Selection of aluminum casting process 1

[tgmcg]

Hi,

I have some very basic questions and will appreciate any and all help. We're tooling up for production of a new product we've developed. The product can be seen at (

http://www.tailboats.com)

There are three of four major aluminum parts that should probably be made using some form of casting technology. The largest part runs approx 12" x 3" x 6". Fatigue resistance, corrosion resistance and tensile strength are primary considerations. The parts will require some finish machining operations after casting and will then be powder coated.

We'll be ordering the parts in batches of less than 50 parts each.

I'm not clear on the relative technical and cost benefits between the various casting processs...eg. permanent mold, investment casting and die casting.

What casting processes should we be considering first? What kind of tooling and production costs should we expect...order-of-magnitude? Do you have any recommendations as to who we might contact?

Thank you for your help.

Best regards,

Tom

 

[BillPSU]

Tooling costs are going to be the deciding factor on what process you use. Typically in short run parts a wood pattern is made and parts are cast using green sand or no-bake sand. Tooling cost $1000-$3000 depending on the complexity of the part. Aluminum patterns will increase the tooling cost to $2000-$4000. Permanent mold casting is the next level of tooling but size control and repeatability is excellent. Injection molding is the next step. Tooling cost escalate in each process. Permanent mold and injection molding only work for the light metals, aluminum and zinc and magnesium.
Investment casting creates a wax pattern of the part, the part is attached to a sprue and then coated with refractory
several times until the shell is strong enough to support itself and the molten metal. The shell is de-waxed and sintered/preheated and the metal is poured into the shell. Extremely tight tolerances can be achieved using investment casting and all the machining would probably be eliminated.
I would think you would not use aluminum for this part due to corrosion. I would think you would be looking at stainless.
For your first runs of parts I would probably try to make the parts weldments or machine from solid. Prove out the design and see how the market for the product is deveoping and then go to castings.

Hope this helps
Bill

 

[etch]

For this type of run it would have to be sand casting id say.

 

[arunmrao]

For a short run of 50 pieces per batch, sand casting is advisable. Machining the sample out of a block may probably be good if it is difficult to find a foundry to process small batches.

 

[tgmcg]

Gentlemen,

Thank you all so much for your input.

The tooling costs for sand casting are less than I anticipated. However, a concern I have with sand casting is surface finish and fatigue. One of the parts is subject to +/-5 ksi fully-reversing alternating stress per FEA. In your experience, will this be a problem for a sand casting?

The lost foam process sounds interesting.

Powder coated aluminum should be fine for a marine environment. I will take a close look at Almag 35.

We may be able to go for larger quantities.

We've already built and tested a full-scale prototype, so are satisfied with the overall kinematics. The parts for the prototype were all milled from standard aluminum sections. Now we need to redesign all the parts for the selected manufacturing processes.

Best regards,

Tom

 

[MikeHalloran]

Most aluminum founders will put on a big song and dance number about helping you choose from the thousands of alloys that are available. They will then scientifically arrive at the best alloy and temper choice for your particular application.

It always turns out to be 356-T6. ;-)

;---

Lost foam is particularly good at difficult parts, where it would be impossible to remove the pattern from the mold. The patterns aren't removed in the usual way; they're consumable, so you have to tool up to make them in quantity. Your parts look simple enough to do with reusable wood patterns in classical sand molds.

;---

Until your volumes go way up, you should seriously consider hogging the parts from thick bar or plate. If you can make them with a relatively small number of relatively large diameter tools, a CNC mill can achieve astonishing metal removal rates, and the mill's owner can offer competitive pricing. Also, 'billet' parts have a certain snob appeal to the gearheads who might buy your boats.

Don't even try to start with flimsy rolled sections; they'll move away from the cutter and 'sing' even at low feed rates. If you've got the spindle speed and feedrates high enough, milling is almost silent, the chips hit the ceiling, and they leave a mark...











Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[tgmcg]

Mike,

The parts are currently milled from billet. This approach is too expensive, even at high metal removal rates.

My primary reason for evaluating various casting technologies is to slash parts manufacturing cost by at least a factor of 3. With die-casting, I can achieve a 15:1 cost reduction on the larger parts...but die-casting is a bit of a wild card in terms of fatigue strength, or at least it presently appears so. So I'm focusing on other casting technologies.

Agreed...356-T6 looks to be the best all around casting alloy for these parts. The LF process is intriguing.

Best regards,

Tom

 

[swall]

Tom--Lost foam is a great casting process for making near net shape castings.BUT--you really need to find a foundry that knows this process. The key is controlling the characteristics of the foam pattern, such as density and moisture content.Also control of the pattern wash coating.Most foundries don't have the expertise to do this, and you end up with lap type defects in the castings.Another problem with lost foam is that you can't control the thermal gradients during solidification, as the process does not lend itself to the placement of chills. 356-T6 is a good alloy for sand castings and if you need higher mech properties you can upgrade to A356-T6, or even 357-T6, which is basically a jazzed-up 356.

 

[MikeHalloran]

For the same money it would cost you to start up die casting, you could start up with plastic foam.




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[tgmcg]

Swall/Mike,

An attraction of LF is that there may be more flexibility on section thickness than with die casting.

If you can recommend any foundries who've mastered the lost foam technique, I'd be most grateful.

Mike, I'm not sure what you mean by "For the same money it would cost you to start up die casting, you could start up with plastic foam". Are you suggesting the tooling cost for LF is comparable to that of die casting?

Many thanks,

Tom

 

[MikeHalloran]

I'd expect the tooling costs to be similar.

For each process mentioned, you have to sink a die. The die doesn't know what you're going to do with it.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[mcguire]

Casting experts
Is it possible that investment casting (lost wax) may offer the best economy for the short run size in that a scaled-up (for shrinkage) model of the part could be used as a pattern and then some or all machining may be eliminated?

 

[tgmcg]

Mike,

Tooling cost is an area that seems to offer a fair bit of flexibility...bridge tooling, permanent tooling, rapid tooling. I'd have thought that a tool for foam or plastic might be made from aluminum or some type of SLA/SLS material, kirksite, etc. Whereas a die for molten aluminum would require cast iron or steel...though Laserform might also work here.

My budget is limited (whose isn't? ) and for current needs I'd be satisfied with a tool life of 1,000's of pieces, not 100,000's. By the time we sell 1,000's of units, tool cost will no longer be a major issue.

Sooo many options. Many foundries seem to have a vested interest in the traditional tooing technologies and tooling suppliers. These might work if I were Ford or GM, but I need to economize aggressively....which means becoming familiar enough with all the options to be able to start excluding them from further consideration.

Best regards,

Tom

 

[arunmrao]

You can consider using ceramic molds and cores . this would be true to cast and reproducibity is good. Wooden or metal patterns can be used. This technology is being offered for around 15000 US$ by a company in US. It is pretty straight and simple. I too have been considering using this for making thin walled impellers.

mcguire this partly answers your question, wherein all the advantages of investment casting is offered without the need for large investments.

 

[MikeHalloran]

Tools for rigd polyurethane foam can be made from soft materials. I wouldn't use RPU in an outdoor application.

Other resins require more pressure, or are abrasive and/or corrosive.

I wouldn't count on lost wax getting you close enough to net shape to eliminate machining. I assume you're talking about machining bearing surfaces of the moving parts. Because the product is human- powered, there's not a lot of power to waste, and excesive friction or loose fits will cause perceived quality problems.

;--

Will you ever have the volume to pay for hard tooling? Frankly, I think not; you've got a niche product there. I'd be inclined to go with the sand casting for now, and as time permits, produce interchangeable part _designs_ optimized for the different processes that have been suggested here. Do conjectural designs for different production volumes, and get quotes on them, just in case Sharper Image or Wal-Mart calls. ;-)





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[swall]

Tom--ten years ago when I was involved with a couple of lost foam aluminum projects, the captive foundries of the automotive OEM's were the ones with the necessary expertise. Also, the outboard motor manufacturers had it as well. With job shop foundries, it was hit or miss. (My experience was that it was mostly miss).I'm sure the job shop foundry lost foam situation is better today, but off hand, I don't know of any that I could recommend. The more I look at your needs and volumes, the more I agree with the other posters that sand casting is best suited to your needs.Sand casting is very versatile and you can do any aluminum casting alloy with it.

 

[tgmcg]

Thank you all for the input. I believe I'm leaning towards sand casting, rubber-plaster mold and similar low-tooling cost processes. The savings should still be substantial w.r.t. machining from billet and allow us to meet our cost target.

Mike H...we're planning on secondary machining for the critical bores. The bores hold rolling element bearings or bushings. The only concern is fit.

Best regards,

Tom

 

[unclesyd]

May not be applicable but very interesting.

http://www.tpicast.com/about.asp

 

[tgmcg]

Unclesyd,

Thank you.

I just got back from the Pacific Design & Mfg show in Anaheim. Spoke with the folks at Harmony/TPI who are working up a quote for us. Interesting process. Tooling costs look reasonable for smaller quantities. Surface finish is pretty decent too.

The rubber plaster mold process also looks interesting. Came away with a lot of new insights and options.

Best regards,

Tom

 

[unclesyd]

From the windmills of my mind here is another process that might be viable. I shouldn’t tell you that I first worked on this process in 1953.

http://www.portshellmolding.com/home.htm

As a matter of interest.

http://site.cubictechnologies.com/3d/PAGE27.HTM