For previous article, please refer to The world's top ten mold types ranking-Die-casting mold,  Forging die, Plastic mold, Stamping die.
Molds that make metal and non-metal powders form or shape into blanks and parts through powder metallurgy processes, including pressing molds, isostatic pressing molds, injection molds, extrusion molds, shaping molds, etc.
Principle of powder metallurgy molds
Principle of powder metallurgy molds mainly includes pressing and demolding process of powders, as well as design and manufacture of molds.

 

 

 

Basic principles of powder metallurgy
Powder metallurgy is a technology that uses metal or non-metal powders to manufacture parts through forming and sintering.
Specific process includes:
Powder mixing: Mix raw material powders evenly.
Pressing: Put mixed powder into mold and apply pressure through a press to shape it.
Sintering: Heat pressed parts in a controlled environment to make particles bond together to form required parts.
Design and manufacture of powder metallurgy molds: There are mainly following types of powder metallurgy molds:
Unidirectional pressing: Apply pressure in one direction.
Bidirectional pressing: Apply pressure in two directions at the same time.
Floating pressing: Mold can adjust its position during pressing process to adapt to different pressing needs.
Application areas and advantages and disadvantages: Powder metallurgy technology is suitable for manufacturing parts of various shapes and complex structures, such as gears, bearings and special-shaped structural parts.
Its advantages include: ‌High flexibility‌: It can manufacture parts of almost all shapes.
‌High material utilization‌: Powder metallurgy can make full use of materials and reduce waste. ‌Low cost‌: It is suitable for mass production and has low cost.
Disadvantages include: ‌Material limitation‌: Some high melting point materials are not suitable for powder metallurgy.
‌Precision limitation‌: Although precision is high, there is still a gap compared with some traditional processing methods.

 

‌Powder metallurgy mold materials are an indispensable part of powder metallurgy process and have various types and characteristics.
‌Type‌: Powder metallurgy molds can be divided into compression molds, finishing molds, composite compression molds, forging molds, extrusion molds, etc. according to their uses; they can be divided into steel molds, carbide molds, graphite molds, etc. according to manufacturing materials.
‌Characteristics‌: Powder metallurgy mold materials are obtained through processes such as gas atomization powder making and hot isostatic pressing to obtain uniformly distributed fine structures, and have characteristics of high wear resistance, toughness, fatigue resistance and isotropy.
‌Application‌: Powder metallurgy mold materials are widely used in the fields of automobiles, aviation, aerospace, etc., especially in manufacture of large-scale, complex or special-shaped special molds.
In addition, powder metallurgy mold materials also have characteristics of high hardness, strong wear resistance, good toughness, strong stability, and long service life. They are suitable for processing of various workpieces such as gears and oil-containing bearings. Powder metallurgy manufacturing process mainly includes following steps:

1. Raw material preparation‌: Raw materials are prepared into metal powders by oxide reduction and mechanical methods.

‌2‌. Preparation of blank powder‌: According to product requirements, powders are mixed evenly in a certain proportion by wet, dry or semi-dry methods to form blank powder.
‌3. Mold forming‌: Mixed blank powder is loaded into powder metallurgy forming mold. Desired shape is obtained by pressure forming or pressureless forming.

4. Sintering: Molded model is subjected to multi-element or unit sintering to form physical and mechanical properties of final part product. This is the most important step in the entire process and determines quality of product performance.

5. Post-processing: According to product precision requirements, detailed processing such as finishing, oil immersion, electroplating, machining or heat treatment is carried out to improve stability and hardness of product. This manufacturing process ensures high quality and performance stability of powder metallurgy mold materials.
Design of powder metallurgy pressing molds for powder metallurgy parts with complex structures

 

Basic requirements of mold design should not only make mold have a long service life, but also pressed product should have good quality stability. In order to extend service life of mold, it is necessary to find ways to reduce occurrence of wear during working process. In terms of mold materials, due to low hardness of tool steel, service life of mold processed with tool steel is relatively short. In process of large-scale mass production, mold made of metal ceramic carbide material with higher hardness can greatly increase service life of mold.
Manufacturing net-shaped parts and components is an important feature of powder metallurgy technology. Therefore, in order to obtain powder metallurgy products that meet technical requirements, structure and dimensional accuracy of mold must be strictly controlled. Considering that pressing pressure is limited by shape structure and mold material of mold, it is also necessary to pay attention to pressing amount, stroke, product structure control and mold surface roughness of punch. When designing a mold with complex movements, it is necessary to consider characteristics of pressed powder and structural requirements of prepared product. Some special movement designs can also be added when designing mold, such as using an overfilling process to remove excess powder before upper punch enters die cavity, so that a compact with better performance can be obtained. This method is particularly suitable for production of small parts pressed with powders with poor fluidity. In contrast, underfilling process of powder can be used in pressing process with complex and fragile upper punches. Underfilling process can make outer punch (especially thin-walled punch) less likely to be damaged because it is supported by inner wall of die cavity. Generally, when powder compact has a more complex shape structure, such as a multi-step structure compact or a compact with a large thickness along pressing direction, in order to meet requirements of compact's external dimensions, each action stroke of die must be independently controlled. For example, for double-step compact shown in figure below, if lower die is designed as an integral structure, powders in two step parts will be subjected to different amounts of downward pressure, resulting in an uneven distribution of compact density.

 

Density p1 of compact is closely related to bulk density p2, filling height h1, and downward pressure h2. Specific relationship is: p1 = p2h1/h2; downward pressure can also be expressed by difference between filling height h1 and height variable h3, that is: h2 = h1-h3. Magnitude of downward pressure changes with change in distance between upper and lower die punches, so a simple relationship between compact density and change in spatial position of upper and lower die punches can be obtained, that is: p1 = (p2*h1)/(h1-h3).

 

As shown in figure above, two steps in die cavity have different heights. Since lower die punch is an integral structural part, stroke and h3 value of die punch are same for two parts, resulting in inconsistent compaction density between high and low steps, and density of low step part is significantly higher than that of high step part. If lower die punch is designed as a split structure, same compression ratio can be provided for high and low step parts in conjunction with independent control movement of left and right lower die punches, so that compact can obtain a more uniform and consistent density distribution.

 

Summary: Reasonable and practical pressing process design and mold design work require engineers and technicians to have rich experience and a solid theoretical foundation. In design process, it is necessary to understand in detail properties of various powders, properties of products, and dimensional tolerance requirements of products. High-quality molds not only require long life and low wear, but also need to be able to work under high pressing pressure (sometimes up to 700Mpa) while meeting dimensional accuracy requirements. Therefore, in pressing process design, requirements for mold design are very high. Main points to note in powder metallurgy mold manufacturing process include following:

1. Product size confirmation: Powder metallurgy molds are precise, different product structures and sizes require different molds. Before opening mold, product size, tolerance and chamfering, etc. must be confirmed.

2. Material selection: Different materials have different shrinkage rates. Appropriate materials must be selected according to product requirements. For example, copper and iron have different densities and hardnesses, and subsequent processing requirements must be considered.

3. Demolding method: Choose a reliable demolding method to reduce probability of part cracks and defective products, control costs, and improve mold manufacturing efficiency.

4. Uniform density: During pressing process, force must be uniform to avoid affecting density of parts. If problems occur, part structure must be adjusted in time.

5. Accuracy and density guarantee: Mold is made in strict accordance with technical requirements to ensure density, shape, accuracy and surface roughness of parts. Following above precautions can ensure smooth progress of powder metallurgy mold manufacturing process, improve product quality and production efficiency.