I have been using "Styrofoam" heads for over thirty years and never hand any problems. The expandable beads used to make
them emit a gas over two or three days them the product is inert. There are over two-hundred types of this product manufactured
one of which is Styrofoam. I know of many old time collectors that have always used these heads for many years, all of them
never experienced problems. Many expanded bead products in use are "acid free" and arcival safe as indicated in museum catologues. The only known problem to me was a US WWII visor cap with a carmelized lining. It stuck to the top of the head.
It this case it would of course stick to anything.
this is another way to display visors w/o having any pressure on the visors:
(Credit: Marc Walker)
NEC SOLI CEDIT
To follow up on my other comments, I have a few approaches as I don't have all my visors in one place
I've taken 2 inch thick foam padding cut to two pieces to the shape of each visor to create a pancake like "Form". Snug but not too tight. The 2 inch thick pancake is the perfect size to fit inside the visor as it is generally equal to height of the sweatband.
For the first "Pancake, I then cut out an old cotton tee shirt and cover it. I then put this inside the visor to maintain the visors shape.
finally I put Visor with the inserted pancake on top of the extra pancake to raise the visor and brim off the shelf. This method allows me to fit the visors in my bookcase cabinets which have a low height but elevates them just enough to be seen better.
I do have a few Styrofoam heads. For these I take the sleeve of the same old tee shirt (waste not want not) and stretch it over the head. Almost like a knit ski cap. With the Visor on the head, you can hardly see it.
All in all the cost of this is dirt cheap (maybe 50 cents a visor plus the cost of an old [but washed] tee shirt) versus custom hat stands which run you $30+ each.
One day I will invest in these custom hat stands but only when I have a permenant room to display everything and install custom shelving.
P.S.: To come clean, I only put the cut out tee shirts on the foam padding recently.
My greatest fear is that one day I will die and my wife will sell my guns for what I told her I paid for them
"Don't tell me these are investments if you never intend to sell anything" (Quote: Wife)