CAN INVESTMENT CASTING SOLVE YOUR PUZZLE
The part on the design screen is full of perplexities. The lead engineer can’t find a way to keep technical requirements and cost to manufacture in balance, and the buyer is wringing her hands. Fabrication is their usual go-to process, but this time, no matter how they look at it, it would be an impractical money pit.
It’s a common enough scenario, yet even with their thin wall sections, deep internal passages, and unbendable material, some parts are nonetheless pushed through fabrication like proverbial round pegs through square holes, and it doesn’t end well. The truth is, whatever your training, not everything can be machined out of a billet and hold up to the function for which it’s intended.
But at the fateful moment when part purpose, producibility and price are all locking horns, the often-overlooked process of investment casting may just be the process that saves the day.
Incentivized to secure the lowest price, buyers might be tempted to make the dollar their primary criteria, but potentially to the detriment of the part’s success in some cases. In fact, price often becomes a barrier to discovering the superior results that only investment casting can deliver––for the right application.
It won’t be the cheap answer, but in some cases, it may be the only answer. And buyers who are savvy enough to learn about this process and resourceful enough to pose questions to their foundry partners are the unsung heroes and problem-solvers in parts manufacturing.
“It’s probably hard for OEMs to quantify, but a good casting buyer gives a lot of value to their company,” said Jay Harmeyer, owner at AFS Corporate Member Fisher Cast Steel in West Jefferson, Ohio. “They know who to go get costs from, what foundries are a fit, and they can screen [casting sources] better. If they go with the lowest cost, that could work sometimes, but it can also be a complete mess if they didn’t ask the right questions.”
Investment casting should be carefully explored when the part requirements include very thin wall sections, high internal intricacy and tight tolerances.
“One of the things that drives us to investment casting is part geometry.” said Steve Shade, general manager at AFS Corporate Member Craft Pattern & Mold Inc. in Montrose, Minnesota. “So that encompasses wall section—in investment casting, you’re pouring the metal at higher temperatures ... and you’re able to fill the mold with thinner sections. That’s one big consideration. We’re also generally able to hold a tighter tolerance with an investment casting over other gravity casting methods like sand and permanent mold.”
Parts requiring a lot of detail are less compatible with other casting processes such as sand, but they’re no problem for investment casting. A part you can hold in your hand is an ideal size, though one-off parts as big as 300 lbs. are doable. Low to medium volumes are typical, and jobs exceeding 10,000 are likely better suited for sand or permanent mold––the exception for high volume is only when one of the attributes above are non-negotiable.
The process gets top marks for superb surface finish, which can eliminate a good deal of post-process machining, and thereby remove cost. Another major advantage: While ferrous material is most common for investment casting, foundries with this capability can pour almost any alloy imaginable, with the exception of some super alloys.
The Right Call
Applications and industries well-suited to investment casting are too numerous to cite, but a few examples sources pointed to include aerospace, wear parts for glass-making, food handling equipment, dental and optometry, impellers for flow applications like oil and gas, as well as chemical processing’s small valves and intricate components. In fact, the process can bring efficiencies to countless parts––all it takes is some out-of-the-fabrication-box thinking at both the design engineering and buyer levels.
“There is a tremendous amount of fabrications out there, especially in the aerospace industry, that could be easily converted to really any kind of casting process,” said Steve Olson, president of AFS Corporate Member Bimac Precision Castings near Dayton, Ohio. “When you do a fabrication, it could involve stamping the part out, bending the part and welding the part, and it’s unbelievably labor intensive. Whereas with a casting, you just pour it. The other advantage when you go to investment casting is ... let’s take the welding process, for example: Maybe you’re making a part where the wall thickness is 100,000ths. Those parts are very difficult to weld because they’re so thin; you need a really, really good welder and a high technology welding process. But we can cast it with no problem, no welding, no worry about structural integrity.
“And most times we can add features to the part that they wouldn’t be able to do as a fabrication,” Olson added. “The biggest thing that comes to my mind are internal passageways. You cannot do that in fabrication unless you put it on a mill and drill those passages out, and even then, you can’t get blind passages. It’s only in the investment process that you can do that.”
Of course, fabrications involve welding pieces together, Harmeyer noted, and where there’s a weld there’s a way for cracks to occur along weld lines.
“Whereas, if you have a casting, it’s one continuous piece of metal; you’re not putting it together,” he said. “So, the tolerances are going to be better as well, because you’re not welding multiple pieces of metal together.”
Thanks to that one-piece nature of casting, manufacturers can actually realize significant cost savings on things they might ordinarily add in separate post-processing machining steps––things like a logo, knurling or threading. These types of parts characteristics can be incorporated right into the mold and thus cast into the part for free, Shade explained.
Investment casting can cure a lot of fabricating production pain but traditionally comes with a cost that’s both monetary and time-related––in the end, buyers and designers have to weigh the total cost of acquisition, Shade said. You can achieve those internal pockets like no other process can offer, but the costs of tooling, for example, and the unique, complex steps of investment casting contribute to a price tag that may cause a little sticker shock. Again, the operative word is: weigh ... all factors of both fabricating and casting as you tally up the true costs, value, abilities and limitations.
“If you’re looking at a fabrication and what it’s going to take to make it,” said Shade, “you have the material cost, you have the processing cost, you have the feasibility of even making it as a fabrication with some of the different cavities within the part––can you even do it in that fashion? Investment casting might hold an inherent advantage over a fabrication process based on that alone because it affords the ability to do some things you can’t with machining operations or other fabrication operations.
“But the cost of making an investment casting––well, it’s a fairly labor intensive process, although automation and robotics are finding their way into it very rapidly.”
Complexity parlays into time, too, and the investment casting process has historically required eight to 10 weeks for tooling lead time, although foundries continue to nudge that down to just a few weeks.
Bottom line, the efficiencies of investment casting could easily outdo fabricating on cost, so buyer research––even a phone call or two with a foundry rep––is worth the effort.
“From shooting of the wax to shelling of the mold, flashout, pouring, to gating and heat treat might take a couple weeks to work that through a typical shop,” Shade said. “If that was a machined or fabricated component, it’d be a function of the amount of time spent in a machining center and the number of operations. And while that process might go quicker, once started, it’s most likely much more expensive and not as cost effective.”
While buyers are certainly tasked with counting the cost, Olson sees another priority that matches, if not overtakes, the numbers side of the buying equation.
“Back in the day, you competed on price, and then you competed on quality,” he said. “Now, when you quote work to a customer, those two have become commodities. Everybody is just assumed to be at some level there ... What we compete on now is lead time and delivery.”
3D & the Great Time-Shrink
3D printing in investment casting is rewriting the utilization gamebook for this process right now, and has the staying power to alter the future, as well.
Finishing his thought about time and delivery––in light of 3D printing, which replaces lengthy tooling creation––Olson added, “What we’ve become competitive on is, ‘Hey, I can produce first articles in two or three weeks versus 15 weeks. And because they’re all going to just in time, my lead times go from eight to 10 weeks to four to six weeks. And so that’s where foundries are pushing in order to be more competitive, because [manufacturing] companies don’t want to carry inventory anymore. It costs money.”
Fisher Cast Steel recently 3D printed 300 parts, 1 in. by 1 in. and a 10th of a lb., for less cost than building a tool, said Harmeyer. “You can do these low volumes, but it’s faster and at less cost compared that to traditional investment casting where you would be doing the expensive tooling.”
One-off prototyping has become more commonplace among investment casting foundries, with lead times dropping to two weeks or less.
Shade says the bulk of Craft Pattern & Tool’s investment casting sales is now prototype work done using 3D patterns, with volumes ranging from a one-off to 10–25 pieces. “Early on in the design phase, where the design isn’t completely validated and ironed out, the customer wants to validate some of the part properties or characteristics or the performance and assembly,” said Shade.
Olson said his company casts individual 150- to 200-lb. custom impellers as rapid prototypes. Similarly, he says many foundries are doing away with all their tooling and molds and printing all their patterns, even for high volume jobs. Aerospace, primarily air foils for turbine jet engines, are going more toward 3D printed patterns instead of injected wax patterns, he said.
His company has been able to shrink the process down to 10 days, and in one extreme example of “babysitting” the part––literally walking it through the shop by hand––they even achieved a six-day turnaround ... for a premium price.
Olson predicts in 10 years 75% of work run in an investment foundry will go to 3D printing.
Now Ask Yourself
Bring your questions and ideas to the foundry early in the design stage, sources advise. Have a conversation and educate yourself as much as possible to determine whether investment casting really is the more attractive and viable alternative to fabrication.
But before sending out the RFQ, buyers should coordinate with parts design engineers and together ask themselves: What do we really need on this part? What tolerances are required? What surface finish, strength, weight and alloy are important? Let these variables drive the process, and strike out hollow wish lists that merely add cost and complexity with no substantive benefit. Think it through, and avoid waste.
“What’s critical?” said Shade. “Sometimes we see castings that are almost 100% machined after they are cast––it kind of destroys the whole point of the casting option. Look at where the application is, start asking questions; it may turn out that 70%, 80%, or 90% of the design isn’t even critical. It’s just a case of someone wanting it to have that machined look.
“The other thing is, ask yourself, ‘How much simplification can we do to the part so the casting process will be successful?’”
Kim Phelan - CS
Source: Casting Source
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