A Series of Questions

[amphoterik]

Hello Everyone, after reading through the first page of posts in this forum, I have some questions:

Currently, I am using the Oomoo 30 Silicone and resin found in this kit here . I am noticing the resin is very plyable even hours after casting. Is there a harder resin I should be using, or is all resin the same?

I am attempting to cast pieces with a lot of sharp corners and spikey bits. I am cutting airways, but bubbles keep getting stuck in my 2-part molds. The problem is that air is having trouble escaping from the ends of longer spikes, or spikes that face perpendicular to the base. Would pressure casting fix that?

I read the posts about air compressors and Mang's setup, but I guess I am still a little unsure of what I would need. I gather that I need a pressure pot, but what kind of compressor? I looked at a local sttore and found a compressor that was 2.0H, but it also listed a volume. Is there a specific volume I would need? Why do compressors even have volume? Is the compressor and the pressure pot the same thing?

Whats the difference between using a pressure system and a vacuum system for casting? When are they usefull?

How do you have time to pour the resin, seal a pressure tank, and pressurize it before it sets?

Any help would be greatly appreciated.

Thank you

[paulson games]

I use the OMOO silicon and smooth cast 300 resin for my parts, they are usually only slightly pliable when they first come out and within half an hour of casting they should be quite rigid. Is your mix a proper 1:1 ratio? If that's off it might be causing them to cure much slower which could account for the pliablity.

Micromark.com has a really good resin but it's slightly more exspensive.

The differance between how the pressure system and vaccum system works is that the pressure system crushes the bubbles down to a tiny size and at the same time it alter the viscosity characteristic of the resin which helps the bubbles flow up and out of the mold. It's not perfect and you still need well placed vents for the air and resin to escape from in order to get a good cast.

With Vaccum it sucks the air out of the mold literally, allowing the former air pockets to be filled completely with resin. It also helps suck the air bubbles out of the resin. You don't need as many vents on molds that are going to be used in a vaccum system as each part only needs one sprue connection, where pressure parts need at least two or more sprue connections.

Using either system will probably help get clean casts that are free of bubbles, another trick to try is applying baby powder to the mold and then shaking it off so only a very light dusting remians, this will usually help break up bubbles.

It's possible to combine both systems to get maximum effect, do the vaccum first and then pressurize second. But that's probably overkill for what most people will need. It also requires a resin with a slightly longer pot life so that you have time to get a proper vaccum and additional time for the pressure to be applied.

The air compressor and pressure pot are two completely seperate items. (you'll need both for a pressure system)

The pressure pot is a heavy duty metal pot that has screw down clamps and a rubber seal on the iside of the lid. You can place pobjects inside bolt the lid on and feed compressed air into it via hoses.

This is what I use (removing the spray gun and extra hoses)
They sell this same item on ebay slightly cheaper.
http://www.tcpglobal.com/spraygundepot/ ... TCP+PT8312



Air compressors have a built in tank that stores the compressed air which is why they have a volume listed. (you can't open or close the tank as it's sealed) The larger the air tank the larger it's volume is, basically what that lets you do is fill more space or use an air tool longer before the compressor has to turn on again. If you are using a 2.5 gallon pressure pot then you will want a compressor with at least a 4 gallon capacity, that will let you pressurize the tank very quickly. I use a 6 gallon compressor and it takes me 5-10 seconds for my tank to pressurize to 42 PSI.

If I used a smaller tank, say 2 gallons it doesn't hold enough air to completely fill my 2.5 gallon tank so that means that the compressor engine will kick on and take a minute or two to get to the proper pressure level. The engine will run more often and it'll take longer to get things to the correct level.

In short you want a compressor that is at least a 4 gallon capacity, if there's not a huge price gap I'd suggest a 6 gallon one.


Here's the compressor that I use, picked it up at home depot:
http://www.homedepot.com/h_d1/N-5yc1vZ5 ... ogId=10053


If you are on a tight budget this one will also work well:
http://www.homedepot.com/h_d1/N-5yc1vZ5 ... ogId=10053

There's only a price differance of about $40, and honestly it's worth it to get the larger 6-gallon tank. It'll pressurize faster and run less often which is very helpfull as they are nosiy.

Horsepower isn't really that important for casting so long as you have a proper size capacity, a 1.5 HP will take longer to fill up the tank than a 2 HP compressor, but since you will want to be using a completely filled compressor tank each time it won't really matter how long it takes to get to that pressure level.


The compressor volume has a huge impact on getting things pressurized quickly and trying to beat the pot life of the resin.

The smoothcast 300 resin has a pot life of 3 minutes so you have to work fast, before you even start mixing make sure to have the molds sealed up and ready to go. Have the compressor charged to a proper level, and use quick release fitting for the hose connections so they can be popped on and off rather than having to mess around with thread fittings.

Mixing should only take 10 seconds which gives you about a minute and a half to get the pouring done and get the tank sealed. You have to work quickly and it's something you'll get used to with practice.

The smoothcast-305 has a pot life of about 7 minutes so it gives you a bit more time, the downside is that it takes twice as long to cure so it'll take 60 minutes rather than the 30m inutes to cure like the 300 does. Another trick that can be done is to refigerate the resin as the curing process needs heat. If the resin is chilled it'll set up slower giving you slightly more pot life time, but it also lengthens the curing time as well.


How you posistion your part can have a big impact on how and where the bubbles gather, never palce a part so it's completely horizontal as that will tend to trap air. Always give the part a slight upwards angle on one side if possible that way the air will flow up and out.


From the tutorial section: viewtopic.php?f=3&t=80

"The position of the parts is also important as it will help control where air pools and collects, causing trap points which results in bubbles in the part. Air will always rise to the highest point in the cavity; this is where you will want to attach the vent channels so that the air will leave through the channel rather than being trapped in the cavity. Also it is generally a good idea to place the wider or thicker points of the model at the bottom as it'll force the air up into the thinner area where you'll only need a single vent rather than 2 or more that a wide piece would need.

Here's another example:



On each of the parts air will rise to the top and trap in the upper surface of the cavity, so we attach vent channels to those areas. On the left and rightmost triangles all of the air rises up into the upper tip and then out through the vent. The middle triangle however has a large flat edge at the top of the piece which can cause air to trap all along that edge, you can add a vent at each corner but air may still trap in the middle of that edge, so you want to avoid positioning parts like that. By paying attention to the angles and position of the piece you can save a lot of headache over trapped air bubbles."

[amphoterik]

Thanks for the great response. That helps a lot.

If you can, would you walk me through this thought process so I can better understand how to cast odd shaped parts. Let's say you wanted to cast a 40K power fist (its the only thing I could think of right now that has multiple direction of "points". Also, I know you don't condone, and I am not asking to, cast a copyrighted part). If you don't know what one looks like, check here

Since one finger points straight, and the others point downward:

-What type of mold would you make? 2-part, split, or some other type I don't know of
-How would you angle it or make air vents
-Would you vacuum cast or pressure cast?
-Also, how would you mold the "gap" between the elbow and the power line

Thanks. I am just trying to figure out how things work before I make the decision to buy this more expensive equipment.

[mangozac]

Wow Jon has really answered all of your questions there so I've not got any more to add....

[paulson games]

Here's a rough idea of how I'd approach it. The lighter grey areas would only be added if I was experiencing bubbling in those areas.



I would build it as a two part mold. Casting it under pressure as I do with the majority of my parts.

All that'd have to be done for the space between the cable and the arm is to make sure the clay is at the same level line as the rest of the mold.

Due to the shape of the cable it's likely create a high point where air would gather which is why I'd expect to add a vent somewhere on there, same with the chain/hook on the underside of the fist. If using a pressure system It need not be a complete vent a "tail" is basically an intentional pocket that as the air compresses it shinks and pulls resin up from the surrounding piece, effectively eliminating the bubble and creating an etxra bit of flash hanging off the part which will have the bubble in it instead of the part.


With a vaccum system you could probably build the mold like this and not need an intake branch on the sprue.

[amphoterik]

Thanks for the awesome information. So, basically, with a pressure system, I would have to use a 2 part mold, but with a vacuum I could use a split mold like mangozac described in a previous post?

It seems that a vacuum is better than a pressure system, so why use a pressure system at all?

[amphoterik]

Regarding a compressor and a vacuum, assuming I get a pot that is 2.25 gallons, I would want a vacuum or compressor that is 4 gallon atleast, correct?

Regarding mold making, what do you use to make your air vents? I know sprue for the major channels, but what about the small air releases? Do you cut channels or use toothpicks (etc)? If you cut the channels, do you use a knife or some other tool? I have tried cutting channels with a knife it it is very inaccurate and tedious.

Regarding the clay. What brand of clay do you use for your 2-part mold base? I have tried sculpey and fimo, and both have left a residue on the parts, crumbled, got stuck on the pieces, and in times when left together longer, began to melt the plastic sprue.

[mangozac]

You don't need to use vacuum on split moulds, but it makes life a little easier and you don't have to worry about secondary air channels.

For small parts vacuum is generally better, but for larger parts the vacuum is unsuitable. Remember that when you put the resin under vacuum it will expand so if you don't have a small reservoir at the top of your mould then it will spill out of the top. Plus it's easier and cheaper to get hold of a vacuum pump

Don't get the wrong impression though - although the vacuum does remove bubbles, you can still have issues with air pockets if there aren't adequate channels for the air removal.

Vacuum pumps don't have a tank, only compressors do.

Secondary air channels are just cut into the silicone using a hobby knife/scalpel. For best results use a new blade and it should cut very easily. make two perpendicular cuts at 45° so that the channel ends up with a "V" shape. It is slightly inaccurate and sure you won't end up with super clean lines, but it doesn't really matter.

Those are terrible choices for clay! You need to get some non-hardening modelling clay (also known as plastecine) such as . Alternatively Newsagents sell small packs of various colours, but it's nicer to have just the one neutral colour like tan. I think I've got about half a kilo (one pound) of the stuff and that's suited me for everything.

[amphoterik]

You have all been very helpful. Thank you. So it sounds like I should go with a vacuum since I will be doing smaller pieces. Any vacuum recommendations for a 2.25 gallon pot?

Also, Mang, what type of resin do you use?

Thanks again

[silent35]

Welcome amphoterik. I recently went through the process of putting together a pressure casting system. I went to Harbor Freight to get most my major equipment and ended up with a 2-1/2 gallon paint pot:
[url]http://www.harborfreight.com/2-1-2-half-gallon-pressure-paint-tank-66839.html[url]
And a 2 HP 8 gallon compressor:
[url]http://www.harborfreight.com/air-tools/oil-compressors/2-hp-8-gallon-125-psi-portable-air-compressor.html[url]

Which ever pressure pot you go with, inspect the air outlet under the lid. Some have a side-way diffuser, others shoot the air straight down. This could mean you have an air jet shooting straight into your mold or resin while your pressurizing. The one I got has a side-ways diffuser. You may also want to make sure you have a small round flat board that fits inside. The bottom of these pots are slightly rounded.

And that is not the end of the purchases. You will also need an Air Hose, quick disconnect fittings for the hose to the compressor and pot, fittings to seal off the pressure pot, and compressor oil. I had to go back and forth to the local Sears next to me for different fittings.

Just my 2 cents.

[amphoterik]

Thanks silent. I have a sears and Harbor freight next to me as well (you dont happen the live in canton ohio do you?) I will have to check them out.

[mangozac]

[amphoterik]

Thank you Mang.

How close do you need to be with measuring the resin? So far, I just use clear cups and eyeball it so that there is less cleanup. Should I be using measuring devices to be 100% accurate?

[blind pig]

[amphoterik]

Thank you for the reply, blind.

As far as mold release goes. Do you apply mold release then talc, talc then mold release, or is talk a sufficient mold release?