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Roto-Molding with a Composite

08/20/2012 7:54 PM

I'm looking to try roto molding a few things. I really want the strength and flex of polyethylene, but would like to do the molding without heat. So I am trying to figure out if a composite could be used with. Would polyester with short glass strands, making up maybe 40-50% of the mix, stay fluid enough to work with roto molding? Has anyone used any kind of composite with this process? Thanks.

Joe

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#1

Re: Roto-molding with a composite

08/20/2012 7:59 PM

This might work.

Gel coat the inside of the mold, spray chopped fiber or glass strands over that and pour in resin and spin it.

Can you tolerate holes?

How big?

How big are the parts?

Polyester will be much more stiff and not as forgiving.

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#2

Re: Roto-Molding with a Composite

08/21/2012 1:58 AM

Have you see any of the smooth on products from U S A or look at A,M,T French product both have a range of two part resins for a wide range of uses.

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#3

Re: Roto-Molding with a Composite

08/22/2012 12:09 AM

What are you thinking of making? Just curious.

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#4

Re: Roto-Molding with a Composite

08/22/2012 8:47 AM

Roto-molding, in the classic sense, uses thermoplastic powder (usually polyethylene)that melts and fuses as it gets hot. There's no crosslinking as with polyester. This material is usually homogeneous and not re-inforced with glass.

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#5

Re: Roto-Molding with a Composite

08/22/2012 10:22 AM

Not too familiar with the rotomolding process, but I do know that any form of glass reinforcement is going to significantly increase both the compressive strength & the rigidity of your product. In other words, you lose the flex & gain strength. Also, 40- 50% is a huge amount of glass & will reduce the flowability of the uncured material a LOT, which I do know will affect the consistency of your finished product. I have parts made with PPE homopolymer/ copolymer blend, same part both in unfilled & with a 25- 30% glass fill- the GF material is over twice the compressive strength, 1/2 the flexibility & less than 1/2 the melt flow of the unfilled. Maybe you'd be better to look further into your polyester selection?

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#6

Re: Roto-Molding with a Composite

08/22/2012 11:24 AM

It would be hard to get the mechanical properties of polyethylene with a glass reinforced composite. Polyethylene is very low tensile strength, but very flexible and tough. Glass in polyethylene is much stiffer and stronger but not any where near as impact resistant.

The fundamental difference between rotomolding and polyester resin molding is that in rotomolding, the polyethylene that has not yet adhered to the mold is fully fluid (as beads or powder). In polyester molding the resin all "goes off" at once, so you can imagine that you could have a partly covered mold and a large puddle, all hardened at once.

I think it would be extremely difficult to get an even, thick coat of material on the inside of a rotating mold using a thermoset resin. If you have the mold for rotomolding, then spraying up fiberglass with a chopper gun will give you a finished product, without the need for rotating the mold.

Several layers of glass cloth could absorb the resin, but adhering these dry layers to the gelcoat (that you could spray inside the mold) would be a huge challenge, especially because the mold rotates. If the glass is pre-saturated, then hand layup could be used to get the glass into place (and you'd need to remove bubbles as you would in building a hand layup boat -- squeegeeing, etc), but then you've eliminated the need for rotation, and you've incurred the same labor as for a hand layup.

The kayak industry tells the story. High quality sea kayaks (which rarely get bashed into rocks) are still built in composites and are expensive, as a result. They are stiff and light, both good features for a sea kayak. White water kayaks and almost all recreational kayaks (even pretty good sea kayaks) are rotomolded, all from polyethylene. They are impact resistant and cheap, but heavy. You could talk to the people at Wilderness systems, (or other kayak builders who use or have used both systems). All these builders (and builders of small sailboats) would love to be able to build composite boats in a fashion like rotomolding, which involves almost no hand labor, but they have been unable to do so.

As it is, in boat building (going from expensive to cheap) you have hand-laid cloth (with or without cores) and either hand saturation or various resin injection systems, or prepreg. With carbon cloth and good resin control, these can be very light and strong and expensive. Next you have hand layup and hand saturation using cloth and mat, which produces a heavier (but cheaper structure. Next you have chopper gun applied resin and glass -- cheaper yet, and even heavier and weaker. Finally you have rotomolding, which is even weaker, but which has the huge advantage of high impact resistance.

Rotomolding with polyethylene has additional advantages in vapor control, ease of recycling (and using recycled feed stocks), etc. Tooling and machinery is much more expensive, of course.

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#7

Re: Roto-Molding with a Composite

08/23/2012 11:46 AM

Thanks for all the help. I am thinking that this may not be possible. I have found products that will work, but they are too cost prohibitive.

What I am looking to build is a bear proof box. About 2'x1'x1' in dimensions. The other systems out there are either too heavy or expensive. Also looking to make it hollow, so as to insulate it.

The main reason that I'm looking to do this with some sort of composite or thermosetting plastic is to avoid the cost of building an oven to melt polyethylene. I'm really starting to think this may be the only real option, though. Every, and I mean every, bearproof anything that I can find is either HDPE or metal.

I'm thinking that using a light resin outer shell with a reinforcing foam center could be better. I just don't know if the outer skin would bond enough with the foam to prevent it's being ripped off.

Thanks again for all the help,

Joe

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#8
In reply to #7

Re: Roto-Molding with a Composite

08/23/2012 11:54 AM

Go Metal.

Killbear Provincial Park (in Canada) can show you lots of pictures of what coolers look like after a black bear has been at them. Any sort of glass etc. will make this too brittle to stand up- that's why HDPE would be better than PP even though it's not as strong.

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#9
In reply to #7

Re: Roto-Molding with a Composite

08/23/2012 10:31 PM

I wonder how a graphene reinforced material would hold up to bear claws?

You building this for fun or business?

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#10
In reply to #7

Re: Roto-Molding with a Composite

08/24/2012 1:39 PM

I just don't know if the outer skin would bond enough with the foam to prevent it's being ripped off.


I doubt that it would. The actual failure would be of the core, just inside the bond line. Using the boatbuilding analogy again: cored composites are light and stiff as overall structures, but poor in impact resistance and point loads. Anywhere that hardware is attached to a cored composite, the core must be removed and replaced with wood or aluminum. A bear's tooth makes for a very high point loading.

For bear protection, I'd go with steel. My dog has perforated metal door knobs, (when home alone and panicked by a thunderstorm) so even steel needs to be reasonably thick.

Tensile strengths:
Core materials: about 30 psi to 500 psi

Wood: about 4000 psi

Chopped strand fiberglass: about 15,000 psi ?

Fiberglass cloth in epoxy: 25,000 psi

Steel: 60,000 psi or more.

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#11
In reply to #10

Re: Roto-Molding with a Composite

08/24/2012 9:45 PM

My dog has perforated metal door knobs....

That's a very high spec dog. My dog doesn't even have a door let alone door knobs.

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#12
In reply to #11

Re: Roto-Molding with a Composite

08/25/2012 12:00 PM

We had him modified to permit cheaper routine surgery. Head door for brain replacement in case he gets too cranky. Belly door for fat removal if he gets obese, etc. The knob perforations are, of course, for lightness.

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