Introduction to Sizing Process in Powder Metallurgy

1. Introduction to Powder Metallurgy
Powder Metallurgy (PM) is a manufacturing technology that forms parts by pressing metal powder and sintering. Compared with traditional casting and forging processes, powder metallurgy can accurately control material composition, save raw materials, and reduce production costs. It is especially suitable for manufacturing parts with complex shapes and high precision requirements. However, due to the limited powder forming during the pressing process, the blank often has a certain dimensional deviation after sintering, so it is necessary to adjust the precision through the **sizing process**.

2. Overview of Sizing Process in Powder Metallurgy

**Sizing process** is an important subsequent processing method in powder metallurgy, which is mainly used to improve the dimensional accuracy and shape accuracy of parts. It is usually carried out after sintering, and the size and shape of the parts are corrected by applying pressure to the parts in a special mold to meet the required tolerance requirements. This process can not only eliminate the dimensional deviation generated during the sintering process, but also improve the surface quality and mechanical properties of the parts.

3. The principle of sizing process in powder metallurgy
In powder metallurgy, the size of sintered parts often changes due to the bonding and shrinkage of powder particles. The principle of sizing process is to use a mold to re-press the sintered parts and eliminate the dimensional error through plastic deformation or slight compaction. The sizing process can not only accurately control the size of the parts, but also eliminate the pores or other defects generated during the sintering process, thereby improving the density and strength of the parts.

There are usually two ways to sizing in powder metallurgy:

（1） **Cold sizing**: Apply pressure to the parts at room temperature. This method is suitable for parts with higher strength and density, can effectively improve dimensional accuracy, and is widely used in the final finishing of parts.

（2） **Hot sizing**: Apply pressure to the parts at a higher temperature. It is suitable for parts with complex shapes, high dimensional requirements and brittleness. Hot sizing can achieve dimensional correction under mild conditions to avoid damage to the parts.

4. Advantages of Sizing Process in Powder Metallurgy
The sizing process has many advantages in powder metallurgy:
（1） **Improve dimensional accuracy**: During the sintering process, due to uneven material shrinkage, errors may occur in the size of the parts. The sizing process can effectively correct these errors so that the size of the parts meets the design requirements, and the accuracy can usually be controlled within a few microns.

（2） **Improve surface quality**: Through the high-precision pressing of the sizing die, the surface finish and surface defects of the parts are significantly improved, making them more in line with the requirements of subsequent assembly and use.

（3）**Improve part strength**: The pressing during the sizing process can increase the density of the parts, reduce the porosity, and thus improve the mechanical strength and wear resistance of the parts.

（4）**Save subsequent processing costs**: The sizing process can improve the dimensional accuracy of the parts in advance during the manufacturing process, reduce or eliminate the subsequent mechanical processing steps, and significantly reduce the manufacturing cost and time.

5. Application of Sizing Process in Powder Metallurgy

The sizing process is widely used in the manufacture of powder metallurgy parts in the automotive, aerospace, electronic and electrical, medical equipment and other industries. For example:

- **Gears and bearings**: These parts usually require high dimensional accuracy and surface finish to ensure their smooth operation and durability. The sizing process can effectively improve their dimensional accuracy and surface quality.

- **Mechanical parts**: Some complex-shaped parts made of powder metallurgy can be accurately formed by sizing, reducing secondary processing steps, especially in situations where high-precision tolerances are required.

6. Challenges of sizing process in powder metallurgy
Although the sizing process has significant advantages, it also faces some challenges:

（1）**Mold life**: The sizing process relies on high-precision molds. If the mold quality is poor or severely worn, it will directly affect the final quality of the parts. The life and maintenance of the mold are issues that need attention.

（2）**Limitations of part materials**: Different materials have different deformation capabilities during the sizing process. Brittle or high-strength materials may crack or be damaged during the sizing process. Therefore, the parameter control of the sizing process (such as pressure and temperature) needs to be precisely adjusted according to the material properties.

7. The role of sizing process in powder metallurgy process
（1）Improve dimensional accuracy

During the forming and sintering process of powder metallurgy, the actual size of the parts often deviates from the designed size due to material shrinkage, stress release and other reasons. Through the sizing process, the parts can be pressed for a second time after sintering to return their size to the design tolerance range. The sizing process can control the dimensional deviation within a small range, usually up to micron-level accuracy.

（2）Correct shape error
During the sintering process, the shape of parts made of powder metallurgy may have some slight deformation or irregularity. The sizing process can correct these shape errors through precise pressing of the mold to ensure that the geometry of the parts meets the design requirements, especially in the precision control of complex structures and key parts.

（3） Improve the density and strength of parts
The sizing process reduces the porosity inside the material by pressing the parts again, further compacts the material, and increases the density of the parts. This can not only improve the mechanical strength and fatigue resistance of the parts, but also improve their wear resistance, corrosion resistance and other properties, thereby improving the overall quality.

（4） Improve surface quality
The surface of the parts after sizing treatment is further compacted and smoothed by the precision mold, which greatly improves the surface quality. After the surface roughness is reduced, the performance of the parts, such as friction coefficient, wear resistance, fatigue strength, etc., can be improved, which is particularly suitable for parts with high requirements for surface finish.

（5）Reduce subsequent processing steps
The sizing process can enable powder metallurgy parts to achieve higher size and shape accuracy, so many traditional machining steps can be reduced or even eliminated. By reducing subsequent processing, production costs and time can be significantly saved, and production efficiency can be improved, especially in mass production.

（6） Stabilize product consistency
In large-scale production, the sizing process can better control the quality consistency of parts. Even if there is a dimensional deviation caused by material and process fluctuations during the sintering process, the sizing process can be corrected by precise mold pressing, thereby ensuring the consistency of size and shape of each batch of products and meeting the design requirements.

（7） Improve the assembly accuracy of parts
Many powder metallurgy parts, especially gears, bearings, bushings and other parts, require high precision to ensure the fit with other parts. The sizing process can effectively improve the tolerance matching accuracy of these parts, thereby improving assembly efficiency, reducing wear and noise during operation, and improving the performance of the overall equipment.

（8） Eliminate sintering residual stress
During the sintering process, residual stress may be generated inside the parts, affecting their service life and mechanical properties. Through the sizing process, the parts can effectively release some stress during the compression process, thereby improving the stability and service life of the parts.

8. Conclusion
The sizing process plays a key role in the powder metallurgy manufacturing process. It can effectively improve the dimensional accuracy, shape accuracy and mechanical properties of the parts, and meet the needs of modern manufacturing for high-precision and high-quality parts. With the continuous development of powder metallurgy technology and the expansion of application scenarios, the sizing process will play an increasingly important role in future high-precision manufacturing.