process parameters. The initial average SR (R_a) value is measured between 390 and 430 nm. The SJ-400 Mitutoyo surface roughness profilometer with a 0.25 mm cutoff length is used to measure the SR. The initial experiments are then used to select the process parameters which are listed in Table 1. Design of experiments (DOE) is utilized to examine the influence of various factors on the interior surface of the EN-8 mold workpiece. Response surface methodology (RSM) is used to analyze the individual and combined effects of process factors. The regression model's main objective is to examine the decrease in process variability in % ΔR_a and to improve the effectiveness of the inner surface fine finishing of the EN-8 mold steel using the current RMRH process.

Table 1.

Process parameters and their range.

Finishing parameters	Range
	−2	−1	0	1	2
Rotational speed of the magnetic tool, T (rpm)	250	350	450	550	650
Reciprocation speed of the magnetic tool, A (cm/min)	35	55	75	95	115
Rotational speed of the EN-8 steel mold workpieces, W (rpm)	5	15	25	35	45

In the present study, three parameters and five levels of design are utilized to study the experimentations with the help of the central composite design (CCD). The CCD has planned a total of 20 trials as reported in Table 2. The dimension of the internal cylindric EN-8 work samples used for the experiment is 38 mm in diameter and 30 mm in length. Each experiment is done for 20 minutes and after each experimentation, the MRP fluid is changed over the magnetic tool surface. This is done to keep the abrasive cutting edges sharp throughout the surface finishing of the EN-8 mold's interior cylindric surface. Further, to verify the efficiency of the present model, the confidence level of the regression model is kept at 95%. Next, to strengthen the relationship between the output response (% ΔR_a) and the process parameters, the F-level test using analysis of variance (ANOVA) is performed. To demonstrate the effectiveness of the method for finely finishing the interior EN-8 mold surface, the circularity, waviness, and surface characteristics are also examined using the coordinate measuring machine (CMM) Accurate Spectra 5.6.4, Mitutoyo Surftest SJ-400, and scanning electron microscopy (SEM) JEOL JSM 6510 LV, respectively.

Table 2.

Plan of experiment and their response % ΔR_a after 20 minutes of the RMRH process over the inner surface of the EN-8 steel workpieces.

Sr. No.	Magnetic tool rotation	Workpiece rotation	Magnetic tool reciprocation	Initial average surface roughness	Final average surface roughness (SR) value			Final average SR value	Response (% ΔRa) (Equation 1)
	T (rpm)	W (rpm)	A (cm/min)	Rain value (nm)	R1 (nm)	R2 (nm)	R3 (nm)	Raf (nm)
1	450	25	75	390	130	120	110	120	70
2	450	25	115	410	180	170	160	170	59
3	450	25	75	400	110	120	130	120	70
4	450	25	75	390	120	120	120	120	69
5	550	35	55	400	150	140	130	140	65
6	350	15	95	390	160	150	140	150	61
7	250	25	75	420	250	260	270	260	38
8	650	25	75	400	160	170	180	170	57
9	350	15	55	410	190	200	210	200	50
10	450	45	75	420	160	150	140	150	64
11	450	25	75	430	170	160	150	160	63
12	550	15	95	410	160	170	180	170	59
13	450	25	35	390	220	230	240	230	41
14	550	35	95	400	130	140	150	140	65
15	350	35	55	420	210	220	230	220	48
16	450	25	75	390	110	100	90	100	74
17	450	25	75	410	140	120	100	120	71
18	550	15	55	420	250	240	230	240	43
19	350	35	95	430	200	220	240	220	49
20	450	5	75	410	260	240	220	240	42

Results and discussion

Twenty experiments are conducted in total to investigate the effect of process factors on the outcome response (% ΔR_a). The experimentation data after performing finishing is reported in Table 2. Further, the average surface roughness (SR) value is taken at three points (R1, R2, and R3) over the internal cylindric surface after the RMRH process and given in Table 2. By averaging the three SR values, one arrives at the final average SR value, which is then used in Equation (1) to get the % ΔR_a. Next, the analysis of variance (ANOVA) is utilized for creating the statistical model as given in Supplementary Table S2. The ANOVA model revealed that the anticipated model's p-value (PV) is smaller than 0.05. This demonstrates the significance of the current model. Further, the PV of the lack of fit is higher than 0.05 (0.6047), confirming that the lack of fit is not significant. Thus, parameters with a p-value lower than 0.05 are deemed significant, whereas parameters with a p-value higher than 0.05 are deemed non-significant. In Supplementary Table S2, the ANOVA with the significant parameters’ effects on the output response (% ΔR_a) is presented. Equation (2) provides the second-order mathematical regression model that is used to examine the effect of the process factors on % ΔR_a.

$$\text{\%}\mathrm{\Delta }{R}_a=-141.51+0.417T+0.494W+2.50A-5.73\times {10}^{-4}{T}^2-2.86\times {10}^{-2}{W}^2-1.24\times {10}^{-2}{A}^2+5.50\times {10}^{-3}TW-1.75\times {10}^{-2}WA$$

(2)

where T is the tool's rotation (rpm), A is tool reciprocation (cm/min), and W is the workpiece rotation (rpm). Further, Equation (2) is used to compare the % ΔR_a predicted for each experimentation trial which is then compared to the experimental data. The model can be used to verify the closeness between anticipated and experimental values as the R-squared value is 93.52% and Adj R-squared is 88.81% value showing the accuracy of the model. Next, each significant parameter's effect on the output response (% ΔR_a) in the terms of the percentage contribution (% cont.) has been analyzed using Equation (3) and reported in Supplementary Table S2.

$$\text{\%}\mathrm{cont}.={\displaystyle \frac{\mathrm{Sum}\mathrm{of}\mathrm{square}\mathrm{of}\mathrm{parameter}}{\mathrm{Total}\mathrm{sum}\mathrm{of}\mathrm{square}\mathrm{of}\mathrm{model}}\times 100}$$

(3)

The % cont. is useful to indicate the process parameter effect on the output response, that is, % ΔR_a. From the values of % contribution (Supplementary Table S2), it can be observed that the % cont. of tool rotation (T) contributes as 8.17%, reciprocation speed of tool (A) as 8.17%, and workpiece rotation (W) as 6.67%. Further, following the regression analysis, various significant parameters’ results have been addressed in this section.

Effects of the significant process parameters on % ΔR_a

Figure 4 shows the interaction effect of T and W on the % ΔR_a when the tool's reciprocating speed (A) is kept constant. This figure shows that with the initial rise in tool rotation (T) and workpiece rotation (W) concurrently, the improvement in surface finishing increases. This happens up to T = 550 rpm and W = 35 rpm. But, above these values of W and T (35 rpm and 550 rpm, respectively), the % ΔR_a starts decreasing. The net relative motion of AAPs increases when the tool and workpiece are rotated concurrently in opposing directions, resulting in a rise in the net tangential shear force. This results in the higher shearing off roughness asperities from the mold's internal surface as depicted in Figure 3. Additionally, when the T and W rise, the helical path length followed by the AAP increases, resulting in fine finishing. This causes the trendline to rise of the % ΔR_a. However, above the best values, the relative motion of AAPs becomes very high at the simultaneous higher tool and workpiece rotation, reducing the amount of interaction required for AAPs to remove asperities from the surface of the mold work component. This reduces the amount of material being removed from the work component surface. Also, when the speed increases, the net tangential shear force increases to such an extent that the EIPs in the columnar structure of the MR polishing fluid become unstable, lowering the finishing performance. This decreases in the trendline of the % ΔR_a. Thus, the optimum value of T and W are found as 550 rpm and 35 rpm, respectively, over which maximum surface finish is achieved.

Figure 4.

Effects of the tool's rotational speed (T) and the mold workpiece's rotational speed (W) on % ΔR_a.

Figure 5 shows the interaction effect of the W and A on the % ΔR_a when the tool rotational speed (T) is kept constant at 550 rpm. From this figure, it is found that with the initial increase in the values of W and A simultaneously the trend line of % ΔR_a increases. This is because the simultaneous initial rise of W and A causes the upsurge in overall cutting shear force by increasing tangential and axial shear forces as the relative speed of AAPs increases along rotating as well as reciprocating direction as depicted in Figure 3. But after achieving maximum % ΔR_a at W = 35 rpm and A = 75 cm/min, the % ΔR_a starts decreasing. This is due to an excessive increase in these values the time for which AAPs interact with the roughness asperities present on the finishing surface reduces. As a result, the AAPs move from asperity without much abrading it. Therefore, the optimum value of A and W are found as 75 cm/min and 35 rpm, respectively, over which maximum surface finish is achieved.

Figure 5.

Effects of the tool's reciprocation speed (A) and the mold workpiece's rotational speed (W) on % ΔRa.

Next, five further experiments were conducted to ensure the repeatability of the regression model that was developed in this study by the application of the RSM technique. In these studies, the asperity of the surface before and after the RMRH process was measured. The predicted and experimental % ΔR_a was compared using % error as reported in Supplementary Table S3. The % error is calculated using Equation (4).

$$\begin{array}{rl}& \text{\%}\mathrm{error}=\\ & {\displaystyle \frac{(\mathrm{Experimental}\text{\%}\mathrm{\Delta }Ra-\mathrm{Predicted}\text{\%}\mathrm{\Delta }Ra)\times 100}{\mathrm{Experimental}\text{\%}\mathrm{\Delta }Ra}}\end{array}$$

(4)

The proposed regression model is well suited for repeatability because the discrepancy, that is, the error between the experimental and anticipated (predicted) % ΔR_a is within the range of −3.69% to 4.06% (Supplementary Table S3).

Analysis of the finished internal cylindrical surface of the EN-8 steel alloy mold with optimal process parameters

Now, the internal surface of the real-time industrial EN-8 steel alloy mold (Figure 1(a)) has been finely finished utilizing the RMRH process with the optimal parameters (T = 550 rpm, A = 75 cm/min, and W = 35 rpm). The finishing operation on the surface of the real-time EN-8 steel alloy bottle cap mold was carried out with respect to time until a significant enhancement in the surface finish was noticed. The surface finish was measured following each 20-minute RMRH process on the internal surface of the mold. After 40 minutes of finishing on the mold's work part inner surface, a significant improvement in the surface finish was attained. The surface roughness (SR) value of the EN-8 steel alloy mold is decreased from 430 nm to 60 nm. The SR profiles are demonstrated in Figure 6. Figure 6(a) depicts the SR profile of the initial surface, and Figure 6(b) depicts the SR profile of the final MR-finished inner surface of the bottle cap mold workpiece after 40 minutes of finishing with the RMRH process. The parameters of the SR profile of the initial mold's inner surface were found as Ra = 430 nm, Rz = 2300 nm, and Rq = 520 nm and the parameters of the SR profile of the MR finished mold surface is obtained as Ra = 60 nm, Rz = 410 nm, and Rq = 80 nm.

Figure 6.

Profiles of surface roughness (a) initial ground, and (b) final finished internal surface of 38 mm diameter and 60 mm length of the real-time EN8 steel alloy mold using the RMRH process with the optimal parameters (T = 550 rpm, A = 75 cm/min, and W = 35 rpm) in 40 minutes of finishing.

The % drop in parameters of the SR profile (Ra, Rz, and Rq) of the MR finished real-time EN8 mold surface is achieved as 86.05, 82.17, and 84.62, respectively. This significant drop in parameters of the SR profile shows the fine finishing on real-time EN8 mold surface utilizing the RMRH process with the optimal parameters. Figure 3 illustrates the distinctive finishing mechanism used for these fine finishes. A greater percentage drop in the SR profile parameters (Ra, Rz, and Rq) of the MR-finished real-time EN8 mold surface results from the higher tangential shear force when the cylindric mold rotates counterclockwise to the tool. The surface finish of the mold workpiece was significantly improved by comparing the roughness values of the initial ground and the final MR-finished mold's internal surface. The reduction in the SR profile parameter's (Ra) value from 430 nm to 60 nm (86.05%) shows the surface finish's fineness while lowering average peaks and valleys. However, it does not distinguish between peaks and valleys. Consequently, the decrease in the SR profile parameter (Rq) from 520 nm to 80 nm (84.62%) shows the fine surface finish with greater sensitivity to peaks and valleys than Ra because of the squared amplitudes in this. Also, the decrease in the SR profile parameter (Rz) from 2300 nm to 410 nm (82.17%) also illustrates the fine surface finish with the greatest decrease in profile heights. Thus, these greater percentage decreases in the SR profile parameters (Ra, Rz, and Rq) can extend the life of the real-time EN8 steel alloy mold for producing plastic bottle caps in industries.

Further, the surface waviness of the mold's inner surface was assessed on its initial ground and final finished surfaces to confirm the improvement in straightness attained after using the present process for its finishing as shown in Supplementary Figure S1. The initial inner ground surface of the cylindrical bottle cap mold workpiece component has waviness parameters of Wa = 120 nm, Wz = 560 nm, and Wq = 150 nm, as shown in Supplementary Figure S1(a). The waviness parameters were achieved as Wa = 40 nm, Wz = 180 nm, and Wq = 50 nm after 40 minutes of finishing with the RMRH process on the inner surface of the bottle cap mold using the best process parameters as shown in Supplementary Figure S1(b). The bottle cap mold's increased straightness on the interior surface increases its dimensional precision which results in dimensionally correct end products (plastic bottle caps) cast from this mold.

The functional performance of this mold is to produce the cap of the bottle as much as possible dimensionally accurate with aesthetically best surface quality. Such requirement of the end product relies on the minimum circularity deviation of the cylindric mold which may cause to provide the fewest possible resistances as well as avoids the product to become a distorted shape. ²⁹ Therefore, deviation in circularity is analyzed. To validate the enhancement in the circularity on the inner surface of mold achieved, the circularity assessment was carried out before and after the RMRH process. The test of circularity is performed to evaluate how near the true circle is to the actual circle of the inner cylinder surface which determines the work part's dimensional stability. ³⁰ The circularity image of the initial ground surface of the EN8 real-time bottle cap mold workpiece inner surface is shown in Figure 7(a) with a circularity deviation value of 0.2109 mm. Most of the points highlighted on the ground surface are deviated from the true circle of the circularity diagram, as can be seen in this figure. Whereas, Figure 7(b) depicts the circularity image of the inner finished surface of the bottle cap mold with the circularity deviation value of 0.0421 mm using the RMRH process. Major improvement in circularity can be clearly verified as most of the points reported either are on the true circle or are very close to it. The circularity deviation value was reduced to 0.0421 mm with 40 minutes of finishing on the internal surface of the bottle cap mold workpiece using the RMRH process. The results show that the RMRH process is considered to be helpful for increasing the circularity of the inner surface of the present considered real-time EN-8 steel alloy mold for the plastic bottle caps.

Figure 7.

Images of circularity of (a) initial interior surface and (b) final MR-finished internal surface of 38 mm diameter and 60 mm length of the real-time EN8 steel alloy mold using the RMRH process with the optimal parameters (T = 550 rpm, A = 75 cm/min, and W = 35 rpm) in 40 minutes of finishing.

Figures 8(a) and (b) depict the scanning electron micrographs of the before and after 40 minutes of the RMRH process with the optimal parameters over the inner surface of the real-time EN-8 steel alloy. Figure 8(a) illustrates a ground surface (Ra = 430 nm, Rz = 2300 nm, and Rq = 520 nm) that exhibits a variety of surface flaws, including deep grinding lays, surface irregularities, scratches, scratching marks, etc. This might be because of the significant normal force that was used during the grinding operation. However, while performing the RMRH process on the EN-8 steel alloy inner mold surface, the controlled magnetic normal force was used during the abrasion action of the micron-sized abrasive particles on the surface to abrade the roughness peaks. This abrasion action is caused by the tool rotating and reciprocating at the same time as the workpiece rotating under a magnetic indentation force, which yields the resultant finishing force ( $F_{\mathrm{r}}$ ) (Figure 3). As a consequence, after 40 minutes, the surface quality of the inside surface of the real-time EN-8 steel alloy mold improves as illustrated in Figure 8(b). The MR-finished surface (Ra = 60 nm, Rz = 410 nm, and Rq = 80 nm) exhibits a noticeable smoothness with a few finishing lines as compared to the initial surface in Figure 8.

Figure 8.

The scanning electron micrograph of (a) initial ground surface, and (b) final MR-finished interior surface of the real-time EN8 cylindrical mold surface (diameter of 38 mm and length of 60 mm) using the RMRH process with the optimal parameters (T = 550 rpm, A = 75 cm/min, and W = 35 rpm) in 40 minutes of finishing.

Thus, the significant improvement in the interior surface's fine finishing, texture, circularity, and waviness on MR-finished surfaces of EN-8 steel alloy mold for plastic bottle caps indicates the RMRH process's efficacy. This can be a considerable potential for application in industries to fine-finish a variety of industrial real-time cylindrical molds. Another advantage of the RMRH process is that because finishing is less time consuming, the various industrial real-time cylindrical molds can be fine finished with high productivity to improve their functionality.

Conclusions

In the current work, the inner surface of the EN-8 material mold for plastic bottle caps is finished using a rotating magnetorheological honing (RMRH) process. The influence of finishing process parameters on the % ΔR_a is investigated using the response surface methodology approach. This approximates the optimal process parameters for efficiently finishing the mold's inner surface with the present process. The following conclusions demonstrate the efficacy of the MR-finished surface of the mold in improving the effectiveness of its widespread applications for producing plastic bottle caps.

The study's anticipated optimum RMRH process parameters of 550 rpm for tool rotation, 75 cm/min for tool reciprocation, and 35 rpm for workpiece rotation are demonstrated to be in excellent order for improving the internal surface finish of the industrial real-time EN-8 steel alloy cylindrical mold.

The error between the experimental and anticipated % ΔR_a in the current study is attained within a range of −3.69% to 4.06%, indicating that the obtained regression model is reasonably accurate; therefore, the anticipated optimal process parameters for fine finishing the inside surface of the EN-8 mold for plastic bottle caps is found in good order.

After 40 minutes of finishing across an inner surface (38 mm of diameter and 60 mm of height) with the RMRH process, the average roughness value was lowered to 60 nm (86.05%) from the initial ground value of 430 nm. The result signifies that the present process is capable of fine finishing the EN-8 molds’ inner surface efficiently and increasing their usability.

The internal surface of the EN-8 mold's surface waviness and circularity deviation was found to be reduced by 40 nm and 0.0421 mm, respectively, from the initial values of the ground surface's waviness of 120 nm and its circularity deviation of 0.2109 mm after 40 minutes using the RMRH process. These results show that the inside surface of the real-time industrial EN-8 mold has improved in terms of dimensional and geometric accuracy. It also reveals that when plastic bottle caps are produced from the EN-8 mold surface that has been MR finished, the dimensional accuracy improves.

The improvement in surface properties of the mold surface following the RMRH process confirms the elimination of surface flaws like grinding lays with optimal parameters that further help molds’ smooth operation during their industrial applications.

The successful finishing execution of the RMRH operation on the inner cylindric EN-8 mold surface in less time reveals the usability and process effectiveness for the fine finishing of the variety of industrial real-time cylindrical molds.

Supplemental Material

sj-docx-1-pie-10.1177_09544089221143887 - Supplemental material for Experimental investigation on magnetorheological fine finishing of EN-8 steel alloy mold surface for cylindrical plastic bottle caps

Supplemental material, sj-docx-1-pie-10.1177_09544089221143887 for Experimental investigation on magnetorheological fine finishing of EN-8 steel alloy mold surface for cylindrical plastic bottle caps by Sunil Kumar Paswan, Kunal Arora and Anant Kumar Singh in Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering