The comparison for CNC and MIM

Analysis and Comparison of the Advantages and Disadvantages of Mechanical Machining and Powder Injection Molding (MIM) Processes

Machining and Powder Injection Molding (MIM) are two crucial component forming processes in manufacturing, each suitable for different complexity levels, production volumes, and material requirements. The following will provide a detailed analysis based on their process principles, advantages, and disadvantages, and present the core differences through comparison charts.

I. Overview of the Core Principles of the Processes

Before analyzing the advantages and disadvantages, it is necessary to first clarify the essential differences between the two processes, which is the foundation for understanding their distinctions:

• Machining: This falls under "subtractive manufacturing", using metal/non-metal blanks as raw materials. Through cutting operations with tools such as turning, milling, planning, grinding, and drilling, or through energy removal methods like electrical discharge machining or laser technology, excess materials are removed, resulting in parts that meet dimensional accuracy requirements.

• Powder Injection Molding (PIM): This belongs to "near-net-shaping". It combines the advantages of powder metallurgy and injection molding - first, the metal/ceramic powder is mixed with a binder to form a "feedstock", which is then injected into a mold by an injection molding machine to form a "blank", followed by degreasing (removal of the binder) and sintering (powder densification) to obtain the part, requiring only a small amount of subsequent processing to reach the finished product.

II. Analysis of the Advantages and Disadvantages of the Two Processes

1. Advantages and Disadvantages of Machining Process Advantages

1) Advantages

• High material adaptability: It can be processed into almost all metals (steel, aluminum, copper, titanium alloys, etc.), plastics, wood, stone, etc. It is particularly suitable for high-hardness and high-melting-point materials that are difficult to handle in MIM (such as high-speed steel, hard alloy).

• High dimensional accuracy and surface quality: The dimensional tolerance of precision mechanical processing (such as CNC machining centers, grinding) can be controlled within ±0.001mm, and the surface roughness (Ra) can reach the minimum of 0.025μm, meeting the requirements for high-precision parts (such as precision bearings, core components of medical devices).

• High process flexibility: No need for custom molds. By adjusting the tool path and program, the type of parts can be quickly switched, suitable for small-batch, multi-variety production, and can process complex irregular structures (such as parts with deep holes and thin walls).

• Stable finished product density and mechanical properties: The raw material is a dense blank, and after processing, the parts have no internal pores. The mechanical properties (strength, toughness) are close to the theoretical values of the material itself, without performance fluctuations caused by sintering.

2) Disadvantages

• Low material utilization: The "removal" characteristic leads to a large amount of materials being cut into waste (the waste rate for complex parts can reach 50%-80%), especially for precious metals (such as gold, silver, and titanium), the cost is extremely high.

• Low production efficiency and high costs with increasing complexity: The more complex the part structure (such as multiple curved surfaces, micro-holes), the more times the tool needs to be changed and the process needs to be adjusted, resulting in longer processing time; and in small batch production, there is no scale effect, and the unit cost of each part is high.

• Difficult to process extremely complex structures: For internal intersecting holes, tiny internal cavities (diameter < 1mm), and ultra-thin walls (thickness < 0.5mm) parts, the reachability of the cutting tools is poor, the processing is extremely difficult and even impossible to achieve.

2. Advantages and Disadvantages of Powder Injection Molding (MIM) Advantages

1) Advantages

• High material utilization rate (close to 100%): The "near-net-shape" feature enables the parts to closely resemble the final product, requiring only a small amount of subsequent processing, with a waste rate of less than 5%. This is particularly suitable for cost control of precious metals and rare alloys (such as titanium alloys, stainless steel).

• Adapted for mass production of complex parts: Once the mold development is completed, the injection molding - degreasing - sintering process can be standardized for mass production (with a daily output of up to tens of thousands of pieces); it can form complex structures (such as multiple steps, micro-holes, hollowing, internal cavities) in one go, without the need for multiple processes to be joined together.

• High dimensional accuracy and good consistency: The dimensional tolerance after sintering is usually within ±0.1% to ±0.5% (adjustable according to the part size), and the dimensional and performance consistency of the parts in mass production is better than that of mechanical processing (reducing human operational errors).

• Enable multiple material composite molding: It can mix different components of powders, or achieve "two-color/multi-color" composite during injection molding (such as the combination of hard and soft alloys), expanding the function of the parts (such as a wear-resistant head + a ductile matrix).

2)Disadvantages

• High mold cost and large initial investment: MIM molds need to be compatible with injection molding machines and must consider the sintering shrinkage rate (usually 3%-20%). The mold design and manufacturing process is long (1-3 months) and costly (ranging from tens of thousands to hundreds of thousands of yuan), making it unsuitable for small batch (<10,000 pieces) production.

• Material selection is limited: It is only applicable to powdered raw materials (such as stainless steel, iron-based alloys, ceramics). It is difficult to sinter densify high-hardness and high-toughness materials (such as certain high-speed steels), and there are a few pores in the sintered parts (the density is usually 95%-99% of the theoretical density), with mechanical properties slightly lower than forged parts / machined parts.

• Complex process flow and strict parameter control: It requires multiple steps such as "feeding preparation - injection - degreasing - sintering", and each step's parameters (such as injection temperature, degreasing speed, sintering atmosphere) will affect the quality of the parts (improper degreasing can cause cracking, and deviation in sintering temperature can lead to dimensional deviations). High production management requirements are also necessary.

• Part size is limited: It is restricted by the tonnage of the injection molding machine and the size of the sintering furnace. MIM is suitable for small and medium-sized parts (usually weighing <500g and maximum size <200mm), and the sintering uniformity of large parts is difficult to guarantee.

III. Comparison Chart of Core Indicators between Mechanical Processing and MIM Technology

IV. Process Selection Suggestions

1. Prioritize scenarios for mechanical processing:

   o The parts are in small batches (<1000 pieces) or have customized requirements (such as samples, spare parts);

   o High precision (tolerance <±0.01mm) or surface quality (Ra <0.8μm) is required;

   o The materials are of high hardness and non-sinterable types (such as high-speed steel, hard alloy tools);

   o The part sizes are large (weight > 500g, size > 200mm).

2. Prioritize scenarios for powder injection molding (MIM):

   o The parts are for mass production (>10,000 pieces) and have complex structures (multiple steps, micropores, internal cavities);

   o The materials are precious metals or rare alloys (such as titanium alloys, stainless steel), and cost control is required;

   o The part sizes are small (weight < 500g), and high consistency is required (such as batch electronic components);

   o Composite materials or special functions (such as wear resistance + toughness combination) are needed.