WO1997038811A1

WO1997038811A1 - Injection moulding processes, especially metal injection moulding processes

Info

Publication number: WO1997038811A1
Application number: PCT/GB1997/001015
Authority: WO
Inventors: David John Stephenson; Jeffrey Robert Alcock
Original assignee: Apv Uk Plc; Cranfield University
Priority date: 1996-04-13
Filing date: 1997-04-11
Publication date: 1997-10-23
Also published as: GB9607718D0

Abstract

A method for producing a sintered component comprises moulding a compact which incorporates a binder, substantially removing the binder from the compact, and sintering the resulting debound compact. The compact is produced by an injection moulding process employing at least two different mixes (A, B), each mix comprising a preferably powdered binder and a powdered filler material. The binder comprises a polymer-based material. The filler material comprises a metal and/or ceramic filler material. The mixes are injected either sequentially or simultaneously into a mould (14) so that said first mix forms an outer layer of the compact and said second mix forms an inner layer of the compact.

Description

INJECTION MOULDING PROCESSES. ESPECIALLY METAL INJECTION MOULDING PROCESSES

BACKGROUND TO THE INVENTION This invention relates to injection moulding processes and particularly, but not exclusively, to metal injection moulding processes.

Metal injection moulding (MIM) is a technique which can be used for the mass production of intricately-shaped metallic components. Metal powder is mixed with a binder comprising a polymer, and the mixture is heated and forced to fill a mould cavity. The moulded compact is debound and then sintered to the required density. Though the compact shrinks, the original shape of the mould is retained in the final component, even when the mould shape is complex.

Although the present invention will be described herein primarily in relation to the use of metal powder, it should be appreciated that ceramic powders may be used instead of, or in addition to, metal powders.

Many engineering components must operate under severe conditions of mechanical contact, wear and corrosion. It is desirable therefore to tailor the component structure and properties such that the surface properties are optimised to improve performance and life-time whilst the substrate uses low cost but mechanically adequate materials. For simple shapes PM (Powder Metallurgy) and HIP (Hot Isostatic Pressing) processing to produce functionally structured parts has been shown to offer many technical advantages over conventional surface engineering processes, such as thermal spraying. However, limitations exist when manufacturing components of complex shape. Large amounts of machining are undesirable due to the high cost of removing hard surface material and the possibility of removing the surface region in its entirety. Hence, a near- net-shape fabrication approach is a desirable aim.

We realised that it might be possible to modify the technique known as 'polymer co-injection moulding' to combine this in some way with the known MIM process so as ultimately to produce sintered components having a layered structure.

Polymer co-injection moulding is a process for producing polymer sandwiches using a twin-barrelled injection system.

One method of operating the polymer co-injection moulding process is as follows:

Stage 1 - material from barrel A is injected into a mould to form the skin of the compact.

Stage 2 - a selector valve is repositioned to allow injection of the core material from barrel B whilst shutting-off barrel A. This forces the skin material to the outer parts of the mould cavity.

Stage 3 - the valve is re-set to A for a short shot of material from barrel A to complete the skin at the injection point.

The mould is then packed under pressure.

A second method of operating the polymer co-injection moulding process is as follows: 1) Injection of material from barrel A to constitute the skin,

2) Simultaneous injection of a co-axial stream of both materials from barrels A and B,

3) Completion of core injection of material from barrel B.

SUMMARY OF THF. PRESENT INVENTION

According to one aspect of the present invention a method of producing a sintered component comprises moulding a compact which incorporates a binder, substantially removing the binder from the compact, and sintering the resulting debound compact, in which the compact is produced by an injection moulding process from at least two different mixes, each mix comprising a binder and a powdered filler material, the filler material being a metal and/or ceramic material, a first of said mixes being injected into a mould to incompletely fill the mould, followed by injection of a second of said mixes, the arrangement being such that said first mix forms an outer layer of the compact and said second mix forms an inner layer of the compact.

A suitable ceramic filler material may comprise calcined alumina. The particle size is preferably less than 20 microns.

The binder may comprise a polymer or a wax. If a polymer, the binder may be in liquid form.

The step of injecting the second mix may be accompanied by simultaneous injection of the first mix as a co-axial stream, with the first mix encircling the second mix in said stream. The step of injecting the second mix into the mould is preferably followed by injection of some more of the first mix into the mould so as to provide a complete skin of material formed from the first mix.

Thus, we have adapted polymer co-injection to MCM (Metal Co-injection Moulding), and the ceramic equivalent thereof, by replacing the pure polymers with high metal and/or ceramic filler content, polymer-binder systems. Surface engineering of the component is achieved by replacing the metal powder of the skin powder-binder system with a more wear or corrosion resistant metal and/or ceramic powder, and/or by addition of a secondary phase, for example to improve wear resistance.

We have found that it is very desirable to arrange for substantially equal sintering rates of the core and skin of the debound compact. That is, we have found it desirable that the core and skin sinter at similar rates and at similar positions in the sintering temperature profile, so as to minimise delamination which might otherwise occur in regions close to the interface between the core and skin.

If the two filler powders were of the same material, but of a different particle size, then the material with the finer particle size would sinter first and a method of retarding the sintering of the finer particles, or/and increasing the rate of that of the coarser particles would be required.

Retardation of the sintering rate can be achieved by either adding a second non-sintering phase or decreasing the green density. The mechanical keying of the faster sintering material to the slower one is known to reduce densification, so there will also be some 'built-in' tendency to equalise the densification rates. Acceleration of the sintering rate could be achieved by the use of dopants, liquid phase sintering etc.

When the filler powder of one of the mixes is finer than the filler powder of the other mix, it can sometimes be preferable to employ a lower volume percentage of filler powder in said one mix than in the other mix, when this assists in achieving comparable sintering rates.

Then, preferably the ratio of the volume percentages of the filler powder in the mix having coarser filler powder to that in the mix having finer filler powder is in the range 1.25. to 1.50 and is most preferably in the range 1.30 to 1.45.

The invention also comprises a sintered compact produced by the inventive method.

DETAILED DESCRIPTION OF THE DRAWINGS

The various aspects of the present invention will now be described by way of example only, with reference to the accompanying drawings, wherein:

Figure 1 shows measurements of the apparent viscosity of powder-binder feed-stocks and polypropylene in accordance with the invention,

Figure 2a is a copy of a scanning electron microscope image of a co- injected metal compact, thermally debound and pre-sintered to 850°C (2 hrs) in hydrogen, in accordance with the invention,

Figure 2b is a similar microscope image of a co-injected metal compact, sintered to 1300°C (4 hrs) in vacuum, in accordance with the invention, and Figure 3 illustrates a suitable injection moulding machine.

DETAILED DESCRIPTIONS OF THE VARIOUS ASPECTS OF THE INVENTION

Examples Some examples of the process conditions for processes in accordance with the invention will now be given:

The fabrication of a model MCM system with a skin of 316L stainless steel (Osprey Metals), containing 25 μm alumina particles as a secondary, wear- resistant phase, and a core of OM grade carbonyl iron (BASF) was investigated. The powder properties are given in Table 1. Particle size was determined by X-ray Sedigraph. Both powders had a spheroidal morphology.

TABLE I Powder Characteristics.

Bulk Cumulative Particle

Density Size (μm)

The binder system, a combination of carnuba and paraffin wax, Stearic acid and polypropylene, was adapted from Wiech. The properties of the constituents are given in Table II. The waxes impart low viscosity to the binder-powder mix and the carnuba wax also acts as a lubricant and mould release agent. In addition, the waxes are soluble in organic solvents and so may be removed prior to thermal debinding.

Stearic acid aids dispersion of the powder and also improves powder- polymer adhesion. The high molecular weight polypropylene functions as a backbone polymer, imparting green strength to the polymer-metal compact. The range of melt and degradation temperatures of the polymer and waxes produces a more uniform debinding rate during subsequent thermal debinding.

TABLE EE. Binder Characteristics

Density Melt Temperature

(g cm^'3) (°C)

Polypropylene 0.9 160

Carnuba wax 0.97 84

Paraffin wax 0.9 59

Stearic acid 0.85 74

Table III shows the composition of the feed-stocks produced for injection moulding of core and skin of the compact. The weight ratio of binder constituents was constant for core and skin. However, the volume fraction of powder in the core was held at 12.5% less than in the skin, to prevent delamination adjacent to the core-skin interface caused by the higher sintering rate of the finer particle size iron powder.

TABLE m. Composition of Feed-stocks

A: B:

Skin Core

Binder (wt%)

Polypropylene 60.0 Carnuba Wax 7.5

Paraffin Wax 31.5

Stearic Acid 1.0

Powder loading (vol%)

316L stainless steel 60.0 particulte alumina 5.0 OM carbonyl iron 47.5 Feed-stocks were initially blended in a shaker-mixer for 30 minutes at room temperature. Each was subsequently compounded in a twin-screw co- rotating extruder, at a screw speed of 300 rpm and temperature of 200°C. The resulting feed-stock was granulated. The apparent viscosity of the polymer-binder mixes was measured at 200°C, using a capillary rheometer of die length 2 cm and capillary radius 0.5 mm. The shear rate was varied between 533 and 12000s^"1. The results are shown in Figure 1. The apparent viscosity of the stainless-steel powder-binder mix was greater than that of the iron powder-binder mix, owing to the higher powder loading. Studies indicate that in polymer co- injected systems, this is a requirement to prevent break through of core material to the skin of the compact. The high wax content of the binder system produces apparent viscosities for both the metal powder-polymer systems which are similar to that of pure polypropylene.

Disks, of diameter 76 mm and thickness 3 mm were injection moulded on a 30-ton, twin-barrelled injection moulding machine. Moulding conditions are given in Table IV. Compacts were sectioned for debinding and sintering.

Figure 3 illustrates a suitable injection moulding machine.

TABLE IV. Injection Moulding Conditions Barrel A Barrel B

Barrel temperature range (°C) 175 - 200 175 - 200

Maximum injection pressure (MPa) 10.0 7.5

Maximum screw speed (rpm) 160 175

Shot size (mm) Al = 30 - 55 12 -37

A2 = 5

Nozzle temperature (°C) 200

Packing Pressure Range (MPa) 1.0 - 6.0

Sintering and debinding conditions are given in Table V. The wax binder constituents were removed by solvent debinding in heptane at 85°C. After 6 hours a maximum of 2.8 wt% of the compact, 87 wt% of the soluble wax content, was removed depending on the size and geometry of the compacts. TABLE V. Sintering and De-binding Conditions

R: ramp rate; T: temperature; D: dwell time.

Debinding or Atmosphere Rl TI Dl R2 T2 D 2

Sintering step (° C/min) (°C) (hrs) (°C/min) (°C) (hrs)

Solvent Heptane in 85 6 Debinding Air

Thermal Hydrogen 2 650 2 2 850 2 Debinding

Sintering Vacuum 5 850 2 1300 4

A two stage thermal debinding and sintering schedule was used. Initially the compacts were debound and part sintered in a hydrogen atmosphere. Figure 2a shows a part sintered compact. Discrete stainless steel particles can be observed in the skin. In contrast the core has sintered and densified. However, there is no evidence of delamination at the interface.

The compacts were sintered under vacuum to 1300°C, followed by furnace cooling. Figure 2b shows a sintered compact. No delamination is visible at the skin-core interface. The dark contrast particulates in the skin are alumina. The core shows some porosity of size less than 10 μm. The density of the sintered compacts measured by a liquid immersion method was 90%.

Two ceramic materials may be co-injected, preferably two different particle sizes of alumina.

Details of the processing conditions are given below:

1 ) Ceramic powder size: 0.5 μm (skin) and 1.0 m (core). 2) The binder systems vary from some of those used previously, notably in the use of polyethylene and high wax and high Stearic acid content in some mixes.

Binder Systems A B c

Low Molecular Weight Polypropylene 20 30 High Density Polyethylene 10

Paraffin Wax 60 60 30

Carnuba Wax 15 10 10

Stearic Acid 25 10 20

The injection moulding temperature used is much less for these systems, being 140° to 180°C for the LMWPP based systems and 90° to 120°C for binder system A, which contains no backbone polymer.

3) Some samples were solvent debound, whilst others were thermally debound only. A powder-bed support was used to stop slumping of the high wax content parts during debinding.

4) Sintering in air was performed up to a peak time and temperature of 1550°C for 4 hrs, for the co-injected compacts. (Alumina usually requires a higher sintering temperature than carbonyl iron or stainless steel)

Use of a relatively coarse alumina powder in the core which would sinter by liquid phase rather than solid state sintering is contemplated. The skin would remain fine grained alumina, with sintering by a solid state process.

Figure 3 illustrates an injection moulding machine 10 with twin barrels, namely 1 1 and 12, and a control valve 13, through which moulding materials A (barrel 1 1 ) and B (barrel 12) are discharged to a mould 14, by way of a passageway 15. The mould 14 is provided with cooling/temperature control ducts 16.

Sequential injection comprises use of material A, followed by B, followed by A. Simultaneous moulding comprises use of material A, followed by A plus B, followed by A.

Where a ceramic filler material is used by the invention, a suitable filler comprises calcined alumina, with a preferred particle size of less than 20 microns.

The invention can be used to produce a component having a skin of ceramic or ceramic-based material and a core of metal or metal-based material, or vice versa.

In summary, metal co-injection moulding (MCM), a new method of surface engineering of powder metallurgical components has been established. MCM of a model system composed of a stainless steel skin and iron core has been investigated and sintered densities of 90% can be achieved without delamination of the sandwich structure.

A wax may be used as a binder material.

A non-powder, that is, a liquid binder material, may be employed.

Claims

1. A method of producing a sintered component comprises moulding a compact which incorporates a binder, substantially removing the binder from the compact, and sintering the resulting debound compact, in which the compact is produced by an injection moulding process from at least two different mixes, each mix comprising a binder and a filler material, the filler material comprising a metal and/or ceramic filler material, said mixes being injected into a mould so that said first mix forms an outer layer of the compact and said second mix forms an inner layer of the compact.

2. The method claimed in claim 1 , wherein the binder comprises a powdered binder material.

3. The method claimed in claim 2, wherein the powdered binder material comprises alumina.

4. The method claimed in claim 3, wherein the particle size of the alumina is less than 20 microns.

5. The method claimed in any one of claims 1 to 4, wherein a first of said mixes is injected into the mould so as to incompletely fill the mould, followed by injection into the mould of a second of said mixes.

6. The method claimed in any one of claims 1 to 4, wherein the step of injecting a second mix into a mould is accompanied by simultaneous injection of a second mix, whereby the two mixes form a co-axial stream, with the first mix encircling the second mix of said stream.

7. The method claimed in any one of claims 1 to 4, wherein the step of injecting the second mix into the mould is followed by injection of some more of the first mix into the mould so as to provide a complete skin of material formed from the first mix.

8. The method claimed in claim 4, wherein the first and second mixes are of ceramic powder material.

9. The method claimed in claim 8, wherein the particle size of the first mix differs from that of the second mix.

10. The method claimed in claim 9, wherein the particle size of one mix is 0.5 μm and that of the other mix is 1.0 μm.

1 1. The method claimed in any one of claims 1 to 10, wherein the sintering rate of the mixes are substantially equal.

12. The method claimed in any one of claims 1 to I I , wherein the filler and binder materials comprise powders, the powder of one of said mixes being finer than the powder of the other of said mixes.

13. The method claimed in claim 12, wherein there is a lower volume percentage of filler powder of said one mix than in said other of said mixes.

14. The method claimed in claim 12, wherein the ratio of the volume percentages in the mix having coarser filler powder to that in the mix having finer filler powder is in the ration of 1.25: 1.50.

15. The method claimed in claim 14, wherein said ratio is in the range of 1.30: 1.45.

16. The method claimed in any one of claims 1 to 15 , wherein one mix comprises particles of stainless steel, plus particles of alumina, and the other mix comprises particles of ferrous material.

17. The method claimed in claim 16, wherein said one mix comprises 316L stainless steel.

18. The method claimed in claim 16 or 17, wherein said other mix comprises OM carbonyl iron.

19. The method claimed in claim 16, 17 or 18, wherein the binder used is a combination of carnuba wax, paraffin wax, Stearic acid and polypropylene.

20. The method claimed in claim 19, wherein the densities of the binder materials comprise: Carnuba wax 0.97 gem³

Paraffin wax 0.9 gem³

Stearic acid 0.85 gem³

Polypropylene 0.9 gem³

21. The method claimed in claim 19 or 20, wherein the weight percentage of the binder materials comprise:

Carnuba wax 7.5%

Paraffin wax 31.5%

Stearic acid 1.0%

Polypropylene 60.0%

22. The method claimed in any one of claims 16 to 21 , wherein said one mix comprises 60% stainless steel by volume and 5% particulate alumina by volume.

23. The method claimed in any one of claims 16 to 22, wherein said other mix comprises 47% by volume of OM carbonyl iron.

24. The method claimed in any one of claims 1 to 23, wherein the injection temperature range comprises 175 to 200°C.

25. The method claimed in any one of claims 1 to 24, wherein sintering takes place under vacuum to 1300°C.

26. The method claimed in claim 25, wherein sintering takes place in a furnace and is followed by furnace cooling.

27. A sintered component produced by a method claimed in any one of claims 1 to 26.

28. A sintered component as claimed in claim 27, having a skin of ceramic or ceramic-based material and a core of metal or metal-based material.

29. A sintered component as claimed in claim 27 having a skin of metal or metal-based material and a core of ceramic or ceramic-based material.

30. A method of producing a sintered component, substantially as hereinbefore described with reference to the accompanying figures.