Injection Moulding and 3D Printing

Injection moulding is the most common production method for plastic parts and items in the world. As such, it’s common to portray injection moulding and 3D printing as mortal enemies. But that’s not true. While there’s certainly competition between companies …

Ile Kauppila

September 27, 2021

Injection moulding is the most common production method for plastic parts and items in the world. As such, it’s common to portray injection moulding and 3D printing as mortal enemies.

But that’s not true. While there’s certainly competition between companies working with the two technologies, combining them actually benefits both.

Additive manufacturing can help mitigate the most significant shortfall of injection moulding and make it more widely available. At the same time, using a 3D printer in injection moulding processes affirms additive manufacturing’s status as a valid manufacturing technology.

Let’s take a look at what injection moulding is and how it can work together along an SLA 3D printer, like Formlabs Form 3.

What is Injection Moulding?

The traditional injection moulding process is rather straightforward as far as industrial manufacturing goes. At its simplest, injection moulding consists of four process steps:

  1. Machining a part mould from metal such as aluminium or steel.
  2. Injecting molten plastic into the mould cavity with an injection moulding machine.
  3. Waiting for the plastic to cool and harden.
  4. Removing the part from the mould and post-processing it if necessary.

The simplicity of the process plays a major part in injection moulding’s popularity. That said, injection moulding offers other significant benefits to plastic parts producers, as well. They include:

  • Cost-efficiency: The machines used in the injection moulding of plastics are highly automated, and as such the labour costs are low. Although the upfront costs of manufacturing the metal moulds are high, they’re provide an excellent cost-per-part in mass-production, allowing manufacturers to cheaply churn out massive numbers of plastic parts.
  • Detail and quality: An injection moulding machine pushes the molten plastic into the mould at an extremely high pressure, which causes the plastic to fill the mould completely. This results in parts with high tolerances and great surface detail, often completely eliminating the need for post-processing.
  • Speed: Injection moulding is an incredibly fast process. The speed naturally depends on the machine you use, but most machines can fill a new mould every 15-30 seconds.
  • Versatility: Injection moulding isn’t tied to one type of plastic. It’s easy to create parts from different materials — or even use multiple plastics in one injection. This allows manufacturers to quickly alter part properties.
  • Part strength: Different additives and fillers can greatly increase part strength. In fact, with the right material and additive combinations, you can even manufacture plastic parts that are capable of replacing metal ones.
Plastic Injection Moulding

What Are the Disadvantages of Injection Moulding?

All of these benefits make injection moulding sound like the Holy Grail of plastic part production. But there is one major disadvantage to the process that we already mentioned in passing.

That disadvantage is that making the necessary moulds for plastic injection is expensive. Very, very expensive.

Of course, the cost of the mould depends on the part it’s used to make. You can get a small, single-cavity metal mould for a couple thousand pounds, but the average price per mould is closer to £10,000. Very large and complex moulds can push the cost up to hundreds of thousands of pounds.

This exorbitant cost stems from the fact that injection moulds are CNC machined out of a solid metal block. The extreme precision and detail required doesn’t come cheap, and it also leads to long lead times on moulds.

The high up-front cost puts traditional injection moulding far out of reach of small producers. This is also why it’s mostly used for large-scale mass production — the high number of produced parts makes it more likely that producer sees ROI on the mould.

We should also mention that CNC machining is a rather wasteful production method. It could be that only a fraction of the original metal block makes up the final mould, while the rest ends up as shavings on the factory floor.

How Can 3D Printing Injection Moulds Help?

With the moulds demanding such high prices, how can we make injection moulding more accessible? The answer is simple — 3D print the moulds.

SLA (or stereolithography) 3D printers — such as Formlabs Form 3 — are able to print with materials that are fully solid and isotropic. This means the printed part has the same mechanical strength across all dimensions. Combine this with the materials’ ability to withstand temperatures of more than 200°C, and you have a functional, cheap injection mould.

Indeed, SLA 3D printing makes it quick and easy to create resin moulds that can withstand the high heat and temperature of the plastic injection moulding process. The technology produces moulds with surface quality and detail comparable to metal moulds, making it an ideal solution for producing small prototypes or functional parts in small batches.

But the best thing of all is the price. Desktop-sized, professional-quality 3D printers generally cost no more than a few thousand pounds — the same as one tiny metal injection mould. Only, with a 3D printer, you can create an endless number of resin injection moulds.

Prototyping injection moulds with a 3D printer

How Can I Solve the Challenges of 3D Printing Injection Moulds?

Of course, 3D printing is not a silver-bullet solution. Metal moulds have higher thermal conductivity, which means the injected plastic cools down slower and increases production time per part.

You must also account for special considerations when designing 3D printed injection moulds, such as part shrinkage and the type of resin material. Finally, 3D printed injection moulds degrade faster than metal moulds due to the heat and pressure of the moulding process.

These, however, are all manageable downsides. Nobody is born a 3D design master, and designing the moulds becomes easier with experience.

Protective coatings can extend the longevity of 3D printed moulds, and even if one mould breaks, you can print a new one in a matter of hours. As to the issue of thermal conductivity, it’s possible to optimize the production process to allow for longer cooling times.

You also have to consider that the option is usually not using injection moulding at all. 3D printing moulds makes injection moulding available to producers who couldn’t even dream of affording metal moulds.

Thanks to SLA 3D printers, even small operations can expand their product catalogue to include functional injection moulded parts.

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If you’d like to learn more about how to combine injection moulding with 3D printing, contact the SolidPrint3D team. Call us on 01926 333 777 or email us at info@solidprint3d.co.uk.

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