Polymer bonded magnets are compounds consisting of a polymer matrix with embedded hard magnetic filler particles. These materials are mainly used in applications in actor or sensor technology. One example is the application as multipolar encoder wheel in magnetic sensors. Depending on the application different requirements have to be fulfilled, such as a high pole length accuracy and repeatability. This investigation deals with the production of multipolar rings in the injection molding process for sensor applications and influences of the design of the gating system on the pole length accuracy. It is shown that the number of injection points and developing weld lines, as well as the positioning of the injection points, has a major influence on the magnetization characteristics of the molded rings. In general, a positioning of injection points and weld lines in the pole pitch and higher number of injection points lead to rings with a high reproduction accuracy of the pole length of the mold.
Magnetic sensor system applications are further increasing, in particular in the automotive industry, but also in mechanical engineering and consumer electronics [
Polymer bonded magnets consist of hard magnetic filler particles embedded in a polymer matrix consisting of a thermoset, thermoplastic, or elastomeric material [
The high variety of magnetic field configurations can be achieved by the orientation of anisotropic particles during the process, such that arcuate, convergent, or divergent distribution of the polarization is realizable [
Prior research on the production of bipolar and multipolar, rectangular shaped polymer bonded magnets shows the influence of the processing conditions during the injection molding process on the magnetic properties of these parts. The flux density in the cavity and the viscosity of the melt are the main influences on the degree of particle orientation [
This paper deals with the influence of the location and number of the injection points and developing weld lines and, thus, the melt flow behavior on the pole length of multipolar bonded magnets. Further, the influence of the melt flow behavior is analyzed by evaluating short shots, as well as the magnetizing structure in the complete part by using magnetooptic analysis.
The materials used for this investigation are a polyamide 12 (Vestamid BS1636, Evonik Industries AG) and a SrFeO (OP-71, Dowa Holdings Co., Ltd.). Figure
Strontium ferrite particles using scanning electron microscopy (SEM).
For this research, multipolar magnetic rings with a pole length of 4 mm and different gating systems are investigated. A defined magnetic field is integrated in the mold (mold dimensions: outer diameter: 30.6 mm, thickness: 4 mm, and height: 5 mm) by arranging 24 permanent magnets around the outer cavity surface. Thus, the embedded hard magnetic particles are oriented and partially magnetized during the injection molding process. For the further evaluation, the poles are numbered from 1 to 24 with the same location of each pole number in the mold. Thus, influences of the mold design, such as differences in the magnetic field inside the cavity, can be eliminated.
The evaluated gating systems include a film gate and gating systems with different numbers of injection points located laterally at the side surface (Figure
Film gate system and pin-point gating system with two injection points.
Location of the injection points in relation to the pole numbers.
For injection molding of the multipolar rings, an injection molding machine of the company Sumitomo (SHI) Demag Plastics Machinery GmbH (Demag 25/280-80) with a screw diameter of 18 mm is used. All rings are produced using the same processing parameters. This includes a melt temperature of 280°C, a mold temperature of 80°C, and a holding pressure of 350 bar. The injection speed is varied for the different pin-point gating systems, such that the melt flow rate inside the ring remains constant. Thus, the injection speed for the gating system with two injection points is 40 mm/s, with four injection points 80 mm/s and for eight injection points 160 mm/s. As there is no continuous melt front developing for the gating system with 12 injection points, the injection speed is also set to 160 mm/s (compare to Figure
Short shots of the multipolar rings with different pin-point gating systems; material: PA12 + 50 vol% SrFeO.
For the gating system with two injection points, short shots with five different changeover points without holding pressure are produced additionally in order to identify the pole length accuracy during the filling phase.
Furthermore, a mold insert with integrated sensors, but without integrated bonded magnets, is used in order to identify the pressure distribution inside the ring. Thus, two pressure sensors (Type 6157B, Kistler Instrumente GmbH, Sindelfingen) and one combined temperature sensor (MTS 408-IR-STS, FOS Messtechnik GmbH, Schacht-Audorf) are integrated in the mold. One sensor of each type is located at one injection point, oppositely from each other. For the gating system with two injection points, the second pressure sensor is located in between the two injection points where the weld line develops; for all other gating systems only the resulting pressure at the injection point location is evaluated. For comparison reasons to other sensor systems, the thermocouple measurement signal will be further evaluated.
The produced rings are evaluated regarding their magnetic properties. For the measurement, a certain characterization device is set up: the rings are fixed to a rotating shaft with attached high-accuracy angular position encoder (RON 786, Dr. Johannes Heidenhain GmbH). The magnetic flux density
Location of the injection points in relation to the pole numbers.
As encoder wheel one important quality criterion is the pole length
Further, rings with different gating system are prepared in order to analyze the filler particle orientation using a scanning electron microscope (SEM) (Ultra Plus, Carl Zeiss AG, Oberkochen, Germany). For the investigation cross section polishes of rings embedded in a thermoset resin were made. The polished surface, located at the ring’s half height, is sputtered with platinum and palladium. Beforehand, the rings have been demagnetized. The particle orientations of the rings with different gating systems are analyzed at the pole with integrated injection point and weld line, as well as in the pole pitch at the outer ring surface structure as well as 800
Furthermore, using the magnetooptic analysis method, the magnetization characteristics of the rings are evaluated. For the evaluation the magnetooptic analyzation device “MiniMo” (Matesy GmbH, Jena) with the sensor type B is used. The magnetooptic principle is based on the Faraday effect, which describes the polarization plane of linearly polarized light when passing through a magnetooptical medium. This device can be used for a qualitative examination of hard magnetic material structures. However, due to the noncalibrated visualization, the shown grey values can only be evaluated qualitatively. For the evaluation, grinded sections of one sample of each processing variation in the rings’ half height are prepared. The rings are then analyzed at different locations, which include the injection point, weld line, as well as the poles in between these two characteristic locations.
The pole length is one important quality criterion of multipolar bonded magnetic rings used as encoder wheels. Hereinafter, the results for the pole length of the multipolar rings with different gating systems and for short shots are presented.
Figure
Influence of the injection point location on the pole length deviation of the rings with a gating system with twelve injection points in comparison to the rings with film gate.
For the gating systems with four, eight, and twelve injection points (Figures
The pole length deviation of the poles with integrated injection point or weld line is summarized in Figure
Influence of the injection point location on the pole length deviation of the rings with a gating system with twelve injection points in comparison to the rings with film gate.
Figure
Deviation of the pole length for short shots with a gating system with two injection points, each located in the pole center, in comparison to the rings with film gate.
The filler particle orientation has been investigated for different locations of the rings, as well as different number of injection points and their position. The particle orientations for different locations of the rings with two injection points, which are located in the pole center or pole pitch, are shown in Figure
Filler particle orientation for rings with a gating system with two injection points; material: PA12 + 50 vol% SrFeO.
Figures
Magnetooptic analysis of the ring with a gating system with two injection points, each located in the pole center.
Magnetooptic analysis of the ring with a gating system with two injection points, each located in the pole pitch.
Magnetooptic analysis of the ring with a film gate.
The temperature and pressure distribution during the injection molding process inside the cavity for two different gating systems are exemplarily shown in Figure
Pressure and temperature courses during the injection molding process.
The increase in pressure starts after the increase in temperature and, thus, after the polymer enters the ring cavity. As the time difference in between the increase in temperate and the increase in pressure varies for the different gating systems, it can be concluded that the filling of the rings is conducted without pressure inside the ring, whereas the holding pressure leads to a pressure of almost the set pressure level. For the gating system with two injection points, the pressure distribution close to the injection point and weld line is very similar regarding height and course. For this reason, the pressure distribution for the different gating systems was only evaluated for the location close to the injection point.
The results of the magnetic properties show a certain pattern for the deviation of the pole length for different number of injection points, as well as their location. It is shown that the pole length is significantly reduced in the pole with injection point and increased in the pole with weld line. This deviation is quantitatively reduced by locating the injection point and weld line in the pole pitch, as the deviation is split into the two adjacent poles. Furthermore, this deviation pattern is already existent in the short shots. Consequently, it can be concluded that this effect is related to the filling and, thus, melt flow behavior of the compound. Due to the pressureless filling of the ring, pressure related effects on the deviations in the pole length, in particular in the pole with centrally located injection point, can be excluded. The magnetooptical analysis shows a developing surface layer with a shifted magnetization to the center, which qualitatively fits to the measured results of the pole length deviation. Thus, it is assumed that the developing surface layer leads to the presented pole length deviation. Furthermore, the particle orientation at different locations is presented. A frozen nonoriented layer at the outer ring surface can be identified. In a certain distance to the surface layer, the particle orientation corresponds to the integrated magnetic field inside the mold. Differences in regard to the flow length, number of injection points, and location of the injection points cannot be optically identified. Presumably, the presented effects are caused by the freezing of the melt and, thus, filler orientation in combination with the magnetization of the particles, during the filling phase.
The orientation of nonmagnetic platelet-shaped particles during the injection molding process is mainly shear induced [
Scheme for the comparison of the ideal filler particle orientation only due to shear during mold filling or the magnetic field.
These investigations show that the production of multipolar signal transmitter for magnetic sensors with integrated magnetization during the injection molding process is possible. Further, the design of the gating system and the position of the injection points have a major influence on the pole length accuracy. It can be shown that the magnetization of the surface layer is shifted compared to the inner part due to the melt flow during the filling phase. Furthermore, this surface layer strongly influences the pole length measurements. For this, a smaller pole length in the area of the injection point and a broader pole length in the area of the weld line can be identified. The pole length accuracy can be improved by increasing the number of injection points, as well as the positioning of the injection points and weld lines in the pole pitch. For the rings used in this investigation with 24 poles, good results were achieved using a film gate or a 12 pin-point gating system with the location of the injection points in the pole pitch, such that each pole pitch includes either an injection point or weld line.
In future work, it shall be investigated if the development of the surface layer can be prevented by adapting the process conditions (e.g., cooling of the melt at the cavity surface). Further, it shall be evaluated if the distance of the flux density measurement is determinant for the influence of the developed surface layer. The aim would be the production of rings with a high pole length accuracy and a gating system, which can be cost-efficiently removed after the injection molding process. Furthermore, the influence of different matrix materials, as well as number of poles, on the pole length accuracy shall be investigated.
The authors declare that they have no competing interests.
The authors would like to thank the German Research Foundation (DFG) for funding this project (DFG/DR 421/12-1). They also extend their gratitude to Evonik Industries AG for providing the polymer that was used as matrix material.