Patent Application
20190119802
The present invention relates to methods for producing a milling roll used in a milling process of cereals such as wheat and the like.
Cereals such as wheat and the like are subjected to a milling process after undergoing a selecting process, a tempering process and the like. Currently, milling of cereals such as wheat and the like is mainly carried out by a roll crushing method. This system is said to have been developed in the latter half of the 19th century, and thereby productivity and quality are greatly improved as compared with a stone milling system, contributing to industrialization of the milling industry.
In the milling process according to the roll crushing method, firstly, the cereal is roughly broken by a brake roll to remove the skin, then, the operation of crushing and classifying with a smooth roll is repeated, and finished as the final product. Generally, with respect to the brake roll, the surface is subjected to a dressing process to form grooves having a pitch of about 10 mm, and with respect to the smooth roll, a medium is inserted between the two rotating rolls and a mat processing is performed on the surface by doing so, to form a pearskin-like uneven surface. The mat processing for the smooth roll is described in Patent Literatures 1 and 2.
For these milling rolls, chilled steel with a surface hardness in terms of Vickers hardness HV of about 600 is used. If milling rolls made of chilled steel are used for a long period of time, unevenness present on the surface will be worn out, and cereals will not be able to be crushed into an appropriate size and shape, and powders having satisfactory quality will not be obtained. In order to restore the surface roughness of the milling roll, it is necessary to repair and process the surface, however, in the current method, polishing is performed first, then, repair and processing are performed, which is very troublesome and time consuming. Hence, it tends to be a case of exchanging the roll itself, which increases the cost.
Patent Literature 3 describes a food processing roll capable of enhancing biting property of a raw material, capable of efficiently treating the raw material and keeping the satisfactory biting property for a long time. In this literature, a plurality of grooves are formed on the roll surface by laser processing, or an overlay welding layer is formed on the roll surface beforehand, and a plurality of concave portions are formed on the surface by laser processing. A shape for biting food is defined as a groove or a concave portion recessed from the smooth outer peripheral surface of the roll, so that even if wear of the roll is caused by long-term use, it is possible to keep the biting property of the food.
[Patent Literature 1] Japanese Laid-Open Patent Publication No. H10-131948
[Patent Literature 2] Japanese Laid-Open Patent Publication No. H11-028621
[Patent Literature 3] Re-Published Patent Publication WO2013/179356
In the milling process of cereals, there are specific problems other than wear of the surface unevenness of the milling roll. For example, dried wheat is hard, scattered when crushed, and powder of the intended shape and size cannot be obtained, so when crushing the wheat, a certain amount of water is contained. When the smooth roll described in Patent Literatures 1 and 2 or the food processing roll described in Patent Literature 3 is used for the milling process of wheat, the roll gradually heats up due to friction between the rolls through the wheat and friction between the roll and the wheat. When the heat is stored in the roll, the water of the wheat is evaporated by the influence of the temperature, and it is impossible to retain an appropriate water content in the wheat and to obtain powder with a desired shape and size.
In view of the above-described problems of conventional technologies, an object of the present invention is to provide a method for producing a milling roll capable of obtaining satisfactory cereal powder by preventing drying of cereals in a milling process and keeping an appropriate water content.
The present inventors have investigated a means for preventing the drying of cereals in the milling process and resultantly succeeded in retaining a suitable water content in cereals by thermally spraying the surface of the milling roll to form a thermal sprayed coating having pores to retain water, leading to solving of the problem.
That is, the method for producing a milling roll of the present invention is characterized by comprising: a roughening process for blasting a surface of a substrate; and a coating process for spraying a thermal spray material over a roughened surface of the substrate so as to form a thermal sprayed coating having pores that retain water.
According to the present invention, since the thermal sprayed coating is formed on the surface of the substrate, even if the milling roll is used for a long period of time, unevenness of a surface layer can be kept and crushing performance is not impaired. In addition, according to the thermal spraying, the size and presence ratio of pores in the coating to be produced can be controlled by adjusting thermal spraying conditions. Since the thermal sprayed coating formed according to the present invention has pores for retaining water, heat can be hardly transferred to the cereal, and water retentivity in the surface layer of the milling roll can be enhanced, so that cooling property for the cereal can be obtained. In addition, since the thermal sprayed coating is formed on the substrate having surface area and surface roughness increased by blasting, peeling of the thermal sprayed coating can be prevented. Hence, it is possible to eliminate the problems of wear on the surface layer of the milling roll and peeling of the thermal sprayed coating, and it becomes possible to retain the water content appropriate for the cereal, thus, satisfactory cereal powder can be obtained.
A surface roughness Ra of the milling roll suitable for crushing cereals differs according to the purpose of use. When using the milling roll as a smooth roll, the surface roughness Ra after the coating process is preferably 5-15 μm. On the other hand, when using the milling roll as a brake roll, the roughness like that of the smooth roll is not required since crushing is possible by grooves inscribed on a substrate, and the surface roughness Ra after the coating process is preferably 2-8 μm in view of durability.
Since thermal conductivity of ceramics and cermets is generally lower as compared with metals, a milling roll produced by thermal spraying and coating a ceramic or cermet material on the surface of a substrate made of a metal material tends to accumulate friction heat generated during a crushing process. Hence, it is better not to set the thickness of the thermal sprayed coating formed by thermal spraying too large. On the other hand, however, unevenness having a surface roughness of a certain level or more is required for crushing of wheat and the like, and in order to realize such unevenness by the thermal sprayed coating, a coating thickness of a certain level or more is required. This problem can be solved if the surface roughness of the substrate is reflected in the surface roughness of the milling roll. That is, the surface roughness Ra of the substrate is preferably adjusted within the range of −2 μm to +8 μm with respect to the surface roughness Ra of the milling roll after the coating process. Thus, it is possible to suppress the thickness of the thermal sprayed coating to minimum and to impart necessary surface roughness to the milling roll.
Vickers hardness HV of the surface of the milling roll after the coating process is preferably greater than 1000. Thus, wear resistance of the milling roll is markedly improved.
A thermal spray material is not limited, but in particular a carbide cermet is preferred. By using the carbide cermet as the thermal spray material, a high surface hardness of the milling roll (specifically, a value larger than 1000 in the Vickers hardness HV) can be easily obtained and satisfactory wear resistance can be imparted.
An average thickness of the thermal sprayed coating is preferably 10-150 μm. If the average thickness of the thermal sprayed coating is less than 10 μm, the durability when used for a long period of time is concerned, whereas if it is more than 150 μm, the problem of accumulation of friction heat is concerned.
The method for producing a milling roll may include an adjusting process A for shot blasting as a post treatment after the coating process so as to smooth fine unevenness in undulation of a surface of the thermal sprayed coating. Thus, it is possible to prevent adhesion and clogging of crushed cereals in the undulation of the surface of the thermal sprayed coating.
The method for producing a milling roll may include an adjusting process B for peak cutting as a post treatment after the coating process so as to planarize a tip of a convex portion on a surface of the thermal sprayed coating. Thus, the number of parts that make point contact with the cereal on the surface of the thermal sprayed coating decreases, and the wear resistance can be improved.
According to the milling roll produced by utilizing the present invention, the cooling property for cereals can be obtained by the thermal sprayed coating having water-retaining function existing in the surface layer. The thermal sprayed coating is formed on a substrate having surface area and surface roughness increased by blasting, so that peeling of the thermal sprayed coating can be prevented. Due to the high cooling property of the thermal sprayed coating, cereal is prevented from getting heated in the milling process and drying of the cereal can be suppressed. This makes it possible to retain an adequate water content in the cereal, and to obtain satisfactory cereal powder.
Embodiments of the present invention will be described with reference to the drawings.
In the smooth roll 1 of this embodiment, any cereal can be milled. Specific examples of cereals include wheat, barley, oat, pearl barley, corn, rye, buckwheat, barnyard millet, foxtail millet, Chinese millet, sorghum, sorghum bicolor, macomo, and the like. These cereals may be used each singly or two or more of them may be used in combination. Application to wheat and barley, among these cereals, is particularly preferred. Cereal flour derived from wheat includes hard flour, semi-hard flour, medium flour, soft flour, whole cereal flour, and durum semolina.
The size and shape of each part such as a roll diameter and a length of the smooth roll 1 are not limited. A material constituting the roll substrate 2 on which the thermal sprayed coating 3 is formed may be any material as long as it can be applied to a milling roll and is capable of forming various thermal sprayed coatings. As the material constituting the roll substrate 2, a metal material is suitably used. Specific examples of the metal material include metals selected from, for example, Fe, Cr, Ni, Mo, Co, Cu, Mn, Zn, Ta, W, Al, Ti, and Mg, or alloys such as chilled steel, stainless steel and the like, containing one or more of these metals. Such a metal material is formed by extrusion molding, cutting processing, plastic processing, forging, or the like.
The thermal sprayed coating 3 is formed by collision of softened or molten various thermal spray materials with the surface 2a of the roll substrate 2 at a high speed and deposition of these materials thereon. The thermal spray material is not limited, but ceramics or cermets having high hardness are preferable.
Specific examples of ceramics as a thermal spray material include oxide-based ceramics, nitride-based ceramics, carbide-based ceramics and boride-based ceramics, containing at least one element selected from the group consisting of Ni, Cr, Co, Al, Ta, Y, W, Nb, V, Ti, B, Si, Mo, Zr, Fe, Hf and La, and mixtures thereof.
The oxide-based ceramics include Al2O3, Cr2O3, HfO2, La2O3, TiO2, Y2O3, ZrO2, Al2O3.SiO2, NiO, ZrO2.SiO2, SiO2, MgO, and CaO. The nitride-based ceramics include TiN, TaN, AlN, BN, Si3N4, HfN, NbN, YN, ZrN, Mg3N2, and Ca3N2. The carbide-based ceramics include TiC, WC, TaC, B4C, SiC, HfC, ZrC, VC, and Cr3C2. The boride-based ceramics include TiB2, ZrB2, HfB2, VB2, TaB2, NbB2, W2B5, CrB2, and LaB6.
Cermet materials which are composites of metal materials and ceramic materials may be used as the thermal spray material. The cermet materials include a composite material of the ceramic material selected from the group consisting of Cr3C2, TaC, WC, NbC, VC, TiC, B4C, SiC, CrB2, WB, MoB, ZrB2, TiB2, FeB2, CrN, Cr2N, TaN, NbN, VN, TiN and BN, and the metal material selected from the group consisting of Ni, Cr, Co, Al, Ta, Y, W, Nb, V, Ti, B, Si, Mo, Zr, Fe, Hf and La; and the like. Among them, carbide cermets are particularly preferable because a coating with high hardness is easily obtained.
The surface of the thermal sprayed coating 3 is not sealed and a large number of pores 5 are present inside the thermal sprayed coating 3. By retaining water inside these pores 5, the thermal sprayed coating 3 has high water retentivity. Thus, water retentivity is given to the surface layer of the smooth roll 1, and the cereal is prevented from getting heated in the milling process, and simultaneously, cooling property for the cereal is obtained and drying of the cereal can be suppressed. Then, the cereal is crushed by the surface unevenness of the smooth roll 1 while retaining an appropriate water content in the cereal, whereby satisfactory cereal powder can be obtained.
An average porosity of the thermal sprayed coating 3 may be about 0.5-15%, and preferably 2.0-10%. Adjustment of the average porosity is made by selection of thermal spraying methods and thermal spraying conditions. The average porosity at which the pores 5 inside the thermal sprayed coating 3 can well retain water is 2.0% or more. However, if the average porosity is increased, although the water retentivity increases, there is a concern that the wear resistance will decrease. Hence, from the viewpoint of maintaining the wear resistance, the average porosity is preferably 10% or less.
The following methods are effective for adjusting the porosity in thermal spraying construction. That is, effective are selection of particle diameter of the thermal spray material, adjustment of particle velocity during thermal spraying, and adjustment of spraying distance, which are used for coating formation. For example, in the case where the porosity of the thermal sprayed coating is reduced to form a dense coating, it is preferable to use powder having a fine particle diameter as the thermal spray material, and furthermore, it is preferable that the particle velocity is higher. It is also possible to select a method for setting the thermal spraying distance to be short so that heat flux is obtained at which the coating does not break due to heat effect. On the other hand, in order to increase the porosity and form a porous coating, forming of a coating by a method opposite to the above method is effective. However, the above adjustments need to be changed only to the extent that does not impair other coating properties such as hardness, wear resistance, and surface shape retention.
In order to verify heat transfer characteristics based on the water content of the thermal sprayed coating, the following experiment was conducted. For the experiment, two samples of each of the following test materials A to C and one sample of the following test material D were firstly prepared.
Substrate: Stainless steel material
Substrate size: 5 cm square, 5 mm thickness
Blasting conditions: Alumina particles (#60), 0.4 MPa pressure
Thermal spray material: WC—CrNi
Thermal spraying method: HVOF
Cross section was observed by using SEM with 200 times. There was variation depending on measurement place. From sectional SEM-BEI image, black parts inside the coating were considered as pores and the ratio of the pores to the entire coating was calculated. Thermal sprayed coating thickness:
50 μm (test materials A1, A2)
100 μm (test materials B1, B2)
150 μm (test materials C1, C2)
No thermal sprayed coating (test material D)
Tap water was dropped on the surface of each of test materials A1, B1, and C1, and the thermal sprayed coating was sufficiently moistened. The water remaining on the surface was lightly wiped off with Kimtowel. The test materials A1 to C1 containing water were placed on a plate heater kept at 100° C., and the surface temperature of each test material was measured with a contact thermometer. The time until the surface temperature reached 40° C., 60° C., and 80° C. was measured and recorded.
The test materials A2, B2, C2, and D were placed on the plate heater kept at 100° C., and the surface temperature of each test material was measured with the contact thermometer. The time until the surface temperature reached 40° C., 60° C., and 80° C. was measured and recorded.
From these measurement records, heat transfer characteristics based on the coating thickness difference and the water content difference were verified. As shown in Table 1 and
An average thickness t of the thermal sprayed coating 3 is appropriately set, and is preferably 10-150 μm, more preferably 20-100 μm. When the average thickness t of the thermal sprayed coating 3 is too small, the durability is concerned, while when too large, heat amount to be accumulated in the roll substrate 2 is increased due to frictional heat, to promote drying of the cereals in the milling process. Especially, since ceramics and cermets generally have lower thermal conductivity as compared with metals, when ceramics or cermets are thermally sprayed on metallic materials, this point should be noted.
The surface roughness Ra of the roll substrate 2 is adjusted so as to be in the range of −2 μm to +8 μm with respect to the surface roughness Ra of the smooth roll 1 finally aimed at. In other words, the surface roughness Ra of the smooth roll 1 reflects the surface roughness Ra of the roll substrate 2. The thermal sprayed coating 3 of the present embodiment is formed so as to have uniform thickness t so that the surface roughness Ra of the roll substrate 2 is reflected in the surface roughness Ra of the smooth roll 1. Here, the “uniform” means that the maximum thickness and the minimum thickness of the same coating are included within ±30% of the average thickness, respectively.
Since the thermal sprayed coating 3 is formed on the surface 2a of the roll substrate 2 having surface area and surface roughness increased by adjusting the surface roughness, peeling of the thermal sprayed coating 3 can be prevented. An undercoat layer may be provided between the roll substrate 2 and the thermal sprayed coating 3 for the purpose of improving adhesion and the like.
The surface roughness Ra of the smooth roll 1 is set to 5-15 μm from the viewpoint of crushing and classification of cereals. Thus, it is possible to obtain satisfactory cereal powder. The surface hardness of the surface 1a of the smooth roll 1 is high, and the surface hardness thereof is adjusted to a value greater than 1000 in terms of Vickers hardness HV. Thereby, the wear resistance of the smooth roll 1 can be improved.
An example of a method for producing the smooth roll 1 in which the thermal sprayed coating 3 is formed on the surface will be described. There are conducted in this order, a roughening process for roughening the surface 2a of the roll substrate 2, a cleaning treatment of the surface 2a of the roll substrate 2, and a coating process for spraying the thermal spray material onto the roll substrate 2 to form the thermal sprayed coating 3 having the pores 5 for retaining water. Other processes such as a preheating process and the like may be included depending on the type of the substrate and the kind of the thermal spray material.
In the roughening process, unevenness is formed on the surface 2a of the roll substrate 2 by blasting for causing a shot material 21 to collide with the surface 2a of the roll substrate 2 from a blast nozzle 20, as shown in
A thermal spraying method for obtaining the thermal sprayed coating 3 in the coating process includes an atmospheric plasma thermal spraying method, a low-pressure plasma thermal spraying method, a high-speed flame thermal spraying method, a gas flame thermal spraying method, an arc thermal spraying method, a detonation thermal spraying method, and the like. In order to make it possible for the thermal sprayed coating 3 to retain water, these thermal spraying methods are appropriately selected, and further appropriately set according to the thermal spraying method are thermal spraying conditions such as the kind of the thermal spray material, heat source temperature, thermal spraying angle, thermal spraying distance, and the like.
In the case of thermal spraying the cermets, the high-speed flame thermal spraying method (HVOF) is particularly suitable. This thermal spraying method is a thermal spraying method using combustion energy of combustion gas as a heat source. By raising pressure in a combustion chamber, high-speed flame comparable to explosive combustion flame is generated, and a thermal spray material is supplied to the center of this combustion flame jet. The thermal spray material is accelerated, molten or semi-molten, and continuously sprayed at a high speed. Since the molten thermal spray particles collide with the substrate at supersonic speed, the thermal sprayed coating 3 having high adhesion can be formed. For fuels used as the heat source, used are kerosene, and as combustion gas, acetylene, ethylene, propane and the like, which are mainly composed of carbon and hydrogen, in addition to H2 gas.
In the case of thermal spraying the ceramics, the plasma thermal spraying methods are particularly suitable. The plasma thermal spraying method is a thermal spraying method in which a thermal spray material is heated by plasma and molten into liquid thermal spray particles, and the thermal spray particles are caused to collide with coating-forming surface of the substrate at high speed by plasma jet. The plasma thermal spraying method using electric energy as a heat source is a method in which a coating is formed using argon, hydrogen, nitrogen, and the like as a source of plasma. Since the method has a high heat source temperature and a high flame rate, it is possible to form a coating having a high melting point.
As described above, after the thermal sprayed coating 3 is formed on the surface 2a of the roll substrate 2, an adjustment process for adjusting the surface shape of the thermal sprayed coating 3 may be performed. As a result, it is possible to obtain surface properties according to the intended use.
After the thermal sprayed coating 3 is formed, the shot blasting is preferably carried out (adjusting process A). The “shot blasting” refers to a technique for spraying spherical particles onto the surface of a substrate with compressed air or the like to adjust the surface shape of the substrate. In the undulation on the surface of the thermal sprayed coating 3, fine unevenness as shown in
After the thermal sprayed coating 3 is formed, the peak cutting may be carried out as necessary (adjusting process B). As a method for peak cutting, buffing or the like can be exemplified. By the peak cutting, the tip of the convex portion on the surface of the thermal sprayed coating 3 can be planarized as shown in
Such shot blasting and peak cutting can be carried out one by one against the milling rolls. As a result, since the surface shape of each milling roll can be adjusted uniquely, the design can be flexibly changed according to the purpose of use. Depending on the peak cutting, the surface roughness Ra of the thermal sprayed coating 3 will change. However, it is preferable to keep the surface roughness Ra within the numerical range as described above even after the peak cutting so that crushing performance for the cereal is not imparted.
According to the method for producing a milling roll of the present embodiment, since the thermal sprayed coating 3 is formed on the surface 2a of the roll substrate 2 by the thermal spraying, unevenness of the surface layer can be kept even if the smooth roll 1 is used for a long period of time and the crushing performance is not impaired. In addition, according to the thermal spraying, it is possible to control the size and presence ratio of the pores 5 in the coating to be formed by adjusting the thermal spraying conditions.
Since the thermal sprayed coating 3 formed by this method for producing a milling roll has pores 5 for retaining water, it is possible to make it hard to transmit heat to cereals and the water retentivity in the surface layer of the smooth roll 1 is enhanced, so that the cooling property for the cereals can be obtained. Further, since the thermal sprayed coating 3 is formed by thermal spraying on the roll substrate 2 having surface area and surface roughness increased by the blasting, peeling of the thermal sprayed coating 3 can be prevented. Hence, it is possible to eliminate the problems of wear on the surface layer of the smooth roll 1 and the peeling of the thermal sprayed coating 3, and the water retentivity is improved and the cooling property for the cereals is enhanced. This high cooling property prevents the cereals from getting heated in the milling process, and it can suppress drying of the cereals. This makes it possible to retain an adequate water content in the cereals, and to obtain satisfactory cereal powder.
The method for producing a milling roll of the above embodiment is an example and is not restrictive. In the first embodiment, the method for producing a milling roll was applied to a smooth roll, but the method may be also applied to a brake roll.
Also appropriately set within the above-described ranges are surface hardness of the surface 10a of the brake roll 10, and adjustment of surface roughness Ra of the roll substrate 13 to be within the range of −2 μm to +8 μm with respect to surface roughness Ra of the brake roll 10 finally aimed at. The surface roughness Ra of the brake roll 10 is set to 2-8 μm from the viewpoint of removal of the skin of cereals, which is a pre-process of the process using a smooth roll.
The method for producing the brake roll 10 in which the thermal sprayed coating 14 is formed on the surface thereof is the same as in the first embodiment. That is, carried out in this order are a roughening process for roughening the surface 13a of the roll substrate 13, a cleaning treatment of the surface 13a of the roll substrate 13, and a coating process for spraying a thermal spray material on the roll substrate 13 to form the thermal sprayed coating 14 having pores 15 for retaining water. Other processes such as a preheating process and the like may be included depending on the type of the substrate and the kind of the thermal spray material. Also contained in the processes of this embodiment can be adjusting processes A and B for adjusting surface shape of the thermal sprayed coating 14 after the thermal sprayed coating 14 is formed on the surface 13a of the roll substrate 13.
Depending on specification and construction embodiment of the milling roll, other processes may be included in the method for producing the milling roll. The configurations and processes described in the above embodiments can be changed as long as effects of the present invention are not impaired, and other configurations and processes to be provided as necessary are not limited. The scope of the present invention is defined by the claims, and it is intended that all modifications within meaning and scope equivalent to the claims are included.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-111410 | Jun 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/019721 | 5/26/2017 | WO | 00 |
