JP3326072B2 - Iron-based mixture for powder metallurgy and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an iron-based powder mixture for powder metallurgy, in which, among other things, the segregation of copper powder additives and the generation of dust are small, the fluidity and the change over time after production are small. It relates to the manufacturing method.

[0002]

2. Description of the Related Art An iron-based powder mixture for powder metallurgy is prepared by adding zinc stearate to an iron powder in addition to an alloy powder such as copper powder, graphite powder, iron phosphide powder and, if necessary, a powder for improving machinability. ,
It is common to mix lubricants such as aluminum stearate and lead stearate. Such a lubricant is selected based on the mixing property with the metal powder and the fugitive property during sintering.

However, such a mixing method has the following disadvantages. First, a major drawback of the mixing method is that the raw material mixture undergoes segregation. Talking about segregation, powder mixtures contain powders of different sizes, shapes and densities, so that segregation can easily occur during transport after mixing, loading into a hopper, discharging, or molding. I will. For example, it is well known that a mixture of an iron-based powder and a graphite powder segregates in a transport container due to vibration during truck transport, and the graphite powder emerges. It is also known that the graphite charged in the hopper has a different concentration of graphite powder in the initial, middle and final stages of discharge when discharged from the hopper due to segregation in the hopper.

[0004] Due to these segregations, the composition of the product varies, causing dimensional changes and variations in strength to increase, causing defective products.

[0005] Further, since graphite powder and the like are all fine powders, the specific surface area of the mixture is increased, and as a result, the fluidity is reduced. Such a decrease in the fluidity lowers the filling speed of the molding die, and thus has the disadvantage of lowering the production speed of the green compact.

As a technique for preventing such a segregation of the powder mixture, JP-A-56-136901 and JP-A-58-136901 are known.
There is a technique using a binder as disclosed in Japanese Patent Publication No. 28321, but when the amount of the binder is increased so as to sufficiently improve the segregation of the powder mixture, the flowability of the powder mixture decreases. There is.

The present inventors have previously described Japanese Patent Laid-Open No. 1-1657.
No. 01 and JP-A-2-47201 proposed a method using a co-melt of metal soap or wax and oil as a binder. These can remarkably reduce segregation and dust generation of the powder mixture, and can improve fluidity. However, these methods have a problem that the fluidity of the powder mixture changes with time due to the means for preventing the segregation described above. Then, the present inventors have further developed a method using a co-melt of a high melting point oil and a metal soap as a binder, as proposed in JP-A-2-57602. The technique is such that the change with time of the co-melt is small and the change with time of the fluidity of the powder mixture is reduced. However, this technique has another problem that the apparent density of the powder mixture changes because the saturated fatty acid having a high melting point and metal soap which are solid at normal temperature are mixed with the iron-based powder.

In order to solve this problem, the present inventors disclosed in Japanese Patent Application Laid-Open No. Hei 3-162502, after coating the surface of an iron-based powder with a fatty acid, added an additive to the surface of the iron-based powder with a mixture of a fatty acid and a metal soap. A method was proposed in which it was adhered with a melt and metal soap was further added to the outer surface.

[0009]

Although the problem of segregation has been considerably solved in these methods, only the copper powder, which is the most important additive alloy component, is insufficiently fixed to the iron-based powder even in these methods and segregates. This was a serious problem.

An object of the present invention is to solve this problem. The present invention has been made based on the finding that the relationship between the primary particle size and the agglomerated particle size of copper powder greatly affects the degree of adhesion. In addition, it was made as a result of finding that the degree of adhesion is further increased by treating the copper powder surface.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to achieve the above-mentioned object, and comprises an iron-based powder, at least one alloy powder containing at least a copper powder or a cuprous oxide powder, and an iron powder. In an iron-based mixture for powder metallurgy comprising an organic substance for bonding an alloy powder to a base powder, the copper powder or cuprous oxide powder has an agglomerated particle size of 5 to 28 μm evaluated by a microtrack method, the primary particle size as evaluated by the BET method is Ri 0.2~1.5μm der, iron for the powder metallurgy
325 of the mixture relative to the copper content in the whole mixture
The ratio of the copper content of the portion passing through the mesh is 2 or less
A powder metallurgy iron-based mixture, characterized in that that.

[0012] In this case, the pre-Symbol organics are preferred as a co melt or partial melt of two or more different waxes having a melting point of a fatty acid and a metal soap.

In yet said powder metallurgical iron-based mixture, the polyvinyl butyral of 0.1 to 2 wt% on the copper powder or cuprous oxide Powder surface is deposited, the copper powder, the surface 0. Preferably, 1 to 2% by weight of a Si-based or Al-based coupling agent is attached, or 0.1 to 2% by weight of graphite is attached to the surface of the copper powder. It is preferable that the reduced powder be copper oxide.

Next, the following is provided as a method for producing the iron-based mixture for powder metallurgy of the present invention. That is, in the production method of the present invention, a fatty acid which is liquid at normal temperature is added to an iron-based powder and primary mixed, and then a metal soap is added to one or more alloy powders including at least copper powder or cuprous oxide powder. During the secondary mixing step or after the secondary mixing, the temperature is raised to form a co-melt of the fatty acid and the metal soap, and then the mixture is cooled with tertiary mixing. Production of an iron-based mixture for powder metallurgy in which the alloy powder is fixed to the surface of the iron-based powder particles by cooling and fixed, and the metal soap or wax is added during cooling and quaternary mixing is performed by the bonding force of the co-melt. In the method, the copper powder or the cuprous oxide powder has an agglomerated particle size of 5 to 28 μm evaluated by a microtrack method, and a BET.
Primary particle size evaluated by the method is 0.2 to 1.5 μm ,
With respect to the content of copper in the entire iron-based mixture for powder metallurgy
Containing copper in the portion of the mixture that has passed through 325 mesh.
A method for producing an iron-based mixture for powder metallurgy , wherein the ratio of the ratio is 2 or less .

Further, as another method for producing the iron-base mixture for powder metallurgy of the present invention, two or more kinds of powders for alloys having different melting points from one or more alloy powders containing at least copper powder or cuprous oxide powder are added to the iron-base powder. The wax is first mixed, and the temperature is increased during or after the primary mixing step to generate a partial melt of the wax, and then cooled while secondary mixing, and the partial melt is cooled. A method for producing an iron-based mixture for powder metallurgy in which an alloy powder is fixed to the surface of iron-based powder particles by the bonding force of the partial melt, and a metal soap or wax is added during cooling and tertiary mixing is performed. In the above, the copper powder or the cuprous oxide powder has an agglomerated particle size of 5 to 28 μm evaluated by a microtrack method, and 1 to be evaluated by a BET method.
Having a secondary particle size of 0.2 to 1.5 μm ,
32 of the mixture relative to the copper content in the total base mixture
The present invention provides a method for producing an iron-based mixture for powder metallurgy , wherein a ratio of a copper content of a portion passing through 5 meshes is 2 or less .

The technical concept of the present invention and the reasons for limitation will be described below. As described above, copper is an element necessary for increasing the strength of a sintered body manufactured by sintering iron-based powder,
When a mixture of the copper powder and the iron-based powder or the like is formed in advance, the adhesion to the iron-based powder is not always sufficient, and there has been a problem of segregation or dusting in the mixture. The inventors of the present invention have made intensive studies and focused on the shape of iron-based powder, and solved this problem by attaching copper powder preferentially to the recesses of these iron-based powders.

That is, the particle diameter of the iron-based powder generally used in powder metallurgy is 80 μm on average, and the diameter of the concave portion is 5 to 20 μm.
It is about μm. In order to enter the recesses, the agglomeration particle size, which is the apparent particle size of the copper powder, must be 5 to 28 μm. If it exceeds 28 μm, it becomes too large to fit into the concave portion, which is unsuitable. Copper powder of less than 5 μm is expensive and unsuitable for practical use.

Further, it is necessary that the copper powder is uniformly and uniformly coated with an organic substance to be a binder around the copper powder in order to increase the adhesion strength. When the thickness is set to 0.5 μm, the molten organic material permeates into the voids of these primary particles due to capillary action, thereby achieving the above object. It is difficult to obtain copper powder of less than 0.2 μm at low cost, and if it exceeds 1.5 μm, the adhesion rate decreases. For the above reasons, the agglomeration particle size of the copper powder was 5 to 28 μm by the microtrack method, and the primary particle size was B
The thickness was 0.2 to 1.5 μm by the ET method. The aggregate particle size evaluated by the microtrack method referred to in the present application refers to a 50% particle size measured by a laser diffraction type microtrack particle size analyzer as described in Examples, and a primary particle size evaluated by a BET method. Is a particle diameter calculated from the specific surface area measured by the BET method, assuming that all particles are spherical and have the same particle diameter.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Whether or not copper powder adheres to an iron-based powder is determined by the ratio of copper in a portion of the iron-based mixture passing through a 325 mesh (45 μm) to the copper content in the entire iron-based mixture. It is evaluated by the content ratio. That is, -32
The ratio of 5-mesh iron-based powder is small, and the agglomeration particle size of copper powder is 5 to 28 μm. Therefore, if all the copper powder uniformly adheres to the iron-based powder regardless of the iron-based powder diameter, this ratio becomes 1 ,
The ratio is greater than 1 as the amount of free copper powder increases. The present inventors have examined segregation, dust and the like, and as a result, it has been confirmed that there is no practical problem if this ratio is 2 or less.

As an organic substance for fixing the iron-based powder and the copper powder, a co-melt of fatty acid and metal soap or a material having a different melting point is used.
It is preferably a partial melt of one or more waxes.
The method of using a co-melt of a fatty acid and a metal soap disclosed by the present inventors in JP-A-3-162502 is disclosed in Japanese Patent Application Laid-Open No. HEI 3-162502. Ideal for coating the whole. A partial melt of two or more kinds of waxes having different melting points is also preferable because it similarly coats the copper powder uniformly.

When polyvinyl butyral of 0.1 to 2% by weight adheres to the surface of the copper powder, an organic substance contained in the powder mixture and PVB form a co-fusion compound, and the adhesion to the iron-based powder is further improved. I do. If the content is less than 0.1% by weight, the adhesion rate is low, and it is difficult to attach the polyvinyl butyral exceeding 2% by weight.

Further, 0.1 to 2% by weight of S
When the i-type or Al-type coupling agent is surface-treated,
The organic substance contained in the powder mixture and the coupling agent are chemically bonded to each other, and the adhesion to the iron-based powder is further improved. 0.
If it is less than 1% by weight, the adhesion rate of the copper powder is low, and even if it exceeds 2% by weight, the effect does not change, and only the addition cost increases.

Further, since graphite powder has a stronger adhesion to iron-based powder than copper powder, by attaching 0.1 to 2% by weight of graphite to the surface of copper powder, The powder can be more firmly adhered to the iron-based powder.
If it is less than 0.1% by weight, the adhesion rate of the copper powder is low, and it cannot be adhered in excess of 2% by weight.

As a method for producing the iron-based mixture for powder metallurgy, a fatty acid which is liquid at room temperature is added to the iron-based powder and firstly mixed, and then the aggregated particle size evaluated by at least the Microtrac method is 5 to 28 μm. A secondary soap is added by adding a metal soap to one or more alloy powders including a copper powder having a primary particle diameter evaluated by a BET method of 0.2 to 1.5 μm,
During the secondary mixing step or after the secondary mixing, the temperature is raised to form a co-melt of the fatty acid and the metal soap, and then the mixture is cooled with tertiary mixing, and the co-melt is cooled and fixed. The powder for alloy is fixed to the surface of the iron-based powder particles by the bonding force, and then, when cooled, a metal soap or wax is added.
Subsequent mixing is preferred.

The iron-based powder contains at least a copper powder having an agglomerated particle size of 5 to 28 μm evaluated by a microtrack method and a primary particle size of 0.2 to 1.5 μm evaluated by a BET method. One or more kinds of alloy powders and two or more kinds of waxes having different melting points are added and primary mixed, and the temperature is increased during the primary mixing step or after the primary mixing to generate a partial melt of the wax. Cool while mixing, fix the partial melt by cooling, fix the alloy powder on the surface of the iron-based powder particles by the bonding force of the partial melt, add metal soap or wax at the time of cooling, and perform tertiary mixing Is preferably performed.

As described above, 0.1 to 2% by weight of polyvinyl butyral is adhered to the surface, or 0.1 to 2% by weight of a Si or Al coupling agent is surface-treated, It is more preferable to use copper powder to which about 2% by weight of graphite is attached. These specific implementation methods will be described in detail in Examples.

As the copper powder, there are electrolytic copper powder, copper oxide reduced powder, and the like, and any type of copper powder can be used. Is more preferable because it has a small cavity inside. FIGS. 1 and 2 show electron micrographs of the copper powder. As shown in FIG. 1, the primary particles of the electrolytic copper powder are bonded in a dendritic manner, while the reduced copper oxide powder is formed by loosely binding whisker-like fiber shapes such as “mayu” as shown in FIG. are doing. For this reason, the primary particle size of the reduced copper oxide powder becomes smaller with the same aggregate particle size. At the time of mixing, the copper powder adheres to the iron powder while repeatedly colliding, while the reduced copper oxide powder adheres while deforming so as to fit into the concave portion of the iron powder. On the other hand, in the case of electrolytic copper powder, such deformation during mixing does not occur. For this reason, the reduced copper oxide powder improves the adhesion of copper to the electrolytic copper powder.

Although the above description has been limited to copper powder, further experiments have shown that the same results can be obtained by using cuprous oxide powder (Cu ₂ O) instead of copper powder. Was. This is presumably because the surface of the cuprous oxide powder is more excellent in adhesion to organic substances than the metal powder. Therefore, the above description also applies to the case of cuprous oxide powder.

[0029]

【Example】

Examples 1 to 6, Comparative Examples 1 to 3 Oleic acid was added to iron powder for powder metallurgy having an average particle diameter of 78 µm.
3% by weight was sprayed and uniformly mixed for 3 minutes (primary mixing). Thereafter, 1% by weight of natural graphite powder having an average particle size of 23 μm,
0.4% by weight of zinc stearate, agglomerated particle size shown in Table 1,
After adding 2% by weight of copper powder having a primary particle size and mixing well,
The mixture was heated and mixed at 110 ° C. (secondary mixing), cooled to 85 ° C. or lower while further mixing (tertiary mixing), graphite powder and copper powder were combined with iron powder particles by eutectic bonding of oleic acid and zinc stearate. A powder mixture fixed by the agent was produced. Further, 0.3% by weight of zinc stearate was added and mixed uniformly (fourth mixing), and then discharged from the heating mixer. This is designated as mixing method 1.

A powder of iron powder for powder metallurgy having an average particle size of 78 μm, 1% by weight of natural graphite powder having an average particle size of 23 μm, a mixture of stearamide and ethylenebisstearic acid.
4% by weight, 2% by weight of a copper powder having an aggregate particle size shown in Table 1 and a primary particle size are added, and after sufficient mixing, they are mixed and heated at 110 ° C (primary mixing), and further mixed at 85 ° C. After cooling, a powder mixture in which graphite powder and copper powder were fixed to iron powder particles and graphite particles by a eutectic binder of stearic acid amide and ethylene bisstearic acid amide was produced (secondary mixing). Further, 0.3% by weight of ethylenebisstearic acid amide and 0.1% by weight of zinc stearate were added, mixed uniformly, heated, and discharged from the mixer (tertiary mixing). This is designated as mixing method 2.

In Examples 1 and 2, electrolytic copper powder classified by air was used. In Examples 3 and 4, copper powder produced by reducing copper oxide was used. In Example 5, cuprous oxide was used. The graphite adhesion, copper adhesion, and fluidity of each mixture were evaluated, and the results are shown in Table 1 together with the mixing method. The graphite adhesion, copper adhesion, and fluidity are defined as follows. Graphite adhesion = (C content in mixed powder of -100 to +200 mesh) / (C content in total mixed powder) * 100
(%) Copper adhesion = (Cu content in mixed powder of −325 mesh) /
(Cu content in all mixed powder) Fluidity: according to JIS Z2502.

The agglomerated particle size of the copper powder is the 50% particle size measured by a laser diffraction type microtrack particle size analyzer, and the primary particle size is the specific particle size measured by the BET method. It is expressed by the particle size calculated assuming that In Examples 1 to 6, the agglomerated particle size of the copper powder used in any of the mixing methods 1 and 2 evaluated by the microtrack method was 5 to 28 μm, and the primary particle size evaluated by the BET method was 0.2 μm. 2
If it is 1.5 μm, the adhesion rate of copper is as good as 1 to 2. When Examples 1 and 2 are compared with Examples 3 and 4, the latter has more copper adhesion (the smaller the degree of copper adhesion, the better the adhesion). This is related to the property that the copper oxide reduced powder has small cavities inside. Comparative Example 1 is an electrolytic copper powder commonly used in iron-based powder metallurgy, which has a large agglomerated particle size and a large primary particle size, and has poor adhesion of copper. In Comparative Examples 2 and 3, the agglomerated particle size and the primary particle size were large, and the adhesion of copper was poor. In each of the examples, graphite adhesion and fluidity are both good.

Examples 7 to 13 and Comparative Examples 4 to 6 Using the copper powders used in Examples 2 and 4 and Comparative Example 1, powder mixtures were produced by mixing methods 1 and 2. However, the copper powder was mixed with a predetermined amount of a 10% ethanol solution of polyvinyl butyral (PVB) with the copper powder, dried, and further crushed to reduce the PVB to 0.1%.
08 to 0.5% by weight. Examples 7 to 13 and Comparative Examples 4 to 6 are summarized in Table 2. The graphite adhesion, copper adhesion, and fluidity are the same as in Table 1. Mixing method 1, 2
Regardless of Example 7, as in Examples 7 to 12, the copper powder used had an agglomerated particle size of 5 to 28 μm as evaluated by the microtrack method and a primary particle size of 0.2 to 0.2 μm as evaluated by the BET method.
When a powder having a thickness of 2 to 1.5 μm and having 0.1 to 2% of PVB adhered to the surface is used, copper powder adheres more than in Examples 2 and 4 (copper adhesion is small). Incidentally, PV B coating weight is less than 0.1% (Example 13) In the improvement effect is not observed. In Comparative Examples 4 to 6, since both the aggregated particle diameter and the primary particle diameter of the copper powder were large, even if PVB was adhered, the adhesion of copper was poor. Note PV B is adhered amount exceeds 2% was not adhered.

Examples 14 to 21 Powder mixtures were produced in the same manner as in Example 1 (mixing methods 1 and 2). However, copper powder to which various amounts of a coupling material were adhered was used. The coupling material was attached to the copper powder by mixing a predetermined amount of a 10% ethanol solution of the coupling material with the copper powder, drying at 100 ° C. for 1 hour, and then crushing. Table 3 collectively shows Examples 14 to 21 . The graphite adhesion, copper adhesion, and fluidity are the same as in Table 1.

Regardless of the mixing method 1 or 2, the copper powder used has an agglomerated particle size of 5 to 5 as evaluated by the microtrack method.
28 μm, the primary particle size evaluated by the BET method is 0.2 to
In the case of a powder in which 0.1 to 2% of a Si-based coupling material and an Al-based coupling material are surface-treated on the surface of a 1.5-μm copper powder, the copper powder adheres more than in Examples 2 and 4. I do.

In Example 20, the amount of the coupling material attached was 2
%, The addition effect is not recognized, which is not preferable in terms of cost. In Example 21 , the adhesion amount of the coupling material was small and the adhesion of copper was not improved.

Examples 22 to 25 Copper powder to which various amounts of graphite powder were adhered was used as the copper powder. Adhesion of the graphite powder to the copper powder was performed by dispersing the graphite powder in a 10% ethanol solution of PVB, mixing with the copper powder, and drying while mixing. Table 4 shows Examples 22 to 25 . The copper powder used has a coagulated particle size of 5 to 28 μm evaluated by a microtrack method, and a primary particle size of 0.2 to 1.5 μm evaluated by a BET method. ~ 2%
When the graphite powder is adhered, the adhesion of copper is improved as compared with Example 2. In Example 25 , the adhesion amount of graphite was small and the adhesion of copper was not improved. In this test, more than 2% of graphite could not be adhered.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

The iron-based powder mixture for powder metallurgy according to the present invention comprises:
Since it is formed as described above, it has an effect that, in particular, segregation of copper powder additives and generation of dust (dust) are small, fluidity is not changed, and change with time after manufacture is small. This iron-based powder mixture for powder metallurgy can be easily produced by the production method of the present invention.

[Brief description of the drawings]

FIG. 1 is an electron micrograph (× 1000) showing the shape of an electrolytic copper powder.

FIG. 2 is an electron micrograph (magnification: 3000) showing the shape of the copper oxide reduced powder.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-47201 (JP, A) JP-A-2-57602 (JP, A) JP-A-62-80208 (JP, A) JP-A-4- 285141 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22F 1/00 C22C 33/02

Claims (8)

(57) [Claims] 1. A powder metallurgy comprising an iron-based powder, at least one powder for an alloy containing at least a copper powder or a cuprous oxide powder, and an organic substance for bonding the powder for an alloy to the iron-based powder. In the iron-based mixture, the copper powder or the cuprous oxide powder has an agglomerated particle size of 5 as evaluated by a microtrack method.
~ 28 μm, primary particle size evaluated by BET method is 0.2 ~
Ri 1.5μm der, of the powder metallurgical iron-based mixture overall in
Pass 325 mesh of the mixture for copper content
An iron-based mixture for powder metallurgy, characterized in that the ratio of the copper content in the selected portion is 2 or less . 2. The method according to claim 1, wherein the organic substance is a co-solvent of a fatty acid and a metal soap.
The iron-based mixture for powder metallurgy according to claim 1, which is a melt or a partial melt of two or more waxes having different melting points . 3. The method according to claim 1 , wherein 0.1 to 2% by weight of poly
3. An iron-based mixture for powder metallurgy according to claim 1, wherein vinyl butyral is attached . 4. The method according to claim 1, wherein 0.1 to 2% by weight of Si
The iron-based mixture for powder metallurgy according to claim 1, wherein an iron-based or Al-based coupling agent is attached. 5. 0.1 to 2% by weight of graphite on the surface of copper powder
The iron-based mixture for powder metallurgy according to claim 1 or 2 , wherein iron is adhered. 6. The copper powder is copper oxide reduced powder.
The iron-based mixture for powder metallurgy according to claim 1, 2, 3, 4, or 5 . 7. Addition of fatty acid which is liquid at normal temperature to iron-based powder
Primary mixing, then at least copper powder or cuprous oxide powder
Addition of metal soap to one or more alloy powders including powder
To perform secondary mixing and raise during or after the secondary mixing step.
Heat to form a co-melt of fatty acid and metal soap, then
Then, cool while tertiary mixing, and cool and fix the co-melt
To the surface of the iron-based powder particles due to the binding force of the co-melt.
The powder for the alloy is fixed, and when cooled, metal soap or
Iron-based mixture for powder metallurgy with addition of wax and quaternary mixing
In the production method, the copper powder or cuprous oxide powder,
Aggregated particle size evaluated by microtrack method is 5-28μ
m, the primary particle size evaluated by the BET method is 0.2 to 1.5 μm
m, the content of copper in the entire iron-based mixture for powder metallurgy
The portion of the mixture that passed through 325 mesh, relative to the percentage
Method of producing a powder metallurgical iron-based mixture ratio of content of copper is characterized and this is 2 or less. 8. The iron-based powder may contain at least copper powder or sub powder.
Different melting point from one or more alloy powders including copper oxide powder
The primary mixing is performed by adding two or more kinds of waxes and mixing the primary mixture.
During the process or after the primary mixing, the temperature is raised to partially melt the wax.
And then cooled with secondary mixing,
The molten material is cooled and fixed, and iron is
The alloy powder is fixed on the surface of the base powder particles, and
Powder to which metal soap or wax is added and tertiary mixing is performed at the time of disposal
In the method for producing an iron-based mixture for powder metallurgy, the copper powder or
Is a cuprous oxide powder that is evaluated by the microtrack method.
Primary particle size evaluated by BET method with collected particle size of 5-28 μm
Is 0.2 to 1.5 μm, and the iron-based mixture for powder metallurgy is
325 mesh of the mixture against the copper content in the whole
A method for producing an iron-based mixture for powder metallurgy , wherein a ratio of a copper content of a portion having passed through a metal shell is 2 or less .