Heads up: Barnes 4PU Polyurethane NEW FORMULA

[mangozac]

Hi guys, I just wanted to give our Australian RA members a heads up that around 6 months ago Barnes have changed the formula of their 4PU polyurethane resin. I only this week started using a fresh batch that I bought late last year and was having some issues with it.

I then discovered that they are using a new formula and instead of the 1:1 by weight mix ratio in the past it now has a 10:9 mix ratio (which corresponds to 1:1 by volume! I was quite annoyed by this (both the fact that I wasn't made aware of the change before I bought it and that they changed what was already a great product). 10:9 is such a pain, and mixing by volume isn't really practical at very small quantities. So I emailed Barnes with a complaint.

I received an informative reply from Ross and he explained that the reason for the change is that the old product cost had suddenly risen by 250%
He also said that the new product was far superior when cured and gives an extra minute of working time. Plus many of the users are mixing larger quantities by volume so it suits them better.

Initial tests show that unlike the old 4PU the mix ratio is really critical (get it off by much and it never cures to a hard state but rather a flexible, rubbery state). It also seems to generally take longer to cure. I need to do some more test casts and get used to the 10:9 mix ratio...

[Munkey Joe]

Understand the reasoning BUT 10-9 seems like a super PITA.... Jeez. At least they were fairly nice about explaining it!!

[mangozac]

Yeah tell me about it - at least 10:9 is only a 10% difference so not difficult to calculate mentally. Ross was very pleasant about it - made me feel guilty for sounding a little short in my first email to them!

[johncast]

Good to read and get some info on the 4PU . i was informed it was to change when I bought my last batch in Nov. I got one of the last bottles and the lady there at Barns mentioned that the mix ratio was going to be different with the new batches..

Looks like it is back to the ready reconer on the wall again. Few years ago the Easy cast I think it was was a ratio of 1 to 1.13, now there is complexities. I made a ready recona wall chart that went from 3 grams to 100grams simlpy mix this much A to This much B . Better mix ratios are acheived with a good scale too , I can weight to .2 of a gram accuratcy . kitchen scale don't cut it unless you are mixing bigger volumes.

Johncast

[mangozac]

Yeah I run a nice set of electronic scales with a 0.1g resolution, so I can get appropriately close. A wall chart is definitely a good idea!

I have to say though, after more testing of the new formula I'm still really totally pleased with it. Demould time is honestly about twice as long as it was before (for my kine parts at least). Also, the mixed viscosity is really thin so it's really easy to spill it as you're moving moulds around. I also found that it requires much larger reservoirs on the split moulds, since under vacuum it seems to expand a lot more and spill out of the top (my temporary solution is to not take the vacuum too deep).

John do you know of any other products similar to the old 4PU?

[johncast]

Hi Mangozac

Only other products I've used that are similar are Easy cast and Super cast both from Barns.
the only reason I used these is that Barns was out of stock of the easycast so I used the Super cast and then the 4pu , The 4pu was pretty good, I see how things are when I get in a new shippment which will be in the next week of so.

Back in the early 90s when I started to cast resin models i used a resin called UC 30 a 5 to 1 mix I think or 10 to 1 . took a while to set and killed of the mould pretty quick too . The barns rubbers are better today, do like the 4601 Pink and some moulds in the blood red 4642.

Johncast

[mangozac]

OK thanks for that. I tried procast before but I found it nowhere near as good as the 4PU.

This is the problem when they change something that was already perfect! Looks like I'm just going to have to get used to longer cure times

[mangozac]

I can't do it. The new 4PU is just unacceptable for casting models. While the pot life is nice and long (really, far longer than necessary), the demould time for fine parts is at least 2 hours and it takes over 24 hours for it to cure decently. But even the cured state isn't as hard as I'd like. The new stuff is 65 Shore D while the old stuff was 75 Shore D and evidently that's a significant difference.

At the suggestion of some of my good casting mates I'm going to give Smooth Cast 305 a go. 7 minute pot life and 30 minute demould time sounds amazing. 70 Shore D is inbetween the new and old 4PU hardnesses. And it's actually comparably priced to the 4PU. I can't see any disadvantages other than the fact that Smooth Cast is only available in 0.86kg and 7kg quantities which are too small and too large for my requirements...

[SSB]

In the technical bulletin of smooth cast 305 they say that after initial curing, you have to do a heat curing of 65°C for 4-6 hours.

Do you do that ?

[mangozac]

Oh I didn't see that

No, I don't do that. The guy from Barnes suggested the same thing for the new 4PU, but it's such a pain in the butt.

[mangozac]

Reading the post curing via heat is only suggested - it is not stated as being necessary.

I'd say it's certainly worth getting a trial pack to test out.

[SSB]

[mangozac]

That's only to speed up the final curing. It should still be demouldable within an hour and Deadmeat assures me that after 25 hours it will be nice and hard even without the postcuring.

[johncast]

Hi All

Thanks guys for posting this info and letting us know regard the new 4PU mix . I am out of resin and am off to Barns this morning to get more, The longer work time sounds good but not the hardness.
With all the problems accociated with this resin casting , casting procedures and all the other difficulties as if we need these changes to the resins. All this post curing routine stuff is just another headach we could do without. After removal from the mould , my castings sit on the work bench , a smooth flat bench for a day at what ever the outside temp is. has worked for the last 10 years. and thats for different types of resin from barns . 4PU, super cast, Easy cast,
The only problem I get after demoulding is i find sometimes the last few drops out of the mix pot when pouring the mould pot land in the mould and these leave a sticky area in the casting if I don't remember to stir the resin a little after pouring into the open mould.

johncast

[SSB]