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Metal parts have different characteristics and applications depending on the industry and design. To create metal parts, engineers and designers need to know the materials, manufacturing processes, and use cases of the parts. There are many ways to make metal parts, and each one has its own advantages and disadvantages. This article will give a brief introduction of the main metal part fabrication methods and their pros and cons.
The way metal parts are manufactured can vary greatly. Each process has its own advantages, compatible materials, and trade-offs. A deeper understanding of the range of manufacturing methods will improve this decision-making process. This is an overview of 8 different manufacturing processes that can be used to create custom metal parts.
These are:
CNC milling and CNC turning
Extrusion
Metal Casting
Die Casting
Metal Injection Molding
Forging
Sheet Metal and Stamping
Metal 3D Printing
There are two main types of CNC machining technology. CNC milling machines are automatic cutters. They use a rotating spindle head to cut away unwanted material. The rotation causes the material to rotate against a stationary tool to remove the material into the desired shape. They are suitable for a wide range of materials including plastics, aluminum, stainless steel and titanium.
CNC turning involves the use of CNC lathes or multi-axis CNC turning centers. Manufacturers use CNC lathes to turn materials with cylindrical and concentric features on parts. Power lathes incorporate end mills and drills to produce off-axis features without changing platforms. Specialized CNC lathes, often referred to as Swiss machines, are designed to quickly produce small parts with complex features by combining multiple tools and spindles within the machine.
A typical lathe will rotate the part on a center axis to remove features by engaging rigid cutting tools. Other functions performed by a lathe include creating internal and external threads, creating flange features, O-ring grooves, and knurling.
CNC lathes and CNC mills have different axis configurations. a CNC lathe is typically a 3- or 4-axis machine with a single spindle. 3-axis CNC machining processes material in the X, Y, and Z axes and removes shavings in all three directions. 4-axis tooling combines the concentric features of a lathe with the tilted out-of-plane motion of a mill. Lathes are ideal for round parts and are more intensive for irregular shapes and sharp edges, while milling machines are the opposite.
More modern CNC technology supports up to 5 axes. These tools add tilt and rotate capabilities to the traditional X, Y, and Z axes, allowing for precise cutting of more detailed parts. 5-axis machining is also more efficient because it allows for the creation of more part features in a single operation. Milling machines are available in a variety of configurations, typically utilizing 3-, 4-, and 5-axis.
CNC stands for Computer Numerical Control and operators control CNC mills and lathes using G-code, a programming language that provides instructions for machine motion. Modern programming instructions are created using Computer Aided Manufacturing (CAM) software. This code instructs the machine where to move the cutterhead. It also controls its speed and feed, i.e., rotation, depth of cut, and workpiece movement.The complexity of the G-code depends on the number of axes in the machine and the tool set used.
CNC mills and CNC lathes are suitable for similar materials. These include aluminum, brass, bronze, copper, steel, stainless steel, titanium and zinc alloys. These tools are also suitable for plastics and composites such as acetal, ABS, G-10, and high performance materials such as PEEK or PTFE.
CNC milling machines are a manufacturing workhorse because they are both accurate and repeatable. This means they are ideal for rapid prototyping and small to high volume production. Their versatility with materials makes them ideal for almost any job.
CNC lathes are equally versatile. Production with lathes also requires fewer hands-free operational setups, making them an effective choice for a range of use cases. The shape of the part and specified tolerances usually dictate which of two methods to use - concentric parts with a need for a lathe and corner pieces that require milling.
Aerospace and defense
Automotive
Consumer products
Electronics
Industrial
Medical and Dental
Robotics
Extrusion involves pushing heated metal or plastic through a mold. In practice, it is similar to squeezing a tube of toothpaste. The die creates a net shape, such as a tube, L structure, or more complex feature. Extrusion of metal parts usually requires post-processing, such as cutting, drilling or machining. It is ideally suited to large quantities of parts that require a constant cross-section.
This is because extruded profiles can be almost any shape with a continuous cross-section. A good example is window frames, with multiple functions to hold different panes in the assembly. They can even be hollow, such as square, round or hexagonal tubes. The manufacturer determines their shape when creating the shape of the mold.
The three types of extrusion are hot extrusion, cold extrusion and friction. Hot extrusion involves high temperatures to prevent hardening of the work material. Cold extrusion involves near room temperature, which offers some advantages over hot extrusion. The material may be stronger, less oxidized, or have smaller tolerances. Finally, friction extrusion involves the use of force to push the charge into the die.
Extrusion materials can be plastic or metal. About 80% of extruded metal parts are aluminum alloys. Meanwhile, polyethylene is most common in plastic extrusion.
Extrusion is a simple process compared to other methods. Its tooling costs are 80 to 90 percent lower than injection molding and die casting. Extrusion also provides a smoother surface for paints and finishes. This makes extrusion ideal for precision and decorative parts. Flooring, windows and railings are ideal applications. Automotive and aerospace parts are also ideal for extrusion.
Metal casting is a long-established manufacturing process. It involves pouring liquid metal into a mold. The liquid metal hardens into the desired shape. It is then cooled and removed from the mold.
Modern metal casting is precise, automated and uses advanced tools. However, the principles remain the same. Its widespread use is a testament to the success of this method.
The metal casting process begins with mold making. Before pouring liquid metal, the pattern creates partially shaped voids in the mold. Modern moldmaking methods use precise calculations to achieve the desired shape. This can include scaling to account for shrinkage as well as preparing excess material thickness for later CNC finishing.
In many cases, molds are destroyed during the casting process. This is a planned step in sand casting where the casting formed from sand is broken down to reveal the finished product. New sand casting molds are easily made and sand is often recycled to make new molds.
Metal casting may also involve the use of wax in a process called investment casting. The manufacturer first builds a wax model of the final product. The wax is covered by a ceramic layer before the wax is heated and removed. The ceramic shapes the mold, which is imprinted on the inside with a pattern shaped by the wax.
There are benefits to each of these two techniques. For example, sand casting is a simpler process and is easily repeatable. Investment casting requires more preparation, but may perform better in creating complex parts. Sand casting is usually more costly; investment casting requires more labor to change a given design. Manufacturers must determine the best process based on their budget and labor constraints, as well as the quality of the parts they want.
Metal casting is a versatile process. It supports any metal stock part that can be realized in liquid form. That's why engineers in all industries use a variety of materials in metal casting. Aluminum, magnesium and copper alloys are the most common alloys. But manufacturers also use zinc, steel and other metals.
Almost all commercially produced machinery and equipment today uses some form of casting. In mass production, casting is often superior to CNC machining in terms of cost and throughput. Metal casting enables high-tolerance structural components for a variety of use cases. Washing machines, automobiles, and metal pipes all use metal castings. 4.
Die casting is ideal for large quantities of complex metal parts. Die casting uses steel molds and low melting point metals as materials. Engineers use die casting for complex projects where accuracy, reliability and production level throughput are critical. Die casting uses reusable, rigid tools similar to the injection molding process to give parts a smoother surface finish while keeping costs low for high volume.
In die casting, liquid metal is forced into a mold by high hydraulic or pneumatic pressure. This is different from traditional metal casting where the metal is poured. Manufacturers prefer die casting when creating parts with intricate details. Using pressure creates intricacies more efficiently.
Hot chamber or "gooseneck" die casting is the most popular method. A "gooseneck" describes the shape of the metal feed system that delivers molten metal to the mold. Manufacturers use cold chamber die casting to limit machine corrosion. This method involves pouring molten metal into an injection system. In each of these methods, the process is analogous to plastic injection molding, where the steel tool is closed and the part material flows into the mold cavity in the form of a hardened part. Once hardened, the tool opens and the part is either mechanically dropped or manually removed.
Die casting manufacturers typically focus on individual materials, including aluminum, zinc or magnesium. This is because the raw materials are molten metals that are juxtaposed with specialized die casting machines. Approximately 80% of die castings are made of aluminum. Zinc alloy is another low melting point metal that is also commonly used in die casting.
Die cast parts are used in a wide variety of applications. They are strong and resistant to high temperatures. They also have a smooth or textured surface. Die casting facilitates high production volumes and can often outperform CNC and investment casting. This supports a wide range of paints, plating and finishes. Nevertheless, die casting is ideal for use with high-impact, high-stress equipment where strength is critical.
Injection molding is most commonly used to make plastic parts. However, manufacturers also use injection molding services for metal parts. It is cost-effective even for large projects with high precision. While it is ideal for projects that require small parts, metal injection molding or MIM can be used for parts of any size.
It features extremely high flow rate and mass volume production of metal parts with intricate shapes. The flow rate of MIM is comparable to plastic injection molding, so it can produce parts in any shape that can be achieved by plastic injection molding. However, the size and thickness of MIM parts are limited, which affects the sintering time and increases the costs. The more complex the part, the more suitable it is for MIM process. MIM parts are expensive, but they have high relative density, so they have no pinholes even after polishing, and can be electroplated. MIM parts have a relative density of over 99%, while P/M can only reach 85% to 90%.
Unlike die casting, metal injection molding uses a feed of a polymer-metal mixture in which molten plastic allows the material to flow as it is heated. During this process, the material is also pressurized. A machine injects the liquid material into a mold. The material cools and the part is manufactured in the form of a mold.
After molding, the parts are in a "green state," meaning they are correctly shaped but very fragile. The post-sintering process completely ablates the plastic, leaving only the molten metal behind. During this furnace process, which is usually done in a vacuum furnace, the part shrinks a considerable amount.
MIM is a technology that can be applied to various materials, such as black and ferrous metals (such as stainless steel and high-speed steel), nickel iron, tungsten carbide, fine ceramic and magnetic materials, etc. Metal Injection Molding can be used with metals commonly found in other manufacturing processes. However, the process requires these metals to be powdered and mixed with a polymer for injection. In this way, parts can be quickly molded and produced in large quantities.
The relative density of a material affects its mechanical properties and strength. MIM has high performance in terms of tensile strengths, desirable hardness and elongations compared to other methods. MIM can also reduce the costs of producing metal parts compared to other industries such as: Precision Machining, CNC Machining, Die-Casting, Precision Casting, Screw Machining, Traditional P/M, Stamping or Forging or any other Sintering Components. The cost savings can also be achieved by eliminating Secondary Operations, such as Machining, Sizing or Coining, Oil Impregnation, Tumbling, or other Surface Treatment.
Metal injection molding is similar to the plastic injection molding process used to make parts. However, the high-pressure nature of injection molding adds a key advantage. It is effective for parts with small, intricate details. This is too costly for large-scale standard CNC machining processes. That's why metal injection molding is ideal for the medical, aerospace, automotive and defense industries!
Compared to die casting and other metal fabrication methods, injection molds last longer, allowing for a larger number of parts before replacement or maintenance. MIMs often outperform die casting in mass production or where finer part detail is required. This makes it suitable for large-scale repetitive processes. It also provides manufacturers with greater flexibility in terms of strength and unique characteristics.
Like metal casting, forging has been used for centuries. It is the process of forming metal parts by forced heating and molding. Familiar images of blacksmiths and anvils come to mind. Today, forging is widely used in automated industrial processes.
Modern forging uses high-impact machines to shape metal into the desired result. Forging produces less waste than other methods, which makes it more cost-effective in practical applications.
Forged parts are often stronger than those made by other methods. That's because forging utilizes the natural texture of its material. When molded in a forge, the material doesn't need to be reduced to a liquid, it just needs to be heated to a malleable state.
Stainless steel is one of the most common materials used in forging. Aluminum and bronze are also common forging materials.
Forging is ideal for many industries. Its benefits and limitations make it an ideal process for use with other manufacturing materials. Forged tools, such as hammers or wrenches, are common examples of lifetime parts made using this method. Manufacturers should identify use cases where forging advantages contribute to better business and production results.
Sheet metal fabrication involves cutting parts from sheet metal. The punched sheet can then be processed with brakes and die presses to produce angled bends and shapes to build 3-dimensional structures. Sheet metal services feature stamping to produce these parts quickly. In fact, stamping is faster than any other metalworking process.
A stamping press cuts and bends parts from sheet metal. Workers supply the stamping press with cuts or coils of sheet metal. The machine straightens the metal as it is fed into the press. The strategic application of force allows manufacturers to adjust the shape of the part. Bending applies force at an angle, for example, to create the desired angle in the part. Manufacturers use bending machines in this process, which are available in different sizes and lengths to suit the manufacturer's needs. Sheet metal parts can be welded or riveted to create structural elements. press-fit inserts, such as PEM inserts, allow mating features such as bosses, threads, etc. to be added without the need for custom fabrication.
Sheet metal is typically made of aluminum, copper or steel. Sheet metal is also available in a variety of finishes. This includes anodizing, plating, powder coating and painting.
Stamping makes sheet metal fabrication highly scalable in any industry. It is ideal for high volumes and low unit costs. High-volume functional components such as housings, chassis and brackets are often made from sheet metal.
However, the tooling costs for stamping are usually higher than for other processes. Despite this, manufacturers still make hundreds of millions of parts each year in the appliance, electronics and automotive industries. Sheet metal and stamping are optimal for robotics.
3D printing, also known as additive manufacturing (AM), refers to processes used to create a three-dimensional object in which layers of material are formed under computer control to create an object. Objects can be of almost any shape or geometry and are produced using digital model data from a 3D model or another electronic data source. Thus, unlike material removed from a stock in the conventional CNC machining process, 3D printing or AM builds a three-dimensional object from computer-aided design (CAD) model (STL, STP, STEP, SLDPRT) by successively adding material layer-by-layer.
The term “3D printing” originally referred to a process that deposits a binder material layer-by-layer onto a powder bed with inkjet printer heads. More recently, the term is being used in popular vernacular to encompass a wider variety of additive manufacturing techniques. United States and global technical standards use the official term additive manufacturing for this broader sense. ISO/ASTM52900-15 defines seven categories of AM processes within its meaning: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination and vat photopolymerization.
There are many metal additive manufacturing methods and will be the subject of a separate post.
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