The PPI rating is the number of pores per inch. 20ppi has only 20 pores to the inch but HiFi35 has 35 so the pores are obviously smaller. In theory larger pores makes it more open but in practice I doubt anyone can hear any difference. It's all meant to be acoustically transparent.
Personally I like the appearance of the HiFi35 better, but it comes in smaller sheets (2000 x 1000). The 20ppi is the same price but sheets are 2000 x 1200 so can work out more economical.
The grill is usually screwed thru to batten around the edge of the baffle. You can then use self adhesive velcro on the grill.... the foam adheres to the hooks on the velcro and can be taken off and re-attached at will.
It's not standard velcro though, it's moulded and has double hooks. Although "self adhesive" it doesn't stick particularly well to grill as it's so open and has so little contact area. So, I usually stick it onto the grill as normal then re-inforce with a hot glue gun from behind, thru the grill holes.
What is the maximum thickness of reticulated accoustic foam that you can get?
I,m thinking about Earl Geddes and his work on wavegude horns. - His HF wavegudes are filled with 30PPI foam to stop resonances.
Obvously this would idealy need a piece of foam 100mm or so deep - that could be cut to fill the horn. Apparently this really does make a big difference!
You seem to be the best source I've found so far, and quite close to me!
First appologies I seem to have missed this question. The max thickness I usually keep is 20mm. In fact I have a special deal on some full sheets at the moment. They are 2000mm x 1200mm and as stated 20mm thick. The grade is 20ppi (20 pores per inch) which is quite open. They usually cost £35.00 for a full sheet but at the moment I have a clearance deal at just £20.00. They are quite large so have to go by courier (£10.00) but you are more than welcome to pick it up.
As a matter of interest I believe Adam Hall sell this foam for in excess of £50.00 per sheet AND they put VAT on top.