The metal stamping industry is vast, and the methods are diverse. And so, it’s not surprising how baffling picking one from the other can be. The two primary metal stamping treatment categories, cold stamping, and hot stamping, usually have a thin line setting them apart.
Hot and cold stamping use similar metal forming procedures but vary in the amount the recrystallization temperature used. The recrystallization temperature is usually higher for hot stamping but lower for cold stamping. The heat treatment is typically a result of recrystallization annealing, making the blanks more malleable and soft.
Knowing how these processes work is only a scratch on the surface, and there’s more than meets the eye. This article offers insight into hot and cold stamping and which is the most efficient between the two.
Stamping Vs. Forging: Process Overview
Hot and cold metal stamping are the two broad metal treatment categories typically used in large-scale metal shaping industries. Hot stamping uses heat treatment on metals, raising the temperatures above the melting point. The metal then passes through the stamping dies and gets shaped as the Computer-Aided Designing (CAD) instructs. Therefore, creating the dents and logos with sharp, smoothed-out edges becomes more ameliorated.
On the other hand, cold stamping uses mere pressure to shape metals below the recrystallization temperature. That requires intensive pressure since deforming and shaping metals in their natural state is quite a task. Thicker blanks are instrumental for sturdier metal parts for a more effective process. One of the reasons cold stamping has become sought-after is the process’s uncanny ability to result in anisotropy in the microscopic structure, creating more resistant materials. Besides, that’s essential in minimizing metal mass.
The Cold Stamping Process
Cold stamping is the conventional way of shaping metal blanks into desired shapes and designs. However, instead of heating the metal, it undergoes intensive pressure by hydraulic or mechanical presses. The processes use various blank materials, particularly steel, aluminum, copper, and non-brittle alloys. It’s technically an umbrella term for metal forming processes such as deep and semi-deep drawing and progressive deep stamping. Here are the various types of cold metal stamping.
Deep and Semi-Deep Drawing
Deep and semi-deep drawings refer to the ramming of metal blanks to form cup-shaped stamp parts. The blank usually gets subjected to intensive pressure as the positive die rams a cavity. Deep drawing can also involve punching to produce more complex machine parts, including automotive engine piston cylinders and kitchen appliances. Semi-deep drawing – similar to micro drawing – involves making shallower dents while maintaining the material thickness.
Numerous factors must check off the list for a successful metal drawing operation. Essentially, these factors all play a part in guaranteeing a seamless metal flow during the drawing process. They include the draw radii, die temperature, press speed, and blank thickness. Better yet, it’s still possible to create complex design shapes, thanks to the numerous simulation software available.
Stamping with Progressive Dies
Progressive die stamping metal sheet strips to continuous ramming, shaping, and cutting. The sheet gets dragged inside the machine per ram and pulled along the stations to undergo deformation and cutting. Usually, products are ready for use when they emerge on the other side. However, a few minor finishing touches can come in handy for more accomplished products. Typical metal blanks like aluminum, steel, and copper create a vast array of metal parts for numerous industries in progressive die stamping.
Fine blanking is a high-precision process in cold stamping and ensures that the edges are well smoothed out and are up to scratch. The procedure applies equal pressure to the top and bottom of the blank, ensuring a high accuracy by using V-ring planes, punching, and counterpunching. This cold stamping produces stamps as closely similar to hot stamped products, thanks to their high pinpoint accuracy and incredible precision.
Considerations and Success Factors in Cold Stamping
For cold stamping to be successful, numerous factors usually come into play. And while most factors solely influence the production process, others complement each other and contribute to high-quality production. The considerations and success factors in cold stamping include:
The temperature used must click right during cold stamping. Usually, moving machine parts generate significant chunks of heat, leading to minor dimensional changes. That can affect the general production accuracy due to metal expansions. Besides, dies susceptible to heat changes can expand in marginal amounts, causing inconsistencies. There’s also the idea that heat changes lubricants’ viscosity; hence, it’d be challenging to counter production setbacks such as friction and corrosion of metal parts.
Blank Material Type and Thickness
Cold stamping uses incredible die pressure to shape metal blank sheets, and thinner sheets should never be out of the question unless they’re for shallow drawing. Besides, the material type matters in resisting pressure, withstanding the forces, and of course, persevering the presses. Some alloys can be ideal blanks, but others are too brittle to undergo deformation without heat treatments.
Unlike hot stamping, blank shape matters a lot in cold stamping operations. That influences the material flow, especially during drawing processes. Round-shaped blanks are best for most cold stamping operations since they have a better material flow. That way, it’s more straightforward, preventing imminent rippling, which is usually the case in squared metal blanks. Unless these blanks pass through heating, material flow is generally restricted and might lead to unneeded material deformities.
The Hot Stamping Process
Hot stamping uses recrystallization annealing to produce high-precision metal parts. It involves heating the metal blanks beyond the melting point before being shaped into the desired form using dies. Afterward, these heated and shaped blanks cool rapidly to contain the shapes, edges, and curves. Hot-stamped metal parts are usually resistant to stress and pressure due to their improved structural forms. Hot metal stamping divides into two processes.
Direct Hot Stamping
Direct hot stamping is the standard metal forming process involving preheating metal blanks before quenching in closed dies. The product then gets trimmed by laser and undergoes thorough finishing processes to obtain perfect metal parts. Heating temperatures range between 900 and 760 K, perfect for transcending the recrystallization temperatures. The blank herein achieves a martensitic high-strength structure
Indirect Hot Stamping
In indirect hot stamping, the blank gets pre-cold formed before heating it in the furnace to austenite temperatures. Afterward, it gets transferred for cooling as it undergoes forming and quenching concurrently. This method is ideal for creating complex metal parts than direct hot stamping. It ensures seamless metal flow and increases the tensile strength; hence, a perfect way to solve the cracking and brittle metal issues in direct hot stamping.
When to Use Hot Stamping
Hot stamping products have a height tensile strength and are generally hardened enough to withstand extreme pressure. This property makes them incredibly useful in producing sturdy metal bars for car bodies and structural pillars. Such sturdy metals are impossible to form at standard room temperature, and heat plays a significant role in improving their structural integrity at the atomic level. It mainly uses alloys, particularly steel and low carbon boron alloyed steels, ideal for making white body parts as they meet the stringent property requirements.
These metals are usually low-weight and durable, making them ideal for the aviation industry. During the production and stamping, a sort of “weight compression” occurs. That reduces the overall mass making the metals less heavy and more robust, hence incredible usable in aerospace and combat automotive manufacturing.
Considerations in Metal Hot Stamping
A lot goes behind the scenes to determine whether hot stamping is the ideal alternative for producing ductile and malleable metals with high resistance to shear and tear. Cheaper options like hot metal forging can easily replace hot stamping, which may demand extra effort and costs of laser trimming. Other issues include the production volume, rates, and part function, usually minimal in cold stamping processes.
Besides, the materials available dictate whether or not hot stamping is to be picked instead of the standard cold stamping. This process usually requires sturdier metal alloys, including copper-infused steel alloys that are strong against extreme pressure. That enables the selection of the appropriate grade material to prevent getting into more gripping expenditures since weaker materials can lead to mass losses. However, there’s an exception; cold stamping can still be helpful and can be an ideal alternative if the materials formed are ductile at room temperature. That means they won’t require annealing and can produce accurate parts.
Success Factors in Hot Stamping
Unlike cold stamping, hot stamping is an incredibly selective process only successful under stringent production factors. And while most of them are a precursor for incredible material strength, flexibility, and malleability, it all boils down to the economics of the production process. Here are the essential success factors in cold stamping.
Form and Quench Tooling
Form and quenching tools must be up to scratch for a more successful hot stamping operation. Properly designed tools don’t cave into the repeated thermal cycling and help produce reliable and high-strength stamp products. It ensures that the high strength zones quench quickly and the isolated component zones cool at slowed pace for an ideal microstructure. And to lengthen the tool life, nitriding the surfaces to resist abrasion is a no-brainer.
The incredibly high 960+ K temperatures used in recrystallization annealing make the steel sticky. Besides, friction is a problem, and lubricant is usually a far cry. Material coating to cut the decarburization in blank heating increases friction. Therefore, it’s essential to consider these factors during the tooling design and production.
Hot stamping can be an excruciatingly slow process with more complex procedures. That means it can eat away significant chunks of time, leading to low production rates. To counter that, adding silicon to the alloy compositions allows for higher temperature exits. Consequently, it facilitates quicker multi-forming with trimming and press punching. Quenching and laser trimming drag the time, making the cycle rates longer than they should be.
Material properties are the drivers and motivation for hot stamping processes. Most industries rely on metal parts from hot stamping processes, so their requirements are usually cross-cutting. Technically, these product parts need to be sturdier and lightweight, perfect for the automotive and aviation sectors. Such properties improve the fuel economy and help with crashes and accidents since high-strength materials don’t break quickly on impact. Therefore, the blanks, tooling, and procedures must be up to that standard.
Differences Between Hot and Cold Stamping
Cold stamping and hot stamping operations are almost similar, including the tooling and materials. And although hot stamping blanks are traditionally metal alloys, the process still uses pure forms of blank materials. Here are the differences between the two processes
Hot Stamping Is More Complex
The hot stamping process is incredibly complex than cold stamping. The former usually uses the highly advanced laser trimming technology and recrystallization furnace in addition to the standard metal stamping procedures. It usually requires adequate know-how that isn’t generally necessary in the typical cold stamping process.
Hot Stamping Uses Heat Treatments
High temperatures are conventional to hot stamping and involve soaring furnace temperatures to shape metals. The heated metals usually get shaped while still hot in the direct hot stamping. However, indirect hot stamping pre stamps metals before subjecting them to the high furnace heat. That makes the post-stamping operations possible for high accuracy.
Advantages and Disadvantages of Hot and Cold Stamping
Like every metal forming process, hot and cold stamping operations have pros and cons. These include the following.
Cold stamping Advantages
- Cold stamping requires less complex processes and is quicker than hot stamping.
- It’s an ideal process for drawing metals.
- You can work with a plethora of metal materials compared to hot stamping with more limited options
Cold Stamping Disadvantages
- Cold stamping products can be bulky and heavy for the automotive, aviation, and defense industries, limiting the market.
- Some materials used in cold stamping aren’t as sturdy and may not be preferable in a broad scope of application.
Hot stamping Advantages
- Hot stamping produces more lightweight and high-tensile, shear resistant products, applicable in many products.
- It’s a sure way of guaranteeing high-precision products, although that’s still achievable in fine blanking.
Hot Stamping Disadvantages
- The process is usually complicated and incredibly strenuous. Besides, it’s time-consuming with a longer lead time.
- The process has a limited scope of blank materials, hence too specific.
Hot and cold stamping are closely similar metal forming operations, but the only difference is recrystallization temperature in manufacturing. Both processes have their unique advantages and a fair share of their setbacks. Therefore, the relevance of each of these processes depends on the end users’ needs and requirements, but both metal forming operations serve their industry diligently.