Fastening to LDPE

Recidivist

Super Anarchist
I had a new bench seat made for the aft end of the cockpit in an inboard diesel powered launch. The original seat had a plywood base, but we decided to go with 10mm Polyethylene sheet for lightness. It's either low density or maybe medium density - I'd suspect LDPE.

I need to attach hinges to the seat base so the seat can hinge out of the way for access to the steering flat. My question is, what sort of fastener to use?
Does the LDPE hold a thread well - Can I just tap it? Or is there some other fastener I should use?

I can't use a fastener that will protrude into the foam as it would create a hard spot when someone sits on the seat.

Ideas?

TIA
 

Recidivist

Super Anarchist
Thanks sailak - I think that's the answer I was looking for. I now remember the upholsterer saying that's how they do it.

IStream - I was originally going to do that using captive bolts inserted from the upholstery side (and using pan heads to avoid the hard spot issue) and a nyloc nut on the accessible side, but then the upholsterer mentioned the helicoils. It's taken me over a year to fit the new seat and in the meantime I forgot about the inserts.
 

DDW

Super Anarchist
6,239
972
Stainess T nuts will work a lot better than helicoils, even the ones for wood (with teeth to prevent rotation). Alternatively since the piece is likely thickish, spot face the back side with a Forstner bit so the nuts can be buried. LDPE is very soft, but if thick enough would hold threads for non-loaded hinges.
 
Polyethylene is easily split by fasteners that are tight. Flathead countersinks are the worst. If using carriage bolts, I’d chisel out a square countersink so the square bit of the bolt isn’t trying to split the plastic.
 

Recidivist

Super Anarchist
Thanks guys. But the upholstery is already fitted, so access to the "back" of the panel is not available. The advantage I saw with helicoils is that the coils are inserted from the same face as the fastener, whereas T nuts are inserted from the opposite face (which, as I said, is not accessible). But my "local" bolt shop doesn't carry helicoils so I'm still shopping around.
 

2airishuman

Anarchist
They make rivets for soft materials that are designed to split rather than bulge, so there isn't as much stress on the hole. Less likely to crack plastic. That's what I'd use. Here's a link.


I've tried threading PE and I've tried gluing it with Gflex and standard epoxy (after flame treatment). While these techniques do work they result in low-strength connections that aren't especially reliable.
 
Do some research.
Read about G/Flex and polyethylene before rejecting glueing.
Read the current thread on a plywood-cored hatch before using plywood fastening pads.
Try planning before creating another problem like this.
 

SCARECROW

Super Anarchist
5,928
655
Melbourne, Aus
Do some research.
Read about G/Flex and polyethylene before rejecting glueing.
a glued G/flex joint has less than half the tensile strength of a welded joint in PE and then only if prepped really well. But more importantly G10 is going to be significantly stiffer than the LDPE material that is being discussed. So every time the seat gets stepped on and flexes the bond will break a little.
G/flex has its use but for this particular job the thread and helicoils are the right solution.

Follow up info.... use an imperial (BSW) thread it will be coarser and work better in the soft plastic.
 

Recidivist

Super Anarchist
Do some research.
Read about G/Flex and polyethylene before rejecting glueing.
Read the current thread on a plywood-cored hatch before using plywood fastening pads.
Try planning before creating another problem like this.
Wow, who pissed in your wheaties this morning?

Scarecrow happens to be a friend ("mate" in Ozspeak) and a naval architect who I know to be very knowledgeable - when he chips in with free advice, I tend to take notice.

I haven't read the thread on plywood cored hatches, and it's not clear to me what the relevance is to this issue - thanks for the tip though.

"Try planning before creating another problem like this" - WTF! The upholsters had the original plywood based seat as a pattern. They didn't transfer the hinges to the new base. I am now seeking advice from the smart people here who have undoubtedly dealt with similar issues before as to how to approach the matter - and I'm "creating a problem"?! This is my first foray into the use of LDPE and I know little about the material other than that it is lighter than the product it is replacing. As the late Ben Lexen said, "weight is only useful in steam rollers".

Have a nice day.
 
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