Elsevier

Polymer Testing

Volume 91, November 2020, 106820
Polymer Testing

Detecting shrinkage voids in plastic gears using magnetic levitation

https://doi.org/10.1016/j.polymertesting.2020.106820Get rights and content

Highlights

  • Propose a magnetic levitation device for detecting shrinkage voids.

  • Establish the theoretical model for analysing shrinkage voids of plastic gears.

  • Verify the accuracy of magnetic levitation with CT detections.

  • Calculate the porosity levels with the average error less than 7%.

  • Analyze the distributions of shrinkage voids with a correlation coefficient of 99.8%.

Abstract

Shrinkage voids have a large influence on the quality of plastic gears, and it is still a problem to detect the voids inside gears accurately and conveniently. This paper presents a novel method for detecting shrinkage voids via magnetic levitation. The porosity levels of plastic gears can be calculated using magnetic levitation because the density of plastic gears is influenced by the shrinkage voids. The distribution of shrinkage voids is quantified by the moment of volume, hence a theoretical model for the distributions of shrinkage voids and levitating posture can be established. Computer tomography (CT) detections were also carried out to verify the accuracy of magnetic levitation for detecting the shrinkage voids. Experimental results show that the average relative error of calculated porosity level is less than 7%, and the theoretical model for distribution of shrinkage voids agrees well with the results from CT detections, with the correlation coefficient being up to 99.8%. The proposed method has great potential for mass detection of plastic gears.

Introduction

Plastic gears, serving as a competitive alternative to traditional metal gears, have been widely used in many fields ranging from the automotive industry to household equipment [1,2]. Since research into the characteristics and behavior of plastics in gearing systems has been advanced, more advantages of plastic gears compared to traditional metal gears have been shown: (1) Plastic gears have lower density and lower inertia than metal equivalents, which guarantees a quieter runtime environment [3]. (2) They have the ability to run without grease or oil lubrication because of the self-lubrication of materials such as polyformaldehyde (POM) or poly tetra fluoroethylene (PTFE) [3,4]. (3) Based on the manufacturing method of injection molding, plastic gears can realize lower unit cost and more complicated shapes [5]. Furthermore, high corrosion resistance and high strength can be achieved by using fillers and developing new materials [6]. However, the extensive promotion of plastic gears is threatened by fracture during practical use, since data indicates that the most frequent failure mode of gears is fracture (61.2%) [7], which originates from internal microcracks or voids and propagates under the working stress [5,8]. Plastic gears are more vulnerable to fracture failure due to the manufacturing process: during injection molding, the plastic is first melted and then injected into a closed metallic mold. When the samples cool down, shrinkage voids can form induced by non-uniform core-to-skin volume ratios [9,10], as shown in Fig. 1. The shrinkage voids break the symmetry of plastic gears and weaken the loading capacity of the tooth root under medium or heavy loading conditions [11], which can result in fracture failure. To maintain reliability and consistency of plastic gears, a non-destructive method for detecting internal voids is desperately needed.

Generally, the most common non-destructive method for detecting internal voids in plastic gears is computed tomography (CT) [12]. The CT device generates an image of samples from X-ray projections at different angles, with grey values corresponding either to the solid parts or the embedded voids. This can ensure high detection accuracy of internal defects of plastic gears [13]. However, CT detection needs experienced workers and expensive instruments; the cost of the CT detection machine is commonly over 1000 RMB (1 RMB = 0.1408 USD on May. 15th, 2020), which is too high to apply for the batch inspection of plastic gears. Moreover, it is tedious and complicated to realize a full-size image of samples since the maximum size for valid void detection covered by the micro-CT detector is limited to several cubic millimeters [13]. As a result, it usually takes more than 4 h to complete the detecting process. Hence, it remains necessary to develop a low-cost, portable and high-accuracy non-destructive method for detecting internal voids in plastic gears.

Magnetic levitation serves as a truly novel method for analyzing physical properties, which can levitate various diamagnetic materials in a paramagnetic medium under the effect of an applied magnetic field. The configuration of this method was originally two rectangular permanent magnets (50 mm × 50 mm × 25 mm) with the same poles facing each other at a distance of 45 mm, which can guarantee an almost linear relationship between the material's levitation height and its density. This method was widely available and benefited from its high density resolution and portability. Studies have demonstrated the use of magnetic levitation in the analysis of forensic evidence, food, minerals, etc [[14], [15], [16]]. Other applications based on the principles of magnetic levitation include 3D culture of living cells [17], separation of mixed waste [18] and self-assembly [19]. The Zhao research group also made contributions to the improvements of this novel method. Zhang et al. [20] analyzed the feasibility of axial magnetic levitation for improving the sensitivity of measurements, and the manipulation of three-dimensional self-assembly was also demonstrated [21]. Xie et al. [22] demonstrated the influence of different distances between the two magnets on the linear relationship of the device by establishing a theoretical model. According to the model, the phenomenon that samples assemble along the centerline can be explained. The study of Xia et al. [23] applied the magnetic levitation method to diagnose internal voids in 3D printed parts, which illustrated the potential for non-destructive testing of products. These studies improved and broadened the applications of the magnetic levitation method in different fields.

In this paper, a non-destructive measurement based on the magnetic levitation method is proposed for analyzing internal defects in plastic gears. The samples were POM gears produced by injection molding, which serve as the transmitting gears in a mechanical system. The distribution of internal voids affects the centroid of the gear, which may lead to a reduction of the mechanical strength and life span. In this study, the levitating height and posture of the gears were recorded to analyze the quality non-destructively. The different levitating heights can reflect the porosity level of each gear, while the levitating posture shows the distribution of shrinkage voids. To verify the testing capacity of the magnetic levitation method, conventional CT measurements were also conducted to provide validation of the levitation method. In general, the magnetic levitation method is proven convenient, accurate, fast and low-cost.

Section snippets

Measurement device

Fig. 2 shows a diagram of the magnetic levitation device. The original device for magnetic levitation method contains two identical N45 magnets, which are oriented with the same poles facing each other, as shown in the figure. The sample is levitated in the transparent container full of paramagnetic medium with high susceptibility. Theoretically, the equilibrium state of the sample between two magnets results from a combination of gravity, buoyancy and magnetic force, which are expressed as eq.

Device analysis

Two concentrations of paramagnetic solution were prepared: 3.5 M Mncl2 aqueous solution and 2.5 M Mncl2 aqueous solution mixed with 1.5 M Cacl2. Cacl2 is a chloride whose magnetic susceptibility is much lower than that of Mncl2. The density of samples levitating in the magnetic levitation device with d = 60 mm can be determined by eq. (7), which is a third order expression. The measuring characteristics of these two mediums are shown in Fig. 5(a). The densities ρm of 2.5 M and 3.5 M Mncl2

Conclusions

A novel method for detecting the internal shrinkage voids of plastic gears using magnetic levitation method is proposed. The density variations caused by different porosity levels in the samples can be detected by magnetic levitation precisely, and the distribution of the internal shrinkage voids can be quickly analyzed by the levitating postures of the samples. A set of experiments was carried out to verify the proposed method by comparison with CT detections. Based on the experimental results

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

Daofan Tang: Conceptualization, Methodology, Experiment, Writing - original draft. Peng Zhao: Conceptualization, Writing - review & editing. Yaqiang Shen: Resources. Hongwei Zhou: Resources. Jun Xie: Conceptualization, Methodology. Jianzhong Fu: Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (No. 51875519 & No. 51635006), the Zhejiang Provincial Natural Science Foundation of China (No. LZ18E050002), and the Key Research and Development Plan of Zhejiang Province (No. 2020C01113).

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