JP2004345317A - Molding machine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain to the utmost the occurrence of hydrogen embrittlement separation on the ball abutting face (transfer face) of a ball screw mechanism. <P>SOLUTION: In a molding machine provided with a servomotor as the drive source of a molding operation and a ball screw mechanism for converting the rotation of the servomotor into a linear motion, an oxide film comprising, for example, tri-iron tetroxide film is formed on at least either one of the ball abutting face of the nut body of the ball screw mechanism or that of a screw shaft. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a molding machine such as an injection molding machine or a die casting machine, and more particularly to a molding machine that employs a servomotor as a driving source of a molding operation and includes a ball screw mechanism that converts rotation of the servomotor into linear motion. Things.
[0002]
[Prior art]
In a molding machine such as an injection molding machine driven by a servomotor, the rotation of the servomotor is converted into a linear motion by a ball screw mechanism in order to perform a mold opening / closing operation or an injection operation, and this linear motion is applied to a driven member. Thus, the driven member is moved linearly.
[0003]
In the above-mentioned ball screw mechanism, the number of operation times of about several million is usually guaranteed at least for one durable movement as one operation (that is, one shot operation as one operation) as its durability life. It is common.
[0004]
However, in a ball screw mechanism that converts the rotation of a mold opening / closing servomotor having a large load capacity (driving torque) into linear motion, before the endurance life described above is reached (for example, in some cases, about 400,000 operation times). It was reported that the ball screw mechanism was damaged.
[0005]
[Problems to be solved by the invention]
The cause of the damage of the ball screw mechanism described above is difficult to grasp accurately after the ball screw mechanism has been damaged, and it is not clear whether grease used in the ball screw mechanism is the cause or other factors are the cause. It was required to analyze the factors and take measures against them.
[0006]
The inventors of the present application have made various investigations and found that even if the grease was changed to various types, the ball screw mechanism would suddenly wear, and that the ball screw mechanism of the mold opening / closing system would cause rapid wear. Before the occurrence of the phenomenon, the ball screw mechanism of the mold opening / closing system was observed and measured. As a result, it was clarified that rapid wear of the ball screw mechanism of the mold opening / closing system was caused by hydrogen embrittlement separation.
[0007]
Hydrogen embrittlement peeling means that under severe operating conditions such as excessive load, vibration, rapid acceleration and deceleration, not only rolling, but also slippage between the ball contact surface (transfer surface) of the nut body and screw shaft and the ball. It is thought that tribochemical reaction is promoted by the generation of active new surface, hydrogen is generated by decomposition of water in grease, microstructure changes due to penetration of hydrogen into steel, cracks are generated and brittle peeling is considered. It is something that can be done. This hydrogen embrittlement is caused by a change in white structure and cracks as a precursor of hydrogen embrittlement on the transfer surface (transfer portion) of the steel, and an increase in the hydrogen content of the transfer surface (transfer portion) of the steel. You can check.
[0008]
In fact, in the study of the present inventors, after operating the ball screw mechanism of the mold opening / closing system about several hundred thousand times, the load side transfer portion (the transfer portion on the side where the ball is pressed) of the nut body of the ball screw mechanism is observed in cross section. As a result, white structure changes and cracks were found at a depth of 0.15 to 0.20 of the unpeeled portion, and it was observed that the cracks became branch-like and extended in the depth mm direction. Furthermore, as a result of measuring the residual hydrogen amount in the transfer portion of the nut body of the ball screw mechanism of the mold opening / closing system, the residual hydrogen amount was 0.63 ppm in the non-load side transfer portion, but the residual hydrogen amount was in the load side transfer portion. The amount was 1.55 ppm, which was determined to be sufficient to cause hydrogen embrittlement delamination. In addition, as a result of measuring the residual stress on the load side transfer surface of the nut body, the stress applied to the load side transfer surface of the nut body is estimated to be about 2300 MPa. It is extremely unlikely to cause premature detachment or tissue change.
[0009]
As described above, the reason why hydrogen embrittlement is concentrated on the nut body side of the ball screw mechanism of the mold opening / closing system is that the load capacity (drive torque) of the mold opening / closing servomotor is large, and the mold opening / closing servomotor having a large load capacity is used. A large static electricity of about 50,000 V is generated in the high-strength timing belt used to transmit rotation to the ball screw mechanism. Therefore, the nut body side to which rotation from the high-strength timing belt is transmitted. It is presumed that this is because a negative DC voltage is generated at this point, and positively charged hydrogen ions (H ⁺ ) are attracted. Further, it is considered that the reason why the white structure caused by hydrogen is generated in the load-side transfer portion of the nut body is that the residual hydrogen amount of the load-side transfer portion is larger than the residual hydrogen amount of the non-load-side transfer portion ( That is, it is considered that a large amount of hydrogen is generated in the load-side transfer portion to which the maximum shear stress is applied, and that the maximum shear stress is applied, so that hydrogen easily diffuses inside.
[0010]
The present invention has been made in view of the above points, and an object thereof is to provide a molding machine having a servomotor as a driving source of a molding operation and a ball screw mechanism that converts the rotation of the servomotor into a linear motion. Another object of the present invention is to minimize the occurrence of hydrogen embrittlement peeling on the ball contact surface (transfer surface) of the ball screw mechanism, thereby guaranteeing the durability and reliability of the ball screw mechanism.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a molding machine having a servomotor as a driving source of a molding operation and a ball screw mechanism for converting the rotation of the servomotor into a linear motion. At least one of the contact surface and the ball contact surface of the screw shaft is formed with an oxide film made of, for example, triiron tetroxide film.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0013]
FIG. 1 is a diagram showing a servomotor for opening and closing a mold and a ball screw mechanism driven by the servomotor in an injection molding machine according to an embodiment of the present invention (hereinafter, referred to as the present embodiment).
[0014]
In FIG. 1, reference numeral 1 denotes a ball screw mechanism, which includes a nut body 2, a screw shaft 3, and a ball 4. The nut body 2 is rotatably mounted on a holding plate (not shown) via a bearing (not shown). The nut body 2 is integrally formed with a pulley portion 2a. The screw shaft 3 is held so that it can only move forward and backward, and the rotation of the nut body 2 causes the screw shaft 3 to move forward or backward, thereby performing a mold opening / closing operation. Reference numeral 5 denotes a mold opening / closing servomotor, and a timing belt 7 is stretched between a small pulley 6 fixed to an output shaft 5a thereof and a pulley portion 2a of the nut body 2, whereby a mold opening / closing servomotor is provided. 5 is transmitted to the nut body 2.
[0015]
In the present embodiment, an SCM alloy steel is used for the nut body 2 and the screw shaft 3, and a bearing steel (SUJ alloy steel) is used for the ball 4. As the timing belt 7, a high-strength timing belt having high strength and high reliability is used although the carbon content is small and an antistatic effect cannot be expected.
[0016]
In the present embodiment, in order to prevent the above-mentioned hydrogen embrittlement peeling, an oxide film made of a triiron tetroxide film is formed on the entire surface of the nut body 2. An oxide film composed of triiron tetroxide film is formed by immersing the nut body 2 in an alkaline aqueous solution (a main component of the aqueous solution is sodium hydroxide to which sodium nitrate or the like is added as an oxidizing agent). Its film thickness is about several μm. It is sufficient that the oxide film made of the triiron tetroxide film is formed only on the ball contact surface (transfer surface) of the nut body 2. ) Is rather troublesome to form a film on the entire surface of the nut body 2 for manufacturing reasons.
[0017]
FIG. 2 is a diagram illustrating a relationship between the number of operations in the ball screw mechanism 1 of the present embodiment and the iron powder concentration in grease. As shown in FIG. 2, in the present embodiment, the iron powder concentration is 0.02% wt when the number of operations is 53.5 million times, which is a value considered to cause almost no wear. When the ball screw mechanism 1 was disassembled and the state of the load-side transfer portion of the nut body 2 was checked at the number of operation of 53.5 million times, no abnormality was recognized (white texture and cracks were not recognized at all). Did not.) Further, in FIG. 2, the number of operations is 603,000 times, and the iron powder concentration is 0.8% wt. This is considered to be normal initial wear in the ball screw mechanism 1, and a numerical value of this degree is considered. Is not an iron powder concentration that is a problem in practice. Although the test result of FIG. 2 shows only up to 1,000,000 times, the iron powder concentration from 603,000 to 908,000 times is stable at less than 0.10% wt. Therefore, it is expected that about 10 million operations can be guaranteed.
[0018]
FIG. 3 is a diagram showing the relationship between the number of operations in the ball screw mechanism 1 and the iron powder concentration in the grease under the same conditions as in the present embodiment except that no oxide film is formed on the nut body 2. is there. As shown in FIG. 3, the iron powder concentration was 0.74% wt at the operation number of 205,000 times, and at the time point of the operation number of 205,000 times, the ball screw mechanism 1 was disassembled and the nut body was disassembled. When the state of the load-side transfer portion of No. 2 was confirmed, generation of a white structure and cracks was recognized, and it was confirmed that the load-side transfer portion of the nut body 2 showed signs of hydrogen embrittlement peeling. Note that the experiments after the number of operations of 205,000 times are stopped because if the ball screw mechanism 1 is broken, the cross-sectional analysis of the nut body 2 cannot be performed.
[0019]
As is clear from the comparison of the experimental results in FIGS. 2 and 3 described above, in the present embodiment, it is possible to suppress the occurrence of hydrogen embrittlement as much as possible, and it is possible to prevent excessive load, vibration, rapid acceleration / deceleration, etc. It is possible to sufficiently guarantee the durability performance of a ball screw mechanism used under severe operating conditions.
[0020]
In the above-described embodiment, the oxide film made of triiron tetroxide film is formed on the nut body 2 side. However, in the configuration in which the rotation of the servomotor is transmitted to the screw shaft 3 via the timing belt, An oxide film made of a triiron tetroxide film may be formed on the screw shaft 3 side. In some cases, an oxide film made of a triiron tetroxide film may be formed on both the nut body 2 and the screw shaft 3.
[0021]
Further, in the above-described embodiment, an oxide film is taken as an example of a triiron tetroxide film, but other than the triiron tetroxide film, it is dense and has abrasion resistance, and prevents hydrogen from entering the steel. If it is an oxide film, it can be replaced with a ferric oxide film.
[0022]
Further, in the above-described embodiment, a high-strength timing belt having a small carbon content is used. However, when an antistatic belt having a large carbon content is employed, generation of hydrogen ions (H ⁺ ) itself can be suppressed. This can be greatly expected, and when the oxide film is formed on the ball contact surface (transfer surface) of the ball screw mechanism, the occurrence of hydrogen embrittlement peeling in the ball screw mechanism can be further reduced.
[0023]
Further, in the above-described embodiment, the ball screw mechanism of the mold opening / closing system of the injection molding machine is taken as an example. However, the ball screw mechanism of the injection system of the injection molding machine or the ball screw mechanism of the mold opening / closing system of the die casting machine is also used. It goes without saying that the invention is applicable.
[0024]
【The invention's effect】
As described above, according to the present invention, in a molding machine including a servomotor as a driving source of a molding operation and a ball screw mechanism for converting the rotation of the servomotor into a linear motion, a ball contact surface (transfer The occurrence of hydrogen embrittlement peeling on the surface) can be suppressed as much as possible, whereby the durability reliability of the ball screw mechanism can be guaranteed.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing a mold opening / closing servomotor and a ball screw mechanism driven by the same in an injection molding machine according to an embodiment of the present invention.
FIG. 2 is an explanatory diagram showing the relationship between the number of operations and the concentration of iron powder in grease in the ball screw mechanism of FIG.
FIG. 3 is an explanatory diagram showing the relationship between the number of operations and the concentration of iron powder in grease in a conventional ball screw mechanism.
[Explanation of symbols]
Reference Signs List 1 ball screw mechanism 2 nut body 2a pulley section 3 screw shaft 4 ball 5 mold opening / closing servo motor 5a output shaft 6 small pulley 7 timing belt

Claims (3)

In a molding machine having a servomotor as a driving source of the molding operation and a ball screw mechanism that converts the rotation of the servomotor into a linear motion,
An oxide film is formed on at least one of the ball contact surface of the nut body of the ball screw mechanism and the ball contact surface of the screw shaft. In claim 1,
The molding machine, wherein the oxide film is a triiron tetroxide film. In claim 1 or 2,
A molding machine, wherein a timing belt for transmitting rotation of the servomotor to a rotating portion of the ball screw mechanism is an antistatic belt.

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