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 engineerguy.com/preview. Thanks for watching!