The advent of robots carried forward a legacy that originated with the Industrial Revolution. Since the introduction of the first industrial robot in the 1950s, robotics has undergone several decades of evolution. Since then, these conventional machines have permeated almost every industry, with the innovation of soft robots changing the game.
Soft robots are composed of compatible materials and focus on technologies that more closely resemble the physical characteristics of living organisms. Machines are transforming into humanoids capable of performing human-like tasks. However, the design of each robot ultimately depends on its purpose.
Robotics has become a truly multidisciplinary field that involves an intricate fusion of components, which form a functional entity. Among the key considerations challenging developers is how to create a reliable and efficient robot that can do more. The choice of construction materials when creating a robot is a fundamental factor, having a profound influence on the overall durability of the final product.
As each robot has a unique purpose, design and working environment, different types and materials are used in its development. Durability and precision are essential. For example, an industrial robot must navigate and process materials without damage or failure. A rover must maintain efficient mobility while enduring harsh climates.
Humanoid and bioinspired robots must maneuver without falling, dropping objects or colliding with other people. In biomedical robotics, movement must be precise without risking the subject. There is simply no room for error.
The construction materials used to construct each robot are selected according to its intended purpose and specific design requirements. Each material has unique physical, chemical and mechanical properties.
In this article, we will cover some building materials used to make robots, including conventional and soft robots.
Metals
The first robots presented to the world were industrial robots and metals were the ideal material to build these machines. Although not all metals are suitable. Some are more affordable, with ideal mechanical and chemical properties for the job.
Steel is the most common and least expensive material for building robots. It is readily available, strong yet lightweight, and easy to design. Steel properties can be customized through various processes to meet the requirements of a specific robot. For example, unhardened carbon steel can withstand pressure of 30,000 to 50,000 PSI. It can be hardened to 100,000 PSI for structural purposes and up to 300,000 PSI for toolmaking.
Steel can also be shaped in a variety of ways, typically through heat treatment hardening. With a melting point of 1400˚C, a robot's steel body can sustain most environments. Steel sheets can be easily cut, bent, or joined together by welding or low-temperature processes such as brazing.
Steel is sustainable and is the most recycled material on the planet.
Aluminum is another popular metal used in building robots. It's as strong as steel, but three times lighter. Just like steel, it is rustproof and non-magnetic. It corrodes in damp or humid conditions, but can be easily protected by an aluminum oxide coating.
Aluminum alloys are commonly used for robots. Aluminum can withstand 10,000~40,000 PSI and melt at a temperature of 600~660˚C. Compared to steel, it is much easier to machine aluminum sheets. Although it is not possible to weld aluminum, special techniques can be used to weld aluminum parts.
Often, aluminum parts are mounted using threaded fasteners. One disadvantage of using aluminum as a building material for robots is its cost – and the cost typically increases as the size of the part decreases.
Copper, brass, and bronze are metals used for niche applications in robotics. Often its alloys are used for structural purposes. Alloys of these metals offer a resistance of 25,000 to 60,000 PSI. Bronze and brass have a melting temperature of 900˚C and copper has a melting temperature of 1080˚C.
Like steel and aluminum, these metals are corrosion-resistant and non-magnetic. It is also easy to cut and shape these metals. Brass is often used for bearings, while copper is used when conductivity is required. Just like aluminum, it is not possible to weld copper, brass or bronze alloys, so they are assembled with threaded fasteners.
Stainless steel (SS) is the structural material of choice for robots when corrosion resistance is essential. SS has the same strength and temperature resistance as carbon steel. Stainless steel is made up of steel produced by adding a high percentage of nickel and chromium and elements such as vanadium or molybdenum. For this reason, the magnetic properties of SS can change during bending. For example, a non-magnetic SS can turn into a magnetic one when bent or deformed.
Compared with other materials, it is more challenging to machine SS for robotic parts. It requires special welding techniques to join the pieces together, and the sheets are subject to breaking or chipping during cutting.
Titanium is used to build biomedical robots as it is a bioinert material. Titanium alloys offer strength of up to 150,000 PSI, with a melting temperature of 1,670˚C. As it is a lightweight, corrosion-resistant and high-strength material, it is also used in various aerospace applications. A disadvantage of titanium alloys is their flammable nature. Titanium is also expensive and quite difficult to machine.
Zinc, lead and magnesium are other metals used to build robotic parts. Zinc is often used as a structural material in recreational robots. Zinc has low temperature resistance and low mechanical strength, despite being as heavy as steel. But it is cheap, so it is often used to build low-end or low-cost robotic toys and robots. Magnesium offers low corrosion resistance and is expensive, but it is sometimes used in robots as a structural material when a lightweight design is essential.
Plastics
Although plastics are no match for metals in terms of strength and toughness, they offer unique characteristics as a construction material for robots. Most plastics can only withstand 3,000~12,000 PSI and a temperature of up to 100˚C. However, they are lightweight, flexible, corrosion resistant, waterproof, and chemically unreactive to most acids and bases.
They are also easy to mold, shape and machine, enabling the production of uniquely shaped parts. For example, certain plastics can ensure better flexibility for maneuverability in humanoid and bio-inspired robots. Transparency is another unique property offered only by plastics.
Plastics are more likely to be used in indoor robots that require little heat exposure or mechanically intensive tasks. This is because most plastic materials are prone to photodegradation by UV rays and have low heat resistance. Mostly polymers are used, but sometimes monomers.
Let's review some of the most common plastics used as structural materials in robots.
Polycarbonate is the most common plastic used in building robots. It is practically indestructible, with a power of 10,000 PSI. It can be fully (100%) deformed without breaking. Polycarbonate is well known for its use in making bulletproof windows. It is non-reactive to most acids, bases and oils. It is also easy to machine with few adjustments. Robot parts constructed from polycarbonate are often held together by glue.
ABS (Acrylonitrile-Butadiene-Styrene) plastic is known for being rigid but not brittle. ABS can withstand pressures of up to 5,000 PSI and tolerate temperatures from -40˚ to 80˚C. It can be deformed by up to 20% before breaking. The copolymer offers improved characteristics of three monomers combined.
PVC (Polyvinyl Chloride) is similar to ABS plastic in properties. It is strong, rigid and can withstand up to 7,000 PSI. It can also be deformed by 10 to 20% without breaking, making it quite easy to mold or machine into different shapes. PVC is also very durable and impact resistant, but it can become brittle. PVC parts for robots can be assembled easily with glue or fasteners. This plastic is typically used in plumbing (and now robots) because it is durable and economical.
Nylon is a synthetic polyamide plastic. Typically used as fabric, solid nylon is useful for robotic structures and is often used as a material for gears. Nylon has high tensile strength and can withstand pressures of up to 12,000 PSI. It is flexible, but may break if subjected to repeated stress. The main advantage of nylon is its low cost, which makes it a good alternative to metals such as aluminum. Unlike other plastics, it is not possible to glue nylon parts together; they are often assembled using threaded fasteners.
Composites
A composite is a material that contains two or more simpler substances with different properties. When combined as a composite, the materials exhibit properties not available individually. Several materials fall into this category. Some of the composites commonly used for structural purposes in robotics include carbon fiber, Kevlar, wood, fiberglass, and glass-filled plastics.
Wood
Wood may seem like a strange material for robotic structures, but several species of wood are used in engineering applications. Wood is more robust than plastic. Many types of wood have a stiffness-to-weight ratio comparable to steel and aluminum. Wood is tough, strong, a good insulator and heat resistant, making it an ideal choice for some robotic applications. One disadvantage is its fragility. Any force applied perpendicular to its grain can break it.
Carbon fiber
Carbon fiber refers to various carbon composites generally formed by mixing carbon atoms in plastic resins. This low-cost material is highly durable, exhibiting a tensile strength of 300,000~600,000 PSI. Its temperature resistance depends on the plastic resins used to form the carbon fiber.
Most composites with standard epoxy resins can withstand temperatures of 150˚ to 200˚C. Carbon fiber composites with phenolic or polyamide resins can withstand temperatures above 300˚C. In low oxygen environments, this material can tolerate temperatures as high as 400˚ to 500˚C.
Although carbon fiber composites can easily handle rapid temperature spikes, they can degrade with continued exposure to heat. Overall, this material is light, strong, rigid, heat resistant and chemically non-reactive. The only downside is the dangerous manufacturing and handling of carbon fiber. It can be extremely dangerous to machine carbon fiber without proper protective equipment.
Ceramics
Ceramics are rarely the ideal material for robotic structures due to their brittle nature. However, some high-tech ceramics are more resistant and less brittle than conventional ceramics. Although these high-tech ceramics are not as strong and tough as metals, they are highly resistant to wear and tear and are decent insulators.
Elastomers
Elastomers are rubber materials that recover their shape after deformation. Two types are used in robotics: polymeric rubber with a carbon structure and polymeric rubber with a silicon structure. The metal parts of robots are typically unable to perform tasks flexibly. Incorporating rubber into a robot's joints and other moving parts makes them less rigid and more flexible. Rubber also adds texture to robotic arms.
Rubber parts are often used in cobots that share a workspace with humans. Rubber parts are also commonly used in pick-and-place robots. A disadvantage of using rubber in robotic structures is its low heat resistance. This is why, in many circumstances, a robot's exposed rubber parts are protected by a protective covering.
Biodegradable plastics
R&D in bioplastics is still at an early stage. It is a positive initiative in the development of environmentally friendly robots. Biodegradable plastics are produced from food waste. As biodegradable materials, they degrade under certain circumstances. These materials are typically used to build robots that require no further use after completing a task and often disintegrate independently without external effort. But bioplastics are also used as a material in robotic skins, and some are tough enough to make internal parts.
Building Materials for Lightweight Robots
Soft robots are often designed with smart, multifunctional materials. These include bioplastics, fabrics, gels, elastomers, natural rubber, synthetic rubber, silicones and soft composites. Special techniques such as additive manufacturing or 3D printing, 4D printing and cast molding manufacture soft robots.
Conclusion
Each robot is unique in terms of purpose, expected tasks, work environment and design. This is why so many different materials are used to build them. There are many circumstances in which more than one material may be suitable. However, materials should be chosen solely based on the requirements and design purpose of the robot.
Most industrial robots today are constructed from metals and metal alloys. Plastics are an option for low-cost, indoor, light-duty robots. Composites such as carbon fiber and Kevlar, as well as ceramics, bioplastics and rubber are typically used for specific parts or unique robotic structures.
