Fanuc CNC, CNC Turning, Vacuum Casting, or Injection Molding? A Buyer's Guide
There's no single right process
I'm the office administrator for a 40-person product development company. That title doesn't sound like it should involve manufacturing decisions, but I manage roughly $300,000 in annual vendor spend across 12 suppliers, process about 60–80 purchase orders a year, and report to both operations and finance. When an engineer asks 'vacuum casting vs injection molding?' or says 'we need brass parts,' I'm the one who turns that into a PO. And the hardest lesson I've learned is that there is no single right answer.
It took me four years and a lot of POs to understand that process selection is a cost problem, not a technology problem. The cheap unit price rarely wins if you count setup surprises, inspection failures, rework, and delayed schedules. I still compare prices. I just calculate total cost first.
Three scenarios I keep coming back to
In my experience, most custom part orders fall into three buckets. If you can recognize your bucket, the decision usually gets easier.
Scenario A: Low-to-mid volume metal parts — CNC turning
When the part has to be metal, the quantity is between 25 and 500, and the tolerance is tight, I start with CNC turning. Brass is a common material because it's dimensionally stable, easy to machine, and works well for fittings, valve parts, and electrical terminals. I've used several CNC turning brass suppliers over the years, and the good ones always ask about material grade, surface finish, and thread callouts before quoting. That attention is worth money. Honestly, it saves me from having to chase down a bad batch.
For quick sourcing, a CNC turning online quote request is the fastest way to compare shops. Upload a STEP file and a PDF with tolerances, and most suppliers reply within a day or two. But don't treat the first number as final. In January 2025, according to public quote listings on major on-demand manufacturing platforms, a simple brass bushing varied by 38% between the lowest and the highest quote, for the exact same drawing. The low quote didn't include deburring or a material certificate. The high quote did. The real cost difference wasn't the part price.
I also pay attention to the controls on the shop floor. A lot of the shops we use run Fanuc CNC controls. That matters because if a dimension changes mid-order, a shop that knows Fanuc CNC control screens can usually adjust the program and keep the job moving. Many operators keep a Fanuc CNC controller PDF with alarm and setup instructions on a tablet or computer instead of flipping through paper manuals. Fanuc isn't the only control in the world, and some of our best parts came from Siemens-controlled machines. But if I see a supplier with Fanuc experience and good documentation, I count that as lower risk.
Scenario B: Mid-volume plastic parts — vacuum casting vs injection molding isn't the right question
For plastic parts in quantities of 20 to 200, vacuum casting often beats injection molding, despite what most online guides say. I kept reading that vacuum casting is just for prototypes. In practice, it carried us through our 2024 vendor consolidation project when the design wasn't locked. Injection molding would have meant a $10,000 mold before we had a final geometry. Vacuum casting used a silicone mold for about $2,000, and we got 80 parts in three weeks.
The material was a polyurethane casting resin, not a production plastic, and the silicone molds wear out after maybe 15–30 pulls depending on the part. If you need 10,000 parts, vacuum casting is the wrong process. But for a bridge run while production tooling is being prepared, it's genuinely useful. The correct comparison is not vacuum casting vs injection molding in general. It's whether the volume and design stability justify hard tooling. Honestly, I wasn't expecting vacuum casting to hold up that well in field testing, but it got us to production without a tooling disaster.
Scenario C: High volume, stable design — injection molding
When quantities go beyond a few thousand and the part is locked, injection molding is usually the value winner. I know that sounds obvious, but I had to do the math myself before I believed it. A molded part might cost $0.80 per piece at 20,000 units instead of $4.00 at 500 units. The mold might cost $15,000. If the design doesn't change, the total cost math points to injection molding.
Just be careful to include the mold, the minimum order quantity, and secondary operations in your calculation. Some injection molders quote a low unit price but require a 10,000-piece minimum, which shifts your cash flow and your inventory risk. The per-piece price is never the whole story. I use a simple mental formula: tooling cost divided by expected quantity, plus unit price, plus the cost of a design change. If that total beats the next option, that's the process I recommend.
How to tell which scenario you're in
Start with four questions:
- Does the part need to be metal? If yes, CNC turning or machining. If plastic, keep going.
- What's the expected quantity over the product's life? Under 500, CNC or vacuum casting. Over 2,000, injection molding is worth a serious quote.
- Is the design stable? If engineering is still making changes, avoid hard tooling.
- What does the part have to do? Tight tolerance threads and metal-to-metal sealing require CNC turning. Cosmetic, complex, plastic-like shapes at low volume work with vacuum casting.
If you're still unsure, get quotes side by side. For metal parts, request a CNC turning online quote from two or three machine shops. For plastic parts, ask one supplier for a vacuum casting quote and another for an injection molding quote. Then compare tooling cost, piece price at expected volume, lead time, and risk. That exercise has killed more bad ideas than any spreadsheet I've built.
One more thing about price
Let me finish with a story. In 2021, I found a new supplier who was $200 cheaper per order than our regular brass shop. They couldn't provide full material certificates, but I was trying to look good to finance. When the parts reached assembly, 18 out of 250 failed the pressure test because the thread form had been cut inconsistently. The line stopped, we paid for an inspector, and the project lost more than $2,000. The $200 savings became a $2,000 problem.
Now I verify material certifications, invoicing, and quality documentation before I sign the order. That's not because I'm against negotiating price. It's because value over price means paying for what's required so you don't pay for it twice.
So which process should you choose?
It depends. I can only speak to my context: a 40-person company making custom electro-mechanical products, with unpredictable design cycles and a finance team that asks questions. If you're running a high-volume product with a stable design, my advice changes completely, and injection molding will likely win. If your part is metal with tight tolerances, don't let anyone sell you a plastic alternative just because it's cheaper per piece.
Trust me on this one: calculate the total cost, ask about machine controls and documentation, get quotes from capable Fanuc shops, and avoid the trap of the lowest quote. The right process is the one that gets you a usable part, on schedule, without making you explain a $2,000 problem to your VP.