Understanding the Basics of Injection Molding

Welcome to Thomas Insights — every day, we publish the latest news and analysis to keep our readers up to date on what's happening in industry. Sign up here to get the day's top stories delivered straight to your inbox.

"There are injection molding parts everywhere," says Kyra Stawson. The technical sales engineer at Xometry, the world's largest digital manufacturing marketplace, goes on to explains that injection molding parts can be found in "anything that you can see around your house, from consumables, and medical, to aerospace and defense." (See more injection molded products here.)

Injection molding is a manufacturing process in which molten plastic is injected into a mold tool and then ejected as a solidified part. Although it might seem like the latest buzz phrase in manufacturing, it's actually been around for more than a century. Brothers Isaiah and John Hyatt invented the first molding machines in 1872 to create hair combs, buttons, and other small products.

"It's the most commonly used plastics processing method for high volume of parts," Stawson says.

Today, you can upload a 3D CAD file to Xometry's website to get a quote in just a few seconds. The global company, headquartered in Gaithersburg, Maryland, provides 24-hour responses on quotes with design-for-manufacturing (DFM) feedback. Lead times for Xometry's injection molding parts start at 10 business days.

The available injection molding machines include single, multi-cavity, and family molds. Aluminum and steel mold types are available. The mold cavity tolerances are +/- 0.005" when the mold is being machined. When calculating for the shrink rate, there's a further +/- 0.002." That said, for critical features, Xometry can machine to a steel-safe condition if an even tighter tolerance is needed.

Production grades are usually Class 102-104, with Class 101 used for an extremely high production mold and Class 105 for a simple prototype. For more information on Xometry's injection molding mold classes, see the handy chart on our website here. There, you can also find a list of our plastic injection molding materials, which includes rigid and flexible resins, elastomers and synthetic rubbers for molding, and custom sourced materials. You can also find a guide to Xometry's wide variety of injection mold finishes.

Kyra Stawson (KS): There are many differentiators between us and our competition. The biggest one is our ability to operate off of a partner network. Our network really brings a lot of benefits to the table, including capabilities.

A lot of our competition is limited to what capabilities they have under one roof, while we can go out and talk amongst our 5,000-plus partners, who could do something when maybe other molders can't. Diving a little bit deeper into that, if you're limited to just what you have under one roof, you might not be able to do complex side actions or geometries like we do.

In addition to that, another significant differentiator is material selection. You might have a standard list of materials that you keep in stock and you don't really strive outside of that list. Here at Xometry, we order the material for the project so we're able to source any material out there as long as it's commercially available in the manufacturing location.

KS: Mostly, it's just: "Can you make my parts?" We have much broader capabilities when it comes to producing parts than some others do. They come to us saying, "I got this feedback somewhere else that they couldn't make these. Are you able to make them?" And oftentimes, we say yes because we try to have a solution for every project out there.

We want to strive to be able to not provide any no quotes. We want to find a solution. Our objective is to produce the part as close to the initial design intent of the customer.

KS: There are a few factors that go into choosing injection molding.

One is going to be the number of parts that you want to produce. Even though you know for less than 100 parts that 3D printing might be the way to go, there's a breakeven analysis that each customer has to calculate when switching to injection molding to see which one will be more cost-effective over time.

Materials selection plays a part when selecting injection molding.

Injection molding does have a lot of design requirements that go into designing for this process. It's not suited for really thick parts. So it has to make sense for the design material and your volumes mostly.

KS: Both injection molding and 3D printing are more of an additive type process. Essentially, you start with nothing and end up with a part after the process is complete. This is unlike CNC machining, where you start with a big block of material, and you machine it down to what you need your part to be, so it's more of a like a negative or takeaway process.

CNC machining and 3D printing are both very commonly used processes to prototype for the injection molding process, which plays a much bigger role in the production environment when making plastic parts.

Typically, anywhere you see injection molded parts they had to start somewhere with another process like a prototyping process such as CNC machining or 3D printing.

KS: Injection molding is best used for a prototyping process whenever our customers have to do material testing, as you can't get the exact material in any other prototyping process. Even if you're prototyping in nylon, and then you want an injection molded nylon, that material isn't going to be the exact same. So if you actually need the material that you plan on using long-term, it's good to use injection molding for prototyping.

The same thing goes for any type of specific testing that needs to be done. If you're doing impact testing or tensile testing, you want to make sure that you're actually using an injection molded part because the difference in properties going from injection molding parts to 3D printed parts, for example, is going to be much different.

KS: Here at Xometry we have a minimum order quantity of 50. However, we can service projects ranging from a quantity of 50 to millions parts.

Image Credit: Draftangle / Shutterstock.com