by Darren Pierce : Densha Blues : 01.27.06 7:48am EST


I've put words about plastic on the Internet in the past, but times have changed, and so has plastic, and I think the material needs an update. Because I work quite a bit with the plastics industry, writing software that controls the manufacturing process, I've come to know something about simple consumer applications of plastic (that means toys), and I tend to get asked a basic set of questions quite a bit. Hopefully this knowledge will come to some benefit.

What is Plastic

The word "plastic" essentially means "pliant". It refers to things that can be shaped or formed. But when we refer to plastics in the materials sciences, we're talking about a very specific thing. We're talking about a type of product that manufacturing companies use to create a great many of the things we see around us, and that is the subject of this article.

A plastic is composed of two major components: a polymer and a set of additives. The polymer is the main ingredient, providing the bulk of the final product. The names of common polymers are probably what you associate most closely with plastics. Some polymers you might've heard of are polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC). One or more additives join the polymer. Additives sound trivial, but they are equally important, giving plastic a wide range of characteristics that make it useful in daily life.

I'll break down both polymers and additives, and explain the role of each.


There is an enormous set of science and research behind polymers, but a basic overview will give you everything you need to know to understand your toys a whole lot better. The word "polymer" means "many parts", and is used to refer to any large molecule that is a repeating chain of the same elements. As you can imagine, that covers quite a bit of ground, as the world is filled with molecules that meet that description.

Your DNA is a polymer of nucleotides. Proteins, which are the building blocks of your entire body, are polymers of simple amino acids. Other examples of common polymers in nature are latex and cellulose. But these are all so-called "natural" polymers. I'm using them to make the point that just because it carries the name "polymer" it doesn't mean it's "plastic", and it doesn't mean it looks or acts anything like the toys on your shelf. The term polymer covers a very wide range of things indeed.

Synthetic Polymers

Since the topic is toys, of more interest to us here are "synthetic" polymers, that is, polymers strung together in a lab. Synthetic polymers are useful because we can make them in a very controlled and consistent manner with a tailor made set of properties. This is a basic requirement of manufacturing: our materials have to look and act exactly the same way, whether we order them from China or buy them from across the street.

We've mentioned some common synthetic polymers earlier, now let's expand the list. We can string together ethylene and create polyethylene. We can make polypropylene by stringing together propylene. Chains of styrene make polystyrene, etc. Get the pattern?

The reason we have different kinds of synthetic polymers is because they each have very different properties, not the least of which in our real world is cost. Let's look at some of these:

PE - Polyethylene - Good old bottom of the ladder PE has the advantage of being cheap. High density PE (HDPE) is used to make hard things like milk bottles. Low density PE (LDPE) is used to make softer things such as sandwich bags.

PP - Polypropylene - Moving up the price range a little bit, PP is a little more brittle than PE, but stronger and more heat resistant, too. Can easily be used to make stain resistant fibers used in carpet. The heat resistant features make it the most common plastic used in automotive interiors.

PVC - Polyvinyl Chloride - Similar to PE chemically, but with chlorine atoms interlaced in the mix. PVC is stiff and brittle compared to plain PE. When it was widely used to make water pipes replacing copper plumbing, PVC got to be a very common household name. If you mix in the right additives, it becomes soft enough to make shower curtains. PVC is often used as a functional replacement for rubber.

PS - Polystyrene - Moving up the price chart again, PS is considerably stronger than PE, and transparent. If it needs to be thin, rigid and clear, PS is a good choice. Ever gotten a sandwich in a clear plastic shell? Polystyrene can also be puffed to make styrofoam.

ABS - Acrylonitrile Butadiene Styrene - Now we're getting slightly more advanced. ABS is a copolymer, meaning it is composed of more than one sort of repeating sub-molecule. ABS is the next level up in price, but we also take a jump in strength and survivable temperature range. ABS holds great surface detail, which means amongst other things, we can manufacture a glossy or textured matte surface.

PETE - Polyethylene Terephthalic Ester - Can be used to make everything from eyeglass lenses to permanent press pants to two liter Coke bottles. Not found much in our toys.

POM - Polyoxymethylene - Also known as Acetal. Acetal was one of the first high performance engineering plastics developed in the 50's that could be used as a lightweight replacement for metal. It's slightly springy. This is the low end of the expensive range, and you'll only find it in toy parts that have to take a lot of stress for their size.

PA - Polyamide - Also known as nylon. Nylon is very tough stuff. You'll find it in carpet and astroturf, but more indicative of its character is that it's found in gear sets. It's expensive, but this is the high end for consumer goods. Inline skate wheel assemblies often use PA, because it's exactly the kind of thing you don't want unexpectedly breaking.

Why Bother?

All of the above properties of these polymers are useful, but stringing together chemicals in a lab is certainly a complex process, and sounds like an awkward thing one might do simply to make a milk jug. The container sounds more complicated than its contents!

Why bother? If I need a hard shell, certainly I can use steel or stone or glass. Perhaps, but whatever you choose to use as your material for construction, you need to be able to find vast quantities of it and get it to your factory cheaply. You need it to be very consistent, delivery after delivery. It usually needs to be light weight. Most importantly, you need to be able to work with it.

Let's think about a world without plastic. If we didn't have plastic to make half the stuff around us, what would we use? What DID we use? Well, think about toys as an example. Before plastic, there were metal, wooden, and porcelain toys. Metal is heavy, and expensive to get in quantity due its weight. Wood is a very inconsistent material, impossible to mold, and requires lots of trees. Porcelain breaks. Glass is heavy.

Synthetic polymers really revolutionalized the whole world.


Polymers have a key property that makes them super useful. It's something we found useful in metal as well. Many polymers can be molded. If it weren't for this property, it wouldn't be easy for a factory to crank out hundreds of action figures per hour.

Let's talk about ability to mold polymers. There are two big classes of polymers: thermosets and thermoplastics.

A thermoset is a polymer that can be heated until soft, poured into a mold, and allowed to cool. Once cooled, the thermoset will retain its shape permanently. Rubber and Bakelite are thermosets.

A thermoplastic is just like a thermoset but with a special twist. It can be reheated and softened over and over again, each time re-molded, and cooled into shape. This wonderful trait is ultimately why these compounds came to be known generically as "plastics" in the first place. These thermoplastics are the basis of our toys, and are what we will talk about from this point forward.


After selecting a polymer, we need to mix in additives to get the plastic we want. There are wide ranges of additives for different purposes.

Additives are sometimes used for an immediate effect, like coloring. But they're also used to prevent problems down the road. This section is a brief outline of common additives, but to really understand why they're important, don't miss "Deterioration" below.

Some important additives are:

Antimicrobials - Prevents mold (biological mold, not what molds the plastic) from sticking to the plastic.

Antistatics - Prevents static buildup in the plastic, which is hazardous when your plastic is going to be used to make fuel containers.

Antioxidants - Prevents the oxidation cycle from running a wild rampage through your plastic. See "Deterioration" for more information on the important topic of oxidation.

Antislip - Makes the plastic's surface tacky.

Antitack - The opposite of antislip.

Deodorants - Deodorants create charged spaces in the compound that attract and hold onto certain gasses. These can be used in applications such as food storage, where the odor of the food or the plastic can be undesirable. Often in the chemical soup used to make polymers, a certain amount of chemicals aren't fully reacted in the manufacturing process. Those leftovers stick around, and "outgas" from the plastic later on at their leisure. This makes some plastics smell, possibly for years until all the leftovers have vaporized. Do you own any vinyl products?

Flattening Agents - Controls excess gloss.

Flexibilizers - As the name implies, this makes the plastic more flexible.

Flourescent Whiteners - This additive traps UV light and bounces it back as white light. This can cause an already white plastic to "glow", or look "whiter than white". It's good in high visibility applications. This has nothing to do with the yellowing problem. See "Deterioration" for more information on that.

Heat Stabilizers - Heat can negatively affect plastic. A heat stabilizer can make your compound survive higher temperatures longer. See "Deterioration" for more information.

Impact Modifiers - Enhances the plastic's overall toughness, specifically, it's ability to absorb an impact.

Light Stabilizers - Prevents UV light deterioration. See "Deterioration" for more information.

Pigments - Adds color. Without it, you have clear or gray plastic.

Plasticizers - We think about plastic in three typical temperature states. The hottest state is where it freely flows. This is the temperature range we mold in. Next is a semi-solid state. This is the range we use the plastic in normally at room temperature. A plasticizer can keep the plastic soft and flexible at this temperature range. If we go colder, we get to the third state where the plastic is fully solid. This is an undesirable state, because our plastic is brittle in this condition. (You've probably seen a dashboard that's cracked in the winter.) A plasticizer can lower the temperature in which the plastic will enter this third state.

As you can see, not all additives are applicable in all situations. Some of these you'd never use with toys, and some you'll find in all toy plastics.

Plasticizers are especially used quite often. They make the polymer the substance you know and love. Let's consider what the raw polymer would be like without great additives like plasticizers. As a toy fan, you've probably heard of "resin" as a toy ingredient. Resin is the name for the raw polymer soup without any additives. You've probably experienced first hand, or heard through the grape vine, how fragile "resin" is. That's the difference additives make.

The mixture of additives isn't random or done on the fly. As we said before, an important aspect of plastics is their consistency. Once we get a combination of additives that works for a specific application, we hold onto the formula, and we give that formula a name: the masterbatch. A masterbatch is the recipe of additives you intend to add to your polymer.

In summary, polymer plus masterbatch yields plastic. That's what plastic is.

But when do we mix the two together, and who does the mixing? The answers are found in the manufacturing process.

Manufacturing with Plastic

If we take a peek at a typical high-volume manufacturer of plastic products, we'll notice a few large silos outside the factory building. These silos hold plastic. They're delivered to the factory via truck in the form of pea sized pellets, and loaded into the silos via vacuum.

But what exactly are we getting on the truck? It's certainly possible to order very raw bulk polymer (resin), but we typically order a specific material from our vendor that's mixed with the basic additives we need in every production run. For example, "PEHD01-1" from Alastian is a high density PE with impact modifiers and antioxidants added in.

It's important to use materials that are appropriate to the job. Put differently, simply calling for "PE" isn't descriptive enough. It's fine for the recycling information necessary on the back of the toy's package, but isn't useful enough for manufacturing. You want to order good materials from good vendors. You want to order the right materials -- with the right mix of additives, and the right specification for your molding application.

For example, you might sell durable and non-durable goods. Non-durable goods, that is, stuff that's meant to last less than three years, may do just fine with plastic that's not strong on antioxidants. After all, we're not concerned if a shampoo bottle turns yellow or breaks after 10 years. But this material would not be suitable for a computer keyboard. Since thermoplastics may be re-molded over and over again, it's even possible to order cheap waste plastic, which is plastic that's been recycled from previous consumer use. You don't really know what mix of additives you're getting with waste plastic.

Inside the factory, overhead tubes use suction to carry the plastic from the silo to the various molding machines. There are several different ways to mold or form plastic. There are stamping processes used to make things like halloween masks, extrusion processes used to make sheets and tubes, and blow molding and roto molding that can make hollow things. Here, we'll concentrate on injection molding.

Injection Molding

An injection molding machine draws its input material (the "media") from a hopper, where it is fed into a barrel. Inside the barrel is a screw that heats the media using friction and pressure. The media is heated until it's soft enough to flow.

The hopper may include a dosing device that measures in extra additives. Of course, we'd like to buy media that already has the additives we want, and for most applications, this is possible. But we might have custom applications that require a little extra. The most common additive mixed in at this stage is color. It's possible to buy pre-colored media, but if you're working on a large scale production with a central silo, it's more efficient to buy uncolored media, and add the color you need at each machine.

After the media is heated to its flow temperature, it's rammed under pressure into the mold. Next, the media is given a heartbeat or two to cool until it's firm enough to be released. The mold opens, and the final product is dumped into a bin. This is a complete "cycle" of the machine.

The molding machine's cycle is a complex little orchestra that has to be pre-programmed for each job. There are several parts to the dance. The media has to be heated to the correct temperature. If it's too cold, it won't flow properly, if it's too hot, you might damage the media. The right volume of media has to be rammed into the mold, and at the right pressure. Too much pressure can damage the mold, too little won't fill out the cavities of the mold. The media has to be allowed to set in the mold for the right amount of time. Too little, and it'll come out deformed, too much, and you're wasting valuable machine time. The halves of the mold have to be clamped at the right pressure. Too much, and you'll damage the mold, too little, and you'll get too much flash where the halves meet.

All these settings will change based on the size and strength of the mold, and the media that you're using. As you can see, there are quite a few things that can be configured improperly. Older machines were setup using a system of dials or levers. Newer machines accept a floppy disk or centralized programming to avoid human error. The human factor is important because the reality of the situation is that the job of monitoring the machines is fairly unskilled labor. In fact, this is a job that the mentally disabled can be trained to do.

These are all considerations to take into account when choosing a factory. They're certainly not all the same. Do they use good media? Do they have good labor? Do they use good machines? Machine quality is important not only for the sake of your output product, but for the sake of the mold! Your mold is an important asset you send to and entrust to the factory. If it's put in an ancient machine that clamps incorrectly or violently or is set to overpressure, you can ruin the mold. If that happens, your project is out of commission until you affect repairs, and you've missed your deadlines.

The Mold

An injection mold is referred to in the trade as a "tool". I've used the word "mold" thus far, because it's common and well understood in toy circles. In a factory, you'd hear people talking about the "tooling" more than "the mold". A tool is a fairly large, heavy, shining block of metal that looks like something from Cyberdyne Systems. It's kept in a tool room with all its breathren, waiting to be used. A manufacturing company that produces its own stock might have a giant tool room with lots of molding that it owns. A company that does projects for clients might hold onto and maintain its customer's tooling.

In certain parts of the world, tooling is something that's a candidate for theft. This isn't so much of a concern for the toy world, as it would be pretty easy to track down the thief after his products got out, but for generic sorts of product, like washers or bottlecaps, it's a problem.

A tool will generally have more than one "cavity", that is, the hollow spot in the mold where the plastic is poured. It would be inefficient for a large machine with a giant tool to make one tiny bottlecap in each cycle. You might try to get 12 or 24 bottlecap cavities out of a single tool.

Your tool might have some circuitry and coils built into it to keep heat on the more remote parts of the cavitation, so that the flowing media doesn't cool down too much before it fully fills out the tool.

There's a fairly wide range of materials and quality used in mold making. If you want something that will last, you'll spend the money and have your tool made out of hardened stainless steel. Home projects can be done with something as horrible as Bondo, although it may not last 2 cycles. It depends on how long you want the tool to last, and what kind of machine you'll be putting it in. There's no need to spend the money on an outrageous set of tooling for a very short run toy, but common Lego bricks, which will essentially have unlimited production, will use tooling meant to last.

The better the tool is, the more pressure it can handle. The more pressure it can handle, the larger and more complex the cavitation can be, as it takes considerable pressure to get media into every nook and cranny of a large, complex mold. Newer molding techniques require better tooling as well.

Some very modern machines inject media at extreme pressure into the mold, rather than using a screw to heat it for a low pressure ram. The advantages are two fold. First, less energy is used overall as the heating process is a big power expense for the company. Secondly, the media is injected into the tool through a tiny port, which leaves a smaller scar on the final product. Without a doubt this technology requires strong tooling.

When the final product is ejected from the tool into a hopper, box, or bag, it might still be attached to a sprue or "runner". The sprue represents the guide channel that the media took on its way to each cavity. There are techniques that allow sprueless molding, but we don't see that much in the toy world yet. Some nicer machines can automatically cut the parts from the sprue, and eject the sprue and parts to different output bins. Otherwise, it has to be done by hand.

There is no more boring and thankless job than de-sprueing parts. If you think the sprue scars on your toys are ugly, consider the mindset of the possibly impoverished guy who has spent his life de-sprueing toy parts dawn till dusk sitting on a crate in a sweaty factory. It's a more typical scenario than you might realize, and might help you understand why love is seldom put into this chore.

So what happens to the sprue once it's removed? The answer is recycling at its best. Remember one of the best properties of thermoplastic is that it can be remolded over and over. The sprue goes into a big chipper where it's cut up, and dumped right back into the machines. Nothing goes to waste.

Mold Misconceptions

Here are few little nit-picky points that I thought would be worthy of bringing up after hearing years of Internet chatter. Hopefully, it'll lead to a better understanding of the overall process.

A tremendous number of people seem to refer to the toy design as "the mold". Everyone knows exactly what the speaker means, but it does sound funny if you've ever seen a 300 pound mold. If one of these monsters fell on your foot, you'd lose the foot. After putting on a hardhat and steel toed boots to work in a tool room, you'd never misuse the word.

There also seems to be a conception that a toy is associated with a single mold. For many modern toys, that may be the case, but for high production items, it's too limiting. If your part can only be made by one mold, you're bound to one machine. Going back to the common Lego brick example, you can bet that there's more than one mold that can make the basic 2 x 4 brick.

Also on this point, it's important to understand that most toys are actually made using several different molds to create all the parts. It's not a mold, but a "mold set", or more generically "tooling". Find a basic toy nearby and try to figure out how many molds were used to create the toy.

Each set of parts that was molded in a different color used its own tool. Some very advanced machines can mold a single cycle in multiple colors, but this technique isn't that common today. Most injection molders flow media from a single hopper, this means one color is going into the tool.

For this reason, the manner in which the toy's various parts were broken out amongst the tooling puts big restraints on your ability to design a color scheme. Ideally, you'd like the product to be molded in its final colors. If that can't be done, you'll have to settle for some sort of paint process later on in manufacturing, driving up cost.

Each set of materials that was used came from a different tool. For the same reason one tool uses one color, one tool uses one material. So clear parts, PVC parts, and ABS parts all probably use separate tooling.

There's also a misunderstanding about individual parts. "Why do I have to buy a whole new robot to get one with a red gun? Why can't they just make a bunch of red guns?" Remember that the gun is probably a single cavity on a tool with a bunch of other parts. You can't make the gun without making all the other parts. It is possible to gate off the other cavities so that only the gun cavity is poured, but it's just not the kind of thing that's likely to happen unless the tooling is on its last legs anyway.

Finally, there's a notion about the cost of a mold. Two decades ago, the cost of tooling was a company's major capital expense. Nowadays, tooling just isn't that hard on the pocketbook, especially when you consider the size of companies such as Bandai and Takara Tomy.

There are many reasons tooling has gotten cheaper. So called "insert molding" uses tools that have a common framework, but with cavities formed from modular interchangeable inserts. With this technology, you're only paying to have an insert made, rather than an entire tool. CAD has dramatically reduced the labor involved in making the tooling. The global market plays a big role as well. The more people who are into the tooling business, the cheaper getting tooling made becomes.

Imagine the sheer range of new products Bandai puts out every month. Don't just think about Gundam, think about all the many myriad ranges of products that you never hear about. There's girls' stuff, baby stuff, gashapon stuff, school supplies, hobby stuff, bug collecting stuff, R/C stuff, and so on. Every new product uses one or more tools. The cost of any single tool just isn't a big deal for the big guys.

If a big company wants to reissue an old product, they can. Don't get too wrapped up about the status of the old tooling. It's a factor, but it's not the end-all / be-all factor.


As much as we'd like our plastic products to last at least for our lifetime, it often just isn't the case. Plastic can go bad. There are some kinds of deterioration problems toy owners won't much have to face, such as environmental weathering, but a few you may see a lot. I'm going to explore two of the most common.


Oxygen is a thief. It steals electrons from other atoms. It begins a vicious cycle in many things, including plastic, that results in a permanent chemical change. Like most chemical reactions, heat and time assist it. This is thermal oxidation. Moisture assists oxidation as well. The results are that the plastic becomes inflexible and brittle. But the most alarming symptom for toy collectors is a color change, most noticeable in white areas where darkening is more obvious. A substance's chemical nature gives it its color, and when the chemical nature changes, quite often the color changes as well.

What can you do to stop it? You can't retard time, so your best actions are to reduce heat and humidity. Even a reduction of ten degrees of heat can make a big, big difference.

There's another kind of oxidation you'll find more often than thermal oxidation, and that's photo oxidation. Rather than feeding on heat and moisture, photo oxidation feeds on high energy photons, more specifically, UV light.

UV light comes from two big sources in daily life: the sun and florescent lights. This is why we don't put our toys near a window, and we don't keep florescent lights in the toy room. A plain old incandescent light bulb will do. Halogen lights are fine, but they generate a lot of heat -- keep them at a distance.

Photo oxidation is completely preventable by you. Thermal oxidation can be greatly prevented by you. Whether or not sufficient anti-oxidants and stabilizers were put into your toys' plastic at the factory is something you really can't control.


With the vast soup of additives that may be mixed in your plastic, there's a possibility that some of those additives may leach out over time. Plasticizers are especially known to do this. The symptoms often include a waxy or oily film on the outer surface of the toy. It's ok to wash this film off, if you're careful to dry afterwards. If this happens, you should be a little more careful with the part than you are normally, since plasticizer is what keeps (or kept!) the plastic flexible and strong. Anti-oxidants might've been carried away in the leaching, making the toy more susceptible to oxidation as well.

It's important to get rid of the external film because the chemicals in plasticizers are harsh, and can easily "burn" through other petrochemical products, such as some rubbers or styrofoam. It's not uncommon to see cheap PE pack-in parts that have "melted" into their styrofoam sockets.

Like most chemical processes, leaching is assisted by heat. This is another reason to keep your toy room dry and cool.

A Parting Note

A few final words relevant to the situation in the world today. We've talked about how chemicals are turned into polymers, but we didn't mention where those chemicals come from. Most all of the chemicals used in the production of thermoplastics are petrochemicals. That is, they come from crude oil.

In fact, for every one pound of plastic you have in your hand, it took two pounds of crude to make the raw materials. Fortunately, nearly all plastics can be recycled, but the next time you're thinking about oil, think about plastic. Your toys, high tech gadgets, clothing, and fast food containers all benefit from plastic, and thus oil. Of course, the truck that brought the plastic goods to you uses oil as well.

So the next time you think your plastic feels a little oily, you may be right in more ways than one.

4 July 2007 -- Edited for readability