US20140023547A1

US20140023547A1 - Magnesium alloy chips and process for manufacturing molded article using same

Info

Publication number: US20140023547A1
Application number: US14/009,861
Authority: US
Inventors: Yutaka Mitooka; Makoto Hino; Koji Murakami; Hikaru Uchiyama; Yoshiaki Hashimoto
Original assignee: Okayama Prefectural Government; STU CO Ltd
Current assignee: TERUMA Co Ltd; Okayama Prefectural Government
Priority date: 2011-04-08
Filing date: 2012-04-06
Publication date: 2014-01-23
Also published as: CN103079725A; KR101310622B1; KR20130041354A; DE112012001625B4; DE112012001625T5; WO2012137907A1; CN103079725B; JPWO2012137907A1; JP5137049B2

Abstract

There is provided chips for injection molding wherein the surfaces of chips made of an aluminum-containing magnesium alloy are coated with carbon powder. A molded article produced by injection molding of such chips for injection molding had excellent bending properties and tensile strength, which vary in a small range. Furthermore, a scrap formed during injection-molding of the chips for injection molding has improved recyclability.

Description

TECHNICAL FIELD

The present invention relates to magnesium alloy chips for injection molding and a process for manufacturing a molded article using same.

BACKGROUND ART

A magnesium alloy has a high specific strength since it is most lightweight among practically used metals, and exhibits excellent heat radiation and better recyclability than a resin. Thus, molded articles made of a magnesium alloy are used in a wide variety of applications such as electric devices, automobiles and leisure goods.
Injection molding is a common molding methods for a magnesium alloy. In general, a magnesium alloy is injection-molded by heating chips made of a magnesium alloy in a cylinder into a molten or semi-molten state (coexistence of a solid and a liquid phases) and then injecting the molten or semi-molten magnesium alloy into a mold. Here, since the magnesium alloy is injected into a mold at a relatively higher pressure, injection-molding is preferable for forming a thin-wall article such as a casing of an electric device. A so-called thixomolding method, that is, inter alia, a molding method in which a semi-molten material is injected into a mold, is a typical injection molding method for a magnesium alloy and is used for producing various molded articles.
Conventionally, a Mg—Al alloy having excellent mechanical properties, particularly a Mg—Al—Zn alloy being well-balanced between mechanical properties and processability and exhibiting improved corrosion resistance have been widely used as a magnesium alloy for injection molding. Recently, further improvement in mechanical properties of a molded article has been needed in order to allow for thinning of a molded article made of a magnesium alloy and increasing an yield.
There has been known a method for adding carbon to an alloy for the purpose of improving mechanical properties of a molded article made of an aluminum-containing magnesium alloy. Addition of carbon to a magnesium alloy makes crystals finer, resulting in improved mechanical properties. It is believed that such reduction in a crystal size may be caused by Al₄C₃formed by a reaction between carbon and aluminum which are added to a magnesium alloy. Conventionally, carbon is added to a magnesium alloy by adding C₂Cl₆to a molten magnesium alloy. Such a method, however, has an environmental disadvantage that C₂Cl₆added is decomposed to generate harmful substances such as chlorine gas, and thus an alternative method has been needed.
A known alternative method for adding carbon to an aluminum-containing magnesium alloy is addition of carbon powder (for example, see Patent Reference Nos. 1 and 2). However, when carbon powder, is directly added to a molten magnesium alloy, the carbon powder tends to agglutinate, so that a molded article obtained may have insufficiently improved or have varied mechanical properties.
Patent Reference No. 3 has described a process for manufacturing a carbon-containing magnesium alloy comprising mixing 5 to 30 parts by weight of at least one of carbon powder, carbon nanofiber and carbon nanotube with 100 parts by weight of a magnesium alloy to prepare a master batch and then mixing the master batch with a 3- to 20-fold parts by weight of a magnesium alloy. In the examples therein, there is described a magnesium alloy produced by processing magnesium alloy powder and carbon powder in a ball mill to prepare a mixed powder, sintering the mixed powder to prepare a master batch, adding the master batch to a molten metal and homogenizing the molten metal with stirring. There is described that the magnesium alloy thus produced contains uniformly dispersed carbon and has improved tensile strength and a higher Young's modulus. The process is, however, troublesome and thus at a disadvantage in terms of cost.

PRIOR ART REFERENCES

Patent References

Patent Reference No. 1: Japanese published unexamined application No. 1994-73485.
Patent Reference No. 2: Japanese published unexamined application No. 2004-156067.
Patent Reference No. 3: Japanese published unexamined application No. 2007-291438.

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

To solve the above problems, an objective of the present invention is to provide chips for injection molding allowing for forming a molded article made of a magnesium alloy which has excellent bending properties and tensile strength which vary in a small range. Another objective of the invention is to provide a process for manufacturing a molded article made of a magnesium alloy using such chips for injection molding.

Means for Solving Problem

The above problems can be solved by providing chips for injection molding wherein the surfaces of chips made of an aluminum-containing magnesium alloy are coated with carbon powder.
Herein, a content of the carbon powder is preferably 0.01 to 3% by weight. Furthermore, the carbon powder is preferably carbon black. Here, the carbon black more preferably has an average primary particle diameter of from 5 to 100 nm and a DBP absorption of from 40 to 200 mL/100 g.
The above problems can be also solved by providing a process for manufacturing the chips for injection molding, comprising mixing chips made of an aluminum-containing magnesium alloy with the carbon powder.
A preferable embodiment of the present invention is a process for manufacturing a molded article made of a magnesium alloy, comprising charging the chips for injection molding in an injection-molding machine and then injection-molding the chips.
Here, in the molded article, a complex of aluminum and carbon is dispersed in a magnesium matrix.
Another preferable embodiment of the present invention is a process for manufacturing an ingot made of a magnesium alloy, comprising heat-melting a scrap formed during injection-molding of the chips for injection molding in the presence of a flux and then cooling it. Here, it is preferable that a ratio (C₂/C₁) of a carbon content C₂(% by weight) in said ingot to a carbon content C₁(% by weight) in said scrap is 0.1 or less.

Advantageous Effects of the Invention

A molded article produced by injection-molding of chips for injection molding of the present invention has excellent bending properties and tensile strength which vary in a small range. Furthermore, a process for manufacturing a molded article made of a magnesium alloy of the present invention can conveniently provide a molded article made of a magnesium alloy having excellent bending properties and tensile strength which vary in a small range. It thus allows for making a molded article thinner and improving an yield. Furthermore, a scrap formed during injection-molding of the chips for injection molding has improved recyclability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exterior photo of a test piece in a tensile test and an exterior photo of a testing machine on which the test piece is set.

FIG. 2 shows an exterior photo of a testing machine on which the test piece is set, in a bending test.

FIG. 3 shows micrograms of a cross section of a molded article cut in a direction vertical to a flowing direction of a molten metal in Example 1 and Comparative Example 1.

FIG. 4 shows a relationship between a displacement and a load at break of a test piece as determined by a tensile test in Example 1 and Comparative Example 1.

FIG. 5 shows a relationship between a displacement and a load at break of a test piece as determined by a bending test in Example 1 and Comparative Example 1.

FIG. 6 shows an elemental map of an area containing a complex of aluminum and carbon in the surface of a molded article in Example 1.

FIG. 7 shows measurement points in determination of content distribution of each of aluminum and zinc in a molded article in Example 1 and Comparative Example 1.

FIG. 8 shows content distribution of aluminum in a molded article in Example 1 and Comparative Example 1.

FIG. 9 shows content distribution of zinc in a molded article in Example 1 and Comparative Example 1.

FIG. 10 shows a 0.2% proof stress of a molded article in Examples 1 to 3 and Comparative Example 1 as determined by a tensile test.

MODE FOR CARRYING OUT THE INVENTION

The present invention provides chips for injection molding wherein the surfaces of chips made of an aluminum-containing magnesium alloy are coated with carbon powder.
The chips to be coated with carbon powder must be made of an aluminum-containing magnesium alloy. In other words, the chips must be made of an alloy containing aluminum as a component in addition to magnesium. Aluminum is effective for improving tensile strength and corrosion resistance of a magnesium alloy. Furthermore, as described in Examples later, a complex of aluminum and carbon is formed in a molded article produced by the present invention. Formation of the complex would lead to a molded article having excellent bending properties and tensile strength.
An aluminum content in a magnesium alloy used in the present invention is preferably 1 to 15% by weight. If an aluminum content is less than 1% by weight, tensile strength and corrosion resistance of a molded article produced may be reduced. Furthermore, formation of a complex of aluminum and carbon may be inhibited in a molded article produced, thus bending properties and tensile strength may not improve. An aluminum content of more than 15% by weight may lead to brittle failure.
The magnesium alloy can contain zinc, wherein a zinc content is 3% by weight or less. In a case that zinc is contained therein, toughness of magnesium alloy and fluidity during molding further improve. A zinc content is preferably 0.1 to 3% by weight. If a zinc content is less than 0.1% by weight, toughness of a molded article produced and fluidity during molding may be reduced. If a zinc content is more than 3% by weight, hot tearing may occur.
The magnesium alloy can contain manganese, in which a manganese content is 1% by weight or less. In a case that manganese is contained therein, corrosion resistance of a magnesium alloy further improves. A manganese content is preferably 0.05 to 1% by weight. If a manganese content is less than 0.05% by weight, corrosion resistance of a molded article produced may be reduced. If a manganese content is more than 1% by weight, compression strength and tensile strength may be reduced.
The magnesium alloy can contain beryllium, in which a beryllium content is 0.003% by weight or less. In a case that beryllium is contained therein, flame resistance during melting of a magnesium alloy improves. In a case that beryllium is contained therein, brightness of a molded article produced also improves. A beryllium content is preferably 0.0001 to 0.003% by weight. If a beryllium content is less than 0.0001% by weight, it may fail to improve flame resistance or brightness. If a beryllium content is more than 0.003% by weight, crystals may be coarse, leading to reduction of tensile strength and a higher cost.
The magnesium alloy can contain calcium, in which a calcium content is 3% by weight or less. In a case that calcium is contained therein, flame retardancy of a magnesium alloy improves. A calcium content is generally 0.5 to 3% by weight.
The magnesium alloy can contain elements other than those described above as long as they do not reduce the effect of the present invention. Such elements can be willingly added or contained as inevitable impurities. A content of such elements is generally 1% by weight or less. The balance of a magnesium alloy used for the chips is magnesium, whose content is generally 80% by weight or more.
The magnesium alloy can be specifically selected from magnesium alloys such as AZ91, AM50, AM60 and AZ31 in accordance with the ASTM Standard. Among others, AZ91 is preferable, which is well-balanced between mechanical properties and processability and exhibits higher corrosion resistance.
There are no particular restrictions to a process for manufacturing the chips. Generally, an ingot made of the above magnesium alloy can be cut into the chips. There are no particular restrictions to the shape or the size of the chips, which can be appropriately selected, depending on, for example, the specifications of an injection-molding machine used for producing a molded article. Chips with a length of from 1 to 10 mm is generally used. Herein, a length of a chip denotes a distance between the furthest positions in the chip.
The carbon powder used in the present invention can be selected from, but not limited to, carbon black, graphite such as scaly graphite, coke or the like.
In the present invention, the carbon powder is preferably carbon black. When carbon black is used as the carbon powder, carbon black and the chips are mixed by, for example, a mixer to coat the surfaces of the chips with carbon black. The use of chips for injection molding coated with carbon powder in injection molding would allow carbon powder to easily disperse in a magnesium alloy.
There are no particular restrictions to the type of the carbon black. Examples which can be used include furnace black, thermal black, channel black, acetylene black ketjen black and the like, which can be used in combination.
Preferably, the carbon black has an average primary particle diameter of from 5 to 100 nm and a DBP absorption of from 40 to 200 mL/100 g. A DBP absorption is a parameter corresponding to a volume of voids in a bunch of grapes when fused primary particles of carbon black, so-called “aggregate”, is assumed to be a bunch of grapes. As the aggregate grows, the void volume increases and thus a DBP absorption increases. Formation of a complex of aluminum and carbon may be influenced by a primary particle diameter of carbon black and also a growing state of the aggregate. It is, therefore, preferable that an average primary particle diameter and a DBP absorption are within a certain range. A DBP absorption can be determined in accordance with JIS K6217.
In the light of higher proof stress of a molded article produced, the carbon black preferably has a DBP absorption of from 40 to 200 mL/100 g, more preferably 60 to 200 mL/100 g, further preferably 80 to 200 mL/100 g.
The carbon black can have functional groups in its surface. Examples of such functional groups include hydroxy groups such as phenolic hydroxy group, carboxyl groups and quinone groups.
The surfaces of the chips are coated with the carbon powder to produce chips for injection molding having surfaces coated with the carbon powder. There are no particular restrictions to a method for coating the surface of the chips with the carbon powder. Generally, the chips and the carbon powder can be mixed using a mixer to produce chips for injection molding having the surfaces coated with the carbon powder. A mixing ratio of the chips to the carbon powder can be appropriately adjusted, depending on the amount of carbon contained in a molded article to be produced. The amount of the carbon powder in the chips for injection molding coated with the carbon powder is preferably 0.01 to 3% by weight, more preferably 0.01 to 0.5% by weight.
The chips for injection molding having surfaces coated with the carbon powder are charged in an injection-molding machine and then injection-molded to provide a molded article. Generally, the chips for injection molding charged in an injection-molding machine are heated in a cylinder while being fed via a screw in the cylinder to an injection nozzle. Then, a molten or semi-molten (coexistence of a solid and a liquid phases) magnesium alloy fed to the vicinity of the injection nozzle is injected into a mold to be shaped. In general, a cylinder temperature in an injection-molding machine is 530 to 700° C. and a mold temperature is 160 to 240° C.
Thus, the use of the chips for injection molding having surfaces coated with the carbon powder allows the carbon powder to be homogeneously dispersed in the molten or semi-molten magnesium alloy in the injection-molding machine, so that a molded article in which a complex of aluminum and carbon is homogeneously dispersed is obtained. In the injection-molding machine, it seems that a magnesium alloy molten or semi-molten by heating could be so efficiently stirred by rotation of a screw that the carbon powder can be homogeneously dispersed in the molten or semi-molten magnesium alloy. It is surprising that the carbon powder is homogeneously dispersed in a magnesium alloy in spite that a cylinder temperature is not so high and a time taken for injection after charging the chips in the cylinder. In the present invention, preferred is a molding process where chips introduced in an injection-molding machine are semi-molten and then injected in a mold, a so-called thixomolding method.
In a molded article produced by a manufacturing process according to the present invention, a complex of aluminum and carbon is dispersed in a magnesium matrix. A complex of aluminum and carbon can be observed by element mapping using, for example, an X-ray microanalyzer. In the complex region, aluminum and carbon are detected in higher levels than surrounding regions. A magnesium matrix denotes regions other than the complex of aluminum and carbon, and the major part of the matrix contains magnesium as a main component.
In the present invention, a complex of aluminum and carbon would be formed by bond formation between the carbon powder and aluminum in the chips during injection molding. Our analysis of a molded article produced have confirmed that a large part of carbon in the molded article forms a complex with aluminum. Although it is not confirmed whether Al₄C₃is formed in the complex, formation of such a complex would allow a molded article of the present invention to have excellent bending properties and tensile strength. Furthermore, according to the manufacturing process of the present invention, the carbon powder can be homogeneously dispersed in a magnesium alloy, so that the complex is homogeneously dispersed in a molded article. Therefore, variation in bending properties and tensile strength in the molded article is reduced.
Furthermore, in a molded article of the present invention, defects are reduced and a segregation level for each component is low. It would be because fluidity is improved by dispersing carbon powder in a molten or semi-molten magnesium alloy during injection molding. Reduction of defects and a lower segregation level also contribute to smaller variation in bending properties and tensile strength.
A carbon content in a molded article produced according to the present invention is preferably 0.01 to 3% by weight. If a carbon content is less than 0.01% by weight, bending properties and tensile strength of the molded article may be insufficiently improved and fluidity may be insufficiently improved. If a carbon content is more than 3% by weight, carbon powder may agglutinate, leading to tendency to crack formation and thus causing variation in tensile strength. A carbon content is more preferably 0.5% by weight or less.
A molded article thus produced has excellent bending properties and tensile strength which vary in a small range. A molded article can be, therefore, thinner and produced in an improved yield. A molded article produced by a manufacturing process of the present invention can be suitably used in a variety of applications including electric devices such as a cell phone, a personal computer, a video camera, an optical disk player, a display and a projector; automobiles; welfare devices such as a wheel chair; and leisure goods such as fishing goods and a bicycle.
It is preferable that a scrap produced during injection molding after feeding the chips to the injection-molding machine is heat-molten in the presence of a flux and then cooled to produce an ingot made of a magnesium alloy. Such a manufacturing process can provide an ingot with a reduced carbon content.
Examples of a scrap produced during injection molding include alloys solidified inside of the injection-molding machine such as a sprue, a runner and an overflow unit, and non-standard molded articles.
The scrap is charged in a melting furnace to be molten. Here, the scrap is preferably charged in a pre-heated melting furnace. Furthermore, a molten-metal temperature is preferably adjusted to 600 to 750° C.
There are no particular restrictions to timing of adding a flux to a scrap, but it is preferably added after the scrap charged in a melting furnace has been molten. After a flux is added, a molten metal is preferably refined by stirring. A refining temperature is preferably 600 to 750° C. and a refining time is preferably 3 to 300 min.
There are no particular restrictions to a flux used in a manufacturing process for an ingot of the present invention, and those commonly used for refinement of a magnesium alloy can be used. An example is a flux containing a halide of a metal belonging to Group 1 or 2 in the periodic table as a main component. The term “main component” as used herein means a component contained in a content of generally 50% by weight or more, preferably 80% by weight or more. The metal halide is preferably at least one selected from magnesium chloride, calcium chloride, barium chloride, potassium chloride, sodium chloride and calcium fluoride. The amount of a flux to be added is preferably 0.3 to 45 parts by weight to 100 parts by weight of a scrap.
A molten metal after refinement is preferably allowed to stand. A settling temperature is preferably 600 to 750° C. and a settling time is preferably 3 to 300 min. A clean part in the upper layer in the molten metal after refinement is cast in a mold and then cooled to give an ingot.
A ratio (C₂/C₁) of a carbon content C₂(% by weight) in the ingot to a carbon content C₁(% by weight) in the scrap is preferably 0.1 or less, more preferably 0.06 or less.
It is generally difficult to remove carbon from a molten metal after melting of a scrap. For example, a carbon content is high in an ingot produced by heat-melting a scrap wherein carbonized materials adhere to its surface and then cooling it. Thus, a molded article produced from such an ingot has insufficient functions such as corrosion resistance. Carbon in such an ingot is not dispersed, so that properties such as corrosion resistance would be deteriorated. Thus, for example, when a scrap having carbonated materials on its surface is regenerated into an ingot, it is necessary to remove the carbonized materials before melting, leading to a high cost, and the carbonated materials cannot be adequately removed. In contrast, a scrap produced during molding the chips for injection molding of the present invention can be generated into an ingot having a low carbon content by a convenient method as described above. Chips produced from such an ingot has a low carbon content, so that it can be used in combination with chips free from carbon and can be used after being again coated with carbon powder with higher recyclability. Furthermore, a molded article produced from an ingot of the present invention exhibits good corrosion resistance and excellent mechanical properties.

EXAMPLES

The present invention will be described with reference to Examples.

Tensile Test

A tensile test was conducted using a universal material testing machine “3382 Floor Model Testing System” from Instron Japan Company Ltd. A test piece was a plate-type molded article with a thickness of 2 mm which has a parallel portion with a width of 20 mm and a length of 60 mm in the center and grips at both ends. The test piece was prepared by injection molding using a mold for forming a test piece which has a shape corresponding to that of the test piece. FIG. 1 shows an exterior photo of the test piece in the tensile test and an exterior photo of the testing machine in which the test piece is set. Measurement was conducted at a tension rate of 5 mm/min.

Bending Test

A bending test was conducted using a universal material testing machine “3382 Floor Model Testing System” from Instron Japan Company Ltd. A test piece used in the bending test was a plate having a width of 20 mm, a length of 70 mm and a thickness of 2 mm prepared by partly cutting the grips in the molded article formed by using a mold for forming a test piece for the tensile test. FIG. 2 shows an exterior photo of a testing machine in which the test piece was set in the bending test. A distance between two supports was set to 60 mm. A test was conducted by pushing down a former at a rate of 5 mm/min. The test was terminated either when the test piece was broken or when a displacement of the former reached 20 mm.

Element Mapping

Element mapping in the surface of a molded article was conducted by using an X-ray microanalyzer “JXA-8500FS” from JEOL Ltd. An acceleration voltage and a sample irradiation current were set to 15 kV and 1×10⁻⁸A, respectively for measurement.

Determination of a Chemical Composition

A chemical composition of a molded article was determined using an optical emission spectrometer “PDA-7000” from Shimadzu Corporation. A diameter of a measuring spot was 5 mm. However, a carbon content was measured as described below.

Measurement of a Carbon Content

A carbon content in a molded article was measured by using a carbon/sulfur analyzer “EMIA-920V” from HORIBA Ltd. Measurement was conducted in accordance with JIS Z2615 “General rules for determination of carbon in metallic materials” (infrared absorption spectrometry (integration)).

Microscopy of a Cross-Section

A molded article was cut vertically to a flow direction of a molten metal. After the piece obtained was embedded in a resin, the cut surface was polished. The cross section after polishing was observed using a light microscope.

Example 1

An ingot made of AZ91D (specifications; Al: 8.5 to 9.5% by weight, Zn: 0.45 to 0.9% by weight, Mn: 0.17 to 0.4% by weight, Be: 0.0008 to 0.0012% by weight, Si: 0.05% by weight or less, Fe: 0.004% by weight or less, Cu: 0.025% by weight or less, Ni: 0.001% by weight or less, balance: Mg) was cut into cylindrical magnesium alloy chips with a radius of about 0.5 mm and a length of about 4 mm. 100 kg of the magnesium alloy chips obtained and 100 g of carbon black (furnace black “#30” from Mitsubishi Chemical Corporation, average primary particle diameter: 30 nm, DBP absorption: 113 mL/100 g) were separately introduced in a V-type mixer, and mixed at a rotation number of 30 r.p.m. for 20 min to give chips for injection molding in which the surfaces of the magnesium alloy chips were coated with carbon black. Here, the chips for injection molding obtained were visually observed, showing that the surfaces of the chips were substantially homogeneously coated with carbon black. The chips for injection molding obtained were introduced in an injection-molding machine for thixomolding (“JSW JLM220-MG” from The Japan Steel Works, Ltd.) and injection-molded. During injection-molding, a melting temperature and a mold temperature were set to 610° C. and 225° C., respectively. The mold used was a mold for producing a test piece for a tensile test. Thus, a plate-type molded article with a thickness of 2 mm which had a parallel portion with a width of 20 mm and a length of 60 mm in the center and grips at both ends was produced. The molded article obtained had an aluminum content of 8.9% by weight, a zinc content of 0.68% by weight, a manganese content of 0.26% by weight, a beryllium content of 0.0011% by weight, an iron content of 0.002% by weight, a copper content of 0.003% by weight, a nickel content of 0.001% by weight and a carbon content of 0.085% by weight. FIG. 3 shows a microgram of a cross section of a molded article cut in a direction vertical to a flowing direction of a molten metal. As shown in FIG. 3, there were no large cavities in the molded article.
A tensile test and a bending test were conducted for the molded article obtained. For each test, a plurality of samples were used. FIG. 4 shows a relationship between a displacement and a load at break of the test piece as determined by the tensile test. FIG. 5 shows a relationship between a displacement and a load at break of the test piece as determined by the bending test. When a sample was not broken at the end of the bending test (a displacement is 20 mm), a load at the end of the test is given. FIG. 10 shows a 0.2% proof stress as determined by the tensile test.
Element mapping was conducted for the surface of the molded article obtained. FIG. 6 shows an elemental map of an area containing a complex of aluminum and carbon.
Distribution of a content of each of aluminum and zinc in the surface of the molded article were determined. FIG. 7 shows measurement points in determination of content distribution of each of aluminum and zinc in the molded article. FIG. 8 shows content distribution of aluminum in a molded article while FIG. 9 shows content distribution of zinc. The number of sample was three for each.

Comparative Example 1

A molded article was produced using chips for injection molding which was not coated with carbon black. A molded article was produced as described in Example 1, except that carbon black coating was not conducted. The molded article obtained had an aluminum content of 9.2% by weight, a zinc content of 0.78% by weight, a manganese content of 0.25% by weight, a beryllium content of 0.0010% by weight, an iron content of 0.002% by weight, a copper content of 0.004% by weight and a nickel content of 0.001% by weight. A carbon content was below a detection limit (0.0001% by weight). FIG. 3 shows a microgram of a cross section of the molded article cut in a direction vertical to a flowing direction of the molten metal. As shown in FIG. 3, relatively larger cavities were observed in the molded article.
A tensile test and a bending test for the molded article were conducted as described in Example 1. FIG. 4 shows a relationship between a displacement and a load at break of the test piece as determined by the tensile test. FIG. 5 shows a relationship between a displacement and a load at break of the test piece as determined by the bending test. FIG. 10 shows a 0.2% proof stress as determined by the tensile test. Furthermore, distribution of a content of each of aluminum and zinc in the surface of the molded article was determined as described in Example 1. FIG. 8 shows relationship between a measurement point and an aluminum content, while FIG. 9 shows a relationship between a measurement point and a zinc content.
As shown in FIG. 4, the molded article in Example 1 produced by the manufacturing process of the present invention had excellent tensile strength. Furthermore, variation of tensile strength between samples was small. In contrast, the molded article in Comparative Example 1 produced using the chips for injection molding which were not coated with carbon black exhibited large variation of tensile strength between samples. Furthermore, as shown in FIG. 5, the molded article in Example 1 had excellent bending properties. In the test, any of the measured samples (four) was not broken at the maximum displacement (20 mm). In contrast, the molded article in Comparative Example 1 was broken even at a small displacement and variation of bending properties between samples was large.
Element mapping of the molded article in Example 1 was conducted, and a complex of aluminum and carbon as shown in FIG. 6 was observed. Such a complex was substantially homogeneously dispersed in the surface of the molded article.
As shown in FIGS. 8 and 9, the molded article in Example 1 had lower level of segregation of aluminum (FIG. 8) and zinc (FIG. 9) than the molded article in Comparative Example 1.

Examples 2 and 3

A molded article was produced as described in Example 1, except that a different type of carbon black was used. In Example 2, carbon black “#45L” from Mitsubishi Chemical. Corporation (average primary particle diameter: 24 nm and DBP absorption: 53 mL/100 g) was used. In Example 3, carbon black “#3050B” from Mitsubishi Chemical Corporation (average primary particle diameter: 50 nm and DBP absorption 175 mL/100 g) was used. For the molded articles obtained, a tensile test was conducted as described in Example 1. FIG. 10 shows a 0.2% proof stress as determined by the tensile test.

Example 4

An ingot was produced using a scrap formed during injection-molding chips introduced in an injection-molding machine as described in Example 1. After the injection molding, 100 kg of an alloy (carbon content: 0.16% by weight) solidified in a sprue in the Injection-molding machine was placed in a pre-heated melting furnace. A temperature was regulated to make a temperature of the molten metal 650 to 700° C. After the alloy introduced was completely molten, 2 kg of a flux (Dow 310: MgCl₂50 parts by weight, KCl ₂20 parts by weight, CaF ₂15 parts by weight and MgO 15 parts by weight.) was added to the molten metal. After stirring for 30 min, the molten metal was allowed to stand for 30 min. A clean part in the upper layer in the molten metal was cast in a mold and then cooled to give an ingot. A carbon content in the ingot was 0.003% by weight. The molded article produced from chips obtained by cutting the ingot had comparable corrosion resistance and mechanical properties to those in the molded article in Comparative Example 1.

Claims

1. Chips for injection molding wherein the surfaces of chips made of an aluminum-containing magnesium alloy are coated with carbon powder.

2. The chips for injection molding as claimed in claim 1, wherein a content of said carbon powder is 0.01 to 3% by weight.

3. The chips for injection molding as claimed in claim 1, wherein said carbon powder is carbon black.

4. The chips for injection molding as claimed in claim 3, wherein said carbon black has an average primary particle diameter of from 5 to 100 nm and a DBP absorption of from 40 to 200 mL/100 g.

5. A process for manufacturing the chips for injection molding as claimed in claim 1, comprising mixing chips made of an aluminum-containing magnesium alloy with said carbon powder.

6. A process for manufacturing a molded article made of a magnesium alloy, comprising charging the chips for injection molding as claimed in claim 1 in an injection-molding machine and then injection-molding the chips.

7. The process for manufacturing a molded article as claimed in claim 6, wherein in said molded article, a complex of aluminum and carbon is dispersed in a magnesium matrix.

8. A process for manufacturing an ingot made of a magnesium alloy, comprising heat-melting a scrap formed during injection-molding of the chips for injection molding as claimed in claim 1 in the presence of a flux and then cooling it.

9. The process for manufacturing an ingot as claimed in claim 8, wherein a ratio (C₂/C₁) of a carbon content C₂(% by weight) in said ingot to a carbon content C₁(% by weight) in said scrap is 0.1 or less.