Metal Injection Mold

 

 

What Is Metal Injection Molding?

Metal Injection Molding (MIM) stands out as a revolutionary metalworking process that seamlessly integrates the design adaptability of plastic injection molding with the durability and strength of metal materials. By blending finely-powdered metal with a binder material to create a feedstock, MIM enables the efficient production of intricate parts with complex geometries.

Metal injection molding is used in the high-volume fabrication of small, geometrically complex parts. It can precisely produce parts with delicate and intricate details without machining. It can be used with a variety of ferrous and non-ferrous metals. This process is also more economical than forging, casting, and machining processes. The parts produced from the MIM process are used in numerous industries, such as automotive, aerospace, electronics, telecommunications, medical, dental, sporting goods, consumer products, and weapons.

Stages of the MIM Process

 

 

The steps involved in the metal injection molding of metal parts are the following:

Feedstock Mixing

 

Your desired blend of metal powders is combined with binders to create a “feedstock” mix.The feedstock preparation starts with the powdered metal and the suitable binder. The size of powdered metal is 20 microns in diameter, which is finer than those used in conventional powder metallurgy processes. Precise amounts of these materials are combined to achieve a metal-to-binder volume ratio of 60:40. These materials are mixed at an elevated temperature to produce a homogenous feedstock. They are then fed to a granulator and cooled to form the feedstock pellets acceptable by the injection molding machine.

 

Injection Molding

 

Metal injection molding takes place in standard injection molding machines similar to those used in manufacturing plastic parts. The homogenized feedstock pellets are melted and injected into a mold. The melt takes the shape and volume of the mold cavities. The molten feedstock cools and solidifies inside the mold cavities. However, the volume of the mold cavities must be larger than the size of the final product to compensate for the shrinkage during sintering. The products from the injection molding step are called “green parts.” The green parts are 20% larger than the final part, but they are geometrically comparable. The green part is like a “scaled up” version of the final part. The green parts are ejected from the mold after a sufficient dwelling time. The excess materials resulting from the flow of the molten feedstock are finally separated from the part.

 

 Injection molding machine.

The standard injection molding machine consists of the following units:

Clamping Unit

The clamping unit is responsible for generating and applying sufficient clamping force to keep the mold halves closed during the feedstock injection and part dwelling. It houses the ejection system, which removes the green parts after dwelling on the mold cavities. It is also responsible for opening and closing the mold halves between molding cycles and maintaining their proper alignment during operation.

Injection Unit

The injection unit is responsible for heating and injecting the feedstock into the mold cavities. The injection unit consists of the following components:

Hopper

The hopper is where it all begins. This is where the plastic material is poured before it goes into the molds. The hopper has specific features and components within itself to ensure that the material is not contaminated and the molds are top-of-the-line. A drying unit is installed to ensure no water or moisture is added to the material. Several magnets are also installed to prevent unwanted metal shavings from entering the mold.

Barrel

The barrel is the next stop. The barrel is where the plastic material is transported, compacted, melted, agitated, and finally pressed into the injection mold. The temperature must be regulated in the barrel to maintain the appropriate temperature for different materials.

Reciprocating Screw

The reciprocating screw feeds the material from the hopper to the barrel. The turning motion allows flights of material to be added to the barrel. This allows for a more uniform heating process. The reciprocating screw produces most of the heat for melting, and the turning motion pushes flights together, creating friction and allowing the pellets to melt.

Heaters and Nozzle

The heater does just that, heats the barrel to the correct temperature to create a liquefied plastic material inside. There are many types of heaters, all used for different types of materials. The nozzle is the final stop before entering the injection mold. Located at the bottom of the barrel, the nozzle pushes the material into the molds. It can also filter the material used and even shut off the flow if any leaks are detected.

A mold is a tool that gives the feedstock its shape. It is divided into mold halves. The front mold half is stationary and adjacent to the injection unit. The rear mold half is attached to a movable plate that opens and closes the mold and is adjacent to the ejection system.

The mold cavity is the space created when the mold halves are closed. A single mold tool can have multiple mold cavities. The dimensions of the mold cavity give the dimensions of the green part. The injection unit fills the mold with the shot. From the nozzle, the viscous feedstock flows to the sprue, to the runners, and finally to the gates. The gates introduce the feedstock to the mold cavities. After the molding cycle, the sprue, runners, and gates are filled with the solidified feedstock. The excess materials formed due to the feedstock flow in these channels are removed by trimming. Trimming takes place on stand-alone equipment.

Air vents remove the entrapped gasses inside the mold. A cooling system is also present to dissipate heat during cooling and dwelling.

Debinding decomposes and removes most of the organic binders from the green part. The binders degrade the mechanical properties if left on the metal part. The resulting products from this step are called the “brown parts.” The brown part has an interconnected pore structure and is less dense than the final part. However, the geometry obtained from the injection molding step is retained and not altered. The brown part still has some binders that hold the metallic particles together. The porous structure allows the escape of the remaining binders during sintering through evaporation.

 

      Debinding

 

    Thermal Debinding

    Solvent Debinding

    Catalytic Debinding

After debinding, the part is sintered, where the remaining binding agent is removed and the part fuses all metal particles together to meet final density and strength requirements.

Hot isostatic pressing (HIP) is a secondary process after sintering. It is employed to increase the density of the part up to 100% of the theoretical density of the material. It further reduces the porosity of the part and eliminates defects such as internal and external cracks, voids, and pores. It also increases the strength, fatigue resistance, and ductility of the material.

In this process, the sintered part is compressed at high pressure (5,800 to 30,000 MPa) and high temperature (up to 3600°F [2,000°C]) in a gas-tight chamber. The HIP temperature is 70-90% of the solidus temperature of the material. An inert gas, usually argon, compresses the part. The gas pressure acts uniformly on all surfaces of the material. The densification mechanisms during HIP are plastic deformation, creep, and diffusion. Since the gas pressure is higher than the yield strength of the material, the part will undergo plastic deformation, and its internal voids will collapse. Creep and diffusion mechanisms close the remaining pores in the part.

Metal injection molding has applications in a range of industries for different parts. Examples of the uses of metal injection molding are listed below:

Medical Manufacturing

 

Hospital beds, x-ray machines, prosthetics, pacemakers, surgical instruments

Industrial and Consumer Applications

 

Drives, controls, material handling, scales, drill chucks, motors, plumbing components, latches, spraying systems, and hand tools

Aerospace and Defense

 

Weapon systems and misc. control components

Orthodontics

 

brackets and hooks

What Are the Advantages of Metal Injection Molding?

Metal injection molding has many advantages including:

1. There can be a large volume of metal parts produced at one time. Commonly, these have complex geometries and detailing. The result is the production of small, precision parts that have tight tolerances.

2.Few restrictions are placed on the design of the end piece which allows manufacturers the freedom to produce a variety of shapes.

3.High-quality surface finish, with the potential for further enhancements after the initial process.

4.Multi-component parts can be manufactured as a single piece.

5.Components are produced with high-quality mechanical properties in terms of hardness and strength.

6.The production process produces less material waste and scrap when compared to a machining process. Up to 95–98% of the material can be transformed into usable metal parts, making it cheaper when using expensive materials such as superalloys or specialty metals.

7.Cheaper in comparison to investment casing, machining, and stamping over a long period.

8.Used across a wide variety of metals, such as copper alloys, nickel alloys, and iron.