3D printing offers a fast way to trial new part designs. With a few hundred dollars for your own 3D printer, or by working with a vendor with a larger commercial system, you can repeatedly test out a new concept and tweak it until the functionality is dialed in.
But what you get in fast lead time and low capital outlay you pay for in part cost. Why? Because while injection molding has much higher mold costs, and takes much longer to get the first part, each subsequent part is produced much faster.
A typical 3D print may take several hours to complete a single part. An injection mold will complete the same part in ten or twenty seconds. And for higher-volume products, they can be made with multiple cavities, so more than one part is produced per cycle. It’s typical to see injection molds produce at rates that are 1,000 or more times faster than 3D printing the same part. A four-cavity tool making a thin part could produce 30,000 parts every day.
But it’s also important not to leap too quickly into injection molding. When is it time to switch?
1. You have tested and gained internal and customer validation on part design.
When you’re still finding the right part design, it’s too early to build an injection mold. Take your 3D printed parts and show them to customers. Try them out in their intended use case and see if they work the way they’re intended.
It’s expensive and not easy to change an injection mold once it’s built, and some changes will not be possible without major portions of the mold being replaced. So it’s best to be reasonably sure that you won’t be needing to change your part design before committing to a mold.
That’s one of the major benefits of 3D printing: it allows you to iterate designs repeatedly, and quickly, until you find the one that works perfectly.
2. Your volume demands are increasing.
An injection mold doesn’t make financial sense for very low quantities, because injection molds involve significant up-front costs in exchange for much lower per-piece costs. A requirement for 50 parts is going to be very expensive if you have to build a $15,000 mold first.
But once volume requirements increase to 500, 1,000, or tens of thousands and up, 3D printing will become much more expensive than injection molding.
Take a case of a simple industrial part around 5”x5x5” in size. It may cost $50 per piece to 3D print it. At 1,000 pieces, an injection molder may quote $5 per piece, with an up-front cost of $15,000 for the mold. Which is a better deal?
The injection molded rate is much better: the total cost is $5,000 for parts, and $15,000 for the mold, or $20,000 in total. To 3D print all 1,000 pieces would cost $50,000.
However, if you need ten parts to show to customers, it will be cheaper to 3D print them until your volume quantities increase.
These numbers are made up for the exercise, but it’s representative of the math for many similar parts.
The break-even point varies by product, but will typically be when your run quantity exceeds several hundred parts.
Volume requirements are the biggest reason product makers switch from 3D printing to injection molding.
3. You need special functionality or cosmetic appearances.
3D printed parts can achieve similar appearance to injection molded parts, but they can take a lot of post-processing work to get there. Surface texture such as gloss, or a special stipple pattern, will be difficult to achieve or very expensive with a 3D printed part, but can be polished or etched into the surface of an injection mold.
Similarly, certain product functionality can be more difficult to achieve with 3D printing, because layers of plastic melted together (either through fused deposition modeling, or laser sintering of plastic powder) do not have the same strength properties as an injection molded part, which is made from a single shot of molten plastic all at once.
Similarly, use of glass beads (for abrasion resistance) or glass fiber (for bending resistance) is more feasible with injection molding than 3D printing.
