Best resin for Pressure casting

[johncast]

New forum for me and have enjoyed the reading of some interesting posts. over the last few days.

Not so new to resin castings as I have been in the casting game for 25 years and 18 of those are with resin, mostly white metal spin casting, but half of my full time work is now resin casting. Model Trains, Car parts, etc
Have only worked on the pressure casting side of things 3 years and would like to know 2 things

Is there is a good resin for this casting method. I have bought a slow set resin but who has 3 hrs to wait for each run of a particular mould. Most of the resins i have used are from barns, easy cast, 4PU , super Cast all very good for most things. the problem with slow set( Barns TC 891) is the aggressive nature of this resin I found destroying the mould in only 7 or 10 castings.Not after the usuall 30 or more. The Moulds are in the Barns 4601 pink, a good rubber after all the other rubbers I have used. Castings I did were Ok and no holes but the 3 hrs of the resin in the moulds just killed the mould .

2 With the slow set resin placed into a pressure chamber, what would be the approx time it takes for the process of the air bubbles to be eliminated from the casting . I have tried the 4PU resin in the pressure chamber on the expectation that the removal, or should that be depursal of air bubbles throughout the casting maybe a rapid action. Quick enough to be complete before the resin sets. On my experiments it indicates I am wrong . The dispercial of air is a slow process. Unlike the vacuume procedure .So if any resin is used with a set time of 15 min or less the pressure pot tec is not going to work .
any surggestions would be appreciated.

Thanks Johncast
http://www.orncastings.com

[Anvils Hammer]

surely the qualities of the resin make no difference at all to the pressure casting process..

Im not sure what you mean by "approx time for the air bubbles to disperse". Pressure does not remove air bubbles, it crushes them to a microscopic size, so the process is instantaneous, then to keep the part bubble free, you have to maintain the pressure untill the resin is cured.

I use a Resin from a Germany company, pot life is a little under 3minutes, they are "cured" but still a little soft after 15 minutes, so thats when I take the pressure off, but I will not demold untill 30 mins have passed.

I think the rubber giving you 7-8 pulls is nothing to do with the cure time of the resin, leaving the resin in the molds longer doesn't cause ant additional damage AFAIK, and a slow cure resin is still the same ammount of chemical reaction, just over a longer period..

I cant comment on the brands you are using but I know otehrs here can so im sure they will chip in.

AH

[mangozac]

Hi Johncast, it's great to have established professionals like yourself joining!

I did a bit of sampling in my early days of casting and found 4PU to be the best of a bunch of resins. Others were much more prone to bubbles (although admittedly that was using purely pressure casting rather than vacuum + pressure that I adopted since). I find the working life is long enough that I can fill several moulds, do my vacuum and pressurise comfortably without too insane a rush (of course that just depends on how many "several" is ). For the tiny parts I do I like to leave it one hour for demoulding, but larger parts could get away with 30 minutes.

Personally I don't like to leave parts in the moulds longer than they need to be and will definitely avoid leaving them for several hours. I find that demoulding while the resin is still slightly soft helps in prolonging mould life, since the small amount of flex helps release the silicone and minimises the risk of tearing.

I'm not familiar with the TC891, but reading the datasheet it doesn't sound like the most user friendly product. Do you really need that long a working time?

Like AH says, casting at pressure requires the pressure to be maintained until the resin has cured, it's not an instantaneous action. You might get some bubbles dislodging due to the rapid expansion, but the majority will still stick around.

For reference here's what I do in the 4 minute pot life window of the 4PU:
1. Mix 4PU
2. Pour into all moulds (generally half a dozen)
3. Squeeze moulds to manually get most of the air out
4. Place into chamber and set lid on top
5. Start vacuum pump and while evacuating tighten up all of the clamps on the chamber
6. Shut of vacuum and apply pressure (50PSI)
7. Leave at pressure for 1 hour, then demould

It's all a matter of being efficient in your actions
Does this clarify things?

[johncast]

Hi Anvil hammer and Mongozac
Thanks for the help and prompt replies. Anvil , and mongozac,The mentioned , "approx time the air bubbles to disperse" was a question on the pressure cast process. I understand the air is not removed ,the pressure just crushes them into micoscopic size. But is this intantaneous ?? as you mentioned Anvil. I woundered this , and have tried this with a problem mould this last weekend and had no success at all with the small items I am casting for this job at present. The mould was filled with 4Pu quick set resin and placed in the pressure tank and pressured to 50 psi and was left for 15 min as this is all the time the resin would take to set. removed the mould and items still had holes around the sprues and lower parts of the items. This is why i question whether it is instantaneous . when I used the long set resin i got good results with all I tried, big and small items. so I came to conclusion that the dispersal or crushing of air bubles in the liquid resin when under pressure takes time. With a resin that remains in a liquid state long enough for this process to take affect. Barns don't seem to have a that set around the 20 min mark it was 4min With 4PU or the 3 hr 891 stuff.
Could you clarify the meaning of AFAIK

Thanks for the run down on your procedure of casting Mongozac. With that casting procedure I am assuming you have a vacuume chamber that doubles as a pressure chamber as well . Is this correct. You do several casts at once vac then pressure the moulds with 4PU Must be a quick vacuume you have , mine takes 20 sec to full vac , Moulds removed topped up and thats it. And with you being in QUEENSLAND thats real good in the summer. My resin casting stops at 12pm here in Sydney in the summer hot days as the resin sets too quick. Or do you have AC in the wokshop.

Thanks guys
Johncast

[mangozac]

[paulson games]

The air bubbles being crushed happens as quickly as the chamber pressurizes, so usually in about 5-10 seconds.

Large pockets of trapped air can still stick around which is why you need to have it vented properly, or use the squeeze method to help work the larger air bubbles out.

Part of how the pressure works is that it alters the material viscoisty, which helps the air flow out and escape the surface of the resin easier. If there are small bubbles remaining they get squeezed down to a very small size.


You don't need to have the resin set up under pressure the entire cure time. I use stuff that takes about 30 min to fully cure and I can take it out at around the 7-10 minute mark without any issue and let if finish curing at room pressure.

While under pressure any left over air is squeezed down to about 1/60th it's former volume so a small bubble would barely even be a pin prick in size. If the resin is still very soft when it's depressurized the air will expand and stretch the surrounding resin. The resin should be strong enough to resist expansion onces it's it's passed about 1/3rd to 1/2 it's cure time.

When I let the chamber de-air I let the air out very gradually, if you do it too rapidly the uneven pressure on the mold can cause it warp slightly or force air into the resin (if it's still soft)

Every brand of resin has it's own handling properties so this may not work the same with yours, but I'd suspect it's somewhat simular so don't be afraid to play around with it a bit and see how it works for you.

Try letting it set up for about half the time before letting the pressure off slowly. Then on the next batch try cutting down the time slightly until you can find the perfect mark point that works for you. (or increase the time if it proves to be too early)

[johncast]

Firstly , thanks to all the people above for their valued help. sorry to keep you on the phone too long Mangozac. and again thanks for the call. You are never too old to learn,and after so long resin casting there is alway a part that posses problems that have to be sorted out due to shape or size of the part. Like my old boss said, If nothing goes wrong you don't learn anything. i never fale to here those words said when a problem in the casting arrises whether metal spin casting or resin casting.

Now what happened in my workshop yesterday. With the mould of 5 small items each 15mm high 10mm W and 5mm Thick I was having probs with, I cut some small air leads to the out side of the mould from each cavity. I knew this would be a necessity if the cavities in the mould were to be filled from the sprues. I had previously been filling the mould halfs with the mould flat on the table, vacuuming and squashing the 2 half of the mould together and clamping. worked ok. but still the odd bubble and castings not perfect. and locking in large air bubbles if you muckup the closing of the mould.

Filled each cavity in the mould by the sprues , squashed the mould together a few times as Mangozac surrgested and placed ito the Pressure chamber and pumped to 50 psi and left for 20 min removed and let set for another 40min on the workbench... Just perfect. not a bubble. 5 perfect castings, a little skin thin flash but thats no prob.

Went on the do 2 moulds of the same item and got the same result only 10 good casts.
Didn't vacuume the resin before pouring the mould as I don't think I would have time.
So as mentioned by paulson games the bubbles are crushed in 10 sec. I can use the slow set for other things.
So the 4Pu resin can be put into the pressure pot. I can see the casting procedures getting a little altered to suit the pressure casting way of things.
Also Mangozac I also tried the other box items I have had trouble with, however i did vacuume them first and made sure there was a good reserve of resin in the cavities so that when the vacuume was released the cavity remained full of resin. Placed a perpex piece in top of the mould to flatten the resin and then pressurised , perfect, with no trapped air down the thin side of the smaller box. All a matter of getting the right procedure for that part.
Johncast

[mangozac]

Well congratulations on getting some success!

Like we discussed on the phone, casting often requires so much trial and error to work out the best way to do things. I'm surprised that you still get good results after just 10 minutes in the pressure pot - I'll have to try it out myself one time. Although generally I'm continually casting the same parts from the same mould so I'd have to wait before I can demould anyway!

Have a look at for an example of a mould I use with the vacuum + pressure technique. The mould shown opened up at the bottom of the post is for the main hull piece shown in the second pic. You can see the reservoir in the top.

Admittedly that's not the best example of a mould, as it was always very prone to alignment issues. Ideally it would be made from a lot more silicone to help keep the alignment, but that would have increased the mould cost by quite a bit and ultimately I worked out the best way to use it to avoid miscasts. In that case I have to put boards on all four sides of the mould.

Also, that mould was back when I was using Ultrasil - for the last 18 months I've been using Pinkysil as my moulding product of choice

[johncast]

Nice castings mangozac. Looks like a split , cut down mould.
Ive always liked the 2 part mould with deep cut retangle , square or triangle alignement keys.
NEVER ROUND KEYS, that allows movement.
I will see if i can post a pic of one of my moulds. On larger moulds I like to feel the mould lock together nice and tight. The other issue with moulds is getting the correct volume for each half.
If you skimp on the rubber volume the mould is pron to distorting. I know we all want to save money. This rubber issue is also important for large metal spin casting moulds too, Ive seen people make flimsy RTV moulds for spin casting and resin casting and they put themselves behind the 8 ball right from the start. About 10mm of rubber over the item and 15mm around the item is good ,more on large items.

johncast