Resin mixing - hardener question

[Locke_333]

Hi, this seem to be a simple question and certainly hope the answer will be simple.

i've bought a new sort of clear resin because the store doesn't carry the one i used to use. The amount of hardener to be added depends on the thickness of the part to be cast. What would happend if i put more hardener than the recommended dosage? by memory, it's 8 drop per onces for 1/2 inch thick and 15 drops for 1/4 thick.

I know that not putting enough would mean resin not curing properly but what can to much hardener do?

Thanks

[modelnutz]

If you are speaking of a polyester resin ( the type used with fiberglass )
then too much resin will do a number of things....

1) Too much heat will be generated causing the resin to shrink exsessivly
perhaps even cracking from internal stresses

2) The clear resin will get cloudy

3) the heat generated could yellow the resin.

There may be more issues that what I've stated....I'll let others chime in with their experiences.

Hope this helps !

Modelnutz

[Umi_Ryuzuki]

Also the more harder you put in, the more brittle the piece may turn out.

I gave up on clear cast polyester resin.
1. The brand I had was an "air inhibited" cure. The dealer told me that despite being encased in the mold, it would not cure properly in a silicone mold.
Apparently there was an issue there. It did not make sense.
2. The resin would shrink up to 14% thus leaving an air gap between the mold sides and the part. Any tacky resin would "string" between the mold sides and part, and drop onto the part making a horrible mess.

I finally switched to a polyurethane clear casting resin.

[modelnutz]

Umi...I finally got very good, consistent results using a casting resin from
LBI, inc. 973 North Road...Groton Ct. 06340
It's called 303 clear casting resin
I had to do a good amount of experiments to arrive at the proper resin to catlyst ratio to ensure a clear cast as well as a complete cure.
I found that it worked best to use very little catlyst...in platinum silicone molds...molds pre-heated...as well as post cured in a light box at 85 - 90 degrees. About 1 hour cure time with 1 hour post cure...tack free.....minor shrinking.

Sounds like rocket science, I know.
Once I got the system down...it went really well and I got very consistent results that only reqired some buffing to shine the pieces.

FYI...I was encapsulating ( trapping ) marshmallo's in 2" sq. cubes....
Boy, the things you get into when your trying to learn casting !

Keep asking questions....we'll get ya' there !

Modelnutz

[Locke_333]

yes, it's polyester resin. i've got a hard time having it curing. Out of 3 pull, only the first came out clean.... i was a bit frustrated by that and now, since it's warmer time, i'm retring my luck. The 2 bad pull where made at a lower temperature so it may also have played agaisnt me at the same time...

Thanks for your input guys!!

[modelnutz]

Keep in mind that the silicone robs heat from the resin...
Heat is what cures the resin....the catalyst is there to create heat.

Also, you can get a negative reaction from the silicone as well.( Thats why I use platinum cure silicones for clear parts...no alcohol )
Standard silicone gives off alcohol all the time...this could also be a reason for the partial cure.
You can avoid this issue by heating the mold in an oven ( set to it's lowest setting ) for about 1/2 hour or till it feels warm. This tends to dry off the alcohol and can really help. It will also help to speed the cure of the resin so you could potentially use less catlyst for a clearer final piece.

Help ? That remains to be seen...if you are sucessful using our info...we'ved helped.
Me, I'm just payin' it forward

Modelnutz

[Chacal]

I also use polyester resin (that's the only kind readily available near me) and Modelnutz is right: too much catalyst will overheat the resin, make it crack (or at least develop hairline cracks, invisible to the naked eye which will become brittle, like Umi said), maybe even ruin the mold. Worst-case scenario, resin will crack and "explode". Fires aren't out of the question.

To keep shrinkage at a minimum, I use mineral fillers (calcite, ground up to a powder). Those are there just to add volume, so I use less resin per pour. Mineral fillers will not shrink, so the overall shrinkage is reduced, and they sand more easily (if required later). I need however, to add a little bit more catalyst when I use fillers (if I use 50% by volume, I'll have to think that the resin layer I'll actually lay will be 50% thinner, so I use – as in your example – 15 drops per ounce if I pour a 1/2 inch thick layer), because the filler will not only not produce the heat required, but it will draw some heat from the curing resin.

[Locke_333]

well, i just pulled the pieces i made yesterday and surprise!, uncured resin left in the mold, i'll clean it up tomorow when it has dried out a bit. The "core" of the piece is fully cured so it seems that i have a problem mainly from the mold taking out the heat of the curing process. Next batch tomorow mornig, i'll heat it up a bit to see if i have better succes. i'm guessing that the main problem since the left over resin of the batch completely cured in the mixing pot.

[modelnutz]

Don't forget about heating the molds.....it will help

About 1/2 hour in an oven set on the lowest setting.

[Chacal]

well, i just pulled the pieces i made yesterday and surprise!, uncured resin left in the mold, i'll clean it up tomorow when it has dried out a bit. The "core" of the piece is fully cured so it seems that i have a problem mainly from the mold taking out the heat of the curing process. Next batch tomorow mornig, i'll heat it up a bit to see if i have better succes. i'm guessing that the main problem since the left over resin of the batch completely cured in the mixing pot.

How big is the part being cast? What is its shape? Is the resin left in the mold from thinner parts or the entire surface layer? You might be using too little hardener per ounce of resin, OR you may have an incompatibility problem between the resin and the mold. It has been discussed in other threads (I just can't remember which one or when – blame it on my fourth-decade-syndrome™). Just do a search in here for "sticky resin". As far as I remember, that expression was used.

[Umi_Ryuzuki]

If the problem occuring is similar to my attempts with polyester resin, then more catalyst or heating the molds will not help.

It is worth trying these methods, it might yet work.
I tried them all before I switched resin formulas and stopped using polyester resins.

[Locke_333]

Thanks all for the coment but i'm about to give up on this combination. I've tried everything mentionned above but nothing will give me good results( heating the mold, more hardener, less hardener, 48 hours to cure ... ). i still get uncured resin on the surface that is in direct contact with the mold... i have one last batch in curing and if it ain't good, i'm going back to envirotex lite.

Can you add a dye to color the Envirotex Lite?

maybe i should tri one last bastch with a most release agent... baby powder or petrol gel...

[modelnutz]

Sounds like a case of cure inhibition caused by an adverse reaction between the resin and silicone.

You are using "casting" resin and not laminating resin right ?
Laminating resin is supposed to stay sticky so the next layer will bond.
Casting resin is more like gel coat resin...it has some wax included in the mix so it will cure to a sandable plastic.

The only other suggestion I have is PVA.
It's a sprayable coating that fiberglass guys apply to molds.
It's ment to allow easy removal, but in your situation, it would work as a "barrier coat" keeping the resin from coming into contact with the silicone.

Or......use clear urethanes ( which have their own issues )

Modelnutz

[Locke_333]

well, in the end, i gave up on the combination, i've switched back to epoxy resin for now until i have more time to loose with this "thing"

Thanks all for you help, it was appreciated!