UKC

Fibreglass (cast ) advice

New Topic
Please Register as a New User in order to reply to this topic.
 Rick Graham 06 Jul 2024

Advice requested from the hive mind, please.

I want to make some diy craft thingies.

Inspired by the nhs plasterman putting a walking cast on my broken ankle 10 years ago, I think buying some nhs style materials might just do the job in mind.

Remembering that I have never used glassfibre personally just vague memories of a lot of mess watching a friend repair a canoe or car years ago, a lot of mixing, brushing and messy bits. The nhs man just seemed to wrap special tape around my leg and told me to wait a few minutes before weighing it.

Is this just a pre coated glassfibre tape or something else? 

Any type of glassfibre construction stronger than others or does it just depend on the thickness?

 Mr Lopez 06 Jul 2024
In reply to Rick Graham:

If you want strength then casting tape is not really the thing. Also keep in mind strength is not the same as rigidity.

> Any type of glassfibre construction stronger than others or does it just depend on the thickness?

2 things make what is referred to as fiberglass. The glass fibers themselves that come as a cloth, and the resin.

With regards the cloth, fibre orientation, cloth weight, number of layers and final thickness are the variables. (More thinner layer vs less thicker ones). Depending on application you tailor the variables for the desired result. 

With regards the resin, epoxy is strongest, PU is somewhat more versatile/forgiving and cheaper, vinylester nobody really uses it anymore

Post edited at 21:01
OP Rick Graham 06 Jul 2024
In reply to Mr Lopez:

> If you want strength then casting tape is not really the thing. Also keep in mind strength is not the same as rigidity.

> 2 things make what is referred to as fiberglass. The glass fibers themselves that come as a cloth, and the resin.

> With regards the cloth, fibre orientation, cloth weight, number of layers and final thickness are the variables. (More thinner layer vs less thicker ones). Depending on application you tailor the variables for the desired result. 

> With regards the resin, epoxy is strongest, PU is somewhat more versatile/forgiving and cheaper, vinylester nobody really uses it anymore

Thanks .

Presumably epoxy is two part, needing to be mixed a few minutes before brushing in.

Does the NHS use PU precoated tape or something else?

 jkarran 06 Jul 2024
In reply to Rick Graham:

What do you want to make? Easy composites have a website with great educational content and obviously, the materials and equipment.

Jk

 Mr Lopez 06 Jul 2024
In reply to Rick Graham:

> Thanks .

> Presumably epoxy is two part, needing to be mixed a few minutes before brushing in.

Yes, all resins are 2 part. Depending what you are doing it doesn't necessarily get brushed in. You can soak the cloth in the mixed resin and then lay it wherever you need it for example. That's what i prefer to do whenever possible

> Does the NHS use PU precoated tape or something else?

They use a water based resin-catalyser impregnated cloth that gets activated when soaking it in water. Probably more to it but that's the basic of it

P.s

> With regards the resin, epoxy is strongest, PU is somewhat more versatile/forgiving and cheaper, vinylester nobody really uses it anymore

I meant polyester not PU

Post edited at 22:13
OP Rick Graham 07 Jul 2024
In reply to Mr Lopez:

Thanks both for the info and knowledge.

Think I will get whatevers cheapest and easiest to use and make a prototype. If thats not robust enough will either overlayer it or get better materials.

 deepsoup 07 Jul 2024
In reply to Rick Graham:

> Think I will get whatevers cheapest and easiest to use and make a prototype.

That's quite likely to be polyester and chopped strand mat.  (Depending on what you're trying to make.)  I use it quite regularly for minor kayak repairs - love a bit of rockhopping and I'm not always the carefullest.

You'll need a good sized bottle of acetone for cleaning etc. as well as the resin and catalyst, a syringe for measuring out the catalyst and I use digital kitchen scales (to 0.1g) to weigh out the resin and work out how much catalyst to add. (Unlike epoxy there's a bit of wiggle room there to use more or less catalyst so that it'll go off faster on a cold day or slower on a hot one.)

I find Tommee Tippee baby feeding bowls are nicer to use than disposable plastic cups, and when the unused resin has gone off you can more or less just pop it out of the bowl and it's clean.

You really don't want to be breathing in the fumes if you're working with polyester, so best to work outdoors or somewhere extremely well ventilated.  Oh, and the reaction as it goes off is exothermic - no problem if you're using it with glass mat to make a 'shell' but if you're tempted to try to cast a big block of the stuff it'll get very hot and might even spontaneously catch fire.

If you're making something in a mould you'll need to look into waxes and release materials, and you might want to start with a layer of gelcoat before the first layer of fibreglass goes on for a better & more waterproof finish.

Youtube is a good source of info on how to work with the stuff, there's lots of good (and a little bit of very bad) advice on there.

 kevin stephens 07 Jul 2024
In reply to Rick Graham:

I’ve found East Coast Fibreglass to be a particularly good source of information and materials

https://www.ecfibreglasssupplies.co.uk/

 jon 07 Jul 2024
In reply to Rick Graham:

Pah, in my day we used papier maché...😎

 Mr. Lee 07 Jul 2024
In reply to Rick Graham:

I use the type of impregnated fibreglass rolls that you're referring to. I use them to casting in order to make various types of orthoses. They're available in different degrees of rigidity but the process of application is the same. You'll probably find the narrower roles easier to apply if going around corners or unusual shapes. The brands that I've used seem to have similar strength. 

 CantClimbTom 07 Jul 2024
In reply to Rick Graham:

There are several YouTube channels just on this very subject... Enjoy!

 jkarran 07 Jul 2024
In reply to Rick Graham:

The best material and process does heavily depend on what you're making.

Jk

OP Rick Graham 07 Jul 2024
In reply to jkarran:

> The best material and process does heavily depend on what you're making.

Thanks JK and everybody else for the advice.

Would rather not say what prototypes I intend to make atm. Will tell, if they work after testing.

 Toerag 17:26 Mon
In reply to Rick Graham:

There's a fibreglass group on FB. quite USA-oriented, but might be useful.  https://www.facebook.com/groups/1128565927491051

 jkarran 10:32 Tue
In reply to Rick Graham:

> Would rather not say what prototypes I intend to make atm. Will tell, if they work after testing.

Then to answer your initial question broadly.

The stiffness of the part in particular is strongly related to the fibre stiffness (there are lots of different fibers available even within a broad group like glass or carbon), fiber fraction (fiber volume vs resin/matrix) and in particular the fibre orientation which is really critical. Some reinforcements (like chopped strand matting, loose fibres and some woven products) have multiple fiber directions and require little thought with regard to alignment, some (most cloths) have 2 fiber directions and some even just 1 (unidirectional ribbon/tape and tow) so have to be placed with increasing care but to potentially much greater effect. Likewise for strength (to yield and ultimate) the layup (fiber volume, fraction and particularly orientation) matters. Also different fibres and resins fail in radically different ways (particularly post yield or matrix failure). Some fibers like carbon and to a lesser degree glass produce composites exhibiting high stiffness but brittle failure, others like kevlar and the uhmwp's produce tougher composites that tend to be like for like less stiff but strongly resist fracture.

Resins differ quite dramatically in performance across various metrics, in general the epoxies will tend to win on most but within that class there are lots of options, for outdoor products you may be concerned with UV resistance and for sporting goods subject to impact, toughness. Epoxies don't stink like vinyl resins.

Some reinforcements are intended for specific resin systems. The most likely issue here is trying to use epoxy with chopped strand matting, you can do it but you need to buy the right sort. Some of the polymer fibres will only really work with epoxy.

If you're thinking of making something akin to a medical 'plaster' cast that will see similar service then those medical products probably are a good choice being easy to work with but they won't deliver the best performance structurally if that's of interest.

jk

In reply to jkarran:

Thanks for the detailed and informative reply. Ukc usually delivers

 mutt 13:29 Tue
In reply to Rick Graham:

> Advice requested from the hive mind, please.

> I want to make some diy craft thingies.

> Is this just a pre coated glassfibre tape or something else? 

Its called pre-preg and used widely in the boat building industry around here.  The benefit is that the epoxy mix is perfect (any mismatch in the resin and catalyst will significantly weaken the cured laminate. The downside is that you will need to buy a 200m roll and keep it refrigerated so its not much use for low volume manufacturing or small items . The resin goes off eventually even in the fridge so you materials won't last.

> Any type of glassfibre construction stronger than others or does it just depend on the thickness?

Yes, the weave materials, glass carbon and kevlar have differenct properties. the aircraft industry tend to mix glass and carbon to get the stiffness and lightness of carbon but with the durability of glass  (carbon snaps easily). The marine industry usually uses glass but for vessels that ground kevlar can be added. Stiffness is enhanced buy laminating up a PU sheet 10mm or so thick but that makes it more difficult to conform to complex shapes.

 Crest Jewel 14:23 Tue
In reply to Rick Graham:

Is out sourcing a 3D printed solution beyond the pale and anathema to DIY self-reliance?

Post edited at 14:23
 tingle 15:07 Tue
In reply to Rick Graham:

If you are making an intricate product with tight angles, use a low weight glass 300/450g and a gelcoat depending on your moulding style. Air is your enemy, learn what it looks like and remove it before it goes hard. DM me any specific problems you have, expect to have problems ha!

 jkarran 15:48 Tue
In reply to mutt:

Easycomposites have pre-preg glass and carbon by the meter. Having to bag/bind and cook it would count as the main drawback in my book!

The OP seems to be looking at some sort of water-cured polyester-glass tape aimed at the medical market. For the right type of part/job it does look very user friendly.

jk

Post edited at 15:49
In reply to Crest Jewel:

Its an intricate complicated 3 d shape outside the capabilities of 3d printing in the correct materials.

The right materials, structural strength, the correct ratio of stiffness to flexibility.

Doing it in gf means I can fine tune the prototype if required. Very likely. Or possibly just throw it in  the bin as impractical. 

Its a concept idea  I had 55 years ago, thought it about time to give it a go

 Mr Lopez 10:58 Wed
In reply to Rick Graham:

> Its an intricate complicated 3 d shape outside the capabilities of 3d printing in the correct materials.

Sounds like the kind of thing it'd be extremely difficult to do with fibreglass.

You probably know it, but fibreglass is a cloth that gets laid onto a form/mold/substrate and kept there until is cured.

One issue for it to be laid on an 'intricate complicated 3d shape' is that the cloth isn't very pliable in the grand scheme of things, unless you use a very thin veil, and that brings another issue, which is how you keep it there till it's cured. The wetted cloth isn't sticky at all, in fact it's very slippery, plus there's some shrinkage and dilation when curing that tends to lift up the cloth wherever is a radius too tight, and specially when there's curvatures in 2 planes.

There's a few techniques like vacuum bagging (complex, relatively expensive and needing suitable experience/knowledge), waiting for the resin to go sticky before laying it on the mold (it becomes even less pliable) or some more gangster tricks like spraying contact adhesive for example, but they all have their limitations.

Filling the mold with gelcoat to fill all the gaps, corners and crannies only has a cosmetic effect and does nothing for strength. The closest you could come to, which helps structurally, is spraying finely chopped glass fibres mixed up with resin, but besides it again being expensive to set up, etc. the fact the fibres are short rather than continous lengths and so not mechanically connecting the 'face' to the main body means that section is likely to crack and fail as the elasticity won't be homogenous.

Willing to be proven wrong though. Messing about trying to work what works and what doesn't is 90% of the fun (and 100% of the frustration, haha).

Good luck

Post edited at 11:00
 jkarran 11:34 Wed
In reply to Rick Graham:

> Its an intricate complicated 3 d shape outside the capabilities of 3d printing in the correct materials.

Are you sure? https://www.easycomposites.co.uk/learning/mechanical-properties-of-forged-c...

The link mostly covers compression moulding quasi-orthotropic solid epoxy-carbon parts in printed moulds but it also looks in some depth at conventional FDM 3d printing with fiber-reinforced filament to surprising effect.

I'd say the take home is that up to a certain scale if you can glass* it you can print it at home. Modelling complex surfaces to print may remain a barrier though, sculpting in clays or wrapping existing forms is more intuitive with a much lower bar to getting started. Beyond the home shop, sintered nylon parts are affordable, tough and finish well. We mostly use this process at work now rather than injection moulding plastic parts, the results are excellent where you don't need a smooth high gloss surface.

*FDM is still no match for more refined composite materials and designs.

I've just bought myself a printer, basically off the back of that page and the availability of these enhanced semi-structural filaments.

jk

Post edited at 11:37
In reply to Mr Lopez:

Thanks, again.

Not too intricate or tight radii, I was just being deliberately vague.

Cheers Rick

 Toerag 12:59 Wed
In reply to Rick Graham:

Please show us it when you're done, it sounds interesting.

In addition to what the others have said about the problems with getting the cloth to conform to complex shapes and small radii, do other things exist in fibreglass which have similar curves and radii? For example, those ride-on machines in amusement arcades are normally fibreglass.  If so, your design should be possible.  What sort of loads is it planned to experience? Is the weight of it a consideration?  I suspect hand layup with glass cloth and polyester resin is the easiest / cheapest thing for you to do in terms of a prototype, especially if you want to keep the design secret. Make a wood / foam / clay mould/plug, wax it, put gelcoat on, then veil/tissue, then CSM, then more layers of CSM / cloth / tows as necessary to give the required directional strength and point impact resistance.  See if you can make the shape with that method. If so, then for a commercial design, improvements would be made in the use of stronger/stiffer fibres (carbon), tougher fibres (kevlar), better resins (epoxy), more aesthetically pleasing fibres (carbon/kevlar cloth, spread tow carbon, helical wrapping of tube), different manufacturing techniques (hand layup is expensive).

 Toerag 14:24 Wed
In reply to Rick Graham:

> Any type of glassfibre construction stronger than others or does it just depend on the thickness?

The philosphy behind fibre-reinforced plasic composites is that structures fail by cracking / tearing, and the design of the composite works to deal with those things

Imagine making a thick sheet of solid glass and bending it. It breaks by a crack starting on the side under tension and propagating through the whole thickness of the sheet.  That crack invariably starts at a surface defect, so any surface defects massively reduce the bending strength of the sheet.  The ultimate glass would be a single crystal with no defects, but making a single crystal big enough to use is impractical.  However, if you make ultra-thin fibres the number of surface defects is reduced compared to thicker glass, and thus the opportunity for a crack to start is reduced. so that's one reason why fibres are used rather than sheets.

Now, if you make the thick sheet out of lots of layers of thinner glass stuck together with resin, what happens is that the crack propagates through the first layer then stops in the softer resin, or is forced to divert sideways along the interface between the glass and resin. Because the forces acting on the sideways crack are not in the same plane the crack stops propagating.  The crack will propagate sideways across the sheet of glass, breaking it completely across it's width.  Eventually the resin will fail and the next sheet will crack and so on until total failure, but the key thing is that loads are normally temporary and the structure will survive the original peak load.

Now, If, instead of a sheet you use loads of fibres, not only do you stop the cracks propagating downwards, but you also stop them propagating sideways, thus curing/reducing the cracking problem.

Fibres are strong in tension and the composite is relatively stiff along their lengths. Therefore you need to align the fibres with the expected direction of force, like the fibres in trees and bamboo.  The prime example of this would be one of those roll-up beach mats or a wood& wire fence - stiff across the width, flexy across the length.  Plywood combines wood fibres in two directions to make it stiff 'across the grain' as well as along it.  CSM goes even further and is equally strong in all directions due to the random fibre arrangement.  If you made a tube from long fibres it would be strong in tension, but if you tried to bend it it would go oval, then flatten and lose the strength it gets from being a tube. Thus tubes (e.g. fishing rods) have fibres running circumferentially to prevent that ovalising / crushing.

Resistance to tearing - obviously the fibres provide this along their length.  CSM or other short fibres ('whiskers') have the issue that their short length means that the bond between them and the resin only exists along that length, and thus they can suffer 'fibre pullout'. This manifests itself in poor impact strength - if you hit something made of thin CSM with a hammer it will punch through it - the resin crumbles and it pulls any fibres that didn't break out of the resin around the edge of the damage. Thus for good impact resistance you need to use long fibres (woven cloth or unidirectional bundles / tows).  Kevlar has a high resistance to cutting, thus is used to prevent impact damage like this - performance speedboats will often have long fibre kevlar in the construction in the front half of the hull to deal with damage coming from hitting floating logs etc.

Fibre-reinforced plastic's weakness is compression strength - fibres buckle relatively easily, especially in thin-walled constructions. The only solutions are to make thick cross sections, or add webs to prevent buckling.

Increasing layup thickness increases strength/stiffness, but with a weight penalty.  You may gain an advantage of radii decreasing / disappearing and the ability to place fibres in more orientations.

In terms of particular fibreglassing methods being stronger or not, the following principles apply:-

The majority of the structural performance comes from the fibres, the resin is just there to hold them in place and provide surface protection / finish. Thus you can retain structural performance whilst losing weight by improving the fibre:resin ratio, or what is know as the 'fibre volume fraction'.  This means using vacuum bagging or resin transfer moulding techniques which reduce the amount of resin used for a given volume of fibres.  There is a limit though, and a lack of resin manifests itself as a lack of toughness / resilience - high fibre volume parts tend to me more brittle and vulnerable to unexpected damage such as impacts.

Bubbles in the resin reduce matrix strength by decreasing the amount of resin/fibre bond length and reducing the thickness of the resin 'layer' between fibres.  Bubbles can be dealt with by particular resin mixing and application to the matrix techniques; applying vacuum to runny resin before cure to make them 'boil off', or applying pressure to the matrix before and during cure to shrink them (autoclaving).

So, reducing resin content makes things lighter for the same strength, removing bubbles makes things stronger.

Post edited at 14:34

New Topic
Please Register as a New User in order to reply to this topic.
Loading Notifications...