Comparing Injection Moulding and 3D printing: Factors to consider for Small and Large Volume Production

Where injection moulding is already a popular manufacturing process for medical devices, 3D printing is becoming increasingly popular. While both approaches have advantages and disadvantages, businesses must carefully consider several factors to determine which method best suits their needs. This article will discuss critical factors comparing injection moulding with 3D printing and metal versus 3D printed moulds.

Small vs Large Volume Production of plastic parts

Production volume is one of the primary considerations when choosing between injection moulding and 3D printing. Injection moulding is typically more cost-effective for large-volume production runs, whereas 3D printing is more suited for small-volume production. 3D printing has a lower upfront cost but is usually slower and more costly per part.

Small Volume Production

Small-volume production is typically used for prototypes, specialised products, or niche products with limited market demand. Small-volume production can be done using traditional manufacturing techniques such as injection moulding, casting, and CNC machining or with newer methods such as 3D printing.

The benefits of small-volume production include flexibility, customisation, and cost-effectiveness. With small-volume production, manufacturers can quickly adjust their production line to accommodate changes in demand or design specifications. Furthermore, small-volume production can be more cost-effective than large-volume production due to lower mould expenses or set-up costs.

However, the downside of small-volume production is the higher unit cost of production. Since the production runs are smaller, the price per unit is more elevated. Small-volume production also tends to have longer lead times and may not be suitable for products that require high-speed manufacturing or economies of scale.

Large Volume Production

Large-volume production is used for producing high quantities of products. The manufacturing process is automated and optimised for efficiency, allowing fast and cost-effective production.

Large-volume production often uses assembly line, injection moulding, and stamping techniques. These methods allow for high-speed production and can be adjusted to produce different types of products.

The benefits of large-volume production include lower unit cost, shorter lead times, and consistent quality. Since the production runs are larger, the price per unit is lower, making large-volume production ideal for products with high demand. The shorter lead times make large-volume production ideal for fast-production products.

However, the downside of large-volume production is the need for more flexibility and customisation. The manufacturing process is optimised for efficiency, making it difficult to adjust the production line to accommodate changes in demand or design specifications. Large-volume production also requires significant upfront investment in machinery and tooling, making it less suitable for specialised or niche products.

Micro-moulding versus 3D printing

Micro-moulding has become increasingly popular in recent years, allowing smaller parts to be produced at a lower cost than traditional moulding methods. This has made injection moulding a more viable option for smaller production runs and has narrowed the gap between injection moulding and 3D printing for low-volume production of small parts. Another upside of micro-moulding is the possibility of scaling up the output to large volumes with an unchanged design.

In the picture below, one can see two tabletop micro-moulders on the top row (semi-automatic and manual), the bottom left is a FormLabs 3D printer (SLA), and the bottom right is an injection moulding machine.

One of the reasons that micro-moulding is getting more popular is the choice of mould materials. In general, two types of mould materials are available.

1. Plastic 3D printed moulds.
2. Metal moulds.

Plastic moulds

Plastic moulds are usually used for very low-volume production. The exact number of products made from the mould depends on different aspects (material, temperature, shape, required accuracy), but one should think in the range of 100s before the mould is worn out. The moulding rate and accuracy are generally lower than with metal moulds. However, 3D-printed moulds are improving in accuracy and can produce moulds with tolerances suitable for many injection moulding applications. The accuracy of 3D printed moulds can be enhanced using higher-end 3D printing technology, such as stereolithography (SLA) or digital light processing (DLP), which can produce moulds with smoother surfaces and finer details.

The lead time for designing and making 3D-printed moulds is much shorter than for metal moulds. So, during prototyping and initial sample production, 3D-printed moulds can be considered. Formlabs has a nice white paper on this topic on their website.

Metal moulds

Metal moulds are always used for high-volume production with one or multiple cavities per mould. Steel is more expensive, as well as mould production. The accuracy of metal moulds is higher than that of 3D-printed moulds. The complexity of the part geometry, the desired tolerances, and the quantity will play a role in determining which type of mould material is best suited. Metal moulds are known for their long lifespan (hundred-thousands or millions of cycles)  and durability. Aluminium moulds are less expensive and can be used for medium-sized volumes of tens of thousands to a few hundred-thousands parts. The lifespan of a mould depends on the mould material, the polymeric material being moulded, the operating conditions, and the maintenance schedule. Metal moulds are made of high-strength materials such as steel or aluminium, which can withstand the high temperatures and pressures required for injection moulding.

So, in summary, micro moulding has higher start-up costs than 3D printing but allows for a more comprehensive material selection and higher speed. Still, the startup costs can be lower compared to regular injection moulding. However, micro moulding has a limit in the dimension due to smaller (tabletop) injection moulders with limited pressure.

3D printing provides better flexibility and is the choice for larger parts. Jointly they are complementary.

Upscaling

Upscaling can be tricky for medical devices. When changing from 3D printing to injection moulding, proving that the product performs precisely the same in clinical conditions is a challenge. Possibly, a clinical evaluation is required. This is very expensive.

Upscaling from micro-injection moulding to injection moulding might reduce this problem. Validation is always needed, but as the technology before and after scaling is the same, it is easier to prove that the product functionality and safety are equivalent or better.

Overall, both injection moulding (or micro moulding for low-volume production of small parts) and 3D printing have advantages and disadvantages. For high-volume production, injection moulding is preferred, but for low-volume production, the choice between micro-moulding and 3D printing depends on the design and specific needs and requirements.