3D printing/additive manufacturing and injection molding do not have to be competitors. In fact, 3D printing can aid injection molding. Formlabs recently commented that 3D printing is more of an accelerator for injection molding. 3D printers can introduce a hybrid approach where molds can be created fast and at low cost. Here are key points from an interview with Kathy Bui, engineering vertical lead at Formlabs.

Injection molding is a tried-and-true process for very high-volume production of plastic parts. But it’s not without drawbacks, one of which is the length of time it takes to create a mold from metal, such as aluminum or tool steel. It usually involves traditional machining and can take up to four to eight weeks. The cost can range from several thousand dollars to more than $100,000 depending on the complexity of the mold.

3D printing can be used to reduce both time and cost. A 3D printed mold can be turned around within 24 hours, or a few days at most, at a cost of a few hundred dollars, labor included.
An advantage of beginning with 3D printing to make molds for injection molding is the ability to iterate quickly. With a 3D printed mold taking roughly a day to build, and then molding the part, you can perfect the design in far less time than it takes to machine one metal mold for an initial pass at a design.

Another advantage is the cost savings in engineering changes and retooling that come with traditional methods of machining and injection molding. “With 3D printing you can feel more comfortable iterating while you’re trying to ramp up your manufacturing process because the cost is much lower,” said Kathy Bui.

Once the part is finalized, then a traditional mold can be machined if an adequate number of final parts is needed.

The use of 3D printing in this way becomes a hybrid process, where 3D printing handles the initial design, iterations, and even low volume production, all to get to market faster. 3D printed molds are usually not meant for high-volume production, but they serve well in low volumes.

More materials are available for 3D printing a mold. Some SL materials—plastics or composites of plastics with a filler—can handle a number of low-run injection molding applications. Formlabs, for example, offers a high-temp resin that can withstand temperatures upwards of 238 degrees Celsius. Another option is a rigid 10K resin that can handle limited-run production molds because it is very stiff. The 10K stands for 10,000 megapascals–the stiffness of the material. It comes close to the stiffness of metals, such as aluminum. Plus, it allows for better accuracy in a part.

When considering a material to use for a mold, consider the pressure the mold will endure as material is pushed into the mold. If the chosen mold material is too flexible, the mold will flex, create flash, or result in inaccurate parts. With a stiffer material, the mold can retain the geometry as the pressure is introduced during injection. So, part number one versus part 1,000 can still be fairly close and fewer variabilities.

A mold made from a plastic material is not a one-for-one drop-in replacement of a more rugged mold. There are certain design considerations to make, such as creating chamfers or reducing the angles of the mold. The number of shots you can get from a mold depends on the geometric complexity of the part. For example, how many pieces need to come together to make this mold? Are there inserts? And so on.

Formlabs worked with Braskem, a leading petrochemical company, in developing a mold for a part it needed. The Formlabs engineers used one of the high-temp resins to iterate a mold that would be used to make mask straps for their workforce at the beginning of the pandemic. Each mold was used for more than 1,500 shots. Some models went longer, depending on the design and the pressure and the types of plastic being injected.

Design tips for 3D printing molds follow similar tips for designing any mold that will be used in injection molding. For example, pay attention to draft angles, because the sharper the angles, the higher the stress risers in those angles, making it easier for the mold to fail under high pressure
Recently, some companies have developed a material suitable to create molds that closely mimic aluminum. These materials will likely offer more shots but there are still cautions. Any porosity or defects in the 3D printed mold with affect the life of mold.

You can either develop a mold yourself on your 3D printer or work with service providers/injection molders. Not all injection molding service providers are familiar with working with a 3D printed mold.

Noted Bui, “Sometimes there is a level of convincing that you need to do before they accept the job. But we have had success where people have been skeptical and tried it and said, ‘Actually, this works out pretty well.’

“We have a couple of other companies like Novus Apps, that are a design consulting firm, and they do their own injection molding in-house. They use our printers and our materials to design the molds and do limited-run production.”

Thus, if you have the 3D printers and the injection molding capability, this is an option you can consider when you’re developing your designs?

Formlabs offers guidelines on working with materials for 3D printed molds.