Metal Injection Moulding (MIM), is a manufacturing process that is a variation of the traditional plastic injection moulding and die casting. It helps with the fabrication of solid metal parts by combining two separate processes – injection moulding and powdered metallurgy sintering.
On the other hand, CNC machines are manufacturing systems controlled by pre-programmed software fed into a computer. The setup enjoys immense versatility as it can perform a wide range of functions depending on the tools mounted on the CNC machine.
We look at the differences between MIM and CNC machining and which one is the better alternative between the two:
Differences Between MIM and CNC Machining
The differences between MIM and CNC machining can be encapsulated against the following variables:
At a higher level, the difference between the manufacturing processes of the two is quite straightforward. MIM follows an additive process while CNC follows a subtractive process. In the case of MIM, molten metal comes together to form the final product. While for CNC, the excess material is removed until you are left behind with the final product.
MIM requires powdered metal feedstock, which is processed in high-pressure, high-temperature conditions to produce small, precise, and high-performing parts. This metal feedstock is a powdered mixture of metal and polymer. For CNC machines, the input material is direct – the raw material is the metal that needs to be processed to produce the end product.
Both MIM and CNC machining can produce parts having unique geometry. However, the degree to which they can achieve it may vary. For instance, MIM can satisfy complex capability requirements through parts consolidation that helps produce a single part having the desired features. However, CNC machining limits design freedom, intricacy, and flexibility as it is a factor of the software and the skills of the individual designing the software.
Strength and Performance
Once again, MIM and CNC machining can produce high-strength, high-performance parts. However, MIM parts are not subject to internal pressure or induced stress during manufacturing, making them inherently stronger than their counterparts produced over CNC. Even though this variability may be minor, it plays a critical role in the cases of complex designs with no tolerance for deformation or failure.
Depending on the feature size, the tolerance in the case of MIM can range from +/-2% to +/-0.4%. However, the drawback of MIM lies in the fact that these tolerances increase in proportion to the size of the features. Such an event occurs due to shrinkage, which can be pegged at 15 to 18% during the sintering process. At the same time, such deformities are not consistent and can be non-isotropic due to factors like gravity, drag forces, and mould shape.
In contrast, CNC machining tolerances can be as low as +/-0.005, with capabilities as low as +/-0.001. As a result, CNC machines can produce less error-prone parts.
The production cycle in the case of MIM varies depending on the part design and complexity. For generic parts where moulds are readily available, the MIM production process can have a periodicity of a few hours. However, in the case of specialised parts, the same can be stretched out to several weeks.
In this aspect, CNC machines offer greater reliability as the manufacturing process is not a factor of external requirements (except maybe any specialised tools, if necessary). The delay only corresponds to the designing of the CAD/CAM software. Once it is up, production can commence at an accelerated pace.
Cost of Production
Putting aside the upfront capital investment necessary for the tools, equipment, and setup, the production cost may be higher for MIM production. This is because it involves the mould, causing the corresponding delays. Further, the metal feedstock works out to be costlier than direct raw input as required in CNC machining. Not to mention that CNC machines deliver the job on time and without compromising on quality. This mitigates the cost of production corresponding to delays in production.
However, this comparison is purely subjective as MIM may prove to be more cost-effective in certain applications, especially when the mould is readily available.
As seen above, MIM follows an additive approach while CNC machining entails material removal. Resultantly, it is only intuitive to understand that CNC machines will generate more scrap waste than MIM machines. Hence, you will have to implement a waste handling and disposal guideline to prevent such waste from occupying inordinate floor space.
MIM vs. CNC Machines: Which One is Better?
The choice between MIM and CNC machining depends on various factors.
As such, you will have to weigh the pros and cons of each before picking one. MIM is more suited for you if you are looking for high repeatability, improved cycle time, parts consolidation, and reduced dependence on secondary operations. On the other hand, if you desire greater accuracy, greater versatility, and higher capacity, then a CNC machine is the way to go.
About the Author:
Peter Jacobs is the Senior Director of Marketing at CNC Masters. He is actively involved in manufacturing processes and regularly contributes his insights for various manufacturing blogs.