Time to read: 6 min
Both 3D printing and injection molding are used to produce plastic parts. However, these processes are not mutually exclusive, as some projects that start with 3D printing may eventually transition to injection molding.
3D printing is typically used for early-stage prototypes, evolving designs, and part geometries that cannot be produced by other methods. Since this form of additive manufacturing does not require metal tools, it is cost-effective for low-volume prototyping and production. 3D printing also offers shorter lead times. However, it has longer cycle times, fewer available plastic materials, and may require post-processing—especially to improve surface finish. Additionally, 3D-printed parts are generally less strong than injection-molded parts.
Injection molding is suitable for late-stage prototypes, mature designs, and parts with tighter tolerances. Although it requires tooling, prototype injection molding is ideal for designs that need real-world testing. Low-volume injection molding is used for 100 to 10,000 parts, while high-volume injection molding can produce hundreds of thousands of parts. Injection molding may have longer lead times but shorter cycle times, with a wider range of available materials. Post-processing may not be necessary since parts can be molded to specific surface finishes. Generally, injection-molded parts are stronger than 3D-printed ones.
This article provides a detailed comparison of 3D printing and injection molding to help you make the best choice for your next project. If you already know which process you need, create a free Unofactory account and upload your part design. In addition to a quote, you’ll receive expert design for manufacturability (DFM) feedback.
3D Printed Parts
This 3D-printed part is based on a solid model file.
A Deeper Dive into 3D Printing
3D printing uses computer-aided design (CAD) software and printers to build three-dimensional (3D) plastic parts layer by layer. Different 3D printing technologies are used, such as stereolithography (SLA) and fused deposition modeling (FDM).
The following sections describe core 3D printing concepts. You’ll also learn about the advantages and disadvantages of additive manufacturing.
STEP Files, STL Files, and G Code
Designers use CAD software to create 3D part models, typically in the form of STEP files or STL files. STEP files contain precise measurements in an X, Y, Z coordinate system. STL files approximate a part’s surface with triangles and are represented as a mesh.
Designers can create either type of file, but it’s not the CAD file that controls the 3D printer. G code, a type of computer numerical control (CNC), is what actually commands the machine.
Slicing and Printing
Slicing is the process of converting a CAD file into G code using specialized software. Typically, slicing also considers geometric dimensioning and tolerancing (GD&T). The 3D printer then builds the part by depositing plastic material layer by layer until the entire object is formed.
Each layer is a sliced cross-section of the part, but 3D printing technologies vary. For example, FDM forces polymer-based filaments through a heated nozzle to melt the material. SLA pours liquid photosensitive resin into a tank and uses ultraviolet (UV) light to cure the resin layer by layer until the final part is complete.
Post-Processing
After 3D printing is complete, post-processing may be required. Sometimes this involves removing support structures that held the part during production. Post-processing may also involve sanding or polishing the 3D-printed part to achieve a specific surface finish. Curing, thermal annealing, or applying laminated epoxy may be needed to enhance the part’s strength.
Advantages of 3D Printing
The advantages of 3D printing start with its low barrier to entry. Unlike injection molding, 3D printing does not require thousands of dollars in tooling. As a result, 3D printing supports on-demand prototyping and production. Shorter lead times and setup times make this additive manufacturing cost-effective for lower part quantities.
Importantly, 3D printing’s support for rapid, iterative prototyping can reduce project costs and time. For example, when TransMed7 wanted to quickly prototype a new set of biopsy tools, Unofactory’s 3D printing services helped the medical device company save millions of dollars. TransMed7 also reduced new product introduction (NPI) time from 10 years to 2 years.
For designers, the greater design freedom offered by 3D printing is particularly appealing. From complex geometries to personalized products, designers can bring ideas to life that are technically impossible or economically unfeasible with other manufacturing methods. Examples include lattices, honeycomb structures, and personalized products like shoes and clothing.
Disadvantages of 3D Printing
Despite its advantages, 3D printing has several key limitations. Compared to injection molding, there are fewer plastic materials available. As a result, designers may not be able to 3D print with their intended production material. Even so, 3D-printed and injection-molded versions of the same polymer differ in end-use performance, including strength, as 3D-printed parts may have weaknesses between layers.
Additionally, 3D-printed parts cannot achieve the tighter tolerances possible with injection molding. Injection molding also enables more detailed and smoother surface finishes. Furthermore, post-processing operations to remove support structures, polish parts, and enhance strength add cost and time.
Due to its longer cycle times, 3D printing is not recommended for high-volume production. 3D printers also have limitations in build size. In other words, a specific 3D printer may not be able to produce the larger or smaller parts you need. While large and miniature 3D printers exist, these machines tend to be specialized.
Injection-Molded Parts
Unofactory injection-molded these parts for TransMed7.
A Deeper Dive into Injection Molding
Injection molding uses plastic resin, molds, and a machine called a press to produce parts in low or high volumes. Like 3D printing, this process has advantages and disadvantages.
Parametric Files and Plastic Resin
Designers use CAD software to create parametric part models, typically in non-mesh formats. These CAD files usually contain a single solid. Injection molding can support many different types of parametric files and even more types of plastic resins, including glass- or fiber-filled materials for enhanced strength.
Tooling and Tooling Costs
Injection molds consist of a core and cavity clamped between metal plates. Other components, such as part ejection mechanisms, are also part of the injection mold. Molds are typically made of steel or aluminum. Steel molds are more expensive but produce higher-precision parts and last longer in high-volume production.
Molding Under Heat and Pressure
Injection molding machines operate under heat and pressure. First, plastic pellets are fed into a hopper and melted in a heated barrel. Next, a rotating screw pushes the molten material into the mold cavity, which remains clamped. Injection molding equipment varies, but the basic process is the same.
After injection, the molten plastic is held under pressure to ensure complete flow and filling of the mold. The plastic material cools and solidifies according to the mold’s shape and finish. The tool opens, and the finished part is ejected.
Advantages of Injection Molding
The advantages of injection molding start with faster cycle times and higher precision. Compared to 3D printers, injection molding machines can produce more parts in less time. The tighter tolerances achievable with injection molding equipment—especially with steel molds—support greater part consistency.
Importantly, injection molding supports a wider range of materials, including silicone rubber and thermoplastic elastomers. Designers can use the same polymer for prototyping and production, facilitating testing and approval. Additionally, the high level of automation in injection molding helps control labor costs. Injection-molded parts are also denser and more uniform, enhancing strength.
Moreover, most injection-molded parts require minimal post-processing. Depending on the mold quality, parts emerge from the tool with the desired surface finish and dimensions. Well-established injection molding design guidelines, along with high-quality molds and skilled toolmakers, help designers achieve their goals for low or high part quantities.
Disadvantages of Injection Molding
The disadvantages of injection molding start with tooling costs. If you need fewer than 100 parts, using soft or semi-hard steel molds may not provide sufficient return on investment (ROI). Aluminum tools cost less but cannot match steel molds in performance. Aluminum molds also wear out faster, especially for parts made from abrasive materials.
Since injection molding always requires tooling, changes to part designs may require cutting new molds. This makes injection molding a riskier choice for low-volume and early-stage prototyping where designs may change. Even if tools can be modified instead of scrapped, part quantities need to be high enough to amortize tooling costs.
Finally, injection molding has longer lead times because mold manufacturing requires machining. Injection molding also offers less design freedom and cannot match some of the complex features (such as lattices) possible with 3D printing. Although most plastics are recyclable, failing to follow injection molding design guidelines can lead to part defects.
For information on injection molding design guidelines, download this free resource.
[Download Injection Molding Design Guidelines]
Plastic Parts
Plastic parts like these can be produced by injection molding or 3D printing.
Injection Molding vs. 3D Printing: Key Considerations
Now that you know more about the differences between injection molding and 3D printing, what’s the right choice for your project? Remember to consider all these factors:
- Part quantity
- Cost
- Lead time
- Cycle time
- Part strength
- Design complexity
The following sections explain these factors.
Part Quantity
Injection molding can be used for low or high part quantities, primarily depending on tooling. Low-volume injection molds made of soft or semi-hard steel are used for 100 to 10,000 parts. High-volume injection molds made of hardened steel can produce hundreds of thousands of parts.
For 3D printing, part quantities are smaller. While part quantity isn’t the only reason to use 3D printing, its cost-effectiveness for low-volume prototyping and production is a key consideration.
Cost
Injection molding has high upfront costs due to the need for metal tooling. When production volumes are low, the cost per part for tooling is relatively high. However, when part quantities are high, the cost per part for tooling is relatively low.
At sufficient production volumes, the cost per part for injection molding is lower than what 3D printing can offer. Identifying this crossover production volume is critical, especially if you plan to use 3D printing for early prototyping and injection molding for late-stage prototyping, low-volume production, and high-volume production.
Lead Time
Since no tooling is required, 3D printing can produce part samples and initial order quantities faster, typically with a lead time of one week. Injection molding has longer lead times, but Unofactory can provide low-volume soft or hardened steel molds in about two weeks.
Part Strength
Generally, injection-molded parts are stronger than 3D-printed parts. This is due to how the parts are manufactured. The layer-by-layer process of 3D printing can create weaknesses between layers. In contrast, injection molding uses high pressure and heat to produce parts with higher density and uniformity.
Design Complexity
3D printing supports greater design complexity than injection molding. While injection molding can produce relatively complex parts in high volumes, it cannot create the same intricate structures. For designers planning to start projects with 3D printing and transition to injection molding, it’s critical to understand that each process has different design guidelines.
Getting the Plastic Parts You Need
Like all manufacturing processes, 3D printing and injection molding have advantages and disadvantages. Unofactory has engineering experts who can recommend the right process. We also work with a rigorously vetted network of manufacturing partners and offer both molding services.
Whichever decision you make, you’ll receive design for manufacturability feedback along with your quote. Ready to get started? Create a free Unofactory account and upload your part drawings.