Investment Casting 101

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Are you searching for parts with higher performance? Are you looking to push the limits of what’s possible in casting? And to do it all in a fraction of the time of old-school alternatives?  

Whether you are new to the investment casting process or you’re interested in learning how Signicast does it better, download our free, online introduction to investment casting seminar.

Gain insight on:

  • The investment casting process
  • Continuous flow manufacturing
  • Unique Signicast automation

Fil out the form below to access the slides and video recording. 


Investment Casting 101 Transcript

Fill out the form to download the entire seminar and see the answers to the live Q&A portion of the presentation. 

Taylor Topper:

I’d like to introduce Adam Curtis, our presentor for today’s seminar. Adam has been with Signicast for 17 years. He is currently the Estimating Manager where he and his team develop proposals for new projects, including piece and tooling price and lead times, as well as engineering design and feedback. Adam graduated with an industrial engineering degree from Milwaukee School of Engineering. Adam, thanks so much for presenting our first Signicast webinar.

Adam Curtis:

Thanks, Taylor. I thank everybody for attending. I’m looking forward to this. We do have a quick agenda here. We’re just going to...the goal for today is to get you acquainted with the process of the investment castings just so you can have a feel for all the steps and some of the tooling that’s required and some of the things we think about as we were processing these parts. So we’ve got a few things here, some alloy options, some of the lead times will end up working when we’re done, and then a few things we do throughout the process.

So, the first step for us is to actually build a wax tool. Now, we have some flexibility in tool design. We can do manual tools, semiautomatic tools, automatic tools, so depending on part shape, part volume, complexity, we can design different types of tools to handle that. Now the tool itself is made out of 7075 aluminum in a T6 heat-treated condition. The tool is made from aircraft carrier aluminum because the cavity really doesn’t wear out over time. So the wax itself is low pressure, low temperature, no abrasion, so the cavity will last essentially for the life of the project. 

So, I’ll point out a few things on the cavity and on the tool here so you can see what I’m talking about. So the cavity itself is in here and this is just the aluminum that’s been cut to form the feature. Now there are a few other things, the tool that you can see to make some other features, so there are slides and things like that. Now those do wear out over time, so we’ll add some features to those to give it a robustness, so there will be hardened gibs and rails in here on the slide, dissimilar metals on the wear plates, things like that that will reduce flash and extend the life of the tool. We also add some features like robust leader pins and bushings. Those reduce party-line mismatch over time, you know, giving us a little better quality pattern. We do implement a cleaning and repair schedule throughout the life of the project, so as some of these wear items do actually wear we’ll put in a process to make sure we’ve got a good tool.

Now, there are some features that we add to our tool to improve the process control. You see out to the side here, there’s some connections for water lines in a thermocouple? These are added to give us some control over the temperature of the dye, so we want to maintain the dye temperature from the first shot to the last shot throughout the lot for this part, so it’ll give us some consistency there and the less variation we have in temperature of our wax tool, the less variation we’ll see in the size of our wax pattern. It’ll give us a little more control of our metal parts in the end.

Now, we typically do not have to add draft to the design of the tool or the part. So, as this wax pattern is sitting in here the wax will shrink just a little bit as it’s cooling and that will allow this ejector plate here to push the pattern out of the tool with typically no draft, so it’s pretty good on the design side when you don’t have fuss over adding draft to your parts. 

There are some things we do on the tool design to give us some flexibility in part scheduling. I’ll show you some of the things we do in our plant where we like to do smaller lot sizes more often, more of a true just-in-time manufacturing philosophy, and the way we can do that, these clamp plates, PLC bundles, airline bundles, all those features allow us to quickly tear down and set up the next job on the wax press. So with short changeover times we can do smaller lot sizes more often.

Now the tools themselves, the cavities, we make most of our tools in our tool shop and the way we do that is we take the customer’s native file, Pro/E, Solidworks, Unigraphics, or a STEP file, and well apply a shrink factor to that model. We’ll then use that model to develop the cutter paths after the tool is designed in our CAD system. We’ll then send a cutter paths to our 5-Axis CNC machines where we’re able to cut the entire cavity, cut the mold base, cut all the water lines in place so we can put in a block of aluminum and we can pull out a finished dye cloth, so an efficient way to cut tools and to build these tools into the process.

So now we’ll go into injecting the wax pattern. So after the tool is built it’s designed to inject this wax pattern, and we’re going to use this wax pattern to make our metal part. So we need to inject a wax pattern for every metal part that we’re going to ship, and I’ll show you some of the features on our wax pattern. So the pattern itself is an exact replica of the metal part that we want to make, so all the features and details you want to see in your metal piece we’re going to put into our tool and then they’re going to show up on our wax pattern.

So, for this first step we use what we call virgin wax, so this is formulated specifically to make these patterns and we’ll use this wax one time. That virgin wax gives us a little more dimensional control throughout the process of injection in making the parts. If you’re at Signicast we actually formulate our own wax. We have a facility that can make it, pump it down to the different foundries. We use it, we recycle it, and it never really leaves our plant, so we have an efficient way of producing wax within our internal plant here at Signicast.

Now I’m going to have Taylor play a video for us where we can see the injection process. We’re going to do the semiautomatic process first. Okay, so what you see here is the wax press opening up and then the robot will reach inside and grab that pattern, and one opportunity we have here is the flexibility of using the robot. We can also use an operator to reach in there and grab the pattern. After the pattern is picked out of the tool it’ll be placed into the water bath to give it a soft position to land, and then the robot will finish the cycle by spraying a little mold release into the dye which is what you’re seeing now. Now up above the press is a wax temperature heat exchanger where we’re controlling the wax temperature to make sure each shot has the same wax temperature going in.

So, this is a shot of the automatic wax press where you’ll see the dye open up. The ejection of the patterns will just come out and there’s a water bath down below that the parts land into. The parts are then conveyed up to the operator where they will be assembled onto the mold.

So, a couple other features to note. The wax pattern that you’re looking at, this little section here on the backside, this is our gate. Our engineers will design a gate that will fill and feed the casting correctly, and then on the contact here where the gate meets the part we put a little notch and we use that notch to remove the part later in the process, and I’ll show you how we do that as we get to part removal 

Now that we’ve injected our wax patterns we’re going to assemble as many wax patterns onto a sprue as we can. So the square piece of wax that you see on the lower right-hand portion of the picture, that’s our sprue. The white portion at the top here, this forms our cup, so when we attach our cup, our sprue and the pattern itself this becomes our mold. This mold becomes the one piece flow throughout our plant, so we can track everything by part number, lot number and mold number.

Now what you see here is the operator welding the gate onto the sprue and the little welding torch he has there, it’s called a water welder, so we’re just using hydrogen and oxygen, we’re just burning it to make water, but it’s hot enough to melt some of that wax so we can make a little puddle of wax, so the gate attaches and adheres to the sprue. So this is quite a talent to be able to create a good seal here of where this gate attaches, so we have a good contact that the shell builds correctly around this mold.

 

So now our sprue is made from recycled wax where the pattern itself is made from virgin wax, so the dimensions and size we’re not as fussy about on the sprue. We really want to control dimensions and size on the padding because that’s ultimately the part that we’re producing.

 

What you’re looking at is they’re assembling three to six of these molds onto a cluster and there’s just a large bracket up on top where we can assemble these, and this becomes our minimum lot size to run this job through the facility.

We’ll play a video here of showing the cluster getting assembled. Now, three to six of these assembled molds will be assembled to this cluster hanger. Once these molds are assembled this becomes the minimum lot size to run this job through the facility. What you see now is the robot picking up that cluster that’ll deliver it to the next work cell. So, all the material handling is handled with an automated either conveyors or robots to move from cell to cell throughout the plant.

For the shell building process, we’re going to run another video here. So what you see here is that cluster then being dipped or covered with ceramic shell, and this is actually where the term “investment casting” comes from, as we’re investing this mold into the ceramic slurry. Now, there’s a custom robotic program that’s going to coat this entire mold with this ceramic. We’re going to then back this ceramic slurry up with sand and that’s going to happen behind the robot. There’s a rainfall sander constantly showering sand down that’s going to coat this mold. This first layer of sand that we’re going to apply is a real fine powdery sand that’ll give us the surface finish and part detail we’re looking for out of the casting .

After that sand is applied we’re going to then open up the doors behind the robot where we’re going to dry that mold. It’ll take us about an hour to dry that mold, and then we’ll repeat this process of ceramic and sand till we get six to eight coats about a quarter inch to three-eighths inch thick. As we add layers of sand the sand will become more coarse. That’ll give the mold some permeability that we’ll use later in the process. Now that window that the robot’s placing those molds into, that’s where we have the ability to dry those molds in about an hour. Typically it takes 24 hours for that drying process to take place. So, Signicast has taken the shell building process from a week long down to about one day.

All right, so now to remove the wax from our mold we’re going to use a pressure/steam autoclave, so before we go into the burnout of and that you’re looking at we’re going to put the mold into a vessel where we’re going to pressurize that vessel before we introduce the steam. The pressurization will prevent the wax from expanding and cracking our shell, and then we’ll use the steam to melt the wax out, so it’ll be set on a grate kind of like upside-down the way you’re looking at it right now and most of that wax will just pour out of the cup. The wax that poured out of the cup then becomes our sprue, so that’s how we recycle our wax right away. We’ll de-water it, we’ll filter it, and then we’ll use it for our sprues. What we don’t use for our sprues we’ll send back up to our wax plant, we’ll reformulate it back into virgin wax, and then pump it back down to the facility to be used again.

Now that our mold is evacuated of most of our wax, now with that permeability in our mold some of the wax is going to leach into the shell itself, so we’re going to put each mold into this burnout oven. So this oven is a long tunnel, or it could be a batch oven where you just place the molds in the oven and run it through its cycle, so this oven is at 1800 degrees and usually for about four hours and we’re going to burn up any of that residual wax that’s stuck in the shell. We’re then going to cure our ceramic. We want a phase change to happen to our ceramic to make it stronger, and we’re also going to preheat the mold. That preheated mold we’re going to pour our metal into while it’s still hot. 

So we’ll move on. So, this tunnel that you’re looking at, or this oven that you’re looking at is a long tunnel, so the molds will go in this end and then they’ll come out on the other side out to the melt deck here. So on the melt deck we’ll have an operator over here on the right and what he’s doing is he’s prepping this furnace where he’ll prep about 700 pounds of metal for this heat, and he has a couple different ways of producing the alloy. We can start with like 1010 and add the elements we need to make it carbon steel, 4140, 8620. We can start with like a 304, you know, stainless steel to make 316, things like that. We could also buy ingot of the exact alloy that we want to pour, so that’s used a little more on the higher alloys and for some cosmetic parts. But for the most part, what we’ll do is buy 1010 material, add the elements we need, and then produce and pour that alloy that we make. 

But now that the molds are preheated the robot will reach into this long burnout oven and grab the preheated mold. We’re going to use the preheated mold to allow us to do some thinner walls and it also will prevent a thermal shock from cracking our mold as we’re pouring the 3000-degree metal in. Now up on the melt deck the operator is preparing a heat, he’s got about a 700-pound furnace for each heat, and he’s going to use that heat to fill the ladle. He’ll finish up deslagging the ladle before the robot comes over and pours it into the mold. Down at the base of our sandbox you can see a fire has started. That’s intentional. We put some sawdust down there. So, after the mold is poured this can cover will go over the top of our mold burning up any oxygen underneath that can. That’ll give an oxygen-depleted environment allowing the metal to cool and prevent decarburization on the outside layer of our metal, so we’ll get some better metal by letting it cool underneath that can. 

Up on each of the melt decks there’s a spectrometer, so we’re checking every heat before we pour it for alloy chemistry to make sure we’re within spec. So that our shell is designed to be strong at 1800 degrees, so what happens when the mold comes down to room temperature our shell then becomes very brittle so we use water to blast most of that shell off the mold, and that’s what you’re looking at here is that shell has been cleaned off the mold and now we’re ready to remove the parts. So, one of the ways to remove the parts is what you see here with a large abrasive wheel where we can just target the gate and just cut the parts off the mold.

Another method we have, when we looked at our wax patterns we have that little notch on our gate contact and what we can do is submerge the mold in liquid nitrogen and then use that notch as a little stresserizer and then we can just simply remove the parts by just tapping in with a little mallet or using a vibratory hammer to just quickly remove the parts from the mold, so we have an efficient way to remove most of our parts from mold. The trick with this is you need the right alloy, so carbon steel alloys, 400 Series stainless steels, that’ll allow us to use this cold break method. 300 Series stainless and some aluminums and things like that become a little more difficult to be able to remove when using the break-off method. 

All right, to remove from the mold, we’ll then go to a finishing cell where the parts will get blasted, inspected and gate ground. So this you can see here the parts just set into a fixture. The fixture is then presented to a large grinding wheel, it’s a big time-saver belt sander where that part made contact with the gate, we’re then grinding that gate contact off. So we’ll have another picture of the finishing area. So the part is getting set up in the sandblaster. That’s what the gal on the right is doing. It’s a spinner hanger. So, the parts will get blasted. Any of the shell will be blasted off there, and then the picture on the left, the robot is setting up for grinding that gate off. We could also load the fixture manually where you’d have a tote of parts next to the gate grinder and the operator would be setting the parts in the mold, or we can do it the way you’re looking at with the robot setting the parts in the mold.

Now we also have...for parts that don’t blast clean, so we’ve used the water blast and sand blast to remove as much shell as we can from the parts. For tight areas and internal geometries where we can’t blast, one method we have of cleaning the parts, we call it caustic cleaning and here we’ll actually melt salt. We’ll use a sodium hydroxide and a sodium carbonate mixture of molt and salt, and that salt will leach the excess ceramic off the outside or the inside of the part to remove that shell, and then it gives us just the metal part. We can then sand blast the part to remove any of the excess shell and any of the, I guess from the salt it’ll kind of look not as clean so we’ll blast that off, and typically our blast will leave about 125 rma surface finish, we’ll use a sandblaster, Grip Blaster to do that. We can also do some things with stainless steel shot blasting, glass bead, you know, whatever the customer might require to finish the part.

It takes about four and a half days to get through the facility. Now that’s pretty fasts for the industry. We also have some parts that take weeks to get through. We had talked about before the drying time, things like that, so there are parts and operations that do take longer, but typically we can do what you saw in about four and a half days. 

So, after the parts get through the inspection we’ll 100 percent visually inspect every part and then we’ll be very product-specific or part-specific for secondary operations. So we have machining capabilities, you’re looking at the paint line where we can finish the parts with a powder coat. The picture on the left is a heat treat oven so we can send the parts right over to be heat treated with like a normalizing, quenching temper, nitrocarburizing, things like that, so we have all those types of operations that we can do as the customer requires.

So here’s just a couple examples of some finished parts where we’ve done the powder coating. The one piece on the right gets a little pin assembled to it after we powder coat, so we’ll over-form that pin on afterwards. This piece on the left will broach the spline and tap the hole and then kernel plate the part as a finishing operation. Or this piece in the middle where we grind the surface flat and then the as-cast kind of forms some dimension there to give it a good look.

Now we have just some quick notes on some alloy options because this is kind of an introduction to investment castings. We’re also going to host another seminar where we’re going to get into the design of investment castings. So, here’s just a quick overview of some alloys that we pour. This information is on our website as well, signicast.com. So next time we meet we’ll be going over the design of investment castings, you know, how to gate, how deep of a hole can we cast, the different...we’ll explore some of these alloy options as far as carbon steel, stainless steels, nickel-based, cobalt-based alloys, so we have a wide variety of alloys that we can pour in the commercial markets that we serve.

Now the process itself is very flexible in the alloys that it can pour, so aerospace foundries will pour titaniums and aluminums. Jewelry makers use this process for precious metals, gold and silver. These are just the alloys that we focus on with the commercial markets that we serve, this as we went through the process just some of the advantages of the Signicast Hartford manufacturing complex where we can speed up the process by controlling our in-house tooling, getting parts into production. You can see the new product launch lead time, and then as we go through the process all of the steps are continuous flow, so as we do those smaller lot sizes more often it doesn’t take a long time to de-wax just a handful of molds, where if you’re trying to do a large batch you could be spending a long time at one operation just to get that large lot through the facility, but the smaller lot sizes can go through it much quicker.

So, those are just some quick notes on kind of the lead time of making parts, so one to four week lead time with your secondary operations just adding days because we do manage and have that equipment in our facility to be able to run.

So I think at this time hopefully Taylor’s gotten a handful of questions and everybody’s got a good overview of the investment casting process.

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Taylor Topper:

Yeah. We have just a few questions, so just a reminder if you have any questions for Adam we have time now for a live Q&A. On your dashboard here there’s a questions pane, enter your question in there and then we’ll answer it.

Adam, the first question I have, how do you vent the mold?

Taylor Topper:

And we have another one. What method of blasting is the most effective at removing the remaining shell?

Taylor Topper:

Great. And the next one is, what’s the benefit of the investment casting process, or benefits I suppose, of the investment casting process compared to machining a part?

Taylor Topper:

That’s very cool. I actually didn’t know that little bit of history. The next question here is from John. What is the range of part size you can produce?

Taylor Topper:

So adding to that our next question is, the opposite side I guess. Does Signicast do smaller fine detail parts? So while we can do the large ones, how small are we comfortable going?

Taylor Topper:

Great. Next question is how do we minimize porosity?

Taylor Topper:

What is the tool capacity? What is the volume? So, kind of a dual question.

Taylor Topper:

Great. The next question is what process can you offer for low volume, quick turn prototype parts?

Taylor Topper:

Here’s another one. Is one material more difficult than another to pour or control dimensions, or do all materials behave the same?

Taylor Topper:

What inspection methods or what standards do we use to qualify parts?as we expected, and we are making some castings within the customer’s specification.

Taylor Topper:

We have a two-part question here. Can aluminum be investment cast, and then also what is the best surface finish that can be achieved. 

Taylor Topper:

We’re back to prototypes here. What do you do differently when you cast prototype parts compared to when you cast production runs?

Taylor Topper:

Great. We have one here with injection. What is the advantage of using a robotic arm to remove a wax pattern from the tool versus an automatic process with only the ejector pins?

Taylor Topper:

Speaking of optimizing mold flow, this is what I think we were talking about before. So do you use MAGMA flow prediction modeling software or something similar?

Taylor Topper:

If you’re going to machine in investment cast component, how much material is typically needed for sufficient machining clean up? 

Taylor Topper:

Can you produce internal threads or undercuts in your wax pattern?

Taylor Topper:

How do small investment cast components fare better than MIM components?

Taylor Topper:

Yeah, and I think I’ll eventually write a blog post about that, so check back with us and maybe we’ll have some more answers. Next question is, are sprues always one vertical bar? 

Taylor Topper:

Next question is, can voids or inclusions occur?

Taylor Topper:

Sure. What is the lowest annual production quantity for Signicast?e.

Taylor Topper:

So this goes off of order quantities. Can a cluster assembly mold be “closed off” to produce a smaller minimum order? 

Taylor Topper:

How drastically does the volume of the production run affect the unit cost?

Taylor Topper:

This one will probably be a good question to go over again more in depth during our design webinar, but what is the minimum outside radius you can consistently hold on an outside edge?

Taylor Topper:

We have time for one more question. What advantages do investment castings have over dye castings?

 

 

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Last updated 12.18.2019