3D Printing, CNC Machining or Molding: What’s Best for Your Prototype?
James Anderton posted on February 26, 2016 |

Whether you’re an inventor looking to create a first prototype or a company doing a short production run, choosing the right process for your needs is no mean feat.

At the most general level, computer numerical control (CNC) machining, injection molding and 3D printing can all be used for short runs, but those technologies each encompass multiple processes.

For example, stereolithography, selective laser sintering and direct metal laser sintering are all types of 3D printing, but each use different types of materials. Similarly, injection molding can be used for plastics, silicone rubber or metal.

Metal parts made with (left) direct metal laser sintering, (middle) CNC machining and (right) injection molding. (Image courtesy of Proto Labs.)
Metal parts made with (left) direct metal laser sintering, (middle) CNC machining and (right) injection molding. (Image courtesy of Proto Labs.)

Factors to Consider for Prototyping and Short Runs

When presented with so many options, it can be difficult to decide between them. Fortunately, there are several considerations that can help narrow down the choice.

“Some people know what they want, and others come to us saying they need a part without knowing what material they need, let alone the process,” said Tony Holtz, technical specialist at Proto Labs.

“We try to start with how many parts they need,” Holtz continued. “After quantity, I might ask what type of materials or material properties are needed for prototyping or testing. From there, it may go all the way to the cosmetics. Finally, it comes down to cost.”

Taking these factors—quantity, material properties and cosmetics and cost—into account can greatly simplify the decision between 3D printing, CNC machining and injection molding.


Quantity

The number of parts you need is likely to be the biggest factor when deciding between these three technologies. Obviously, injection molding is the best bet if you’re planning on making several thousands of parts, but it is less cost-effective for smaller quantities.

Plastic part made with selective laser sintering. (Image courtesy of Proto Labs.)

Plastic part made with selective laser sintering. (Image courtesy of Proto Labs.)

“If you only need twenty parts per year, we would suggest CNC machining or 3D printing if the material properties and cosmetics all play well with your part,” said Holtz.

Ordering injection-molded parts typically involves a setup and production fees for the mold, so even if your parts only cost three dollars per piece to produce, total cost, including the mold itself, could be reason enough to choose a different process if you only need ten parts at a time. 

The point is, if you’re really set on injection molding but only planning on ordering 20 parts, you might as well order 200.

As a reference, Holtz offered the following rule of thumb: “3D printing is typically a cost-effective option for up to about 50 parts and CNC machining is economical for up to about 200, but our typical single-cavity molds can typically produce up to 10,000 parts.”

It’s worth talking to an expert if you’re uncertain of the best process for your project, especially if scalability is a concern.

Injection molded plastic part. (Image courtesy of Proto Labs.)

Injection molded plastic part. (Image courtesy of Proto Labs.)

“For injection molding, we highlight areas where draft is needed or where wall thickness is needed. If it’s too thick, we can apply a mold flow analysis of select materials to show where possible knit lines will be. If there’s any concerns for ejection or gating, we’ll highlight them. If you can’t have ejector pins on your part, we might go back to the design and revise it.”

Someone who’s inexperienced with injection molding may not take gates or ejector pins into account during the design process. That’s why it’s worth contacting an expert as early in your design process as possible.


Material Properties and Cosmetics

Historically, prototyping often involved using different materials compared to the final product to perform fit checks and similar tasks. Currently, that same strategy is available thanks to 3D printing technologies, which are ideal for testing fit and form of parts.

Thermoplastic-like chess board made with stereolithography—ant for scale. (Image courtesy of Proto Labs.)

Thermoplastic-like chess board made with stereolithography—ant for scale. (Image courtesy of Proto Labs.)

“Using stereolithography, with secondary processes you can get a part that looks like it’s been injection molded,” said Holtz. “You can use selective laser sintering to get better material properties than stereolithography, but then you’re limited to nylon materials.”

As always, there are trade-offs between different production processes in terms of achievable material properties and cosmetics. 

However, secondary processes can go a long way toward improving the cosmetics of a part.

“For molding, we apply the surface finish to the mold,” said Holtz. “We have seven that we offer, from an unfinished part to an SPI-A2, so you get the nice clear finishes or even matte finishes, like a light and a medium bead blast.”

“For 3D printing with direct metal laser sintering or stereolithography, you have a support structure built underneath the part,” Holtz continued. “After curing or heat treating, we hand remove those with cutters or saws or with wire EDM for metals.”

You might think that certain designs, such as the living hinge, would be restricted to a particular process. However, that’s not the case for Proto Labs, according to Holtz.

“We can accommodate that in all three services,” Holtz said. “In injection molding, we’re running ABS and polypropylene as well as high-temp materials like PEEK and Ultem. We have all production-grade resins available in thermoplastics.”

Aluminum part turned with CNC machining. (Image courtesy of Proto Labs.)

Aluminum part turned with CNC machining. (Image courtesy of Proto Labs.)

Holtz continued, “With CNC machining we can mill very small spaces. As long as it’s horizontal in the machining plane, we can get down to 0.010” – 0.015” wall thickness, so you can test your living hinge in polypropylene.”

“Even with 3D printing, we have thermoplastic materials like our nylon that will work as a living hinge,” Holtz concluded.

All this goes to show that the material and cosmetic requirements for your prototype are just as important as your application when deciding between 3D printing, CNC machining and injection molding.


Cost

Although it shouldn’t necessarily be the deciding factor, it’s always worth considering your upfront budget. An entrepreneur with limited funding for prototypes will most likely find 3D printing or—if it’s possible with the part geometry—machining to be the best option in terms of cost. 

In contrast, a company with a more substantial budget might find injection molding to be more cost-effective.

Injection molded rubber silicone part. (Image courtesy of Proto Labs.)

Injection molded rubber silicone part. (Image courtesy of Proto Labs.)

“Our injection molds start at $1,500 for the simplest parts that don’t contain side actions or complicated mold designs,” said Holtz. “Our standard delivery time is 15 business days or less; we have even turned around injection molds in as little as one day.”

This suggests that time is less of a factor than you might think when deciding between CNC machining, 3D printing and injection molding for your prototype. Instead, consider how your product will be manufactured and work backwards from there.

“Design something that can be manufactured with injection molding, then 3D print it,” Holtz suggested. “We’ve had people 3D print five parts and end up needing many more parts through traditional high-volume manufacturing.”

Being able to produce parts that can’t be manufactured using traditional means is usually seen as an advantage of 3D printing, but it can be a double-edged sword if it forces you to go back and redesign your product.

The advantage of starting with injection molding is that you’ll already have a backup mold in case your production tooling wears out.

“If customers started prototyping with us and still have parts running off a production mold, we’ll contact them and ask if they still need our mold,” said Holtz. “We encourage them to say yes, since we can store it until it’s needed. You don’t know when a production mold is going to crash. If you have an assembly line going and have a supply chain emergency, we can cover you.”


Prototyping: Where to Begin?

Holtz had one last piece of advice for inventors and design engineers who are just getting started:

“Contact us as early as possible. As soon as you have your first 3D CAD, even if it’s undrafted, we want to get that into our system and get you talking to an engineer. If you come to us three months into your design and you need parts in a week, you might hit a roadblock and have to go back to square one. But if you’re working with us from the beginning, when you need those parts, everything’s going to be in line.”

In fact, you can upload a 3D CAD file of your design directly to the Proto Labs website for a free design for manufacturability analysis and an interactive quote. The company has over 400 CNC mills, more than 70 molding machines and more than 50 3D printers, so capacity is never an issue.

“The scalability we have and the speed of our turnaround is what sets us apart,” said Holtz. “There’s also no cost or commitment upfront. All of our quoting is free of charge.”

For more information, visit the Proto Labs website.


Proto Labs has sponsored this post.  It had no editorial input into this post. All opinions are mine. --James Anderton

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