JPH07314456A - Porous material for mold, and its production - Google Patents

Abstract

PURPOSE:To develop a porous material for a mold realizing the easy removal of gas produced at the time of injection molding of a plastic and having a wear resistance. CONSTITUTION:In a mold material made of a tool steel and having such mechanical characteristics as a flexural strength of 500MPa or more and a hardness HRC of 35 or more, open pores having an average pore diameter of 5-20mum are formed in a ratio of 20-40vol.%, and the pore is passed through from one surface to the other surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold porous material such as a mold for plastic injection molding and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, plastic injection-molded products have tended to be thinned, and as the resin material, a difficult-to-mold material having high strength and heat resistance is being used. These materials are not only difficult to mold, but also generate a large amount of gas during injection molding. Therefore, especially in the case of thin-walled products with a wall thickness of 0.5 mm or less, the resin material itself may deform due to such gas generation, such as warpage and bending, and surface defects such as wrinkles and cloudiness. Is a cause of.

The means for preventing such deformation of the injection-molded product and the generation of surface defects is basically to suppress the generation of gas from the material, but at present, it is necessary to select a proper resin material for the product. , It is difficult to prioritize its molding conditions etc. Therefore, in order to prevent the occurrence of defects that occur during the production of thin-walled injection-molded products, it is necessary to remove the gas generated during molding.

Also in the prior art, as a means for removing gas, a method of forming a gap in a part of the mold, a method of utilizing the clearance of the ejector pin, and a method of making the mold into a nested structure and utilizing the parting line thereof. and so on.

However, these gas removal methods are
Burrs easily occur, many pins cannot be provided,
It cannot be used particularly in the case of a thin-walled molded product because the parting line itself is not desirable.

The most effective method for this is to use a porous material for a part of the mold of the thin portion, and force the gas generated at the time of injection molding to the back surface through the mold surface by a vacuum pump. It is a method of suction and removal. However, since the porous mold material currently used in this method (see JP-A-3-28303 and JP-A-3-159708) is a stainless steel alloy, it has poor wear resistance. . Therefore, when it is used for injection molding of difficult-molding plastic, plastic containing filler, etc., the life of the mold is very short, and maintenance such as mold correction and replacement must be performed every time a fixed number of injection molding is performed.

As described above, the currently used porous die material has an effect of removing gas, but is inferior in wear resistance to the ordinary plastic die material and has a short die life. There are drawbacks.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a mold porous material which facilitates the removal of gas generated during injection molding and is excellent in wear resistance, and a method for producing the same. .

[0009]

[Means for Solving the Problems] Here, the present inventor has conducted various studies to solve the above problems, and continued research and development focusing on the use of tool steel as a die material. The following findings were obtained. By the way, the conventional tool steel has not been used because it is difficult to process and there is a problem of corrosion due to gas generated during injection molding of plastic.

It has been found that the determinant of mold life is wear rather than the traditionally considered corrosion. Based on that, when the material of the die material is high speed tool steel or alloy tool steel, a die with bending strength of 500 MPa or more and hardness of HRC35 or more is obtained, and contrary to what is generally recognized, It was found to have a longer life than a mold made of stainless steel material. This is because the tool steel was able to fully exhibit the wear resistance characteristics of the tool steel.

Conventionally, the sintered product of metal powder has been considered to have low strength, but unexpectedly, in the sintered product described later,
It was found that it is possible to make a mold having a bending strength of 500 MPa or more and a hardness of HRC35 or more.

In order to maintain the mold life and ensure a predetermined air permeability, when the tool steel as described above is used as the mold material, the average diameter of the pores is 5 to 20 μm, and the existence rate is 20 to 20 μm.
It should be 40 vol.%.

In order to provide continuous pores from any surface to another surface, the size of the metal powder used as a raw material, the particle size distribution, the resin blending amount, etc. are critical.

As the metal powder, a raw material powder having an average particle size of 7 to 20 μm and a narrow particle size distribution is suitable, and the binder resin to be added and mixed is EVA (ethylene-vinyl acetate copolymer), polyethylene, paraffin wax. A mixture of resins is suitable, and the mixing ratio of the resin is preferably 1 to 10 wt%.

In the present invention, the material is tool steel,
A mold material with mechanical strength of 500 MPa or more and hardness of HRC 35 or more, having open pores with an average pore diameter of 5 to 20 μm and abundance of 20 to 40 vol.%, And 1 pore. It is a porous material for molds having a continuous pore structure from the surface of to the opposite surface.

Another aspect of the present invention is that the present invention has a composition of tool steel, an average grain size of 7 to 20 μm, and a ratio of steel powder having a grain size of 10 to 20 μm is 30 to 40% by weight based on the whole steel powder. %, Particle size 10μ
Resin 1 to 10 is added to steel powder whose m is less than 40 to 50% by weight.
A method for producing a porous material for a die, comprising blending wt%, molding the obtained mixed powder into a green compact, and then sintering the green compact.

According to a preferred embodiment of the present invention, there is provided a method for producing the above-mentioned porous material for molds, which uses a press or CIP when forming the mixed powder into a green compact. According to another preferred embodiment of the present invention, in order to impart the above-mentioned required mechanical properties to the porous material for a mold, sintering and quenching without exposure to the atmosphere,
You may make it temper in one furnace. Examples of the tool steel include high speed tool steel and cold tool steel.

[0018]

Next, the operation of the present invention will be described more specifically. TECHNICAL FIELD The present invention relates to a porous mold material capable of removing gas (degassing) and having wear resistance for plastic injection molding, and a method for producing the same.

The porous material for a mold according to the present invention may be incorporated in a part of the mold, or may be used for a small diameter / long ejector pin. At times, there is a risk of damage such as chipping or breaking. Therefore, the porous mold material according to the present invention also requires bending strength, that is, toughness. As a result of examining the required strength at that time, if the material has a bending strength of 500 MPa or more, since it is possible to prevent breakage even when used for an ejector pin, in the present invention, the bending strength is 500 MPa or more, Preferably 700 MPa or more, more preferably
800MPa or more.

Recent plastics tend to have higher hardness, and some of them have fillers added for the purpose of improving strength. Therefore, the die material is also required to have abrasion resistance. However, the porous materials currently used have a matrix hardness of about HRC30 (HV300) at the maximum, so that they have a short life and increase maintenance time. Therefore, the hardness of the die material according to the present invention is limited to HRC 35 or more, and more specifically to HRC 35 to 50. Preferably HRC 36
~ 42.

Here, as the tool steel used in the present invention, high-speed tool steel (SKH) already specified in JIS etc.
51, SKH57), cold tool steel (SKD11) and the like, and is not limited to a specific composition as long as it has been conventionally used as a tool steel, but in a broader sense, the tool steel according to the present invention is It has the following steel composition.

C: 0.5 to 2.5 wt%, Si: 0.5 wt% or less,
Mn: 0 to 1.2 wt%, Cr: 0.2 to 15.0 wt% (preferably
3.0 to 5.0 wt%), Mo: 0 to 10 wt%, W: 0 to 19 wt%,
V: 0 to 6 wt%, Co: 0 to 11 wt% More preferably, it has the following steel composition. C: 0.95 to 1.35 wt%, Si: 0.37 to 0.40 wt%, Mn: 0.35 to
0.38 wt%, Cr: 4.0-4.3 wt%, Mo: 3.5-5.1 wt%,
W: 5.8-6.1 wt% Here, each component has the following actions.

C: The hardness is limited to the above range in order to secure a predetermined hardness and strength (particularly hardness). Si: Although it is effective in improving the hardenability, Si is a substitutional atom, so if the amount of Si added is too large, the hardenability deteriorates, so the range is limited to the above range.

Mn: MnS has a function of fixing S which deteriorates the steel, and MnS has a function of improving machinability. Further, it has the effect of refining the structure within the above range, and thus has an effect of improving toughness. Cr: limited to the above range in order to improve hardenability and hardness.

Mo: Limited to the above range in order to improve hardness and toughness. Mo is an element that causes precipitation hardening and has a great effect on ensuring hardness. W, V, Co: To improve hardness, it is limited to the above range.
W, V, and Co are carbide-forming elements, and the carbides have extremely high hardness.

In order to enable gas removal from the back side with a vacuum pump during injection molding, there are pores in the mold material, and the pores are located from the surface of 1 to the back surface (opposite side or side surface). It must be continuous (connected) to the other surface.

If the average diameter of the pores is too small, the gas removal ability becomes insufficient, while if it is too large,
Pore marks may be transferred to the skin of the molded product, and the strength may be reduced, causing damage to the mold. This is also related to the abundance ratio of the pores in the whole material, and if the abundance ratio is low, the gas removal capacity becomes insufficient, and if it is high, the material strength decreases.

Here, the existence rate is determined by using an electron micrograph.
It is the value obtained as follows. Existence rate = (total area of pores occupying arbitrary cross section / area of entire arbitrary cross section) ^1.5 × 100 (%) From the above, in the present invention, the average diameter of pores is 5 to 20.
μm, abundance rate limited to 20-40 vol.% respectively. Preferably, they are 12 to 17 μm and 34 to 36 vol.%, Respectively.

Next, a method of manufacturing the porous mold material according to the present invention will be described. Generally, although there are some differences in the sintered metal produced by the powder metallurgy method, pores always exist. However, most of these pores are closed pores independent of each other and are not continuous with each other.

This is because a metal powder used as a raw material has a relatively small particle size and a wide distribution, and a binder (resin) is added only in a necessary minimum amount in order to facilitate degreasing during sintering. In order to increase the density of the above, the reason is that the metal powder is heated and sintered under atmospheric conditions so that mutual diffusion of the metal powders sufficiently occurs.

Originally, in powder metallurgy, in order to improve mechanical properties and the like, except for special applications such as oil-impregnated bearings,
This is taken as a matter of course because it is a major premise to reduce the resulting pores as much as possible.

However, in the case of the plastic mold material, continuous pores (open pores) as described above, which are also continuous from the surface of 1 to the back side (opposite side or side) thereof, are required. Further, in order to efficiently remove the gas, it is desirable that the pores communicate with the thick portion from the front surface to the back surface side.

Here, according to the present invention, it is possible to form a mesh-like pore structure having a branch derived from the gaps between the powder particles by producing a sintered body by the following operation.

(I) The average particle size of the raw steel powder is set to a relatively large particle size of 7 to 20 μm, and the average particle amount in the whole is 40% by weight or more, preferably 10 to 20 μm. 30-40% by weight, particle size less than 10 μm is the same
The distribution is narrowed to 40 to 50% by weight. More preferably, particles having a particle size of 11 to 16 .mu.m make up 15 to 20% by weight of the whole powder, and particles having a particle size of 8 to 11 .mu.m make up 14 to 19% by weight of the whole powder.

(Ii) 1 to 10% by weight of binder (resin)
And

As described above, according to the present invention, the metal powder raw material has 1) a relatively uniform shape, and 2) an average particle size of 10 to
20 μm, 3) Select a metal powder raw material that also has conditions such as a narrow particle size distribution range, that is, an average particle amount of 40% by weight or more.

That is, press molding of powder (mold molding)
In CIP molding, etc., if the particle shape of the powder is uniform and the particle size distribution is narrow, bridging of the powder particles easily occurs and the molding density becomes low.

Therefore, in the present invention, since the powders have the same shape and particle size, the gaps between the powders also have a certain shape and size. Further, the size of this gap can be controlled by determining the average particle size of the metal powder.

Incidentally, in general powder molding, a powder having a wide particle size distribution and a non-uniform particle shape is used.
Fine powder penetrates into the gaps between the coarse-grained powders, and as a result a high compaction density is obtained, but the pore formation is not sufficient, let alone pores that penetrate the front and back cannot be obtained.

According to the present invention, as the tool steel powder having the above characteristics, the metal powder having the above composition ratio is used as the starting powder raw material. When using high-speed tool steels (SKH51, SKH57) and cold tool steels (SKD11), which have particularly excellent wear resistance, the matrix hardness after sintering of these alloys is unexpectedly the lowest in the example of SKH57. But with HRC40 and conventional products
Higher hardness was obtained (about HRC30). It should be noted that this conventional product is a porous mold material that is currently commercially available as one type, and is made of stainless steel.

In the production method according to the present invention, when a resin is used as a binder to be added when molding powder, it exists in the gap between the powder after molding and is gasified in the subsequent degreasing step. Since the gap is larger than in the case of the general powder metallurgy method, the gasified resin is easily removed. Therefore, even if a large amount of resin is added, it is unlikely to remain as free carbon, or to be damaged or cracked due to deposition expansion of gasification.

In the practice of the present invention, it is possible to control the abundance of pores by adjusting the amount of resin added. This is because the resin existing portion inside the molded body becomes voids after degreasing. The amount of resin added is, as described above,
1-10 wt% is suitable.

The type of resin to be added is preferably EVA (ethylene-vinyl acetate copolymer), polyethylene, paraffin wax and the like, and the mixing ratio is EVA: polyethylene: paraffin wax: other = 0 to 5: 0
The optimum range is from 5: 1 to 5: 0-2. Specific examples of other resins include polypropylene, polystyrene, dibutyl phthalate, and stearic acid.

These resins are mixed with the above-mentioned steel powder in a heating temperature range of room temperature to 180 ° C., cooled and solidified into granules, which are then subjected to a die press or CIP method at a temperature of room temperature to 180 ° C. Mold.

The molding pressure is not particularly limited, but for example,
If it is less than 98 MPa, the strength of the molded body is low, and there are many voids even after sintering, which is not practical. On the other hand, when the molding pressure is 200 MPa or more, cracks due to backlash are likely to occur in the molded body, and the density after sintering becomes high, which is also not practical.
From the above, a molding pressure of 98 to 196 MPa is suitable. It is preferably 120 to 180 MPa, more preferably 145 to 150 MPa.

For degreasing the mixed resin, it is effective to flow argon or nitrogen gas as a carrier gas.
The atmosphere pressure is most appropriate in the range of 260 Pa to 4000 Pa,
The degreasing rate is also high at almost 100%.

The powder compacting means, conditions, sintering conditions and the like are not particularly limited in the present invention, but the powder compact density is preferably 60 to 80%, preferably 65 by press molding or CIP molding.
~ 75% green compact is molded and then sintered under normal vacuum atmosphere or reducing gas atmosphere. At this time, it is preferable to select sintering conditions so that the powders are not completely integrated. If these conditions are combined and produced, a sintered body having continuous open pores can be obtained.

Sintering of the molded body according to the present invention may be performed in one furnace from degreasing to sintering and cooling. That is, since the green compact that has been molded can be heat-treated (quenching and tempering) without exposing it to the atmosphere, not only is material deterioration prevented, but also the process is shortened.

The sintering holding temperature varies depending on the material of the metal material, but is a temperature of 75 to 85% of the melting point, for example, SKH57.
Alternatively, in the case of SKD11, it is desirable to set the temperature at 1050-1150 ° C, which is lower than the sintering temperature by the general powder metallurgy method.
This is to prevent the inter-diffusion between the metal powders that are in contact with each other when sintering is performed at a general sintering temperature, and the pores intentionally provided are contracted.

Following the sintering and holding, the processing temperature is further reduced to 0.
After raising the temperature in the range of ~ 100 ° C and holding it for a certain period of time,
Introduce argon or nitrogen gas to a pressure of ~ 0.6MPa and quench. Since the die material according to the present invention is a porous body, it cannot be subjected to a normal quenching method, that is, oil cooling or water cooling. This is because oil or water penetrates into the alloy through the pores. Therefore, gas cooling is required.

Therefore, in the manufacturing method of the present invention, it is preferable to carry out direct quenching from sintering so that the thermal energy at the time of sintering can be used as it is. Further, after cooling, the temperature is maintained at a certain temperature or lower, then reheated and tempered.

According to this method, it is possible to minimize the deterioration of the material without exposing the treated material to the atmosphere. In addition, the process will be greatly shortened. Next, the function and effect of the present invention will be described more specifically based on Examples.

[0053]

【Example】

(Example 1) High speed tool steel with an average particle size of 9.6 μm as a metal powder (material SKH57) (35.5 with a particle size of 10 to 20 μm)
%, Particle size distribution in which the particle size is less than 10 μm occupies 47.6% by weight), EVA, polyethylene, and paraffin wax are mixed at 95% by weight, 2% by weight, 2% by weight, and 1% by weight, respectively, and the resulting mixture is heated. Was pressed at a pressure of 147 MPa in a mold heated to obtain a green compact. The green density was 68%.

Next, this green compact was sintered at 1150 ° C. under N ₂ gas flow maintained at a vacuum degree of 660 Pa, held for 2 hours, and then a pressure of 0.5 MPa was applied to it at 2 ° C./s. It was quenched at a cooling rate of.
Then, after holding at 150 ° C for 1 hour under a vacuum of 1.3 × 10 ^-3 Pa,
After tempering at 550 ° C., it was taken out of the furnace. During this time, the processed material never exited the furnace.

The matrix hardness of the sintered body of this example, which has been thus sintered and heat-treated, is HRC 41 to 45 (HV 400 to
450) and commercial stainless steel porous product with the same composition (HV300)
It was significantly higher than that, and a bending strength of 800 MPa was obtained.

The average diameter of all pores is 15
μm, the abundance rate was 35 vol%. It was confirmed that the pores were communicating pores by injecting air from one surface and releasing air from the other surface in water.

The thus obtained sintered body was processed into a predetermined size, incorporated into a mold for injection molding, and subjected to plastic injection molding having a 0.5 mm thick portion. Conventionally, warpage and bending were observed. It was possible to prevent deformation of the thin portion where
Further, in this example, the number of continuous uses was 30,000 shots when the commercially available porous material made of stainless steel was used. About 2.7 times more products could be injection molded.

In addition, the total number of times of use is 400,000 shots in the case of this example, and 30 times in the case of the conventional example.
It was 10,000 shots. In the prior art, a warp of about 3 mm was observed every 5 cm, but no warpage was observed in this example as in the case of other porous materials. No rough skin was observed as well.

(Example 2) Steel powder having an average particle size of 9.9 μm (material SKD11) powder (particle size distribution: 38.2% by weight of particle size 10 to 20 μm, 45.8% by weight of particle size less than 10 μm) and paraffin wax 98.5 Knead at a ratio of wt% and 1.5 wt% and CIP this with a rubber mold at a pressure of 196 MPa to a size of 6.5 × 120 mm.
Molded. The green density was 72%.

Next, the green compact was sintered at 1150 ° C. under a N ₂ gas flow kept at a vacuum degree of 260 Pa, held for 2 hours, and then held at 1050 ° C. for 30 minutes to give 0.5 Apply pressure of MPa 2
It was rapidly cooled at a cooling rate of ° C / s. Next, after holding at 150 ° C. for 1 hour under a vacuum of 1.3 × 10 ⁻³ Pa, tempering was performed at 500 ° C.

The matrix hardness of the sintered body of this example which has been sintered and heat treated is HRC 48 to 51 (HV 480 to 530), and the transverse rupture strength is
It was 1020 MPa. The average diameter of the pores present in the whole was 10 μm, and the abundance was 30 vol%. As in the case of Example 1, it was confirmed that the pores were communicating.

Ejector pins were machined from this sintered body and incorporated into an injection molding die so that degassing could be performed through the pins. As a result, degassing was better than when conventional clearances were used. Not only was it possible, but the occurrence of burrs was also suppressed. The number of continuous uses before maintenance is extended from the conventional 30,000 shots to 60,000 shots. The service life of this example was 400,000 shots after 7 maintenance cycles.

[0063]

As described above, according to the present invention,
Conventionally, tool steel can be used as a porous mold material for injection molding, which had a short mold life and was problematic, and it is superior to conventional melt products in terms of not only breathability but also hardness and strength. Thus, the life of the porous mold can be remarkably extended, and great practical advantages can be obtained.

Claims (2)

[Claims] 1. A mold material having a tool steel as a material, a mechanical strength of 500 MPa or more and a hardness of HRC 35 or more, and an average pore diameter of 5 to 20 μm and an abundance of 20 to 40 vol.%.
And a porous material for a mold having a pore structure in which the pores are continuous from the surface of 1 to the surface on the opposite side. 2. The composition of tool steel has an average grain size of 7 to 20.
1 to 10 wt% of resin is mixed with steel powder having a ratio of steel powder of μm and a particle size of 10 to 20 μm of 30 to 40% by weight to the entire steel powder and a particle size of less than 10 μm of 40 to 50% by weight A method for producing a porous material for a die, comprising molding the obtained mixed powder into a green compact, and then sintering the green compact.