WO2002098588A1 - Method for the rapid fabrication of mold inserts - Google Patents

Method for the rapid fabrication of mold inserts Download PDF

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Publication number
WO2002098588A1
WO2002098588A1 PCT/US2002/018032 US0218032W WO02098588A1 WO 2002098588 A1 WO2002098588 A1 WO 2002098588A1 US 0218032 W US0218032 W US 0218032W WO 02098588 A1 WO02098588 A1 WO 02098588A1
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WO
WIPO (PCT)
Prior art keywords
inserts
cavity
core
die
mold
Prior art date
Application number
PCT/US2002/018032
Other languages
French (fr)
Inventor
Herman E. Amaya
Dennis K. Crounse
Original Assignee
Amaya Herman E
Crounse Dennis K
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to US09/875,472 priority Critical patent/US20020187065A1/en
Priority to US09/875,472 priority
Application filed by Amaya Herman E, Crounse Dennis K filed Critical Amaya Herman E
Publication of WO2002098588A1 publication Critical patent/WO2002098588A1/en

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Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F7/00Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression
    • B22F7/06Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression of composite workpieces or articles from parts, e.g. to form tipped tools
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F3/00Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/22Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip
    • B22F3/225Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip by injection molding
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F5/00Manufacture of workpieces or articles from metallic powder characterised by the special shape of the product
    • B22F5/007Manufacture of workpieces or articles from metallic powder characterised by the special shape of the product of moulds
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F5/00Manufacture of workpieces or articles from metallic powder characterised by the special shape of the product
    • B22F5/10Manufacture of workpieces or articles from metallic powder characterised by the special shape of the product of articles with cavities or holes, not otherwise provided for in the preceding subgroups
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F3/00Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/24After-treatment of workpieces or articles
    • B22F2003/247Removing material: carving, cleaning, grinding, hobbing, honing, lapping, polishing, milling, shaving, skiving, turning the surface
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F2998/00Supplementary information concerning processes or compositions relating to powder metallurgy
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F2998/00Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10Processes characterised by the sequence of their steps
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F2999/00Aspects linked to processes or compositions used in powder metallurgy
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B29WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29CSHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C33/00Moulds or cores; Details thereof or accessories therefor
    • B29C33/30Mounting, exchanging or centering
    • B29C33/306Exchangeable mould parts, e.g. cassette moulds, mould inserts
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B29WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29CSHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C45/00Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
    • B29C45/17Component parts, details or accessories; Auxiliary operations
    • B29C45/26Moulds
    • B29C45/2673Moulds with exchangeable mould parts, e.g. cassette moulds
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B33ADDITIVE MANUFACTURING TECHNOLOGY
    • B33YADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y80/00Products made by additive manufacturing
    • CCHEMISTRY; METALLURGY
    • C04CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04BLIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/60Aspects relating to the preparation, properties or mechanical treatment of green bodies or pre-forms
    • C04B2235/602Making the green bodies or pre-forms by moulding
    • C04B2235/6026Computer aided shaping, e.g. rapid prototyping

Abstract

A method for the rapid fabrication of mold inserts for molds and molds is disclosed; wherein high machinability rates and time and cost savings along with increased tool life and material savings are obtained through the use of blank die inserts formulated from material comonly used in the metal injection molding process of complex shaped parts. The method involves first the creation of cutting path programs developed from CAD files of the part; direct machining of the cavity and core inserts as described to predetermined sizes; processing the cavity and core inserts to convert the soft material - initially consisting of fine metal powders in a matrix of binder compounds - into a dense, fully hardenable material comparable to the material used in conventional toolmaking; and performing any necessary finishing and fitting operations to fit the resulting dies into a base that can be used as part of an injection mulding tool.

Description

TITLE: METHOD FOR THE RAPID FABRICATION OF MOLD INSERTS

CROSS-REFERENCE TO RELATED APPLICATIONS: Not applicable

BACKGROUND - FIELD OF THE INVENTION

This invention relates to the field of rapid tooling manufacturing, particularly for mold inserts that can be incorporated into injection molds, die cast molds or tools for related forming methods, and to the rapid process for manufacturing same.

BACKGROUND OF THE INVENTION - PRIOR ART

Metal molds for forming processes such as injection molding, blow molding, die casting, sheet metal forming and die casting are made from conventional machining techniques, EDM, casting and electroforming.

The standard method for the fabrication of a molding tool begins with the splitting of a three dimensional CAD representation into the two above mentioned cavity and core halves, and proceeding to create a positive or male version of the parts. The positive version of the part is normally machined or ground as a set of carbon electrodes for each mold half, and these set of carbon electrodes are then used to burn a negative or female representation of the parts into a block of steel, one for the cavity half of the tool, the other for the core.

These two tooling halves can then be mounted on on a standard injection molding machine to mold the part the actual parts from plastic, metal, ceramic or composite material formulations. Hard tooling for injection molding such as described above, is also used to produce patterns for the investment casting process as well as several powder metallurgy processes.

The present state of the art in moldmaking demands skilled labor and the use of folly automated equipment which can cost upwards of $100,000 per unit. Tool shops generally have a multitude of cutting, milling and grinding equipment to deal with the different tool materials that are cut into dies and molds. It is because of that that the moldmaking industry is both a capital and labor intensive process, that has been experiencing pressure from intense international competition.

What this means in terms of technology is a greater emphasis on the development of computer driven applications and less emphasis on the artisan skills demanded of toolmakers in the conventional tooling industry. It is important to consider that of the $3.2 billion in sales that the moldmaking industry reported for 1999 in the US, fully a third of that or $1 billion was spent in the skilled labor cost area. International competition addresses this equation by radically lowering the cost of labor while using conventional tooling methods and equipment.

The largest time factor in mold construction is the time that must be taken to cut, mill, grind or EDM steel. Molds must be durable enough to last for the production of hundreds of thousands of parts with nrinimal maintenance, and for that performance the types of steel that must be used are the tool steel alloy grades that have great wear and impact resistance. The problem is that because of their desirable mechanical properties, these tool steels are often the hardest to cut, mill or grind and hence take more time to process.

Softer materials such as aluminum or prehardened steels are easier to cut, and this ease translates into a time factor reduction and hence a lower cost. Aluminum for example, cuts 50% faster than an S -7 or D-2 tool steel. Clearly aluminum tools do not last as long as the corresponding tool steels.

The reduction in process time for the cutting or milling operations is then a recognized bottleneck for cost improvements in moldmaking. In a high volume moldmaking operation, this means that a great many machines performing these operation must be used, and this then translates into a higher overhead structure which can buckle a company if the high volume demand disappears. While there have been advances made in developing high speed milling machines to directly cut into steel and to form the carbon electrodes, the high cost of the equipment and the concomitant increase in the overhead cost remains. In addition to that is the increased cost of the tooling itself, that must still withstand the abrasiveness and wear of the tool steel that it is cutting.

One advantage of the instant invention is that it takes advantage of the heretofore unexploited conversion characteristic of the material commonly used for the metal injection molding, from soft, pliable and hence machinable, to dense and fully hardenable upon full processing. As a reference point, while aluminum cuts perhaps 50% faster than tool steels, MM block material cuts 50% faster than aluminum, and all without the use of cutting fluids which in standard operations are required to cool the steel as it is being cut. Softer more machinable material translates to increased rates as well as increased tooling life. One way to analyze the conventional moldmaking process is to break it down into cost factors, where 15% of the cost and time is invested in the design phase of the tool, 15% of the is invested in the cost of materials, and the remaining 70% of the cost is invested in the machining and labor. Increasing machinability of a material reduces cost in three ways; firstly by reducing the time required to complete the operation, secondly by reducing the overhead cost of the operation by using less capital intensive equipment and thirdly by reducing the tool replacement cost for milling cutters and related items.

Due to the recognized limiting time and cost factors in conventional moldmaking technology as described, several rapid tool manufacturing technologies have been developed. There are three generally recognized processes used. The first uses some of the established rapid prototyping technologies to directly develop molds. The second copies a rapid prototype form into metal for instance by investment casting. The third directly manufactures hard metallic molds directly with adapted prototyping systems.

An example of the first type of rapid tooling system is U.S. Patent No. 5,458,825 which describes the use of stereolithography to directly produce blow molding tooling for rapid container prototyping. This method of direct tooling, so called because a pattern is not required in the building of a mold, can produce tools of high accuracy but limited durability, so the volume runs are short. One of the issues is that the choice of materials for the stereolithographic process, referred to

J as SLA(stereolithography apparatus), is limited, and these materials have to be able to withstand higher molding temperatures to accommodate a wider range of plastics for sampling. U.S. Patent No. 5,641,448 takes the "soft" tooling produced by the any of the solid modeling technologies such as SLA, and selectively deposits layers of nickel around the inner mold surfaces. The nickel coated mold is then fitted into a base for the injection molding operation. This process does harden the tool to increase the tooling life, nonetheless, molding parameters must be controlled towards the lower end of the molding pressures to maintain the nickel deposited layers intact.

An example of the second process is described in U.S. Patent No. 4,220,190, where the investment cast shell serves as a means to form the functional cavity surfaces when the metal alloy is cast. In a variation on this, SLA patterns are being used for the direct casting of the injection tooling molds. The main issue with this fabrication method has been the inherent surface quality of the casting and the amount of work required to bring the cast mold or die halves to specifications for use in injection molding tools.

The third type of process has several variations beginning with the use thermal spraying, 3D printing technologies, laser sintering of powder metals, hot isostatic processing and other variations of the use of powder metallurgy technologies including the use of metal or powder injection molding. Methods of thermal spraying of metal have been developed to directly produce prototype parts and more recently to form "hard" molds, die and tools as described in U.S. Patent No. 5,609,922. The patterns in this case are support members constructed not only to form the desired shape of a cavity or a core, but also to promote optimum heat exchange properties for the thermal spray deposition process. In one recent variation disclosed by patent # 6,074,194, the liquid material consisting not only of molten metallic alloys but also polymeric compounds are atomized into fine droplets by a high temperature, high velocity gas and deposited onto a pattern The tools have the same issues of fragility as the other thermal spray method that uses patterns from stereolithography to serve as a base for the thermal spray deposition process.

The use of powder metallurgy takes advantage of the fact that powders can conform to the shape of any given pattern when they are flowed in. Variations in the application of the process can be identified by the way the powders are consolidated so they can maintain the desired shape. For the purposes of forming complex metal molds, the advantages of powder metallurgy lie not only in the forming of complex shapes facilitated by the flow of powders, but also by the fact that a great deal of material waste can be avoided by processing net shape or near net shape molds when compared to the other metal working processes.

A means of forming the die cavities through the use of conventional powder metallurgy is described in US Patent No. 4,327,156. The practice of this invention involves flowing in refractory powders around a flexible rubber mold that has been previously conformed from a replicating master. To keep the powders in place, a binder is mixed with the powders and molded or compressed into shape, followed by a curing period to allow the binder to harden and hold shape. The next step is remove the cavity or core mold and to burn off the binder once it has accomplished its purpose in an oven, thereby leaving a porous metal skeleton that can be closed off by infiltrating a low melting point metal such as copper. This method does provide "hard" tooling that will last longer than the "soft" tooling of the other rapid prototyping technologies and introduces the use of powder metallurgy as a means to form the "hard" tooling.

A variation of this process as described in U.S. Patent No. 5,507,336, casts a ceramic compound over a pattern to form the cavity or core half. The procedure is to take the cavity impression on the ceramic casting and place in a tubular container so that loose metal powder can be poured into the container. Instead of binding the powders together with a binder as in US Patent No. 5,507,336, the whole tubular container is placed in an oven and a low melting metal such as copper is melted over the powder to bind the whole shape. The next step is to remove the original ceramic pattern to leave exposed the desired cavity or core mold half, which can then be assembled into a complete tool for injection molding.

Improvements have been commercially incorporated into this methodology by coating the fine metal powders by a proprietary polymer and selectively laser sintering the coated powders around a given pattern. In this case the laser serves to fuse the polymer and holds the shape of the part, thereby eliminating the need for any tubular shaped container to hold the powders together. This "green" part is subsequently impregnated with a low-melt binder system and heated in an oven before sintering at higher temperatures to provide a metal skeleton, that in the final steps is infiltrated with copper. This process is know as "RapidTool-Long Run(LR)" and is practiced by DTM Corporation in Austin, Texas. Some of the patents covering this process are patent #'s 5,648,450, # 5,733,497, # 5,749,041.

A variation of the use of laser sintering uses a 200 W laser known as Direct Laser Sintering, to act directly on metal powder. This metal powder consists of a mixture of bronze and nickel and some additives and as a result has the unique property that it shows very limited shrinkage during sintering. Some of the patents by EOS covering this method are patent # 5,876,767 and # 5,908.569.

The three dimensional printing process developed by MIT works much like an ink-jet printing by spreading a thin layer of powder over a platform. Directed by a computer file, the electrostatic ink jets are selectively sprayed with a colloidal acrylic binder onto stainless-steel powder to create the green part. Debinding, sintering and infiltration follow the printing process to make the part more robust. Like the Rapidtool process, the problem lies in the unpredictability of the shrinkage and infiltration process, resulting in poor surface finishes and propensity for warpage.

The above mentioned approaches have addressed the issues of tool longevity by using powder metals to form "hard" metal dies. Though the resulting molds are more permanent in nature, there are two main issues which prevent these tools from being considered permanent hardened tools. The first is that the tools are difficult to polish due to the coarse nature of the base powders. This means that the surface finish on parts produced from these tools may not be adequate. The second issue is that the high copper content- necessary to close the porosity in the initial metal skeletons- reduces the attainable hardness of the composite to about Rockwell B75, which is softer that similar tools machined from aluminum. Tool life and wear resistance remains a major issue when compared to tools manufactured from conventional methods that can be hardened above Rockwell C60.

A recent application of powder metallurgy as a method for producing dies is described in U.S. Patent No. 5,435,824. It applies hot isostatic compacting to develop a fully dense mold and die block that does not need to be copper infiltrated to achieve full density. Hot isostatic compacting consists of using a rubber container which has the general shape desired, to hold the powders together while they are compacted into shape by high pressures. The process includes removing the rubber container once the mold can hold its shape, and then heating or sintering the "green" article in a furnace to consolidate the metal powders. Several alloys can be processed from this method that can attain harnesses equivalent to those of the wrought materials commonly applied in the toolmaking process.

Another related application disclosed in patent # 5,937, 265 also uses the combination of cold and hot isostatic pressing, with the difference of using master parts produced by stereolithography, followed by the creation of a flexible mold from these master parts, which are then filled with metal powders that are first cold isostaticaily pressed and then hot isostatically pressed. The main issue with both of these processes for the construction of molds and mold components, is that the methods are inherently limited in the complexity of components that it can reproduce as well as issues having to do with dimensional accuracy, since compaction and forming of the molds and dies occurs in a directional basis.

A variation of the use of powder metallurgy as a forming method for tool inserts is described in patent #'s 5,976,457 and # 6,056,915 respectively. Both patents disclose a method that takes advantage of the forming capabilities of the metal or powder injection molding process. In each case material is molded around master cores, machined from aluminum or other materials to produce a version of the cavity and core that can be later processed using standard powder injection molding parameters, to produce a final sintered or fused steel part with all the properties and performance of a wrought or standard tool material. One potential drawback of these methods, is that the dimensional accuracy depends on the compounded tolerances of producing master cores and cavities through one method, on top of the inherent process variation of sintering and shrinking the parts to attain the final part sizes and properties. The instant invention also uses less process steps to accomplish the end result of obtaining a mold insert.

Each of the above mentioned inventions has improved the development process by reducing the elements of time and cost, yet each has issues that detract from its adoption as production and extended run tools.

Most of the rapid tooling methods use variations of the powder metallurgy process, however many of these have issues relating to surface finish that may detract from form and function evaluations on certain parts applications. In addition to this dimensional tolerances of the resulting tools may vary because in some methods the copper infiltration process causes some expansion of the mold or the method of compaction provides a directional bias, as in hot isostatic compacting, or in others, the final shrinkage after sintering is not reproducible.

Continued improvement of the rapid tooling methods has to rely on reduction of processing times, increased compliance with cosmetic and surface finish requirements, as well as developing dies that have comparable dimensional reproducibility and hardenability as the materials used in conventional moldmaking. The instant invention reduces processing times, meets cosmetic requirements and can be finished to match the dimensional requirements of the conventional tools.

BRIEF SUMMARY OF THE INVENTION - OBJECTS AND ADVANTAGES

The instant invention provides a method for the rapid fabrication of mold tooling inserts that can be incorporated in a mold base for use in forming processes such as plastic injection molding, metal injection molding, ceramic injection molding, metal die casting and other related forming processes, wherein high machinability rates and time and cost savings along with increased tool life and material savings are obtained through the use of blank die inserts forumlated from material comrnmoly used in the methal injection molding process of complex shaped parts.

Accordingly several objects and advantages of the instant invention are:

1-) To provide a method for the rapid fabrication of metal die inserts used in injection molding or die casting tools with a minimum of production steps to meet and exceed the time & cost requirements of rapid prototype tooling.

2-) To take advantage of the soft nature of mold insert blocks molded or cast from material commonly used for powder injection molding, in order to obtain increased machining rates, and to then exploit the ability of the material to be converted into a heat treatable tool steel metal inserts that last longer than the present state of the art "soft" tooling, thereby allowing greater flexibility in time and cost for the production of hard tooling.

3-) To increase tooling life in the machhiing process as a result of using a soft machinable material not requiring the need of coolants, during the macliiriing. The material chosen for machining has a self-lubricating nature.

4-) To reduce material waste by recycling the raw material being machined.

5-) To produce single or multiple tool steel or related alloy metal inserts that can meet the dimensional and surface finish requirements of permanent tooling - which the present state of the art rapid fabrication "hard" tooling cannot.

6-) To add additional flexibility in the rapid prototyping tool manufacture by choosing related metal alloys such as hardenable stainless, carbon steel or other ferrous or non-ferrous powder materials that can be premixed with the appropriate binders to provide cost & time savings advantages.

7-) To add features to the die that can facilitate fabrication and assembly. These could include water channels and coordinate referencing features, ejector locations etc., which would normally have to be machined into a conventional tool.

These together with other objects and advantages of the invention will become more readily apparent to those skilled in the art when the following general statements and descriptions are read in the light of the appended drawings and claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The invention will now be described in connection with the accompanying drawings, FIGS. 1- 6, which include a flowchart and preferred embodiments of the invention.

FIG. 1 is a process flow chart for the fabrication of the hardened steel inserts, from the machining of the soft die blocks to the sintering and assembling of the die inserts.

FIG. 2A to 2B show a representative 3-D CAD drawing of a part with a view of the core or interior features of the part, and the cavity or external features of the part.

FIG. 3 A & FIG. 3B shows the machining or milling of the core pattern into a die block made from soft machinable MLM material, in the initial and final stages respectively.

FIG. 4A to 4B shows the machining or milling of the cavity pattern into a die block made from soft machinable MLM material, in the initial and final stages respectively.

FIG. 5A to 5B show a representative mold base where the resulting cavity and core inserts will be assembled.

FIG. 6A to 6B show a fully assembled mold half and a view of the mold assembly showing the cooling water lines.

DETAILED DESCRIPTION OF THE INVENTION

The instant invention describes a novel method shown in Fig. 1, with identified steps 10-14 for the rapid fabrication of hardened tool steel or comparable alloy molding dies and mold components. These tools would have the added advantage of being incorporated as production tools after any key design and functional advantages were incorporated into any particular parts, hence it also addresses the time and cost factors involved in manufacturing tooling through the conventional means that have been described.

The method of forming the mold die inserts as shown in Fig. 1, involves creating cutting path programs from CAD files; applying these programs to a blank die block that has been formulated from material commonly used for in the powder injection molding process; processing the machined blocks - in this case consisting of the cavity and core halves - first through a debfnding process to remove the binder constituents of the material, and then though a high temperature sintering process to convert the powders into a dense fully hardenable tool steel or similar alloy, followed in the last step by performing secondary operations on the mold inserts to fit them into a master mold base as part of a complete molding tool.

The first step referenced as step 10 in the flowchart of Fig. 1, is the starting point for all of the rapid tool manufacturing processes as well as for the conventional toolmaking technologies. One difference however, with the rapid tool manufacturing processes is that cutting path programs are developed for direct machining of the cavity and core halves, instead of going through an intermediate step of developing master cores and cavities in order to form the actual core and cavity inserts. The process is similar to conventional toolmaking in that cutting path programs are often applied directly to blank tool steel mold inserts, hut differs in the important respect that the material that the machining is being performed on is in a soft state when it is being machined, but can be subsequently converted to a fully dense and hardenable tool steel or similar alloy. In this way, the process realizes the immediate advantage of time and cost savings by increasing machinability, while achieving in the end the manufacturing of tool steel mold inserts that are indistinguishable from the tool materials used in the conventional tooling process.

The blank mold insert blocks upon which the cutting path programs are applied are formed as referenced in steps 11& 12 of the flowchart in Fig. 1, by either casting onto a die block mold or by directly molding in a molding machine. The material used for casting or molding the insert blocks is formulated typically from a material combination of extremely fine metal powders, in the size range of 2-30 microns and binder compounds known to the art of powder injection molding. In their standard application, the material and process of powder injection molding is used for the manufacturing of small and intermediate sized components to near net shape parts. The soft nature of the molded material and the machinability advantages is a side benefit that has heretofore not been applied towards the production of mold inserts. As a reference point, while aluminum cuts from 30- 50% faster than the tool steels used in conventional moldmaking, the powder injection molding material cuts in turn from 30-50% faster than aluminum without the need of using cutting fluids, which in standard operations are required to keep the material cool as it is being cut. The reason for this, is that the lubricants of the powder injection molding material, such as wax, are reducing the heat effects of friction. Cutting or macm'ning rates of the flowchart shown in Fig. 1, is therefore very efficient.

Because the mold inserts undergo a conversion process, the cutting path programs referenced in step 10 of the flowchart shown in Fig. 1, have to compensate for the shrinkage experienced during the sintering/consolidation steps. This means that the cutting path programs are made to a specific oversize shrink factor that depends on the type of alloy powder being processed, the amount of binder that is required to be mixed with the powders to obtain a moldable material, and the degree of densification the parts experience during sintering. While in the normal practice of powder injection molding, molds are cut to an oversize factor ranging from 15-20%, application of the instant invention will require programming to compensate both for the expected shrink of the material being processed as well as the material that will be molded into the final core and cavity halves. For example, if the part is being tooled for use in plastics, then the 2-6% shrink rate normally experienced upon molding will have to be added onto the standard 15- 20% shrinkages experienced by the PLM blocks when they are sintered or fused to steels. In another application, if the mold inserts are being developed for use to manufacture a powder injection molded part, then the oversize factor would be 15-20% for the material plus an additional 15-20% for the part, for a total oversize range depending on the material of 30-40%.

One example of powder injection formulation used in the instant invention, would be to use M-4 high speed tool steel powders. These are spherical gas atomized powders produced and sieved by Anval/Carpenter Corporation to a particle size less than 30 microns, and mixed with a polymer/wax binder so that the premixed material has a 94% by weight powder loading with the remaining 6% by weight being binder. Other metal powders such as carbon steel, stainless steel, copper, or bronze, will have different binder requirements depending on their size and shape characteristics, as well as the type of binder chosen by those versed in the art. The shrink rate for the M-4 material with the powder characteristics and size as defined above, would be 1.15.

Conventional tool making practices incorporated the intermediate use of carbon electrodes for use in a process called electrode discharge machining, to compensate for a known problem in direct machining of wrought steel molds, of not being able to produce sharp corners. The milling or cutting action occurs normally in a radiused manner, and this problem would be expected in the instant invention, however, due to the high machinability of the material used in the instant invention, additional cutting processes such as honing can be programmed to adjust for these desired features. Cutting rates can also be adjusted as well as the design of the cutting tools themselves .

Processing of the machined core and cavity insert halves as referenced in step 13 of the flowchart in Fig. 1, is accomplished within the standard process parameters known to the art of powder injection molding. A two step debinding process to remove the low melting portions of a typical binder compound is accomplished through the combination of solvent or thermal means leaving only the presintered powder skeleton that is later consolidated in a high temperature sintering step.

In the example for an M-4 tool steel molded material using a thermoplastic polymer/wax binder, the wax can be eliminated using a heated solvent in liquid or vapor form. This would be followed by the removal of the remaining polymer in an inert atmosphere furnace, thereby producing a debound mold or part consisting only of the metal powders. This debound part will continue to hold its shape due to the fact that the elimination of the polymer portion of the binder, will also allow the powders to presinter or weld together.

The debound mold is finally put through a sintering or high temperature consolidation to produce the final near full density mold article. The sintering, can be carried out in any high temperature vacuum or atmosphere furnace. To process M-4 tool steel in the reference example, the preferred mode of sintering would be. in a vacuum furnace at a maximum temperature of 2240°F(1220°C) for 10 minutes. This will yield a mold or die with a sintered density of approximately 8.0 g/cc, which is about 99% of the theoretical density of 8.1 g/cc. In achieving this high density, the part will have shrank as noted approximately 15% from the green molded state. Different materials such as carbon steel, bronze, copper or stainless steel, will have different sintering temperatures and hence different shrink rates.

The sintered mold or die can be finished by any number of secondary operations and fitted into an injection molding frame in the final step 14, as referenced in the flowchart in Fig. 1. Secondary operations can include heat treating, polishing, and the addition of slide, ejection pins, and other mold accessories that will create a functional mold tool for injection molding any materials.

Figures 2-11 expand and exemplify the results of the process steps outlined in the overall flow diagram shown in Fig. 1.

A representation of a 3-D CAD file drawing is shown in Figs. 2A & 2B. The goal at this stage of the process is to split the part into two corresponding halves, the interior or core 20 half shown in Fig. 2A, and an exterior or cavity 21 half shown in Fig. 2B. These core 20 and cavity 21 representations will form the basis for the design and manufacture of the mold cavity/core patterns, which will be the mirror images of the CAD representations. Once the split of the part has been defined, it is now possible to use the same database to generate a cavity/core sets of CAD representations, which are the inverse of the original representations shown in Fig. 2. These representations are then translated into cutting path programs after adjusting for the desired size dimensions that are to be cut. In the example of a mold insert being developed for use in plastics molding, an allowance for shrinkage of the plastic part - for example 2% - would be given on top of the expected shrinkage of the blank block, which can vary from 15-20%.

Figs." 3 A shows the application of the cutting path programs to the core half 33 by machining with a CNC type milling cutter 32 on a soft die block 30 cast or molded using metal injection molding material. Fig. 3 A is a representation of the insert as it is being machined or milled, while Fig. 3B shows the completed insert 31.

Figs. 4 A shows the corresponding process for the cavity half with the milling cutter 42 machining on the cast or molded die block 40. Fig. 4B shows the completed cavity insert 41.

The blank blocks cast or molded from the powder injection molded material are normally squared and ground. The goal is to cut the block to conform to the general dimensions of an insert pocket shown in 51 of Figures 5 A & 5B without having to arrive at the exact dimensions as finishing and fitting operations can guarantee that that will occur. Cutting, grinding or milling can occur with a variety of tools and at rates dictated by the geometry of the part. Speeds can be adjusted to obtain rough cuts, followed by finer cuts that will give the tool a better surface finish. Secondary finishing operations, after the cavity and core blocks are sintered, can render a tool with the required surface finish requirements.

Fig. 5 A shows a standard mold base 50 with the insert pockets 51 to accept the processed inserts 53, shown in Fig. 5B . Fig. 5B show the inserts fitting into a yoke 53 that holds the inserts to facilitate the location of water lines around the inserts. The actual dimensions of the base 50 depend on the size of the part being replicated, as the die pocket 51 and/or yoke 52 can be easily varied to allow the insert 53 to fit in.

Fig. 6A shows the assembled insert mold in this case for the core half of the mold halve with the insert 53 assembled into a yoke 52. This exemplification is for a four cavity tool. Fig. 6B is a cross-sectional representation of this tool to highlight the assembly of the different components as well as to demonstrate the location of the water lines 61 within the yoke 52 containing the core insert 53. This core die 53 will undergo a number of secondary operations that will include heat treating to harden the metal, and grinding and polishing, to assure a tight fit in the die pocket 51.

The major advantage, however, is that this method of manufacture can yield, with a finalized part design, a fully production ready multi-cavity tool at a fraction of the time and cost to produce a comparable tool using carbon electrodes, CNC machining and other standard tooling and rapid fabrication practices.

Claims

We claim:
1-) A method of fabricating mold inserts comprising the steps of:
a-) creating a blank die block from a metal alloy material, consisting of fine metal powders bound together by any combination of appropriate binders that form a material compound commonly used in powder injection molding processes.
b-) applying a cutting path program developed from the CAD files or similar of the part application, and directly macliήiing said die block by milling or related means corresponding cavity and core die inserts.
c-) processing said cavity and core inserts respectively to consolidate the fine metal powders including a means to remove said binder of said metal alloy material and forming a sintered metal core and/or cavity insert.
d-) finishing said sintered metal insert to fit into a mold base or master mold for molding of metal, ceramics, plastic, die casting and related processes.
2-) The method of claim 1, wherein the material used to produce the die blocks that result in cavity and core inserts are from the group of tool steels, carbon steels, stainless steels and other ferrous powders and alloys that can be processed to near full density and heat treated.
3-) The method of claim 1, wherein the material used to produce the die blocks that result in cavity and core inserts are form the group of any non-ferrous powder such as copper and bronze, that can be processed to near full density.
4-) A method for the fabrication of mold inserts comprising the following steps:
a-) creating a CAD or computer model of the tooling die inserts from a part configuration.
b-) scaling the said model to allow for shrinkage resulting from subsequent processing plus expected shrinkage for the tooling application.
c-) developing cutting path programs for CNC, milling or other related machining processes for the corresponding cavity and core die inserts.
d-) fabricating blank die blocks from a metal alloy material consisting of fine metal powders bound together by any combination of appropriate binders that form a material compound commonly used in powder injection molding processes. e-) applying a cutting path program developed from the CAD files or similar of the part application, and directly machining said die block by milling or related means said cavity and core die inserts.
f-) processing said cavity and core inserts respectively to consolidate the fine metal powders including a means to remove said binder of said metal alloy material and forming a sintered metal core and/or cavity insert.
g-) fmishing said sintered metal inserts by grinding, polishing, assembling of mold components and related activities to fit into a mold base or master mold for molding of metal, ceramics, plastic, die casting and/or related processes.
5-) the method of claim 2, wherein the machining step (e) includes the addition of ejector hole locations, coordinate reference points, water cooling channels and other features normally required in an injection molding or die casting tool.
6-) the method of claim 2, wherein the cooling channels, are included in a component refered to as a yoke.
PCT/US2002/018032 2001-06-06 2002-06-06 Method for the rapid fabrication of mold inserts WO2002098588A1 (en)

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