DIY Roto-casting vinyl like head thread

[Tothiro]

After posting in another thread this youtube DIY roto-casting video (and there are others of use that will pop up with it - I think a straight up gear assembly would be better than his band system for interior rotation - his seems a little sticky) I got to thinking about Smooth-On Product.

There's a project I'd been working on for a bit which would require vinyl-like casting for the heads (with rooted hair) and I had been planning on calling Smooth-On up and grilling some customer service people on what blends of flexible material to put together to achieve a barbie/mego-like result. I remember there was a user here that was doing fairly pricey cast runs using this material on eBay... seem to recall they'd claimed to have been a former smooth-on rep. I also recall they were being pretty tight with information to protect their niche.

Has anyone done any footwork in that direction? I know it will probably fall to me, but I haven't been able to scrape the cash together recently for product experiments, and I'd like to know if anyone has in case we can share notes.

Anyway my first idea brought to the table was this completely water-soluble sculpting wax I'd found... I figured in the absence of roto-casting, I could make a water-soluble wax blank to suspend inside the head mold and cast around it, then soak the cast to empty the interior. Low tech enough I guess... but pricey and complicated for anything bigger than a run of two or three.

Now that I've got some idea of how to build a DIY roto set-up, I think it's just a matter of time until I go this direction instead...

I'd love to get some brainstorming about this going on. Anyone?

[jimsmegos]

I would also like to see this thread expanded on. I am very curious about the process myself.

[Tothiro]

After jogging my memory around the block I recalled the user that was eBaying the material - but he's long since gone I guess. Here's a thread with an example and I know a few members were buyers...

The roto set-up would also be great for torsos... I've got a modified female body working just as darklord1967 had started way back here and the thing with that sculpt is the spine is so far inset into the body that you can't drill straight through from one shoulder to the other... being able to roto cast it would be a great time-saver.

[rche]

couple of questions for you Robert,

For the torso. Wouldn't it be easier to cast a two part torso and glue it together?

As for the rotocasting. Could you just skip the roto machine and put a jig on the end of a drill. You could create the y axis by rotating the drill slowly by hand as the resin set. Should work with a fast setting resin. I wouldn't want to do that for more than a few minutes, but if you were just testing things to see how they set up, it might be easier than contructing an entire 2 axis rig.

[Tothiro]

Torso:
You would think. DM's and CAW's bodies are like that. It requires two separate sculpts with a number of keys and channels to make sure the halves lock together securely and the interior bands or elastic don't travel down between the halves at the ribs. So there's a bit more engineering, and then you have a separation line which is I suppose similar to a regular mold line, but still. It also increases the weight because of the thickness needed to make secure keys and channels.
Many people wouldn't find this a big deal, but I like the look and lighter heft of a blow-molded-like torso. I did a slush mold for my HB and Roger that I did back in 2002, and liked how they turned out:


Roto:
Faster setting resins are best (two to five minute set time) but I suppose you could leave a rig on higher rpms for a long time with a slow set. That would push all the material to the walls evenly through centrifugal force. You'd want a really secure rig for that though. I'm not sure there'd really be benefits either - I'm just saying it'd be Mr Wizard worthy experiment to see what you got. Though if one were using a semi rigid vinyl like compound as suggested here, the higher speed would be a major plus, because in that case you want the material pushed out as hard as possible to stay thin against the walls.

What the rig is normally doing is already exactly what you'd do by hand with a slush mold - just rotating it enough to coat the walls by action of gravity on the liquid, not forcing the material out centrifugally. 25-30 rpm is usually optimal for that, which is about as fast as it gets turned by hand.

The difference is consistency of motion. This is important because when doing hand slush molds, often the material won't spread thin enough and what you're doing is sloshing one large mass of material around which leaves a trail behind it. This can leave large sections which are too thin, as well as cross-connected areas that happen when the resin begins to set up but contacts another wall - it's not moving as fluidly and has multiple contact points when it hardens, so you've got a random mass of plastic in the center. This is only an issue for some things - like helmets, etc. So for those (and what I did with HB & Co above) what you do is multiple castings... one to get a good fast skin layer on the inside of the mold, and then turn it upside down to drain out while everything is semi fluid - followed by multiple castings the same way to build the cast walls up on the inside. This uses a ton of resin.

The problem with setting a rig up on the end of a drill is weight. The mold, resin and mother-mold plus drill get super heavy really fast, and it's awkward to control. The other problem with that is if you manually turn it on a y axis, you 1) don't have a constant angle for the axis because it wobbles all over, and 2) you are, by physics and physical probability, not turning it on the center of gravity for the mold. The last part is pretty critical... because what you'd have to do is turn the drill in rotation around the almost stationary mold, not the mold around the drill. Otherwise the centrifugal force is once again pushing the resin out to the end of the big stick you're spinning, and all the material ends up on one side of the mold.

Also I'm not a big fan of the drill itself in this guy's set-up. I imagine it'd be pretty hard to get something as low as 30 rpm from a cordless like that. You'd have to use reduction gears and blah blah blah...
Even the digital dremels I've read about have 1,000 rpm increments... so I'm thinking a hand-crank would be the way to go... then you'd also have more intuitive control of the speed. Or the best of both worlds would be a removable hand crank you could swap out for the drill if the recipe for some vinyl-esque material were worked out.
I think a gear system for the y axis instead of the pulley he has would be better too - you can see in the video where the y axis on the rig wants to stop and start out of sync because of gravity, inertia and the jerky, slippy nature of the rubber band pulley get up.

So in brief:
I really like this rig idea and I think there's a way to simplify it, perfect it, and build cheaply

[Merlyn1976]

Would a rock tumbler work?

[megozilla13]

I sort of remember that before Doc Mego started producing stuff in China he was experimenting with using a small roto cast machine you crank by hand. I remember someone posting a picture of it a very long time ago.

mikej

[rche]

The problem with setting a rig up on the end of a drill is weight. The mold, resin and mother-mold plus drill get super heavy really fast, and it's awkward to control. The other problem with that is if you manually turn it on a y axis, you 1) don't have a constant angle for the axis because it wobbles all over, and 2) you are, by physics and physical probability, not turning it on the center of gravity for the mold. The last part is pretty critical... because what you'd have to do is turn the drill in rotation around the almost stationary mold, not the mold around the drill. Otherwise the centrifugal force is once again pushing the resin out to the end of the big stick you're spinning, and all the material ends up on one side of the mold.

ahhh, yes

all the goop going to the end of the lever.

[spamn]

I saw this video a few months ago, but haven't found a cheap construx lot yet to build this contraption and start playing. And now that everyone knows, I'm betting that contstrux lots will be more expensive for awhile. So be prepared to spend $40-$60 for a lot with enough pieces to built what's pictured in the video.

Small price to pay if customizers can start making squishy heads and hollow bodies, though.

[spamn]

Hey, wasn't there a toy that had two handles on each side that you could pull, then push, over and over again to spin something in the center? If we could use those handles to base a design around, then imagine how much fast resin casting you could do while watching an hour of TV!

[jessica]

OK...how about a DIY hand cranked skein winder? Same kind of principle...you would just have to rig it so that you can rotate your mold to various degrees to keep the resin contained in the mold...

[SlipperyLilSuckers]

^ ingenious Jessica. This thread is really interesting!

[mego73]

After jogging my memory around the block I recalled the user that was eBaying the material - but he's long since gone I guess. Here's a thread with an example and I know a few members were buyers...

The roto set-up would also be great for torsos... I've got a modified female body working just as darklord1967 had started way back here and the thing with that sculpt is the spine is so far inset into the body that you can't drill straight through from one shoulder to the other... being able to roto cast it would be a great time-saver.

I have his email but haven't heard from him in a while. That head is solid but it is a more flexible plastic. A hole is drilled in the neck and it makes the neck plug even more flexible.

[jessica]

I'm thinking there's got to be a way to McGuyver something simple...perhaps with the aide of some bevel gears to be able to hand crank something and yet keep the resin from sloshing out by providing just the right angle:

[Tothiro]

I'm thinking there's got to be a way to McGuyver something simple...perhaps with the aide of some bevel gears to be able to hand crank something and yet keep the resin from sloshing out by providing just the right angle:

Well you can make a cap plug for the mold really easily... In fact that's what I'd want to do anyway with head molds. It just requires removing the sprue you suspend the original part on before cutting the mold open, slathering everything in mold release and pouring a third mold section (the plug) until it's level with the rest of the mold wall.
You'd only need to add a second set of outer plates or mother-mold walls to keep the plug in while it's spinning (so one set of plates to hold the mold together, a second set of plates to keep the plug in place). I would think some quick release velcro straps would be ideal to replace the typical rubber band set-up. They could even be riveted onto one plate of the mother-mold walls if one were to go with standardized mold box sizes so they could be re-used for any mold set-up and be fast assembly.
This'd be fairly no fuss - with a pour-to-spin time of just a few seconds.

If you check out this DIY vid from a Rochester design school, they've set-up their system as a gear assembly - you just have to counter-weight the other side of the outer frame with the same amount of weight as the extra gears - so everything is in balance.

I found their mold suspension system interesting if impractical for what we'd be doing... it's just elastic bungie cords since they seem to be focused on vacuformed molds.
I was thinking yesterday that there must be some kind of slide system or something you could make to secure the mold which would auto-center it at the same time. Securing the mold in the center is the only part I'm still a little fuzzy on...

Here's another site: Some Instructions

I like that the rochester vid uses a bike chain instead of a rubber belt, because that would keep the belt from slackening over time.

I have his email but haven't heard from him in a while. That head is solid but it is a more flexible plastic. A hole is drilled in the neck and it makes the neck plug even more flexible.

How relatively flexible would you say the material is? Is there a commercial object or toy part you could compare it to? I'm curious if this stuff might be re-formulated for casting belts and things... Would also want to know if it might get fragile or brittle at thinner thickness... It wouldn't work for hollow casting or belts if it doesn't have a decent tear-strength.