How 3D printing work's -Engineering behind 3D printers

3D printers

Even the best artists struggle to show us what real-world objects look like in all their three-dimensional (3D) glory. Most of the time that doesn't matter—looking at a photo or sketch gives us a good-enough idea. But if you're in the business of developing new products and you need to show them off to clients or customers, nothing beats having a prototype: a model you can touch, hold, and feel. Only trouble is, models take ages to make by hand and machines that can make "rapid prototypes" cost a fortune (up to a half million dollars). Hurrah, then for 3D printers, which work a bit like inkjets and build up 3D models layer by layer at up to 10 times the speed and a fifth the cost. How exactly do they work? Let's take a closer look!

Read more about 3d printing at Engineering stuff

From hand-made prototypes to rapid prototyping

Before there were such things as computer-aided design (CAD) and lasers, models and prototypes were laboriously carved from wood or stuck together from little pieces of card or plastic. They could take days or even weeks to make and typically cost a fortune. Getting changes or alterations made was difficult and time-consuming, especially if an outside model-making company was being used, and that could discourage designers from making improvements or taking last-minute comments onboard: "It's too late!"

With the arrival of better technology, an idea called rapid prototyping (RP) grew up during the 1980s as a solution to this problem: it means developing models and prototypes by more automated methods, usually in hours or days rather than the weeks that traditional prototyping used to take. 3D printing is a logical extension of this idea in which product designers make their own rapid prototypes, in hours, using sophisticated machines similar to inkjet printers.

How does a 3D printer work?

Imagine building a conventional wooden prototype of a car. You'd start off with a block of solid wood and carve inward, like a sculptor, gradually revealing the object "hidden" inside. Or if you wanted to make an architect's model of a house, you'd construct it like a real, prefabricated house, probably by cutting miniature replicas of the walls out of card and gluing them together. Now a laser could easily carve wood into shape and it's not beyond the realms of possibility to train a robot to stick cardboard together—but 3D printers don't work in either of these ways!

A typical 3D printer is very much like an inkjet printer operated from a computer. It builds up a 3D model one layer at a time, from the bottom upward, by repeatedly printing over the same area in a method known asfused depositional modeling (FDM). Working entirely automatically, the printer creates a model over a period of hours by turning a 3D CAD drawing into lots of two-dimensional, cross-sectional layers—effectively separate 2D prints that sit one on top of another, but without the paper in between. Instead of using ink, which would never build up to much volume, the printer deposits layers of molten plastic or powder and fuses them together (and to the existing structure) with adhesive or ultraviolet light.

Q: What kind of "ink" does a 3D printer use? A: ABS plastic!

Where an inkjet printer sprays liquid ink and a laser printer uses solid powder, a 3D printer uses neither: you can't build a 3D model by piling up colored water or black dust! What you can model with is plastic. A 3D printer essentially works by extruding molten plastic through a tiny nozzle that it moves around precisely under computer control. It prints one layer, waits for it to dry, and then prints the next layer on top. Depending on the quality of the printer, what you get is either a stunning looking 3D model or a lot of 2D lines of plastic sitting crudely on top of one another—like badly piped cake icing! The plastic from which models are printed is obviously hugely important.

When we talk about plastic, we generally mean "plastics": if you're a diligent recycler, you'll know there are many types of plastic, all of which are different, both chemically (in their molecular makeup) and physically (in the way they behave toward heat, light, and so on). It's hardly surprising that 3D printers use thermoplastics (plastics that melt when you heat them and turn solid when you cool them back down), and typically one called ABS(acrylonitrile butadiene styrene). Perhaps most familiar as the material from which LEGO® bricks are made, ABS is also widely used in car interiors (sometimes in outside parts such as hubcaps too), for making the insides of refrigerators, and in plastic computer parts (it's quite likely the mouse and keyboard you're using right now are made from ABS plastic).

So why is this material used for 3D printing? It's really a composite of a hard, tough plastic (acrylonitrile) with a synthetic rubber (butadiene styrene). It's perfect for 3D printing because it's a solid at room temperatures and melts at a little over 100°C (220°F), which is cool enough to melt inside the printer without too much heat and hot enough that models printed from it won't melt if they're left in the Sun. Once set, it can be sanded smooth or painted; another useful property of ABS is that it's a whiteish-yellow color in its raw form, but pigments (the color chemicals in paint) can be added to make it virtually any color at all. According to the type of printer you're using, you feed it the plastic either in the form of small pellets or filaments (like plastic strings).

You don't necessarily need to print in 3D with plastic: in theory, you can print objects using any molten material that hardens and sets reasonably quickly. In July 2011, researchers at England's Exeter University unveiled a prototype food printer that could print 3D objects using molten chocolate!

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Engineering behind Headphones - How noise cancelling works?

Noise-cancelling headphones

Why do people on trains and buses have to play their music so loud? Look at it from their point of view: they're trying to enjoy a nice bit of Beethoven or Schubert (as if!), but all they can hear is the deafening throb of the engine—so they turn the volume up as far as it will go. But don't worry, because there is a solution to this problem for both of you: noise-cancelling headphones. These amazing gadgets block out the background noise, allowing people to listen to their music without unwanted distractions. Since there is no competition between music and noise, they can set their MP3 players to a much lower volume, which is better for the people around them as well. Let's take a look at how noise-cancelling headphones tell the difference between the sounds you want to hear—and the ones you don't.

If you want to find out about how ordinary headphones work first, take a look at our separate articles on loudspeakers and headphones.

Two kinds of noise reduction

There are two ways to reduce the noise in your headphones, one simple and one complex.

Passive noise reduction (noise isolation)

The simplest kind is called passive noise reduction or noise isolation. The headphones are designed so the earpieces fit snugly into your ears. No sound can escape to bother the people around you and no background noise can get in either. The Etymotic headphones shown in our top picture work this way. They have earbuds with large pieces of soft, viscoelastic foam built around them, much like foam earplugs. You wear them by squeezing the foam so it makes a perfect seal with your ear canal. They also come with plastic reusable earpieces a bit like the ear plugs you can use for swimming.

Active noise reduction

A much more advanced way of getting rid of the noise is called active noise reduction, and it's used in the sophisticated noise-cancelling headphones that pilots use. Headphones like this have a small microphone built into their case. The microphone constantly samples the background noise and feeds it to an electronic circuit inside the headphone case. The circuit inverts (reverses) the noise and plays it into the loudspeaker that covers your ear. The idea is that the noise you would normally hear is canceled out by the inverted noise—so all that's left (and all you hear) is near-silence or the music you want to listen to. Headphones that work in this way include the Bose QuietComfort®, which uses a system called Acoustic Noise Cancelling®.

How active noise reduction works

Suppose you have the noise of a pneumatic drill (jackhammer) driving you mad. You put on your noise-cancelling headphones, switch them on, and the drilling noise virtually disappear. How does that work? We've already seen that the headphones superimpose a reversed version of the drilling noise on top of the original noise, but why doesn't that simply make the noise twice as loud?

Sound is energy traveling through the air in waves. Sound waves don't look like the waves on the sea—indeed, you can't see them at all. If you could see sound traveling, you'd see it squeezing air molecules together in some places and stretching them out in others. In other words, sound travels by making the air pressure change. Now suppose there's a sound wave traveling between a pneumatic drill and your ear. At any given moment, the air between the drill and your ear has areas where the sound is compressed (compressions) and areas where's it's stretched out (rarefactions). Suppose you could exactly reverse the sound made by the drill and superimpose it on top. Now the original compressions would be replaced by rarefactions and vice versa. Two waves that are precisely reversed in this way are said to be in antiphase. Adding an original sound and the same sound in antiphase would, in theory, make the two sounds completely cancel each other out—leaving nothing but silence!

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Engineering behind deodorants cans - How Aerosol cans works?

Aerosol cans

Pssssssssssssst! Everyone loves the convenience of aerosol cans. They make it easy to paint, polish, and lubricate—and lots of other household chemicals, from deodorants and hairsprays to air fresheners and detergents, come in them too. Let's take a quick look at the two main kinds of aerosol dispenser (cans and misters) and find out how they work.

What are aerosols?

Aerosols aren't aerosols at all. No, really, let's be clear about this. An aerosol is really the cloud of liquid and gas that comes out of an aerosol can, not the can itself. In fact, to be strictly correct about it, an aerosol is a fine mist of liquid, or lots of solid particles, widely and evenly dispersed throughout a gas. So clouds, fog, and steam from your kettle are all examples of aerosols, because they're made up of water droplets dispersed through a much bigger volume of air. Smoke is an aerosol too, though unlike those other examples (which are liquids dispersed in gases) it's made up of solid particles of unburned carbon (soot) mixed through a cloud of warm, rising air. Even candles make aerosols: the smoky steam swirling above a candle flame consists of soot and water vapor dispersed through hot air.

How do aerosol cans work?

Now you know what an aerosol is, you can see what an aerosol can is all about: it's a mechanism designed to turn a liquid, such as paint or polish, into a finely dispersed mist. So how does it work?

If you've ever read the back of an aerosol can, you'll have noticed messages such as "pressurized container" and "contents stored under pressure." What's that all about? To ensure that something like paint comes out evenly when you press the button on the top of an aerosol can, the manufacturers have to squeeze the contents inside with a pump or compressor (a bit like inflating a bicycle tire). Typically, the contents of an aerosol are stored at 2–8 times normal atmospheric pressure (and usually the lower end of that range). That's why aerosols really rush out when you press their buttons.

Propellants

Now we can't easily pressurize liquids, so just pumping something like liquid paint into a can isn't going to make an aerosol that works properly. Fortunately, we can pressurize gases very easily. So, in practice, aerosol cans contain two different substances: the liquid product you're interested in releasing (the paint, detergent, hairspray, or whatever it might be) and a pressurized gas called a propellant that helps to push the liquid product into the air and turn it into an aerosol cloud. The propellant gas usually turns into a liquid when it's forced inside the can at high pressure during manufacturing. That makes the propellant and the product mix together (and you can help to ensure they do so by shaking the can before you use it). The propellant turns back to a gas (evaporates) when you push the nozzle and the pressure is released. It disappears into the air leaving behind the product you're really interested in.

Evaporation is the main reason why aerosols feel really cold when you spray them on or near your body. In the case of a typical aerosol (such as a deoderant or perfume mister), the contents aren't gases but (usually) volatile liquids (ones that evaporate at everyday temperatures). When they leave the nozzle, they immediately evaporate. The molecules of liquid have to be ripped apart from one another and separated to turn them into a gas, which takes lots of energy (technically known as the heat of vaporization). This energy is sucked into the liquid/gas from its surroundings, causing them to cool. Evaporative cooling also makes aerosol or mister spray feel cold when you blow it on your skin: your body provides the energy that turns the spray from liquid to gas. The pressure inside the can is reduced slightly each time you spray some contents out of it, which means there's some evaporation and cooling going on inside as well as outside, causing the can to cool down too.

Until the 1980s, chlorofluorocarbons (CFCs) were widely used as the propellants in aerosol cans, but they were banned after scientists discovered conclusively that they damaged Earth's ozone layer. (No wonder, really, when you consider that something like 10 billion aerosol cans are used and thrown away each year.) Now other chemicals are used as propellants instead, including the gases propane and butane. Although these gases don't damage the ozone layer, they do have other drawbacks: they can be harmful to inhale and they are highly flammable.

Valve

An aerosol can would be entirely useless if there weren't some way of allowing its contents to escape in a very controlled way. That job is done is by the valve at the top of the can—just underneath the button you press—which has a spring to stop it staying permanently open. When you force the button down against the pressure of the spring, the valve opens and reduces the pressure at the top of the can, allowing the contents to escape as an aerosol. Release the button and the spring closes the valve again.

How does an aerosol can work?

What happens when you press down on the little button?

1. The button at the top of the can is normally in its "up" position.
2. The exit tube on the side of the valve is safely closed.
3. Just inside the can, the valve is tightly closed.
4. A spring holds the valve tightly in place.
5. The can is perfectly and safely sealed. No product or propellant can escape.
6. Press down on the button and everything changes!
7. Pushing down on the button pushes the valve down too.
8. Under the valve, the spring is tightly compressed. (When you release the button, the spring will expand again and close the valve for you.)
9. The pressurized product and propellant escape through an opening at the top of the valve.
10. As they leave the nozzle, the product and propellant form an aerosol (mist spray).

How are aerosol cans made?

Aerosol cans are made in various ways, normally from metals that can safely contain pressurized liquids and gases. Most everyday, household aerosols are made from a thin sheet of steel coated with another material to stop it rusting or reacting with the product or the propellant. Traditionally the coating is tin, often applied by electroplating, which turns the steel into a material called tinplate. The tinplate is wrapped into a cylinder that has a top and a bottom welded on to ensure the can is completely leakproof. Environmentally friendly cans are increasingly being made with plastic (polymer) linings instead of tin, which is often cheaper and makes them easier to recycle. Other cans are made by pressing a small lump of aluminum through a ring-shaped tool, called a die, so a cylinder forms from a single piece of metal. The product labeling and instructions are placed onto the metal can by offset printing. Some aerosol "cans" are also made from glass—theoretically an ideal material (because it doesn't corrode or otherwise react with the product it's containing), but problematic given that it can break under high pressure or if it's knocked or dropped.

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Engineering behind 3D TV - How 3D phenomenon works?

3D TV

Back in the 1890s, when French cinema pioneers Auguste and Louis Lumière screened their early movie of a train arriving at a station, legend has it that terrified members of the audience dived to one side to get out of its way. Despite all kinds of technological innovations in the decades since then, movies and TV never again seemed quite so convincing. Just over a century later, it seemed affordable three-dimensional television (3D TV) would usher in a new age of super-realistic entertainment, and the big TV manufacturers, such as Sony, Panasonic, LG, and Toshiba, soon started throwing their weight behind 3D technology. Unfortunately, consumers haven't been anything like as keen on watching 3D pictures at home and, for now at least, the revolution seems to have stalled: in 2013, The New York Times officially declared it "an expensive flop"; by 2017, leading makers such as Sony, LG, and Samsung had decided to drop the technology entirely. Despite that, 3D TV is still a fascinating technology, so let's take a closer look at how it works!

How and why do we see in three dimensions?

It's easy to forget you're an animal, pre-programmed to survive as long as you can by millions of years of evolution. Eat or be eaten is the basic law of a world most of us no longer live in (or want to)—but it's the world your body was built for. Some 25–50 percent of your (relatively) huge brain is devoted to processing information sucked in by your eyes and rebuilding it into a colorful three-dimensional "movie theater" that wraps around your head. But your visual system is much more than just a fancy interior designer: it's main purpose is to help you survive. Seeing things in three, rich and deep dimensions is a shortcut to understanding how the world is laid out: how close that 'gator is and whether you can escape it or, in modern terms, how far you need to stretch your hand to lift that cheeseburger to your lips!

Our brains generate a 3D picture largely by having two eyes spaced a short distance apart. Each eye captures a slightly different view of the world in front of it and, by fusing these two images together, our brains generate a single image that has real depth. This trick is called stereopsis (or stereoscopic vision) and it's the seeing equivalent of using two loudspeakers (or a pair of headphones) to hear three-dimensional, stereo sound.

We take seeing in three dimensions for granted, largely because it's almost impossible to view the world any other way—even if you have just one eye. Binocular vision (seeing with two eyes) is only part of how we perceive depth. With one eye closed, you'll find you can still get a pretty good idea of how the world is laid out thanks to many other "depth cues": lines recede into the distance in what we call perspective; closer objects are generally bigger than distant ones and move more rapidly as you move your head (an effect called motion parallax); nearer things have different surface textures than further ones; and so on. That's why even flat, two-dimensional images (photographs and moving images on movie screens and TV) give a reasonable approximation to depth.

But if you really want to see a convincing image of the world, nothing beats putting a slightly different 2D picture in front of each of your eyes to jolt your brain into seeing them as a single, 3D image. And that's exactly the trick the latest 3D televisions are designed to play!

How does a 3D TV work?

There are several different ways of making a 3D TV, but all of them use the same basic principle: they have to produce two separate, moving images and send one of them to the viewer's left eye and the other to the right. To give the proper illusion of 3D, the left eye's image mustn't be seen by the right eye, while the right eye's image mustn't be seen by the left.

With glasses

The simplest way of achieving this is to display two different images on the TV screen (one for the left eye and one for the right) and make viewers wear special glasses so each eye sees only one of them.

The 3D technology most people have seen involves wearing eyeglasses with colored lenses, one red and one cyan, also known as anaglyph glasses. Why these two colors in particular? The red lens is a light filter that allows only red light to pass through, while the cyan lens (cyan is an equal mixture of blue and green) allows through any color of light except red. The point is simply that each eye isn't being allowed to see parts of the image that are being viewed by the other one, so each eye gets a slightly different picture of its own. While simple and inexpensive, this technique produces a relatively poor quality, monochrome picture and often makes viewers feel nauseous.

Photo: You'll need to wear anaglyph (colored) glasses to see images like this in three dimensions.

A much more promising system uses polarizing lenses in the glasses instead and two pictures that are projected from the screen using differently polarized light. Normally, light is made up of waves that vibrate in multiple directions at once as they shoot in straight lines through the air, while polarized (or "plane polarized") light is filtered so its waves vibrate in one direction only. If you fit glasses with two different polarizing filters, so the left lens receives only light vibrating up-and-down while the right lens receives light vibrating side-to-side, you can easily beam a different image from a TV set to each lens. The main drawback is that the TV set has to be fitted with polarizing filters as well, which bumps the cost up quite considerably.

A third option is more ingenious and involves people wearing active-shutter glasses. Battery powered, these have an electronic shuttering system that opens and closes the left and right lenses, in alternation, at very high speed. At a certain moment, the left lens is "open" and viewing the left-eye image on the TV screen while the right lens is blocked. A fraction of a second later, the glasses reverse: the right lens opens, the left lens is blocked, and the TV picture changes so your right eye gets a slightly different image (or "frame") to your left eye before the whole process reverses again. And goes on reversing dozens of times each second. This technology is also called alternate-frame sequencing.

Although the "shutters" work like little blinds that open and close in front of your eyes, they're actually optical rather than mechanical: each lens of the glasses is fitted with a liquid crystal display that instantly turns transparent (clear) or opaque (dark) when it receives an electronic signal. The glasses are linked by infrared, radio wave, or Bluetooth to the TV set so they synchronize precisely with the rapidly changing pictures on the screen. Active shutter glasses like this are more expensive than passive ones (with colored or polarizing lenses), but give a better 3D image and can be worn for far longer without tiring people's eyes or making them feel sick. But the shutter system causes a significant loss of brightness and the flickering can be noticeable under certain conditions.

Without glasses

All three types of glasses can be a nuisance—especially if you wear glasses already—and that's why some people think TV manufacturers will need to be more ingenious and develop 3D technologies that don't need them. One option might be to use something like holography, but that would probably mean redesigning TV cameras so they use complex laser mechanisms that can capture depth, as well as patterns of light and color—and who knows how many decades that could take?

Another possibility is to use lenticular TV screens, which work like those ridged plastic, lenticular printed book and magazine covers you see that show slightly different images as you tilt them back and forth. Lenticles are simply rows of very thin, parallel plastic lenses that bend images either to the left or the right. Place them in front of a TV screen and you could, relatively easily, send two slightly different pictures to a person's left and right eyes so they see a single fused image in three dimensions. Although some TV manufacturers have successfully developed lenticular 3D TV, it does have a big drawback: you have to be sitting in exactly the right place, at exactly the right distance from the screen, so your two eyes receive their intended images in just the right way. Move too far to the side, the illusion will collapse, and you'll probably just see a messy blur. While it doesn't sound that practical for family viewing, lenticular 3D could work extremely well with laptop computers (generally viewed by one person at a very precise position and distance) and handheld DVD players.

Why hasn't 3D TV caught on?

If manufacturers and program makers are going to have any incentive to make 3D TV sets and programs to show on them, consumers have to get behind the technology too—and, so far, that's not happening. Many people simply don't like the 3D experience or find they can't watch it for long. And there's just not enough compelling 3D content (TV programs and movies) to persuade people the investment is worth it. That leaves 3D TV technology in something of a Catch-22 limbo: it's not worth developing programs if people won't buy the sets; and it's not worth buying a set if there aren't any programs to watch on it. For now, at least, it seems unlikely that 3D TV will graduate beyond short-lived novelty—but it was an interesting experiment, while it lasted.TV makers would love us to think we can instantly get 3D versions of all our favorite movies and shows just by shelling out for a more sophisticated box of electronics—but, of course, it's not quite that simple. To start with, the world has about a century of 2D movies and TV programs; any new 3D equipment that's developed for the foreseeable future will need to be able to play all this stuff as well. More seriously, imagine the extra cost of producing 3D programs and (worse still) outside broadcasts. You want to watch superbowl in 3D? Fine! But that means you'll need at least two cameras (and maybe two trained operators) on the pitch for every one that's there today. So, potentially at least, 3D TV programs could be much more expensive to make.

A summary of how 3D TV works

Here's a quick summary of the four most common 3D TV technologies. In these diagrams, we're looking down on a person's head from above and comparing how two different images enter their two eyes in each case:

• Anaglyph: You have to wear eye glasses with colored lenses so your brain can fuse together the partly overlapping red and cyan pictures on the screen.
• Polarizing: You wear lenses that filter light waves in different ways so each eye sees a different picture.
• Active-shutter: The left and right lenses of your glasses are fitted with liquid crystals that effectively "open" and "close" at high speed, in rapid alternation, so your two eyes view separate images (frames) shown on the same screen.
• Lenticular: You don't need glasses with this system. Instead, a row of plastic lenses in front of the screen bends slightly different, side-by-side images so they travel to your left and right eyes. You must sit in the right place to see a 3D image.

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Engineering behind Electroplating

Electroplating

There's no such thing as alchemy—magically changing common chemical elements into rare and valuable ones—but electroplatingis possibly the next best thing. The idea is to use electricity to coat a relatively mundane metal, such as copper, with a thin layer of another, more precious metal, such as gold or silver. Electroplating has lots of other uses, besides making cheap metals look expensive. We can use it to make things rust-resistant, for example, to produce a variety of useful alloys like brass and bronze, and even to make plastic look like metal. How does this amazing process work? Let's take a closer look!

What is electroplating?

Electroplating involves passing an electric current through a solution called an electrolyte. This is done by dipping two terminals called electrodes into the electrolyte and connecting them into a circuit with a battery or other power supply. The electrodes and electrolyte are made from carefully chosen elements or compounds. When the electricity flows through the circuit they make, the electrolyte splits up and some of the metal atoms it contains are deposited in a thin layer on top of one of the electrodes—it becomes electroplated. All kinds of metals can be plated in this way, including gold, silver, tin, zinc, copper, cadmium, chromium, nickel, platinum, and lead.

Electroplating is very similar to electrolysis (using electricity to split up a chemical solution), which is the reverse of the process by which batteries produce electric currents. All these things are examples of electrochemistry: chemical reactions caused by or producing electricity that give scientifically or industrially useful end-products.

How does electroplating work?

First, you have to choose the right electrodes and electrolyte by figuring out the chemical reaction or reactions you want to happen when the electric current is switched on. The metal atoms that plate your object come from out of the electrolyte, so if you want to copper plate something you need an electrolyte made from a solution of a copper salt, while for gold plating you need a gold-based electrolyte—and so on.

Next, you have to ensure the electrode you want to plate is completely clean. Otherwise, when metal atoms from the electrolyte are deposited onto it, they won't form a good bond and they may simply rub off again. Generally, cleaning is done by dipping the electrode into a strong acid or alkaline solution or by (briefly) connecting the electroplating circuit in reverse. If the electrode is really clean, atoms from the plating metal bond to it effectively by joining very strongly onto the outside edges of its crystalline structure.

Now we're ready for the main part of electroplating. We need two electrodes made from different conducting materials, an electrolyte, and an electricity supply. Generally, one of the electrodes is made from the metal we're trying to plate and the electrolyte is a solution of a salt of the same metal. So, for example, if we're copper plating some brass, we need a copper electrode, a brass electrode, and a solution of a copper-based compound such as copper sulfate solution. Metals such as gold and silver don't easily dissolve so have to be made into solutions using strong and dangerously unpleasant cyanide-based chemicals. The electrode that will be plated is generally made from a cheaper metal or a nonmetal coated with a conducting material such as graphite. Either way, it has to conduct electricity or no electric current will flow and no plating will occur.

We dip the two electrodes into the solution and connect them up into a circuit so the copper becomes the positive electrode (or anode) and the brass becomes the negative electrode (or cathode). When we switch on the power, the copper sulfate solution splits into ions (atoms with too few or too many electrons). Copper ions (which are positively charged) are attracted to the negatively charged brass electrode and slowly deposit on it—producing a thin later of copper plate. Meanwhile, sulfate ions (which are negatively charged) arrive at the positively charged copper anode, releasing electrons that move through the battery toward the negative, brass electrode.

It takes time for electroplated atoms to build up on the surface of the negative electrode. How long exactly depends on the strength of the electric current you use and the concentration of the electrolyte. Increasing either of these increases the speed at which ions and electrons move through the circuit and the speed of the plating process. As long as ions and electrons keep moving, current keeps flowing and the plating process continues.

Electroplating plastics

Inexpensive, easy to form into different shapes, lightweight, and disposable, plastics rapidly became the world's most commonplace and flexible materials in the 20th century. But, to many people, that's as much of a drawback as a benefit: plastics are cheap and cheerful—and that's exactly what they look like. One solution is to coat a cheap plastic with a thin layer of metal to give it all the benefits of plastic with the attractive, shiny finish of metal. Many different plastics can be plated this way, including ABS, phenolic plastics, urea-formaldehyde, nylon, and polycarbonate. You'll often find parts on cars, plumbing, household, and electrical fittings that look metallic but are, in fact, plated plastic. They're lighter, cheaper, rustproof, and don't require any polishing after plating.

Why use electroplating?

If you know anything about plastic, you'll spot the obvious problem straightaway: plastics generally don't conduct electricity. In theory, that should completely rule out electroplating; in practice, it simply means we have to give our plastic an extra treatment to make it electrically conducting before we start. There are several different steps involved. First, the plastic has to be scrupulously cleaned to remove things like dust, dirt, grease, and surface marks. Next, it's etched with acid and treated with a catalyst (a chemical reaction accelerator) to make sure that a coating will stick to its surface. Then it's dipped in a bath of copper or nickel (copper is more common) to give it a very thin coating of electrically conducting metal (less than a micron, 1μm, or one thousandth of a millimeter thick). Once that's done, it can be electroplated just like a metal. Depending on how much wear and tear the plated part has to withstand, the coating can be anything from about 10–30 microns thick.

Electroplating is generally done for two quite different reasons. Metals such as gold and silver are plated for decoration: it's cheaper to have gold- or silver-plated jewelry than solid items made from these heavy, expensive, precious substances. Metals such as tin and zinc (which aren't especially attractive to look at) are plated to give them a protective outer later. For example, food containers are often tin plated to make them resistant to corrosion, while many everyday items made from iron are plated with zinc (in a process called galvanization) for the same reason. Some forms of electroplating are both protective and decorative. Car fenders and "trim," for example, were once widely made from tough steel plated with chromium to make them both attractively shiny and rust-resistant (inexpensive and naturally rustproof plastics are now more likely to be used on cars instead). Alloys such as brass and bronze can be plated too, by arranging for the electrolyte to contain salts of all the metals that need to be present in the alloy. Electroplating is also used for making duplicates of printing plates in a process called electrotyping and for electroforming (an alternative to casting objects from molten metals).

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