Trends Biomater. Artif. Organs, Vol 18 (2), January 2005

http://www.sbaoi.org

Injection Moulding of Titanium Metal and AW- PMMA Composite Powders P. Divya, A. Singhal, Deepak K. Pattanayak and T.R. Rama Mohan Dept of Metallurgical Engineering & Materials Science Indian Institute of Technology Bombay, Powai, Mumbai E-mail: [email protected]

Abstract: Complex / Intricate and near-net shape forming advantages of Powder Injection Moulding (PIM) has led to this process being applied to metals, ceramics as well as composites processing. In the present work, biocomposites containing bioglass ceramic Apatite Wollastonite (AW) and Polymethamethacrylate (PMMA) were processed successfully using PIM. The composites with composition varying from 1040% PMMA were tested for modulus of rupture (MOR). The density of these composites was in the range 1.3-1.5 g/cc. Pure titanium was also injection moulded and the process parameters such as pressure, temperature etc. were optimized. Keywords : Powder injection molding, AW glass ceramics, PMMA

developing stage, it would be appropriate to infer that PIM is an ever-evolving manufacturing process.

Introduction: Powder Injection Moulding (PIM) refers to the processing of any powders viz. metal, nonmetal, ceramics or composites. PIM is referred as Metal Injection Moulding (MIM) when dealing with metal or alloy powders and as Ceramic Injection Moulding (CIM) when ceramics are processed. PIM is an innovative and cost effective manufacturing process commonly used for complex, high-quality medical components such as surgical instruments and implants for orthopaedic and dental use. It is a high-volume, high quality, cost-effective process that helps eliminate secondary machining operations for metal part production. It is very efficient for manufacturing small, intricate and complex components with excellent mechanical properties and high geometrical accuracy. As Powder injection moulding is still in a

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(b) Fig 1: CIM dental parts (a) alumina sleeve (b) zirconia screw [1] 247

Injection Moulding of Titanium Metal and AW-PMMA Composite Powders

CIM has found great success in processing of zirconia, alumina and stellites for applications as diverse as implants, medical devices like nozzles, wire guides, optical instruments, semiconductor and microelectronics devices (Fig 1). The most commonly used materials for MIM are stainless steel, chrome steel, precipitation-hardened stainless steel, nickel, iron and titanium. The initial use and broad success of metal-injection-moulded orthodontic brackets in the 1980s demonstrated the biocompatibility and corrosion resistance of injection-molded stainless steels and led to early instrument applications, including scalpel handles, bipolar forceps, and jaws and clevises for biopsy forceps (Fig 2).

New applications of metal-injection-moulded components include smaller, more-complex devices for minimally invasive surgery, especially laparoscopic instruments for grasping tissue, cutting, and suturing. Such devices are being designed for greater freedom of movement, and, have increased the numbers of metal parts used in the assembly. Metal injection moulding has provided the design freedom to be able to produce such parts costeffectively. A new area of exploration and research of the process is the production of micro sized parts, which should help meet future medical needs as parts continue to shrink for minimally invasive surgery. The parts have dimensions in the order of micrometers and need require specialized tooling system. The process is generally best suited to parts measuring less than 6 mm thick and weighing less than 100 grams. Newer techniques, however, seemed to have enabled the processing of cross sections above 12.5 mm and up to 400 grams [2]. In the present work titanium was injection moulded and debinding studies were carried out. Composites of bioglass ceramic Apatite Wollastonite with poly Methamethacrylate (PMMA) were successfully injection moulded. The MIM Process: The MIM process has four processing steps [3]: 1) Mixing -compounding the metal powder and organic binder into feedstock; 2) Moulding – shaping the parts from feedstock as in plastic injection molding; 3) Debinding – removing the binder in the moulded parts;

Fig 2: MIM parts-scalpel handle, forceps, jaws and clevice [2]

4) Sintering – densifying the debound parts to a high final density. 248

P. Divya, A. Singhal, Deepak K. Pattanayak and T.R. Rama Mohan

for 30 minutes to obtain AW-PMMA fine granules.8 ml of plasticizer – poly Ethylene Glycol [PEG] was added to the composites of AW-PMMA with 10-40 wt % PMMA.

Prior to mixing, the feedstock composition and the right powder loading has to be decided. Mixing is done to obtain the metal powders and binders together in a granular form. These granules are fed into the hopper of the injection-moulding machine, which is followed by moulding at temperature high enough for the binders to flow along carrying along with them the metal powders. The next step is that of debinding of the binder system. This is achieved by solvent extraction, thermal debinding, catalytic debinding or vacuum debinding. Sintering is the final step, which is achieved by heating of the debound parts at high temperature. It affects the final density as well as the mechanical properties. Studies have shown that the important factors of the sintering cycle are: heating rate, sintering time, sintering temperature and sintering atmosphere.

The composites were injection moulded in Eisinger Semi-Automatic Injection Moulding Machine. The dimensions of the composites were 40mmx4mmx3.5mm.After a number of trials injection temperature, injection pressure, and clamp pressure and injection time were optimized for different powder loadings. The injection parameters used for the experiment are given below: • Injection temperature 1500 C • Injection pressure 0.41-0.84 MPa • Clamp time 4 s • Clamp pressure 0.27 MPa • Injection time 2 s

Experimental:

The apparent density of the injection moulded composites was calculated. Three point bend test was performed to check the modulus of rupture (MOR).The test was done using Instron UTM1195 with a crosshead speed of 0.5mm/min and a span length of 30 mm at ambient temperature. The MOR was calculated according to the following equation:

Apatite Wollastonite (A-W) bioactive ceramic powder was synthesized by sol gel method. It was dried on a hot plate and then calcined at 6000 C for 2 hours. The calcined A-W was dry ball milled for 2 hours in a centrifugal ball mill at 200 rpm using 1:5 charge to media ratio. The grinding media used was Zirconia. Particle size analysis was carried out in a particle size distribution analyzer. The powders were characterized to conform the presence of the Apatite and Wollastonite phases using Philips PANalytical X-Ray Diffractometer.

where, P = Fracture Load kN L = Span length [mm], b = breadth [mm], d = depth [mm]

The reinforcement content i.e. PMMA for the composite preparation was set at 10, 20, 30 and 40% weight. PMMA was dissolved in acetone and fine AW powders were added to this. This dispersion was stirred till the AWPMMA solidified into flakes. These flakes were subsequently ground in a mortar-pestle

Results and Discussions: Low-pressure injection of the feedstocks at the injection parameters mentioned above yielded the following results. In the feedstock 249

Injection Moulding of Titanium Metal and AW-PMMA Composite Powders

containing 60 wt% AW, the density increased slightly on increasing the injection pressure. However, there were difficulties in the injection process. Addition of 40 wt% PMMA content caused the viscosity to increase thereby impeding the injection process. This was contrary to the expected ease in moulding due to increase in the plastic content of the feedstock. High PMMA could have caused segregation of the AW Powders, while it itself sticking to the walls of the injection chamber. Thus its purpose as a carrier for the powders was not being met. The feedstock could not flow into the mould cavity at the moulding temperature even on increasing the pressure. The density changed from 1.26 g/cc at injection pressure of 0.41 MPa to 1.37 g/cc at injection pressure of 0.48 MPa. The corresponding values of MOR were 8 and 14 MPa respectively.

Fig 3: Variation of density of 70 – 30 feed stock

The variation in MOR values were consistent with the densities obtained and is shown in Fig 4. The change in MOR was found to be insignificant at various injection pressures.

The flowability of the feedstock containing 70 wt% AW and 30 wt% PMMA was found to be better than that of observed for the 60-40 feedstock. Injection of the former feedstock was easily possible at pressures greater than 0.48 MPa, however there was a decrease in density. This indicates that there was no particle rearrangement which lead to decreased densification. The highest density obtained (1.49 g/cc) indicated the optimized injection pressure to be 0.48 MPa for the 70-30 feedstock (Fig 3). Density of these composites was higher than that exhibited by the 60-40 composites at the pressures of 0.41 and 0.48 MPa.

Fig 4: Variation of MOR with injection pressure

The third feedstock containing 80 wt %AW and 20 wt %PMMA showed moulding difficulties at low pressures. The minimum pressure at which injection was possible was 0.60 MPa. At higher pressures, the density decreased indicating that the optimum injection pressure for this composition was

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0.60 MPa. The density of this 80-20 feedstock injected at 0.61 MPa was 1.53 g/cc [Fig 5] as compared to 1.41 g/cc of the 70-30 feedstock. The MOR for the injection-moulded samples of the same feedstock also indicated a decrease with increasing pressure [Fig 6].

The final feedstock containing 90wt%AW and 10wt% PMMA showed moulding difficulties similar to those observed with the first feedstock of 60AW-40PMMA [Figs 7 and 8].The PMMA content evidently was insufficient to wet the AW powder particles. The range through which successful injection moulding was achieved indicated that flowability of the feedstock was not a major concern but wet ability was the problem. The MOR obtained for these composites were extremely poor as expected. 10wt %PMMA did not provide enough cushioning to the AW to counteract the poor flexure of AW ceramics.

Fig 5 : Variation of density with injection pressure

The difference in the MOR values was found most likely due to the improper mixing of the hard and brittle constituent AW, which was present up to 80 wt %. Fig 7 : Density variation with injection pressure

Fig 6 : Variation of MOR with injection pressure Fig 8 : MOR changing with injection pressure 251

Injection Moulding of Titanium Metal and AW-PMMA Composite Powders

In another study based on injection moulding of pure titanium [4], the optimized injection pressure was found to be 0.41 MPa. Thus, for the composites used in the present experiments the order of injection pressures was higher. The other optimized parameters that produced parts with reproducibility are as follows: • • • •

Conclusions: There are several process parameters associated with PIM; hence the need to set optimum operating conditions acquires immense importance. In the AW-PMMA feedstocks considered in the study MOR decreased as the amount of the ceramics in the feedstock increased. The highest strength of 14.6 MPa was found in 60AW-40PMMA injected at 0.48 MPa and the lowest was 2.6 MPa in 90AW-10PMMA injected at 0.62 MPa. CIM requires higher injection pressures as compared to MIM, due to the “springback effect” of ceramics.

Injection temperature 1600 C Clamp pressure 0.16 MPa Clamp time 4 s Injection time 1-2 s

The challenge in the MIM of titanium was the removal of binder, which had wax as its primary component [Fig 9]. Solvent debinding of the samples using various solvents showed that combination of solvents showed higher binder removal than the pure solvents employed [4].

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(c) Sa - 30 min in Sol-A+4hrs in Sol-B Sb - 45 min in Sol-A+4hrs in Sol-B Sc - 90 min in Sol-A+4hrs in Sol-B Fig 9: Debinding characteristics of (a) solvent A (b) solvent B and (c) combination of solvents A and B [4] 252

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References: 1. www.g1.spt-cim.com/applications 2. J. L. Johnson, “Mass Production Of Medical Devices By Metal Injection Molding”, www.devicelink.com/mddi 3. R. W. Messler, “An Industrially Sponsored Research Programme in PIM”, Metal Powder Report, 363-369, 1990. 4. P.Divya, A.Datar, B.T.Rao, T.R.Rama Mohan, “Studies in Injection Moulding of Metallic Powders” Transactions PMAI, Vol. 29, pp. 166 (2003).

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