JP6086311B2 - Evaluation method of polishability of steel for mold and evaluation method of polishability of mold - Google Patents

Description

  The present invention relates to a method for evaluating the polishability of a steel material by measuring the waviness of the surface after the polishing of a steel material used for manufacturing a mold having a polished work surface. Or, it relates to a method for evaluating the polishability of a mold by measuring the waviness of the work surface of the mold, which is the surface after polishing, for an actual mold whose work surface is polished. is there.

  A mold for molding a product having a high-quality surface property, such as a plastic product, requires a mirror-finished skin on its work surface. Such a work surface is finished to a mirror surface by, for example, preparing a pre-hardened steel material that has been adjusted in advance to use hardness, cutting the surface into a predetermined shape, and then polishing.

  The above polishing process is generally performed in the final process of mold production. The polishing process is usually carried out in the order of rough polishing with a grindstone, followed by intermediate finish polishing with abrasive paper, and finally with final polishing with diamond paste. And the specification according to an average particle diameter exists in the abrasive grain contained in each of a grindstone, abrasive paper, and a diamond paste. For example, an abrasive having an average particle size of about 15 μm is expressed as “# 1000” in accordance with the standards of JIS R 6001 (grain size of abrasive for grinding wheel) and JIS R 6010 (grain size of abrasive for abrasive cloth). To do.

Meanwhile, by using a mold having finished the work surface to a mirror surface by polishing process described above, for example, actually performs plastic forming shapes, "unevenness in the appearance on the surface of the obtained molded article thereby (plaques; shading) "May occur. This unevenness is a poor surface quality due to the difference in the degree of light reflection at each part of the surface of the molded product, and is one factor that deteriorates the appearance quality of the molded product. And it has been confirmed that the cause of this unevenness is “waves” present on the work surface of the mold from which the properties of the molding surface are transferred (Patent Documents 1 and 2). ). Waviness is a periodic undulation that occurs at intervals greater than the surface roughness defined in JIS B 0601: 2001 (ISO 4287: 1997) (Non-patent Document 1).

Japanese Patent Laid-Open No. 01-212789 JP 2010-242147 A

JIS B 0601: 2001 “Product Geometric Specification (GPS) —Surface Properties: Contour Curve Method—Terms, Definitions and Surface Property Parameters”, JIS Handbook, 2005

  The undulation that occurs when the mold steel is polished affects the quality of the molded product. Therefore, before processing the mold steel material into an actual mold, the polishability of the mold steel material, that is, the degree of undulation that occurs when polishing under specified conditions is performed in advance and numerically. If it can be grasped, it is possible to determine whether or not the mold steel is an appropriate material, or the optimum use and polishing conditions for the mold steel. And even if it is in the state of the mold after it has already been manufactured, it can be generated on the surface of the molded product without using the mold if the degree of undulation occurring on the work surface can be numerically evaluated. The degree of unevenness can be predicted, and the soundness of the mold can be evaluated.

  However, in reality, even with a plurality of mold steels having the same level of waviness after polishing, the degree of unevenness produced between the molded products obtained by using these molds There was a case where there was a difference enough to pass or fail. Therefore, the evaluation of the polishability of the steel for molds is finally based on the work surface of the mold when a good molded product is obtained, and the polished surface to be evaluated. There was no choice but to rely on an empirical and sensory method of comparing with the naked eye or photograph (Patent Document 2), resulting in a difference in the evaluation results.

  The object of the present invention is to measure the surface properties of the molded product obtained by the mold at an optimal measurement condition before the mold steel material is processed into an actual mold, or at the time of the mold after manufacture. It is to provide a method for evaluating the polishability of steel for molds or molds that can be numerically predicted by the above.

  The inventor has intensively studied the correlation between the degree of waviness measured on the work surface of a polished mold and the degree of unevenness of a molded product obtained by the mold. As a result, if the surface roughness element corresponding to the size of the abrasive grains is removed from the surface properties of the work surface of the mold polished with abrasive grains, and only the undulation element is evaluated, It was found that there was a correlation between the undulation value and the degree of unevenness. If this correlation can be grasped, the degree of unevenness can be predicted numerically with the measured undulation value, and the present invention has been reached by ascertaining the undulation measurement conditions for this purpose.

  That is, the present invention is a method for evaluating the polishability of the steel for molds by polishing the surface of the steel for molds with abrasive grains and measuring the waviness of the surface after the processing, The waviness curve is obtained by setting the cut-off value λc to a value satisfying the relationship of (average grain size of abrasive grains used for polishing × 10) <λc <(average element length WSm). This is a method for evaluating the polishability of a steel material for molds, characterized by measuring using Preferably, the waviness is measured using the average height Wc as an index.

  And this invention manufactures the metal mold | die which finished the surface of the steel material for metal molds with an abrasive grain, and was finished so that it may become a work surface of a mold, and measured the waviness of the work surface of the mold after the manufacture This is a method for evaluating the polishability of the mold, and the measurement of the waviness is made by measuring a cutoff value λc (average grain size of abrasive grains used for polishing × 10) <λc <(average of elements) This is a method for evaluating the polishability of a mold, characterized by performing measurement using a waviness curve obtained by setting a value satisfying the relationship of length WSm). Preferably, the waviness is measured using the average height Wc as an index.

  ADVANTAGE OF THE INVENTION According to this invention, the polishability of the steel material for metal mold | die conventionally evaluated empirically or an actual metal mold | die can be evaluated by the numerical value processed appropriately. In addition, the polishability can be evaluated simply and at low cost because the polishability can be evaluated even if a small test piece is collected from the steel for the mold before being processed into the mold without using an actual mold. It becomes technology.

In Example 1, the cross-sectional curve measured on the surface of the mold steel after finishing polishing with # 3000 diamond paste and the undulation curve when the cut-off value thereof is 0.160 mm. FIG. In Example 1, it is a metal micro photograph which shows the surface of the steel material for metal mold | dies after performing finishing polishing with the diamond paste of # 3000. In Example 2, it is a metal micro photograph which shows the surface of the steel material for metal mold | dies after finishing polishing with # 2000 abrasive paper. In Example 2, it is a metal micro photograph which shows the surface of the steel material for metal mold | dies after finishing polishing with the diamond paste of # 5000. In Example 2, it is a metal micro photograph which shows the surface of the steel material for metal mold | dies after finishing polishing with the diamond paste of # 14000.

  The feature of the present invention is that the surface of the mold steel before being processed into a mold is polished in advance, or the work surface when processed into an actual mold (hereinafter, also referred to as “surface”). )) Can be numerically evaluated for the degree of unevenness occurring in the molded product by appropriately measuring the swell. That is, when waviness is measured according to the above JIS B 0601: 2001, the cross-sectional curve to be measured corresponds to the size of the abrasive grain and is an element of “surface roughness” that does not contribute to the occurrence of unevenness. Therefore, the undulation measured without excluding this element could not evaluate the unevenness of the molded product in a correlated manner. Therefore, the present invention obtains a cross-sectional curve in which only the waviness elements that are greatly involved in the occurrence of unevenness are properly extracted by excluding the surface roughness elements from the cross-sectional curves in the surface properties after polishing. The “true” waviness value is measured from the curve.

  The steel material for metal mold | die which concerns on this invention, or the evaluation method of the polishability of a metal mold | die is demonstrated concretely. The undulation that causes unevenness of the molded product on the surface of the mold steel after polishing or the actual mold (hereinafter referred to as “mold steel”) is mainly caused by the mold steel. Due to the variation in hardness, it is an undulation formed by a portion that is easily polished and a portion that is difficult to polish. And since the period of this undulation is generally larger than the abrasive grain used for polishing, this becomes a big wave | undulation and the nonuniformity which can be visually confirmed is transferred to a molded article.

  On the other hand, the abrasive grains used for the polishing process form grinding marks on the surface of the mold steel. This grinding mark is fine, so to speak, it is an element of surface roughness specified in JIS B 0601: 2001. This grinding mark also forms the undulations on the surface, but since it is fine, it does not become a major factor causing unevenness in the molded product. In spite of this, if this surface roughness factor is also included in the measurement of waviness for predicting the unevenness of the molded product, even if the above-mentioned undulations are on the same level (unevenness generated in the molded product). However, the measured waviness values differ depending on whether there are many grinding traces or few grinding traces, and it is impossible to grasp the numerical relationship with the generated unevenness.

  Therefore, in the present invention, the influence of grinding marks, that is, the element of the surface roughness is excluded from the cross-sectional curve for measuring the waviness. Specifically, it is a method for polishing the surface of a mold steel material with abrasive grains and measuring the waviness of the surface after the processing to evaluate the polishability of the mold steel material, which is described above. Waviness is a method for evaluating the polishability of steel for molds, which is measured by a waviness curve with a cutoff value λc exceeding the average grain size of the abrasive grains. The cut-off value refers to a predetermined wavelength that is removed from the cross-sectional curve. Specifically, by setting the cut-off value to 10 times the average particle size of the abrasive grains used in the polishing process, the degree of unevenness occurring in the molded product is sufficiently expressed as the number of waviness. Preferably, the swell index (parameter) to be measured is an average height Wc that allows the undulations themselves to be grasped as a whole. The average height is an average value of the heights of the contour curve elements at the reference length. The waviness measured in this way indicates that the lower the value, the better the polishability of the steel for molds at that time. And, for example, if you know the value of the waviness of the work surface of the mold when a good molded product is obtained, you can measure the waviness of the surface to be evaluated, Judgment can be made.

The average particle size of the abrasive grains will be described. Conventionally, fine powder for precision polishing has been used as abrasive grains for polishing steel for molds. Fine powder for precision polishing is standardized in fine categories according to its particle size distribution (average particle size). In the display method, numbers such as # 1000, # 2000, and # 3000 are generally used in accordance with the above-described JIS R 6001 and the like (the larger the number, the finer the particles). In addition to the standardized abrasive grains, finer abrasive grains with larger numbers have also been proposed. The average particle diameter of these abrasive grains can be measured by a sedimentation test method and an electrical resistance test method according to JIS R 6002. The sedimentation test method is a method in which the particle diameter is measured from the sedimentation speed of particles that settle in the dispersion medium. The electric resistance test method is a method of measuring the particle diameter by utilizing the fact that the electric resistance value when the particles in the electrolytic solution pass through the pores is different. The relationship between the abrasive grain number and the average grain size is generally as follows. Among these abrasive grains (for example, abrasive grains of # 500 to # 14000), those having numbers (average particle diameter) suitable for the purpose of polishing are appropriately selected and used.
# 500: 30-36 μm
# 1000: 14-22 μm
# 2000: 5-10 μm
# 3000: 4-8 μm
# 4000: 3 to 6 μm
# 8000: 2-4 μm
# 14000: ~ 2μm

  The cut-off value needs to have an upper limit corresponding to the average length WSm of the elements included in the undulation curve at that time. The average element length is the average of the lengths of the contour curve elements in the reference length, and so to speak, indicates the period (wavelength) of undulation. Therefore, if the cut-off value exceeds this undulation period, even the undulation element related to the occurrence of the undulation will be excluded. As a result, even when noticeable unevenness occurs in the molded product, the waviness curve after the cut-off becomes flat, and a numerical evaluation correlated with the degree of unevenness cannot be made. Therefore, when measuring the swell, by measuring WSm in advance or simultaneously, if the cut-off value is set smaller than this WSm, a sectional curve consisting exclusively of the swell element to be measured can be obtained. More accurately, the accuracy of the evaluation method of the present invention is improved.

  In addition, it is preferable to match | combine the conditions of the polishing process implemented to the steel material for metal mold | die used for evaluation according to it at the time of manufacturing an actual metal mold | die. This is particularly effective when the polishability is evaluated at the time of the mold steel before being processed into an actual mold. Usually, the working surface of an actual mold is a curved surface, whereas a sample collected quickly and simply from a steel material for molds is exclusively a flat surface. And when the conditions of polishing process differ greatly between these, even if the value of the waviness measured after the polishing process is the same level, the degree of unevenness of the molded product may be different.

  P21 steel, which is a standard steel type of AISI, and SCM440 steel, which is a standard steel type of JIS, were prepared, and their polishability was evaluated. These steel types are materials having different surface properties after processing (that is, polishing properties are different) even if polishing is performed under the same conditions at normal pre-hardened hardness applied to each steel type. P21 steel is a steel material for molds that is generally considered to have good polishability. And SCM440 steel is a steel material for metal mold | die considered to be inferior to polishability compared with P21 steel.

  First, these steel materials were tempered and tempered, and each sample was adjusted to 40 HRC for P21 steel and 26 HRC for SCM440 steel, each having a normal prehardened hardness, and a sample having a size of 10 × 10 × 20 mm was obtained. Next, a polishing process was performed on one plane of these samples under the conditions used for actual mold production. For details of the polishing process, polishing is performed in the order of a grindstone, abrasive paper, and diamond paste on one side after two 10 x 20 mm surfaces are polished in parallel, and finally with a # 3000 diamond paste. Finished. In the case of # 3000 diamond paste, the average grain size of the abrasive grains is about 6 μm.

  And according to JIS B 0601: 2001, the average height of the swell when various cut-off values are applied to the surface after polishing using a surface roughness shape measuring instrument (Surfcom 570A manufactured by Tokyo Seimitsu Co., Ltd.). Wc and the average element length WSm were measured. At this time, the arithmetic average roughness Ra (cut-off value 0.08 mm) was also measured. These results are shown in Table 1. FIG. 1 is a cross-sectional curve measured on the surfaces of both samples after polishing and a undulation curve when the cut-off value is 0.160 mm. And since the optical microscope photograph by the differential interference function utilized by patent document 2 for evaluation is also image | photographed, it shows in FIG.

  First, since the Ra value measured under conditions with a small cut-off value depends exclusively on the size of the abrasive grains used, no difference in value was observed between the two steel types. However, by observing the optical micrograph of FIG. 2, undulation was confirmed on the surface of the SCM440 steel by visual observation, and it was confirmed that the polishability of the P21 steel was superior to that of the SCM440 steel. And this difference in polishability can be confirmed numerically by the Wc value of P21 steel appearing smaller than that of SCM440 steel by measuring the swell under appropriate conditions.

  Therefore, for the size of the abrasive used, No. with a small cut-off value was set. In the case of 1, since the waviness curve used for the measurement also includes an element of surface roughness, as a result, the SCM440 steel had a smaller Wc value, and the correct evaluation was not shown. In addition, No. The reason why the WSm value of the SCM440 steel could not be measured in No. 1 was that the cut-off value was too small, and it contained many elements of surface roughness, and it was difficult to handle the obtained value as WSm. On the other hand, by setting the cut-off value large, the reliability of the Wc values of both steels increases, and they show the correct relationship of SCM440 steel> P21 steel, and the cut-off value is the abrasive grain. No. which is 10 times the size of From 3 onwards, the relationship was fully demonstrated.

  However, if the cutoff value becomes too large, the undulation element to be measured begins to be excluded from the undulation curve, and the undulation value loses accuracy. And No. From 8 onwards, in SCM440 steel, the cutoff value at that time exceeded the WSm value, so the WSm value could not be specified, and the Wc value also showed a low value that was inaccurate. No. In 9, the cut-off value exceeded the WSm value in both steels. As a result, the Wc values of both steels were in the relationship of SCM440 steel <P21 steel, and correct evaluation was not shown.

  An improved steel of SUS420J2 steel, which is a JIS standard steel grade, and the above P21 steel were prepared, and their polishability was evaluated. First, these steel materials having a size of 100 × 100 × 10 mm were quenched and tempered, and the SUS420J2 improved steel was adjusted to 30 HRC and the P21 steel was adjusted to 40 HRC. Next, one plane (100 × 100 mm) of these samples is divided into three regions A, B, and C of 50 × 30 mm with a marking line, and then each region is subjected to actual mold production. Polished according to conditions. As for the details of the polishing process, the polishing process was carried out in the order of the grindstone and the abrasive paper in the region A, and finally finished with # 2000 abrasive paper (the average particle diameter of the abrasive grains was about 10 μm). The remaining regions B and C are polished in the order of a grindstone, abrasive paper, and diamond paste. Region B is finished with # 5000 diamond paste (the average grain size of the abrasive is about 5 μm). Finished with # 14000 diamond paste (average grain size of the abrasive grains was about 1 μm).

  Then, on each surface of the areas A to C after polishing, the average swell height Wc and the average element length WSm when various cut-off values were applied were measured in the same manner as in Example 1. The arithmetic average roughness Ra (cut-off value 0.08 mm) was also measured. The results in region A (average grain size of abrasive grains; about 10 μm) are shown in Table 2, the results in region B (about 5 μm) are shown in Table 3, and the results in region C (about 1 μm) are shown in Table 4, respectively. . Moreover, the optical micrograph by the differential interference function which patent document 2 utilizes for evaluation is also shown to FIG. 3 (area | region A), 4 (area | region B), and 5 (area | region C).

  As in Example 1, the Ra values measured under conditions with a small cut-off value showed no difference between the two steel types in all regions A, B, and C after polishing. However, by observing the optical micrographs of FIGS. 3, 4, and 5, the surface of the SUS420J2 improved steel is visually confirmed to be wavy, and the fact that the polishability of P21 steel is superior to that of SUS420J2 improved steel is confirmed. It was done. Then, if it was selection of the cut-off value which concerns on this invention, it was confirmed whether the Wc value of P21 steel measured the relationship smaller than that of SUS420J2 improved steel, and showed the said fact.

<When polishing paper # 2000 is finished (average grain size of abrasive grains; about 10 μm)>
No. measured with a cut-off value of less than 10 times the average particle size with respect to the size of the abrasive used. In A-1 to A-3, the Wc value of P21 steel excellent in polishability was larger than that of SUS420J2 improved steel, and the relationship of the correct Wc value was not shown. On the other hand, by carrying out the method of the present invention, the cut-off value was set to be larger than 10 times the average grain size of the abrasive grains. After A-4, the Wc value of P21 steel was smaller than that of SUS420J2 modified steel, and the correct evaluation result of polishability was shown. However, if the set cutoff value exceeds the WSm value, In A-9, the WSm value could not be measured in any steel type.

<Diamond paste # 5000 finish (average grain size of abrasive grains; about 5 μm)>
No. measured with a cut-off value of less than 10 times the average particle size with respect to the size of the abrasive used. In B-1 and B-2, the Wc value of P21 steel excellent in polishability was larger than that of SUS420J2 improved steel, and the relationship of the correct Wc value was not shown. On the other hand, by carrying out the method of the present invention, the cut-off value was set to be larger than 10 times the average grain size of the abrasive grains. After B-3, the Wc value of P21 steel became smaller than that of SUS420J2 modified steel, and the correct evaluation result of polishability was shown. The cut-off value exceeded the WSm value. After B-8, it was difficult to measure the WSm value.

<Diamond paste # 14000 finish (average grain size of abrasive grains; about 1 μm)>
For the size of the abrasive grains used, the method of the present invention was carried out, and the cut-off value was set larger than 10 times the average grain size of the abrasive grains. After C-1, the Wc value of the P21 steel was smaller than that of the SUS420J2 improved steel, indicating a correct evaluation result of the polishability. The cut-off value exceeded the WSm value. In C-9, it was difficult to measure the WSm value.

Claims (4)

The surface of the mold steel is polished with abrasive grains, and the swell of the mold steel is evaluated by measuring the waviness of the surface after the processing, and the measurement of the waviness is cut Measure using an undulation curve obtained by setting the off value λc to a value satisfying the relationship of (average grain size of abrasive grains used for polishing × 10) <λc <(average element length WSm). A method for evaluating the polishability of steel for molds characterized by 2. The method for evaluating the polishability of a steel for molds according to claim 1, wherein the measurement of the waviness is measured using the average height Wc as an index. The mold is made by polishing the surface of the mold steel with abrasive grains to produce a mold work surface and measuring the undulation of the work surface of the mold after the manufacture. The waviness is measured by measuring the cut-off value λc as follows: (average grain size of abrasive grains used in polishing process × 10) <λc <(average element length WSm) A method for evaluating the polishability of a mold, characterized by performing measurement using a waviness curve obtained by setting a value satisfying the above. 4. The method for evaluating the polishability of a mold according to claim 3, wherein the waviness is measured using an average height Wc as an index.