Abstract: The process and characteristics of metal powder injection molding (MIM) technology are described. The applications of MIM technology on medical products such as orthodontic brackets, surgical tools, knee implant parts and hearing aid acoustic tubes are summarized. The application of two new metal powder injection molding technologies, metal micro-injection molding and metal co-injection molding, on medical products is described.
Keywords: metal powder injection molding; medical products; metal microinjection molding; metal co-injection molding
China is a populous country, and the aging phenomenon is becoming more and more serious, coupled with the excessive damage to the environment in the past, the pursuit of economic development, so that people’s health is facing a huge challenge, which stimulates the whole society’s demand for medical products.
And 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 high demand for medical products and the complexity of many of the products themselves require a new manufacturing technology to replace traditional production.
Metal powder injection molding (metal Injection Molding, MIM) is a new type of near-net molding technology, can be in a shorter period of mass production of products with complex shapes, in line with the requirements of the medical product manufacturing, has become an ideal manufacturing method.
1 MIM Technology
1.1 The process of MIM technology
MIM is a fast-developing near-net molding process in the 20th century, the general process is: powder + binder → mixing → injection molding → degreasing → sintering.
First of all, the use of polymers and powder mixing, mixing under certain conditions with sufficient fluidity, mixing and meet the injection requirements of the feedstock, followed by the selection of the appropriate injection temperature, injection pressure and injection speed and other process parameters for injection molding, and then is to degrease the injected billet binder sintering, so that the powder to form a metallurgical bond, and finally to get the requirements of the parts.
1.2 Characteristics of MIM technology
MIM is a combination of plastic molding technology, polymer chemistry, powder metallurgy technology and metal materials science and other multidisciplinary combination of the formation of a new type of parts and components of near-net forming technology, has the following characteristics:
① The parts formed by MIM technology do not require subsequent processing or very little subsequent processing, high material utilization, belonging to the near-net forming technology, can produce high-performance, complex shape parts.
② The filling process of feeding material and sintering of products can be simulated by computer, which can optimize the process in the early stage [1-2] and get the best design scheme.
③ The pressure is equal at all points inside the cavity during the injection process, and the density is equal at all places under the premise of uniform mixing of the feed material, and there is no density gradient, which makes it easy to realize large-scale production.
2 MIM technology in the application of medical products
2.1 Medical products manufactured by MIM technology
Medical products are generally required to have good usability and long enough service life, and flexible design in structure and shape design .
MIM technology was first applied to medical products in the early 1980s, and has since become the fastest growing segment of the MIM market.
Figure 1 shows the share 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, and the main grades are 316L and 17-4PH; there are also titanium alloys, magnesium alloys, gold, silver, tantalum and so on .
Fig.1 Approximate partition of applications for MIMin North America for 2015 in different industriesFig.1 Approximate partition of applications for MIMin North America for 2015
2.1.1 Orthodontic Brackets
MIM technology was first used in medical treatment to make some dental orthopedic appliances, these precision products are very small in size, biocompatibility and corrosion resistance, the main material used is 316L stainless steel, and orthodontic brackets are still the main products in the MIM industry.
Germany Forestadent company with MIM technology to produce a bidirectional inverted hook type orthodontic brackets, mechanical retention force can be increased by 30%, the use of MIM once formed and polished, can make the bracket on the archwire friction is greatly reduced, the product has been confirmed by BjornLudwig has a positive role in orthodontic surgery .
2.1.2 Surgical tools
Surgical tools require high strength, low blood contamination and the ability to achieve aggressive sterilization procedures. MIM technology has the design flexibility to meet the majority of surgical tooling applications, as well as the process superiority to manufacture a wide range of metal products at low cost, and is gradually replacing traditional production techniques as the primary manufacturing method.
Harber Metal used MIM technology to develop a stainless steel claw , produced from 17-4PH stainless steel, with a density greater than 7.5 g/cm3, which can be used to grasp objects inside the body during surgery, functioning as a forceps. Its design is quite complex and requires high production accuracy.
By utilizing MIM technology for forming and then sintering, it is possible to achieve a high level of tolerance without extensive subsequent processing, avoiding disruption of the claw’s line orientation and geometry.
It is difficult to produce these complex shaped stainless steel jaws using casting or machining methods, requiring long lead times and high costs, whereas using MIM technology to manufacture them can result in cost savings of up to 60%.
Disposable surgical tools need to develop a low-cost mass production process, Smith Metal Products, Inc. to use MIM technology to produce a shaft assembly , applied to a new type of disposable surgical instruments, the cost of using Swiss CNC machine tooling only 1/4 ~ 1/5, the density of 7.5 g/cm3, the ultimate tensile strength of 1190MPa, yield strength 1090MPa, elongation is 6.0%, maximum hardness is 33 HRC.
The manufacturing process of this product is as follows: firstly, the two shaft parts of 178mm length are formed by MIM technology, then the two parts are laser welded, followed by machining and heat treatment, and in order to achieve better tolerances, shot peening and passivation are also required.
2.1.3 Knee implant parts
MIM technology in the field of human implantation is relatively slow progress, mainly because of the product certification and acceptance of a longer period of time.
Currently, MIM technology can be utilized to produce parts that partially replace bones and joints, and the metal material used is mainly Ti alloy .
In terms of biocompatibility, Chen Liangjian et al  used MIM technology to prepare porous titanium with a porosity of 60%, and gelatin slow-release microspheres were prepared and coated on the porous titanium surface using a modified condensation polymerization cross-linking method.
The results showed that the gelatin slow-release microspheres coated porous titanium were not cytotoxic and could be well used as materials for medical implants.
MaettaSciencesInc, Canada, successfully utilized Ti-6Al-4V to produce a knee sample part for human implantation , which is mainly subjected to pressure after entering the human body and has to be very biocompatible. MIM was utilized for shaping followed by hot isostatic pressing and subsequently shot peening, polishing and anodizing to obtain better surface properties, reduced friction with the human body and improved compatibility and service life.
2.1.4 Hearing aid sound tubes
MIM technology can also be used to produce components for a variety of medical devices.
Indo-MIM utilizes MIM technology to produce a hearing aid acoustic tube for the German company Phonak , which has the effect of enhancing sound rate and promoting hearing.
The complex shape of this hearing aid tube is obtained by sintering the MIM form, which is followed by a glass bead blasting process in order to give the tube a surface finish.
The tube has a density of more 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 is able to reduce costs by up to 20% compared to conventional production processes.
MIM technology can also be used in medical applications to produce many products, including interventional stents, radiation shielding for tungsten high-density alloy syringes, microsurgical manipulators, micropump endoscopic parts, and drug inhalers .
2.2 New MIM Technologies for Medical Product Applications
2.2.1 Metal Microinjection Molding
Metal micro injection molding (μMIM) is a molding technology developed by the IFAM Institute in Germany, which is the organic application of MIM technology to the preparation of parts with dimensions up to the micron level.
In general, μMIM can be used to produce two kinds of products:
(i) Parts with micron-sized dimensions that are as light as a few milligrams in mass;
(ii) Parts with microstructures whose external dimensions are similar to those of conventional injection molded parts, but whose local structures have micron-sized dimensions.
In recent years, micro-injection molding has become a research hotspot in the field of injection molding, and with the development of modern machinery in the direction of miniaturization, the application of micro-injection molding will be more and more extensive .
At present, Karlsruha Research Center has successfully applied μMIM technology to the production of tiny parts for medical instruments , such as spectrometers, titration plates, etc., and the structural dimensions of the products have reached the micrometer level, with a minimum wall thickness of 50 μm.
Fig. 2 shows the μMIM technology used by IFAM in Germany to produce a suture anchor for surgical use , which is only the size of a match head.
Fig.2 Suture anchor
2.2.2 Metal co-injection molding
Metal co injection molding (Co-MIM) originated in the 1990s as a sandwich type of powder injection molding technology.
The process is the same time or batch of two materials with different characteristics will be injected into a mold for a composite injection molding, it can be metal and a completely different properties of the material in the same parts combined together.
In this way, functional and complex core/shell structures can be obtained without the need for subsequent processes such as coating, heat treatment and assembly. The result is a functional gradient material that can be prepared in a single process, which greatly reduces the number of processes and lowers costs.
Co-MIM technology provides a new way of thinking for the development and design of functional parts. Li Yimin et al  have proposed a new biological implant structure using Co-MIM technology, which is widely used in dense cortical bone structure and cancellous bone structure with outer hole and inner solid.
This structure facilitates the interfacial stress transfer between the implanted bone and the surrounding bone structure, and the porosity volume ratio of the outer porous structure ranges from 5% to 60%, with the largest pore having 400 μm.
Read more: [Technology] New Metal Injection Molding Technology: Introduction to μ-MIM and 2C-MIM Processes
According to a recent BCCresearch market study on metal and ceramic injection molding, it is predicted that the global market value of metal and ceramic injection molded parts will grow from $1.5 billion in 2012 to nearly $2.9 billion in 2018, an average annual growth rate of 11.4%.
Meanwhile, as automotive sales decline, MIM technology will move more into the medical, aerospace and electronics sectors.
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 part of the injection molding industry .
As the market continues to expand, the application of MIM technology in the medical field will become more and more in-depth, and a variety of new materials and processes based on MIM technology will continue to be developed.