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 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
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 backward because
when the screw pushes forward, a “check ring” is shoved against a “thrust ring”
to block that backward 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 becomes stuck tightly on the core half of the mold. Molds have built-in
ejector pins that push the part of 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 its 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 connects 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 item 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 its 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 that 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. 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. To learn the full story of 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.
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