What do I Need To Know About Induction Heating?

I'm no expert on induction heating by any means, but it seems to me that even though the exterior of the mold is uniformly cylindrical, the interior is not, resulting in a varying cross-section through which the induced current flows. This, in turn, may result in non-uniform heating, ie, hot spots and 'cold' spots. Something you may wish to ask the vendor about, if it could pose a problem.

The heating comes from eddy currents on the surface of the metal so the thin spots will heat faster but only because they are thin, you need to cycle the device on and off to 'soak' the heat into the thicker areas. Uneven heating is more likely if you have a varying distance object.

I read an article on case hardening that indicated the strength of the current is a factor in the depth of the heating metal but that isn't a factor for me because I am heating the entire mold and will need to cycle it so the mold doesn't get too hot on the surface or thin sections.

Drew K

Ah, okay. Thanks for clarifying.

Just a simple question: does the induction heater's frequency also figure into depth?

The third paragraph here says that low frequency is best for deep heating. It make sense because the current will travel farther before reversing direction as the A/C cycles.

http://www.acrossinternational.com/Induction-Heaters_c75.htm

Drew K

Makes sense. Thanks!

The main thing you need to know about induction heating is it crap for bulk heating of large objects.

The energy level inputs and cost associated behind them are way higher per unit of energy than what any basic fuel fired heat system is and that doesn't even begin to touch the costs and complexities of the actual induction units purchase and operating costs.

You could easily be into the $100,000+ cost range with a $10's of dollars an hour operating cost for what bulk heating effect can be achieved with a common weed burner propane torch that costs under $50 that only costs a few dollars an hour to run.

The vendor I spoke with yesterday talked of operating costs of around a dollar an hour for a 10kw unit and electricity at $.16 (U.S.).

The only prices I have seen so far (waiting on a quote) are from the link on my other post which showed melters in my capacity range for $20k (U.S.) and heaters for $3/4k and they can customize the coils for your needs. I might use them as a comparison for the quote I get.

Drew K

Is that 10 KW input power of 10 KW inductive heating power?

By the numbers if its 10 KW input power the obvious operating cost electricity wise would be $1.60 an hour however if it was inductive heating power figure 1.5 times that being most in use induction heaters are lucky to get 2/3's of their input energy into the actual metal they are heating once all other heat loss and power supply losses are factored in.

Now that said 10KW doesn't do much in bulk heating terms being its only ~34,121 BTU's but given your relatively low melting points of lead and not so large of mass it should work reasonably well.

Given lead has a specific heat of .03 BTU/pound plus has a 9.65 BTU/ pound latent heat value taking 70 pounds of lead from ambient room temp of 70 F to say 700 F would take roughly 2000 BTU or about 3.5 minutes with a 10 KW inductive heating input at 100% efficiency.

Now given that the bigger question is, will you be doing enough lead melting to justify the induction heater rigs up front cost plus realistic operating costs Vs using the present method you have now to see enough gains in cost savings and other expenses in a time frame that's remotely reasonable?

In your application that going the be the primary determining factor of the setup.

The first thing we are trying to do is pre-heat the mold. Currently we use a butane torch, an induction heater would make the heating even and precise across the mold instead of just hot-spotting it with a torch. The guy who does it has considered making a ring-torch to heat the mold more evenly, but being who I am I thought an induction coil would deliver consistent reliable heating and a better molded part.

I did ask the vendor I spoke with so far about the cost of melting the lead too but the only reason I don't like our present set-up is the lead drains through a valve that wears out after several months of use.

Drew K

So why not keep the mould in a preheating oven?

From what I am seeing online a decent 10KW induction heater is $8000 - $10,000+.

You can buy or custom build a heck of a preheating oven for way less than that so if that sort of $8 - $10K investment is justifiable for just preheating your moulds obviously you have the cash flow in the business.

Or is all of this just an excuse to buy an induction heater just to say you have one?

They tried using an oven but the mold cools too fast and is too heavy by itself to carry easily once hot to the lead pot.

Drew K

What's nice is that you can turn it on and off like a light switch, and it only heats what is in the path. Coupling efficiency also depends on what you are heating, it starts looking pretty good when heating iron/steel.

I know nothing about this sort of thing.

But: at what temperature is the mold to get the lead to form correctly, and, at what temperature is it when you remove the molded item? It's just that a cylindrical mold will be twice the volume of the molded item (I'm assuming a cone), and, the specific heat of iron is nearly 3 times the specific heat of lead (0.45 J/°Cg compared 0.16J/°Cg). I know that both these are swamped by the specific latent heat of the lead (23J/g). But if the above differential is only about 6°C you're getting to the point where you're using as much energy to heat the mold as you are the lead.

So maybe the mold shape mimicking the item shape is preferable.

(My dad used to make fishing weights by fitting wire loops in conical molds in clay and pouring in lead: are you fishing for sharks?)

The molded shape is shielding for an x-ray calibration machine. The reason for heating the mold is so that the lead doesn't flash freeze on contact with the mold and leave surface defects or air pockets. Large voids would wreak havoc with the detection equipment.

Drew K

We have considered it but fear the heater would be damaged in removing the part from the mold and we have several different types. We are hoping that the induction heater will be adaptable to the different molds and we can protect the coils better than a wrapped resistance heater. Also, there is the precision and repeat-ability that comes with induction that I like.

Drew K

Most induction pans for cooking use aluminum between the inner and outter Stainless Steel. Improves a more uniform heating. May look at that.

That is probably because the Aluminum will conduct heat about twice as good as most steels and much better than stainless. The steel would heat up where the induced current is greatest and the Aluminum would spread it across the pan better.

Which is why I look for a copper clad or aluminum clad pan for cooking that is polished stainless on the inside.

Drew K