[CNPIM] Innovative metal injection molding process and related mold technology

[CNPIM] Innovative metal injection molding process and related mold technology

The processes listed in this article are not new. Basically, it has very mature applications.

This article refers to the contents of Han Fenglin's books, aiming at systematic inventory for the inquiry and reference of friends in need.

Metal injection molding process is a multidisciplinary technology and one of the advanced precision molding processes for metal parts.

The metal injection molding process has been gradually recognized, accepted and valued by people. In order to achieve more complex parts production requirements, the latest technologies in many fields have been introduced into the MIM industry and have been vigorously innovated. Therefore, new technologies and new processes of metal injection molding process have also been emerging and applied to development and production.

Next, let's make an inventory.

1. Metal micro-injection molding technology（ μ- MIM）

Micro-mechanical or micro-electro-mechanical systems (MEMS) is a new interdisciplinary developed in the late 1980s, and has been recognized as one of the key disciplines in the 21st century.

The practicability of micromechanical or MEMS depends on the progress of micro-machining technology. Metal micro-injection molding technology is the most effective method for mass production of high-precision and high-performance micro-metal or ceramic parts.

Metal micro-injection molding technology refers to a process technology that uses the MIM process to produce metal or ceramic parts with micrometer size or micrometer structure. It generally refers to precision parts with size less than 1mm or local micrometer fine structure.

At present, 25~50 can be prepared with appropriate fine powder μ M thick, local structural details less than 5 μ M. The surface roughness is 2~3 μ M of metal or ceramic parts.

The size of metal injection molding parts has developed to two extremes. Micron-size precision parts have huge market capacity and development potential. The technical added value of these small parts is very high, such as optical fiber metal sleeve, laser catheter, printed circuit micro drill, microelectronic actuator and dental medical parts, and the price is 4000~20000 dollars per kilogram.

Micro injection molding products have broad application prospects in actuators, sensors, pocket consumer goods, weapons, aerospace, electronic assembly tools, oxygen analyzers, filters and medical and health equipment.

The main obstacles restricting the development of micro injection molding technology are the manufacturing of precision micro molds, the filling of narrow gaps and the operation of small parts.

The molds used to produce such high-precision micro-parts are much more precise than conventional molds, and various kinds of cash are needed for fine processing technology, such as photolithography, electroforming, micro-cutting, micro-edm, etc. The above problems can be well solved by using LIGA (German plate making, electroforming and injection molding three abbreviations) and other processes to manufacture plastic lost foam molds.

There are two ways to manufacture plastic lost foam mold by LIGA process:

One process is to mold the PMMA plastic mold core, insert the PMMA plastic mold core into the mold base for direct metal injection molding, the PMMA plastic mold core and the MIM part blank are separated from the mold base as a whole, and the MIM part blank is left in the plastic mold core for direct degreasing and sintering, which becomes a one-step replication process.

The other process is to deposit a layer of metal nickel on the surface of PMMA plastic parts by electroforming process, and then peel PMMA plastic from the nickel shell, and then insert the nickel shell into the metal mold of the mold base process to form the MIM part blank. This becomes a two-step replication process.

The precision of parts formed by one-step replication process is high, and the difficulties of demoulding and subsequent operation of parts are solved, but the cost is high; The precision of parts formed by two-step replication process is reduced, which is suitable for mass production, but there are difficulties in demoulding and subsequent operation of parts.

2. Multi-component material composite injection molding technology

It is difficult for parts made of single chemical composition materials to meet the special requirements of modern manufacturing industry for complex integration of part functions. Different parts of a part are made of different materials, and meeting different functional requirements is a development trend of modern part manufacturing.

[CMPIM] Innovative metal injection molding process and related mold technology Multi-component material composite injection molding technology

The two-color (multi-color) injection molding technology widely used in the plastic industry has been introduced into the field of metal injection molding, making it possible to batch and efficiently treat complex metal or ceramic composites.

The principle of compound injection molding technology is that an injection machine is equipped with two or more sets of barrels at the same time, and the injection materials in each set of barrels are the same. The fixed mold of multi-cavity mold can rotate around the rotating shaft, and different injection materials are injected into different cavities at each position. The initial injection blank is left at the innermost, and the mold is opened after cooling, but it is not immediately demoulded. After the fixed mold is rotated to a certain angle, the fixed mold is closed, and the whole cavity expands outward relative to the first injection blank, and then the second injection molding of different injection materials is carried out. Each part is formed by multiple injection and finally ejected.

The introduction of multi-component composite injection molding technology can meet the requirements of functional and performance integration of single parts, saving valuable raw materials and reducing costs.

Composite technology has broad application prospects in many fields, such as steel-hardened carbide or ceramic cutting tools, precipitation-hardened stainless-steel iron-aluminum alloy nozzle, magnetic and non-magnetic electronic components, and so on.

For the first and second articles, please refer to the more detailed introduction: [Technology] New metal injection molding technology: μ- Introduction to MIM and 2C-MIM process

3. Gas (liquid) assisted molding technology

The working principle of gas (liquid) assisted molding is to inject a certain amount of molten injection material (volume fraction of 50%~80%) into the mold cavity, and then inject pressurized gas or water from the melt to make the product hollow. The molten injection material expands and fully fits with the inner wall of the mold cavity. Since the core of the thicker part of the product is finally solidified, this part is most likely to form hollow..

[CMPIM] Schematic diagram of innovative metal injection molding process and related mold technology gas-assisted molding equipment

Because the volume changes with the pressure, which makes the gas much smaller, the flow of water and the formation of hollow wall thickness are easier to control. With the gas (liquid) body assisted forming process, the design freedom is increased, and the products with large wall thickness differences are easy to form; The injection pressure can be reduced, and the internal pressure distribution of the product is more uniform; The product participates in the reduction of stress, warpage, collapse and surface quality; It can shorten the degreasing time, reduce material consumption and reduce the weight of parts.

The gas (liquid) assisted molding technology has been successfully applied to the fields of golf head, door handle, handicrafts, etc., with remarkable results.

4. Processing and assembly technology of injection blank

Although the strength of the injection blank before degreasing is far lower than that of the sintered metal parts, it still has a certain strength to be processed and trimmed.

The processing technology of adding and subtracting materials can be implemented to change the size and shape of the blank. The injection blank before degreasing can be processed by gate cutting, parting line processing, drilling, chamfering and other material removal.

Because the blank is soft, the wear of the tool is greatly reduced. The strength of the blank is weak and easy to be damaged. It needs high cutting speed and low feed rate to meet the final dimensional machining accuracy.

The traditional assembly process is to connect the sintered parts, and it is also feasible to combine the injection blank parts before degreasing. At present, there are three methods for the assembly process: first, the initial molding blank is used as an insert for the second injection molding; The second is the composite molding of multi-component materials; Third, assemble the single injection blank into a whole before degreasing.

If all blank parts are formed by injection molding of identical injection materials, the matching degrease sintering shrinkage property can ensure their good combination; If each blank is injected by different injection materials, measures must be taken to prevent cracking and deformation.

Using this technology can simplify the mold structure and reduce the mold cost; Parts with more complex shape and difficult to be processed by traditional technology; Forming composite parts with different performance and functional requirements or saving valuable raw materials.

5. Hot runner technology

The hot runner injection mold is a real non-runner solidification injection mold, and the hot runner technology is an advanced technology in the injection process.

Through precise design, manufacturing and control technology, the injection material in the whole flow channel is always kept in a molten state, without producing flow channel condensate, salivation, and overheating, separation or degradation of the injection material.

The hot runner structure is mainly composed of main runner nozzle, runner plate, nozzle, heating and temperature measuring elements, installation and fastening parts.

[CMPIM] Innovative metal injection molding process and related mold technology - hot runner structure

Due to the high technical difficulty, the whole hot runner system is generally designed and manufactured by professional companies. A complete set of complex hot runner mold is jointly designed and manufactured by experienced injection mold enterprises and hot runner equipment companies to ensure smooth injection molding.

The mold structure of the hot runner system is complex and the cost is high, which is suitable for mass continuous production:

-The whole injection process is easier to realize automatic control by using the hot runner system without runner for the process of demoulding;

-There is no mixing of recycled materials in the runner, which improves the stability of the production process and the quality consistency of products produced in large quantities;

-When the pressure loss in the flow channel is reduced, the injection pressure can be reduced, which reduces the tendency of separation and degradation of the injection material, reduces the residual stress of the product, and reduces the deformation;

-The holding time is longer and effective, reducing the shrinkage of the injection part, and the density of each part is more uniform;

-It can manufacture products with larger size, thinner wall thickness, more complex shape and higher precision;

-Combined with the latent gate which can not be used in the MIM mold, the production efficiency can be improved by reducing the processing of the blank gate;

-Energy saving and mass production can reduce costs.

6. Rapid tooling technology

The manufacturing cost of normal production molds is usually high. In many cases, it is necessary to make experimental molds to find problems that may be encountered during the whole process of verification design and production, and the final mold must be modified. In order to adapt to this situation, many rapid or soft mold technologies have emerged to manufacture experimental molds that can meet the trial production of hundreds of parts.

At present, aluminum alloy, particle reinforced epoxy resin, beryllium copper, low carbon steel, stainless steel and cobalt alloy have been used to manufacture soft metal injection mold. Due to its easy forming, zinc, aluminum and bismuth alloys are occasionally used to manufacture test molds and sample prototypes.

However, due to easy scratching and damage, the final production mold will use hard materials.

It is a relatively new mold technology to make MIM plastic injection mold with limited service life based on the process principle of silicone rubber mold. Pour the molten plastic around the cavity of the mother mold. After solidification and hardening, cut the plastic and take out the mother mold. Pressed into the restricted mold base, such plastic mold can be used to withstand hundreds of low-pressure injection tests.

Laser rapid prototyping technology is a very simple method of mold or prototype manufacturing. It uses laser scanning integral accumulation of plastic or metal powder to directly manufacture mold cavity. Another mold manufacturing process of laser rapid prototyping technology is to use the stacked resin or paper model to manufacture the mold cavity by precision casting or electroforming.

The surface of the mould manufactured by these methods is relatively rough and the precision is low, which cannot meet the stringent requirements of the production mould.

The mold cavity or its components used in very large batch production are easy to wear. Rapid tooling technology will be a very effective technological means.

7. Fusible core forming technology

For parts with complex core or special structure that are difficult to be demoulded by conventional methods, fusible core forming technology can solve the molding problem of such parts.

The basic idea of fusible core molding technology is to make the core structure part of the part that is complex or difficult to be demoulded into the insert with resin, paper, low melting point metal and other materials. After the molding and demoulding, the insert remains in the injection blank and does not immediately come out. Then the inserts left in the injection blank are removed before sintering by melting, cracking, solvent dissolution and other methods.

Using this method, it is easy to carry out mass production of parts that are difficult or impossible to be directly manufactured by conventional metal injection molding, such as surrounding inner concave, fine thread and small thread