3D printing, otherwise known as additive manufacturing, doesn’t appear to have much in common with injection molding. 3D printing is primarily a rapid prototyping process for making one-off parts and very small batches, while injection molding is a mass manufacturing process regularly used to make batches numbering millions of units.

And the differences don’t end there. For instance, while 3D printing can fabricate parts with a wide range of geometries — including complex internal cavities — injection molding is generally restricted to simple, thin-walled parts.

But there are some scenarios in which 3D printing and injection molding present themselves as equally viable production processes. For example, a small to medium-size batch of simple parts might have roughly the same production costs with either process.

Furthermore, the disparity between the two processes in terms of production scale is actually getting smaller. Advances in low-cost tooling are making short-run injection molding more cost-effective than ever, while new and improved additive manufacturing technology makes it possible to quickly 3D print large production runs of high-quality parts. In other words, calling 3D printing a “prototyping” process and injection molding a “mass production” process is less accurate than it once was.

This article looks at FDM 3D printing vs injection molding in terms of process characteristics, product design constraints, materials, cost, applications, and other factors.

What is 3D printing?

3D printing is an additive manufacturing process that uses raw material and a computer-controlled print head to build up parts in a layer-by-layer fashion.

The most common plastic 3D printing process is fused deposition modeling (FDM) — also known as fused filament fabrication (FFF) — which uses spools of plastic filament as feedstock. During the FDM process, the 3D printer melts and extrudes the filament and deposits it onto the print bed, where the material cools and hardens. Once the first layer has been deposited, the next layer can be deposited on top of it, with each layer fusing to the next. In this way, a 3D shape is created.

3D printing is an extremely precise and repeatable process because it is a form of digital manufacturing. A 3D model designed using CAD software is converted into machine-readable instructions, so the 3D printer can fabricate the design without any manual supervision.

This article focuses on FDM because it has important parallels with injection molding; for instance, both processes are primarily used with thermoplastic polymers. However, FDM is just one type of 3D printing technology. Stereolithography (SLA), for example, works in a completely different way, using a laser to cure successive 2D layers in a vat of photopolymerizable liquid resin. And some forms of 3D printing, including direct metal laser sintering (DMLS), are used to make production-grade metal parts rather than plastic ones.

An FDM 3D printer

3D printing process:

1. Extruder gears move filament towards hotend

2. Hotend melts tip of filament

3. Filament pushed out of nozzle

4. Printhead moves along X and Y axes to draw 2D layer using molten filament

5. Printhead moves up (or print bed moves down) one step along Z axis

6. Printhead draws next layers until part is finished

7. Finished part is prized from print bed

What is injection molding?

Injection molding is a traditional manufacturing method used for the high-volume production of plastic parts. It is the most widely used manufacturing process for polymers, with the ability to make final parts for a range of industries.

An injection molding machine uses a reciprocating screw or ram to force a precise quantity of molten plastic into the cavity of a metal mold — typically consisting of two halves clamped together — where the plastic material quickly cools and solidifies. The mold is then opened up, and the finished product is ejected from it. Because the material can be injected quickly at high pressure, and because the cooling process is also very fast, injection molding is one of the fastest production methods, hence its suitability for mass production.

The plastic injection molding process is essentially divided into two distinct processes: making the metal mold, then using the mold to make plastic parts. Moldmaking is a laborious and expensive process that can take several weeks, creating long lead times. High-quality tool steels are typically used, as these can survive hundreds of thousands of molding cycles without degradation. By contrast, the actual injection molding process is lightning fast, with some cycles taking well under a minute.

Because the same mold can be used for hundreds of thousands of units, batches tend to have excellent consistency with minimal variation.

An industrial injection molding machine

Injection molding process:

1. Tooling is machined from tool steel

2. Tooling is set up in injection molding machine

3. Hopper is filled with plastic pellets

4. Pellets are melted down in barrel

5. Mold is closed with high clamping forces

6. Reciprocating screw forces melted plastic through nozzle and into mold

7. Plastic cools and solidifies in mold

8. Mold opens and part is ejected

3D printing vs injection molding

This section looks at the key differences between 3D printing and injection molding, from the mechanics of the two production processes to the various real-world applications of each technology.

Part design

One of the major differences between the 3D printing and injection molding processes happens before the machinery has even been switched on.

Product design for 3D printing is vastly different to design for injection molding, and very few designs would be equally suited to both technologies. Although the same CAD (computer-aided design) software can be used to make printed or molded plastic parts, product designers need to consider unique design constraints depending on the chosen manufacturing process. The philosophy of designing parts with the production process in mind is commonly referred to as DfM (design for manufacturing).

In general, 3D printed parts can have a wide range of geometries, though they should be designed with minimal overhanging sections (as these require printed support structures which use up more material) and the walls of the part cannot be too thin. Injection molded parts, on the other hand, are highly suited to thin walls. In fact, thicker walls are prone to warping as the material melts.

The table below shows some of the most important design considerations for 3D printed and injection molded parts.

Process

The 3D printing and injection molding processes differ greatly. While 3D printing builds parts incrementally from the ground up, usually over many hours, injection molding builds the whole part at once in a matter of seconds.

Fundamental differences

Although 3D printing and injection molding use similar raw materials, the processes are not alike. The most significant differences are highlighted below.

• 3D printing is a tool-free process, whereas injection molding requires tooling to form the final product.

• 3D printing builds a part slowly, one layer at a time, whereas injection molding forms the whole part all at once and very quickly.

• The printhead of the 3D printer moves along three axes according to computer instructions so the nozzle can reach different coordinates, whereas the nozzle of the injection molding machine remains stationary. 3D printing is therefore more error-prone.

• 3D printing has almost total geometric freedom, although overhanging sections require support structures. Injection molding is much more limited.

• 3D printing is like building a house brick by brick, with the potential to create multiple rooms and features; injection molding is more like filling an ice cube tray.

Machinery and manufacturing environment

An injection molding machine is a large piece of machinery that can only be used by a trained operative in a factory setting. It may have a footprint of several square meters. Furthermore, due to the large volumes of parts typically produced, further space is required for the finished moldings.

Some types of additive manufacturing machine — industrial DMLS systems, for example — are similarly large and, due to their power usage and harmful emissions, are restricted to factory settings. However, many FDM 3D printers have a compact footprint and can be used in non-industrial environments like offices and even residential buildings. 3D printers are typically classified as industrial, professional, or desktop, which gives an indication of their size and where they can be used, in addition to their overall printing quality.

Injection molding equipment has a large footprint

Ownership vs outsourcing

Companies developing and designing products do not typically have their own injection molding production equipment and will instead outsource the process to a manufacturer. 3D printing can also be outsourced; however, because 3D printers have a low base cost, companies will often buy their own for in-house use.

Materials

FDM 3D printing and injection molding use many of the same plastic materials. Both processes are primarily used for thermoplastic polymers, although some types of injection molding can also process thermosets and elastomers.

The main difference between feedstocks for the two technologies is material form: 3D printing uses filament, i.e. long strands of plastic wrapped around a spool, whereas injection molding uses pellets, i.e. small lumps of raw material. Confusingly, 3D printer filament manufacturers typically make their products by melting and extruding pellets.

On the whole, injection molding is better than 3D printing for making parts from high-temperature materials. Most injection molding machines are capable of melting plastics with high melting points, whereas the hotend of a typical 3D printer is limited in power. 3D printers typically achieve better results with low-temperature materials like PLA, although these parts have inferior mechanical properties. High-end 3D printers have high-temperature hotends and enclosed print chambers and can more easily print high-temperature materials.

Some of the most common 3D printing filaments are ABS, PLA, PETG, and TPU, while performance materials (printable on production-grade printers only) include PEEK and PEKK.

Some of the most common injection molding materials are ABS, PC, PE, PP, and nylon. It is also possible to create hybrid materials, as different pellet types can be easily mixed, including non-plastic materials.[1]

Injection molding uses pellets rather than filament

Post-processing

One of the biggest limitations of FDM 3D printing is that parts often require extensive post-processing due to the poor surface finish it produces. As parts are fabricated layer by layer, the process often leaves visible layer lines on the surface of parts.

To make 3D printed parts look and feel glossy, surface finishing techniques like sanding, polishing, or abrasive blasting may be used. However, this adds time and cost to the process. Other post-processing steps include manual or chemical removal of support structures.

A major difference between 3D printing and injection molding is that surface finishes are typically applied to the metal mold rather than each individual molding. This saves lots of time, as the mold texture is simply transferred to the molding. Some moldings require flash (excess plastic) to be removed manually.

Cost, speed, and scale

When analyzing 3D printing vs injection molding, production scale is one of the biggest differences between the technologies.

3D printing offers short lead times and low upfront costs when dealing with one-off parts and small batches. Because the process does not require tooling, the first part can be fabricated within a matter of hours. The only costs are for materials, energy, and labor. However, 3D printing offers no economy of scale: the cost per part does not decrease in larger volumes.

Injection molding works in a different way. Because metal tooling is required, manufacturers typically need to wait several weeks (and pay several thousand dollars) to have the mold manufactured, and no parts can be fabricated until the mold arrives. However, once the tooling is set up, individual parts can be fabricated in a matter of seconds and at a very low additional cost.

Determining whether a part is best suited to 3D printing or injection molding involves finding the break-even point for the mold. As a very rough rule of thumb, 3D printing is cheaper in batches under 10,000, while injection molding is better value above that number and significantly better value beyond 100,000 units.

For cost and time reasons, 3D printing technology is better suited for prototypes and low-volume production, while injection is better suited to larger batches. However, it is possible to reduce the cost and turnaround times for injection molding by CNC machining the tooling from low-cost aluminum instead of steel. Likewise, it is becoming more and more affordable to 3D print parts in large volumes using production-ready 3D printers with large build volumes.[2] Notably, printers with large build volumes can print several parts simultaneously.

Applications

Common 3D printing applications include prototypes, custom parts for individual users, or other one-off parts. Injection molding is more often used for mass production of products in areas like food, consumer goods, and automotive.

Key Takeaways

3D printing and injection molding both offer unique advantages. 3D printing provides excellent value and turnaround times in low volumes while also offering near-total geometric freedom. Additionally, many 3D printers can be operated in non-factory environments, and the learning curve for the technology is not particularly steep. On the other hand, 3D printing offers no economy of scale and produces parts with limited mechanical properties and a poor surface finish.

Injection molding is compatible with a very wide range of materials and can generally achieve tight tolerances. Once the tooling has been installed, the process is very fast, and the cost per part decreases in larger batches, making it suitable for mass production. Molded parts also require less post-processing than 3D printed parts. Limitations of injection molding include high upfront tooling costs and significant design constraints such as the need for thin walls.

Recommended reading: The pros and cons of 3D printing low-run injection molds

References

[1] Serban D, Lamanna G, Opran CG. Mixing, conveying and injection molding hybrid system for conductive polymer composites. Procedia CIRP. 2019 Jan 1;81:677-82.

[2] Franchetti M, Kress C. An economic analysis comparing the cost feasibility of replacing injection molding processes with emerging additive manufacturing techniques. The International Journal of Advanced Manufacturing Technology. 2017 Feb;88:2573-9.