Plastic Injection Molding

Injection molding is the most common method
for mass manufacturing plastic products. Examples include chairs, toys, cases for consumer electronics,
disposable cutlery, and, my favorite, Lego bricks. Injection molding was invented to
solve a problem for billiards. In the nineteenth century billiard balls were composed of ivory
harvested from the tusks of African elephants. This devastated the elephant population, so
a billiards manufacturer offered a ten-thousand dollar prize for a replacement for ivory.
And this spurred John Wesley Hyatt to develop one of the first plastics — celluloid — to
create billiard balls. He patented an apparatus for molding products plastics from celluloid. This
apparatus was the birth of plastic injection molding.
In principle, injection molding is simple: melt plastic, inject it into a mold, let it
cool and, then, out pops a plastic product. In reality, injection molding is an intricate
and complex process. An injection molding machine has three main parts: the injection
unit, the mold, and the clamp. Plastic pellets in the hopper feed into the barrel of the
injection unit. Inside the barrel, a screw transports the pellets forward. Heater bands
wrapped around the barrel warm up the plastic pellets. As the pellets are moved forward
by the screw, they gradually melt, and are entirely molten by the time they reach the
front of the barrel. Once enough molten plastic is in front of the screw it rams forward like
the plunger of a syringe. In a matter of seconds, the screw injects the molten plastic into
the empty part of the mold called the cavity image. The plastic solidifies in under a minute,
the mold opens and the part is ejected. The mold then closes, and the process repeats.
All injection molded objects start with these plastic pellets, which are a few millimeters
in diameter. They can be mixed with small amounts of a pigment, called “colorant,”
or with up to 15% recycled material, then fed into the injection molding machine.
Before the mid twentieth century injection molding machines used only external heating
of the barrel to melt the plastic before a plunger injected the molten material. But,
because plastic conducts heat poorly, the temperature was uneven in the plunger: either
the middle was too cool and not fully melted or the outer regions were too hot and degraded
the plastic. The solution was this: the reciprocating screw. Often regarded as the “most important
contribution that revolutionized the plastics industry in the twentieth century.”
In the earlier plunger-style machines plastic filled completely the cylindrical barrel,
but as I showed you the plastic was not at a uniform temperature. The reciprocating screw
overcomes this in three ways: First, in modern units, the plastic fills only the space around
the shaft of the screw. This eliminates the cooler central region leaving a thinner, evenly
heated layer of plastic. Second, the screw has “flights” that wrap
around the shaft. As the screw rotates, the flights transport the raw material forward
through the barrel. The flights also serve to mix the plastic. The screw action agitates
the melting pellets within the flights to create a uniform mixture.
And third, the screw action itself heats the plastic throughout. The shaft’s diameter
increases along the screw so that the distance between the wall and the shaft decreases.
The flights, then, squeeze out air as they move the plastic forward and they shear the
pellets and press them against the barrel’s wall. This shearing creates friction and so
heats the plastic throughout. This screw-induced shear supplies a majority of the heat needed
to melt the plastic — between 60 and 90 percent — with the rest from the heater
bands. The molten plastic flows past the front of the screw through indentations or “flutes.”
When there’s enough plastic to fill the mold at the front of the screw, it rams forward
like a plunger injecting the plastic into the mold. The plastic cannot flow backwards because
when the screw pushes forward, a “check ring” is shoved against a “thrust ring”
to block that backwards movement of the molten plastic. This forces the plastic into the
mold. Initially the cavity image is filled with air. As the molten plastic is injected
it forces air out of the mold, which escapes through vents. These vents are channels ground
into the landing surface of the mold. They are very shallow— between five and forty microns
deep. The plastic, which has the consistency of warm honey, is too viscous to flow through
the narrow vents. To speed the plastic’s solidification, coolant, typically water,
flows through channels inside the mold just beneath the surface of the interior. After
the injected part solidifies, the mold opens. As the mold opens the volume increases without
introducing air, which creates tremendous suction that holds the mold together. So at
first the mold slowly opens several millimeters to allow air to rush in and break the vacuum, and
then, the mold quickly opens the rest of the way so the part can be removed. The slow step
is needed to prevent damage to the mold — these precision machines steel molds can cost hundreds
of thousands of dollars. Removing the part from the mold can be difficult. When the plastic
cools, it shrinks and so become stuck tightly on the core half of the mold. Molds have built-in
ejector pins that push the part off the mold. The ends of the pins sit flush with the core
half of the mold, but are not perfectly aligned—sometimes they protrude or are indented slightly. So,
if you look closely you will see circular ejector pin “witness” marks on molded
products. For example, this chair, on it’s bottom, has an array of witness marks.
When the part drops from the mold, an operator has to remove the sprue—that section of plastic that connected the injection unit to the mold. Sprues are manually twisted or
cut off the part. Sprues are attached to objects only in molds that make a single items at
a time — like a chair. Smaller objects are made in multiples in a single mold. In these the sprue connects not to the part itself, but to a network of distribution
tunnels called “runners.” The runners fan out from the sprue and connect to each
cavity in the mold via a small — typically rectangular — entrance called the gate.
You can see the gate on plastic cutlery. The parts for model planes typically come still
attached to their runners. Molds always have at least two parts. And
where the parts of the mold meet is called the parting line. Here on this piece of cutlery
you see the parting line along the side of the fork. When mold halves close they are
never perfectly aligned, nor do they have sharp corners — this creates a noticeable
parting line on the molded object. Another very important aspect of mold design
is the draft angle. If a part has walls that are exactly ninety degrees, it will be very
difficult to eject because it’s inner walls will scrape the core half of the mold. Also,
the vacuum will be difficult to break because air cannot readily enter. However, if the walls
are slightly tapered—even just one or two degrees–-it becomes much easier for the
part to be removed because once the part moves slightly, the walls are no longer in contact
with the core half and air can rush in. One impressive example of injection molding
is the Lego brick. You can see the injection point in the middle of a stud. But this is
not from a gate or a sprue. The Lego molds use “hot runners.” Hot runners are a heated
distribution network. This keeps plastic inside molten, while the plastic in the mold solidifies.
This leaves no gates or sprues to be removed: the molded bricks are ejected ready-to-use.
The downside is that this setup is more expensive than a traditional cold runner system.
On the bottom edges of the brick you can see ejector pin witness marks. And what’s most
clever to me is where Lego designs their draft angle. The outside of a Lego brick must be
square. So, if you cut a Lego brick in half, you can see that these inner supports are
thicker at the top than at the bottom—there is a draft angle of about one-and-a-half degrees.
This helps the ejector pins push the brick off the mold. The core half and the cavity
half of Lego molds are designed so that the parting line is at the bottom edge of the
brick. This hides the parting line. Look around you and see how many injection molded objects
you can find. Likely the device you’re watching this on has injection molded parts! You should
be able to find ejector pin witness marks and parting lines, but you might find something
like this. It’s a date wheel that shows the month and year the item was made. These
are removable inserts and can be changed out for each run of the mold. They are very useful
for tracking down defects. So, to return to where this all started. John
Wesley Hyatt and his injection molded billiard ball did not win the $10,000 prize—his celluloid
billiard balls didn’t bounce quite right—but he did pioneer injection molding, a thriving,
continually evolving manufacturing process which creates many billions of products
every year. I’m Bill Hammack, the engineer guy.
To learn more click on this video overview of injection molding. And this video explains
how the molds are manufactured. Click here to see an injection molding machine produce
plastic bottle caps very rapidly. Finally, this video details the production and automation
of Lego bricks. And to learn the full story of the John Wesley Hyatt’s celluloid billiard
ball listen to the podcast from 99 Percent Invisible, which I’ve linked to in the description
for this video. We’re very grateful for our advanced viewers
who critiqued early versions of this video. Sign up to me an advanced viewer at
Thanks for watching!

100 thoughts on “Plastic Injection Molding”

  1. I used to work for a company that built "Injection Molding Machines", called Engel in Canada, a European based company (that was great to work for) , and the process of building these machines was intricate and varied (depending on size and purpose)
    Sadly, with the slow downs starting in the early 2000's affecting much of the manufacturing sector they had to close many of their plants due to competition and a drop in demand for new equipment.
    I don't know how many divisions still remain, if any, but i hope they are still around, because they were an incredible company to work for, and treated their employees quite well.

  2. Wikipedia says that the $10,000 prize is only $188,000 now–not $3 million.

  3. Bill is such a doucher, just listen to him, he sounds like such a douche. And look at him, he looks like such a douche. And touch him, he feels like such a douche. And lick him, he tastes like such a douche. And watch him eat, he eats like such a douche. And bend him over and insert the part of your body that makes you a male into his butt, he fucks like such a douche.

    Jk, good stuff Bill keep it up. Oh and btw, he fucks like a dream, the inside of this mans backside is as smooth as silk

  4. Molding is boring… I make multiple medical parts including ones that require something being placed into the mold before the shot is made. Almost none have hot runner or tunnel gate's its awful

  5. Being a machine operator in an injection mold factory is the closest to hell I ever hope to come. You sit or stand in front of a hot ass machine for 8 or more hours, every two seconds the door opens up and and a bunch of hot ass parts fall all over and with dykes or a razor you have to trim the shitty parts while they burn your fingers, before you can finish more parts fall out of the machine. Aside from blowing random people in a circle K parking lot for crack money, the worst fucking job of all time. If it comes down to it, just kill yourself and get it over quickly. By the way all the parts you make are killing the world. Seriously, the worst job ever.

  6. This video is near to my heart, it reminds me of my father who was a physicist/plastics researcher and engineer he designed polymers and injection molding processes for American Cyanamid and CYRO industries back in the 70's and 80's.

  7. I came up with a product and have a great name for it that would make me rich if I could mass produce it, and I make it using wood. but I'm sure I could make it out of molded plastic. what is it? can't say, it's a secret.

  8. Well at least you said he was a pioneer of "one of the first". What amuses me is that an Englishman developed Celluloid first and was quite famous for it, but was terrible at marketing it and thus sold it to the aforementioned American.

  9. I have been working in factory that made plastic elements in that way, it cool to see how it works, thanks for video !

  10. After somehow stumbling across plastic injection molding, the Titanic truths and the intricate process of aluminum cans… well I am pretty much an engineer now and have told at least 5 people how many atmospheres an aluminum can can handle.

  11. I find this a bit ironic. So much creativity and engineering behind one of the biggest problems humanity faces: plastic pollution. The plastic industry has made huge money out of excessive plastic consumption and consumerism in general, and they are still able to hire the best engineers and develop the best technologies in order to produce more, make more money, and keep destroying the planet.

  12. My dad was a plastics engineer for General Motors for 20 years. He developed a one shot molded solar collector complete with glass. He used to bring them home from the tool & die shop and spread newspapers all over the living room floor. When the units cured, some popped the glass, most of them at first, until he had the formula just right, there was a lot of tempered glass to pick up. GM cancelled the project after the array in Arizona proved profitable and efficient.

  13. And the molded parts are designed to accomodate the flow of the plastic and cooling of the plastic. The main portion of the part is called the nominal wall. Things which protrude from the nominal wall are called projections, and holes in the nominal wall are called depressions.

  14. Great… an incredibly chain of small changes to get better plastic products… that can last thousands of years… Sadly, this brings the next problem: the world is getting full of plastic… Could it be another billiard ball prize to invent a plastic decomposer?…

  15. Beyond awesome, educational, intuitive learning video! Like my own mechanical, gearhead, brain, I can mentally see how things work. I've always loved step,by, step instruction books, and videos on how things work. Thank you!👍😀

  16. Maybe I miseed it but what is sensing when there is enough plastic to be pushed into the mold? Very nice video, thanks

  17. The Overview video was removed for 'terms of use violation' (how a video on injection molding could manage that is beyond me). Any chance there's an alternative upload to watch?

  18. If you in need of inexpensive and quality mold do not hesitate to contact us at Bison tool and machining. Find us on linked in or

  19. 🤔 How could you be persuaded (😉) to make a video like this about a recommended topic?
    Like Metal casting in general with a zoom on 3d printed parts method? There are tons of videos that show the work being done, but sparsely explain the reasons or measurements for sprue and runners locations and sizes, i figured I'd ask someone who can break down things in informative chunks 😊

  20. Hi everyone hope everything goes well. We are injection molding factory in Xiamen, China. We focus on the production of high quality and low price products hope to get your favor. Email [email protected] please feel free to contact meeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

  21. came here to know more about injection moulding and the detail of the process as I work with design and mech engineers. I knew some of it but this is a very nicely constructed video. Thanks.

  22. Great video, you did forget to mention glass fiber, often added to increase strength. Usually can be found near time/date markings. GF 30% for example and also usually type of plastic is stamped near that as well. Regardless great content

  23. This video is very well done, I have worked in an injection molding company for quite some time as a mold tech meaning I have to know a lot about the process. And he even taught me a good bit. Perfect.

  24. q; did they have blow molding at the time injection molding was invented?
    also, besides getting the plastic temp correct, there is a specific rate to injection that is very important. The rate will control the gas (or air) being expelled by the plastic, usually it starts slow, ramps up then quickly tapers off. If not done correctly the parts will come out under shot and burnt around the edges that get the material last. On older machines this was a very real challenge. Large parts were pretty easy, the small parts are easier now that we can use computers to control the press. Now we have micro-injection machines. They make very tiny plastic parts. Also there a process to inject molten metal.

  25. Great video and explanation of the process – I'm fresh out of school and this video actually helped me with designing one of my first real manufacturing automation fixtures. I was tasked with designing a machine to slice the gate off some plastic injection molded parts (flush within .005") and did not learn enough about it in school. This video helped explain the process and give me some more hindsight – oh and the machine worked out beautifully so thanks again!

  26. “Often regarded as the most important development…. in the plastics industry… in the 20th century.”

    Wanna add anymore qualifiers to that? On a Tuesday? By a left-handed black man? Involving no more than 2 lunch breaks?

  27. Yet, when we run out of crude oil some 25 year in the future max, the plastics party will all be over. Can you imagine? a world with people, all depending on classic, non-synthetic, natural material for everything you can think of?
    There is one little light point though. When all crude has been sucked out of our planet it's also curtains for the whole pharmaceutical industry so starving people don't have to wait that long for their end to come. Isn't that great?

  28. Its just perfect, this guy is going into such a detail in very limited time frame without making a single mistake. There is no room for improvement.

  29. I don't if this is the scope of your channel, but I'd love to see something I've seen written as "learning economy" that arises from tweaking processes like plastic injection – or learning how to add slight changes to the mold that will result in less loss and better products. Thanks!

  30. I wish you were a professor in my college because your explanation is simple,clear and to the point, just the way it should be explained. You helped me for my exam sir. Thank you!

  31. What I was hoping to find is how toy companies make injection molds for action figures and dolls. Machining the mold is one thing, but I've seen toy manufacturers say they start out with a wooden sculpture of the final product, but I never seen how they go from the wooden sculpture to the injection mold. I'm not sure if they just mold the wooden pieces with molten metal then clean out the ash and charcoal afterwards or what, but I assume that's not the case because they have to mold the other side of the piece as well, I don't seem them being very successful with making the back half of the mold when the sculpture is reduced to ash and charcoal.

  32. to be honest, i don't even care for the content. but the way he explain it and those animation makes even stupid people (me) can understand it with just a little effort.

  33. Plastic is the worst thing that has happened to our planet,so now, Engineerguy, design something to rid our planet of plastic waste,just sayin'

  34. I have worked in an automotive injection molding shop for almost 30 years, We supply all oem's, aston martin, and many others. In 9:36 you have provided more basic molding training than my company has done in thirty years, Contact Magna International, you could make a fortune.

  35. Finally~ I'll be able to finish my Lego Mansion. There were a few thousand Legos that I couldn't buy because no body made them. Hope I can move in by December! Thanks gents!

  36. This is great! Cant believe i just found this site. I know what im doing for the next few weeks. Diagrams and explanations are fantastic. Would love to raise my architectural presentations to this level of clarity. Cheers

  37. It's crazy how many people go about their day not knowing how the things around them are made. Once you know the processes behind how things are mass produced, it opens up the possibility that you as a human can create anything. You can revolutionize the world if you invent something and you yourself can figure out how to mass produce it. Knowing how to create and make 3D objects opens up so many possibilities.

Leave a Reply

Your email address will not be published. Required fields are marked *