Discussion on metal powder injection molding technology

Discussion on metal powder injection molding technology

Powder injection molding (PIM) is composed of metal powder injection molding (MIM) and ceramic powder injection molding (CIM) two parts, it is a new metal, ceramic parts preparation technology, it is the plastic injection molding technology into the powder metallurgy field and formed a new part processing technology.

Ceramic powder injection molding (CIM) is a branch of modern powder injection molding (PIM) technology, which has many special technical and technological advantages: mass production can be quickly and automatically, and the process can be accurately controlled; Because of the flow filling mold, the green density is uniform; Due to the high pressure injection, the powder content in the mixture is greatly increased, the shrinkage of the sintered product is reduced, the product size is accurate and controllable, the tolerance can reach ±0.1% ~ 0.2%, and the performance is superior; No mechanical processing or only micro processing, reduce the preparation cost; It can form complex shape, with cross hole, oblique hole, concave and convex surface, thread, thin wall, difficult to cut the ceramic special-shaped parts, has a wide range of application prospects.

First, powder injection molding process

definition

Metal powder injection molding is to select the metal powder and resin binder that meet the requirements, and mix the powder and resin into a uniform injection pellet at a certain temperature. After granulation, the forming blank obtained is sintered and densified after degreasing treatment, and the final product is obtained.

This is very different from traditional injection molding in that metal or ceramic powder is used as raw material. Due to the poor fluidity of the powder itself, it is necessary to add a large amount of binder at a certain temperature to mix into a uniform, flowing feed, and then make a uniform size of the particle.

Application characteristics

1, the mold cost is very high, especially for mass production, the mold needs to be manufactured with high hard wear-resistant materials, and the material cost is very high;

2, high cost of ingredients, many process steps, high process requirements;

3, mainly used in the shape of complex, difficult to process with other methods or even can not be processed products.

Second, the process comparison

Third, granular material

Pellet preparation includes

Powder metal powder

The particle size of metal powder used in MIM process is generally 0.5~20μm; In theory, the finer the particle, the larger the specific surface area, which is easy to form and sintering. The traditional powder metallurgy process uses a coarser powder greater than 40μm.

Organic adhesive

The role of organic adhesive is to bond metal powder particles, so that the mixture has rheological properties and lubricity when heated in the injection machine barrel, that is to say, the carrier that drives the powder flow. Therefore, the choice of adhesive is the carrier of the whole powder. Therefore, the adhesion selection is the key to the entire powder injection molding. Requirements for organic adhesives:

1. Less dosage and less adhesive can make the mixture produce better rheology;

2. No reaction, no chemical reaction with metal powder in the process of removing the adhesive;

3. Easy to remove, no carbon residue in the product.

aggregate

It indicates a balanced relationship between powder and binder, and the proper proportion between the two is the key to determine the success or failure of injection molding. The use of low molecular weight binders reduces viscosity and is easy to form. Qualified particles should be powder evenly distributed in the binder, can not agglomerate or have pores; Uneven powder distribution will lead to inconsistent viscosity of the particles. Not conducive to forming and sintering

(a) too much binder, small particle viscosity, insufficient contact between metal particles, serious deformation after degreasing, and even lead to product collapse;

(b) too little binder, high particle viscosity, very difficult to inject, easy to form pores after degreasing, easy to lead to product cracking after sintering;

(c) Adding criteria: point contact occurs between powder particles, powder particles are stuck together without external pressure, and the gap in the middle is filled with adhesive;

Fourth, granular material - mixing

Mixing is the process of mixing metal powder with binder to obtain uniform granules. Because the properties of the pellet determine the performance of the final injection molding product, the mixing step is very important.

Mixing process

(a) The surface treated metal or ceramic powder is added to the binder, and the two are evenly mixed to obtain a composite powder system;

(b) The composite powder is heated to melt the binder;

(c) The liquid binder enters the powder particle aggregate through the capillary action, lubricates the powder particles, and agglomerates the particles under the action of the screw shear force to obtain the holding block decomposition, and maintains the uniform mixing.

(d) If the alloy powder oxidizes, resulting in mixing failure.

(e) In order to ensure that the particles are uniform, the powder particles are small or irregular in shape, and the mixing time needs to be increased accordingly to achieve uniform mixing. The mixing time increases, the uniformity of the mixture increases, but the resin is easy to oxidize and decompose, and the mixing time is shortened as far as possible under the premise of some metals or uniformity.

(f) The pellets after mixing are processed by the crusher or the granulation machine (generally made into about 3mm particles) into the injection molding feed.

Five, injection molding

Injection molding is under a certain pressure and temperature, through the plunger or screw push, the flow and temperature uniformity of the granular melt into the mold cavity filled, the melt solidified and cooled under controlled conditions until the injection billet out of the mold cavity to form a three-dimensional complex shape and structure. This stage is completely different from press forming in traditional metallurgy and is similar to the forming process in the plastics industry.

1. When injecting, the nozzle is close to the flow path, the screw is pushed forward, the feeding cylinder is extruded after pressure, and the mold cavity is filled; When there is enough feed to fill the mold cavity, the screw stops rotating. The ideal mold filling is to gradually fill the mold cavity along the mold wall, and the thick billet requires the screw to advance faster, and the thin part is vice versa.

> The filling rate is too large, resulting in injection, bubbles, solder marks, or incomplete filling (air can not escape). (Large injection pressure and mold filling rate, low feed viscosity are all causes of injection)

> Too slow filling speed will lead to premature feeding cooling, resulting in incomplete filling and short shooting. (Improper control of pellet injection temperature can also cause this phenomenon)

2. When the screw reaches the top nozzle, the process of pressurizing the feed is the pressure maintaining process. · Injection molding ends with the forming blank being removed from the mold.

> The mold opening temperature should be lower than the critical temperature required to maintain the shape of the blank when it is demoulded.

> The opening pressure must be less than the large pressure required for the forming blank to release without sticking.

> Mold opening pressure and temperature should have a certain range, can not make the product deformation, stick mold, scratch mold or form shrinkage holes or depressions on the surface of the product.

Six, degreasing

Degreasing is a unique step in metal powder injection molding, which requires the removal of about 30%-50%(volume fraction) of the binder from the billet, completely different from the removal of a small amount of surfactants in traditional powder metallurgy.

Two basic processes

(1) Thermal decomposition → chemical reaction process of binder;

(2) Decomposition gas transfer to the surface of the slab into the external atmosphere → physical heat and mass transfer process.
7. Sintering

The traditional powder metallurgy compact generally has a relative density of more than 90% before sintering, and the complete densification only needs to eliminate about 10% of the pores. After degreasing, the relative density of powder injection molding billet is only 60% before sintering, and its sintering nature is loose powder sintering, which increases the difficulty. Metal powder injection molding product sintering success criteria: to ensure the accuracy and performance of the product with controllability and repeatability of the premise, so that its density to meet the requirements.

Large shrinkage occurs in sintering, and although this shrinkage is the main purpose of sintering, it also leads to deformation. → Sintering shaping process to ensure product accuracy

◆ Control the heating speed can promote the compact parts.

1) Slow heating makes the surface diffusion dominate in the low temperature sintering stage, but it is difficult to densify the billet while consuming the sintering driving force.

2) Rapid heating to a certain temperature range, in which the volume diffusion becomes active, and rapid heating can control the grain growth, while the pores are also evolving and shrinking.

◆ Liquid phase is produced in sintering to contribute to the densification of billets.

1) The liquid phase increases the material transfer rate, resulting in faster sintering;

2) The liquid phase exerts a capillary force on the particles, which is the same as a large external pressure;

3) The desired liquid phase may be formed by melting one of the components.

◆ Sintering process

1) Initial stage: sintering neck formation and growth;

2) Intermediate stage: the sintering neck grows, forming a grain boundary connected pore network;

3) Final stage: the pore geometry becomes cylindrical, leaving only a few small pores on the grain boundary.