Metal Injection Molding

Metal Injection Molding

The metalworking and manufacturing industry has come a long way in human history when it comes to mechanical parts manufacturing, there are a number of methods we can choose from, such as machining, die casting, powder metallurgy, and more. Each of the processes has its own pros and cons. Despite the fact that the characteristics of these methods vary, they share the same goal: to produce quality parts and components in the most effective way.

Over the years, Rainbow Ming has put its focus on the improvement of the powder metallurgy process and successfully established a system to facilitate the process. Though the ‘conventional’ powder metallurgy fits most of the general manufacturing requirements, it is not a good fit for high-complexity, tight-tolerance parts. When the production requires a higher level of complexity and quantity, metal injection molding is evidently the best solution.

 

Metal Injection Molding Overview

Metal injection molding (MIM) is a process that combines powder metallurgy and injection molding. Click here to learn more about powder metallurgy and the sintering process. Combining the two processes, MIM acquires both powder metallurgy and injection molding’s advantages. It can produce high-volume metal parts with complex geometries in a cycle. There are several steps during the process, including mixing the feedstock, injection molding, debinding, and sintering. In the following section, we will break down the MIM process and hope you find it useful!

 

Feedstock

The term feedstock refers to the mixture of metal powders and binders. Kneading the metal powder with the binder to make the feedstock is the first step of MIM. The presence of the binder is necessary for MIM because of the functions it serves. Since the metal powders do not liquify and bind on their own, the mixture needs the binder to bind the powders and serves as the liquefier. Adding the binder allows great spreadability to the feedstock and hence it can be injected into the mold easily. There are several types of MIM binders and each of them gives the molded part different qualities.

 

Injection Molding

Injection molding is where the liquified feedstock turns into its solid form. In this process, the injection unit makes sure the feedstock mix thoroughly, heats the feedstock, and forces it into the mold. The amount of feedstock injected into the mold in each cycle is called a shot. Inside the mold, there are cooling lines to accelerate the cooling process and help the shot to solidify. This process usually takes seconds to complete.

In a typical injection molding process where metal is not involved (as opposed to investment casting), the process ends after the part is removed from the mold. However, in the MIM process, injection molding only creates the green part. The green part needs to go through a couple more processes to become the final product. There is another article about injection molding coming up if you want to learn more about it. Now let’s read on.

 

Debinding

As you may already tell, the goal of debinding is to remove the binder from the green part. Though the thermoplastic resin binder is necessary as well as beneficial in assisting the injection molding process, it degrades the part in terms of its mechanical properties if remaining in the final product. The method (solvent extraction, heat, chemical, etc.) used to remove the binder from the green part varies based on the type of binder added into the feedstock. One thing to note here is that the debinding process does NOT take away all the binders. Instead, a small amount remains inside the part and will be removed entirely in the next step.

The reason not to remove all the binder at once is that the part still needs a certain volume of the binder to hold the piece together. The metal powders fuse into one solid piece without the need for the binder only after sintering. After the most amount of binder is taken away from the part, the green part becomes the brown part and the size shrinks. Check the biner article to learn more about the binding and debinding of MIM.

 

Sintering

This step is where Rainbow Ming’s true strength weighs in. The sintering process involves a high level of heat, usually up to around 1300 degrees Celsius (roughly 2500 degrees Fahrenheit). The sintering process takes hours, sometimes even days, to finish. Exposing the brown part to the high temperature over a long period of time in the furnace allows the metal particles in it to melt, fuse together, and densify. During the sintering process, the pores (created by the binder) reduce and the residual binder particles escape from the part. This process leads to a part solid density of over 95%, a density significantly higher than that of a powder metallurgy process.

Sintering also results in the shrinkage of the part by around 20 percent. Also, the temperature in the MIM sintering process is much higher than powder metallurgy, the final product of MIM has a much better physical strength and hence better performance. In most cases, MIM does not need any additional treatments or machining for the end product. However, some MIM parts made for certain applications require additional surface treatment, heat treatment, or hot isostatic pressing to further enhance the tolerance and mechanical properties.

 

Pros and Cons

The metal injection molding process comes with a great many advantages as well as several downfalls. On the bright side, MIM is able to carry out very complex geometries on the end product and the little porosity content materializes high-density components. The average density is about 95 or 96%, but it can reach a 99% density when excellent part consolidation is required. Speaking of geometry, MIM allows for designs such as undercuts, thru-holes, threads, etc., rending the freedom of design. As aforementioned, the process features tight tolerance, fine surface finish, and high accuracy, which diminish the need for secondary operations. Other advantages such as high physical strength, lightweight, high toughness, high fatigue strength, and more make MIM an ideal solution for many applications.

The primary downfall of metal injection molding is its expense. Generally speaking, the MIM process is more expensive than powder metallurgy, investment casting, and other metalworking processes. The reason why it costs higher is mainly because of the tooling, shrinkage, and temperature involved in the process. MIM is genuinely two processes combined into one, which leads to a higher cost of equipment and tooling. Maintaining the high temperature during the sintering process costs a lot of energy and therefore a higher cost. Aside from tooling, the cost also comes from the tremendous amount of material that escapes from the part. That being said, though MIM is expensive, it is still considered cost-effective, given the fact that it produces higher quality parts and does not need secondary operations in most cases. So what are the applications of metal injection molding? Let’s take a look.

 

Applications

Metal injection molding is prevalent across sectors that demand a high level of precision, accuracy, tolerance, and physical qualities. It is also used in some metalworking shops for recreational purposes, given the capability of creating state-of-the-art components. Below lists the fields of application of the MIM process. Contact us right now to learn more about Rainbow Ming’s metal injection molding solutions.



Process information about MIM

tooling detailno. of cavities1~8
tooling life time per cavity200000 shots
cleaning intervalafter every production
molding process temp.80~185
hydraulic injection pressure500~1500 bar
processname of injection molding machineARBURG ; FANUC
name of sintering ovenShimadzu
process timeinjection30 second
debinding12Hr ~ 24Hr
sintering 18Hr ~ 24Hr
other process (total in days)7~14
total capacity of injection per day (pcs)1500~8000
delivery time of optimal batch in days30~45

MIM-011.jpg
Injection Molding equipment
MIM-021.jpg
Injection Molding Equipment
MIM-031.jpg
Debinding and Sintering Equipment (Japanese Equipment – Shmadzu)
MIM-041.jpg
Debinding and Sintering Equipment (USA Equipment – CVI)
MIM-051.jpg
Size of the parts before and after sintering.
MIM-061.jpg
Photo of the MIM samples

Typical material composition and properties
Propertiesdensity (g/cm3)Tensile Strength ( MPa )Elongation ( % )Hardness Before Heat TreatedHardness After Heat TreatedC%Ni%Mo%Cr%Cu%OthersFe
Fe-2% Ni7.535020HRB 85Max.HRC52< 0.152----Bal
Fe-8% Ni7.645025--< 0.158----Bal
4650 *7.515002HRC 18~30Max.HRC550.520.5---Bal
316L7.450040HRB 70-< 0.03112.217-1.4MnBal
440C7.516002-Max.HRC5510.60.7517-1.0 Mn / 1.0 SiBal
17-4PH *7.512004HRC22~33Max. HRC 380.054-174-Bal
M2 *> 8.018000.5-Max. HRC 620.9-0.54-2.0V / 6.0WBal
We still have other materials such as WC, Fe50Ni....Etc.

Product namemetal injection molding part / MIM
production processForming (injection) / debinding / sintering / tumbling / machining
featuresWhen part is too complex and small to be produced by conventional powder metallurgy process, then MIM is a good choice. 
materialfine sintered metal powder with plastic binder : Fe2Ni, Fe8Ni, 4605, 8640, stainless steel 316, stainless steel 17-4, M2, Tungsten carbide / WC and many other materials
applicationselectrical parts, locking system part, industrial machine parts, automobile component, aircraft component, medical appliance component