The benefits of injection molding for mass production have long been clear: the manufacturing process is highly scalable and becomes increasingly more cost effective the more parts are made. Besides the economy of scale, injection molding is also known for its high-quality output, which requires minimal post-processing. These reasons, along with a wide material choice, have made the process one of the most broadly adopted production methods for plastic parts, used for making everything from toothbrushes and Lego building blocks to medical supplies.
But there has long been a gap in injection moldings capabilities. While the process is great for mass production, it has not been viable for smaller production scales. 3D printing has started to change this, not only being used as an alternative to injection molding, but complementing the process with cost-effective mold inserts. In this article, we’re looking at how 3D printing low-run injection molds works and what the technology’s strengths and weaknesses are for this application.
Before we get into how 3D printing can be used to make low-run injection molds, it’s first important to understand the fundamentals of injection molding. Injection molding is a manufacturing process used across the world for the mass production of goods, particularly plastic products (though other materials like glass and metal can also be used).  In the simplest terms, the process consists of injecting molten material (such as a melted thermoplastic) into a mold cavity under pressurized conditions. The pressure is critical to ensuring the melted material fills the entire mold quickly. Built in cooling systems then accelerate the hardening process for the material. Once the material has solidified, the mold can be taken apart and the component removed.
There are many reasons injection molding has become such a dominant production method. For starters, it is highly effective for large-scale production. This is due to the rapid turnaround time for the molding and cooling process (sometimes as quick as seconds) as well as a low cost per part ratio. Another key benefit of injection molding is that it is highly repeatable: manufacturers can reliably make tens of thousands of parts that are all identical in terms of dimension, quality, and properties. Beyond that, injection molding has high tolerances, low material waste, and requires minimal post-processing.
Typically, injection molding requires a high-quality mold made from a durable metal material, such as aluminum or steel. Aluminum molds can be reused as many as ten thousand times, while steel molds can be reused a hundred thousand times before they start to wear. Traditional metal molds are made using CNC machining, which can create tools with incredible precision and dimensional accuracy. Metal molds involve a high upfront cost—due to the materials and labor involved in making them—but this cost is typically amortized by large production volumes. This dynamic has made injection molding the most cost effective manufacturing process for mass production. For low-volume production runs or one-off parts, however, injection molding is not economically viable using traditional tool manufacturing, because the high cost of the tool is not adequately absorbed. That’s where 3D printing comes in.
When small production rates are required—whether you are making a prototype or simply a small batch of parts—traditional metal tools are not the best solution due to high cost and turnaround times. This has meant that injection molding has been largely limited to mass manufacturing. In recent years, however, 3D printing has emerged as a solution for rapidly and cost efficiently creating molds for small batch production runs up to roughly 1,000 components.
Also known as additive manufacturing, 3D printing is suited to producing custom components, such as tools or patterns. For instance, many jewelry makers rely on high-resolution 3D printing technologies, like stereolithography (SLA), to create patterns of their pieces. These high-quality patterns are based on CAD designs and are used to make molds. Jewelry makers can then burn or melt the pattern to seamlessly remove it from the mold and pour molten metal into it. Once the metal has solidified, a near-finished piece of jewelry can be removed. This process is known as investment casting, which is slightly simpler than injection molding, because it does not rely on high pressure and coolants to achieve greater efficiency.
When it comes to injection molding, 3D printing is used in a different way. Rather than 3D print a pattern, the technology is used to produce the mold itself. These molds are designed to fit into traditional injection molding machines. For instance, it is possible to 3D print mold inserts that fit into standard metal frames. These frames (typically made from aluminum) reinforce the plastic mold and provide greater resistance to pressure throughout the injection molding process. The 3D printed mold inserts are simply fitted into the metal frames, which are then installed into the injection molding machine. The inserts can also be easily interchanged for greater production agility.
Typically made from high-temperature polymers, these injection molds are less durable than their machined metal counterparts, but they can withstand tens and even hundreds of injections without fail. They are also significantly cheaper and faster to produce. This means manufacturers can benefit from the speed and reliability of injection molding for low-volume production runs while still maintaining a budget-friendly cost-per-part.
Of course, there are many considerations when 3D printing an injection mold. For example, some 3D printing processes are better suited to the production of molds than others. Material choice is also a big consideration, as is mold design. Here are some things to think about when designing a 3D printed mold and best practices to follow.
One of the primary considerations when choosing a 3D printing process for mold making is resolution. 3D printing technologies that require post-processing are not as efficient for injection molding as those that produce high-resolution parts right out of the build envelope. For this reason, stereolithography (SLA) is among the most popular 3D printing processes for injection molding. The process, also known as vat polymerization, uses UV light to cure layers of a photosensitive resin. Resulting parts have good dimensional accuracy and are characterized by a smooth surface finish—which is important for injection molds. Unlike FDM 3D prints, SLA prints that undergo cleaning and post-curing are also fully dense and isotropic, which makes them more durable under injection molding stresses.
Material jetting technologies, such as PolyJet or HP’s Multi Jet Fusion, as well as selective laser sintering (SLS) can also be used to produce injection molds with success, though SLA’s properties are arguably still the best match. FDM mold inserts can be printed, however post-processing is required and the quality and density may not be suitable for as many production runs as an SLA mold. Manufacturers may also choose to machine 3D printed molds for even tighter tolerances.
When it comes to 3D printing materials for injection molds, there are a couple of things to think about. The first, and most obvious thing, is that materials for mold inserts should be resistant to high temperatures. This is because the mold insert must withstand the heat of the melted thermoplastic that is injected into it. Using a material that warps easily when heated can therefore cause flaws in your final part.
In general, you are looking for a material with a high heat deflection temperature (HDT), glass transition temperature (Tg), and high stiffness. These properties ensure that the mold will remain strong and maintain dimensional accuracy throughout the repeated injection molding process. SLA materials that meet the needs of injection molds include Formlabs’ High Temp Resin and Rigid 10K Resin  and DSM’s Somos PerFORM, among others. 
The process of designing a 3D printed mold insert is not unlike that of designing a traditional metal mold. In fact, it incorporates many of the same design elements, including holes for ejector pins, and mounting holes to attach the insert into the mold frame. You can also choose to pre-integrate other helpful features like channels for reinforcing rods that strengthen the mold.
Before sending the mold design to the 3D printer, it is also worth keeping in mind that the better quality the print, the better quality the final component will be. It is therefore recommended to orient the model so that supports are not needed in the cavity, and to print models with a high-resolution. Finally, it can be beneficial to integrate small vents into the mold insert design to let air escape during the injection molding process. These vents can also reduce the amount of pressure required. 
Now that we’ve seen how 3D printing injection molds works, let’s take a quick look at the hybrid technique’s main advantages and disadvantages.
Enables low-run production
Using expensive traditional metal molds, injection molding only becomes economically viable at mass production scales. 3D printing mold inserts opens up new opportunities for low-run injection molding.
3D printing a mold insert dramatically reduces the up-front tooling costs associated with injection molding. Material, labor, and machinery costs are all significantly lower for 3D printing compared to conventional mold making using metal CNC machining.
A significant advantage of 3D printing injection molds is that it provides greater agility in the product development and production process. Product design changes can be integrated easily, without adding significant time or costs.
Low lead times
Not only are aluminum or steel molds expensive (reaching upwards of tens of thousands of dollars), they also come with long lead times. For comparison: a complex steel tool for injection molding can take months to manufacture and an aluminum mold can take several weeks. A 3D printed mold—even a complex one—can be printed and prepared within just days.
3D printed mold inserts are best suited for small components measuring up to 164 cm3 whereas machined metal molds have a larger capacity for volume. For instance, parts measuring 966 cm3 can be easily injection molded with metal tools. 
Whereas metal molds can withstand the high temperatures of injection molding (even with high-temp polymers), 3D printed inserts will eventually begin to degrade under the extreme conditions. Generally, the hotter the molten polymer used for injection molding, the faster the 3D printed insert will degrade or deform.
While the time to produce a 3D printed mold is significantly faster than a traditional tool, the manufacturing time in the injection molding process can be longer. Because 3D printed molds can’t withstand as much injection pressure as metal molds or the same temperatures, the time to inject and remove molded parts is longer, often taking minutes compared to seconds.
In the end, there is a time and a place for 3D printed injection molds. They will admittedly not replace more traditional tools used for mass production, but they are opening up new possibilities for cost-effective low-volume production runs of end-use parts. Here’s what we learned about 3D printing low-run injection molds.
3D printing is effective at producing mold inserts for plastic injection molding machines. Printed molds can withstand tens or potentially hundreds of injections, though they are not well equipped for high-volume production.
SLA and material jetting technologies are the best suited for high-quality molds due to their good dimensional accuracy and durability.
Temperature resistance and stiffness are key considerations when choosing a mold material.
3D printed mold inserts have significantly lower lead times than machined metal molds and can thus reduce product development and rapid prototyping cycle times.
3D printing can produce mold inserts with highly complex geometries and are significantly cheaper to make than aluminum or steel molds.
The main limitations of 3D printed injection molds are size constraints, faster degradation, and longer manufacturing time.
 “What is Injection Moulding.” TWI. [Accessed February 2022] https://www.twi-global.com/technical-knowledge/faqs/what-is-injection-moulding
 “How to Use 3D Printing for Injection Molding”. Formlabs. [Accessed February 2022] https://formlabs.com/uk/blog/3d-printing-for-injection-molding/
 “3D printed injection molds: Materials compared”. Hubs. [Accessed February 2022] https://www.hubs.com/knowledge-base/3d-printed-injection-molds-materials-compared/#requirements
 “Design for 3D Printed Injection Mold Tools 101” Fortify. [Accessed February 2022] https://3dfortify.com/design-for-3d-printed-injection-mold-tools-101-tips-tricks-and-best-practices/
 “3D-Printed Molds vs. Aluminum Tooling”. Protolabs. June 7, 2021. [Accessed February 2022] https://www.protolabs.com/resources/blog/3d-printed-molds-vs-aluminum-tooling/