By Megan Ray Nichols
Additive manufacturing — especially 3D printing — has captured the attention of hobbyists and manufacturers everywhere. Its capabilities and demand are certainly on the upswing. However, it would be premature to count out the more traditional subtractive manufacturing techniques.
Additive manufacturing builds objects by depositing a chosen material layer by layer. Subtractive manufacturing, including carving, milling, laser cutting and others, removes portions of a solid material until it reaches the desired shape.
There are still many situations where subtractive manufacturing makes better practical sense for the application at hand. Here are a few of them:
1. Time Is of the Essence
Additive manufacturing lends itself well to rapid prototyping. Unfortunately, 3D-printing a product is still not as “rapid” as subtractive manufacturing can be. If the production time is of critical importance, manufacturers should know whether to budget their time for the few minutes injection molding requires or the several hours a 3D printer requires to complete a workpiece.
2. There Aren’t Enough Resources for Retraining
The 3D printers currently on the market are impressively capable machines. However, they do require at least a small amount of retraining before technicians can use them confidently.
Machine calibration for 3D printers covers multiple variables, including bed leveling and temperature, nozzle alignment and temperature and several others. The problem of machine setup and calibration increases manifold if the factory or shop is ramping up for a mixed lot of products with multiple SKUs.
3. The Project Calls for Large Quantities
This point goes along with the time considerations mentioned earlier. 3D printing is most cost-effective in projects involving small batches. For the time being, these machines can’t compete at scale with subtractive manufacturing price-wise.
Post-processing is a related concern. Traditional manufacturing processes typically produce parts that require little or no post-processing. On the other hand, 3D printing may require any number of different steps before the product is ready for sale. These include support removal, sanding, smoothing using acetone, priming and/or painting, hydro dipping, electroplating, polishing and others.
That is not to say machined, milled or injection-molded parts don’t require additional handling to achieve the desired aesthetical or functional effect. But this drawback to additive manufacturing is a common enough headache that some 3D printer manufacturers now claim to be working on models that do away with post-processing requirements for good.
4. The Workpiece Is Very Large
Technology news sites get justifiably excited when large-scale printers turn out bridges and even boat hulls. However, printing massive objects is still severely cost-prohibitive compared to CNC machines and other industrial manufacturing equipment.
A 3D printer capable of producing a five-inch object could cost around $1,000. Upgrading to a 10-inch workpiece means buying a machine in the $7,000 to $20,000 range. Manufacturing a five-foot-long product this way requires a printer in the $250,000 arena.
On the other hand, using a more familiar CNC machining environment unlocks a world of large-scale potential. Some multi-axis CNC machines have room enough for 30-foot workpieces. Naturally, this is not to say that owning such equipment is cost-effective for every manufacturer.
For the largest manufactured products, outsourcing the job to a fully equipped machine shop makes the most financial sense. The equivalent additive manufacturing technology does not yet exist in a widely accessible form.
5. The Machine Shop Is on a Restricted Budget
The cost of raw materials is another reason why 3D printing is cost-effective only for small quantities of parts. Manufacturing industry experts estimate the following price ranges for common raw materials in 3D printing:
- Thermoplastic filaments (ABS, PLA, etc.) — $20-$70 per kg
- Powders (SLS, PA, etc.) — $45 to $75 per kg
- Resins — $50 per kg
- Metal powders — $350 to $550 per kg
The raw materials used in CNC machining and other processes are more accessibly priced — at least for now. If a machine shop spends $90 on CNC-ready stainless steel, an equivalent amount of 3D-printer-ready steel powder could run as high as $1,300.
The difference in plastic prices is less pronounced but still present — $17 spent on CNC-ready ABS or $30 spent on CNC-ready nylon corresponds to a $40 purchase of 3D-printer-ready nylon powder.
Research from the University of Texas at Austin indicates that, while raw material prices are not the most substantial expense in either additive or subtractive manufacturing processes, the cost to manufacture an item through SLM additive manufacturing could be as much as 60% higher than using milling to produce an equivalent piece.
Additive vs. Subtractive Manufacturing
There’s no question that 3D printing has already turned the manufacturing world on its ear. However, it’s premature to count out subtractive manufacturing. Even regulatory institutions like the FDA are busy actively studying additive manufacturing and its implications for human health and a more sustainable approach to conceiving and manufacturing products.
For the above reasons, it’s still a good idea to consider additive manufacturing for your next production run. A day is approaching when 3D printers may be the new normal, but we’re not quite there yet — at least, not for every application.