Using a Ceramic Filter to Prescreen An Iron Pour

I’ve had a few 60mm square ceramic filters sitting around my foundry for a year or more. I decided to try one out on a recent greensand iron pour.

The easiest way for me to give it a try was to make a 4” deep pouring cup with a si-carbide foam filter incorporated in it. The biggest unknown for me was how to install the filter in green sand without at the same time contaminating it with sand. My simple-minded solution was to wrap the filter in a piece aluminum foil and then embed it in the sand. I used a wood sprue plug on top of the filter to initiate the input opening to the filter. Then around that I carved a large pouring basin about 2.5 inches deep with tumble home sides. I scraped out a flat spot around the sprue at the bottom of the cup and set the filter on the flat. Then I back-filled the base of the carved basin with a half inch of pounded sand. Next I cut out the opening to the filter exposing about a 1.5” foil-covered surface of the filter. Under the filter I carve a sort trumpet shaped exit sprue with a similar diameter and cone shaped with increasing diameter toward the bottom. Then I took some care to clean the edges of the sprue of any small bits of sand that might dislodge when pouting and packed the edge with a spatula and my fingers to be as firm as possible. Finally I used a hook bladed knife to trim away the foil faces (could probably have just left them as they would quickly melt through) and checked the filter. It looked nice and clean and was firmly in place.

The filter in place with just a partial rim of residual foil seen along the the right side and top of the filter.

The filter sitting over the mold pouring basin


The underside of the filter/cup after the pour. 50% of the filter channels drained clear. Only a tablespoon of crud was seen on top of the filter—-pics later as somehow the photo I thought I took failed to show on my camera roll.



I then placed the OSB-shell cup on 1” blocks so I could see the metal flow into the actual pouring basin of the mold. Had I just plopped the pouring cup onto the top of the mold I would not have been able to se if the pouring basin was properly choked during the pour.

With some trepidation, I gave it a go. You can see I was so intent on watching the choke I did not realize I nearly tipped over the pouring cup. Incidentally, it is interesting to me to see the vents working as metal is pouring into the larger mold. I poured 25 pounds into the smaller mold and 40 into the larger. Poring temp was 2490 F.


It worked quite well seeming to impede metal flow very little and I was able to monitor the choke and maintain it just fine.

I will be shaking out my castings in a couple hours. Report to follow as well as pic of the top of the filter.
I think I’ll be trying these filters some more. I may try to incorporate them into the mold itself in a more standard fashion though that will take some planning. Incidentally, the cup itself did not suffer much damage. It quickly extinguished as soon as I lifted it off the pouring basin. It is obviously charred but only quite shallowly.

Denis

 

As an aside, here is a link to how ceramic filters are made:
http://www.madehow.com/Volume-5/Ceramic-Filter.html

Denis

 

And here is what was in the filter after the pour. Noteworthy is there was virtually no iron held back by the filter.



And I should add the casting was flawless. That does not prove the filter worked as I have had lots of good ones without a filter. But I am happy.

Denis

 

I ran some simulations with filters a while back. They definitely work, but they work much better at the bottom of the sprue. The idea in industry is that they help reduce the velocity and clean the iron when used at the bottom. When used at the top of the sprue it separates the iron into a million little streams that create a ton of oxides as they free fall down the sprue. Cast iron is relatively forgiving and will mash together with just micro porosity so you see people doing it this way all the time (in cast iron) with no real issues unless they have to be x-ray imaged or anything. They even have special pour cup filters for vertically parted machines which make it easier to insert the filter in the top. They (one or two) want to try them here but our inclusion rate is fairly low so really not justifiable and usually corelates to sand conditions. Im not a big fan on a high volume production line like we have. The cost can be staggering. While we were producing about a tenth of what we do today in 1996 we were averaging around 60K a month for ceramic filters. Today the cost would be ten times that plus the filter is more expensive now. In a hobby foundry or small production run they can absolutely be very helpful in preventing slag or velocity related sand inclusions. Just dont rely too much on them and keep normal good practices and they will do exactly what they are designed to do.

 

Thanks for sharing your experience on this.

With respect to cost, I bought a box of 700 2x2’s
For 40 cents a piece shipped. You can find a dozen or so on eBay for a buck each or so.

The breaking up of the stream is visible when I’ve used my pouring cup. But, one mitigating factor may be that the streams, in my case, fell into the pouring basin and formed a 4x4” puddle which then proceeded to drain down the sprue rather than streamlets cascading down an unchoked sprue. My plan is to use them conventionally at the bottom of the sprue with all future pours, but wanted to give them an initial try in the cup. I think if I make a wooden pattern for the filter and just draw the pattern and stick in the filter, things should work out.

Denis

 

Yeah...put it in the runner between the sprue and ingates for best results. Im betting you did not have a lot of impurities in your iron anyway. Ive seen your castings and know you have pretty clean iron.

 

Here is what my first in-the-cope filter form will look like. The square portion is just a mm larger than the filter. The plastic pieces are made from acetal (Delrin brand). The sprue has a nub on the end which will engage the dome of the filter pattern. The square pattern has draft to allow it to be withdrawn from the drag side of the cope. The filter will reside right over a small splash basin from which the runners take off left and right.



This look right (more or less)?

As an aside: For those who have not machined Delrin, once you have tried Delrin, you will never ever even be tempted to mess with crappy dimensionally unstable UHMW.

Denis

 

It machines great and clean too! It does not like heat too much but that is not of concern with anyone on here...at least I dont think anyone uses pattern heaters on here.
I will see if I can find our sketches with the filter...if not I will sketch something up real quick.

 

This is not ours but is almost exactly how we did them in horizontal. Put it in bottom of sprue if two or bidirectional runner. Put it in runner if singular. I like going in the drag with filter and runner in cope. Gives the best reduction in velocity and in theory any left over impurity will float above the ingates...although I do not think that is always the case..especially in depressurized gating systems.

 

Well, Billy, looks like I more or less did it right according the good illustration you provided. I am anxious to try it out. Thank you.

Akkk! Looking back at my post prior to this one I see I forgot to add images. Fixed it.

Denis

 

I tried using filters with cold iron to correct dirty castings, it just blocked the filter. I have found that pouring hot iron eliminated the dirty castings problem. You should out a camera on your pouring trolley as it captures the action a lot better.

 

You can plug up a filter with hot iron too if it's dirty enough. We generally don't have slag issues here but when we do it's when we are pouring way too hot. Sand inclusions are by far the biggest reason for scrap here.

 

Nice pour. Your new flasks are looking good as well.
Mark

 

Anybody have a reference for suggested filter area vs pour rate/ head pressure for filters? I’ve been looking around online and have yet to find that data which has got to be well tabulated by Selee and others.

I incorporated my filter into the mold yesterday. It worked well. It is a Selee 5x5 cm 15p filter. In order to maximize flow through it I carved out the entry port to the filter leaving only a few mm sand rim around the top and carved a generous splash basin below. I do think the filter retards flow noticeably as it to 20 seconds to pour 35 pounds of hot (2550) iron through it. I am wondering if I need to use two side-by-side filters. I did cut generous gates and runners so that they should not be impeding flow.



Here are a couple pics of the splash basin and filter above it




I had a malfunction of my pyrometer just as I was preparing to pour. Much consternation! Some cussing! Had to guess temp—-not fun. More on that cautionary tale that involves the Seebeck effect in another post.

Denis

 

I use a ceramic filter cloth that is placed at the base of the sprue where it meets the runner. I use a piece of thin plywood to form the pocket and then cut the filter material to fit. This sprue was the only one I could find tonight, but the screen is visible on the bottom side, and the other view shows where the slag was stopped. With cast iron, the choked sprue prevents a lot of the slag inclusion, so this is just insurance. As far as pour rate is concerned, I probably double the sprue cross section at the filter and have never plugged one up.
The sheets come as maybe 16" x 24" with different weave characteristics. Here are two samples. The darker one is very stiff, but the lighter sample is soft as cloth and I have formed it into cups to catch slag in the pouring basin.

 

Very interesting. I was previously unaware of ,refractory filter cloth. Next questions:
Is this Flexsil brand cloth?
Where do you get it?
What does it cost?

The results I’ve gotten with the filters I am using seem very good.

Now that I have 700 rigid reverse-foam silicon-carbide filters, I probably won’t be buying cloth. But I can see how it would be quite easily adapted to various situations.

Good to know.

Denis

 

I'll look for the receipt, I got it in 2015 , I don't recall what it cost.
I bought from Pyro Minerals in Oakland, CA. The only description is GWHT, #28C, GLSWV sheeets.
It was $28 per sheet in 2016.

 

We figured the cross sectional area of each hole in the filter and multiplied it by the number of holes. We used a slight increase on the friction factor because of the size of the holes and the fact that the metal was being split up and touching all the sides of the holes. We did play around with the friction factor quite a bit and found that the friction was not as high as we thought...I guess the difference between sand and ceramic. Your sprue could be much bigger with the filter. Even though you have spread the metal out to the full extent of the filter, the sides around it have very little pressure on them and the holes directly under the sprue are doing nearly all of the feeding. It is a very short sprue so it will not add that much weight and will help utilize each of the filter holes better.

 

Thanks for your insights, Billy. Thinking about your comment concerning widening the sprue, I initially was thinking the iron would flow uniformly through the filter. But, it is heavy and does have quire a bit of momentum, so, I can that I it would favor the center of the filter with the sprue I formed. I’ll try a much bigger and straighter-sided sprue next time.
I’ll video it again to see if the pour time diminishes as expected.

Denis

 

Pyro Minerals sounds like a source (new to me) worth knowing about. Thanks, ESC.

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