Molding and casting

[modelnutz]

Just wanted all to know....
Moldmaking and casting is a major part of my day job....
If anyone has questions about materials or techniques...feel free to ask.

I'm glad to help out.

( It's a Karma thing ;-)

[bwayne64]

Hey Man, thanks for that offer. I'm getting ready to start casting for my own builds, and could use the help. Specifically, I was wondering about Roto casting. What is the best resin for this, and are there any tricks to getting the bubbles out. I heard that they naturally work out as the resin runs around in the mold, but wasn't sure. Will I still need to vacuum degas before molding, or can I skip this ? I like the idea of thin, hollow castings, plus these will be rather large parts. Want to use as little resin as possible for a good part. Thanks in advance,


Joe C

[modelnutz]

Good questions.... i'll try to answer a few as well as toss out a few extra tricks I've picked up over the years.

First...resin. I've used Smooth-on's SC-300 quite a bit and it makes for a decent roto material....have to work fast though.
A better option would be to use Smooth-on's roto plastic.( I'll get some more info on it and post the actual name of the material)
It goes through an interesting and useful transition phase as it cures....it gets thick like soft peanut butter then kicks solid.

As for bubbles...they're not that much of an issue with Roto-Molding.
I would, however, recommend that you dust your molds with Talc
( baby powder works fine...be sure to get Talc based... not cornstarch based) as this step has the effect of reducing surface adhesion, allowing the resin to flow into details as opposed to bridging over them.

You do not have to de-gas your resin when Roto-Molding either.
I'd still recommend de-gassing your silicone for better mold life.

Remember that when roto-molding, you may need to do more than one coat to get a decent wall thickness built up.
Timing is important here...you want to do a second coat as soon as the resin has kicked....but no longer.... you may have adhesion issues if you wait too long.

One final trick that I've learned is that it's very helpful to let the casting "breath"....as resin kicks, it heats up...this causes the air inside the casting to expand....pushing outward against the mold...and can cause distortions in the final part.
I get around this issue by making a pour hole to match the diameter of a silicone tube ( I find a tube that matches the dia. of a drinking straw...more on this in a moment ). When I do a pour...I use the straw as an extention to my pour spout on tne polyproplene bottle ( I use as a pouring bottle ) so I can slide the tube deep into the mold.
Once I've poured the resin, I replace the straw with the silicone tube ( greased up with vaseling ) which I push into the mold so that it extends an inch or two in to the mold cavity. This allows the mold to breath so the ( heating) air can escape. Because it extends deep into the mold cavity...resin doesn't leak out ( much ;-)
Because the silicone tube was greased up with vaseling...it's easy to remove...leaving a clean pour hole for my second ( or third..etc.) pour.

Well, there's a few "pearls of wisdom" for you.....I'm sure they will cause more questions to pop up...but that's cool...ask away !

Best of luck to you on you new endevor...and remember..nothing's easy at first....but stick with it...you'll learn the tricks.

[erospawn]

Hey Ed.. Thanks for that info that really helps! If the parts are really large, you could use the brush rubber right? Mold-Max 30 or 40 brush on version, atleast trom what I've read so far.. and you don't need to degass that do you?
Also, I've looked around, and the only vacuume chamber I could find was the one Smooth-on sells. What other brands are there for this?

[bwayne64]

Thanks Guys,
Great Info there, will be very helpful. Sorry for late reply, computers on the fritz, . Appreciate the help, Cheers,

Joe C

[modelnutz]

(11-17-2009, 02:25 AM)erospawn Wrote: Hey Ed.. Thanks for that info that really helps! If the parts are really large, you could use the brush rubber right? Mold-Max 30 or 40 brush on version, atleast trom what I've read so far.. and you don't need to degass that do you?
Also, I've looked around, and the only vacuume chamber I could find was the one Smooth-on sells. What other brands are there for this?

Mold Max 30 is the way to go...The thickening agent ( Thi-Vex) that Smooth-On sells is ment for Mold Max 20 and 30....not ment for 40

You also might consider getting the fast catlyst with the Mold Max 30
You can really control the speed of cure with that stuff.

As for de-gassing...no, you really don't need to if you're troweling the silicone. But, your first coat or two should have no Thi-Vex in the mix...
You want a thin, runny mix so the silicone will capture all the details.
( this is where the fast catlyst comes in handy...you can speed the cure so the silicole doesn't run off the master so badly )
Once you have these "skin" layers down and cured...then you can start building up to a 3/8" thick layer....the thick silicone will fill in pockets and undercuts real well also.

As for vacume chambers...I'm not that well versed on what's out there....I bought mine when I first started....it was one of my major investments ( $1,000.00 ). My chamber is large enough to hold a 5 gallon bucket. ( I don't mess around ;-)
You may want to look into jewelry supply houses...look for the bell jar type...not so expensive...smaller, but still functional.

[erospawn]

OK, so I've been playing around with this Rebound-25 and this sample resin I have. Need to make some small parts.. So, the problem I am running into is with this brushable Rebound 25 stuff.. Granted I need the mold max non-brushable.. but this stuff should work too..Its pretty thick and guey.. anyway, I have a box with 2 small parts in it.. the box is about .5 cm x 5 cm x 3 cm.. anyway, I pour the stuff in and let it sit for 6 or more hours.. and the problem is, the inside doesn't seem to cure properly so that when i pull the mold up, the part that contains the impression of the part is still wet.. as if it didn't cure. What am I doing wrong?

[erospawn]

Alrighty... what I've done is just to put on a thin layer and Im going to let it cure... then once that dries, I'll layer in some more I spose..

[modelnutz]

ero...if you are still having the same problem ( rubber not curing against the master but curing everywhere else ) you may be experiencing something called "inhibition"....this is where the rubber is reacting to the material in the master causing it to not cure.
The only option is to seal the master .
Some sort of clear coat ( Krystal Kleer, future, clear coat..etc. ) should do the trick.

Platinum cure rubbers suffer from this issue quite regularily.
It's also the reason we use sulfer free clays in sculpting and mold work.

[427 Cobra]

Hey,Ron - try spraying your master with lacquer. I've heard this is the way to keep your paint coating from reacting with the mould,and making it turn out gooey. Make sure that you've sprayed your master with automotive primer,as real lacquer - not acrylic will eat any finish that isn't lacquer based as well. Acyrlic is NOT lacquer,but it will suffice for a barrier against the mould surface while moulding. You just want to make sure that the product IS lacquer,as not everyone who claims it is on the can actually makes true lacquer!

This is highly toxic to the nervous system,and it's something you don't want to spray inside your house! I use plastikote brand,as this IS a true lacquer based coating,and not an acrylic clearcoat in a high solvent base,as most manufacturers make claims to the contrary on their labels today. Most paint is now highly regulated by the EPA,and lacquer is on top of that list. Meaning that this is one of few coatings that most paint manufacturers DON'T MAKE,as this requires special licensing,and manufacturing processes that are extremely expensive. This is also highly flammable - don't use this around any fire producing appliances. Unfortunately,most people don't know the difference between laquer,and acrylic,as they believe everything they read on a label without knowing the truth - lacquer is NOT a generic product any more than a brand name is!

Enamel,and acrylic can be mixed together without fear of ruining paint formulation. Lacquer on the other hand,can't be. It's too acidic,and won't mix with other binders! You can spray it over anything - wood,metal,plastics,but NOT OVER OTHER TYPES OF PAINT. It causes enamel,and acrylic to wrinkle,and lift. Unlike these paints,laquer can be resprayed at any time without fear of wrinkling. The solvents in it need to escape for a few days before attempting to cast anything with it.

I don't know what kind of reaction it may cause if used too early before curing. It actually takes a year to fully cure,but not to use. This just means that full hardness takes six months to a year to achieve. Most finishes take a while to cure,but since lacquer is an organic based finish it takes longer to reach full hardness,but it can be used in just a few days. Normally,you can use it in twenty four hours after use,but something as sensitive as,silicone - I'd wait for the smell to not be as strong before you use it.

[erospawn]

(12-23-2009, 12:59 AM)modelnutz Wrote: ero...if you are still having the same problem ( rubber not curing against the master but curing everywhere else ) you may be experiencing something called "inhibition"....this is where the rubber is reacting to the material in the master causing it to not cure.
The only option is to seal the master .
Some sort of clear coat ( Krystal Kleer, future, clear coat..etc. ) should do the trick.

Platinum cure rubbers suffer from this issue quite regularily.
It's also the reason we use sulfer free clays in sculpting and mold work.

hey thanks.. I just needed "greeblie" peices for the sensor array details on the main master.. So, I figurd these would be a good start to learn with.. so, basically, they are just plastic peices from a model kit .. So, maybe its reacting with the material ..

Cobra, thanks for that info as well.. Unfortunately, the master has been sprayed with Enamel paint.. However, I think Smooth-on sells a sealer though..

[427 Cobra]

Sanding sealer - this is oil based,and it won't eat enamel,or other soft paints like waterbased acrylic. You'll need spray equipment,as this stuff usually comes in a quart can,not a spray paint can. You'll have to thin it with mineral spirits before applying it the model. If this is something that you're not familiar with,I suggest an acrylic based clearcoat from Krylon.

[erospawn]

(12-23-2009, 12:59 AM)modelnutz Wrote: ero...if you are still having the same problem ( rubber not curing against the master but curing everywhere else ) you may be experiencing something called "inhibition"....this is where the rubber is reacting to the material in the master causing it to not cure.
The only option is to seal the master .
Some sort of clear coat ( Krystal Kleer, future, clear coat..etc. ) should do the trick.

Platinum cure rubbers suffer from this issue quite regularily.
It's also the reason we use sulfer free clays in sculpting and mold work.

You mean like Future floor polish? Im confused. Is this like spray paint or floor polish, laquer or something to that effect?

[erospawn]

So yea, I guess it is floor polish (Future).. its acryllic based.. so, you can't paint Lacquer over enamel.. but you can paint Acryllic over enamel.. So, since the master was painted in primer, acrylic is the safest way to go I spose.

[treker2557]

I have a vacuum chamber that is 12.5 x 21 inches high. I was just wondering how big of a vacuum pump I should buy?

[modelnutz]

I'll check the stats on the one that I'm running and get back to you with the info.

[treker2557]

Thanks

[JAWS]

Im pretty sure no matter what size vaccuum chamber you have they all say to use a 9cfm pump. I believe this to be a safe guard for any complaints. If you use the highest pull in a vaccume then that leaves no room for error & complaints. A simple safe guard & a guarantee of pulling as much air out as fast as possible. Honestly if your looking to cut costs your going to pay later down the track.

[treker2557]

Thanks, I guess I will place an order then for a 10 cfm robinair pump then tonight.

Thanks

[427 Cobra]

(12-24-2009, 05:26 AM)erospawn Wrote: So yea, I guess it is floor polish (Future).. its acryllic based.. so, you can't paint Lacquer over enamel.. but you can paint Acryllic over enamel.. So, since the master was painted in primer, acrylic is the safest way to go I spose.

Correct. This is the way to go to prevent future problems, so that you won't have to go over all this again! Sorry - I just realised I said "future" after you said the floor polish version. Anyway, use acrylic spray,or out of an airbrush. This will seal it, and keep it from making your silicone have problems with curing. If you don't use a sealer - you might very well have a gooey result from it. It's better to be safe with acrylic, than without it. Lacquer will not hurt styrene products, or cause blemish spots in silicone, but if you're not sure what the product actually is - use Krylon. I've used it in the past with excellent results

[erospawn]

Well, thanks for that Cobra, although a bit late on the advice.. I pretty much learned that when I was making the small casts for my Voyager. I kept running into the issue with my rubber staying gooey despite 6 hours of curing.. so, ModelNutz turned me on too sealing it with Acrylic.. SO, after that, when I was making the sensor arrays by hand, I sprayed them over with Acrylic which did the trick!

[427 Cobra]

OK. I hoped that this would be in time to help you in your situation. Someone else had a problem like this before on another forum, so I thought that I'd post it here to keep anyone else from facing the same troubles.

[darkimmitator]

Here is a casting rtv silicone question .If the mix ratio is 10 to 1 , would I be correct in saying ,10 ounces of part a + 1 ounce of part b = mix and pour over part being molded ? Or is it more mathematical than that ?

[erospawn]

its pretty much that simple... depends on what type of silicone you get.. its either by volume, or by wieght..

[darkimmitator]

It's CR -purple RTV Silicon mold making rubber .

[427 Cobra]

There should be some kind of instruction manual that came with the mould kit. Usually, it's by weight from most manufacturers. There are some from http://www.smoothon.com that are by volume only, but those are a little more expensive than the regular casting supplies. Do you have a manufacturers' name?

[darkimmitator]

The company is ,BCC Products,Inc/Blehm Plastics.

[ruby1058]

These some are the moulding and casting materials:
1) Hot Pour Vinyl.
2) Polyurethane Resin.
3) Yellow Net.
4) Rhodorsil Silicone compound and catalyst.