As you point out, your discussion with SP Systems indicate that the accuracy
they have been recommending to people in the past is not sufficiently strict
and should be tightened to 1% of the total mix...."
My advice from SP Systems had been that a tolerance on the hardener of no
more than +/- 5% would be acceptable. In other words, assuming that your
resin is 100 parts, your hardener may be 25 parts +/- 1.25 parts. This is
achievable using electronic scales with 2 gram increments provided no less
than 96 grams of resin is used. In this case you could end up with between
95 and 97 grams of resin and between 23 and 25 grams of hardener. With
each component at opposite extremes the max. error of hardener is 5%.
Since your e-mail on the subject I again contacted SP Systems and the story
this time as that a 3& error is the accepted norm. It was expressed as +/-
1 part of resin and +/- 1/2 part of hardener, based on 100 parts of resin
and 25 parts of hardener.
I think it is obvious to all of us that the more accurate an epoxy mix is
made the higher the strength that will be achieved.
Using your scales will require you to make mixes with at least a quantity of
84 grams of resin and 21 of hardener. To achieve better than 1% accuracy
your smallest mix must be in excess of 240 grams of hardener.
To be able to mix smaller batches with the accuracy you are aiming for you
should use a ratio balance such as that shown in the build manual.
Regards
Andy
>Gidday,
>
>I've spent a bit of time talking to SP Systems about Amine Blush and mix
>ratios. I'll talk about each in turn:
>AMINE BLUSH
>Quote: "Amine Blush which you mention in your letter we call Amine
>Bi-Product but it is the same thing. The Amines in the epoxy hardener will
>react with any moisture in the atmosphere as the epoxy cures. This will only
>occur after the epoxy has started to gel and will continue until a
>reasonable state of cure has been reached.
>This bi-product feels greasy or sticky dependant on the system and the
>amount of moisture in the atmosphere. The slower the hardener the longer the
>epoxy has to react wiht the moisture. I WOULD SUGGEST THAT ALL THE AMPREG 20
>HARDENER YOU ARE USING PRODUCES BI-PRODUCT TO A GREATER OR LESSER DEGREE BUT
>THE SLOW HARDENER PRODUCES THAT UCH MORE SO IT IS MORE EASILY DETECTED
>Without curing the epoxy under a vacuum or in a laboratory whth zero
>humidity there are only two ways of overcoming the productionof bi-product.
>1. Use a Peel Ply which a Nylon 66 woven material which is Silicon Free.
>This is applied to the surface of the laminate and 'wetted out' whth the
>epoxy system used.The laminate is then left to cure as normal. When the next
>stage in the laminating /coating is to be carried out, the peel ply is
>pulled from the surface. This leaves a perfectly prepared surface for
>secondary bonding or coating as the bi-product which forms is removed on the
>top surface of the peel ply.
>2. The second is to wet sand the surface after the epoxy has cured, as the
>bi-produce is water soluble, please note that dry sanding only clogs the
>paper and smears the bi-product across the surface but does not remove it.
>THIS BI-PRODUCT MUST BE REMOVED BEFORE THE NEXT COAT OR LAMINATE LAYER IS
>APPLIED AS ADHESION TO THE PREVIOUS COATING/LAYER WILL BE GREATLY EFFECTED.
>
>I later asked him about using vinegar to wash down the surface,prompted by
>other e-mails where it has come up as an option. He explains:
>
>"As to your question over why some people are using vinegar,the reason for
>this is that if the bi-product is left for any length of time it bakes onto
>the surface and can be very hard to remove. Then it is some time necessary
>to a 50/50 mix of vinegar and water. The vinegar is a mild acid which in
>conjunction with a 'scotch Brite' pad will remove even baked on material. If
>the bi-product is not left for any time, it will not be necessary to use the
>50/50 mix but only water.
>
>Thats it! As a comment, there has been extensive dialogue about using Peel
>Ply over the entire surface. The general consensus is DON'T DO IT!! Read
>your Aircraft Spruce and Specialty Catalogue, read previous posted messages,
>speak to Graham Singleton etc.So that only leaves the washdown option. You
>may well ask why this isn't included in our manuals (or at least I don't
>think it is). The a/c no doubt has it's composite design to achieve Ivan's
>goals, and no doubt it does. At the same time,SP have suggested a treatment
>to the surface prior to filling and painting. I know what I will be doing!
>
>RESIN MIX RATIOS
>My other topic Ratio mixing is prompted because I have purchased a set of
>electronic scales accurate to 1/2 gram i.e. in 1 gram readout
>increments.What follows is SP Systems response on the required accuracy of
>mixes required by them. I should add that I made simultaneous enquiries to
>Europa regarding mix ratios,whilst organising the purchase and shipment of
>my wing and fuselage kits. Europa say a maximum of 5%! Does anyone remember
>what I said once about "Believe nothing because a wise man said it" etc
>etc.!!!!Don't stop asking questions! Anyway SP's next response follows to my
>questioning about ratios:
>"Your question about the accuracy of mix with Ampreg 20 has raised a debate
>within our company. This is not the first time this qiestion has come to
>light, but it has never been as fully explored in the past.
>I am sorry to inform you that the current consensus is that the accuracy we
>have been recommending to people in the past is not sufficiently strict, and
>that it should be tightened to 1% of the total mix.This will mean that your
>current scales are sufficient for 100 gram mixed but are not for 50 gram.
>I am sorry that we can not recommend or guarantee that any of our products,
>when mix ( with or without an error in measurement) can be used for a
>specific application without carrying out trials under the same conditions.
>Re Resin Pumps he says:
>"I believe the other measuring device you mention is supplied by a company
>in the US called J&B Products. If this is the case we have a sample which is
>currently inder going trials, but as yet we do not gave a accuracy figure
>which we could give you".
>
>There is more talk to SP coming re postcuring. I have it on good authority
>( an airlines fibreglass workshop staff), that postcuring needs to be done
>in an exact way to maintain stability of the polymers, especially with
>regard to how the piece is brought back to ambient i.e. the temperature drop
>needs to be certainly controlled. This would suggest a more professional
>method of oven design, but I'll reserve my judgement till I know more, and
>I'll post it,hopefully more succinctly..
>
>Regards
>Tony Renshaw
>Builder No.236
>The Aussie Connection
>
>
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