Help with plastic specs

Question

Help with plastic specs

Tokiyata (Materials)

(OP)

8 Oct 14 23:40

G'day, I am just a tinkerer so please forgive my amateurish questions. I simply don't know where to seek the information that I am chasing. There are no plastic engineers where I live. A while back I experimented with making bow limbs (archery) using pre-tensioned mono-filament fishing line on the back of the limb. Many strands set in epoxy directly on the wood core. It achieved the desired result but the tension was just too great for the adhesion between the epoxy and wood and it delaminated rather quickly.

I have been wanting to have another crack at it but looking to find a sheet source of plastic which has similar qualities to the fishing line. I think I used flourocarbon line but nylon should have similar mechanical properties right?

Is anyone here able to recommend a grade of plastic that is available in sheet form that has a high elongation and modulus of elasticity like fishing line? Another question that might save me a great deal of trial and error.... if the material is stretched to say 10% elongation prior to glueing it up, will it retain elasticity over time or creep and relax?

Cheers!

Compositepro (Chemical) · Answer 1 · 2014-10-09 12:57:11

All plastics will creep and loose tension with time. That is why they are called plastics. What you are dong is generally done using glass or carbon fibers. The only reason you need pretensioning is because the plastic filament is not stiff. Glass or carbon are orders of magnitude stiffer so they can store far more energy with less stretch. Also plastics dissipate energy as heat when used as springs.

Tokiyata (Materials) · Answer 2 · 2014-10-09 18:13:20

Thanks for your reply. I am quite familiar with the use of carbon and glass fibre but I am trying to find a synthetic sheet that will act similar to sinew when used on the back of a bow. Sinew is applied wet and as it shrinks it pulls a great amount of pre-tension onto the back of the limbs. I understand that it has similar properties to silk and nylon, except that it is much more susceptible to moisture (too susceptible for where I live).

The problem with carbon and glass is that they are too rigid to be used on the back of the short horn bow style bows. They don't allow the short limbs to flex as much as they need to without stacking (requiring an exponential amount of energy to continue flexing).

So,do you think it is possible to mimic sinew with a synthetic material?

Demon3 (Materials) · Answer 3 · 2014-10-10 09:38:05

Nylon adsorbs a lot of water and polyethylene very little. There are a series of polymer in between so by trying those you could potentially home in on the one with just the right amount of water susceptibility. For example, the standard nylons are 6 and 6,6 but look into nylon 11 and nylon 12 and other higher numbers. Those have intermediate chemical structure and water uptake. Look for example at www.matweb.com

Chris DeArmitt PhD
President - Phantom Plastics LLC

Consulting, ideas and training on plastic materials
www.phantomplastics.com

Tokiyata (Materials) · Answer 4 · 2014-10-10 20:17:41

Wow, thanks for the link. That will keep me busy for a while. The material won't be used in or under the water but I live in the tropical north of Australia and humidity is very high for 1/2 the year and very low for the other half. This has a significant affect on the biological sinew fibres and hide glue. Do you think the affects would be noticeable with nylon?

MikeHalloran (Mechanical) · Answer 5 · 2014-10-12 00:46:38

If the monofilament bonded well to the epoxy, but you got an adhesion failure of the epoxy to the wood, maybe you are working on the wrong problem. Have you tried, e.g., roughening the wood surface, or patterning it, say with milled dovetail grooves? Or braiding epoxy- coated monofilament into milled braided grooves in the wood?

AFAIK, hide glue is used for violins precisely because it doesn't bond well, so it's easy to break the joints without splitting the wood. I can't imagine using it in an archery bow.

Mike Halloran
Pembroke Pines, FL, USA

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Help with plastic specs