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 engineerguy.com/preview. Thanks for watching!