NL2003325C2

NL2003325C2 - Method for producing a powder injection moulded part.

Info

Publication number: NL2003325C2
Application number: NL2003325A
Authority: NL
Inventors: Roland Alexander Korbee
Original assignee: Syroko B V
Priority date: 2009-08-03
Filing date: 2009-08-03
Publication date: 2011-02-04
Also published as: WO2011016718A1; EP2461929A1; JP2013501148A

Description

Method for producing a powder injection moulded part

The invention relates to a method for producing a powder injection moulded part, comprising the steps of: A) mixing powder to be sintered with a binder to provide a 5 moulding mixture, B) injection moulding the moulding mixture to form a moulded part, C) debinding the moulded part, and D) sintering the debound moulded part. The invention also relates to the use of a binder in the method according to the invention.

The invention further relates to a powder injection moulded part obtained by the method according to the invention. The invention moreover relates to an assembly of moulded 10 parts obtained by the method according to the invention.

Powder Injection Moulding (PIM) is an attractive process to complex form shaped parts. Using such a PIM process, metal, glass, or ceramic powder, is mixed with a polymer binder (step A), also referred to as carrier, wherein said binder primarily 15 facilitates injection moulding of the powder. A commonly used binder is formed by (a copolymer of) poly(oxymethylene) (POM), such as the acetal polyoxymethylene copolymer marketed under the trade name Ultraform® available from BASF. Moulding said mixture into a desired shape (step B) will lead to a bound moulded part, also called a "green body". Subsequently, the polymer binder is removed by means of a corrosive 20 solvent, typically nitric acid, commonly at a temperature between approximately 110 and 120 °C (step C). Thereafter, the remaining shaped porous moulded part is sintered to produce the desired shaped article (step D). During this sintering step the moulded part will shrink without affecting the shape of the moulded part. The powder particles will fuse together and the open space between the powder particles disappears. Hence, 25 during sintering the density of the product increases and the product shrinks. The sintering step is commonly completed when the product has reached a density of around 99 volume % of the solid of which the powder is made. Although the known PIM process is very suitable for forming complex and intricate shaped moulded parts, the known process also suffers from several drawbacks. A major drawback of the known 30 process is the required use of the corrosive solvent, typically nitric acid, for chemically decomposing the binding polymer POM. At increased temperature nitric acid will be converted into toxic nitrogen oxides, which is undesirable. Thermally decomposing of the binder, instead of chemically decomposing, will merely be possible at relatively low temperatures (exceeding room temperature). However, this would result in an 2 unacceptable slow debinding process. Thermal decomposition at higher temperatures would, however, lead to the generation of gaseous decomposition products, which accumulate in the moulded product. The accumulation of gases will lead to the build up of pressure inside the product and hence easily to damage caused by deformation and 5 fracture. Hence, thermally decomposing the binder is not a pragmatic option to remove the polymer binder.

It is an object of the invention to provide a more efficient method for producing a powder injection moulded part.

10

To achieve this object the invention provides a method according to the preamble, characterized in that said binder comprises a mixture of at least one thermally decomposable polymer and at least one low molecular weight decomposable solvent for solving said thermally decomposable polymer, said solvent having a melting 15 temperature exceeding room temperature, and in that step C) comprises a first debinding step E) comprising removal of the solvent from the moulded part, and a second debinding step F) comprising removal of the polymer from the moulded part. The incorporation of the low molecular weight (non-polymeric) solvent in the binder applied has several advantages. Due to the presence of the binding solvent, acting as carrier for 20 the binding polymer, the total amount of the binding polymer in the moulded part will be reduced substantially with respect to the amount of binding polymer applied in the process known in the prior art. Removal of the low molecular weight binding solvent during the first debinding step according to step E) will result in a sponge-like (porous) structure of the binding polymer within the moulded part. In this first debinding step the 25 binding polymer remains solid and serves to maintain the shape and size of the green moulded part. This open structure within the green moulded part will substantially facilitate removal of the binding polymer from the moulded part according to step F), and allows more or less free, unhindered flow of liquids and gases through the moulded product prior to sintering according to step D). Both the reduced amount of binding 30 polymer and the open structure within the moulded part after removal of the binding solvent will allow efficient removal of thermal decomposition products that are produced during the thermal decomposition of the binding polymer . Since decomposition gases formed within the moulded part can and will freely flow out of the moulded part without deforming the moulded part, the use of a corrosive substance is no 3 longer required to remove the binder from the moulded part which is favourable from both an economic and an ecological point of view. The improved binder used in the method according to the invention has the additional advantage that the incorporation of the binding solvent in the binder reduces the viscosity of the binder and hence of the 5 moulding mixture, and hence reducing material stresses within the moulded part. Consequently, an improved dimension stability will be obtained allowing a larger degree of freedom of design of the moulded part. The removable solvent will be solid at room temperature to secure a solid dimension stability of the moulded part. Preferably, the removable solvent has a melting temperature of between 50 °C and 300 °C. The 10 lower limit of the range is on one hand sufficiently low to allow liquidizing the solvent at a relatively low temperature, and is on the other hand sufficiently high to exceed the temperature of the mould used in the method according to the invention to allow solidifying of the binding solvent within said mould. Since the binding solvent is a low molecular weight substance, which means of non-polymeric substance in this context, 15 removal of the binding solvent from the moulded part by vaporization or extraction will commonly be relatively easy. Removal of the solvent may imply either physical processes like extraction or vaporization or chemical decomposition (degradation).

In a preferred embodiment the removable solvent is selected from the group consisting 20 of: caprolactam, polyethylene glycol (PEG), acetanilide, benzamide, 4- hydroxyacetophenone, maleimide, and phtalimide. In particular caprolactam, the monomer for the production of nylon-6, has beneficial properties to be applied as binding solvent, since caprolactam is relatively cheap and moreover soluble in almost any organic solvent, such as water and methanol, and even in certain non-polar solvents, 25 such as hexane. It is expected that at least caprolactam is also extractable by means of a supercritical medium, such as supercritical CO2. The application of PEG is beneficiary, since PEG is biodegradable and hence environmentally friendly. As mentioned above, during the first debinding step according to step E) the binding solvent is removed resulting in the formation of open pores. The higher the initial concentration of binding 30 solvent, the higher the pore volume will be. A higher pore volume will also make it easier to remove the remaining the binding solvent particles when the first debinding step has not been finished yet.

4

In another preferred embodiment the thermally decomposable polymer is selected from the group consisting of: poly(oxymethylene) (POM), a copolymer of POM, polystyrene (PS), polyacrylates, polymethyl acrylates (such as polymethyl methacrylate or polybutyl methacrylate), and copolymers of acrylates and styrene. Since the concentration of the 5 decomposable polymer in both the moulding mixture and the moulded part is reduced with respect to the concentration applied in the method known from the prior art, thermal decomposition will be possible at an acceptable decomposition rate. POM is in particularly preferable, since POM can almost fully be depolymerised to its monomer when heated above ca 200 to 250 °C without producing any carbon. POM is a well 10 known crystalline engineering plastic. Different POM’s may be used, such as Delrin (available from Du Pont), Hostaform (available from Ticona), and Ultraform (available from BASF).

The binder preferably comprises at least 5 mass percent of the at least one thermally 15 decomposable polymer. The binder preferably also comprises at least 5 mass percent of the removable solvent. It has been found that efficient debinding of the decomposable polymer will be impeded in case the binder would comprise more than 95 mass percent of the polymer. It has also been found that the application of a binder comprising more than 95 mass percent of the binding solvent would affect the (dimension) stability of the 20 moulded part.

In a preferred embodiment the moulding mixture formed during step A) further comprises at least one surfactant (dispersant) to be able to achieve a substantially homogenous dispersion of powder particles within the moulding mixture, and hence 25 within the moulded part. In particular in case ceramic powder is used in the method according to the invention, the use of one or multiple surfactants will commonly be beneficiary. The surfactant that is chosen depends on the type of ceramic powder that is used. For alumina powder stearic acid is a commonly used surfactant. The concentration of the surfactant in the moulding mixture is relatively low. Preferably, the powder 30 comprises between 0 and 5% volume percent surfactant(s). Alternative surfactants which may be applied are amines and olymeric fatty acid esters. 8. Commonly the surfactant is selected from the group consisting of anionic surfactans, cationic surfactants, zwitterionic surfactants, and nonionic surfactants. Examples of anionic surfactants are surfactants based on sulfate, sulfonate or carboxylate anions. Examples 5 of cationic surfactants are surfactants based on quaternary ammonium cations. Examples of zwitterionic surfactants are dodecyl betaine, cocamidopropyl betaine and coco ampho glycinate. Examples of nonionic surfactants are alkylpolyglucosides, fatty alcohols (cetyl alcohol and oleyl alcohol) and cocamide monoethanolamine (MEA) and 5 cocamide diethanolamine (DEA).

In a preferred embodiment the powder to be sintered comprises at least one compound selected from the group consisting of: metal, ceramic, glass, and mixtures thereof. Glass and glass ceramic materials have beneficial properties for many applications.

10 Outstanding properties such as chemical and physical durability, biological inertness, high temperature stability, and transparency of many glass-based materials have led to wide-ranging applications of such materials in chemical and biological laboratory and production processes. Glass materials have been used or suggested for use in biological well plates, "labs on a chip," microreactors, and in other fluidics and microfluidics 15 applications, for example. Glass and glass ceramic articles have also found broad use in many other industrial applications and in various consumer products. Beside, different kinds of metals may be used, such as iron, nickel, titanium, aluminium or mixtures thereof. As those skilled in the art will appreciate, shaped titanium parts have utility as medical implants, i.e. bone screws and plates, golf club heads, and as aerospace 20 components. Due to the high reactivity of titanium and its susceptibility towards forming solid solutions with commonly occurring elements, such as oxygen, carbon, and nitrogen, the moulding process requires that the carbon containing binder is completely removed from the moulded part at temperatures below 450 °C. Since the method according to the invention is directed to remove the binder completely from the 25 moulded part at relatively low temperatures, the method according to the invention is also very suitable to produce titanium moulded parts. The volume ratio of the powder and the binder mixed during step A) is preferably between 30:70 and 70:30 to be able to provide a moulded part with a desired shape and stability.

30 In particular in case the binder is composed of caprolactam as the solvent and POM as the polymer, the first debinding step E) is preferably carried out at a temperature exceeding 69 °C, and preferably between 69 °C and 130 °C. In this temperature range, the binding solvent, in particular caprolactam, will be liquid, while the binding polymer will still be solid, as a result of which removal of merely the binding solvent will be 6 facilitated. During said first debinding step E) the moulded part may be exposed to a debinding solvent for removing the binding solvent of the binder. More preferably, the debinding solvent is selected from the group consisting of: water, esters (such as ethylacetate and butylacetate), ketones (such as acetone or butanone), hydrocarbons 5 (such as xylene or toluene) and alcohols, in particular methanol and ethanol. As mentioned above, also non polar organic debinding solvents, such as hexane, may be used to remove the binding solvent. However, since PMMA dissolves in molten caprolactam at temperatures directly above the melting point (69 °C), there is no temperature range wherein caprolactam is liquid and PMMA is still solid, as is the case 10 when POM is used. For that reason thermal debinding by vaporization of liquid caprolactam, which requires the existence of such a range, is not possible in this latter case, as a result of which the first debinding step has to be carried out with a debinding solvent, preferably water, organic solvents, or a supercritical medium. The solvent used should be a solvent for caprolactam, but a non-solvent for PMMA. More specific 15 examples in this case are water and alcohols.

The second debinding step E) is preferably carried out at a temperature exceeding 200 or 250 °C. Above this temperature an effective decomposition of the binding polymer can commonly be achieved, wherein, dependent on the binding polymer used, a high 20 yield depolymerisation may be achieved.

In a preferred embodiment the method comprises step G) comprising assembling multiple moulded parts together prior to debinding the moulded parts according to step C). More preferably, during step G) an adhesive is applied between the moulded parts 25 assembled together, said adhesive comprising the same powder and the same binding polymer in the substantially same fraction as present in the moulded parts, said adhesive further comprising an auxiliary solvent being liquid at room temperature, wherein the fraction of the auxiliary solvent of the adhesive and the fraction of the binding solvent of the moulded parts are substantially similar. The composition of the adhesive is in fact 30 similar to the composition of the moulded part, except that the binding solvent (solid at room temperature) is replaced by the auxiliary solvent (liquid at room temperature). As a result the bonded parts will behave as one single part after the first debinding step has been carried out and during subsequent debinding and sintering the shrinkage in the adhesive and in the green moulded parts will also be the same. The materials properties 7 are the same throughout the assembly of moulded parts. Therefore the parts will stay together without deformation or cracks during further debinding and sintering. A suitable auxiliary solvent is formed by methyl ethyl ketone (MEK).

5 The invention also relates to the use of a binder in a method according to the invention, wherein said binder comprises a mixture of at least one thermally decomposable polymer and at least one low molecular weight removable solvent for solving said thermally decomposable polymer, said solvent having a melting temperature exceeding room temperature. Advantages and preferred embodiments of the improved binder 10 composition have already been described above in a comprehensive manner.

The invention further relates to a powder injection moulded part obtained by the method according to the invention. The invention moreover relates to an assembly of moulded parts obtained by the method according to the invention.

15

The following non-limitative examples are presented to further illustrate to persons skilled in the art how to make and use the invention and to identify presently preferred embodiments thereof.

20 Example 1 - The primary process A Feedstock batch of approximately 6.5 Kg was made containing 60 volume % stainless steel (type 316L) powder and 40 volume % binder. The binder was made of 50 wt% caprolactam and 50 wt% Hostaform® C27021. Hostaform® is a polyoxymethylene 25 (POM) copolymer available from Ticona. The materials were mixed on a 5 litre Wemer & Pfleiderer Z-blade mixer at ca 180 °C and the rotation of the rotors was set at 60 to 90 rpm. Because of volatility of caprolactam the mixer has to be kept closed. Mixing was continued for about 1,5 hours to ensure that all POM granules would dissolve in the caprolactam. When the mixture was homogeneous, the mixer was cooled and the mixer 30 blades were rotated at 14 rpm. The mixture solidified and the rotating blades crushed it so that a moulding mixture granulate was obtained. Subsequently, a dosed amount of said moulding mixture is heated to about 160 °C and injected into a cavity of a mould to form a green moulded part (moulded article). The mould itself has a relatively low temperature of between 40 °C and 65 °C to force solidification of caprolactam, and 8 hence of the moulded part. After removal of the green moulded part from the mould, the moulded part is subjected to a debinding process. During a first debinding step, the shape of the green moulded part should not deform, even though considerable shrinkage may take place. Shrinkage can result because material is removed from the green 5 moulded part. In a first debinding step the green moulded part is heated to about 110 °C. At this temperature caprolactam is in a liquid state and will evaporate. A porous moulded part incorporating the binding polymer Hostaform® will remain. During a second debinding step the moulded part is further heated to about 240 °C or higher. At this temperature Hostaform® will be decomposed and in particular depolymerised, 10 wherein gaseous formaldehyde is produced. Formaldehyde and caprolactam can be flared off by blowing these gases through a flame. In a last step, the debound moulded part is sintered at a relatively high temperature of above 1000 °C. At this stage, the binder has been removed completely and the powder particles slowly fuse together and the open space between the powder particles disappears. During sintering the density of 15 the product increases and the product shrinks. The sintering step is completed when the product has reached a density of around 99 volume % of the solid of which the powder is made.

Example 2 - Adhesive bonding of green parts 20

In some cases it is useful to bind two green moulded parts bonded together before sintering. Joining would make it possible to produce parts that are too complicated to make by powder injection moulding. To this end, multiple green moulded parts are put together with a liquid adhesive between them. In this example, the green parts are made 25 of powder combined with a binder made of PMMA and caprolactam. The adhesive composition is almost similar to the composition of the green parts, except that the volume of caprolactam is replaced by the same volume of methyl ethyl ketone (MEK), a solvent being liquid at room temperature. The assembly is immersed in a debinding solvent that extracts the caprolactam and the MEK out of the green parts and out of the 30 adhesive respectively. The debinding solvent should only dissolve caprolactam and MEK, not the PMMA. Propanol is a good debinding solvent and supercritical C02 may also be used. After extraction and drying, the green parts and the adhesive will have the same composition. Both contain the same type and amount of powder, and the same amount of PMMA. Since MEK is a solvent for both caprolactam and PMMA, there will 9 not be a sharp transition in composition or properties at the interface where the adhesive is applied. It is to be expected that no interface is visible in the porous PMMA that is left after extraction. As a result the bonded parts will behave as one single part and the shrinkage in the adhesive and in the original green parts will also be the same. The 5 material properties are the same throughout the product. Therefore the parts will stay together without deformation or cracks during further debinding and sintering.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative 10 embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The mere fact 15 that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

A method for manufacturing a molded part by means of powder injection molding, comprising the steps of:

2. A method according to claim 1, characterized in that the decomposable solvent has a melting temperature of between 50 ° C and 300 ° C.

Method according to claim 1 or 2, characterized in that the decomposable solvent is selected from the group consisting of: caprolactam, polyethylene glycol (PEG), acetanilide, benzamide, 4-hydroxyacetophenone, maleimide, and phthalimide. 25

A method according to any one of the preceding claims, characterized in that the thermally decomposable polymer is selected from the group consisting of: poly (oxymethylene) (POM), acetal copolymers, polystyrene (PS), polyacrylates, polymethacrylates, and copolymers of acrylates or methacrylates and styrene. 30

Method according to one of the preceding claims, characterized in that the binder comprises at least 5% by mass of the at least one thermally decomposable polymer.

A) mixing powder to be sintered with a binder to provide a molding mixture, B) injection molding the molding mixture to form a molded part, C) removing binder from the molded part, and D) sintering the molded part binder-stripped molded article, characterized in that the binder comprises a mixture of at least one thermally decomposable polymer and at least one low molecular weight removable solvent for dissolving the thermally decomposable polymer, the solvent having a melting temperature higher than room temperature and that step C) comprises a first removal step E), comprising removing the solvent from the molding, and a second removal step F), comprising removing the polymer from the molding.

A method according to any one of the preceding claims, characterized in that the binder comprises at least 5% by mass of the decomposable solvent.

7. A method according to any one of the preceding claims, characterized in that the casting mixture formed during step A) further comprises at least one surfactant.

8. Method according to claim 7, characterized in that the surfactant is selected from the group consisting of: anionic surfactants, cationic surfactants, dipoolionic surfactants and non-ionic surfactants .

9. A method according to claim 7 or 8, characterized in that the surfactant is selected from the group consisting of: amines, stearic acid and esterified fatty acid polymer.

A method according to any one of the preceding claims, characterized in that the powder to be sintered comprises at least one compound selected from the group consisting of: metal, ceramic, and glass. 20

A method according to any one of the preceding claims, characterized in that the volume ratio of the powder and binder mixed during step A) is between 30:70 and 70:30.

A method according to any one of the preceding claims, characterized in that the first removal step E) is carried out at a temperature above 69 ° C.

13. Method as claimed in any of the foregoing claims, characterized in that during the first removal step E) the molded article is exposed to a dissolving solvent for removing the solvent from the binder.

A method according to claim 13, characterized in that the decomposition solvent is selected from the group consisting of: water, alcohol, esters, hydrocarbons, and ketones.

Method according to any one of the preceding claims, characterized in that the second removal step E) is carried out at a temperature above 200 ° C.

A method according to any one of the preceding claims, characterized in that the method comprises step G), comprising assembling a plurality of mold parts prior to decomposition of the mold parts according to step C).

17. Method as claimed in claim 16, characterized in that during step G) an adhesive is applied between the assembled molded parts, wherein the adhesive comprises the same powder and the same binding polymer in substantially the same ratio as is present in the molded parts, wherein the adhesive further comprises an auxiliary solvent which is liquid at room temperature, wherein the fraction of the auxiliary solvent of the adhesive and the fraction of the binding solvent of the molded parts are substantially equal.

18. Use of a binder in a method according to any of claims 1-17, wherein the binder is a mixture of at least one thermally decomposable polymer and at least one low molecular weight removable solvent for dissolving the thermally decomposable polymer. wherein the solvent has a melting temperature higher than room temperature.

19. Molded article produced by means of powder injection molding obtained by the method according to one of claims 1-17. 25

20. Assembly of molded parts obtained by the method according to one of claims 16-17.