How to grind MIM metal powder injection molded products?

 

Date：[2024/4/9]
 

Grinding refers to the surface treatment, chamfering, deburring, polishing, etc. of metal materials such as iron, stainless steel, copper, aluminum alloy, zinc alloy, magnesium alloy, titanium alloy, etc. Metal injection molding products, due to their process characteristics, basically require the material to go through a grinding process.

Grinding three elements:

Customers often report to us that their ground product parts are black, not shiny, or have been scratched... After investigation, it was found that:

1. They used inappropriate grinding media or grinding machinery, for example, a customer of luggage hardware,

2. His products are all zinc alloy die castings, and he uses brown corundum grinding stones to grind them, resulting in blackening as they are ground; There is also a stainless steel circular gasket

3. Use a vibration grinder to remove burrs. During the grinding process, many gaskets overlap and the defect rate remains high. These phenomena are closely related to their failure to properly handle the three grinding elements. What are the three grinding elements? Only when machines, workpieces, and grinding media are effectively combined can they achieve twice the result with half the effort.

Grinding techniques:

Below is a brief analysis of the three elements:

1、 In surface treatment, commonly used machines include vibration polishing machines, drum polishing machines, centrifugal grinders, and eddy current polishing machines. Among them, vibration polishing machines and drum polishing machines are the most widely used.

The vibration type polishing machine is easy to operate and is used for processing various parts in small, medium, and large batches; A drum type polishing machine is an economical type of grinding and polishing machine. During operation, the parts and grinding medium rotate horizontally in a closed drum at a slower speed. It is commonly used for small-sized and thinner parts, especially those with larger planes. The use of a vibration type polishing machine can easily overlap and is more suitable for drum type polishing machines;

The centrifugal polishing machine is actually a high-speed drum type polishing machine, which is generally driven by a large disk to rotate four (there are also two small machines) centrifugal polishing buckets at high speed, with high friction force and good polishing effect. It is commonly used for small parts that are difficult to process in vibration type polishing machines and drum type polishing machines;

Eddy current machine is also a high-speed polishing machine that forms strong overcurrent friction motion through the rotation of the chassis, used for removing burrs, burrs, and polishing small parts.

2、 Parts with different materials, specifications, and requirements require different machines and grinding media to be matched. For example, zinc alloy parts produced by die casting need to have their oxide scales removed, and resin grinding stones should be used; The stainless steel parts that are stamped need to be sharpened and deburred, so a brown corundum grinding stone with high cutting force should be used

3、 Grinding media includes grinding stones, polishing stones, abrasives, gloss agents, and other grinding and polishing materials, each with its own application range. For example, plastic grinding stones are used for softer materials such as aluminum, zinc, copper, plastics, etc; Ceramic grinding stones are used for harder materials such as iron, stainless steel, white iron, steel, etc; Polishing parts made of iron requires the use of iron gloss agent. Polishing parts made of copper require the use of copper gloss agent. Hand tool cutting agent is used for various types of wrenches, sockets, and nozzles to remove black film and oxide skin. If used for other parts, it may corrode and damage the parts... Overall, only by fully understanding the performance and characteristics of the three grinding elements can we effectively match and achieve twice the result with half the effort.

Grinding method:

1. Fluid polishing

Fluid polishing relies on the high-speed flow of liquid and its carried abrasive particles to flush the surface of the workpiece to achieve the purpose of polishing.
Common methods include abrasive jet machining, liquid jet machining, fluid dynamic grinding, etc. Fluid dynamic grinding is driven by hydraulic pressure, which causes the liquid medium carrying abrasive particles to flow back and forth at high speed through the surface of the workpiece. The medium is mainly made of special compounds (polymer like substances) with good flowability under lower pressure and mixed with abrasives, which can use silicon carbide powder.

2. Mechanical polishing

Mechanical polishing is a polishing method that obtains a smooth surface by removing the protrusions after polishing through cutting and plastic deformation of the material surface. It generally uses oilstone strips, wool wheels, sandpaper, etc., and is mainly operated manually. For special parts such as rotating surfaces, auxiliary tools such as turntables can be used. For high surface quality requirements, ultra precision polishing can be used. Ultra precision polishing is the use of specially designed grinding tools, which press tightly on the surface of the workpiece in a polishing fluid containing abrasives and perform high-speed rotational motion.

By utilizing this technology, Ra0.008 can be achieved μ  The surface roughness of m is the highest among various polishing methods.

Optical lens molds often use this method.

3. Ultrasonic polishing

Put the workpiece into the abrasive suspension and place it together in the ultrasonic field, relying on the oscillation effect of the ultrasonic wave to grind and polish the abrasive on the surface of the workpiece. Ultrasonic machining has low macroscopic force and will not cause deformation of the workpiece, but it is difficult to make and install fixtures. Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution, causing the dissolution products on the surface of the workpiece to detach and the corrosion or electrolyte near the surface to be uniform;

The cavitation effect of ultrasound in liquids can also suppress the corrosion process and facilitate surface gloss.

4. Electrolytic polishing

The basic principle of electrolytic polishing is the same as chemical polishing, which relies on selectively dissolving small protrusions on the surface of the material to make the surface smooth. Compared with chemical polishing, it can eliminate the influence of cathodic reactions and achieve better results. The electrochemical polishing process is divided into two steps:

(1) Macro leveling dissolution products diffuse into the electrolyte, resulting in a decrease in geometric roughness of the material surface, with Ra>1 μ  M.

(2) Low light flat anodic polarization, increased surface brightness, Ra<1 μ  M.

5. Fluid polishing
Fluid polishing relies on the high-speed flow of liquid and its carried abrasive particles to flush the surface of the workpiece to achieve the purpose of polishing.

Common methods include abrasive jet machining, liquid jet machining, fluid dynamic grinding, etc. Fluid dynamic grinding is driven by hydraulic pressure, which causes the liquid medium carrying abrasive particles to flow back and forth at high speed through the surface of the workpiece.

The medium is mainly made of special compounds (polymer like substances) with good flowability under lower pressure and mixed with abrasives, which can use silicon carbide powder.

6. Chemical polishing

Chemical polishing is the process of allowing the surface of a material to preferentially dissolve the concave parts of the surface that protrude in a chemical medium, thereby obtaining a smooth surface. The main advantage of this method is that it does not require complex equipment and can polish workpieces with complex shapes. It can simultaneously polish many workpieces with high efficiency. The core issue of chemical polishing is the preparation of polishing solution. The surface roughness obtained by chemical polishing is generally around 10 μ  M.

7. Magnetic grinding polishing

Magnetic abrasive polishing is the process of using magnetic abrasives to form abrasive brushes under the action of a magnetic field to grind and process workpieces. This method has high processing efficiency, good quality, easy control of processing conditions, and good working conditions. By using appropriate abrasives, the surface roughness can reach Ra0.1 μ  M.

The polishing used in plastic mold processing is very different from the surface polishing required in other industries. Strictly speaking, mold polishing should be referred to as mirror processing. It not only has high requirements for polishing itself, but also has high standards for surface flatness, smoothness, and geometric accuracy. Surface polishing generally only requires obtaining a bright surface.