Plastic Injection Molding

# The Science Behind Injection Molding: A Comprehensive Guide

Injection molding is one of the most widely used manufacturing processes in the world today. It is employed to create a vast array of plastic products, ranging from disposable cutlery to intricate Lego bricks and even large-scale items like chairs. This process has its roots in solving a pressing environmental issue related to billiards, but it has since evolved into a sophisticated method that shapes countless objects we encounter daily.

## A Historical Perspective

The origins of injection molding can be traced back to the 19th century when billiard balls were primarily made from ivory harvested from African elephants. This practice led to a severe decline in elephant populations, prompting a billiards manufacturer to offer a $10,000 prize for an alternative material. Enter John Wesley Hyatt, who responded to this challenge by inventing one of the first plastics—celluloid—to create billiard balls. His creation not only addressed the environmental crisis but also laid the foundation for modern injection molding.

Hyatt’s invention was groundbreaking, as it introduced the concept of using a machine to mold plastic. This early apparatus marked the birth of plastic injection molding, setting the stage for its widespread adoption in various industries.

## The Injection Molding Process: An In-Depth Look

At its core, injection molding is a relatively straightforward process: melt plastic, inject it into a mold, allow it to cool, and then remove the finished product. However, the actual mechanics of this process are far more intricate.

An injection molding machine consists of three main components: the injection unit, the mold, and the clamp. The process begins with plastic pellets being fed into the hopper, which funnels them into the barrel of the injection unit. Inside the barrel, a screw transports the pellets forward while heater bands warm up the barrel to melt the plastic. As the screw rotates, it moves the molten plastic toward the front of the barrel. Once sufficient molten plastic has accumulated, the screw ram injects it into the mold’s cavity.

The plastic solidifies within seconds, and after a short cooling period, the mold opens to eject the part. This cycle is repeated, with the machine producing high volumes of identical parts efficiently.

## Types of Injection Molding Machines

Historically, injection molding machines relied on external heating to melt plastic before it was injected into the mold. However, this method often resulted in uneven temperatures and degraded plastic due to poor heat conduction. The solution came in the form of the reciprocating screw, which revolutionized the industry by providing a more uniform melting process.

In modern injection units, the plastic fills only the space around the screw shaft, eliminating cooler central regions. The screw’s flights transport and mix the plastic while generating frictional heat that melts the pellets uniformly. This innovation ensures consistent quality and efficiency in the manufacturing process.

## Mold Design and Functionality

Mold design plays a crucial role in the success of injection molding. Molds are typically composed of two halves, with the parting line marking where they meet. The draft angle, or taper, is another critical factor. Walls that are too perpendicular can make parts difficult to eject and may trap air, leading to vacuum issues. Slight tapers (1-2 degrees) facilitate easier ejection by reducing contact between the mold halves and allowing air to flow in freely.

For example, Lego bricks demonstrate excellent draft angle design. The inner supports of a Lego brick taper slightly, making it easier for ejector pins to remove the part from the mold. This attention to detail ensures that the final product is both functional and visually appealing.

## Hot Runners and Injection Points

In some cases, such as with Lego bricks, hot runners are used instead of traditional cold runners. These heated distribution networks keep plastic molten within the runner system, eliminating the need for gates or sprues. This approach streamlines production by delivering parts ready for use directly from the mold.

However, hot runners come at a higher cost compared to conventional systems. Despite this, they offer significant advantages in terms of design flexibility and reduced material waste.

## Real-World Applications

Injection molding’s versatility is evident in its wide range of applications. From small items like disposable cutlery to larger products such as chairs, this process adapts seamlessly to different scales and complexities. Even intricate components for model planes are produced using injection molding, with parts still attached to runners for easy removal.

The use of recycled materials (up to 15%) in the plastic pellets further underscores the process’s sustainability, aligning with modern environmental standards.

## Witness Marks and Mold Identification

When inspecting injection-molded products, subtle features like ejector pin marks and parting lines reveal insights into the molding process. Ejector pins leave circular marks on the underside of objects, while parting lines are often hidden along edges or surfaces. For instance, date wheels on devices may display these marks, serving as identifiers for quality control and defect tracking.

## Conclusion

From its humble beginnings to its current status as a cornerstone of manufacturing, injection molding has come a long way. John Wesley Hyatt’s pioneering work not only solved an ecological crisis but also opened doors to endless possibilities in plastic production. Today, this process continues to evolve, driven by advancements in technology and material science. As you look around, chances are you’ll find countless objects shaped by injection molding—each one a testament to the ingenuity of this versatile manufacturing technique.

For further exploration, educational videos and podcasts delve deeper into specific aspects of injection molding, from machine operation to mold design. These resources offer valuable insights for enthusiasts and professionals alike, ensuring that the legacy of John Wesley Hyatt’s invention lives on in every molded product we use today.

"WEBVTTKind: captionsLanguage: enInjection molding is the most common methodfor mass manufacturing plastic products. Examplesinclude chairs, toys, cases for consumer electronics,disposable cutlery, and, my favorite, Legobricks. Injection molding was invented tosolve a problem for billiards. In the nineteenthcentury billiard balls were composed of ivoryharvested from the tusks of African elephants.This devastated the elephant population, soa billiards manufacturer offered a ten-thousanddollar prize for a replacement for ivory.And this spurred John Wesley Hyatt to developone of the first plastics — celluloid — tocreate billiard balls. He patented an apparatusfor molding products plastics from celluloid. Thisapparatus was the birth of plastic injectionmolding.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 moldingmachine has three main parts: the injectionunit, the mold, and the clamp. Plastic pelletsin the hopper feed into the barrel of theinjection unit. Inside the barrel, a screwtransports the pellets forward. Heater bandswrapped around the barrel warm up the plasticpellets. As the pellets are moved forwardby the screw, they gradually melt, and areentirely molten by the time they reach thefront of the barrel. Once enough molten plasticis 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 cavityimage. The plastic solidifies in under a minute,the mold opens and the part is ejected. Themold then closes, and the process repeats.All injection molded objects start with theseplastic pellets, which are a few millimetersin diameter. They can be mixed with smallamounts of a pigment, called “colorant,”or with up to 15% recycled material, thenfed into the injection molding machine.Before the mid twentieth century injectionmolding machines used only external heatingof the barrel to melt the plastic before aplunger injected the molten material. But,because plastic conducts heat poorly, thetemperature was uneven in the plunger: eitherthe middle was too cool and not fully meltedor the outer regions were too hot and degradedthe plastic. The solution was this: the reciprocatingscrew. Often regarded as the “most importantcontribution that revolutionized the plasticsindustry in the twentieth century.”In the earlier plunger-style machines plasticfilled completely the cylindrical barrel,but as I showed you the plastic was not ata uniform temperature. The reciprocating screwovercomes this in three ways: First, in modernunits, the plastic fills only the space aroundthe shaft of the screw. This eliminates thecooler central region leaving a thinner, evenlyheated layer of plastic.Second, the screw has “flights” that wraparound the shaft. As the screw rotates, theflights transport the raw material forwardthrough the barrel. The flights also serveto mix the plastic. The screw action agitatesthe melting pellets within the flights tocreate a uniform mixture.And third, the screw action itself heats theplastic throughout. The shaft’s diameterincreases along the screw so that the distancebetween the wall and the shaft decreases.The flights, then, squeeze out air as theymove the plastic forward and they shear thepellets and press them against the barrel’swall. This shearing creates friction and soheats the plastic throughout. This screw-inducedshear supplies a majority of the heat neededto melt the plastic — between 60 and 90percent — with the rest from the heaterbands. The molten plastic flows past the frontof the screw through indentations or “flutes.”When there’s enough plastic to fill themold at the front of the screw, it rams forwardlike a plunger injecting the plastic into themold. The plastic cannot flow backwards becausewhen the screw pushes forward, a “checkring” is shoved against a “thrust ring”to block that backwards movement of the moltenplastic. This forces the plastic into themold. Initially the cavity image is filledwith air. As the molten plastic is injectedit forces air out of the mold, which escapesthrough vents. These vents are channels groundinto the landing surface of the mold. They arevery shallow— between five and forty micronsdeep. The plastic, which has the consistencyof warm honey, is too viscous to flow throughthe narrow vents. To speed the plastic’ssolidification, coolant, typically water,flows through channels inside the mold justbeneath the surface of the interior. Afterthe injected part solidifies, the mold opens.As the mold opens the volume increases withoutintroducing air, which creates tremendoussuction that holds the mold together. So atfirst the mold slowly opens several millimetersto allow air to rush in and break the vacuum, andthen, the mold quickly opens the rest of theway so the part can be removed. The slow stepis needed to prevent damage to the mold — theseprecision machines steel molds can cost hundredsof thousands of dollars. Removing the partfrom the mold can be difficult. When the plasticcools, it shrinks and so become stuck tightlyon 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—sometimesthey protrude or are indented slightly. So,if you look closely you will see circularejector pin “witness” marks on moldedproducts. For example, this chair, on it’sbottom, has an array of witness marks.When the part drops from the mold, an operatorhas to remove the sprue—that section of plastic that connected the injection unitto the mold. Sprues are manually twisted orcut off the part. Sprues are attached to objectsonly in molds that make a single items ata time — like a chair. Smaller objects aremade in multiples in a single mold. In these the sprue connects notto the part itself, but to a network of distributiontunnels called “runners.” The runnersfan out from the sprue and connect to eachcavity in the mold via a small — typicallyrectangular — entrance called the gate.You can see the gate on plastic cutlery. Theparts for model planes typically come stillattached to their runners.Molds always have at least two parts. Andwhere the parts of the mold meet is calledthe parting line. Here on this piece of cutleryyou see the parting line along the side ofthe fork. When mold halves close they arenever perfectly aligned, nor do they havesharp 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 thatare exactly ninety degrees, it will be verydifficult to eject because it’s inner wallswill scrape the core half of the mold. Also,the vacuum will be difficult to break becauseair cannot readily enter. However, if the wallsare slightly tapered—even just one or twodegrees–-it becomes much easier for thepart to be removed because once the part movesslightly, 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 injectionpoint in the middle of a stud. But this isnot from a gate or a sprue. The Lego moldsuse “hot runners.” Hot runners are a heateddistribution network. This keeps plastic insidemolten, 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 expensivethan a traditional cold runner system.On the bottom edges of the brick you can seeejector pin witness marks. And what’s mostclever to me is where Lego designs their draftangle. 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—thereis a draft angle of about one-and-a-half degrees.This helps the ejector pins push the brickoff the mold. The core half and the cavityhalf of Lego molds are designed so that theparting line is at the bottom edge of thebrick. This hides the parting line. Look aroundyou and see how many injection molded objectsyou can find. Likely the device you’re watchingthis on has injection molded parts! You shouldbe able to find ejector pin witness marksand parting lines, but you might find somethinglike this. It’s a date wheel that showsthe month and year the item was made. Theseare removable inserts and can be changed outfor 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 billiardball did not win the $10,000 prize—his celluloidbilliard balls didn’t bounce quite right—buthe did pioneer injection molding, a thriving,continually evolving manufacturing processwhich creates many billions of productsevery year. I’m Bill Hammack, the engineerguy.To learn more click on this video overviewof injection molding. And this video explainshow the molds are manufactured. Click hereto 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 storyof 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!\n"