Placeholder Image

字幕表 動画を再生する

  • 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 plasticscelluloidto 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, calledcolorant,” 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 themost 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 hasflightsthat 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 plasticbetween 60 and 90

  • percentwith the rest from the heater bands. The molten plastic flows past the front

  • of the screw through indentations orflutes.” 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

  • ringis shoved against a “thrust ringto 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 shallowbetween 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 moldthese

  • 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 alignedsometimes

  • they protrude or are indented slightly. So, if you look closely you will see circular

  • ejector pinwitnessmarks 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 spruethat 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 timelike 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 calledrunners.” The runners

  • fan out from the sprue and connect to each cavity in the mold via a smalltypically

  • rectangularentrance 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 cornersthis 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 taperedeven 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

  • usehot 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 bottomthere

  • 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 prizehis celluloid billiard balls didn't bounce quite rightbut

  • 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!

Injection molding is the most common method for mass manufacturing plastic products. Examples

字幕と単語

ワンタップで英和辞典検索 単語をクリックすると、意味が表示されます

B2 中上級

プラスチック射出成形 (Plastic Injection Molding)

  • 96 3
    winnilimor に公開 2021 年 01 月 14 日
動画の中の単語