Requesting Suggestions to eliminate porosity

Hi Novak,

The pictures look like pretty classic hydrogen porosity as opposed to folds and oxide lost foam defects, so I'd probably lean toward melt management as the source.

Nothing wrong with a dip out furnace and whether ladle or smaller crucible that all sounds fine. Is it a fuel fired furnace? NG? LPG? Is the flame introduced low in the furnace? Or blows on the melt like a reverb furnace? If so you want a very lean burner tune for aluminum, especially at the elevated pouring temp. A large dip out furnace isnt usually something you find in the hands of a hobbyst. Is it yours and do you do other casting, like green sand?

This is confusing me a bit. Is the furnace electric? How are you setting furnace temp? If you dont know your metal temp, your at a big disadvantage. 1450F is very hot for aluminum. I confess, I pour many lost foam castings at that temp because the added energy to vaporize the foam and the slow mold fill times allows a lot of heat transfer to the mold, but my parts have very high surface area/volume so sometimes it's needed and it insures high success rate. But if it is something where I want good machanical properties I'll back the pour temp to as low as 1300F or until I get cold shuts. If you have a fuel fired furnace that has a rich burner tune, in a humid environment, and metal temp >1450F, you'll likely have H2 porisity. The reason I asked if you do green sand casting was to ask if you poured from the same melt into a green sand mold would you see the same porosity? If so, you know you need to focus onmelt management.

Fomular 150 is preferred but Foamlular 250 is also suitable if you need more pattern stregth. As long as the candle wax is minimal, it is probably not much concern but melt temps can be considerably higher than foam and can cause some flaws.

I usually recommend just enough coating to cover the color of the foam. I actually thinned my PolyCap a bit, about 1/2gallon of distilled water to 30gal of PC600. I can't say that would necessarily be a problem but if you are using gravity feed with a short sprue, you wont have much head pressure to drive diffusion of the foam accross the refractory.

Doesn't give me too much to go on. Like I mentioned above, a little sprue height adds pressure. Are you using a pouring cup? Can you show me the casting position in the flask, relative size of casting and gating cross section?

You don't mention your mold media. Is is dry silica sand or something else? Since you mention you are getting otherwise formed and acceptable castings, I wouldn't get too wrapped around the axle on this until the melt management practices.

Best,
Kelly

 

OK, resistive electric furnace, that's good to know. Can you tell us a little more about it? How many KW? What is a rough estimate of the aluminum capacity of the large crucible and how much are you melting? How about a photo of the furnace and how/where you are measuring temperature?

There is debate about whether charging hot or cold is better but the debate is about how hard it is on the crucible versus melt quality. When you charge the crucible cold the argument is you get a very large temp gradient across the crucible wall thus thermally induced stresses. Problem is, when you charge it hot with cold ingot, the same thing happens when you introduce the cold metal charge so I dont think it matters much and aluminum duty isn't too challenging.

Five hours is a very long heat time thus my opening questions. If you have a very massive furnace there can be very large differneces in temperature within the furnace and the melt depending upon how and where you are measuring temp. I set my smaller resistive electric (8kw up to A20 crucible) at 1800F and measure at the top center of the furnace. When it had more massive refractory, it didn't matter what the set point was because the melt temp always lagged the set point by several hundred degrees and the furnace never reached set point before the melt reached pour temp. In my low mass version, the temp lag of the charge is much greater and the furnace does reach set point well before the charge melts.

My larger furnace was really meant to be fuel fired but I have a 8kw resistive electric insert I made for heat treating that can also melt, but I have ~200-250lbs of metal, crucible, and refractory to heat. The diameter of the furnace is close to the A60 crucible and the heating elements of the electric insert is concentrated near the bottom of the crucible. I measure temp at the top ceneter of the lid. That turns about to be the coldest spot in the furnace, it is likely 2000F + at the bottom/base of the crucible and the melt temperature is actually 100-200F hotter than the indicated temp at the top center of the lid where I am neasuring, so if I didn't occassionally measure metal temp during the heat, I would be superheating the melt well beyond the measured temp at the center of the lid and superheating aluminum will definitely increase the potential for porosity. -See where I'm going?

You really need to measure your metal temp during the heat. If you dont want to build one you can can buy a probe from MIFCO for $75. You dont have to buy their readout. Most inexpensive multimeters have a port for a mini-ktype thermocouple plug (the k-type mini-plugs are cheap, ebay, Amazon) and will readout directly in C of F.

004062 12" reinforced tip thermocouple - MIFCO

I'm surprised you're getting the result that you are with that position. You must be getting the overhung areas to pack well. You really should increase you sprue height and use an offset pouring cup. The contribuition to poroisty could be enrtrapping and aspirating combustion gases into the feed stream, but I'm still more focused on melt mangement and metal temp for now. Video taping your pour would be helpful too. Is there a lot of flame and smoke, and the duration of the pour can be useful information.

It's helpful to state the size of the sprue and runners. It's also good to know the approximate LxWxH dimensions and of the casting. It helps the people that are trying to help you....me!

The sand is fine. I use retail store sand too. It's actually decent quality.

Best,
Kelly

 

That explains a lot. 10 liters is the brimful capacity of my A60. By comparison my large furnace is 8kw on 14" bore, and still grossly underpowered.......but as I said, it was designed to be fuel fired. I just don't do enough 40-60lb melts to run it on a burner when I can just start earlier and have the melt benefits of a resistive electric furnace atmosphere.

This would have been all I needed at the start of the thread to make a reommendation.......Measure your metal temp! Here's mine. I deenergize the furnace coils just swap it with the vent plug, then remove and reinstall vent plug. Takes a minute or two. It's so valuable I built two so I had a spare (sheaths yes), but have never needed the spare in six years. There are build threads if you search.



I mentioned a large dip out is unusual to find in hobby setting, but you can transfer the content as you describe without lifting a large crucible full of molten metal. Do you use its capacity for most melts? .....and I presume you empty it after each melt; correct? That's a lot of mass to heat for each melt, especially with 2.2kw. Still no picture of your furnace?

You're right there.

He's a very good videographer. His 3D printing and lost pla/resin videos are very good. His lost foam casting videos are not at the same standard as far as content and technical approach; I don't recommend them but yu will find them (and one other author using green sand ,....I dont get that one) at the top of any lost foam casting key word search.

6 years ago I built my lift off furnace. I had video and a very detailed build thread at the old Alloy Avenue forum. 4 years ago he made his video on making a resistive electric lift off furnace. I thought hmmmm,that sounds familiar. He incorporated all the same features, same rationale for the features, and same build method and sequence as my instructional build..............naturally I was very impressed . He'll never mention a credit for his content for anyone unless it's for a sponsor......that'sthe world of YT I guess.

Best,
Kelly

 

Sounds about right at copper temps. You can push the coil loading quite a bit harder at lower temps. I started this thread because a lot of people ask me about building a resistive electric furnace so it was targeting someone just starting out. I was going to add some more content but haven't gotten around to it.

Building a Low Mass Resistive Electric Furnace | The Home Foundry

At the bottom of the second post at the link above, I attached a downloadable pdf copy of the Kanthal design handbook. Pages 8-13 are the meat of it. I designed to it way back when and can't remember if coil loading or wall loading was the limiting factor but is in part why I have two 4kw, 14hm, 14ga coils in my 10" bore furnace. I really like it. 8kw in that size and mass furnace melts 10lbs of aluminum in 30 minutes from a cold start which is just enough time for me to pack a mold and stage everything for the pour.

I'd like to see that.

Best,
Kelly

 

I like it! Is 440vac three phase? or other?

Best,
Kelly

 

One vulnerability of lost foam is that it needs higher pour temps, and that alone means more H2 can be soluable in the molten metal, because H2 soluability is a strong function of metal temperature. This is especially so in fuel fired furnaces and if the tune of the burn is not well managed and oxidizing. The problem is, any fuel fired furnace will shove a very large amount of ambient air through the furnace and that air will contain water.....just no way around it. This is true for all types of casting, but as mentioned, you can pour cooler with open cavity casting.

Induction or resistive electric heating in a sealed or N2 or Ar blanketed furnace is best for preventing H2 in the first place, Rotary degassing with inert gas second best. No matter what geen sand casters do, they will always have steam present in their molds due to water content of the sand.

There are two primary reasons LF must be pour hotter. The first is energy is require to evaporate the foam as it goes through two phase changes from solid to liquid, and liquid to gas. But, perhaps the bigger reason is the fact that lost foam pours are often 2x to 3x the duration of a conventional open cavity mold pours because the the fill rate is most often paced by the rate the foam can be evaporated, not gravity/metallostatic head pressure and the cross section of the the runner and gating system. Does it need to be hotter to the tune of 1450F, maybe not, especially for more massive cross section parts. Everything shoul dbe poured as cold as possible....just not colder than possible

Consequently, the metal has much more time to loose heat to the mold, thus requiring higher pour temps. The upshot is, if you have managed the coating permeability and feed system properly, you have a very tranquil pour. If you have violent flame, smoke, and gas evolving through the feed system, it is not managed well, at least for loast foam casting technical parts.

LF casting is thinly practiced in the USA but very widely practiced in Asia, which is where the vast majority of the research and process development work has occurred. Rest assurred, it is being used to produce complex technical castings. Mercuray Marine in FonduLac, WI has developed a very interesting LF process for there V8 outboard marine engines. -Search it. Like all things in industry, it has it's place, and is largely driven by economics. The comparison of the LF and conventional sand casting is an interesting discusison. Not trying to joust with you on the topic other than to say if you just try to apply conventional sand casting process and norms to lost foam, you'll find it to be a very unrewarding experience.

Best,
Kelly

 

Pretty much agree that most problems are melt management and temp related. Curiosity question for you: What are you using to degass? Rotary? and what metrology equipment do you have to measure H2 content/porosity of your castings.

You're obviously are an experienced foundry professional and happy to have you here but I must ask, is lost foam casting something you routinely practice?

You lose me when you start talking about venting lost foam castings especially the use of clothes hangars like poke vents. You do realize these posts are using unbound sand that has been vibrated to compact the mold.....no binder. Removing a wire would just cause the mold media to fill in behind it and cause the mold to be poorly packed in the vicinity of the extraction. If you're using green or some other bound sand, it can certainly work, but it's not standard practice nor utilizing the process to it's advantage.

I fully concede that most people (especially here on a home/hobby casting forum) attempting lost foam casting are willing to accept the gas evolving from the decomposed foam buoyantly rising through the molten metal of their castings and exiting through "vents" or even their sprue and feed system (worse yet). That may be ok for ornamental stuff and trying to insure higher first past success, as long as your definition of success doesn't include high metal quality.

....But, I would say it is certainly not optimal nor any more acceptable practice than allowing the same to occur in a conventional sand casting, and it's inviting folds, flaws, and in general poor metal quality castings. If you want good metal quality lost foam castings that are functioning machine parts with good mechanical properties, you'll need to eliminate that and get the gas to exit through the coating on the pattern wall as it evolves, not through the molten metal.......and it's easier said than done, but can be and is done......I didnt get what I considered good LF castings untill I was able to manage this.

Best,
Kelly