injection molding is the most common methodfor mass manufacturing plastic products. examples include chairs, toys, cases for consumer electronics,disposable cutlery, and, my favorite, lego bricks. injection molding was invented tosolve a problem for billiards. in the nineteenth century billiard balls were composed of ivoryharvested from the tusks of african elephants. this devastated the elephant population, soa billiards manufacturer offered a ten-thousand
types of plastic used in 3d printing, dollar prize for a replacement for ivory.and this spurred john wesley hyatt to develop one of the first plastics — celluloid — tocreate billiard balls. he patented an apparatus for molding products plastics from celluloid. thisapparatus was the birth of plastic injection molding.in principle, injection molding is simple:
melt plastic, inject it into a mold, let itcool and, then, out pops a plastic product. in reality, injection molding is an intricateand complex process. an injection molding machine has three main parts: the injectionunit, the mold, and the clamp. plastic pellets in the hopper feed into the barrel of theinjection unit. inside the barrel, a screw transports the pellets forward. heater bandswrapped around the barrel warm up the plastic pellets. as the pellets are moved forwardby the screw, they gradually melt, and are entirely molten by the time they reach thefront of the barrel. once enough molten plastic is in front of the screw it rams forward likethe plunger of a syringe. in a matter of seconds, the screw injects the molten plastic intothe 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 millimetersin diameter. they can be mixed with small amounts of a pigment, called “colorant,â€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 heatingof 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: eitherthe middle was too cool and not fully melted or the outer regions were too hot and degradedthe plastic. the solution was this: the reciprocating
screw. often regarded as the “most importantcontribution that revolutionized the plastics industry in the twentieth century.â€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 screwovercomes this in three ways: first, in modern units, the plastic fills only the space aroundthe shaft of the screw. this eliminates the cooler central region leaving a thinner, evenlyheated layer of plastic. second, the screw has “flights†that wraparound the shaft. as the screw rotates, the flights transport the raw material forwardthrough the barrel. the flights also serve to mix the plastic. the screw action agitatesthe melting pellets within the flights to
create a uniform mixture.and third, the screw action itself heats the plastic throughout. the shaft’s diameterincreases 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 thepellets and press them against the barrel’s wall. this shearing creates friction and soheats the plastic throughout. this screw-induced shear supplies a majority of the heat neededto melt the plastic — between 60 and 90 percent — with the rest from the heaterbands. the molten plastic flows past the front of the screw through indentations or “flutes.â€when there’s enough plastic to fill the mold at the front of the screw, it rams forwardlike a plunger injecting the plastic into the
mold. the plastic cannot flow backwards becausewhen the screw pushes forward, a “check ring†is shoved against a “thrust ringâ€to block that backwards movement of the molten plastic. this forces the plastic into themold. initially the cavity image is filled with air. as the molten plastic is injectedit forces air out of the mold, which escapes through vents. these vents are channels groundinto the landing surface of the mold. they are very shallow— between five and forty micronsdeep. the plastic, which has the consistency of warm honey, is too viscous to flow throughthe narrow vents. to speed the plastic’s solidification, coolant, typically water,flows through channels inside the mold just beneath the surface of the interior. afterthe injected part solidifies, the mold opens.
as the mold opens the volume increases withoutintroducing air, which creates tremendous suction that holds the mold together. so atfirst the mold slowly opens several millimeters to allow air to rush in and break the vacuum, andthen, the mold quickly opens the rest of the way so the part can be removed. the slow stepis needed to prevent damage to the mold — these precision machines steel molds can cost hundredsof thousands of dollars. removing the part from the mold can be difficult. when the plasticcools, it shrinks and so become stuck tightly on the core half of the mold. molds have built-inejector pins that push the part off the mold. the ends of the pins sit flush with the corehalf 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 moldedproducts. 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 orcut off the part. sprues are attached to objects only in molds that make a single items ata 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 distributiontunnels called “runners.†the runners fan out from the sprue and connect to eachcavity 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 stillattached to their runners. molds always have at least two parts. andwhere the parts of the mold meet is called the parting line. here on this piece of cutleryyou see the parting line along the side of the fork. when mold halves close they arenever perfectly aligned, nor do they have sharp corners — this creates a noticeableparting line on the molded object. another very important aspect of mold designis the draft angle. if a part has walls that are exactly ninety degrees, it will be verydifficult 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 wallsare slightly tapered—even just one or two
degrees–-it becomes much easier for thepart to be removed because once the part moves slightly, the walls are no longer in contactwith the core half and air can rush in. one impressive example of injection moldingis the lego brick. you can see the injection point in the middle of a stud. but this isnot from a gate or a sprue. the lego molds use “hot runners.†hot runners are a heateddistribution 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 mostclever to me is where lego designs their draft
angle. the outside of a lego brick must besquare. so, if you cut a lego brick in half, you can see that these inner supports arethicker 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 cavityhalf of lego molds are designed so that the parting line is at the bottom edge of thebrick. this hides the parting line. look around you and see how many injection molded objectsyou can find. likely the device you’re watching this on has injection molded parts! you shouldbe able to find ejector pin witness marks and parting lines, but you might find somethinglike this. it’s a date wheel that shows the month and year the item was made. theseare removable inserts and can be changed out
for each run of the mold. they are very usefulfor tracking down defects. so, to return to where this all started. johnwesley hyatt and his injection molded billiard ball did not win the $10,000 prize—his celluloidbilliard balls didn’t bounce quite right—but he did pioneer injection molding, a thriving,continually evolving manufacturing process which creates many billions of productsevery year. i’m bill hammack, the engineer guy.to learn more click on this video overview of injection molding. and this video explainshow the molds are manufactured. click here to see an injection molding machine produceplastic bottle caps very rapidly. finally, this video details the production and automationof lego bricks. and to learn the full story
of the john wesley hyatt’s celluloid billiardball listen to the podcast from 99 percent invisible, which i’ve linked to in the descriptionfor this video. we’re very grateful for our advanced viewerswho critiqued early versions of this video. sign up to me an advanced viewer at engineerguy.com/preview.thanks for watching!
