Metal Injection Molding

Application of metal injection molding MIM in medical field

How to continuously improve the quality and reduce the cost of medical products is a topic that scholars at home and abroad have been studying. Medical products are in great demand, and many of the products themselves have very sophisticated and complex structures, requiring a new manufacturing technology to replace traditional production.

Metal Injection Molding (MIM) is a new type of near-net-shape technology, which can mass-produce products with complex shapes in a short cycle, which can meet the manufacturing requirements of medical products and become an ideal Manufacturing method.

At present, most of the medical MIM products use stainless steel materials, the main grades are 316L and 17-4PH; there are also titanium alloys, magnesium alloys, gold, silver, tantalum and so on.

1. MIM technology

1.1 Process flow of MIM technology

MIM is a near-net-shaping process developed rapidly in the 20th century. The general process is: powder + binder → mixing → injection molding → degreasing → sintering.

The first is to use high molecular polymer and powder to mix, under certain conditions, to obtain a feed with sufficient fluidity, uniform mixing and meet the injection requirements, and secondly to select the appropriate process parameters such as injection temperature, injection pressure and injection speed Injection molding is performed, followed by sintering after removing the binder in the injection blank, so that the powder forms a metallurgical bond, and finally a product that meets the requirements is obtained.

1.2 Features of MIM Technology

MIM is a new type of near-net-shape technology for parts and components formed by combining plastic forming technology, polymer chemistry, powder metallurgy technology and metal materials science. It has the following characteristics:

① The parts formed by MIM technology do not require subsequent processing or have little follow-up processing, and the material utilization rate is high.

②The filling process of feeding materials and the sintering of products can be simulated by computer, and the process can be optimized in the early stage to obtain the best design scheme.

③ During the injection process, the pressure at each point in the cavity is equal, and the density is also equal everywhere under the premise of uniform feeding and mixing, and there will be no density gradient, which is easy to achieve large-scale production.

2. Application of MIM technology in medical products

2.1 Medical products manufactured by MIM technology

Medical products generally require good usability and long enough service life, and flexible design in structure and shape design.

MIM technology was first used in medical products in the early 1980s and has since become the fastest growing segment of the MIM market.

Figure 1 shows the proportion of MIM technology in different industries in North America in 2015. It can be seen that medical and dental have become the main application areas of MIM in North America.

At present, most of the medical MIM products use stainless steel materials, the main grades are 316L and 17-4PH; there are also titanium alloys, magnesium alloys, gold, silver, tantalum and so on.

2.1.1 Orthodontic brackets

MIM technology was first used to make some orthodontic appliances in medical treatment. These precision products are very small in size and have good biocompatibility and corrosion resistance. The main material used is 316L stainless steel. At present, orthopaedic brackets are still Is the main product of the MIM industry.

Forestadent Company of Germany uses MIM technology to produce a two-way barb-type orthopaedic bracket, which can increase the mechanical retention by 30%. Using MIM to polish after one-time forming can greatly reduce the friction of the bracket to the arch wire. This product has been proven by BjornLudwig to have a positive effect in orthodontic procedures.

2.1.2 Surgical tools

Surgical tools require high strength, low blood contamination, and the ability to implement aggressive sterilization procedures. The design flexibility of MIM technology can meet most surgical tool applications, and it also has process advantages. The manufacture of various metal products is gradually replacing the traditional production technology as the main manufacturing method.

FloMet Co., Ltd. has developed a stainless steel claw using MIM technology, produced with 17-4PH stainless steel, with a density greater than 7.5g/cm3, which can be used to grasp objects in the human body during surgery, and has the function of tweezers. Its design is quite complex and requires high production precision.

Forming using MIM technology followed by sintering allows high tolerance levels to be achieved without the need for extensive post-processing, which would otherwise destroy the alignment and geometry of the jaws.

It is difficult to produce such complex-shaped stainless steel jaws by casting or machining methods, requiring a long production cycle and high cost. Using MIM technology to manufacture can save 60% of the cost.

Disposable surgical tools needed to develop a process that could be mass-produced at low cost. Smith Metal Products used MIM technology to produce a shaft assembly for a new type of disposable surgical instrument at the cost of Swiss CNC machine tools. The processed 1/4~1/5, the density is 7.5 g/cm3, the ultimate tensile strength reaches 1190MPa, the yield strength is 1090MPa, the elongation is 6.0%, and the maximum hardness is 33 HRC.

The manufacturing process of this product is as follows: first, use MIM technology to form two shaft parts with a length of 178mm, then laser weld the two parts, and then carry out subsequent machining and heat treatment. In order to achieve better tolerance requirements, it is necessary to spray Pellets and passivation treatments.

2.1.3 Knee Implant Parts

The progress of MIM technology in the field of human implantation is relatively slow, mainly because the certification and acceptance of products requires a long cycle.

At present, MIM technology can be used to produce parts that partially replace bones and joints, and the metal materials used are mainly Ti alloys.

In terms of biocompatibility, Chen Liangjian et al. used MIM technology to prepare porous titanium with a porosity of 60%, and used a modified condensation polymerization cross-linking method to prepare gelatin sustained-release microspheres and coat them on the surface of porous titanium.

The results show that the porous titanium coated with gelatin sustained-release microspheres has no cytotoxicity and can be used as a material for medical implants.

MaettaSciencesInc of Canada has successfully used Ti-6Al-4V to produce knee sample parts for human implantation. After entering the human body, the implant is mainly subjected to pressure and must have good biocompatibility. MIM is used for forming and then hot isostatic pressing, followed by shot peening, polishing and anodizing to obtain better surface properties, reduce friction with the human body, and improve compatibility and service life.

2.1.4 Hearing aid tubes

MIM technology can also be used to produce components for various medical devices.

Indo-MIM company uses MIM technology to produce a hearing aid sound tube for the German Phonak company [12], which has the effect of increasing the sound rate and promoting hearing.

After MIM is formed and sintered, this kind of hearing aid sound tube with complex shape can be obtained. In order to make the surface of the sound tube smooth, it only needs to go through a glass bead blasting process later.

The sound tube has a density of more than 7.65 g/cm3, a maximum tensile strength of 480MPa, a yield strength of 150MPa, an elongation of 45%, and a maximum surface hardness of 100HRB. MIM technology can reduce the cost by 20% compared with the previous traditional production process.

MIM technology can also be used to produce many medical products, including interventional stents, radiation shielding for tungsten high-density alloy syringes, microsurgical manipulators, micro-pump endoscope parts and drug inhalers.

2.2 New MIM Technology for Medical Product Application

2.2.1 Metal Micro Injection Molding

Metal micro injection molding (μMIM) is a forming technology developed by the German IFAM Institute, which organically applies MIM technology to the preparation of micron-sized parts.

In general, there are two types of products that μMIM can be used to produce:

① Parts with a size of micron level and a mass as light as a few milligrams;

② The appearance size of the part is similar to that of the traditional injection molding part, but the size of the local structure reaches the micron-level part with a microstructure.

In recent years, micro-injection molding has become a research hotspot in the field of injection molding. With the development of modern machinery towards miniaturization, the application of micro-injection molding will become more and more extensive.

At present, the Karlsruha Research Center has successfully applied μMIM technology to the production of tiny parts of medical devices, such as spectrometers, titer plates, etc. The structure size of the products has reached the micron level, and the minimum wall thickness is 50μm.

Figure 2 shows the suture anchor for surgical use produced by the German IFAM company using μMIM technology, and its size is only the size of a match head.

2.2.2 Metal co-injection molding

Metal co-injection molding (Co-MIM) originated in the 1990s and is a sandwich-type powder injection molding technology.

The process is to inject two materials with different properties into a mold at the same time or in batches, and perform a composite injection molding, which can combine a metal material and a material with completely different properties in the same part.

Using this method, core/shell structures with functional and complex shapes can be obtained without the need for subsequent processes such as coating, heat treatment and assembly of the article. Finally, it is realized that one process can prepare functionally graded materials, which greatly reduces the number of processes and costs.

Co-MIM technology provides a new idea for the development and design of functional parts. Li Yimin et al. [17] have used Co-MIM technology to propose a new biological implant structure, which is widely used in dense cortical bone structure and cancellous bone structure with solid outer foramen.

This structure is conducive to the interfacial stress transfer between the implanted bone and the surrounding bone structure. The porosity of the outer porous structure is 5% to 60%, and the largest pore is 400 μm.

3. Outlook

According to a recent BCCresearch market study on metal and ceramic injection molding, the global market for metal and ceramic injection molding components will grow from $1.5 billion in 2012 to nearly $2.9 billion in 2018, with an average annual growth rate of 11.4%.

At the same time, with the decline of automobile sales, MIM technology will enter more medical, aerospace, electronics and other fields.

In the new edition of the European Powder Metallurgy Industry Roadmap, the European Powder Metallurgy Association points out that the medical market is an extremely important segment of the injection molding industry.

With the continuous expansion of the market, the application of MIM technology in the medical field will become more and more in-depth, and various new materials and new processes based on MIM technology will continue to be developed.