Learn me some basic info on selecting the correct density for seat foam

So this weekend I tore the '73 Capri seats I swapped into my '66 Mustang down to the frame to restore them, this is a picture of the seat "basket" which sits on the frame via springs and a series of wire hooks:

I am not too worried about rebuilding the basket, its easy enough to bend up a new wire frame to replace the lower frame that has rotted away and to use new stainless wire to replace the old rotted wire that used to support the seat bottom foam. I am more worried about the foam currently:

Both driver's and passenger's seat bolster foam is damaged from people sliding in and out of the seats. There is a channel in the foam that the upper portion of the "basket" sits. After I repair and/or replace the foam I will be lining this channel with some ballistics nylon cloth I have purchased to prevent it(or at least vastly slow it) from happening in the future. Its really only the driver's side basket that is completely disintegrated, the passenger side is intact enough to use as a pattern to rebuild both. Anyway, my question today is how to determine what type(s) of foam do I need to buy to carve new seat foam from? In particular what do the different density designations mean and how do I determine what density of foam I have already? I have watched a number of youtube videos on carving and sanding the foam to shape and it doesn't look particularly difficult, but my goal is to retain the comfortable feel the seats have, so I am at a complete loss as to which density I would need to accomplish that.

P.S. I do still need an electric knife for carving the foam, so reccomendations for one good for the purpose is appreciated...as well as a good glue. I know most of the spray glues I have currently melt styrofoam, not sure if that would apply to regular foam as well

I was going to reupholster a seat, so I ordered a bunch of samples and was just going to go with what felt right. The samples seemed really helpful and were  from a company called SunMate. The project never came to fruition, although I still hope it might, so I can't really comment on how effective that method would be.

I worked in the automotive seating industry for 25 years, 16 of them in a foam molding plant. Density will give you durability, firmness gives you comfort. So, try and match the firmness of the old cushions and add a bit assuming the foam cell structure has broken down over time. Just do the Charmin' squeeze test until you find foam that feels right. 

The foam can be readily cut with an electric carving knife from your local second hand store or pawn shop. You can also contour it with any high speed rotary tool and a coarse (30-50 grit) sanding disc. There will be foam dust everywhere if you do, so do it some where where the dust can be cleaned up easily.

I've played with improving my Miata seats since I retired and I've found it's sometimes better to go grab suitable foam cushions from the junkyard. You won't find the Capri cushions, but look for some that are bigger, that way you can cut and sand the bigger piece down to the size you want and you'll have proper automotive grade polyurethane foam. On my previous Miata I used VW Golf GTI seat cushions (3 pieces) on the Miata seat pan and I welded the Recaro bolster supports to the Mazda pan. It made a nice comfortable seat.

I can tell you in a professional reupholstery shop foam density is selected by stacking some off cuts of various foams on the frame and sitting on each type until the one that "feels right" comes along

DeadSkunk (Warren) said:

Density will give you durability, firmness gives you comfort.

Yep.  Density and ILD/IFD measure different things.  You want ILD/IFD.  Ideally you want a fairly high density as well, because that will increase durability as mentioned above.

In reply to white_fly :

Thank you.

I'm looking for some foam to refresh some Recaros. I just contacted SunMate.

I ended up getting a couple of different firmness levels between 35IFD(Medium) and 45IFD(Firm) in several different thicknesses. The idea is to layer the 35 and 45 for the  back and seat bottom with 1" of firm and 2" of medium foam, while making the bolsters entirely from the 35 in an attempt to maximize comfort while still providing needed support, we shall see how that goes

Is that 25% IFD?  those numbers sound way way way low.  Or Metric perhaps?

In reply to ProDarwin :

No, 35IFD and 45IFD. To put that in perspective I emailed TMI and asked what IFD they used in their Mustang seat foam, they have 2 different densities...one is 45IFD and the other is 39IFD. Ford actually specifically made Mustang seat foam a bit softer than usual as a sales gimmick to enhance the feeling of sinking into the seat under acceleration...and the Mustang guys all say the TMI foam is firmer than stock. From that I can extrapolate that Ford definitely used something lower than 45IFD. So with 1 inch of 45 IFD and 2 inches of 35 IFD the seat bottom and back will come out to 38.333 IFD or slightly softer than TMI's softest Mustang foam which should be roughly in line with what Ford used in the Mustang seats. Of course that doesn't give me any reference for what Ford used in Capri seats, but it should translate into some comfortable seats at least. The bolsters I will keep at 35IFD, I think I can get away with that combo.

The IFD rating means how much force in a 7x7" area is required to compress the foam 25% of its height, or so I have read.

Wicked93gs said:

The IFD rating means how much force in a 7x7" area is required to compress the foam 25% of its height, or so I have read.

This is the 25% number I was referring to.  There is a 65% measurement that is common as well.  See ASTM D3574

The force should have units through.  Given they are using imperial units, it makes sense that it would be in lbs.  Those numbers sound really soft.

That said, my experience is in aerospace which is a bit different.

In reply to Wicked93gs :

I will tell you that the density of that slab foam is almost certainly less than the density of a proper molded auto seat cushion, and won't be as durable. Molded cushions will have a denser surface layer due to being close to the heated mold and that layer adds durability.

If a person is into a more diy/cheapskate approach to building out seat padding and reshaping I've had decent success by going to the local pick and pull and finding a rear seat bottom that is of the appropriate firmness.

Most modern rear seats are entirely made of foam and there is typically plenty there to cut,  fit and test.

Usually the rear seats are not very worn and are in practically as new condition along with being more dense than front seats. 

I've mainly used them for motorcycle seats but would certainly not have an issue using them to rebuild seat bottoms or bolstering. 

OK, I circled back to the engineering fellow at the seating company I used to work for. He says that the 45 firmness is the lower end of the range for seat cushions and recommends at least 50. He also reiterated that skived foam will not hold up as well as molded due to the lack of skin. The 35 slab is typical of seat back firmness. He also said most slab stock is lower density that normal molded foam cushion densities.

In reply to DeadSkunk (Warren) :

Possibly, however there is the other side of all this to consider. Seat foam used for couches, recliners, office furniture, etc is often 1.8lb 45IFD and usually made out of slab stock. My couch at home for instance gets daily use and is 7 years old at this point. From what I can feel it does seem like that typical density and it has held up fine for 7 years so far with no collapsing foam. Of course, it is also 5" thick as opposed to 3". 7 years of daily use would equate to 15-20 years of driving a car a few times a week which would be an acceptable lifetime. From researching this on the web(for whatever that is worth) I have seen that people will often use a layer of muslin between the seat foam and the seat cover in an effort to increase durability....I am not sure how well that works, but if it does, I do have enough of the ballistics nylon I bought(going to use this underneath the foam to protect the foam from the seat support wires and inside the bolster foam to protect the foam from the wire frame pushing through) to cover the foam to make an artificial skin if its glued well enough. In any case, I should be ready to try with what I have later tonight and will experiment to see what seems like it will work the best. Worst case scenario I can always get more foam and try again....or try to use an upholstery steamer in an effort to salvage the original foam.

The muslin (or other) layer minimizes the abrasion.  Raw foam is easy to abrade away.  The Nylon you mention is great for this also.  In aerospace fire-resistant foam of some sort is used and fireblock is usually bonded to the outside of it.  Remember that foam is a very effective fuel, exaclty where you don't want it to be when your car catches on fire.

1.8pcf foam will not be very durable in an automotive seat application I imagine, but its worth a shot.

Also your recliner probably has some sort of fabric diaphragm, steel spring, or both below the foam.  What does the bottom of your auto seat look like?

I generally deal with ~3 pcf foams in the 70ish ILD range.  Molded cushions generally come in denser than this.

In reply to Wicked93gs :

Automotive seat foam will break down from ingress/egress over the side bolsters, not from getting up and down the way you do on a couch. The nylon on the B-surface will help keep the slab intact longer, many OEM seat cushions have light fabric molded into high wear areas on the underside for that very reason. Do glue the nylon in place because if it can move around it doesn't prevent wear nearly as well.  Is a '73 Capri the 1st gen version? Are seat cushions not available from some quaint little shop in a corner of England?

I'll add that if this seat has a suspension, like a wire and paper covered "harp" under your butt, then a sturdy piece of sheet plastic can prevent the wire from behaving like a cheese slicer on the bottom of the cushion. My inclination when refurbishing any car seat is to use slab as the last resort. I've spent hours in junk yards measuring OEM seat cushions to find something that's close and easily modified to fit. The hardest part is finding one that has the hog ring wires in the appropriate locations. Back when I was still working I'd take the broken down cushion out to a plant and compare it to every cushion they were currently manufacturing.........Ah...the good ole days. 

Edit: Any photos of the original cushion, A and B surfaces by chance?

Edit again:  I stumbled on this, but can't see some of the pictures {post 26}.......Unobtanium Armegeddon(Or Mk1 Capri seat swap) | Vintage Mustang Forums (vintage-mustang.com) That's a very interesting seat construction method. I started in automotive work in 1983 and things had changed by then. I'm trying to remember what vehicles had seat cushions that were basically a flat bottom with a foam "skirt" around the perimeter like yours.

You should start a build thread on restoring your seats.

In reply to DeadSkunk (Warren) :

Heh, that is my thread on restoring these seats, I am just about finished with the frames and ready to start on the cushions. As for the foam, I haven't found anyone who sells it anywhere(though I am sure someone in the UK makes it), so yeah, the original foam looks like this:

Aside from being collapsed, its also damaged in the same place on both seats from people sliding in and out, causing the frame to puncture the bolster foam...along with the wire support design for the foam:

It prompted the decision to get the 1680 nylon to reinforce the stock design from underneath, now I will make sure to add it in high wear areas on top as well. Since I already have the foam I am going to make an attempt to use it...worst case scenario I will be trying to hunt down new foam in the UK and I will simply pull the covers off and replace the foam at a later date.

So, the seat cushion is two separate pieces, the horse collar around the perimeter and an insert in the center?

In reply to DeadSkunk (Warren) :

Yes, it makes for some tricky carving. The advantage to the design though is it makes it easy to mix firmness so the firmer foam is limited to just the seat and the lower back areas since the seat back foam is a nearly identical design. The listing rods hang in fabric loops and are hog ringed to the lower frame(and in the case of this picture to one of the support wires for some reason):

And a picture of the basket/frame to complete the picture:

That basket needs some improvement so it doesn't cut through the bolster so quickly. My concern is it will make short work of slab foam. Could you get some ABS sheet plastic and make a piece to zip tie to the upper side of each bolster so the foam is supported by a flat surface rather than wires? A separate sheet sitting under the center section would help, too, I think. They would still have some flex but offer more support to the foam. I suspect much of the comfort of these seats is because of the suspension system, rather than the foam firmness . I find the whole seat interesting and it's too bad I haven't got access to a foam plant any longer as I'd be in there trying to find a substitute cushion that could be cut and made to fit.

Yeah, the wire system was the drive behind me getting the 1680 Nylon sheet, glued to the bottom of the foam it will be near impossible to cut. I am replacing the actual wire with stainless steel cable fastened using aluminum ferrules so corrosion is less likely to be a problem in the future...maybe I will even try to find some stainless steel or nickel plated hog rings

In reply to Wicked93gs :

Just keep posting pictures so I can see the results. Press on!

This thread is relevant to my interests, but I don't have a lot to contribute, except for this:

Harder foam is almost always more comfortable in the long run.  Example one:  ever tried a Corbin motorcycle seat?  They're rock hard the first time you sit on one standing still, but after 8 hours in the saddle, there's not many seats that are better.  Example two:  the front seats in our '17 Ram are indistinguishable from the seats in our '21 Grand Cherokee, other than the covering materials.  The design is the same.  The Ram has harder foam, and is more comfortable for long days on the road.

So I did change the plan up a bit....decided to go full 45 IFD for the seat bottom(in three 1" layers since I just have the one sheet of 45 IFD to work with at the moment). Started with passenger seat foam since that basket is still intact:

Made some mistakes, learned some stuff, but for this particular cushion it should work fine. Bolsters are going to be far harder. Since I am still waiting on my electric carving knife I just used a hack saw blade and a cheese grater to do most of the shaping. I don't have the right type of sander to make this easy. Tomorrow I may well buy an air sander to make it easier instead of the electric orbital, which just gets bogged down...will definitley need all the help I can get with the bolsters. I do have to say though, the the foam lock glue I bought really works well