Down tothe basics of it all

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BradG

BradG

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Hello Chaps
Ive been reading information in the library with regards to different types of plastics and molds etc, but i still dont have a great grip on the basics of casting.

Ive been playing with a formula of Polylite, which is needless to say a polyester resin.and i quite like it. easy to turn down and quite forgiving, though a PITA to sand down without it all clogging up the sandpaper.. tried it wet and end up a slurry everywhere lol :biggrin: any tips here?


You may have sen the pics of the rotisserie i have been using which works well but ive had some issues with small bubbles and parts where the resin had ran a small touch and not given me a decent levelled fill in places. I am guessing that a vacuum pot would be the way forward to suck all the air out? also, whats the use of using it as a pressure pot? :rolleyes:

As you cansee... some pretty basics questions though they are some blanks in my head im trying to fill... haha no pun intended.
 
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Hi Brad - Try thinning your resin, degassing the resin before pouring and increasing the rpm of your roto cast unit.

I use sytrene with polyester resin to thin it to reduce the chance of entrapment of air. Not sure if polylite would be the same solvent, but your supplier could tell you. Of course, thinning the resin will increase the amount of ultimate shrinkage, so it's even more important to have a thick layer poured on the blank. (See next 2 paragraphs on higher rpm and degassing.)

it's also important to not apply too thick a layer right a way. This increase the chances of entrapping air. Increasing the rpms (50 - 80 rpm) should allow you to quckly apply a thin initial layer and then build it up. Higher rpms will also allow you to carry a thicker layer of resin on the blank before it starts to fall off. There's a limit to how fast you want to spin though. Too fast and you start to get standing waves that will starve parts of the blank for resin. (If you really go too fast you of course will fling resin all about the shop. Not an optimal outcome by any means!).

Applying in a vacuum while casting would be ideal, but you need to rig up a special setup to rotate your blank and deliver an even coat of the resin in the vacuum. You can't stop rotating until the resin has set (stopping at the gel stage is not a good idea, wait for set up). This seems more trouble than it is worth to me. Degassing the resin with vacuum after mixing in the catalyst will significantly reduce entrapped air in your final castings. Vacuum the (preferably thinned resin) to degas then roto cast.

If you want to abandon the roto casting (something I would not recommend), then using a vacuum to do more traditional "tube in" type casting would also work.

Good luck.

Ed

Who doesn't think using a CNC is cheating......
 
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Kretzky said:
Brad have a look at Curtis Seebeck's website
TurnTex Woodworks - TurnTex Woodworks Penturning Resources
scrolldown & you'll find some videos showing the use of both vacuum & pressure.
Vacuum for drawing air out of wood (when stabilising)
Pressure for forcing bubbles out of pours (when casting)
hope that helps
Click to expand...

Thanks for the link il take a look :) Dont like the idea of using pressure, that wont force air out of pores, it will only compress the air.. it will expand again when the pressures removed.

parklandturner said:
Ed

Who doesn't think using a CNC is cheating......
Click to expand...

Thanks for the info Ed its appreciated. I thought a mold would be better than a rota if using a a pot would force resin into all the gaps so to speak.

And as for the CNC haha i will admit they have a place :biggrin:
 
There can be 2 reasons to get bubbles in castings. The first is due to dissolved gasses in solution which come out when heated or disturbed. Pretty much the same thing as carbonated beverages. These can be minimized by degassing in an ultrasonic bath, carefully in a vacuum, or by thinning the resin by heating or with solvent. The use of pressure while casting keeps the dissolved gasses in solution until it is cured - again think of the carbonated beverage analogy. While sitting on the shelf there are no bubbles as it is still under pressure with the cap closed.

The second type of bubble comes from the resin not wetting the surface. To eliminate these, the surface should be very clean and free of oils, etc.
 
BradG said:
...I thought a mold would be better than a rota if using a a pot would force resin into all the gaps so to speak...
Click to expand...


Hi Brad - Using vacuum with a mold will allow you to get resin into all the little recessed detail areas. There are some things I do where this is the best process (for me anyway). The downside is that it will consume a lot more resin than roto casting. This means extra cost in wasted resin and (more importantly) a lot more work to remove the extra resin to reveal your finished piece. Since you would probably be using casting resin in the mold, it will also take longer to get full cure.

I wasn't satisfied using pressure to hide the bubbles in carved castings. It looks good initially, but I found that over time the pressurized gas could telegraph to the surface of the resin. With thick blocks of cast resin, this is less of an issue. But in the type of work you are looking at doing with thin laysers of resin in shallow recesses, it was problematic for me.

For applications like yours (which is sort of a faux champleve) I find rotocasting and using a laminating (instead of casting) resin to be a much better solution than mold casting. I can get away with using as little as 1/8 teaspoon (about 1/2 ml) of resin to fill recesses in one of my carved blanks. The cure is quick and I can get about 6 carve / cast cycles in a day.

But the big question that I think you need to be asking yourself is "why not go for real champleve using vitreous enamels instead of mucking about with resins to create a faux champleve?". Of course you would probably want to switch to another metal than aluminum (unless you want an extreme challenge). Enamels would be the perfect complement to your etching.

Ed
 
BRobbins629 said:
There can be 2 reasons to get bubbles in castings. The first is due to dissolved gasses in solution which come out when heated or disturbed. Pretty much the same thing as carbonated beverages. These can be minimized by degassing in an ultrasonic bath, carefully in a vacuum, or by thinning the resin by heating or with solvent. The use of pressure while casting keeps the dissolved gasses in solution until it is cured - again think of the carbonated beverage analogy. While sitting on the shelf there are no bubbles as it is still under pressure with the cap closed.

The second type of bubble comes from the resin not wetting the surface. To eliminate these, the surface should be very clean and free of oils, etc.
Click to expand...

Hi Bruce - That's interesting about the dissolved gases. I hadn't thought about that in the context of resins. It does explain some of the different stages of foaming I see when degassing with vacuum. It could also explain why a non degassed resin that I know had no bubbles when poured might develop some in a hot cast. Thanks for that tip!

The problem that Brad is going to face is that all the little recessed areas he is creating will trap air if resin is poured over them rather than into them. Applying pressure to make the trapped air smaller will create problems down the road in the finished pen because the resin layers in the recess are pretty thin. Applying vacuum can get all the trapped air out of the recesses and eliminate any bumps appearing in the finished surface at a later date.

Ed
 
Ed - +1 for champleve - not that it doesn't create problems in itself.

As for pressure making bubbles smaller, they really don't get too much smaller when you do the calculation. The gas laws say volume is indirectly proportional to pressure. Since bubbles are almost spherical, the actual diameter of the bubble doesn't change much under the typical pressure we use. Like you mentioned earlier, thinner layers would be more helpful, but I still like your enamel idea even better.
 
Others have beaten me to it, but here's my 2 cents worth. (must learn to type faster:))

I've done some plastic model casting in the past using Polyurethane Resins and Silicone molds. The ideal was to mix the 2 parts and then use a vacuum to degass the resin or silicone. This would ensure that there would be no bubbles on the surface or in the casting/mold.

I thought that pressure casting was to compress the air bubbles out of existance (or to micorscopic size). Very important if clear casting or you have to turn/cut the casting. I just googled it and found this to confirm. Seems that the pressure allows any air bubbles to dissolve into the the PR giving a bubble free cast. Once the PR is set bubbles can't form.

For your application and equipment I'd suggest that you vacuum the resin once mixed, initially thinly brush it onto the barrels and work it into every nook and cranny. Seperate out some resin for this as the brush will intoduce air into the resin. Let it sit a while so any bubbles can come to the surface, pop them if you see them. Then do very thin pours so minimal air gets trapped and bubbles can come to the surface easily. Again pop any you see.

The other option is to make your own silicone molds or buy some that will suit your barrels and then pressure cast. Less time and mucking about with the PR. I'd still brush some PR on initially, wait, pop and then do the final pour. If you already have a vacuum setup use that as well to degass the PR. If you make your own molds you can make the casting thinner (closer to barrel diameter) and save PR and turning time.

TonyO.
 
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Thanks for the info Bruce, Ed,Tony that gives me plenty to go on, and il certainly have a play with the enamel. never used it before though can see the appeal of it. Can also imagine it being difficult to fill level on a round object lol.

And it seems i have an excuse to own a vac chamber :biggrin: How long does this process take out of curiosity? as im curious about mixing the hardener before or after, though if after i presume that would defeat the object of degassing it.

Bruce id imagine having a clean base may have played a part with the smut involved from the etching. perhaps I should focus a little more on prepping the surface beforehand.

Being a scuba diver im weary of expanding gasses, so most probably too over cautious with pressurising air bubbles in blanks.
 
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Brad - Chamleve enamel uses a mixture of water and enamel with a touch of binder so it had enough body to be used on round pieces. Usually takes several layers to build to final dimension. Then it needs to be sanded down with abrasives designed for glass. While the results can be dramatic, you will likely need all your patience and experimental skills. I suspect like your etching, it won't come out the first time.
 
The degassing time is dependent on the amount and viscosity of the resin. It could take around 5 to 8 minutes for small amounts.

The resin will foam up and may increase up to 5 times the original volume, so use a much larger container. Even so you may have to reduce the vacuum to stop the resin overflowing, and then apply the vacuum again.

If you add the catalyst after degassing the resin you can just stir smoothly to mix, this will not introduce any air. To be sure you could give it a quick degass after mixing. As there should be minimal air in the resin you might be able to get away with a quick degass.

TonyO
 
My guess would be enamel is far harder than Aluminum and if it requires sanding, well we all know if two surfaces are dramatically different hardnesses then its easy to sand it oor. Probably be beneficial to use a glass file and jig it to the cross slide to maintain even filing.

Anyhow, I was intrigued, I searched online but found no reading material at all. I don't know much about it. I'd like to read about it if someone would point me the right direction I'd like to read more. Thanks
 
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