Heat Accelerating PR Cure?

[vtgaryw]

Did a quick search, but didn't turn up anything, although I'm sure it's been covered.

Can I safely accelerate PR curing using heat?

Thanks,

Gary

 

[Ed McDonnell]

I put my molds in a shop dedicated toaster oven at 125f to 150f. Works for me.
PR is flammable. Don't use a heat source that will ignite it. Don't overheat it. I do it outside. I don't heat cast labels because I've had a few failures with that.

Ed

 

[vtgaryw]

I was thinking that would be the temp range to go. My toaster oven doesn't do under 200f (maybe I could leave the door open?)

Otherwise I was thinking just a box (non-flammable of course) with a space heater?

 

[Ed McDonnell]

I use the "warming" setting on mine. I bought an oven thermometer so I could figure out where to set the temp dial.

PR that overheats and starts smoking / burning is really bad news. Don't take a chance with something that could kill you or burn your house down.

For molds that won't fit in my oven, I've put them in a closed box that I painted black and put in the sun. You could try that for pen blanks.

Do not put you resin in your hot car to heat cure it!!! You will regret it.

Ed

 

[ed4copies]

One theory I have seen says PR becomes more brittle, the more heat it generates.

Aren't you concerned that you will make a blank that is more difficult to turn, if you introduce additional heat source(s) to the process?

 

[Ed McDonnell]

Hi Ed - I have not had a problem with brittle blanks. I use the full recommended amount of MEKP (10 drops/oz for siar 41).

Using less (e.g. 3 drops/oz) and then "accelerating" the curing with heat may account for some of the reports you have had.

Other reports of brittle PR are probably due more to tool selection / usage than to curing methods.

All just my opinion, based on my experience. YMMV.

Ed

 

[vtgaryw]

My biggest concern was making it brittle. I don't think I'll risk the blanks I have in the queue right now, but when I have time I'll experiment a bit.

Thanks all,

-gary

 

[sbell111]

My understanding is that you needed enough heat to kick the hardening reaction, but that adding too much heat whether by adding too much MEKP or too much external heat will cause it to be brittle. As I recall, someone did an experiment with a block of PR and way too much MEKP and the block cracked in the mold because it got too hot.

There are a few people in this forum who have actual technical knowledge regarding the properties of PR. Perhaps one will pop into the thread to speak to the issue of how additional heat or MEKP affects PR brittleness.

I'm certainly willing to be wrong, especially if it means that I no longer have to be very concerned about titrating my MEKP.

 

[Akula]

I don't understand the goal of heating it?

 

[ed4copies]

Hi Ed - I have not had a problem with brittle blanks. I use the full recommended amount of MEKP (10 drops/oz for siar 41).

Using less (e.g. 3 drops/oz) and then "accelerating" the curing with heat may account for some of the reports you have had.

Other reports of brittle PR are probably due more to tool selection / usage than to curing methods.

All just my opinion, based on my experience. YMMV.

Ed

Hey Ed!

I believe "brittle" is in the eye of the beholder!!

I have had "comments" about blanks being brittle, where I KNOW the artist uses 3-4 drops per ounce. When I experiment with turning a blank, there is no problem.

So, like you, in my EXPERIENCE, nearly all plastics can be turned. Some are easier than others, but you adjust the presentation of your tool and the material "turns into" a pen.

My comment is made to solicit additional feedback---as one customer said to me "Well, you're a professional turner---of COURSE you can turn it". I found this interesting since his previous statement had asserted that the blank was defective, since it could not be turned.

I rely heavily on the "community wisdom" that comes from reading these threads and soliciting more input.

THANK YOU and all others who contribute.

Ed

 

[sbell111]

I don't understand the goal of heating it?

PR hardens when it is heated. Usually, this heat is created via a chemical reaction from the catalyst, MEKP. People often use heat to augment this process. I suspect that they are doing it to get rid of the stickiness that freshly hardened PR has. (This stickiness is actually a feature of the product.) I occasionally use heat if my pour doesn't harden properly (likely due to old PR or old MEKP or a cold shop).

 

[sbell111]

Hi Ed - I have not had a problem with brittle blanks. I use the full recommended amount of MEKP (10 drops/oz for siar 41).

Using less (e.g. 3 drops/oz) and then "accelerating" the curing with heat may account for some of the reports you have had.

Other reports of brittle PR are probably due more to tool selection / usage than to curing methods.

All just my opinion, based on my experience. YMMV.

Ed

Hey Ed!

I believe "brittle" is in the eye of the beholder!!

I have had "comments" about blanks being brittle, where I KNOW the artist uses 3-4 drops per ounce. When I experiment with turning a blank, there is no problem.

So, like you, in my EXPERIENCE, nearly all plastics can be turned. Some are easier than others, but you adjust the presentation of your tool and the material "turns into" a pen.

My comment is made to solicit additional feedback---as one customer said to me "Well, you're a professional turner---of COURSE you can turn it". I found this interesting since his previous statement had asserted that the blank was defective, since it could not be turned.

I rely heavily on the "community wisdom" that comes from reading these threads and soliciting more input.

THANK YOU and all others who contribute.

Ed

In that guy's defense, your expertise at turning would lead me to believe that you could turn products that a hack such as myself would be completely unable to do. I think that when a blank is sold, it is generally with the expectation that it could be turned by a turner of average experience (a person with skills perhaps better than me, but not requiring the skills of an expert such as yourself).

 

[Ed McDonnell]

I don't understand the goal of heating it?

Hi Bill - By heating the resin (to 125f-150f) as it cures I can have it ready to turn in a couple of hours. I can easily go from the can (of resin) to a finished pen in less than a day. Although that's not why I do it. I create somewhat complicated blanks that may end up being cast and recast many times. I can do that with heat curing in a day or two rather than a week or two.

Ed

 

[Ed McDonnell]

Hey Ed!

I believe "brittle" is in the eye of the beholder!!

I have had "comments" about blanks being brittle, where I KNOW the artist uses 3-4 drops per ounce. When I experiment with turning a blank, there is no problem.

So, like you, in my EXPERIENCE, nearly all plastics can be turned. Some are easier than others, but you adjust the presentation of your tool and the material "turns into" a pen.

My comment is made to solicit additional feedback---as one customer said to me "Well, you're a professional turner---of COURSE you can turn it". I found this interesting since his previous statement had asserted that the blank was defective, since it could not be turned.

I rely heavily on the "community wisdom" that comes from reading these threads and soliciting more input.

THANK YOU and all others who contribute.

Ed

It would be interesting to do an experiment to actually quantify the brittleness of PR varying the MEKP and curing temp. Also to compare with urethane and epoxy casting resins. My plate is full right now, but I should have time during the holidays to try some experiments.

I've always wanted to get a better understanding of the material rather than just doing things because they have worked in the past.


Ed

 

[Akula]

I don't understand the goal of heating it?

Hi Bill - By heating the resin (to 125f-150f) as it cures I can have it ready to turn in a couple of hours. I can easily go from the can (of resin) to a finished pen in less than a day. Although that's not why I do it. I create somewhat complicated blanks that may end up being cast and recast many times. I can do that with heat curing in a day or two rather than a week or two.

Ed

Thanks, I got it now.

I've been casting for years but I'm never in a hurry.

 

[Gary Beasley]

I usually heat my pours for about five minutes in the toaster oven at 200F to get the reaction started. It generally doesn't need any more heat than that to cure properly.

 

[robutacion]

Well, is so much that I could say about all this but, I will try to keep is below "book" size

My experience is with PR (Polyester) only so, keep that in mind.

In general terms, the factory recommendation of between 1% and 2%, is that to account for many factors that will be experience from the user viewpoint.

The very first thing to consider is the age of the resin, only because you purchase it today, doesn't mean that is fresh, fresh resin (made within 30 days) and stored under 20° celsius, will maintain its original viscosity (from factory) and that is a quick thin product, it makes a tremendous difference in casting worthless wood with tight crevasses, after the resin thickens due to age.

This is due to some evaporation of the "solvents" as it ages, this also affects the way the MEKP works where, less hardener is require for fresh resins.

The second factor, is the environment temperature of the location where you are at or were the product will be worked on. The higher the temperature the quicker the MEKP reaction will start.

One other factor is, the correlation in between low and high area volume to cast, using the same MKEP % a small mold with low volume will be OK however, the same mix in a large cast/higher volume, will crack so, the correlation between mold size/pour volume and MKEP %'s are very important.

For example, a normal pour mix put in a container the size of a spray can plastic cap, without anything else in it (wood, etc) it will set without any cracks however, try the same MEKP % mix and pour it into a container, the size of a 1kg coffee jar and you will endup with huge cracks through the PR and that is a guarantee...! I do I know...??? I have had lots of "tests" done...!


I have more pics but, I can't find them, right now...!

Large volume casts (amount of resin needed and not the size of the chunk of wood in need to cast), will require a lot less than the minimum factory recommendation as they don't give you any info on the resin volumes to be used and the high volume of resin produces far too much heat when curing, therefore the resin expansion limits is reached and "separation" is inevitable and most frustrating...!

The only way, you can cast a high volume PR cast is, to use 1/4 of the fact. recc. and let it set slowly for a few days, the heat is minimal but the curing process is happening at a slower pace and the resin won't crack...!

There is one thing that I can say with all certainty is that, heating the mold/PR in a oven over 40° Celsius will harden the resin, the higher the temps, the harder it becomes and therefore brittle as glass...!

There is one test that you can do to see this happening, use a 100% PR pen blank and turn about 1" on it from square to round, this is only to give you the feeling of how it cuts now put it in the oven at any temp. you want over 40°C and check it untill the PR becomes flexible (it will need temps of 60C and above to make the PR flexible. As soon as you can bend it, take it out and let it cool down properly on its own, now mount it in the lathe as before and turn the 1" of the square blank, following the bit you turned before, make sure the tool is sharp equally on each attempt, feel how is reacting/behavior and it will be very easier for you to validate what I said about the brittleness from the heat...!

Is a kind mistake to think that, putting the molds in the oven to accelerate the curing process, as soon as the mix is poured, the last thing you need is to force the hardener to go off before it has time to initiate its gradual heating steps, too fast, is the same as too much MEKP and that means troubles.

And this is when the factory recommendations come into place, they are there to allow the PR to do its thing, depending on the environment temps you have, Summer less MEKP, winter more MEKP...!

For those like myself that, don't like to wait to see the grass growing, and that have the normal issue of the PR deliberate (natural properties of PR) surface stickiness after cured inside, and that do not have 40°C temperatures in the sunshine, you can use the oven then to make the stickiness set hard but for that, you have to make sure you don't abuse the oven temps and keep it under 50°C just long enough to harden the surface resin, if you cut the blanks/blocks immediately, you will notice that inside the resin centre, you can hardly feel any temperature, this is what you should aim for and not letting the blanks in there for any longer than absolutely necessary, as if you do, you will have to deal with a much more brittle resin to work with...!

There are exception to any rule, and there are exceptions to the processes I just mention but, these what I consider the "general" PR rules...!

Hope this make sense to you...!

Cheers
George

 

[79spitfire]

I tried setting some near a wood stove. They received un-even heat. I ended up with deep surface wrinkles. While interesting to look at and not deep cracks into the material, they made the small cross section casts I was trying to do un-usable. I wait for warmer daytime temps and use the proper amount of MEKP.

(yes I was trying to be cheap...)

 

[Gary Beasley]

I tried setting some near a wood stove. They received un-even heat. I ended up with deep surface wrinkles. While interesting to look at and not deep cracks into the material, they made the small cross section casts I was trying to do un-usable. I wait for warmer daytime temps and use the proper amount of MEKP.

(yes I was trying to be cheap...)

Next time warm some bricks and place them around the mold and cover it up. Moderate heating is the key here, just get it warm enough to allow the reaction to start and proceed normally. You don't want to bake it. It also helps to simply warm the mold up before pouring, sometimes that's all it takes.

 

[SDB777]

Confused....a little?

Are you trying to accelerate the time it takes to go from mixing in the MEKP to pouring or the time it takes for the PR to be ready to use?


Either way, it's not a race......







Scott (the time saved isn't worth the troubles) B

 

[Gary Beasley]

Myself I prefer to get the PR to set up fairly soon, 20 to 30 minutes so I can get it out of the mold and do the next set. Doesn't matter to me that it's sticky, I won't turn it till the next day anyway.

 

[Gethenet]

I was thinking that would be the temp range to go. My toaster oven doesn't do under 200f (maybe I could leave the door open?)

Otherwise I was thinking just a box (non-flammable of course) with a space heater?

I have done it in a downstairs bathroom( small room) with a space heater, and the venalation fan running. Using pvc tubing for a mold. Had it cured to the point of being able to demold in less than 2 hrs

 

[Sylvanite]

I'm not a PR expert, but I have friends in the fiberglass industry (one who used to commercially mix resin) who are. They explained it to me thus:

Polyester resins (layup resins as well as casting resins) are generally formulated for use at ambient temperatures of 70-80 degrees F. The curing process is initiated by adding hardener (usually MEKP) but needs some warmth to get started. Once underway, the reaction is exothermic - which means it generates heat. The reaction is also temperature sensitive - the hotter it gets, the faster the reaction proceeds.

Too much hardener and too much heat can result in thermal runaway - and lead to a fire. At 75 degrees F, the typical recommended hardener ratio is 1% - 2% MEKP - more or less depending on pour thickness and any additives (such as pigment) used. In cold conditions, you can help speed the reaction up by adding more MEKP; in hot weather, you can use less. For thin pours (say, 1/8"), you'll need more hardener. For thick pours such as we generally do (over 1/2" thick), the casting will retain more of the heat it generates so you'll need less MEKP. Some pigments retard curing so more hardener is needed.

Now, a fast (hot) cure will have two noticeable effects. First, the chemical cross-linking between polymer chains will be less complete, so you'll wind up with a more brittle blank. Second, hot resin expands more, and therefore will shrink more when cooling down. The cast will pull away from the mold more and may tend to crack.

As previously noted, really thick pours are prone to cracking. You can avoid that by pouring in stages. Tint the full batch of resin, but split it into a smaller amounts. Add the MEKP to one and cast. When it has cured (is solid) but while the surface is still tacky, mix and pour another layer. Continue until the full cast is complete. As long as top is tacky when you add resin, the layers will chemically bond to each other, forming a solid crystal. If you wait until the surface is hard, the layers will not bond chemically (the bond will be mechanical instead, which is not nearly as strong). This is how thick fiberglass layup is done (except that the layers are thinner).

I hope that helps,
Eric