China is a country with a large population, and the phenomenon of aging is becoming more and more serious. In addition to the excessive destruction of the environment in the past, the pursuit of economic development has made people's health problems face enormous challenges and stimulated the demand of medical products for the whole society.

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. The demand for medical products is large, and the structure of many products themselves is very sophisticated, requiring a new manufacturing technology to replace traditional production.

Metal Injection Molding (MIM) is a new type of near-net forming technology that can produce complex shapes in a short period of time and meet the manufacturing requirements of medical products. Manufacturing method.

1 MIM technology

1.1 MIM technology process

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

The first is to mix the polymer with the powder, and mix the feed with sufficient fluidity, uniform mixing and meet the injection requirements under certain conditions, and then select the appropriate injection temperature, injection pressure and injection speed. The injection molding is carried out, and then the binder in the injection blank is taken off and then sintered to form a metallurgical bond, and finally the desired article is obtained.

1.2 Features of MIM Technology

MIM is a new type of near net shape forming technology that combines plastic forming technology, polymer chemistry, powder metallurgy technology and metal materials science. It has the following characteristics:

1 Parts formed by MIM technology do not require subsequent processing or subsequent processing, and have high material utilization rate. They are near-net forming technology and can produce high-performance, complex-shaped parts.

2 The filling process of the feed and the sintering of the product can be simulated by the computer, and the process can be optimized in the early stage [1-2] to obtain the best design.

3 During the injection process, the pressures at the various points inside the cavity are equal. Under the premise of uniform mixing of the feed, the density is equal everywhere, no density gradient occurs, and large-scale production is easy to achieve.

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 have flexible design in structure and shape design [3].

In the early 1980s, MIM technology was first applied in medical products and has become the fastest growing field in the MIM market.

Figure 1 shows the proportion of MIM technology in North America in different industries in 2015 [4]. 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 are made of stainless steel, the main grades are 316L and 17-4PH; there are titanium alloy, magnesium alloy, gold, silver, antimony, etc. [5].

2.1.1 Orthodontic brackets

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

Germany's Forestat Company used MIM technology to produce a two-way inverted hook orthodontic bracket, which can increase the mechanical retention force by 30%. The MIM can be polished after one molding, which can greatly reduce the friction of the bracket to the archwire. This product has been confirmed by BjornLudwig to have a positive effect in orthodontic surgery [6].

2.1.2 Surgical tools

Surgical tools require high strength, low blood contamination, and the ability to achieve aggressive disinfection procedures. MIM technology is designed to meet the needs of most surgical tools, while also providing process advantages and low cost. The manufacture of various metal products is gradually replacing traditional production techniques as the main manufacturing method.

FloMet Ltd. has developed a stainless steel claw [7] using MIM technology, which is produced from 17-4PH stainless steel and has a density of more than 7.5g/cm3. It can be used to grasp objects in the human body during surgery and has the function of scorpion. The design is quite complex and requires high production accuracy.

After forming with MIM technology and then sintering, high tolerance levels can be achieved without the need for extensive subsequent processing to avoid breaking the line and geometry of the jaws.

It is difficult to produce such a complex stainless steel claw by casting or machining, which requires a long production cycle and high cost, and can save 60% by using MIM technology.

Disposable surgical tools need to develop a process that can be mass-produced at low cost. Smith Metals uses MIM technology to produce a shaft assembly [8] for use in a new type of disposable surgical instrument at a cost of only Switzerland. The CNC machine tool is 1/4 to 1/5, the density is 7.5 g/cm3, the ultimate tensile strength is 1190 MPa, the yield strength is 1090 MPa, the elongation is 6.0%, and the maximum hardness is 33 HRC.

The manufacturing process of the product is: firstly, two-axis parts of 178 mm length are formed by MIM technology, then two parts are laser welded, and then subsequent machining and heat treatment are required. In order to achieve better tolerance requirements, spraying is required. Pills and passivation treatment.

2.1.3 Knee implant parts

MIM technology is progressing slowly in the field of human implants, mainly because it takes a long period of time for product certification and acceptance.

At present, MIM technology can be used to produce parts that replace bones and joints. The metal materials used are mainly Ti alloys [9].

In terms of biocompatibility, Chen Liangjian et al [10] used porous MIM technology to prepare porous titanium with a porosity of 60%, and prepared gelatin sustained-release microspheres by modified condensation polymerization cross-linking method and applied to porous titanium surface.

The results show that the gelatin sustained-release microsphere coating porous titanium is non-cytotoxic and can be used as a material for medical implants.

Canada MaettaSciences Inc. successfully used Ti-6Al-4V to produce knee implant parts for human implants [11]. The implant is mainly subjected to pressure after entering the human body and has good biocompatibility. After MIM is formed, hot isostatic pressing is performed, 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 sound tube

MIM technology can also be used to produce parts for a variety of medical devices.

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

After the MIM is formed and sintered, the acoustic tube of the shape of the hearing aid can be obtained. In order to make the surface of the sound tube have a smooth finish, it is only necessary to pass a glass bead blasting process.

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

MIM technology can also be used to produce a wide range of products, including interventional stents, radiation shielding for tungsten high-density alloy syringes, microsurgical robots, micropump endoscope parts, and drug inhalers [13].

2.2 MIM new technology for medical product applications

2.2.1 Metal microinjection molding

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

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

1 parts up to the micron size and light to a few milligrams;

The dimensions of the 2 parts are similar to those of conventional injection-molded parts, but the size of the partial structure reaches micron-sized parts with microstructures.

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 [14].

At present, the Karlsruha Research Center has successfully applied μMIM technology to the production of tiny parts for medical devices [15], such as spectrometers, titration plates, etc. The product has a structural size of micron and a minimum wall thickness of 50 μm.

Figure 2 shows the suture anchor for surgical use produced by IFAM in Germany using μMIM technology [16], which is only the size of a match head.

2.2.2 Metal co-injection molding

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

The process is to simultaneously or batch-inject two materials with different characteristics into a mold for a composite injection molding, which can combine metal materials and materials with completely different properties in the same component.

With this method, a core/shell structure having a functional and complex shape can be obtained, and subsequent processes such as coating, heat treatment, and assembly are not required for the article. Finally, a process can be used to prepare functionally graded materials, which greatly reduces the number of processes and reduces costs.

Co-MIM technology provides a new way of thinking about the development and design of functional parts. Li Yimin et al [17] have used Co-MIM technology to propose a new bio-planting structure, which is widely used in dense cortical bone structure and solid cancellous bone structure in the outer hole.

This structure facilitates 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 BCCresearch's recent market research on metal and ceramic injection molding, the global market value of metal and ceramic injection molded parts will increase from US$1.5 billion in 2012 to nearly US$2.9 billion in 2018, with an average annual growth rate of 11.4%.

At the same time, with the decline in car sales, MIM technology will enter the medical, aerospace, electronics and other fields.

In the new European powder metallurgy industry roadmap, the European Powder Metallurgy Association points out that the medical market is an extremely important part of the injection molding industry [18].

As the market continues to expand, the application of MIM technology in the medical field will be deeper and deeper, and various new materials and processes based on MIM technology will be continuously developed.

Ultrasonic surgical head

Shenzhen Yujiaxin Tech Co., Ltd. is now a manufacturer with rich experience in MIM products for various medical devices. In the future, it will be more dedicated to metal powder injection molding precision products in the medical device industry.

Keywords: metal powder injection molding; medical products; metal microinjection molding; metal co-injection molding; surgical instruments;