Preparing for production is a crucial moment for any company. You’ve refined your design, you know your market, and now you’re faced with one of the most fundamental decisions in production: how do you manufacture your product?
When it comes to plastic components, the discussion is almost always limited to two titans of the industry: 3D printing and injection molding. Both produce high-quality plastic parts, but they are at opposite ends of the manufacturing spectrum.
Choosing the wrong path can mean wasting valuable capital, missing out on market opportunities or ending up with a product that isn’t fit for purpose. This guide is designed to go beyond the superficial summaries.
We’ll take an in-depth look at the nuances between 3D printing and injection molding, providing you with the detailed, practical insights you need to make a sound strategic decision for your project.
What is 3D Printing?
3D printing, also known as additive manufacturing in the industry, is the process of creating a three-dimensional object from a digital file.
Unlike traditional subtractive processes, where you cut material out of a solid block, 3D printing builds the part from nothing, one incredibly thin layer at a time. It’s a truly transformative technology, especially for product development.
For the production of plastic parts, there are three main types of 3D printing that you will probably recognize:
Fused Deposition Modelling (FDM):
This is the most widely used form of 3D printing. It works in a similar way to a sophisticated computerized hot glue gun, where a spool of plastic filament is melted and extruded through a nozzle to pull out the individual layers. It is ideal for producing low-cost initial prototypes to check the fit and form of a design.
Stereolithography (SLA):
SLA was the very first 3D printing technology. It uses a precisely guided ultraviolet laser to cure and solidify liquid photopolymer resin in a vat. The shape of each layer is traced on the resin surface. This creates parts with exceptional detail and a very smooth surface that is ideal for visual models and patterns.
Selective Laser Sintering (SLS):
This is a more industrial process. An SLS machine uses a high-power laser to fuse particles of a polymer powder, usually a nylon material, together. A key advantage is that the surrounding, unfused powder supports the part during the build, eliminating the need for specialized support structures. This enables the production of highly complex and durable functional parts.
Advantages of 3D Printing
The best time to realize the benefits of 3D printing is at the start of a project. It has a huge impact on speed, cost and design flexibility.
The speed of iteration is one of the best things about it. An engineer in Bristol can finish designing a part in the afternoon, send it to an SLS printer and have a working prototype to test the next morning.
The ability to “fail fast” and improve designs in days rather than weeks is very useful. This enables people to develop new ideas and solve problems in ways that are not possible with traditional methods.
Secondly, 3D printing makes complicated things almost free. Printing a simple cube costs about the same as printing a complicated, hollowed-out lattice structure of the same size. This allows engineers to design parts that are lighter and more compact than ever before and that were previously impossible to make from a single piece.
Finally, the fact that you don’t have to pay for molds upfront is a big plus, especially for small businesses and start-ups. Not having to buy a $15,000 mold to make the first batch of production parts for the end user is not only a cost-saving measure, but for many companies, it’s the difference between launching a product and keeping it on the drawing board.
Disadvantages of 3D Printing
Whilst 3D printing is a real step forward, it is not a silver bullet. Its limitations become clear when projects need to be scaled up.
The biggest obstacle is the lack of scalability in terms of speed and cost. Production time is linear; printing 1,000 parts takes 1,000 times longer than printing one part.
Similarly, the cost per part remains relatively static. The first part might cost £7, and the thousandth part will also cost £7. This makes it uneconomical for large quantities and limits its use for mass production.
The mechanical properties of 3D-printed parts are another important consideration. As they are built up in layers, the parts often have “anisotropic” properties. This means that they are significantly weaker when a force is applied across the layer lines than when it is applied along these lines.
A skillful engineer can orient a part to mitigate this, but it is a fundamental weakness that can cause parts to break under load and must be considered in functional designs.
Finally, although the choice of materials is greater, it is more limited and more expensive compared to injection molding. In addition, the parts often need to be post-processed, e.g., by removing supports, sanding or smoothing the vapors, which adds manual labor and hidden costs to the process.
What is Injection Moulding?
When it comes to producing many plastic parts, injection molding is the most effective method. It is a highly advanced and reliable process that has undergone significant improvements over many years. Almost every plastic product that is manufactured in large quantities is produced this way – from the lid of your milk bottle to the dashboard of your car.
A mold is a custom-made, precision-engineered tool used in the manufacturing process. The cavity in it is the exact counterpart of the part to be produced. They are usually made of steel or aluminum. The plastic injection molding process is carried out quickly in a specialized machine known as the injection molding machine.
Clamping: A powerful hydraulic or electric clamping unit presses the two halves of the mold together. The force is measured in tonnes.
Injection: Pellets of a selected plastic polymer are poured from a hopper into a heated barrel. A reciprocating screw melts and mixes the plastic before it is injected into the mold cavity under incredibly high pressure.
Cooling: Water circulates through channels in the mold to rapidly cool the molten plastic so that it can solidify into the final shape of the part. The cooling phase is often the longest part of the cycle time.
Ejection: Once the mold has cooled, it opens, and ejector pins push the solid part out, allowing the next cycle to begin.
Advantages of Injection Moulding
The main advantage of injection molding is its phenomenal cost efficiency on a large scale. While the initial investment in the mold can be significant, the cost per part can be measured in cents once production is underway.
The high speed and automation of the process make it the only viable option for high-volume production. The break-even point at which this investment pays off is an important calculation.
The quality and consistency of the injection molded parts are excellent. As the part is molded from a single, homogeneous mass of material, its mechanical properties are ‘isotropic,’ i.e., it is equally strong in all directions. This reliability is crucial for load-bearing components.
In addition, an injection molding company can achieve incredibly tight tolerances and a wide range of surface finishes, from textured grains to a mirror-like polish, which can be perfectly reproduced on millions of identical parts.
Finally, the variety of materials available is also immense. From flexible TPEs and commercial polypropylene to high-performance PEEK and glass-filled nylon, there is a polymer for almost every application, including those requiring stringent medical device certification.
Disadvantages of Injection Moulding
The biggest obstacle to traditional injection molding is the huge upfront cost and lead time for tooling. A simple, basic mold made from aluminum can cost a few thousand pounds. A complex multi-cavity mold made from hardened tool steel for a large production run can easily cost tens of thousands or even more.
This lead time is not trivial. It involves detailed Design for Manufacturability (DFM) analysis, followed by weeks of precision CNC machining, polishing and assembly before the first test parts or ‘T1 samples’ can even be produced. This entire process can take between 6 and 12 weeks or more.
This leads to another major disadvantage: the design’s rigidity. Once the steel is cut, you are set. Even a minor design change — such as moving a small protrusion a few millimeters — can necessitate costly and time-consuming modifications to the mold.
In the worst case, it can mean you have to scrap the mold and start all over again. The injection molding process is, therefore, completely unsuitable for projects where the design is not 100% finalized and validated.
What is the Difference Between 3D Printing and Injection Moulding?
Choosing your path requires a clear comparison. The difference between 3D printing and injection molding is not simply about which is better but which is best suited to your specific needs at a particular point in the life of your project.
Lead times
This is the most immediate difference. With 3D printing, the time to first part is usually 24-48 hours. With injection molding, it takes at least 5-6 weeks and often much longer before you see the first cast parts.
Set-up costs
Apart from creating the digital model, there are practically no set-up costs for 3D printing. The set-up costs for an injection molding project amount to the entire cost of the mold, which is a significant capital investment.
Volumes
This is the decisive calculation. 3D printing is the champion of small series production, from a single prototype to several hundred pieces. Injection molding is the ideal choice for high-volume production. The break-even point, where injection molding becomes more cost-effective than printing, typically falls between 500 and 2,000 pieces, depending entirely on the part’s complexity.
Materials
Injection molding offers an extensive, proven library of thousands of polymers with well-documented properties. 3D printing materials are more limited and often more expensive on a per-kilogram basis. If your part requires a very specific material quality for legal reasons (e.g., certain medical applications), injection molding is often the only choice.
Size of the parts
Both technologies are versatile, but they have different limitations. 3D printers are limited by their build volume, which is less than one cubic meter for most machines. Injection molding is limited by the size and clamping force of the machine; however, large machines can produce huge parts, such as dustbins or car bumpers.
Part tolerance
While high-end 3D printing can be very accurate, injection molding is generally the winner when it comes to maintaining consistently tight tolerances. A well-made mold can produce parts with tolerances of ±0.1 mm as standard, with even more precision possible through careful quality control.
Part strength
This is an important mechanical difference. As mentioned earlier, solid molded parts are uniformly strong due to their isotropic nature. The anisotropic, layered nature of 3D printed parts creates inherent weak points. For any functional part that is subject to significant load, injection molding offers greater strength and reliability.
Design complexity
3D printing enables immense complexity at no additional cost. Internal channels, hollow structures and organic shapes are easily realizable. With injection molding, complexity equals cost. An undercut, for example, requires a lateral mechanism in the mold, which increases tooling costs by thousands.
Surface quality
Injection molded parts have the exact surface finish of the mold cavity, which can be textured, satin or polished to a mirror finish. 3D-printed parts, especially those produced by FDM and SLS machines, often have a more matte or slightly rough surface that requires reworking to resemble a finished product.
Conclusion
The debate about 3D printing and injection molding is not about declaring one technology superior to the other. It is about understanding that there are different tools for different tasks, often used at different stages of a product’s life cycle.
The smartest approach being taken by innovators around the world today is a hybrid approach. They can utilize the incredible speed and flexibility of 3D printing for rapid prototyping and initial market testing. With a validated design and proven demand, you can then move confidently to injection molding for cost-effective, scalable volume production.
The difference between 3D printing and injection molding in the manufacturing process is clear.
Understanding it will help you make the right strategic decision to take your project from a great idea to a commercial success.