Shelter Pole Material and Diameter

Perhaps one of the most crucial elements of the shelter is finding the right material to use for the structural pole elements of the design.

After rushing in to buying SpeedFit plumbing pipe as a “cheap” and temporary means of creating my first prototype I realized that was a big mistake as the pipe almost relaxes into whatever shape you bend it into, which is great for plumbing, but not so great for making emergency crisis shelters!

This I found out after measuring out all of my poles, bending them into the correct shape, and putting together my first prototype frame, I came back the following day only to find that my shelter looked a little floppy and lacking rigidity – I discovered that the poles had relaxed into their stressed shape and lost all structural integrity…

Homeless Crisis Shelter Prototype Progress Photos - 026 (Project Bivouac)
After a few hours the poles started ‘bulging’ at the sides and all structural integrity was lost.

Having absolutely no previous experience of materials testing, materials research or analysis, I really had no idea what it was that I was looking for and what the technical terms would be to describe what characteristics I needed from the material. I tried Googling various terms but kept going round in circles, I really had NO idea what type of material or search term I could enter in order to find what I was after. Search terms like “plastic rod which returns back to its original shape” etc obviously didn’t yield any good sources of information.

Rather impatiently and somewhat annoyed at my lack of ability to find ANY sources of information that would help me in my quest to find a suitable material, I decided to simply order a range of fiberglass poles in varying diameters and start playing around with them. It felt like I would still be progressing in some way, even if it was down the wrong path.

So I ordered a few different sets of replacement poles from eBay… What a failure that turned out to be. Fiberglass is far to brittle to bend around a curve of such a small radius as I have in my designs. Not only this, but it’s very heavy, expensive and simply not what I need. Back to square one.

Fiberglass Poles Mix
Fiberglass Poles Mix

So, I decided to be a little more methodical about my approach, I didn’t foresee that it could ever be this hard to find a suitable material. So, I draw up a list of criteria I needed fulfilling and eventually decided to contact a few materials manufacturers in the hope that they would be able to help me at least narrow it down to a few choices, then I could order some samples and see which ones I could shortlist for further testing.

Here is what I wanted out of the pole material in a rough order of importance:

  • The structural poles need to be flexible yet return to their previous straight shape even after prolonged periods of being bent.
    • When the shelter is packed up, the poles will be bent and potentially will remain that way for a few weeks or even longer in storage whilst distribution is taking place – they need to return/snap back to their original shape when external stresses are removed so that they continue to make the shelter material taught when unpacked and help to maintain the structures shape and stability.
    • The shelter will also be subject to quite a lot of abuse whilst out on the streets, the structural framework needs to be able stand up to this abuse.
  • Be made from a common and low-cost material to keep manufacturing costs down and allow negotiations with more than one supplier
  • Be readily available in rod/tube form (many plastics are available only in sheet form of varying thicknesses)
  • Not degrade under UV exposure or exposure to hot and cold conditions
  • Absorb minimal water, ideally have zero water absorption
    • Water absorption will increase the weight of the shelter, lead to possible mold/mildew issues and increase the overall diameter of the rod which could affect any fittings which are attached to the end of the pole.
  • Lightweight
  • Sold by a UK supplier

The experiments

Fiberglass

Fiberglass Poles Mix
Job-lot of various size fiberglass rods

As outlined above, my first ‘test’ was with fiberglass poles. It was an absolute failure. Enough said about that.

But what I did realize from the fiberglass failure was that ultimately any other similar material would also be ruled out of the equation. This meant that metal tent poles were also not going to be suitable. They simply don’t have the required ‘flex’ I need in the shelter and most certainly wouldn’t be able to bend past both ends touching without severely deforming the metal. Perhaps it is possible with a certain blend of metals, some kind of alloy, but already I know that this was going to mean big bucks.

So, it was on to the plastics…

Nylon

Nylon 6 - 66 Chemical Structure
Nylon 6 – 66 Chemical Structure

My next thought was that they could be made from nylon. I know that you can purchase nylon rod relatively easily and that nylon is used for for so many commercial applications, it’s cheap and very hard wearing. Initially, my only concern was whether it would have enough material ‘memory’ to flex back to it’s original straight form even after being under flex for extended periods of time.

So I ordered a couple of lengths of nylon rod in various diameters. After giving both rods a quick flex in my hands, I could immediately tell that the material was a bit too ‘soft’, to confirm my thoughts, I taped the ends of each rod together to put them under a high flex and left them for a day. When I came back to them they tried to return back to their normal straight shape but failed, there also wasn’t enough flex resistance whilst bending them to keep the material of the shelter taught.

This wasn’t the end of the Nylon experiments though as I found out there were many different types of Nylon on the market, Nylon-6, Nylon-6-6 and various amounts of glass filled nylon to name a few. (I was hoping the glass filled nylon might be able to increase the stiffness of the rod).

But after checking the price of the glass filled nylon, I kind of knew it wasn’t a winner.

The final nail in the coffin for nylon came when I discovered that nylon soaked up water, as it did so, it expanded and increased the weight of the rod, not to mention it probably would affect it’s structural stability in some way.

So, I had to move on…

Calling in the pros… Nylaplas to the rescue

After ringing around a few local suppliers, many of them simply didn’t have any stock of plastic rods in the diameter range I was after, but I kept ringing around and eventually ended up speaking with a guy from Nylaplas – Thermoplastic Engineer Specialists (http://www.nylaplas.com/) I tried to explain what it is I was after and he said they stock hundreds of different types of material and it was best if I just drive down to see him as he has lots in stock and I could ‘flex a few’ and explain to him in a little more detail what I was after. So I did.

40 minutes later I was there, flexing various materials of various diameters and talking to him about price. Some were clearly out of my price range, it boiled down to a select few materials which felt good…

Nylaplas Stock

PVC Rod

PVC Chemical Structure
PVC Chemical Structure

The PVC rod was the cheapest out of the 3 I was sampling, it also felt the cheapest and softest and didn’t have the stiffness of the other two Acetal types I was also looking at. But it seemed to be better than nylon (both Nylon-6 and Nylon-66) so I thought I’d take it home with me to run some tests on, use it as a control of some sort.

Acetal Co-polymer

The next material I was looking at was the Acetal Co-polymer. It felt good, it was stiff, hard, flexible, seemed to return to its original shape well (material memory) and was very cheap, only slightly more expensive than PVC rod which is perhaps the cheapest of all plastic rods you can buy. I knew I was getting close.

DPP_acetal_copolymer_vs_Delrin_homopolymer_backbone

Acetal Homo-polymer (Delrin)

The final rod I took back was the Acetal Homopolymer (more commonly known as Delrin), which is almost identical to its co-polymer counterpart but is slightly superior in terms of mechanical strength, stiffness & hardness and also offers better wear resistance. Whether or not I’d be able to notice these enhanced characteristics I was unsure, but they were the same price so I thought I would take them back to my workshop for further testing.

Other benefits of Acetal (which I’ve taken from http://www.edplastics.co.uk/Acetal.htm) are as follows (I’ve bolded the ones which are important for the shelter):

  • High mechanical strength
  • High hardness and stiffness
  • High rigidity
  • Excellent resilience
  • High fatigue strength
  • Good creep resistance
  • Good impact strength even at lower temperatures
  • Good wear resistance, though mostly less favourable than the nylons
  • Low coefficient of friction
  • Excellent Machineability
  • High dimensional stability
  • Low moisture absorption
  • Good electrical insulating properties
  • Physiologically inert (Nat.)
  • Natural ERTACETAL® C suitable for food contact (FDA approval)
  • Good chemical resistance
  • Non self-extinguishing
  • Poor weather resistance
  • Poor resistance to high-energy radiation

After flexing and comparing the Acetal Homopolymer to the SpeedFit plumbing pipe, I realized just how much I’d come along…

SpeedFit frame vs Delrin Rod Frame

I briefly went to see if I could find any more information on the difference between Acetel Co-polymer and Delrin rod and came across this pretty informative video (made by Dupont of course!). But still, for the moment I think I’m going to go with Delrin rod for my frame material… UNLESS… YOU CAN ADVISE ANY OTHER POTENTIALLY SUITABLE MATERIALS TO TRY? IF SO, HIT ME UP IN THE COMMENTS SECTION BELOW, IF NOT, SAY HELLO ANYWAY IN THE COMMENTS SECTION BELOW!

DuPont™ Delrin® is more than acetal.

Thanks all,

Paul 🙂

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